Chapter 7 - The Principles of Exchange and Transport

Question 1

Knowledge check 1

Which inorganic ion do plants need to absorb to make amino acids?

Answer 1

Nitrate (and sulfate) ions

Question 2

Knowledge check 2

Calculate the surface area-to-volume ratio of a cuboid body of dimension 1 x 2 x 3.

Answer 2

Surface area = 22
Volume = 6
So the surface area-to-volume ratio is 22:6 or 11:3.

Question 3

Knowledge check 3

Give three reasons why humans need a specialised gas exchange surface (i.e. lungs).

Answer 3

• Humans are terrestrial (with an impermeable body surface)
• They are large (small surface area-to-volume ratio)
• They have a high metabolic rate (large demand for oxygen)

Question 4

What is gas exchange?

Answer 4

The process by which oxygen reaches cells and carbon dioxide is removed from them.

Question 5

What is respiration?

Answer 5

The release of energy from food to synthesise ATP in all living cells.

Question 6

What is an absorptive surface?

Answer 6

An absorptive surface allows the uptake of soluble substances - for example, products of digestion in the ileum or water and ions by root hairs.

Question 7

What is a gas exchange surface?

Answer 7

• A gas exchange surface allows oxygen to move through in one direction and carbon dioxide in the opposite direction.
• It is sometimes referred to as a respiratory surface because the respiratory gases, oxygen and carbon dioxide, are involved.

Question 8

Living cells require certain substances from their environment in order to …

Answer 8

Maintain their metabolic processes

Question 9

Living cells require certain substances from their environment to maintain their metabolic processes, and need to remove …

Answer 9

The toxic by-products of metabolism.

Question 10

Living cells require certain substances from their environment to maintain their metabolic processes, and need to remove the toxic by-products of metabolism.

Animal tissues obtain …

Answer 10

1. Oxygen from the air (or from water if they are aquatic)
2. Glucose, fatty acids and amino acids from ingested food
3. Water

Question 11

Living cells require certain substances from their environment to maintain their metabolic processes, and need to remove the toxic by-products of metabolism.

Animal tissues remove …

Answer 11

1. Carbon dioxide

2. Nitrogenous waste (e.g. urea)

Question 12

Living cells require certain substances from their environment to maintain their metabolic processes, and need to remove the toxic by-products of metabolism.

Plant tissues obtain …

Answer 12

1. Oxygen from the air, especially at night
2. Carbon dioxide from the air during the day
3. Inorganic ions (e.g. nitrate ions to provide nitrogen for amino acid synthesis and phosphate for the synthesis of phospholipids) from the soil solution
4. Water

Question 13

Living cells require certain substances from their environment to maintain their metabolic processes, and need to remove the toxic by-products of metabolism.

Plant tissues remove …

Answer 13

1. Either carbon dioxide or oxygen, depending on the time of day.

Question 14

Surface area definition

Answer 14

Surface area is the total number of cells in direct contact with the surrounding environment.

Question 15

Volume definition

Answer 15

Volume is the total three-dimensional space occupied by metabolically active tissues.

Question 16

What are some of the methods of increasing the area of an absorptive/exchange surface?

Answer 16

1. Evagination (outfolding) of the surface
2. Invagination (infolding) of the surface
3. Flattening of the organism

Question 17

Small organisms have a large …

Answer 17

Surface area-to-volume ratio

Question 18

Large organisms have a small …

Answer 18

Surface area-to-volume ratio

Question 19

As an organism’s size increases, it’s surface area …

Answer 19

Increases less than its volume

Question 20

As an organism’s size increase, it’s surface area increases less than its volume. Why is this?

Answer 20

This is because many cells are not in direct contact with the surrounding environment

Question 21

Why do mammals and flowering plants possess impermeable body surfaces?

Answer 21

To prevent water loss by evaporation

Question 22

Volume of metabolically active tissue influences …

Answer 22

The demand for metabolites

Question 23

Surface area influences …

Answer 23

The rate of supply of metabolites to tissues

Question 24

An organism requires a specialised absorptive surface it if is …

Answer 24

1. Terrestrial (with an impermeable surface)
2. Large (with a small surface area-to-volume ratio)
3. Has a high metabolic rate

Question 25

The external surface of a small organism can be used as …

Answer 25

A gas exchange surface

Question 26

The external surface area of a small organism can be used as a gas exchange surface. Why?

Answer 26

Because the relatively large surface area is able to supply sufficient oxygen to the relatively small volume

Question 27

Give an example of a small organism which uses its external body surface as a gas exchange surface

Answer 27

An earthworm

Question 28

Why do large organisms require a specialised exchange surface to aid passive and active transport?

Answer 28

• A larger organism has a small surface area compared with its large volume, i.e. a small surface area-to-volume ratio.
• The relatively large volume creates a demand for metabolites that the relatively small surface area is unable to supply.
• Therefore, large organisms need specialised permeable surfaces whereby the absorption or exchange area is increased to satisfy the needs of the organisms.

Question 29

The rate at which an organism requires substances depends on …

Answer 29

It’s metabolic rate

Question 30

Cells in living organisms need to be able to obtain essential substances from their surroundings and also be able to remove waste or toxic products. In unicellular organisms this exchange takes place through …

Answer 30

The cell surface membrane

Question 31

What are some of the features of exchange surfaces that aid passive and active transport into and out of organisms?

Answer 31

1. Increasing the surface area of the exchange surface
2. Thin separating surface
3. Large concentration gradients

Question 32

Other than increasing the surface area-to-volume ratio, what does having a flattened shape achieve?

