Co-ordination and Response

You have previously learned that one of the 7 characteristics of living organism is irritability or sensitivity. And this is the ability to detect a change in the outer environment and respond to it. A change in the environment is also called a stimulus (plural stimuli). Actions taken by the body in order to co-operate with a stimuli are called responses. The body detects a stimulus by parts in the body called receptors and is able to respond to it through other parts called effectors. Two organ systems are continuously working to detect and respond to stimuli, these organ system are called the nervous system and the endocrine system.The Nervous System:

 The nervous system is a system of organs working together to detect and respond to stimuli. The nervous system is made up of two systems, the Central Nervous System (C.N.S) and the Peripheral Nervous System (P.N.S) the peripheral nervous system connects the central nervous system to the other parts of the body.
Central Nervous System (CNS):

 The central nervous system is made up of the brain and the spinal cord. The spinal cord is basically a big bundle of nerve cells running through a tunnel inside the backbone which protects it while the brain is protected by the skull. The central nervous system is what gives out orders to other parts of the body to perform certain jobs.
The Peripheral Nervous System PNS:

 The peripheral nervous system is the other part of the nervous system. The main job of the PNS is to detect stimuli and send impulses to the CNS according to the stimuli. The PNS is made of receptors and nerves that carry the impulses.   


Receptor cells are ones whose function is to detect something about its environment. There are many receptors in the body that are able to detect many changes like temperature, touch, light, sound and chemicals. There are some organs in the body that are there to detect just one stimulus, like the eye for example. These are called sensory organs and they can be defined as a group of receptor cells responding to specific stimuli.

Effectors are the opposite of receptors. Receptors are two detect the stimuli while effectors are two respond to it. Effectors are usually muscles and glands.
Neurons (Nerve Cells):

 Neurones are one of the most important structures of the nervous systems. Neurones act as a wire that transmits electrical impulses all over the body. Like a cable that consists of many wires, a bundle of neurones is called a nerve. There are 3 types of neurones, each type is to transmit electrical impulses from a specific place to another.

Motor Neurone: This is a neurone that transmits electrical impulses from the Central nervous system to the effectors.



This neurone is made up of three segments; the cell body which is the start of the motor neurone and is in the CNS, axon which stretches out from the cell body all the way to end of the neuron, and the motor plate which is the end of the neurone and is in the effector muscle.
 Neurones have features that are common between most animal cells like a nucleus, cymiddlelasm and cell surface membrane, but they also have some exclusive features like the axon. The axon is an extended cymiddlelasm thread along which electrical impulses travel. Some motor neurones have axons of length 1 metre. Axons are coated by a layer of myelin called myelin sheath, this is an electrically insulating layer which is essential for the proper functioning of the nervous system.

Another exclusive feature of neurones is dendrites, these are several short threads of cymiddlelasm coming out of the cell body. Their function is to pick up electrical impulses from other cells.

The last exclusive feature of motor neurones only is motor end plate. This is just the end of the axon which is in the muscle. It passes the electrical impulses from the neurone to the muscle fibres.

Sensory Neurones: like other neurones, sensory neurones carry electrical impulses from one place to another. But sensory neurones carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.



The sensory neurone’s shape is unique. This is because it is made of a cell body, with two arms extending out of it. The first arm is the axon which’s other end is in the CNS. The second arm is dendrite which’s other end is in the receptor. The dendrite is similar in structure to the axon except that it joins the receptor with the cell body. The electrical impulses of the sensory neurone flow from the receptor, through the dendrite to the cell body, then from the cell body to the CNS through the axon.

Relay Neurone: Relay neurones are located in the CNS. Their job is to pass electrical impulses from the sensory neurone onto the motor neurone, so it acts like a diversion.



Where neurones meet, they are not actually touching each other. Instead there is a gap called synapse or junction box. When the electrical impulses reach the end of a neurone, the neurone secretes a chemical transmitter which passes by diffusion to the other neurone causing the impulses to be carried from the first neurone to the second.
Reflex Arc (Nervous System in Action):

 If your finger touches a hot surface, receptor cells in the skin of your finger detect a stimulus, which is a sudden rise in the temperature. The receptor uses the energy of the stimulus to generate electrical impulses. These impulses are then carried by the axons of the dendrites of the sensory neurone through cell body to axon and from the axon to the CNS. At the CNS the electrical impulses travel through the synapse to the relay neurone, which passes it onto the motor neurone. The nerve impulses are transmitted through the axon of the motor neurone to the targeted muscle which contracts when electrical impulses reach it, resulting in your finger being pulled away from the hot surface. This pathway is called the reflex arc and happens in about a fraction of a second.



Reflex Arc: RECEPTOR → Sensory Neurone → CNS → Motor Neurone → EFFECTOR

Voluntary and Involuntary Actions:

 The reflex arc is a reflex action. Reflex means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some impulses are carried by other neurones to the brain, and some impulses are passes onto the motor neurone to the effector muscle and the response takes place. The electrical impulses going to your brain are much slower that the ones going to the effector muscle directly. This is why the reflex action takes place before you realise it, it is uncontrollable. Reflex actions are said to be involuntary actions. Involuntary actions start at the sense organ heading to the effector. They are extremely quick. Voluntary actions are the ones that you make the choice to do. Like picking up a bag from the floor for example. Your brain sends electrical impulses to the effector muscles ordering them to contract so you could pick the bag up. Voluntary actions are slower than involuntary actions and they start at the brain.


The Human Eye:

 The human eye is a sensory organ. This means it is an organ of tissues working together to detect and respond to a specific stimulus, which is light.


Features of the Human Eye:
 Lens: changes shape to focus light on retina
Ciliary muscles: contracts and relaxes to adjust thickness of the lens
Suspensory ligaments: loosens and tightens to adjust thickness of lens
Iris: widens and narrows to control amount of light entering the eye depending on light intensity
Choroid: middle layer surrounding the eye. It contains many blood vessels
Sclera: outer most tough, protective layer of the eye.
Retina: inner most layer. It is sensitive to light and it is where the fovea is and it has rods and cons
Fovea: very light sensitive spot
Blind spot: Where the optic nerve touches the eye. No light sensitive cells in this area.
 How We See:

 When the light hits an object, it is reflected in all directions. When a light ray reflected from the object hits your eye you see that object. At the back of your eye, there is a spot on the retina called the fovea (blind spot). This spot is full of light sensitive cells. When the light ray falls on the fovea, the light sensitive cells generate electrical impulses that travel through the optic nerve to brain. When the electrical impulses reach the brain, the brain generates the image you see. This all happens in less than a fraction of a second.But this is the general idea only. Light rays enter the eye from every direction. If they are not focused on the fovea, they will most probably not hit it and we won’t see. Here comes the role of the front part of the eye. When the light ray hits the eye at an angle, it first has to penetrate the cornea which refracts (bends) the light ray inwards. The cornea acts as a converging (convex) lens. Then the light penetrates the lens which refracts the ray a little more inwards focusing the light ray on the fovea. And thus the light ray is focused on the retina. When the ray hits the retina, the closer to the fovea the sharper the image is.   

Accommodation:

 The angle at which the light ray hits the hits the eye depends on the distance of the object. Every light ray that hits the eye needs a certain amount of refraction in order to be directed to the fovea. This is why the lens has the ability to widen and narrow according to the distant of the object you’re looking at in order to make the light ray hit the retina at the right spot. This is called accommodation. Light rays refracted from close objects are diverging (spreading out), they need to be refracted inwards to be focused on the fovea. When you look at a close object, it takes some time till the vision becomes clear. This is because at first, the light ray is not correctly refracted, so it hits the retina away from the fovea. The electrical impulses are generated and sent to the brain which realises that the image is not clear. The brain then sends electrical impulses to the ciliary muscles making them contract. When the ciliary muscles contract the suspensory ligaments become loose, this makes the lens become thicker and rounder for more refraction of the light rays. Now the light rays are correctly refracted and hit the retina at the fovea, the image becomes clear.



