Higher: B2 - Organisation

Question 1

What is a tissue?

Answer 1

A group of similar cells that work together to perform a specific function.

Question 2

What is the function of muscle tissue?

Answer 2

To contract to move what it’s attached to.

Question 3

What is the function of glandular tissue?

Answer 3

It makes and secretes chemicals like enzymes or hormones.

Question 4

Where is epithelial tissue found?

Answer 4

It covers some parts of the body, e.g. the inside of the gut.

Question 5

What is the role of muscular tissue in the stomach?

Answer 5

Moves the stomach wall to churn up food.

Question 6

What is the role of epithelial tissue in the stomach?

Answer 6

Covers the inside and outside of the stomach.

Question 7

Explain what is meant by the term “organ system”.

Answer 7

A group of organs working together to perform a specific function.

Question 8

What does the digestive system do?

Answer 8

Breaks down and absorbs food.

Question 9

What is the role of glands in the digestive system? Give 2 examples.

Answer 9

They produce digestive juices.

E.g. pancreas, salivary glands.

Question 10

What is the role of the liver in the digestive system?

Answer 10

Produces bile.

Question 11

What is the role of the stomach and small intestine in the digestive system?

Answer 11

• They both digest food
• Small intestine absorbs soluble food molecules

Question 12

What is the role of the large intestine in the digestive system?

Answer 12

Absorbs water from undigested food (leaving faeces).

Question 13

1. What are enzymes?
2. What are they made out of?

Answer 13

1. They are large proteins which break down large molecules into smaller, soluble ones.
2. Proteins are made of chains of amino acids.
3. These are folded into unique shapes (i.e. active sites).

Question 14

Why can enzymes be described as biological catalysts?

Answer 14

• The body needs to speed up chemical reactions
• Raising body temperature isn’t a good way of doing this, because this accelerates unwanted reactions and harms cells
• So we have enzymes which speed up reactions instead

Question 15

What is a catalyst?

Answer 15

A substance which increases the speed of a reaction without being altered or used up in the reaction.

Question 16

The substance that an enzyme acts on is called the…

Answer 16

Substrate.

Question 17

Why do enzymes usually only catalyse one reaction?

Answer 17

For enzymes to work, the substrate has to fit into its active site. If it doesn’t, the reaction won’t be catalysed.

Question 18

Why is the lock and key model of enzymes slightly inaccurate?

Answer 18

• The model shows the enzyme remaining the same after a reaction.
• The active site actually changes shape a little as the substrate binds to it (to fit tighter).
• This is referred to as the induced fit model.

Question 19

Enzymes have an optimum ___ and ___, which is often ___.

Answer 19

1. Temperature
2. pH
3. Neutral

Question 20

What is meant by an enzyme being “denatured”? Describe how this happens and what leads up to it.

Answer 20

• An increase in temperature will increase the rate of reaction until the enzyme reaches its optimum temperature.
• At a certain temperature, bonds in an enzyme are broken.
• This changes the shape of the active site- substrate doesn’t fit.

Question 21

The enzyme amylase catalyses the breakdown of starch to maltose. Iodine can be used to detect starch- if starch is present, iodine solution will change from browny-orange to blue-black.

Describe how you could investigate the effect of pH on the rate of amylase activity.

Answer 21

^{_{1) Put a drop of iodine solution into every well of a spotting tile.}}

^{_{2) Place a bunsen burner on a heatproof mat and a tripod and gauze over the bunsen burner.}}

^{_{3) Put a beaker of water on top of the gauze and heat it until it reacher 35°C (measure using a thermometer). Try to keep the water temperature constant throughout the experiment.}}

^{_{4)Put a boiling tube in the beaker. Use a syringe to add 1cm3 of amylase solution and 1cm3 of a buffer solution (pH 5) to the boiling tube.}}

^{_{5) Use a different syringe to add 5cm3 of a starch solution to the boiling tube. Immediately mix the contents and start a stopwatch.}}

^{_{6) Use continuous sampling to record how long it takes for the amylase to break down all of the starch. Do this by using a pipette to take a sample from the boiling tube every 30 seconds and putting a drop into different wells of the spotting tile. When the iodine solution remains browny-orange, starch is no longer present.}}

^{_{7) Repeat the experiment with buffer solutions of different pH values to see how pH affects the time taken for the starch to be broken down.}}

Question 22

The enzymes used in digestion are produced by ___ and then released into the ___ to mix with food.