Answer 32

Decreases the distance over which substances have to diffuse to reach any cell in the organism.

Question 33

Name five examples of efficient absorptive surfaces in flowering plants and mammals

Answer 33

Leaf mesophyll
Root hairs
Alveoli
Capillaries
Red blood cells

Question 34

Leaf mesophyll is an example of an efficient absorptive surface in flowering plants. Describe and explain the structure and function of leaf mesophyll which makes it an efficient absorptive surface.

Answer 34

Structure
• The leaf is a flattened structure (it’s thinness ensures a short diffusion distance) with a tightly packed upper palisade mesophyll layer and a loosely packed lower spongy mesophyll layer.

Function
• The wide expanse of palisade tissue is efficient at trapping light.
• The loose arrangement of the spongy layer provides an air space system through the leaf and creates a huge surface for gas exchange.

Question 35

Root hairs are an example of an efficient absorptive surface in flowering plants. Describe and explain the structure and function of root hairs which make them an efficient absorptive surface.

Answer 35

Structure
• Tubular extensions of the epidermal cells of the young root.

Function
• Increase greatly the surface area of the root for the uptake of oxygen, water and ions.

Question 36

Alveoli are an example of an efficient absorptive surface in mammals. Describe and explain the structure and function of alveoli which make them an efficient absorptive surface.

Answer 36

Structure
• Small (diameter 0.2 mm) sacs, occurring in clusters and in vast numbers within the mammalian lung.
• In human lungs there are 700 million, providing a total surface area of 70 m^2.

Function
• The huge, moist surface area provides for efficient gas exchange.
• Alveolar walls are thin (0.1-1.0 micrometers), so the diffusion distance is short.

Question 37

Capillaries are an example of an efficient absorptive surface in mammals. Describe and explain the structure and function of capillaries which make them an efficient absorptive surface.

Answer 37

Structure
• Small (average diameter 8 micrometers), thin-walled blood vessels, with a total length of 100 000 km and surface area of 1000m^2 in the human body.

Function
• Extensive networks throughout the body represent a huge surface area for exchange of molecules between blood and body tissues.
• The number and distribution of capillaries are such that no cell is further away than 50 micrometers from a capillary.

Question 38

Red blood cells are an example of an efficient absorptive surface in mammals. Describe and explain the structure and function of red blood cells which make them an efficient absorptive surface.

Answer 38

Structure
• Small (diameter around 8 micrometers), flexible biconcave discs, flattened and depressed in the centre, with a dumbbell-shaped cross-section.

Function
• The biconcave disc shape greatly increases the surface area-to-volume ratio for efficient uptake of oxygen.
• The thinness of the cell, particularly where it is depressed in the centre, allows oxygen to diffuse to all the haemoglobin packed into the cell.

Question 39

Knowledge check 4

Which two features of red blood cells provide them with a large surface area-to-volume ratio?

Answer 39

They are especially small and have a biconcave disc (flattened) shape.

Question 40

What are the two gas exchange surfaces in mammals?

Answer 40

1. Between air in the alveoli and blood in the pulmonary capillaries within lungs.
2. Between blood in the systemic capillaries and cells in body tissues.

Question 41

Give an example of an absorptive surface in mammals.

Answer 41

The mucosal layer in the ileum

Question 42

Why does diffusion occur?

Answer 42

• The molecules in gases and liquids move constantly and at random.
• If there is a difference in concentration of molecules within an area, a net movement of the molecules occur, resulting in the molecules becoming evenly distributed.

Question 43

Draw a diagram of a top view and side view (cross-section) of a flatworm

Answer 43

Textbook page 121

Question 44

Sketch a graph showing the relationship between surface area-to-volume ratio and cube size/side length

Answer 44

Textbook page 120

Student Guide AS Unit 2 page 7

Question 45

Draw a diagram of a terminal bronchiole

Answer 45

Textbook page 127
Student Guide AS Unit 2 page 9

Diagram should include:
• A terminal bronchiole (Terminal bronchioles are last generation of conducting airways. Respiratory bronchioles can be identified by the presence of some alveoli along their walls. The respiratory bronchiole splits into a number of alveolar ducts, which terminate in alveolar sacs and individual alveoli).

• Alveolar ducts
• Alveoli
• Capillary network present on the surface of the alveoli
• Capillary network connected to a branch of the pulmonary artery (blue = deoxygenated blood) and to a branch of the pulmonary vein (red = oxygenated blood)

Question 46

Draw a cross-section through a root and draw an individual root hair cell

Answer 46

Textbook page 122
Student Guide AS Unit 2 page 13 figure 6

Diagram should show a cross-section of a root, displaying root hairs extending from the epidermis layer.

Individual root hair cell should contain a vacuole, nucleus, cell wall, cell surface membrane and cytoplasm. The root hair extension should be surrounded by soil particles, air spaces and a soil solution. Movement of O2 should be into the root hair (from the air space) and movement of CO2 should be out of the root hair (into the air space). The root hair cell should be labelled as part of the epidermis layer.

Question 47

Draw a diagram of a top view and side view (cross-section) of a red blood cell

Answer 47

Textbook page 122

Question 48

Diffusion (gas exchange) can take place across surfaces as long as …

Answer 48

The surface is:
• Moist, since gases must dissolve in water before diffusing into tissue cells
• Permeable to the substance(s) in question (e.g. oxygen and carbon dioxide)

Question 49

Transport of substances in larger organisms occurs by …

Answer 49

Mass flow

Question 50

Mass flow is brought about by …

Answer 50

A pressure difference

Question 51

Draw a diagram illustrating the mass flow of molecules

Answer 51

Student Guide AS Unit 2 page 10 figure 3

Question 52

Draw a diagram illustrating the diffusion of molecules

Answer 52

Student Guide AS Unit 2 page 9 figure 2

Question 53

Diffusion stops when …

Answer 53

The molecules have dispersed from an area of high concentration and are evenly dispersed.