For far visions it is the exact opposite. The rays reflected from far objects are almost parallel. Very little refraction should be done. The brain sends electrical impulses to ciliary muscles making them relax, the suspensory ligaments now tighten up and pull the lens which become narrow.




Distance   
Ciliary muscles   
Suspensory
 ligaments   
Lens

Near   
Contract   
Loosen   
Widens

Far   
Relaxes   
tighten   
narrows


Rods and Cones:

 The retina is full of light sensitive cells called photoreceptors. There are two types photoreceptors, they are rods and cones. Rods and cones are specialised types of neurons. They look alike but they are a little different in function.

Rods are sensitive to dim light. At night or in dark places, most light detection electrical impulses transmission is done by rods. Vitamin A is essential for proper functioning of rods, if Vitamin A lacks it can lead to night blindness. Rods are spread all over the retina.

Cones are sensitive to bright and coloured light. All cones are packed in one area, the fovea.
The Pupil:The pupil of the eye is the dark round area in the centre of it. It is surrounded by a coloured ring structure called the iris. The pupil and the together play a big role in protecting the eye from damage by limiting the amount of light entering the eye. If too much light fall on the retina, the rods and cones get damaged. The iris and pupil change their size to smiddle that happening. The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside. When circular muscles contract they make the pupil smaller. When the radial muscles contract the stretch the pupil outwards making it wider.   


In bright light, too much light starts entering the eye, which is dangerous for the rods and cones, which detect the high light intensity. The rods and cones start a reflex arc by sending electrical impulses to the brain via sensory neurone. The brain responds by sending electrical impulses to the muscles of the iris via motor neurone. These impulses make the circular muscles contract and the radial muscles relax limiting the amount of light entering the eye, thus protecting the rods and cones from damage.

If you walk into a dark room, the rods and cones sense the little amount of light. They start another reflex arc and send electrical impulses to the brain which responds by sending electrical impulses the muscles of the iris. The radial muscles contract and the circular muscles relax widening the pupil to let more light in.


Antagonistic Muscles:

 You have just learned that in order for the pupil to get narrower or wider, two muscles work simultaneously, when one contracts the other relaxes. Pairs of muscles like that are called antagonistic muscles.

The most known antagonistic muscle pair is the biceps and triceps of the arm. The bi and the tri for short, they are what causes the movement of the arm. They work simultaneously to bend or straighten the arm. The biceps is located in front of the humerus bone of the upper arm. The biceps is joined to the radius bone of the lower arm and the triceps is joined to the ulna bone of the lower arm. Muscles are attached to bones by strong fibres called tendons.
When you want to bend your arm the brain send two electrical impulses, one to the bi making it contract and one to the tri telling it to relax. When the bi contracts, it becomes shorter pulling the bones to which it is attached close and bending the arm. This causes the fibres of the tri to stretch while they are relaxed.

To straighten your arm, the brain send electrical impulses to both muscles making the bi relax in order to leave the muscle it is attached to free. The tri contracts and becomes shorter pulling the muscle it is attached to into place and straightening the arm.    


The biceps can be called a flexor because it flexes (bends) the arm. The triceps can be called an extensor because it extends (straightens) the arm.
Drugs:

 A drug is a chemical substance that modifies and affects chemical reactions of the body when taken in. Many drugs are useful to us like antibiotics, painkillers and caffeine.

Some drugs however are abused by users to feel relaxed, or reach euphoria. Euphoria is a state of mind at which the abuser feels extremely happy and relaxed. These drugs include alcohol and heroin.
Alcohol:

 Alcohol is a depressant drug. This means that it reduces the activity of the brain and slows down the nervous system and reflex actions. Alcohol can be extremely dangerous when the user is in a situation in which they need fast reflex actions.  Alcohol is addictive. The more you drink it the more you need it. The user may reach a point where they cannot do without alcohol, this is when they become alcoholics. Alcohol is broken down into fats by the liver. If the abuser drinks too much alcohol, the cells of the kidney may die shortening their life.
 Heroine:

 Heroine is a narcotic drug. This means that it relieves pain and induces sleep. Heroine is extracted from a plant called opium poppy. Most heroine abusers become addicts. For the addicts heroine become the number one priority in their lives. They would do anything to get the drug even become criminals and possess a threat to their society. If not rehabilitated, a heroine abuser will end up homeless or dead. Some heroine users inject the drug in their veins by an unsterilized, shared needle, this increases the risk of getting AID/HIV.
The Endocrine System:
You have previously learned that messages are delivered around body as electrical impulses by the nervous system. Another way messages are transported around the body is by chemicals called hormones secreted by the endocrine system.

Hormones are chemical substances produced by a gland, carried by the blood, which alters the activity of one or more specific target organs and is then destroyed by the liver.

Hormones are produced in organs called endocrine glands which make up the endocrine system. The following diagram shows the glands of a human body.

Glands are organs made of secretory cells which’s function is to produce hormones and secret them into the bloodstream. Glands have a dense network of blood capillaries in them to secret the hormones in. hormones are carried around the plasma like all other content of the blood but certain organs are able to use them, these are target organs.   



Gland   
Hormone produced   
Function of hormone

Adrenal gland   
Adrenaline   
Prepares the body for activities that need energy and quick reflex actions

Pancreas    
Insulin   
Makes liver reduce blood glucose level

Glucagon   
Makes liver increase blood glucose level

Testis   
Testosterone   
Produces male secondary sexual characteristics

Ovary    
Oestrogen    
Produces female secondary sexual characteristics

Progesterone    
Helps control menstrual cycle and maintain pregnancy


Adrenaline:

 When you get a fright you feel some changes in your body like a sudden increase in heart beat rate, blood flowing quickly in veins and your breath becomes deeper and faster. This is because the fright you got caused the brain to send electrical impulses to the adrenal glands making them secrete adrenaline hormone in your bloodstream. Adrenaline is a hormone that is secreted from the adrenal glands to prepare the body for situations that need lots of energy and fast reflex action, like fights or running away for example. Adrenaline’s main objective is to increase your metabolic rate so that you have enough energy for fighting or running away etc. This is why adrenaline is called the three Fs hormone (Fight, fright, flight). One of adrenaline’s target organs is the heart. When adrenaline reaches the heart it causes the cardiac muscle to contract and relax much rapider so that oxygen and glucose reach the muscles of the body faster. Adrenaline also makes the liver convert glycogen into glucose and secret it in the blood to be used in respiration. When adrenaline reaches the diaphragm and the intercostals muscles of the ribs, they make it contract and relax faster too to increase rate of breathing. These changes cause an increase in the respiration rate so that lots of energy is being released. Generally, adrenaline is secreted when you are nervous or anxious.
Use of Hormones in Food Industries:

 Technologies and science have advanced enough that we can now gut much more money out of farming and animal keeping. Hormones are now being used in farms to increase milk yields in cows and growth rate in cattle and fish.

In farms, the cows are being injected with a hormone called Bovine Somatotropin or BST. BST is a hormone that is naturally produced in cows. The function of BST is to produce milk. Injecting cows with extra BST will boost milk production and bring in more money for the farmers. Some people however are against the use of BST and claim it is safer for both the cows and the consumer to keep it natural and keep more cows if we want an increased milk yield.

Growth hormones are also being mixed with the food fed to cattle to increase their growth rate and make them grow larger. But again many people are against this and prefer buying meat and fish that were naturally grown.


