Organisation

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Levels of organisation

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Cells

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The basic building blocks of all living things are cells in which important chemical reactions take place within an aqueous environment keeping the cell alive. Different cells are adapted for their function so are described as specialised cells. Red blood cells lack a nucleus providing more space to carry oxygen and root hair cells of plants have a particular shape giving them a large surface area for absorption of water and other molecules.

 

Tissues

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A collection or group of similar cells that work together to carry out a specific function are described as tissue. Cells making up an epithelium (epithelial cells) can be specialised in different ways. Epithelium is tissue that is found lining or covering other cells or tissues of the body. Epithelium lining certain parts of the digestive system is made up of specialised cells. These cells work together to absorb the products of digested food and have projections known as microvilli to increase their surface area. Another important tissue found all over the body is muscle tissue which is made up of cells allowing movement of different parts of the body.  

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Epithelial cells lining the gut

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Note the projections on the ends (microvilli)

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Organs

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Moving higher up in the hierarchy of organisation are organs that are made up of a number of different types of tissue and they work together to carry out a specific physiological function. There are many organs in the body that include: the heart (pumps blood around body), lungs (allows exchange of gases), kidneys (organs of excretion) and liver (many functions including production of heat). In plants the leaf is an organ specialised to carry out photosynthesis.

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Section of a leaf

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Organ systems

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Organ systems are made up of organs that work together to achieve a specific function. Examples of organ systems are as follows:

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• Respiratory system

• Circulatory system

• Digestive system

• Nervous system

• Skeletal system

• Immune system

• Reproductive system

• Excretory system

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Two of these organ systems will be studied in detail, namely the digestive system and respiratory system.

 

The digestive system

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Enzymes

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Enzymes are protein molecules that are vital for digestion of food in the digestive system. They allow chemical reactions to occur without which these reactions would be too slow or would not occur at all. Enzymes can be described as biological catalysts that speed up the rate of a chemical reaction. They all have a cleft region called the active site which has a shape into which the substrate molecule (the molecule to be digested) fits perfectly. The part of the substrate molecule and the active site on the enzymes have complementary shapes.

Enzyme action:

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Exercise 1

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Complete the following paragraph that explains enzyme action in terms of the the lock and key hypothesis:

Enzymes are ____________ ______________ that speed up the rate of a _________________ ___________________ . They have a cleft region to which the molecule to be digested __________ . When the enzyme substrate complex forms ______________ occurs and the substrate molecule is ______________ _____________. The _____________ is formed and released by the enzyme which can now go on and _____________ further reactions.

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catalysis, chemical reaction, catalyse, product, biological catalysts, binds, broken down

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Mouth

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Digestion begins in the mouth where the teeth grind the food into small pieces and saliva lubricate the food which gets pressed against the palate by the tongue to form a bolus. The saliva is mixed with the food and contains the enzyme amylase which starts the digestion of starch to maltose.

 

Oesophagus

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This is a narrow tube that takes food from the mouth to the stomach where the food is stored. The bolus of food is swallowed from the mouth and is prevented from passing into the trachea by the lowering of a flap called the epiglottis. Just like all parts of the digestive system the food moves down the oesophagus by a process called peristalsis that uses circular and longitudinal muscles.

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Stomach

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This is an enlarged part of the digestive system where the food may remain for up to two hours whilst it is being digested. The lining of the stomach is covered with mucus which protects it from the hydrochloric acid produced in gastric pits. Mucus is found throughout the digestive system as it also helps to lubricate the movement of food. The function of hydrochloric acid is to kill the bacteria in food and to provide the optimum PH for an enzyme called pepsin. This enzyme is a protease and released into the stomach from the gastric pits. It is a protease because it digests proteins to polypeptides. Muscles in the stomach contract and relax to help mix the food with enzymes and mucus. The stomach therefore churns the food and the resulting liquid is called chyme.