Question 54

What is the main difference between mass flow and diffusion?

Answer 54

In mass flow all molecules are swept along in the same direction, whereas in diffusion the molecules move at random in all directions.

Question 55

Give some examples of mass flow systems in flowering plants and mammals

Answer 55

Flowering plants
• Xylem system
• Phloem system

Mammals
• Breathing (ventilation) system
• Blood circulatory system

Question 56

Different mass flow systems have …

Answer 56

Different means of generating a pressure difference.

Question 57

What is the method of generating a pressure difference in the xylem system of flowering plants?

Answer 57

Water evaporating from the leaf creates a tension (negative pressure) in the leaf xylem that pulls water up through the xylem as part of the transpiration stream.

Question 58

What is the function of the xylem system of flowering plants?

Answer 58

One-way transport of water and inorganic ions (minerals) from roots to leaves in a flowering plant.

Question 59

What is the method of generating a pressure difference in the phloem system of flowering plants?

Answer 59

Energy expenditure is involved in moving the sucrose into the phloem. ATP is used to move sucrose from the companion cell to the phloem sieve tube.

Question 60

What is the function of the phloem system of flowering plants?

Answer 60

Transport of sucrose (translocation) to roots (for storage of carbohydrate) and to growing regions (to provide energy for growth).
Two-way flow of organic solutes in a flowering plant.

Question 61

What is the method of generating a pressure difference in the breathing (ventilation) system of mammals?

Answer 61

Pressures in the thorax are alternately decreased, causing air to enter the lungs (inducing inhalation), and increased, causing air to be expelled from the lungs (inducing exhalation).

Question 62

What is the function of the breathing (ventilation) system of mammals?

Answer 62

Ventilation of the mammalian lungs, whereby fresh air (rich in O2 and low in CO2) is drawn in and stale air (rich in CO2 and low in O2) is forced out, ensuring that diffusion of respiratory gases can occur (by maintaining the concentration gradient between the alveoli and capillaries)

Question 63

What is the method of generating a pressure difference in the blood circulatory system of mammals?

Answer 63

High pressure is generated by the pumping of the muscular heart.

Question 64

What is the function of the blood circulatory system of mammals?

Answer 64

Circulation of blood carrying oxygen, glucose, amino acids, fats, carbon dioxide, urea etc. in a mammal.

Question 65

Knowledge check 5

How are gases moved across exchange surfaces?

Answer 65

Diffusion

Question 66

Knowledge check 6

List the features of a good transport system.

Answer 66

A good transport system has a pump (or equivalent), vessels and a fluid to transport materials around the body. It also has exchange surfaces to load substances into the transport system and to remove them where required.

Question 67

Knowledge check 7

Explain how thinness helps gaseous exchange and means that a transport system is not necessary.

Answer 67

It increases the surface area-to-volume ratio (more oxygen is absorbed relative to the amount required) and means that all tissues are close to the absorbing surface.

Question 68

Give an example of an overlap between the principles of exchange and the principles of transport.

Answer 68

The flattening of an organism/cell

• Increases its surface area, therefore increasing its surface area-to-volume ratio, therefore increasing its effectiveness in terms of absorption (which may eliminate the need for a specialised exchange surface).
• However, it also decreases the diffusion path, as more cells are in direct contact with the surrounding environment (which may eliminate the need for a specialised internal transport system).

Question 69

Why does a flatworm have no need for an internal transport system?

Answer 69

1. It is thin (short diffusion distance for respiratory gases).
2. It’s digestive system branches throughout its body so that food is absorbed in close proximity to all tissues.

Question 70

Gas exchange across an exchange surface relies on the process of …

Answer 70

Diffusion

Question 71

What law summarises the relationship between factors affecting the rate of diffusion?

Answer 71

Fick’s law

Question 72

State Fick’s law

Answer 72

Rate of diffusion ∝ Surface area X Difference in concentration
—————————————————–
Length of diffusion pathway (thickness of membrane)

Question 73

Fick’s law shows that gas exchange is increased where:

Answer 73

1. The exchange surface has a large surface area.
2. There is a big difference in the concentration of gases on each side of the surface.
3. The exchange surface is thin, with a short diffusion distance.

Question 74

Rate of diffusion ∝ Surface area X Difference in concentration
—————————————————–
Length of diffusion pathway (thickness of membrane)

Fick’s law shows that gas exchange is increase where:
• The exchange surface has a large surface area.
• There is a big difference in the concentration of gases on each side of the surface.
• The exchange surface is thin, with a short diffusion distance.

The higher the ______ ______ of the organism, the greater the need for these principles to be met

Answer 74

Metabolic rate

Question 75

In Fick’s law, the barrier to diffusion is often referred to as a …

Answer 75

Membrane

Question 76

The term membrane includes …

Answer 76

Both the cell surface membrane and the epithelium membrane, i.e. the layer of cells covering an organ.

Question 77

What are the two processes in flowering plants that involve gas exchange?

Answer 77

Respiration

Photosynthesis

Question 78

Where and when does respiration take place in a flowering plant?