Comparing Nervous and Endocrine Systems:
Nervous System   
Endocrine System

Information sent in form of electrical impulses   
Information sent in form of chemical hormones

Information travel neurones   
Information travel in bloodstream

Information travels extremely rapidly    
Information travels relatively slow

Information is headed to one target (effector)   
Information may be used by several targeted organs

Electrical signals have an effect that ends quickly   
Hormones have a longer lasting effect



Coordinates and Responses in Plants:

 Plants cannot move themselves to areas of preferable conditions. This is why plants have the ability to detect a stimulus and respond to it by growing or bending in its direction or away from it. These responses are called tropisms. For example a plant tends to grow its stem in the direction of sunlight for more photosynthesis, this is a tropism. There are two types of tropism, these are phototropism and geotropism.
Phototropism: the response in which a plant grows towards or away from the direction from which light is coming.
Geotropism: the response in which a plant grows towards or away from gravity.

A tropism can be either positive or negative. If a tropism is in the direction of the stimulus, it is positive. If the tropism is away from the stimulus it is negative.

For example, a plant’s shoot tends to grow in the direction of sunlight, this is positive phototropism. But the plant’s root grows in the opposite direction deeply into the soil, this is negative phototropism. However, positive phototropism can also be described as negative geotropism because it involves the plant growing in the direction opposite to gravity. And negative photo tropism can be described as positive geotropism because it involves the plant growing towards gravity.
Auxins:

 Tropisms are controlled by a chemical called Auxin. Auxin is a plant hormone. It is produced by cells at the tip of roots and shoots of plants. At the tip of a shoot, there is an area in which cells are being produced by dividing so that the shoot grows. Old cells do not divide, but they grow longer instead. The growth of these cells longer is controlled by auxins. Auxins is what makes the plant grows this is why a plant doesn’t grow if you cut it’s tip off.
Auxins’ Role in Phototropism:

 If the sun shines on the right side of a plant’s shoot, auxins will accumulate on the dark opposite left side. Auxins accumulating there makes the cells on the left side grow much faster than the cells on the right side. When the left side of the shoot starts growing faster than the right side, the shoot will start to bend to the right side towards sunlight. This is phototropism.


Auxins’ Role in Geotropism:

 Auxins tend to settle at the bottom end of the root. However, this does not make the sells of the tip of the root grow longer. Instead, auxins prevent the cells at the bottom tip of the root from growing, making the cells at the middle of the root grow faster. When the cells of the middle of the root grow faster, they push the root deeper into the soil and the root gets longer. The root grows in the direction of the gravitational pull. This is geotropism.

Roots show positive geotropism and negative phototropism because they grow towards gravity and away from sunlight at the same time. Shoots show positive phototropism and negative geotropism because they grow towards the sunlight and away from gravity at the same time.
Advantages of Positive Phototropism:
 Leaves exposed to more sunlight and are able to do more photosynthesis,
 Flowers can be seen by insects for pollination,
 The plant gets higher for better seed dispersal.
Advantages of Positive Geotropism:
 By growing deeply into the soil, the root fixes the plant into the ground firmly,
 Roots are able to reach more water,
 Roots have a larger surface area for more diffusion and osmosis.
Selective Weed Killers:

 Auxins can be used to kill weeds that grow over grass or cereal crops. If weed grows on crops, auxins are sprayed everywhere. Weeds absorb auxins faster than crops or grass. Auxins accumulate in the weeds making them grow very rapidly. Fast growth of weed kills it leaving the crops or grass alive. Auxins are used ass selective weed killers.

METALS

Metals The Reactivity Series of Metals:

The reactivity series is metal is an arrangement of the metals (and carbon and hydrogen) in order of their reactivity starting with the most reactive metal at the top and ending with the least reactive metal at the bottom.

The reactivity of a metal is determined by its ability to form a positive ion. For example, potassium is extremely reactive because it has only one valence electron, so it is very easy to lose it forming a positive ion.

One the other hand, copper is a weakly reactive metal because it has more valence electrons so it is harder for it to become a positive ion.
 Reactions of Metals:

 The reactivity series of metals was deduced by performing several experiments in the lab which enabled scientists to arrange metals according to their reactivity with dilute acid, oxygen (air), and water.   

Reactions with Dilute Hydrochloric Acid:

 In the previous chapter, you studied those reactions involving a metal and an acid are used to prepare soluble salts and that this method is only suitable for preparing salts of moderately reactive metals (MAZIT). This is because any metal more reactive that magnesium will react very violently with acids which is dangerous.

Metal + HCl → Metal Chloride + Hydrogen
The photo on the right shows magnesium reacting with dilute hydrochloric acid. Those effervescences are caused by the evolution of hydrogen gas, which is a product in this reaction. This reaction was repeated using the other metals of the reactivity series. The rate of evolution of hydrogen gas in each experiment was measured. The metals were arranged in order of reactivity starting with the most reactive metal which had the highest rate of effervescence of hydrogen gas. The rate of effervescence is also the rate of this reaction is measured by measuring the volume of hydrogen produced per unit time.Metals    Reactivity with Dilute HCl
Potassium, Sodium & Calcium    React extremely violently with rapid effervescence and splashing
Magnesium & Aluminum    React violently with rapid effervescence
Zinc, Iron & Lead    React slowly with bubbles
Copper, Silver, Gold & Platinum     Do not react
   



Reactions with Oxygen in Air:

 Most metals react with oxygen from air forming a metal oxide. You have previously studied that metal oxides are basic oxides and that some of them are insoluble in water and some of them are soluble in water forming an alkaline solution. The most reactive metals like potassium, sodium, calcium and magnesium react with oxygen with a very bright flame and producing white ashes and their oxides are soluble. Moderately reactive metals like aluminum and zinc react with oxygen forming white powdered ashes but their oxides are insoluble. Iron and copper react very slowly with oxygen. The result of iron oxygen reactions is rust which is reddish brown iron oxide. When a copper lump reacts with oxygen, a white layer of black copper oxide forms on it. When the lump gets covered by this layer; the reaction stops. Oxides of iron and copper are insoluble. Metals that are less reactive than copper like silver, gold and platinum do not react with oxygen.

Note: When aluminum reacts with oxygen, a layer of aluminum oxide adheres and covers the aluminum. At this point no further reaction can take place.
 Reactions of Metals with Water and Steam:

 Some metals are so reactive that they will just react with water immediately if they come in contact. Other metals will react slowly will cold water, but with steam they react much faster. And other metals can only react slowly with steam. Unreactive metals such as silver and gold do not react with water.

Potassium, sodium and calcium react vigorously with cold water and may catch on fire. The products of these reactions are metal hydroxide and hydrogen gas. If hydrogen gas being produced accumulates it may ignite and cause an explosion.

Metal + Water → Metal hydroxide + Hydrogen

E.g.: 2Na + 2H2O → 2NaOH + H2

Magnesium, aluminum, zinc and iron are less reactive. They react with steam forming metal oxide and hydrogen. Magnesium and aluminum will react vigorously with steam while zinc and iron react slowly.

Metal + Steam → Metal Oxide + Hydrogen

 E.g.: Magnesium + Steam → Magnesium oxide + Hydrogen


Competition Reactions in Solid State:

 Previously you’ve studied displacement reactions which are pre-formed in aqueous states. A very similar reaction takes place in the solid state, it is called thermite reaction. This reaction is used to repair damaged railway lines. In this reaction, aluminum and iron (III) oxide are the reactants. In the reaction, aluminum removes the oxygen ion from iron and bonds with it. This happens because aluminum is more reactive than iron. The products are aluminum oxide and iron in molten form. In the fixing procedure, the reactants are put in the cut in the railway line and the reaction is triggered by heating using a magnesium fuse. The reaction leaves aluminum oxide and molten iron with then condenses in the cut welding it. Like displacement reactions, this reaction is exothermic.