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Small intestine

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The small intestine is named because of the small size of the lumen diameter but in fact it is the longest part (approximately 5 m) of the digestive system. The first part into which the chyme from the stomach is released is called the duodenum. A grey coloured organ just below the stomach called the pancreas releases pancreatic juice into the duodenum through a tube connected to it called the pancreatic duct. This contains 3 different enzymes. Their names and function are as follows:

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Enzymes in pancreatic juice

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Enzyme                              Digestive function

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amylase                              starch to maltose

protease                             proteins to amino acids

lipase                                  lipids/fats to fatty acids and glycerol

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Bile

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Bile is produced in the liver and stored in the gall bladder. It is released into the bile duct and moves into the duodenum. It has two important functions. Firstly, the enzymes in pancreatic juice require a slightly alkaline PH but the acid in the chyme, released by the stomach, makes the environment acidic. The bile neutralises the acid because it is alkaline and creates appropriate conditions for optimum enzyme action. Secondly, bile contains bile salts which emulsify large lipid droplets to small lipid droplets increasing the surface area for effective enzyme action.

 

Other enzymes in the small intestine

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Other enzymes produced in the small intestine complete the process of digestion resulting in soluble products that are absorbed into the bloodstream.

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Summary of the important reactions of digestion

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Large intestine

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Indigestible material will pass into the large intestine and this is where faeces is formed. The large intestine also has the function of re-absorbing water.

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Exercise 2

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1. Briefly explain, using one sentence, the function of the following parts of the digestive system:

     a) mouth

     b) oesophagus

     c) stomach

     d) small intestine

     e) large intestine

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2. State 2 functions of HCl released in the stomach.

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3. Bile is produced in the liver and stored in the gall bladder. State:

     a) where the bile goes when released by the gall bladder.

     b) the 2 functions of bile.

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4. Complete the following table that shows substrate molecules, enzymes and products for the main reactions in digestion:

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The circulatory system

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The heart, blood vessels and blood make up the circulatory system. This is one of the many organ systems which works with other organ systems to maintain important life processes. The heart is essentially a pump that generates pressure to circulate the blood through the blood vessels and allow transport of important substances such as glucose and oxygen. At the same time the blood takes excretory products from the cells and allows them to be transported and removed from the body. 

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Blood

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The blood is a suspension made up of the cells and a liquid called plasma. The cells include red blood cells, white blood cells and cell fragments called platelets. The plasma is mainly liquid with dissolved substances such as amino acids, glucose and salts. 

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Below is a diagram of the heart showing how it is linked to the rest of the circulatory system.

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Pulmonary circulation

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Contraction of the heart causes blood to pass from the right atrium into the right ventricle where it is then pumped to the lungs by via the pulmonary artery. In the lungs the blood picks up oxygen and becomes oxygenated then is carried to the left atrium of the heart via the pulmonary vein. This route which starts from the heart to the lungs and back to the heart is sometimes referred to as the pulmonary circulation. 

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Systemic circulation

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The blood is then pumped into the left ventricle and this has a thicker muscular wall compared to the right side. This allows higher pressure to be generated when the ventricle contracts and pumps the blood into a large artery called the aorta. This is the largest artery in the body and smaller vessels branch of to carry the blood all around the body. The right atrium receives blood via the vene cava which carries blood that has been around the body. The blood is therefore now deoxygenated. This route which starts from the heart to the body and back to the heart is sometimes referred to as the systemic circulation. 

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Double circulation

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Since the blood passes through the heart twice for one complete circulation or in other words it is made up of the pulmonary circulation and the systemic circulation it is described as a double circulation. This makes the circulatory system more efficient as it ensures a substantial amount of pressure driving blood throughout the circulation.

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The heart

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Valves are present in both sides of the heart between the atria and ventricles to stop blood flowing in the wrong direction. The right side has the tricuspid valve whereas the left side has the bicuspid valve.

The heart itself needs a constant supply of nutrients and oxygen as it is cardiac muscle that is working continuously. This is achieved by a blood supply through a network of blood vessels called the coronary arteries. A high fat diet can cause build up of deposit reducing blood supply which can lead to coronary heart disease.

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Blood vessels

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Arteries

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These are blood vessels that take blood away from the heart so the blood will be at high pressure as the blood has not passed through any tissues. For this reason the walls of the arteries are thick and elastic to withstand and maintain pressure. As the heart contracts and relaxes it causes the blood to spurt. When the heart contracts the arteries will stretch to even out pressure. This is what produces the pulse and corresponds to heart rate if counted over a period of a minute.