Answer 78

In all tissues, all the time

Question 79

Where and when does photosynthesis take place in a flowering plant?

Answer 79

Only in green tissues (i.e. those containing chlorophyll), and only during the daylight hours.

Question 80

The rate of photosynthesis in a flowering plant largely depends on …

Answer 80

The light intensity

Question 81

The maximum rate of photosynthesis occurs when …

Answer 81

The light intensity is highest

Question 82

At what time during the day is light intensity the highest?

Answer 82

At midday

Question 83

At what time(s) during the day will light intensity be low?

Answer 83

During dawn and dusk

Question 84

When the light intensity is high (during midday) the rate of photosynthesis greatly (blank) the rate of respiration.

Answer 84

Exceeds

Question 85

During midday, there is a net production of …

Answer 85

Oxygen

Question 86

When the rate of photosynthesis exceeds the rate of respiration there is a net production of …

Answer 86

Oxygen

Question 87

At a specific low light intensity (during dawn and dusk) the rate of photosynthesis (blank) the rate of respiration

Answer 87

Equals

Question 88

What happens when the rate of photosynthesis equals the rate of respiration?

Answer 88

The net exchange of oxygen is zero

Question 89

When the rate of respiration exceeds the rate of photosynthesis there is a net production of …

Answer 89

Carbon dioxide

Question 90

What is a compensation point?

Answer 90

Compensation points - light intensity at which the rate of respiration = rate of photosynthesis

Therefore, the rate of oxygen production (in photosynthesis) is balanced by the rate of oxygen consumption (in respiration).

Question 91

The roots of plants use energy in processes such as …

Answer 91

1. Cell division (growth)

2. The active transport of ions from the soil solution

Question 92

Knowledge check 8

Suggest why plants have a lower metabolic rate than animals.

Answer 92

Plants lack muscle tissue, i.e. they do not move from place to place.

Question 93

In the growth region of plant roots, the epidermal cells possess …

Answer 93

Root hairs

Question 94

In soil that is not waterlogged, root hairs are surrounded by ___ ______ between the particles of soil

Answer 94

Air spaces

Question 95

Diffusion of respiratory gases into root hair cells occur through …

Answer 95

The cell wall and cell surface membrane of the root hair extension.

Question 96

In stems, especially those of woody plants, the most active cells are those …

Answer 96

Under the surface

Question 97

The outer covering of the stem of a flowering plant is …

Answer 97

Waterproofed

Question 98

Why is the outer covering of the stem of a flowering plant waterproofed?

Answer 98

To reduce water loss to the air

Question 99

How does oxygen and carbon dioxide diffuse in and out of the stem of a flowering plant?

Answer 99

Although the stem’s outer covering is waterproofed to reduce water loss to the air (and so is impermeable to gases) there are small pores to allow oxygen in and carbon dioxide out.

Question 100

Plants lack specialised respiratory surfaces and yet can be very large. Why is this?

Answer 100

This is because they lack tissues with a high energy demand and so have low respiration rates.

Question 101

What is the major photosynthetic organ in a flowering plant?

Answer 101

The leaf

Question 102

The leaf is the major photosynthetic organ in a flowering plant. Why does the leaf need a specialised gas exchange surface?

Answer 102

• A high rate of photosynthesis is generated.

* The concentration of carbon dioxide in air is low (400 parts per million or 0.04%)

Question 103

Outline the steps and processes which occur during photosynthetic gas exchange in the leaves of flowering plants.

Answer 103

1. The leaf epidermis, particularly the lower epidermis, possesses guard cells that control the opening and closure of stomata.
2. The stomata open during the hours of daylight.
3. When open, air, containing carbon dioxide, diffuses into and out of the leaf mesophyll.
4. Carbon dioxide diffuses through an air space system provided by the spongy mesophyll.
5. Having diffused through the air space system, carbon dioxide is absorbed by mesophyll cells (spongy mesophyll cells and palisade mesophyll cells) (in which carbon dioxide concentration is low as it is used in photosynthesis).
6. It is the moist mesophyll surface that represents the gas exchange surface.
7. Oxygen produced in photosynthesis diffuses out of mesophyll cells into the air space and then out through open stomata. Some oxygen is, of course, used up in respiration.

Question 104

What are the advantages to the leaf in being broad and thin?

Answer 104

Broad = Large surface area for the exchange of gases
Thin = Short diffusion distance for gases

Question 105

The cell surface membrane of mesophyll cells is ..

Answer 105

Moist

Question 106

Identify the (photosynthetic) gas exchange surfaces in the leaf.

Answer 106

The cell surface membranes of mesophyll cells (spongy mesophyll cells and palisade mesophyll cells)

Question 107

Knowledge check 9

List three features of leaves that facilitate gas exchange.

Answer 107

• Leaves have an air space system.
• They are thin (and broad).
• They have stomata that connect to the atmosphere.

Question 108

There is much (blank) oxygen dissolved in water than there is oxygen in air.

Answer 108

Less

Question 109

Knowledge check 10

Why do floating water plants only have stomata on the upper surface of their leaves?

Answer 109

They allow air to enter from the atmosphere;
Carbon dioxide for photosynthesis;
And oxygen, which may diffuse down to the roots through an extensive air-space system, for respiration. Both oxygen and carbon dioxide are more plentiful in air than dissolved in water.

Question 110

Aquatic flowering plants are called …

Answer 110

Hydrophytes

Question 111

What are hydrophytes?