Answer 22

1. specialised cells in glands and in the gut lining, gut.

Question 23

What do enzymes do to starch, proteins and fats, and why?

Answer 23

They are molecules too big to pass through the walls of the digestive system- they can’t be absorbed into the bloodstream. Enzymes break them down into smaller, soluble molecules.

Question 24

Give 4 examples of smaller molecules that result from enzymes breaking down starches, proteins and fats.

Answer 24

Sugars (e.g. glucose and maltose), amino acids, glycerol and fatty acids.

Question 25

Starch is a….

Answer 25

Carbohydrate.

Question 26

In what three places is amylase made?

Answer 26

• The salivary glands
• The pancreas
• The small intestine

Question 27

What is amylase and what does it do?

Answer 27

It is a carbohydrase and it catalyses the breakdown of starch- a carbohydrate- into maltose and other sugars like dextrins.

Question 28

What do proteases do?

Answer 28

Convert proteins into amino acids.

Question 29

In what three places are proteases made?

Answer 29

• The stomach (it’s called pepsin there - pepsin is one type of protease)
• The pancreas
• The small intestine

Question 30

What do lipases do?

Answer 30

Convert lipids into glycerol and fatty acids.

Question 31

In what two places are lipases made?

Answer 31

• The pancreas
• The small intestine

Question 32

What does the body do with the products of digestion?

Answer 32

• Make them into new carbohydrates, proteins and lipids
• Some glucose is used in respiration

Question 33

Where is bile produced?

Answer 33

In the liver.

Question 34

Where is bile stored before it’s released into the small intestine?

Answer 34

The gall bladder.

Question 35

What does bile do?

Answer 35

• Neutralises stomach acid: hydrochloric acid in the stomach makes the pH too acidic for enzymes in the small intestine to work properly. Bile is alkaline so it neutralises the acid and makes conditions alkaline, which is best for enzymes in the small intestine.
• Emulsifies fats: breaks fats into tiny droplets, giving fats a bigger surface area for the enzyme lipase to work on. This makes its digestion faster.

Question 36

An enzyme controlled reaction was carried out at pH 4. After 60 seconds, 33cm³ of product had been released. Calculate the rate of reaction in cm³ s^-1.

Answer 36

33/60 = 0.55cm³ s^-1

Question 37

What is the formula used to calculate the rate of reaction for a certain pH?

Answer 37

Amount of product formed = change

change/time(s)

(units: cm³ s^-1 or cm³/s)

Question 38

Label the diagram.

Answer 38

Question 39

What enzyme do the salivary glands produce?

Answer 39

Amylase.

Question 40

What digestive enzyme does the stomach produce? What other substance is produced by the stomach and why?

Answer 40

1. Pepsin (a type of protease).
2. Hydrochloric acid, to kill bacteria, and to give the right pH (2) for pepsin to function.

Question 41

What is the function of the stomach?

Answer 41

1. Pummels food with its muscular walls.
2. Produces pepsin.
3. Produces HCl - to kill bacteria, and to give the right pH (2) for pepsin to function.

Question 42

What is the function of the liver?

Answer 42

To produce bile.

Question 43

What is stored in the gall bladder?

Answer 43

Bile.

Question 44

What is the function of the pancreas?

Answer 44

Produces the enzymes protease, amylase and lipase and releases these into the small intestine.

Question 45

What is the function of the small intestine?

Answer 45

1. Produces the enzymes protease, amylase and lipase to complete digestion.
2. Here, nutrients from digested food are absorbed into the blood.

Question 46

What is the function of the large intestine?

Answer 46

This is where excess water is absorbed from food.

Question 47

What is the function of the rectum?

Answer 47

This is where faeces is stored before it is excreted through the anus.

Question 48

Describe the pathway of ingested food as it travels through the digestive system. (6 marks)

Answer 48

1. When the food is chewed in the mouth, it is digested by the enzyme amylase in saliva (produced by the salivary glands).
2. It travels town the oesophagus to the stomach, which pummels the food with its muscular walls. The stomach also produces pepsin, to digest the food, and hydrochloric acid, to kill bacteria and provide the optimal pH for pepsin to function.
3. When the food reaches the small intestine, it is neutralised by bile (produced in the liver and stored in the gall bladder).
4. It is digested by the enzymes protease, amylase and lipase (produced mostly by the pancreas and a little by the small intestine).
5. Nutrients from the digested food are then absorbed into the bloodstream.
6. In the large intestine, excess water is absorbed from the food, leaving faeces.
7. The faeces is stored in the rectum before being excreted through the anus.