2Al + Fe2O3 → Al2O3 +2Fe


Competition Reactions in Aqueous State:

 These are ordinary displacement reactions in which the two positive ions compete for the negative ion. The ion of the more reactive metal wins. Zinc is higher than copper in the reactivity series. If zinc is added to a solution of copper nitrate, a displacement reaction will take place in which the zinc will displace the copper ion from the solution in its salt. The products of this reaction are zinc nitrate and copper. Copper salt solutions have a blue color which fades away as the reaction proceeds because the concentration of the copper salt decreases. This type of reaction also helped in confirming reactivity of metals since the more reactive metal displaces the less reactive one.

Zn + Cu(NO3)2 → Zn(NO3)2 + Cu


Action of Heat on Metal Compounds:

 Applying heat to a metal compound such as potassium nitrate will cause it to decompose into potassium nitrite and oxygen. This is a thermal decomposition reaction.Metal:    Anion:
Nitrate (NO3)    Carbonate (CO3)    Hydroxide (OH)
Potassium
 Sodium    Metal Nitrate → Metal nitrite + Oxygen    NO DECOMPOSITION
Calcium
 Magnesium
 Aluminum
 Zinc
 Iron
 Lead
 Copper    Metal Nitrate → Metal oxide + Nitrogen dioxide + Oxygen    Metal Carbonate →  Metal oxide + Carbon dioxide    Metal hydroxide →Metal oxide + Hydrogen
Silver
 Gold    Metal Nitrate → Metal + Nitrogen dioxide + Oxygen    Metal Carbonate → Metal + Carbon dioxide + Oxygen    -


Silver and gold hydroxides do not exist.

Ions of more reactive metals tend to hold on tightly to their anions and do not decompose easily this is why lots of heat is needed.
Extracting Metals From Their Ores:

 Most metals do not exist in nature as pure elements. Instead, they are found as naturally occurring compounds called ores. Ores are naturally occurring minerals from which a metal can be extracted. Most ores are metals oxide, carbonate or sulfide mixed with other impurities. The extraction of metal from ores begun long ago when people started purifying iron from its iron oxide ore by reducing it using charcoal. This was possible because carbon is more reactive than iron so it can reduce it take the oxygen ion from it. But then other metals were discovered which were higher than carbon in the reactivity series. Those metals were not possibly extracted from their ores until in the 19th century when a method of extracting them by electrolysis was invented. The method extracting a metal depends on its reactivity.Metals - in decreasing order of reactivity    Reactivity
potassium
sodium
calcium
magnesium
aluminium    Extract by electrolysis
carbon   
zinc
iron
tin
lead    Extract by reaction with carbon or carbon monoxide
hydrogen   
copper
silver
gold
platinum    Extracted by various chemical reactions



Extraction of Aluminum:

 Aluminum exists naturally as aluminum oxide (alumina) in its ore, which is called bauxite. Because aluminum is a very reactive metal, it holds on very tightly to the anion it bonds with, which is oxide in this case. This is why the best way to extract and purify aluminum is by electrolysis in a cell like the one below.



In this cell, the electrodes are made of graphite (Carbon). The cathode is a layer at the bottom of the cell and the anodes are bars dipped in the electrolyte. The electrolyte in this process is a molten mixture of aluminum oxide and cryolite. Aluminum oxide by its self has a very high melting point of 2050oC which is higher than the melting point of the steel container in which this process is done. That means the steel container will melt before the aluminum oxide. This is why aluminum oxide is mixed with cryolite which decreases the melting point of it to under 1000oC, thus saving a lot of money because heating is expensive and preventing the steel container from melting. Heat must be continuously supplied to the mixture to keep it molten. Aluminum oxide does not conduct electricity when solid because it does not have free mobile ions to carry the charge.
Aluminum oxide is purified from impurities of oxide by adding sodium hydroxide
 Aluminum oxide is mixed with cryolite and put in the electrolysis cell
 Heat is given in until the mixture becomes molten
 Electrolysis start
 Oxide ions get attracted to the anode and discharged (oxidation); 2O2-, 4e → O2
 Aluminum ions get attracted to the cathode and discharged and settle at the bottom
 of the container (reduction); Al3+ + 3e → Al
 Oxygen gas evolves and is collected with waste gases
 Aluminum is sucked out of the container at regular intervals

Oxygen gas which evolves reacts with carbon from the cathode forming CO2. The cathode gets worn away. To solve this, the cathode is replaced at regular intervals. Heat supply is very expensive; this is why cryolite is used to decrease the melting point of aluminum oxide and this process is done in plants which use hydroelectric energy because it is cheap.


Uses of aluminum:
 Construction of air-craft bodies because aluminum is very strong and very light and it is resistant to corrosion
 Food containers because it is resistant to corrosion
 Overhead power cables because it conducts electricity, is very light, malleable and ductile. Although it is strengthened with steel core


Extraction of Iron:

 The ore of iron is called hematite. It consists of 60% iron in form of Iron oxide (Fe2O3) with other impurities such as silicon
 oxide (SiO2). This process takes place in a tower called a Blast furnace.Substances    Products and Waste Materials
Iron ore (Hematite)
 Coke (heated coal)
 Lime stone (Calcium carbonate)
 Hot Air    Pure Iron
 Carbon dioxide
 Air
 Slag (Calcium silicate)

 Substances are put in the blast furnace
 The process starts by blowing in hot air at the bottom of the furnace
 Coke burns in oxygen from the hot air producing carbon dioxide; C + O2 → CO2
 Heat makes lime stone decompose into calcium oxide and carbon dioxide; CaCO3 → CaO + CO2
 Carbon dioxide produced goes up the furnace and reacts with more coke up there producing
 carbon monoxide; CO2 + C → 2CO
 Carbon monoxide is a reducing agent. It rises further up the furnace where it meets iron oxide and starts reducing it producing iron and carbon dioxide; Fe2O3 + 3CO → 2Fe + 3CO2
 Calcium oxide which was produced from the thermal decomposition of lime stone is a base. It reacts with impurities of hematite such as silicon oxide which is acidic forming calcium silicate which is called slag; CaO + SiO2 → CaSiO3
 Molten Iron and slag produced trickles down and settles at the bottom of the furnace. Iron is denser than slag so it settles beneath it.
 Iron and slag are tapped off separately at regular intervals and pure iron is collected alone
 Waste gases such as carbon dioxide formed in the process and nitrogen and other gases from air blown in escape at the top of the furnace.


Conversion of Iron into Steel:

 Iron produced in the blast furnace is called pig iron. It contains 4% carbon as well as other impurities such as sulfur, silicon and phosphorus which make it hard and brittle. It got that name from the fact that it has to be poured into mould called pigs before it is converted into steel. Most of produced iron is converted into steel because steel has better properties.

Making steel out of pig iron is a process done in a basic oxygen furnace:
 Molten pig iron is poured into the oxygen furnace
 A water cooled lance is introduced which blows oxygen onto the surface of the molten iron
 Impurities start to react
 Carbon is oxidized into carbon monoxide and carbon dioxide and escape
 Sulfur is oxidized into sulfur dioxide and escapes
 Silicon and phosphorus are oxidized into silicon oxide and phosphorus pentoxide which are solids.
 Calcium oxide (lime) is added to remove the solid impurities as slag which is skimmed off the surface
 Throughout the process, sample of the iron are being taken and analyzed for the percentage of carbon present in it. When the percentage of carbon desired is reached, the furnace is switched off and the steel is collected.



There are many different forms of steel.  Each has different components and properties and is used for different purposes.Steel    Composition    Properties    Uses
Mild Steel    99.5% Iron
 0.5% Carbon    Easily worked lost brittleness    Car bodies
 large structures
 Machinery
Hard Steel    99% Iron
 1% Carbon    Tough and brittle    Cutting tools and chisels
Stainless Steel    87% Iron
 13% Manganese    Tough and springy    Drill bits and springs and chemical plants
Manganese Steel    74% Iron
 18% Chromium
 8% Nickel    Tough and resistant to corrosion    Cutlery and surgical tools, kitchen sinks
Tungsten Steel    95% Iron
 5% Tungsten    Tough and hard even at high temperatures    Edges of high speed cutting tools



Extraction of Zinc:

 The ore of zinc is called zinc blende and it is made of zinc sulfide. Zinc is obtained from zinc sulfide by converting it into zinc oxide then reducing it using coke, but first zinc sulfide must be concentrated.