Answer 111

Aquatic flowering plants

Question 112

There is much less oxygen dissolved in water than there is oxygen in air, so the stems and leaves of aquatic flowering plants (hydrophytes) have …

Answer 112

Adaptations to facilitate the uptake and movement of oxygen and carbon dioxide.

Question 113

Draw a diagram of the stem of a hydrophytic plant

Describe the structure and function of the stem of a hydrophytic plant

Answer 113

Normal plant stem = Textbook page 138
Hydrophytic plant stem = Student Guide AS Unit 2 page 14

Diagram should include:
• Epidermis
• Cortex
• Vascular bundles, consisting of phloem (outermost) and xylem (innermost)
• Air spaces - easier for oxygen to diffuse to all tissues, including root. (i.e. Oxygen which enters from stomata on the upper surface of leaf will diffuse down to the roots through an extensive air-space system within the stem).

Question 114

Draw a diagram of a leaf of a hydrophytic plant

Describe the structure and function of the leaf of a hydrophytic plant

Answer 114

Student guide AS Unit 2 page 14

Diagram should include:
• Stoma on upper epidermis - allows gas exchange without the leaf becoming flooded; transpirational loss of water not a problem.
• Air spaces - easier for diffusion of gases and aids the flotation of the leaf.

Question 115

Gas exchange in the cells of mammals occurs by …

Answer 115

Diffusion

Question 116

How is the concentration gradient of respiratory gases induced/maintained in a respiring cell?

Answer 116

In cells respiring aerobically, oxygen is used and carbon dioxide is produced. This affects the concentration gradients and so oxygen diffuses into the cell and carbon dioxide diffuses out.

Question 117

Draw a diagram showing gaseous exchange in a respiring animal cell.

Answer 117

Student Guide AS Unit 2 page 15

Diagram should include:
• Cell surface membrane
• Nucleus
• Cytoplasm
• Mitochondria
• Arrows showing O2 entering and CO2 leaving
• Inside cell:
- high CO2 concentration
- low O2 concentration
• Outside cell:
- low CO2 concentration
- high O2 concentration

Mitochondrion - Site of aerobic respiration: uses oxygen and produces carbon dioxide.

Question 118

The movement of oxygen to cells, and of carbon dioxide out, involves four stages in mammals (humans). What are these four stages?

Answer 118

• Diffusion of gases between respiring cells and blood
• Transport of gases in blood
• Diffusion of gases across the gas exchange surface between alveolar air and blood
• Ventilation of the lungs with fresh air

Question 119

In mammals, there are two mass flow systems involved in the movement of respiratory gases (oxygen and carbon dioxide). What are these two mass flow systems?

Answer 119

• Breathing (ventilation) system

* Blood circulatory system

Question 120

In mammals, there are two mass flow systems involved in the movement of respiratory gases (oxygen and carbon dioxide). These two mass flow systems are known as the breathing (ventilation) system and the blood circulatory system. However, movement of respiratory gases at the two interfaces relies on …

Answer 120

Diffusion

Question 121

Give an account of the structure of the lungs

Answer 121

1. Air is breathed through the nostrils or mouth, and enters or leaves the lungs via the trachea.
2. The lungs are situated within the thorax (also known as the thoracic cavity).
3. The trachea branches into two bronchi (singular bronchus) which further branch into a series of ever-finer bronchioles forming a bronchial tree.
4. Each terminal bronchiole leads to a cluster of alveoli, with an alveolar duct connecting with each alveolus.
5. Each individual alveolus is tightly wrapped in blood capillaries and it is here that gas exchange takes place

Question 122

Draw a diagram of the structure of the lung system

Answer 122

Textbook page 127
Student Guide AS Unit 2 page 16

Diagram should include:
• Nostril
• Mouth
• Oesophagus
• Larynx
• Trachea
• Ring of cartilage
• Rib section
• Left lung
• Pleural membranes
• Pleural fluid
• Right bronchus
• Bronchiole
• Heart/ Space occupied by heart
• External intercostal muscles
• Internal intercostal muscles
• Muscular part of diaphragm
• Diaphragm

Small diagram:
• Terminal bronchiole
• Branch of pulmonary artery (blue)
• Branch of pulmonary vein (red)
• Alveoli
• Capillary network/ capillary

Smaller diagram:
• Capillary
• Arrows showing movement of O2 and CO2 between alveolus and capillary

Question 123

What is breathing?

Answer 123

Breathing refers to the movements of the ribs and diaphragm that cause air to enter and leave (ventilate) the lungs and alveoli.

Question 124

Changing the volume of the thoracic cavity changes …

Answer 124

The air pressure inside the lungs.

Question 125

Air will always move from a region of high pressure to …

Answer 125

A region of low pressure

Question 126

When air moves into the lungs this is known as …

Answer 126

Inhalation (or inspiration)

Question 127

When air moves out of the lungs this is known as …

Answer 127

Exhalation (or expiration)

Question 128

Give an account of the mechanism of inhalation during normal breathing.

Answer 128

• Inspiration (or inhalation) - describes the process of breathing in.
• The external intercostal muscles contract (as the internal intercostal muscles relax) and the ribs are pulled upwards and outwards.
• Simultaneously, the diaphragm muscle contracts, causing the diaphragm to flatten from its domed shape.
• Both these actions increase the volume of the thorax, which in turn reduces the pressure around the lungs.
• The pressure differential between the atmosphere and the lungs causes air to enter the lungs until a pressure equilibrium is achieved.

Question 129

Give an account of the mechanism of exhalation during normal breathing.