Question 49

Before doing a food test, how would you prepare a food sample?

Answer 49

1. Break a piece of food up with a mortar and pestle.
2. Transfer it to a beaker and stir in some distilled water.
3. Filter out any solid bits of food.

Question 50

How would you test for reducing sugars?

Answer 50

Using the Benedit’s test.

Question 51

How would you test for starch?

Answer 51

Using iodine solution.

Question 52

How would you test proteins?

Answer 52

Using the biuret test.

Question 53

How would you test for lipids?

Answer 53

Using the Sudan III test.

Question 54

What would be a positive result for the Benedict’s test?

Answer 54

Testing for reducing sugars.

The solution starts off blue, then can change to green, to yellow and to brick-red, depending on the concentration of sugar.

Question 55

What would be a positive result for the iodine solution test?

Answer 55

Testing for starch.

The solution will change from browny-orange to blue-black.

Question 56

What would be a positive result for the Biuret test?

Answer 56

Testing for proteins.

The solution will change from blue to pink to purple.

Question 57

What would be a positive result for the Sudan III test?

Answer 57

Testing for lipids.

The mixture will separate into 2 layers, the top one (containing the lipids) bright red.

Question 58

What is the thorax?

Answer 58

The top part of your body (bottom of neck downwards), separated from the lower part by the diaphragm.

Question 59

What surrounds the lungs?

Answer 59

1. Pleural membranes.
2. Intercostal muscles.
3. Protected by the ribcage.

Question 60

The air you breathe in travels through the __, which splits into 2 tubes called the __ (each one is a __), one going to each lung.

Each of these splits into progressively smaller tubes called __, which finally end at small bags called __.

Answer 60

1. Trachea
2. Bronchi
3. Bronchus
4. Bronchioles
5. Alveoli

Question 61

Where does gas exchange happen?

Answer 61

The lungs contain millions of tiny air sacs, called alveoli, surrounded by a network of blood capillaries. Gas exchange happens between the alveoli and capillaries.

Question 62

Describe what happens during gas exchange.

Answer 62

Blood passing through capillaries by the alveoli has just returned to the lungs from the body, so it contains lots of carbon dioxide and very little oxygen.

An alveolus contains air just breathed in, so the concentrations of these gases are the other way around.

Oxygen diffuses out of the alveolus into the blood; carbon dioxide diffuses out of the blood into the alveolus, to be breathed out.

Question 63

Describe how gas is exchanged between body cells and capillaries.

Answer 63

When blood reaches body cells, oxygen is released from red blood cells and diffuses into body cells.

Carbon dioxide diffuses out of body cells and into the blood.

The blood is then carried to the lungs.

Question 64

Through which vein does blood flow into the right atrium?

Answer 64

The vena cava.

Question 65

Through which vein does blood flow into the left atrium?

Answer 65

The pulmonary vein.

Question 66

Through which artery does blood flow out of the right ventricle?

Answer 66

The pulmonary artery.

Question 67

Through which artery does blood flow out of the left ventricle?

Answer 67

The aorta.

Question 68

Why does the heart have valves?

Answer 68

To assure that blood flows in the right direction - to prevent it flowing backwards.

Question 69

Humans have a double circulatory system. Describe the flow of blood through each circuit and how they fit together.

Answer 69

1. In the first one, the right ventricle pumps deoxygenated blood to the lungs, to take in oxygen; the blood then returns to the heart.
2. In the second one, the left ventricle pumps oxygenated blood to all our organs; deoxygenated blood returns to the heart to be pumped out to the lungs again.

Question 70

Where in the heart is the natural pacemaker?

Answer 70

In the wall of the right atrium.

Question 71

How does the heart’s natural pacemaker work?

Answer 71

It is a group of cells which produce a small electric impulse which spreads to the surrounding muscle cells, causing them to contract. This causes the heart to beat regularly.

Question 72

Arteries carry blood __ the heart; veins carry blood __ the heart.