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Veins

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Veins are blood vessels that return blood to the heart after it has passed through tissue. They carry blood towards the heart and it is at low pressure as considerable pressure has been lost by passage through tissue (body or lungs). Veins therefore have a thin wall but larger lumen size in comparison to arteries. The low pressure means that blood may be able to flow in the reverse direction. To prevent this veins have valves to keep blood flowing in one direction.

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Capillaries

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Arteries branch and become smaller vessels called arterioles then with further branching they will eventually form the smallest blood vessels called capillaries. Capillaries have blood pressure between arteries and veins and they usually supply resources to tissues. The wall of a capillary is only one cell thick and this is important to their role in the circulatory system. It allows important respiratory gases and other materials to be exchanged between blood and tissue. This is only achieved because diffusion can occur fast enough across the short distance of the capillary wall.

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Exercise 3

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1. The heart is made up of a number of chambers and has valves between them. State:

    a) the names of the 4 chambers.

    b) the function of the valves.

    c) the location of the valves.

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2. The 4 main blood vessels connect the heart to the circulation. State the blood vessel that:

    a) carries deoxygenated blood from the body tissues to the right atrium

    b) carries oxygenated blood from the left ventricle to the body.

    c) carries deoxygenated blood from the right ventricle to the lungs.

    d) carries oxygenated blood from the lungs to the left ventricle.

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3. There are 3 different types of blood vessels that have different functions. State, for each type of blood vessel, how structure is related to function: (Only one structural feature is necessary for each blood vessel type)

    a) artery

    b) vein

    c) capillary

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4. The human circulatory system is sometimes described as a double circulation.

    a) Describe the double circulation

    b) Explain the purpose of double circulatory system

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5. The heart is a muscle that requires resources supplied to it. State:

    a) the blood vessels that supply the heart with blood.

    b) what may cause the build up of deposit in these vessels and the resulting condition.

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Cardiovascular disease

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Cardiovascular disease can occur when fatty deposits build up in the coronary arteries. These arteries supply the heart with blood to deliver a constant supply of nutrients and oxygen. If the coronary arteries become too narrow then blood flow is reduced. This results in less oxygen reaching the heart muscle which can damage the heart or even cause a heart attack.

It is therefore important to have a healthy diet that does not contain too much saturated fat found in certain meats and dairy products. Regular exercise also keeps the heart and body healthy.

nother way of reducing fatty deposit build up is by taking drugs called statins. These do not remove fatty deposit but stop it from forming so it is a preventative measure.

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Lifestyle and non-communicable diseases

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A non-communicable disease is one which cannot be passed on from one person to another such as heart disease and cancer. Risk factors can increase the probability of non-communicable diseases.  

These include aspects of a person's lifestyle and chemicals that may be present in a person's body or in the environment.

Some risk factors are:

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• the effects of diet and smoking on heart disease

• being obese is a risk factor for type 2 diabetes

• the effect of smoking on lung disease and lung cancer

• carcinogens in the environment can increase the risk of cancer

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Cancer

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Benign tumours are caused by growth of cells that are localised to one part of the body. These can easily be removed by surgery and therefore are usually not life threatening.

Malignant tumours are caused by cells that have changed in such a way that they are able to move from one part of the body to another, survive and cause more tumours to develop. The tumours produced are called secondary tumours and it becomes increasingly more difficult to treat this type of cancer as tumours spread throughout the body.

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Exercise 4

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1. The heart itself needs a blood supply so that it can continuously pump blood.

     a) which blood vessels supply blood to the heart?

     b) what can cause build up of fat deposits in these blood vessels?

     c) explain how the presence of fat deposits can result in a heart attack.

     d) state two ways of keeping the heart and body healthy.

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2. Explain the meaning of the following terms providing one example:

    a) non-communicable disease

    b) risk factor

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3. There are two main categories of cancer as given below. State a characteristic feature for each.

    a) benign cancer

    b) malignant cancer 

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Plant tissues, organs and systems

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Plant tissues

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Some important plants tissues include:

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• epidermal tissues

• palisade mesophyll

• spongy mesophyll

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These and other structures can be seen in the cross section of a leaf shown below.