Answer 129

• Expiration (or exhalation) - describes the process of breathing out (expiration is really inspiration in reverse).
• The external intercostal muscles relax (as the internal intercostal muscles contract) causing the ribs to move downwards and inwards.
• The relaxation of the diaphragm muscles causes it to return to its domed position.
• The return of the ribs and the diaphragm to their original positions causes the volume of the thorax to reduce, consequently increasing the pressure around the lungs.
• With the thorax now having a higher pressure than the atmosphere, air is forced out of the lungs.

Question 130

What is forced exhalation?

Answer 130

During forced exhalation, the volume of the thoracic cavity is further reduced by the contraction of the internal intercostal muscles depressing the rib cage.

Question 131

Give two examples of when forced exhalation may occur

Answer 131

During hard exercise

Coughing

Question 132

Knowledge check 11

Explain the advantage of ventilating the lungs.

Answer 132

Ventilation of the lungs replenishes oxygen-rich air in the alveoli, thus increasing the oxygen diffusion gradient into the blood. It also replaces air that has a high carbon dioxide concentration with air that has a low carbon dioxide concentration, thus increasing the carbon dioxide diffusion gradient out of the blood.

Question 133

The alveoli of the lungs exhibit all the features of an efficient gas exchange surface. Name these features.

Answer 133

Permeable
Moist
Large surface area
Thin walls
Close to transport systems

Question 134

Knowledge check 12

Explain this statement ‘inhalation is an active process but exhalation is usually passive’.

Answer 134

Inhalation depends on the contraction of muscles of the diaphragm and the external intercostal muscles of the ribcage, while exhalation normally involves only the relaxation of these muscles.

Question 135

Alveoli consist of …

Answer 135

A single layer of squamous epithelial cells.
Surfactant secreting epithelial cells (septal cells).
Macrophages (phagocytic cell derived from monocyte.)
Alveolar air space.
Moist inner alveolar surface.

Question 136

Gases can diffuse across the moist, permeable walls of the alveoli. Diffusion of gases between the alveoli and the blood is rapid because …

Answer 136

1. Concentration gradients are maintained by ventilation of the lungs and blood flow through pulmonary capillaries.
2. The alveoli and pulmonary capillaries have very large surface areas.
3. The walls of both alveoli and capillaries are thin and therefore the diffusion distance is very short.

Question 137

Draw a diagram of the structure of an alveolus and its associated capillaries

Answer 137

Student Guide AS Unit 2 page 18
Textbook page 127

Diagram should include
• Alveolar air space
• Alveolar duct
• Arrows showing movement of inhaled air and exhaled air
• Moist alveolar surface
• Surfactant-secreting epithelial cells (septal cells)
• Alveolus
• Macrophage (phagocytic cell derived from monocyte)
• Blood capillary, endothelial cells line capillary (which are squamous epithelium)
• Red blood cell(s)
• Epithelial cells of alveolus are squamous epithelium
• Supporting connective tissue
• Diffusion barrier (alveolar wall)
• Arrows showing movement of gases (O2 and CO2) between alveolar air space and blood capillary

Question 138

There is a short diffusion distance between the alveolar air space and the blood capillary because …

Answer 138

The alveoli and capillary walls are thin (one cell thick).

They consist of squamous (pavement) epithelium.

Question 139

Why are alveoli situated deep inside the body?

Answer 139

Since the alveolar surface is situated deep inside the body, evaporation of water from its moist surface is reduced to a minimum.

Question 140

What is lung surfactant?

Answer 140

Lung surfactant is a mixture of lipid and protein. The detergent-like substance reduces surface tension in the fluid coating the alveoli, without which the alveoli would collapse due to cohesive force between the water molecules lining the air sacs.

Question 141

What are monocytes?

Answer 141

A type of white blood cell.

Question 142

What is the function of macrophages within alveolar wall?

Answer 142

Protect the lungs from a broad spectrum of microbes and particles by ingesting them through phagocytosis.

Question 143

The alveolar walls contain …

Answer 143

Elastic fibres

Question 144

What is the function of elastic fibres within the alveolar wall?

Answer 144

The elastic recoil of the alveoli, helping to force air out during exhalation.

Question 145

Knowledge check 13
Babies born prematurely are often deficient in surfactant, causing a condition called ‘respiratory distress syndrome’. From what you know about the role of surfactant, describe and explain the symptoms of this syndrome.

Answer 145

The alveoli collapse and are difficult to expand, making breathing difficult and reducing the efficiency of gas exchange. Blood oxygen levels fall and carbon dioxide levels increase.

Question 146

What is tar?

Answer 146

The collective name given to thousands of toxic substances present in tobacco smoke.

Question 147

What does COPD stand for?

Answer 147

Chronic Obstructive Pulmonary Disease

Question 148

Chronic obstructive pulmonary disease (COPD) is a combination of …

Answer 148

Chronic bronchitis

Emphysema

Question 149

What are the symptoms of chronic bronchitis?

Answer 149

Phlegm production
Coughing
Breathlessness
Increased susceptibility to infection

Question 150

What diseases are associated with smoking?

Answer 150

Chronic obstructive pulmonary disease (COPD)
- Chronic bronchitis
- Emphysema
Lung cancer

Question 151

What affect does tar have on the respiratory system?

Answer 151

• Tar brings about an inflammatory response in which airways narrow and excessive amounts of mucus are produced.
• Furthermore, tar paralyses the cilia that sweep mucus and bacteria away from the lungs, so pathogens and mucus build up.
• This leads to phlegm production, coughing and breathlessness - symptoms of chronic bronchitis
• Inability to clear mucus and bacteria results in an increased susceptibility to chest infections, including pneunomia.