Answer 72

1. Away from.
2. Towards.

Question 73

Describe the structure of arteries and how this relates to their function.

Answer 73

1. Strong, thick and elastic walls to carry blood pumped out by the heart at high pressure. There is a thick muscular layer and elastic fibres.
2. Small lumen (doesn’t need to be big as the blood is at high pressure).

Question 74

Describe the structure of veins and how this relates to their function.

Answer 74

1. Walls thinner than arteries’ as blood is at lower pressure in veins.
2. Elastic fibres.
3. Big lumen to help the blood to flow despite the low pressure.
4. Valves keep blood flowing in the right direction.

Question 75

Describe the structure of capillaries and how this relates to their function.

Answer 75

1. Thin and permeable (1 cell thick) walls allow the diffusion of substances at a high rate.

Capillaries carry blood close to cells to allow food and gases to be exchanged.

Question 76

What are platelets and what is their function?

Answer 76

They are small fragments of cells (no nucleus) which help blood to clot at wounds. This prevents excessive bleeding, bruising and infection.

Question 77

Plasma is the liquid that carries the contents of the blood. Name at least 3 substances carried by plasma (there are 7).

Answer 77

1. Red blood cells, white blood cells, platelets.
2. Nutrients/soluble products of digestion - e.g. glucose.
3. Carbon dioxide, oxygen.
4. Urea.
5. Hormones.
6. Proteins.
7. Antibodies, antitoxins.

Question 78

What is coronary heart disease?

Answer 78

Where the coronary arteries (which supply blood to the muscles of the heart) become blocked by buildup of fatty deposits.

This narrows the arteries, restricting blood flow so that there is a lack of oxygen to the heart muscle, risking a heart attck.

Question 79

Name treatments for coronary heart disease.

Answer 79

1. Stents
2. Statins
3. Artificial hearts (temporary)
4. Transplants
5. Artificial blood (for emergencies)

Question 80

How do stents work?

Answer 80

Stents are hollow tubes inserted inside arteries to push out the wall and keep it open, making sure blood can flow through it. This keeps the heart beating.

Question 81

Give 3 advantages of stents.

Answer 81

1. Lower the risk of cardiac arrest.
2. Effective for a long time.
3. Recovery time from surgery is relatively quick.

Question 82

Give 3 disadvantages of stents.

Answer 82

1. Risk of complications during operation, e.g. heart attack.
2. Risk of infection from surgery.
3. Risk of thrombosis (patients developing a blood clot near the stent).

Question 83

How can cholesterol lead to coronary heart disease?

Answer 83

Cholesterol is an essential lipid that the body needs to function properly, but producing too much LDL cholesterol causes fatty deposits to form inside arteries.

Question 84

How do statins work?

Answer 84

The body producing too much LDL cholesterol causes fatty deposits to form inside arteries. Statins are drugs which reduce the amount of LDL present in the bloodstream, slowing the rate of fatty deposits forming.

Question 85

Give 3 advantages of statins.

Answer 85

1) Reduce risk of strokes, coronary heart disease and heart attacks.
2) As well as reducing LDL cholesterol, statins increase the amount of HDL cholesterol produced, a beneficial type that removes LDL cholesterol from the blood.
3) Studies suggest statins may prevent some other diseases.

Question 86

Give 3 disadvantages of statins.

Answer 86

1) There is a risk that someone forgets to take the drug.
2) Can cause negative side effects, e.g. headaches, kidney failure, liver damage or memory loss.
3) The effect of the drug is not instant.

Question 87

Give 3 disadvangages of heart transplants.

Answer 87

1) Donor organs aren’t always available straight away.
2) A donor heart may be rejected by the receiver’s immune system.
3) Transplant surgery can lead to bleeding and/or infection.

Question 88

Give 2 advantages of artificial hearts.

Answer 88

1) They can be sourced straight away (compared to donor organs) so keep people alive in emergencies.
2) They’re less likely to be rejected by someone’s immune system than donor hearts.

Question 89

Give 3 disadvantages of artificial hearts.

Answer 89

1) Surgery can lead to bleeding and/or infection.
2) They don’t work as well as real hearts - parts could wear out, or the motor could fail.
3) Blood doesn’t flow smoothly through them, causing clots and leading to strokes. (The patient has to take drugs to thin their blood to prevent this.)