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Cross section of a plant leaf

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The upper epidermis is made up of long thin cells that easily allow light to pass through. There is a waxy cuticle on top of the upper epidermis that reduces water loss through the leaf. The palisade cells have a regular shape that is almost cuboidal. These cells have a lot of chloroplast (green dots on cross section) as most of the photosynthesis occurs here. The palisade cells, which make up the palisade mesophyll tissue, are closley packed ensuring efficient use of the light falling on the leaf.

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The spongy cells, that make up the spongy mesophyll tissue, are irregularly shaped spheres and have air spaces between them. Air spaces allow gases to flow easily and as more cell surface area is in contact with air, diffusion of gases occurs more rapidly. On the lower side of the leaf there are pores (holes) called stomata that allow gases to diffuse in and out of the leaf. Transpiration is the loss of water from leaves and stomata are the route of exit of water in the form of water vapour. The lower epidermis is made up of cells similar to the upper epidermis but additionally has guard cells that occur in pairs. The gap between the guard cells is effectively the pore or channel referred to as stoma or stomata (many stoma).

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Stomata and guard cells

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Stomata are actually the spaces between guard cells that can be described as pores on the underside of a leaf. It is important to have stomata for gaseous exchange but water is also lost through stomata. The guard cells are able to control the size of stomata to reduce water loss when there is a shortage of water.

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Stomata

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Guard cells have a more curved shaped when water is available so they fill up with water by osmosis which opens the stomata. During dry periods when there is a shortage of water the guard cells are not so turgid (rigid) and become flaccid (soft) losing their curved shape. This reduces the size of the stoma or close it completely so reducing water loss.

The guard cells are found in the lower epidermal layer and are the only cells that have chloroplast in this layer.

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Vascular tissue

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Xylem

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Xylem tissue is made of dead cells that have lost their end walls forming long tubes through which water can move. The walls of the cell have a substance called lignin that provides strength to the xylem tubes. Usually there will be many tubes running in close proximity in a vascular bundle. The xylem therefore may provide some support to the stem of plants. 

The main function of xylem is to transport water from the roots to the plant. The water moves in an upward direction and is continuously lost from the leaves but is also being drawn in from the roots replacing the water lost by transpiration. The movement of water in this way is called the transpiration stream. The water entering the roots also brings in important minerals required for growth.

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Phloem

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Phloem tubes are similar to xylem tissue but instead of losing their end walls they form sieve plates that have many small holes in them. The cells that make up phloem are described as sieve tube elements which do have cytoplasm but no nucleus. Companion cells are found next to each sieve tube element and do have a nucleus. The companion cells may provide important substances to the phloem cells allowing them to carry out their role. The function of phloem tissue is to carry the carbohydrate (usually in the form of sucrose) from the leaves to all other parts of the plant. A diagram of a phloem tube is shown below.

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Arrangement of vascular bundle in stem

Arrangement of vascular bundle in root

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Exercise 5

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1. For the following 3 cells that make up the tissue of a leaf state their shape and function:

     a) cells that make up the upper epidermis.

     b) palisade (mesophyl) cells

     c) spongy (mesophyll) cells

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2. What is the function of the layer on top of the leaf called the waxy cuticle?

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3. There are 3 different types of cells in a leaf that are capable of carrying out photosynthesis.

     a) which sub cellular structure must be present inside a cell for photosynthesis to occur?

     b) State the 3 cells that are able to carry out photosynthesis?  

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4. Stomata are pores that occur where specific cells occur in pairs.

     a) which part of the leaf are stomata found?

     b) what cells are found surrounding stomata which are always in a pair?

     c) what are the functions of stomata that are important to both photosynthesis and maintaining                                           rigidity/turgidity of plant cells.

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    d) state and explain what happens to the size of stomata:

          i) when their is shortage of water.

         ii) when there is an abundance of water.

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5. Fill in the missing words in the following text based on transport in plants. The missing word are shown in green.

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                               upwards, lignin, end walls, roots, water

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A xylem tube is made up of cells that have lost their _______ ______ . The walls of the cells have a substance called ________ which makes xylem tissue strong. The main function of xylem tissue is to transport ________ from the ________ to the leaves so movement is always in an _________ direction.

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