Question 152

Give an example of a chest infection

Answer 152

Pneunomia

Question 153

What affect does smoke inhalation have on the respiratory system?

Answer 153

• The inflammatory response to smoke inhalation leads to a breakdown of alveolar walls.
• This reduces the surface area available for gas exchange so it becomes difficult to get enough oxygen.
• There is also a loss of elastic fibres in the alveolar walls.
• Therefore, exhalation becomes more difficult because the ability of alveoli (and lungs) to recoil following inhalation is reduced.
• Air in damaged alveoli cannot be effectively expelled during expiration, leaving a layer of residual air in the alveoli that prevents fresh inhaled air reaching the gas exchange surfaces.
• Emphysema is developed.

Question 154

What is emphysema?

Answer 154

• In emphysema, the inflammatory response to smoke inhalation leads to a breakdown of alveolar walls.
• This reduces the surface area available for gas exchange so it becomes difficult to get enough oxygen.
• There is also a loss of elastic fibres in the alveolar walls.
• Therefore, exhalation becomes more difficult because the ability of alveoli (and lungs) to recoil following inhalation is reduced.
• Air in damaged alveoli cannot be effectively expelled during expiration, leaving a layer of residual air in the alveoli that prevents fresh inhaled air reaching the gas exchange surfaces.

Question 155

What are carcinogens?

Answer 155

Substances that can induce cancer

Question 156

Tobacco smoke contains many …

Answer 156

Carcinogens

Question 157

What affect do carcinogens have on the respiratory system?

Answer 157

• Carcinogens can damage DNA in the cells lining the bronchial tubes.
• Cells with DNA damage may divide in a modified and uncontrolled way, producing a mass of unspecialised cells known as a tumour.
• Once the tumour reaches a certain size, it can block the airways and/or damage large sections of the lungs.
• A cancerous or malignant tumour may spread to invade other tissues.

Question 158

Give an example of a carcinogen in tobacco smoke

Answer 158

Tar

Question 159

Knowledge check 14

Explain why a person with emphysema would have difficulty undertaking strenuous activity.

Answer 159

Emphysema reduces the efficiency of breathing (reduced recoil of alveoli due to loss of elasticity) and gas exchange (reduced surface area of alveoli). The overall result is that oxygen uptake is reduced greatly, so that less is available for the production of ATP in respiration - hence the difficulty in undertaking strenuous activity.

Question 160

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

Draw two diagrams of simple respirometers

Answer 160

Textbook page 129

Test tube/ glass container
Tap
Airtight bung
Metal gauze
Living material/organism (e.g. Blowfly larvae or woodlice)
Potassium hydroxide (KOH) or water 
Capillary tubing
Calibrated scale in mm or mm^3
Coloured bead of liquid OR dye

Textbook page 130

Living material
Airtight bung
Glass container
Sealed glass tube
Calibrated scale in mm or mm^3
Potassium hydroxide (KOH) or water (with initial potassium hydroxide/water level labelled)
Levelling arm
- Flexible rubber tubing
- Glass tube

Question 161

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

Give two examples of living material which can be used in this type of investigation

Answer 161

Blowfly larvae

Woodlice

Question 162

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

How can oxygen uptake be calculated?

Answer 162

1. Oxygen uptake in living organisms can be calculated by adding potassium hydroxide (KOH) to the respirometer.
2. When the respirometer is set up, the living organisms respire taking in oxygen from within the system.
3. The carbon dioxide produced is absorbed by the potassium hydroxide.
4. The consequent reduction in pressure cause the liquid/dye in the respirometer to move in the direction of the biological material.
5. The distance (volume) moved represents the oxygen used in respiration.

Question 163

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

Normally respirometers are calibrated so it is possible to calculate …

Answer 163

The initial and final levels of potassium hydroxide

OR

The initial and final levels of dye/ (coloured) liquid

Question 164

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

How can carbon dioxide production be calculated?

Answer 164

1. This can be calculated by repeating the process for calculating oxygen consumption but replacing the KOH with water.
2. The respirometer needs to be left for the same length of time and in the same conditions as that for measuring oxygen uptake.
3. This time the CO2 produced is not absorbed.
4. If there is no change in the movement of the liquid in the manometer or calibrated scale, then the volume of carbon dioxide produced is the same as the volume of oxygen absorbed.
5. If the liquid moves away from the living material, there is more carbon dioxide produced than oxygen taken in.
6. If the liquid moves closer to the living material there is less carbon dioxide produced than oxygen taken in.
7. Volume of carbon dioxide produced = Volume of oxygen consumed in initial test (with presence of KOH) +/- Difference in gas volume in second test (with water, no KOH)

Question 165

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

• Calculating carbon dioxide production

If the liquid moves away from the living material, there is …

Answer 165

More carbon dioxide produced than oxygen taken in

Question 166

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

• Calculating carbon dioxide production

If the liquid moves closer to the living material, there is …

Answer 166

Less carbon dioxide produced than oxygen taken in

Question 167

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

• Calculating carbon dioxide production

If the position of the liquid does not change …

Answer 167

The volume of carbon dioxide produced is equal to the volume of oxygen absorbed

Question 168

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

What is a manometer?

Answer 168

An instrument for measuring the pressure acting on a column of fluid, consisting of a U-shaped tube of liquid in which a difference in the pressures acting in the two arms of the tube causes the liquid to reach different heights in the two arms.