Question 90

How can heart valves become damaged?

Answer 90

1. Can be damaged/weakened by heart attacks, infection or age.
2. This can cause stiffening of tissue, so that it won’t open properly.
3. Or a valve could become leaky, allowing blood to flow in both directions.

Question 91

How does artificial blood keep people alive in emergencies?

Answer 91

1. When someone loses a lot of blood, their heart can still pump the remaining red blood cells to their organs, provided that they have a high enough volume of blood.
2. Artificial blood is a saline solution which is used as a substitute for this lost volume.
3. It keeps people alive, and may give someone enough time to produce new red blood cells (or they’ll need a transfusion).

Question 92

What is health?

Answer 92

The state of physical and mental wellbeing.

Question 93

Give 4 examples of how different diseases can interact.

Answer 93

1) Defects of the immune system make people more vulnerable to communicable diseases.
2) Viruses living in cells can trigger cancers.
3) Immune reactions caused by pathogens can trigger allergic reactions or rashes, or worsen athsma symptoms.
4) Severely ill physical health can trigger mental illnesses.

Question 94

Discuss the human cost of non-communicable (non infectious) diseases.

Answer 94

1. Death
2. Lower quality of life
3. Shorter lifespan
4. Affects loved ones around those suffering

Question 95

Discuss the financial cost of non-communicable (non infectious) diseases.

Answer 95

1. Research and treatment is very expensive for health organisations.
2. Families have to accomodate for someone with a disease (which is costly), e.g. moving house or adapting their home.
3. If someone with a disease has to stop working or dies, their income stops, affecting their family and the economy.

Question 96

What are the 3 main types of risk factor for non-communicable diseases?

Answer 96

1. Lifestyle choices
2. Substance’s in someone’s body or environment
3. Genetics

Question 97

Give at least 3 (there are 6 in total) examples of causual mechanisms (things which are proven to directly cause) for non-communicable diseases.

Answer 97

1) Diet, smoking and exercise risk cardiovascular disease.
2) Smoking risks lung disease and lung (and some other types of) cancer.
3) Obesity risks Type 2 diabetes and cancer.
4) Alcohol risks damaging liver and brain function.
5) Smoking and alcohol affect the health of unborn babies.
6) Carcinogens, including ionising radiation, are risk factors of cancer.
7) UV exposure risks skin cancer.
8) Certain viruses increase the chances of developing certain cancers.
* Many diseases are caused by the interaction of a number of factors.*

Question 98

What is cancer?

Answer 98

Uncontrolled cell division leads to the formation of a tumour. But not all tumours are cancerous - only malignant ones.

Question 99

What are the 2 types of tumour?

Answer 99

1. Benign
2. Malignant

Question 100

What are benign tumours?

Answer 100

Growths of abnormal cells which are contained in one area, usually within a membrane. They do not invade other parts of the body.

Question 101

What are malignant tumours?

Answer 101

Cancerous growths of abnormal cells which invade neighbouring tissues. Malignant cells can break off and spread to different parts of the body, through the blood, where they form secondary tumours.

Question 102

Name 5 risk factors of cancer.

Answer 102

1) Smoking risks lung (and some other types of) cancer.
2) Obesity/diet.
3) Carcinogens (substances capable of causing cancer) e.g. ionising radiation, alcohol, UV exposure (causes skin cancer)
4) Certain viruses increase the chances of developing certain cancers.
5) There are genetic risk factors for some cancers.

Question 103

Where is plant epidermal tissue found?

Answer 103

It covers the whole plant.

Question 104

Where are xylem and phloem tissues found?

Answer 104

Through the roots, stems and leaves of plants.

Question 105

Where is meristem tissue found? What is its function?

Answer 105

1. Found at the growing tips of shoots and roots.
2. It’s able to differentiate into many different types of plant cell, allowing the plant to grow.

Question 106

Name the 4 types of tissue found in leaves.

Answer 106

1. Upper epidermis
2. Palisade mesophyll tissue
3. Spongy mesophyll tissue
4. Lower epidermis

Question 107

How is the structure of plant epidermal tissue related to its function?

Answer 107

It is covered with a waxy cuticle, which reduces water lost by evaporation.

Question 108

How is the structure of the upper epidermis related to its function?

Answer 108

It is transparent so that light can pass through to the palisade layer.