Zinc sulfide from zinc blende is concentrated by a process called froth floatation. In this process, the ore is crushed and put into tanks of water containing a frothing agent which makes the mixture froth up. Hot air is blown in and froth starts to form. Rock impurities in the ore get soaked and sink to the bottom of the tank. Zinc sulfide particles cannot be soaked by water; they are lifted by the bubbles of air up with the froth and are then skimmed off. This is now concentrated zinc sulfide.

Then, zinc sulfide gets heated very strongly with hot air in a furnace. Zinc sulfide reacts with oxygen from the air to produce zinc oxide and sulfur dioxide gas which escapes as waste gas.

2ZnS + 3O2 → 2ZnO + 2SO2

 Sulfur dioxide is used in the manufacture of sulfuric acid.

Zinc oxide produced is put into a furnace with powdered coke. The mixture is heated till 1400oC. Carbon from the coke reduces the zinc oxide into zinc producing carbon monoxide which escapes as waste gas.

ZnO + C → Zn + CO

 Carbon monoxide produced is hot and is used to heat the furnace to reduce heating costs. The pure zinc produced is collected and left to cool down. Zinc is used in many ways like the production of the alloy brass, galvanization and making car batteries.


Extraction of Copper:

 Copper is one of the most popular metals. Native copper occurs in some regions in the world. Otherwise, copper exists in its ore, copper pyrites (2CuFeS2). You have studied before that copper can be purified by electrolysis. It can also be extracted from it ore by converting pyrites into copper sulfide by reacting it with oxygen:

2CuFeS2 + 4O2 → Cu2S + 3SO2 + 2FeO

 Sulfur oxide produced escapes as waste gas and iron oxide impurities are removed by heating the mixture with silicon converting it in to iron silicate which is run off. The remaining copper sulfide is then heated strongly with air. Copper sulfide reacts with oxygen from air producing sulfur oxide which escapes as waste gas and pure copper.

Cu2S + O2 → 2Cu + SO2

 Thus copper is extracted.


Uses of Copper:
In electrical wires because it is a perfect electrical conductor and very ductile, malleable and cheap
Making alloys such as bronze and brass
Cooking utensils because it conducts heat and it is has high melting and boiling points and also resists corrosion
Electrodes because it is a good conductor of electricity
Water pipes because it is resistant to corrosion

Homeostasis

Homeostasis

The human body has the ability to maintain a constant internal environment so that every organ and cell is provided the perfect conditions to perform its functions. This is called homeostasis. There is no organ system for this function. However, every organ plays a role in maintaining a constant internal environment. For example the lungs are responsible for the supply of oxygen to cells. The liver is to maintain a constant level of glucose and amino acids, and so on..
Temperature Regulation:

A healthy human should have a body temperature of 37°C. If the body temperature drops below 37°C, metabolic reactions become slower because molecules move slower and have less kinetic energy. If the temperature rises above 37°C, the enzymes of the body begin to get denatured and metabolic reactions will be much slower.

Sometimes, the temperature of the area you are at is low enough to decrease your body temperature. Sometimes it is high enough to raise your body temperature. This is why the body has the ability to control its body temperature. Our skin is responsible for this process. The Human Skin:

The skin is an organ that coats your entire body. The skin is made up of two layers, the Epidermis and the dermis.

The epidermis’s main function is to protect the dermis which contains most of the structures, and protect the body from ultra-violet rays. The surface of the epidermis is made of tough, dead cells.

The dermis contains many useful structures. Hairs, sweat and sebaceous glands, sense receptors and erector muscles are responsible for controlling the body temperature. Blood vessels transport oxygen and nutrients to the cells of the skin.
A healthy body is continuously gaining and losing heat. Metabolic reactions like respiration release a lot of heat energy, muscular activity increase the metabolic rate and release more heat energy. The body can also gain temperature from the surroundings like the sun or by eating hot food. Heat is lost by the body through exposed skin by conduction. If there is sweat or water on the skin, it will absorb body heat to evaporate which drops the temperature. All these factors are normal however, but it is considered dangerous when the body temperature keeps on dropping or rising severely.

Cooling Down the Body:
When the body is overheated, the body takes several actions to drop it by trying to lose heat in several ways:
Vasodilation: this action causes the body to lose heat quickly. It involves widening the lumen of blood vessels of the skin, this increases blood flow and rate of heat loss. The vessels are also brought near the surface of the skin to reduce the distance heat has to travel to escape.
Sweating: Sweat glands near the skin begin to secret sweat on the surface of the skin through the pores. This sweat acts as a heat consumer to absorb the body heat and use it in evaporation. The activity of sweat glands is increased when the temperature of the body rises.
Hairs lie flat: The muscle erectors of the hairs relax making the hairs lay flat of the skin. When the hairs are erect, they trap air in the gaps between them, this acts as an insulation and prevents heat loss. But when the hairs are flat, less air is trapped between them so there is no insulation and more heat can be lost.

Heating Up the Body:
When the body temperatures drop, the body takes several actions to regulate its temperature by insulation to prevent heat loss and producing heat energy:
Vasoconstriction: this causes the blood vessels to become narrower to reduce heat loss. They also sink deep into the skin to increase the distance heat has to travel to escape thus reducing heat loss.
Shivering: the muscles in the limbs start to contract and relax rapidly, thus increasing the rate of respiration and amount of heat energy released by it.
Hairs become erect: muscle erectors contract and make the hairs erect and stand up vertically trapping air in the gaps between them. This acts as insulation to reduce heat loss.

How the Body Senses Change in internal environment:

When the body’s internal temperature changes the temperature of the blood changes with it. When the blood flows through the brain, a part of it called the hypothalamus detects the drop or rise in temperature. The brain then starts sending electrical impulses to the rest of the body so that it works on heating or cooling its self.

This process is called Negative Feedback. Negative feedback is not for change in temperature only though, it is for any change in the internal temperature including the blood glucose level.
Regulating Blood Glucose Level:

For blood glucose level however, the pancreas is the organ which monitors its level not the hypothalamus. When the blood flows through the pancreas, the pancreas detects the level of glucose in it. If it is higher than normal, the pancreas secretes a hormone called insulin. Insulin flows in the blood till it reaches the liver. When it reaches the liver, insulin hormone will make it convert excess glucose in the blood into glycogen and store it in the liver cells. When the blood glucose level becomes normal, the pancreas will stop secreting insulin so that the liver stops converting glucose. If the blood glucose level decreases below normal, the pancreas secretes another hormone called glucagon. When glucagon reaches the liver, it makes the liver convert the glycogen it made from excess glucose back into glucose and secrete it into the blood stream so that the blood glucose level goes back to normal. When this happens the pancreas stops secreting glucagon.

Normal Blood Glucose Level: 80-100 mg per 100cm3.

Separation Techniques

Lab Skills and Separating Methods

This unit deals with lab equipment and methods of separating solutions.
Element, Compounds and Mixtures:

Speaking about the chemistry of matter, we have only 3 types of matter. These are elements, mixtures and compounds. Long ago, scientists found out that the smallest unit of a matter is called an atom. An element is extremely pure because it is made up of only one type of atoms. For example a pure gold ring has only the element Gold (Au) in it. Compounds are very pure too, a compound is made up of one type of a particle called molecule. A molecule consists of two or more atoms chemically bonded together. Carbon Dioxide (CO2) gas is a compound. A mixture however is not pure at all. A mixture is just two or more elements or compounds mixed together, but not chemically bonded. For example if you dissolve some table salt, which is a compound called sodium chloride (NaCl) in some water, which is also another compound (H2O), you will get a mixture of Sodium Chloride in water, but there are absolutely no bonds between the Sodium Chloride molecules and water molecules. Air is another good example of mixtures. Air is just a mixture of gases floating around each other like Nitrogen and Oxygen, which are pure elements. Air also contains compounds such is Carbon Dioxide.

Iron: lustrous metallic
with a grayish tinge Pure
Calcium Carbonate Sea water is a
mixture of Salts and Water



Elements:

Elements are substances that consist of only one kind of atoms and cannot be broken down into simpler substances by chemical means.

Can you recognise elements, compounds and mixtures?
An element contains just one type of atom.
A compound contains two or more types of atom joined together.
A mixture contains two or more different substances that are not joined together.
The different substances in a mixture can be elements or compounds.

Diatomic Molecules are molecules made of two atoms of the same element, such as Chlorine molecules (Cl2) and Oxygen molecules (O2).

Since particles in mixtures have no chemical bonding between them, they could be easily separated by physical means. The method of separation however depends on the type of the mixture, and some of the physical properties of its components.

We have 4 types of mixtures:
Solid/Solid mixtures
Solid/liquid mixtures
Liquid/liquid mixtures
Gas/gas mixtures


Separating Solid/Solid Mixtures:
By Magnet:

This method is used to separate a mixture of two solids. One condition must be present though. This is that one of the solids is magnetic. For example if we have a mixture of sand and iron chips. We can separate them by:
Pouring the mixture in a dish,
Introducing a magnet just above the mixture. →


The iron chips will immediately get attracted to the magnet leaving sand behind.

By Sublimation:

If we have a mixture of two solids, one of them undergoes sublimation we can easily separate them by heating the mixture using a Bunsen burner.

One solid might melt while the other one will directly sublime into a gas. This process must be done in a fume cupboard in order to collect the gas.

By Solvent Extraction Method:

This method is used one of the solids is water soluble, while the other is insoluble, for example a sand and salt mixture. In this method, the mixture is put in a beaker and water is added to it. The mixture is stirred on gentle heating to make the salt dissolve in the water quickly. Then the mixture is filtered using a filter funnel and filter paper. The residue will be the insoluble sand and the filtrate will be the salt solution. The sand is dried and collected. The salt is obtained from the solution by either the evaporation or the crystallisation method which will be studied later on.


Separating Solid/Liquid Mixtures:
Solubility:

A solution is formed when a solute is dissolved in a solvent.

Solute: This is a substance that dissolves in a solvent forming a solution

Solvent: This is a substance in which a solute dissolves forming a solution

Solution: A uniform mixture which is formed when a solute is dissolved in a solvent.

Dilute Solution: A solution with a small amount of solute/dm3.

Concentrated Solution: A solution with large amounts of solute/dm3.

Concentration: The amount of solute (in grams or moles) that can dissolve in 1dm3 of a solvent.

Saturated Solution: A very concentrated solution with the maximum amount of solute that dissolves in it already dissolved in it.



If you leave a hot saturated solution to cool, crystals of the solute will form. This is because as the temperature decreases the solvent can hold less solute so excess will form in the form of crystals.

The rate of dissolving can be increased by:
Increasing temperature,
More stirring,
Crushed solute (larger surface area).

Solubility: The maximum amount of solute that can dissolve in 100g of water at a particular temperature.

If we want to find the solubility of table salt (sodium chloride) at 30oC, we do the follow these steps:
Use a balance to measure 100g of water accurately,
Pour the 100g of water into a beaker,
Heat the water to 30ºC using a Bunsen burner and a thermometer,
Using a spatula, add a considerable mass of the table salt into the water and stir,
If the mass of salt dissolves completely, add the same amount again and stir, repeat this if the mass keeps dissolving completely until you start seeing excess of the salt not dissolving at the bottom of the beaker,
You have to record the masses of salt you are adding each time and when you start seeing the excess stop adding salt and sum up the amount of salt you added. Call this Mass1,
Filter the solution. The excess of salt will be the residue, dry it and weigh it. Call this Mass2,
The amount of table salt that was dissolved in water is Mass1 - Mass2,
This is the solubility of table salt at 30ºC.

Solubility increases as temperature increases. This is because the intermolecular spaces between the water molecules increase with temperature, giving more space for the solute’s molecules.


By Evaporation (For Soluble Solid/Liquid Solutions):
Put solution in a beaker,
Set the apparatus (Tripod with a gauze above it and a Bunsen burner below it),
Put the beaker on the gauze,
Start heating the solution slowly.

The liquid will evaporate completely leaving the solute behind in powder form.


By Crystallization (For Soluble Solid/Liquid Solutions):
Put solution in a beaker,
Set the apparatus (Tripod with a gauze above it and a Bunsen burner below it),
Insert a glass rod in the beaker,
Turn on the Bunsen burner and continuously dip the glass rod in the solution,
When you see crystals of the solute starting to form on the glass rod, turn of
the Bunsen burner. (This is crystallization point),
Leave the solution to cool,
Filter the solution and take the crystals, which will be the residue,
Wash the crystals with distilled water then dry them between two filter papers.

Note: Do not dry the crystals in oven because it will evaporate the water of crystallization.


By Simple Distillation (For Soluble Solid/Liquid Solutions):
Set the apparatus as shown in the diagram below,
Turn on the Bunsen burner,
The solvent will evaporate and rise as vapor into the condenser,
The cold water surrounding the tube where the water is in the condenser will make the vapor condense into liquid,
The solvent is collected in the tube or beaker on the other side of the condenser, it’s called the distillate,
The solute is collected in the flask as powder,
The thermometer must be where the vapor passes the measure the boiling point of the solvent.



This method is ideal for distilling sea water.
Filtration (For Insoluble Solid/Liquid Mixtures):
Set the apparatus as shown below,
Pour the mixture into the filter funnel,
The solvent will go through and be collected in the beaker as the filtrate,
The insoluble solid will be collected from the funnel as the residue.


Decantation (For Insoluble Solid/Liquid Mixtures):

This method is very simple. It involves letting the insoluble solid rest at the bottom of the beaker. Then pouring the liquid in another beaker leaving the solid behind.




Centrifugation (For Insoluble Solid/Liquid Mixtures):
Put the mixture in a test tube,
Place the test tube in the centrifugation machine,
Start the machine.

The centrifugation force will make the mixture separate into two layers, the liquid at the top, and solid at the bottom. They are then separated by decantation.


Separating Liquid/Liquid Mixtures:Separating Funnel (For Immiscible Liquids):

Immiscible liquids do not mix together; like oil and water.

If they are put in one container, the denser liquid will settle at the bottom and the lighter one will go above it.

To separate and oil and water mixture, we pour the mixture into the separating funnel.

The water is denser than oil, it settles below it.

The tap is opened to let the water flow into the beaker.

The tap is closed when all the water is poured, the beaker is replaced by and empty one and the oil is now poured.



Fractional Distillation (For Miscible Liquids):

Fractional distillation is a method of separating a mixture of two or more liquids provided that they have different boiling points:
The apparatus is set as in the diagram below,
When the heat is turned on the vapor of all the liquids rises,
The liquid with the lowest boiling point goes all the way through the glass beads and into the condenser and out on the other side as liquid. The temperature is constant during this,
The liquids with the higher boiling points condense on the glass beads. When all of the liquid with the lowest boiling point have evaporated and collected, the temperature starts rising again. The liquid with the second lowest boiling point evaporates now, and gets collected on the other side.



The glass beads are to provide a cool large surface area for condensation.