Question 109

How is the structure of the palisade mesophyll tissue related to its function?

Answer 109

It has lots of chloroplasts; most photosynthesis happens here. It is near the top of the leaf, where it can get the most light.

Question 110

How is the structure of spongy mesophyll tissue related to its function?

Answer 110

It contains many air spaces, which increase the rate of diffusion of gases in + out of cells.

Question 111

How is the structure of the lower epidermis (at the bottom of the leaf) releated to its function?

Answer 111

It’s an exchange surface, covered in holes, called stomata, which allow the direct diffusion of carbon dioxide into the leaf (and oxygen + water vapour out).

The opening + closing of stomata is controlled by guard cells in response to environmental conditions.

Question 112

How is the structure of xylem and phloem tissue releated to its function?

Answer 112

These cells form a network of vascular bundles which deliver water and other nutrients to the entire leaf, and take away the glucose produced by photosynthesis. They also help support the leaf’s structure.

Question 113

What is the role of phloem tubes?

Answer 113

They transport food substances (mainly dissolved sugars), made in the leaves, to the rest of the plant for immediate use (like growth) or for storage.

This process is called translocation and transport goes in both directions.

Question 114

What is the structure of phloem tissue?

Answer 114

It’s made of columns of elongated living cells, with small pores in the end walls that allow cell sap to flow through.

Question 115

What is the process of phloem tubes transporting food called?

Answer 115

Translocation.

Question 116

What is the function of xylem tissue?

Answer 116

The tubes carry water and mineral ions from the roots to the stem and leaves. Xylem tubes only transport things upwards.

This is called the transpiration stream.

Question 117

The movement of water from the roots, through the xylem tubes and out of the leaves is called what?

Answer 117

The transpiration stream.

Question 118

What is transpiration?

Answer 118

The evaporation of water from the surface of plants. Most transpiration happens at the leaves.

Question 119

Why is there a constant transpiration stream of water through plants?

Answer 119

1. Leaves have to have stomata for gas exchange and photosynthesis.
2. This, however, means that water is lost through diffusion out of the stomata.
3. So more water has to be soaked up from the roots.
4. This creates a constant stream of water through the plant.

Question 120

What are the 4 main factos that affect the rate of transpiration?

Answer 120

1) Light intensity: the brighter the light, the greater the transpiration rate (photosynthesis can’t happen in the dark, so stomata aren’t opened, so no water is lost).
2) Temperature: the warmer it is, the greater the transpiration rate.
3) Air flow: the more wind around a plant, the greater the transpiration rate (water vapour is constantly blown away, maintaining a high concentration gradient for diffusion).
4) Humidity: the drier the air, the greater the transpiration rate (concentration gradient higher).

Question 121

How can the rate of transpiration be measured?

Answer 121

Measuring the rate of uptake of water by a plant (because you can assume this is directly related to water loss). This can be recorded with a potometer.

Question 122

How are guard cells adapted to open and close stomata?

Answer 122

Their kidney shape opens and closes the stoma (singular for stomata) in the middle.

It has thin outer walls and thickened inner walls to make the opening and closing work.

Question 123

How are guard cells adapted for gas exchange and controlling water loss from leaves?

Answer 123

1) When the plant has a lot of water, the guard cell becomes turgid, opening the stoma for gas exchange (for photosynthesis).
2) When the plant is transpiring faster than water is being taken up by the roots, the guard cell becomes flaccid, closing the stoma to limit water loss and wilting.
3) They are sensitive to light and close at night (when photosynthesis can’t happen) to prevent water loss.
4) There are more guard cells on the underside of a leaf, which is cool and shaded, compared to the upper side. These conditions reduce the rate of transpiration.

Question 124

How is a leaf’s structure adapted to maximise the rate of diffusion and gas exchange needed for photosynthesis? (6 marks)

Answer 124

1) The lower epidermis is an exchange surface, covered in stomata, through which carbon dioxide diffuses directly into the leaf, and oxygen and water vapour diffuse out.
2) The flattened shape of the leaf increases the area of this exchange surface, so that diffusion happens at a faster rate.
3) Air spaces between the walls of cells in the spongy mesophyll layer increase the surface area, increasing the rate of diffusion of gases, and therefore of gas exchange.
4) Therefore, the rate of gas exchange is maximised, so the rate of photosynthesis is also maximised.