Fractional Distillation of Crude Oil:

Crude oil is a mixture of hydrocarbons. It is the major source of fuel. It is refined and separated into several very useful fractions by fractional distillation in a fractionating tower. The higher the fraction is obtained in the fractionating tower the lower its boiling point.



Fuel is a substance that releases energy (E.g.: Coal, Natural gas, Ethanol)
Lubricant is a substance that reduces friction between two surfaces.
Hydrocarbons are organic compounds containing carbon and hydrogen only.

Different hydrocarbons are collected at different levels according to their boiling points. The higher they are collected the lower their boiling point.


Chromatography:

Chromatography is a process used to separate and identify two or more substances from a mixture. This method depends on the solubility of the tested substances. Chromatography, for instance, is also used to find out the number of components in a drink.

Let’s say we want to find the number of colored dyes present in black ink. First we get a piece of filter paper or chromatography paper. We draw a line, in pencil, at the bottom of the paper. This line is called the base line, and the reason it is drawn in pencil is because pencil is insoluble so it won’t interfere with the solubility of the ink. Then we place a spot of the black ink on the base line. The chromatography paper is now put with its bottom soaked in a suitable solvent, which is in our case water. The chromatography paper is going to absorb the solvent, which moves upwards. When the solvent reaches the base line, the spot of black ink will dissolve in it. The solvent will keep moving upwards taking with it the black ink. The more soluble the contents of the ink the higher it will move until it can’t anymore.



Sometimes the substance we are testing is in solid form. In this case we have to crush and dissolve it in water and filter it. We then take the filtrate and evaporate some of it water to get the most concentrated sample. Then we are ready to do the experiment.

When dealing with ethanol in concentrating the sample. We have to heat it in a water bath because it is flammable. And when we use it a solvent in chromatography, it has to be performed in a covered beaker because ethanol is volatile.

The solvent front is the furthest distance travelled by the solvent.

Sometimes, the sample is separated into colorless spots. In this case the chromatography paper is sprayed with a locating agent to that locates the spots. The number of spots indicates the number of components in the sample.

To identify the substances which were formed when the sample was separated, we measure what’s called the Rf Value. The Rf Value is the rate of the distance travelled by the solute (the spot) to the distance travelled by the solvent line. It’s calculated by measuring the distance travelled by the spot (Distance1) from the base line, measuring the distance from the base line to the solvent front (Distance2), and dividing Distance1 by Distance2.



This value is always less than one because the distance travelled by the solvent is always larger than the distance travelled by the spot. Each substance has a different Rf Value.

Chromatography can be used to test purity of substances. If a substance gives only one spot, it means it is pure because it contains one substance.

If two spots have the same Rf value they are made of the same substance.

Plant Nutrition

Plant Nutrition

 Plants are living organisms, they need food in order to keep living. The way they obtain their nutrients however, is completely different than that of ours. Plant make most of their nutrients by them selves, they just need 2 raw materials, these water and carbon dioxide.

The leaf of a plant is considered the kitchen of it. It is where food is made, later on you will see how the leaf is adapted to making food.



Upper Epidermis: it is a layer of cells that cover the leaf and protect it, it is covered by a layer of wax called cuticle.

Mesophyll Layer:
Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis
Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out photosynthesis and store nutrients.
   
Vascular Bundle: it is a group of phloem and xylem vessels that transport water and minerals to and from the leaves.

Lower Epidermis: similar to the upper epidermis, only that it contains a special type of cells called guard cells. Guard cells are a specialised type of cells that control the passage of carbon dioxide into the cell and the passage of oxygen out of the cell by opening and closing the stomata (a hole in the leaf through which gases pass) so guard cells are responsible for the gas exchange.
Photosynthesis:

Photosynthesis means “making with light”. It is the process by which plants make useful glucose out of the raw materials water and carbon dioxide, using light energy from the sun.

Water is essential for photosynthesis, it is sucked up from the soil by the roots and transported up the stem to leaves where it is put into use.
 Carbon dioxide, just like water is essential for photosynthesis. It moves into the leaf from the air by diffusion, through the stomata (tiny wholes in the leaf).

Once carbon dioxide and water are present in the leaf, one condition for photosynthesis is needed, that is light. The two cells in the diagrams are called palisade cells (the rectangular one) and spongy mesophyl cell (the circular one), these are the cells where photosynthesis takeplace. They a structure called chloroplasts, these structures contain a green pigment named chlorophyll, this is to trap sunlight to be used in energy, a large number of chloroplasts is required for photosynthesis.   

How photosynthesis happen:
 Carbon dioxide and water enter the cell
 The cell traps light energy using chloroplasts
 The energy is used to split water (H2O) into hydrogen and oxygen
 The oxygen is excreted outside the leaf to the atmosphere as a waste product
 The hydrogen reacts with carbon dioxide forming glucose.
Overall equation for the Photosynthesis


Carbon Dioxide Supply:

 The carbon dioxide moves to the leaf from the atmosphere by diffusion through tiny holes in the leaf called stomata. Carbon dioxide is not present in a high concentration in air, but compared to its concentration inside the leaf, it is more in the air. This is because the cells inside the leaf are always doing photosynthesis (at daytime), converting the carbon dioxide into the glucose quickly, thus the concentration of it inside the leaf decreases, making a concentration gradient for diffusion from the atmosphere to the leaf.
Water Supply:

 The water is absorbed by the roots of the plants, then they are transported upwards through a hollow tube called the xylem vessel, till it reaches the leaf where photosynthesis takes place, it enters the leaf through holes in the xylem. Excess water leaves the cell through the stomata, this is called “transpiration”
Sunlight Supply:

 The leaves are always exposed to sunlight at daytime. The sun penetrates the transparent layers on the leaf till it reaches the mesophyll layer, where photosynthesis take place. Palisade cells are nearer to the surface of the leaf than the spongy cells, so they receive more of the light and make more photosynthesis.
Factors Needed For Photosynthesis:
 Water
 Carbon Dioxide
 Light
Factors Affecting The Rate Of Photosynthesis:
 Amount of water: the rate increases as it increases
 Concentration of carbon dioxide: the rate increases as it increases
 Light intensity: the rate increases as it increases


Plants at night:

 At night, the plant performs several process to convert the stored starch into many useful nutrients like:
 Sugars for respiration
 Cellulose and proteins for making cells
 Vitamins to help in energy action
 Fats as a long term storage material
 Remaining starch is temporarily stored.
Mechanism of Guard Cells:

 At daytime, the guard cells open the stomata to allow gaseous exchange, this occurs according to the following steps:
 Sunlight increases the potassium concentration in the vacuoles of the guard cells, the water potential decreases making a gradient between the guard cells and the surround epidermal cells,
 Water moves by osmosis into the guard cells from the epidermal cells,
 The water raises the pressure inside the guard cells,
 The cell wall adjacent to the stomata is thicker and less stretchable then the cell wall on the other side,
 The pressure expand the whole cell except for the inner cell wall (adjacent to the stomata) creating a curve and a pore between the two guard cells,
 The stoma opens.

At night however, the mechanism is opposite:
 Potassium level decreases in the vacuole of the guard cells,
 Water potential increases in the cell and water diffuses out of it,
 The guard cells straighten up because of low pressure closing the stoma.
Mineral Requirements:

 The plant is also in need for mineral ions to control chemical activities, grow, and produce materials. The most important minerals are:
 Mg+2  (Magnesium ions): they are important for the production of the green pigment chlorophyll. Lack of it results in lack of photosynthesis and wilting of the leaves,
 Nitrates: these are the sources of nitrogen, they are required to make amino acids and proteins by combining with glucose. Lack of it results in deformation of the plant structure making it small and weak.

Both mineral ions are absorbed from the soil.
Fertilisers:
Sometimes the soil is lacking of the mineral ions needed, this problem can be solved by adding fertilisers to the soil. Fertilisers are chemical compounds rich in the mineral ions needed by the plants. They help the plants grow faster, increase in size and become greener, they simply make them healthier and increase the crop yield. But there are disadvantages of fertilisers, such as:
 Excess minerals and chemical can enter a nearby river polluting it and creating a layer of green algae on the surface of it, causing lack of light in the river, thus preventing the aqua plants photosynthesizing.
 When living organisms in the river or lake die, decomposers such as bacteria multiply and decay, respire using oxygen. Eutrophication takes place eventually.    

Green House:
   
A green house is a placed covered by transparent polythene. In green houses, the limiting factors of photosynthesis are eliminated, and the plants are provided the most suitable conditions for a healthy, rapid growth.

The soil in green houses is fertilised and very rich in mineral ions, assuring healthy, large yields. More carbon dioxide is supplied to the crops for faster photosynthesis. The polythene walls and ceiling allow heat waves and light rays only to enter and prevent harmful waves, thus providing a high light intensity and optimum temperature, sometimes a heating system is used too. A watering system is also present. The disadvantages of green houses are that it is too small to give a large yield and that it is expensive.

Transport in Plants

Transport In Plants

Just like humans, plants have a transport system of vessels and cells that transports water, minerals and other nutrients around the plant.Structure Of Plants:

A plant is divided into two section, whatever is above the soil, is called the shoot, and whatever is below the soil is called the root. The root is simple, it is usually    
    amain root with extensions of thinner ones. The shoot however, is made of several parts. The roots have the specialised cell, root hair cell, which we looked closely before, the root hair cells absorbs water from the soil and fixes the plant into the ground. In the root also, starts the transports system of the plant which extends all the way from the root up to the tip of the stem. The diagram on the left shows a section through the root. The root hairs of the root hair cells are visible. In the centre of the root, is the beginning of the transport system of the plant, which is made of two main transport tissues, the xylem tissue and the phloem tissue.

The diagram below shows a transverse section through a root. We can see than there are two types of hollow tubes, the xylem is the one in yellow and the phloem is the one in blue.    
   
Each  type of these tissues functions adapts differently to the other.

The diagram on the right shows a transverse section of the stem. We can see that the xylem and phloem are still there, but they are arranged differently, they are both put together in an egg shaped structure separated by a cambium.
 Together they are called the vascular bundle, which is surrounded by what is called the cortex.
Structure Of The Xylem Tissue:

The xylem vessels are long hollow tubes made of dead lignified cells arranged end to end forming a continuous motion.

The xylem vessel is specialised to transport water and dissolved minerals from the root up to all the other parts of the plant, and also to helps supporting the stem and strengthening it. These walls of the xylem vessel contains holes called pits which water enters through.

The xylem tissue is adapted to its functions in different ways. For instance, the cell wall of the dead cells of the walls of it is made of lignin, which makes it stronger to support the stem, the fact that they are dead makes all the water absorbed by the root hair cells get transported to the leaves without being used by the cells of the vessel. The tube is also very narrow, to make it easier for the water to be transported upwards.    
    Structure Of The Phloem Tissue:

This is a long tube that runs alongside the xylem tissue. They are made of long narrow tubes with perforated sieve plates along the thin length. The function of the phloem tissue is to transport food nutrients such as glucose and amino acids from the leaves and to all other cells of the plant, this is called translocation.

Unlike the xylem, the phloem tissue is made of living cells, because as we will se later, there are several forces causing the transport of water in the xylem, but there are no forces causing the translocation, so substances need to be moved along using active uptake, which needs energy.

The cells of the phloem vessels contains a cytoplasm but no nucleus, and its activities are controlled by a companion cell next to it which has a nucleus, but companion cells have no function in translocation.
Mechanism Of Water Transport:

 The water reaches the leaves from the soil by several steps, starting at the root:
 The root hair cells have a concentrated cell sap vacuole which means that the water potential is low in it and high in the soil, osmosis takes place and water enters the cell.
 Minerals are also present in the soil but in low concentration, using active up take, the root hair cells takes the mineral ions in.
 The mixture of mineral and water moves from the root hair cells through the other cells by osmosis active uptake till it reaches the xylem vessel in the root, it enters the xylem through pits.
 The xylem vessel transports the water from the root to the stem (forming the vascular bundle with the phloem) and upwards to the leaves.
 The water and dissolved minerals leave the xylem and get absorbed by the cells in the leaves.   

How Water Moves Through The Xylem:

 There are three factors affecting the movement of water:
 In root hair cells, the mineral concentration is high, it helps pushing the water towards the xylem and the stem.
 Capillarity is a factor that helps in the movement of water in the xylem vessels. The water molecules are attracted to each other, as one moves upwards it pulls its neighbouring molecule with it. The molecules are also attracted to the walls of the xylem, the narrower the xylem the easier it is for water to move.
 Transpiration force is the most effective force that causes water movement. In the leaf, the water evaporates and leaves the plant through the stomata, one molecule escapes pulling the other with it, and so on, creating  a suction force. You can think of it as using a straw to drink.
Factors Affecting The Transpiration Rate:
 Humidity: humidity means more water vapour in the air, which means water vapour has a higher concentration in the atmosphere than inside the leaf, so transpiration will be much slower because the diffusion of water vapour outside the leaf will be slow. The higher the humidity the slower the transpiration.
 Temperature: when the temperature is high, molecules move faster and evaporate faster, so transpiration rate increases. The higher the temperature the faster the transpiration.
 Wind speed: when the wind is fast, it takes humid air away from around the leaf, making the diffusion rate faster, so the faster the wind the faster the transpiration.
 Light: when light intensity is high, the stomata will open to let Carbon dioxide in for photosynthesis, the water vapour has an easier chance to escape. In the dark the stomata are closed, the transpiration rate is very slow.

Wilting occurs when the transpiration rate is faster than the rate of water absorption. The amount of water in the plant keeps on decreasing. The water content of cells decreases and cells turn from turgid to flaccid. The leaves shrink and the plant will eventually die.
Translocation:

 This is the transport of organic food such as sucrose and amino acids in the plant through the phloem vessels.

Glucose, the product of photosynthesis is the most important food of the plant. Because from it, it makes most of its other nutrients. Glucose is converted into an other more complex sugar called sucrose. Sucrose in the leaves enter the phloem vessels. The phloem transports it to every other part of the plant where it is made use of. Amino acids are also transported in the phloem.

Sucrose and amino acids are transported to every tissue of the plant, each cell use it in a different way. Root cells convert sucrose into glucose for respiration and store it. Growing cells make cellulose for cell walls from sucrose and use the amino acids to make proteins for growth. And fruits use the sucrose to make the attractive scent and tasty nectar to attract insects.

The areas of the plant where sucrose is made, are called sources, and where they are delivered to and made use of are called sinks.
Pesticides And Insecticides:

 Some insects and pests feed on plants and harm them. A way to prevent this problem is to spray the plant with insecticides and pesticides. But the problem here is that these chemicals also kill insects and pests that are useful to the plant. This is why systemic pesticides are used. When sprayed, they are absorbed inside the plant and distributed all over the plant. When the harmful insects and pests eat a part of the plant, they eat the poison with it, thus they die and harmless ones are safe.
Adaptation Of Special Plants:Desert Cactus:
 Leafs are needle like spines to reduce water loss by transpiration. They are covered with a thick cuticle to insulate heat and prevent water escaping. The stomata of the leaves are sunken into the epidermis to be away from external features that increase transpiration.
 The stem is short to prevent wind from blowing it away. It is round to decrease surface area and transpiration rate. Photosynthesis takes place in the stem.
 Roots are very long and deep into the soil to have access to underground water and rain water. Root hair cells have a very concentrated cell vacuole to increase osmosis rate.   
Pond Plants:
 Wide, broad leaves on surface of water to exchange gases. Stomata on upper side of the leaf to be in contact with air.