Question 169

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

• Calculating carbon dioxide production

If there is less carbon dioxide produced than oxygen taken in, the liquid moves closer to the living material. Why is this?

Answer 169

The carbon dioxide produced does not fully compensate for the pressure reduction caused by the oxygen uptake.

Question 170

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

What is the role of a thermobarometer?

Answer 170

Compensates for changes in temperature and pressure

Question 171

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

What are the controlled variables in this type of investigation?

Answer 171

1. Where possible, use the same apparatus for calculating oxygen uptake (with KOH) and for calculating carbon dioxide production (with water).
2. Use the same living organisms (this ensures that factors such as age and metabolic rate of the living material are controlled as far as possible).
3. Temperature can be controlled by placing the respirometers in a thermostatically controlled water bath for both parts of the investigation.
4. The apparatus must be left for the same amount of time and in the same conditions for both tests.

Question 172

Practical Work - Use of a respirometer to calculate oxygen uptake and carbon dioxide production

Why is temperature a controlled variable in this investigation?

Answer 172

1. It will affect the respiration rate (and hence oxygen intake and carbon dioxide production).
2. Can cause changes in gas volume due to expansion or contraction of gases within the system.

Question 173

What is urea?

Answer 173

Nitrogenous waste product from protein metabolism

Question 174

Cells in living organisms need to be able to obtain essential substances (such as oxygen and glucose for respiration, or nitrate ions in plants to provide the nitrogen needed for amino acid synthesis) from their surroundings and also be able to remove waste or toxic products (such as carbon dioxide from respiration, or urea in many animals as a waste product of protein metabolism). In unicellular organisms this exchange takes place through (blank).

Answer 174

The cell surface membrane

Question 175

Cells in living organisms need to be able to obtain essential substances (such as oxygen and glucose for respiration, or nitrate ions in plants to provide the nitrogen needed for amino acid synthesis) from their surroundings and also be able to remove waste or toxic products (such as carbon dioxide from respiration, or urea in many animals as a waste product of protein metabolism). In unicellular organisms this exchange takes place through the cell surface membrane. Multicellular organisms often have (blank) that take in or remove materials.

Answer 175

Specialised exchange surfaces

Question 176

Cells in living organisms need to be able to obtain essential substances (such as oxygen and glucose for respiration, or nitrate ions in plants to provide the nitrogen needed for amino acid synthesis) from their surroundings and also be able to remove waste or toxic products (such as carbon dioxide from respiration, or urea in many animals as a waste product of protein metabolism). In unicellular organisms this exchange takes place through the cell surface membrane. Multicellular organisms often have specialised exchange surfaces that take in or remove materials. In the more complex organisms, there is a (blank) that links the specialised exchange surface to the cells throughout the organism.

Answer 176

Transport system

Question 177

What is the role of a transport system?

Answer 177

Links the specialised exchange surface to the cells throughout the organism

Question 178

What are the two types of specialised exchange surfaces?

Answer 178

External exchange surfaces
Internal exchange surfaces

(External/internal relative to the body surface)

Question 179

Give an example of an external exchange surface

Answer 179

The folded external membranes of the external gills of young tadpoles

Question 180

Give two examples of internal exchange surfaces

Answer 180

Fish gills

Alveoli

Question 181

The presence of root hair extensions on root hair cells facilitates …

Answer 181

Gas exchange and uptake of mineral ions with the surrounding soil environment (due to the increased surface area).

Question 182

What are some of the adaptations of red blood cells?

Answer 182

1. Biconcave shape of a red blood cell increases its surface area to volume ratio for oxygen uptake.
2. The cell has no nucleus and is tightly packed with haemoglobin.
3. The shape ensures the diffusion distance to any haemoglobin molecule in the cell is very short.

Question 183

To maximise the rate of exchange, gas exchange (respiratory) surfaces:

Answer 183

1. Have a large surface area.
2. Have a moist surface into which the respiratory gases dissolve.
3. Have diffusion gradients for both oxygen and carbon dioxide.
4. Must also be permeable to oxygen and carbon dioxide.
5. Have a short diffusion path - this is aided by exchange surfaces being very thin.

Question 184

What cell type in plants does not carry out photosynthesis?

Answer 184

Root hair cells

Question 185

For a plant to grow …

Answer 185

The production of carbohydrate in photosynthesis must exceed the loss of carbohydrate as a respiratory substrate.

Question 186

For a plant to grow, the production of carbohydrate in photosynthesis must exceed the loss of carbohydrate as a respiratory substrate. In effect this means that over a period of time (for example, a 24 hour cycle) the net intake of carbon dioxide must (blank) the net production of carbon dioxide.

Answer 186

Exceed

Question 187

Draw a graph showing the relationship between the volume of carbon dioxide taken in/given out (in cubic centimetres per hour) over a 24 hour period in a plant

Answer 187

Textbook page 124

Question 188

What are some of the leaf adaptations for gas exchange?

Answer 188

1. Most leaves are usually thin.
   - This ensures that the overall surface area to volume ratio is high but also that there is a short diffusion distance, with no cell being too far from an exchange surface.
2. The large and moist gas exchange surfaces of the spongy mesophyll cells
   - The cell surface membranes of spongy mesophyll are the gas exchange surface (i.e. form the metabolic contact with the air spaces within the spongy mesophyll)
   - Loose arrangement of cells creates air spaces and ensures that there is a large surface area across which gases can diffuse.
   - Due to their positioning within the leaf, the gases involved in photosynthesis and respiration can easily diffuse between the cells of the spongy mesophyll and the palisade cells.
3. The intercellular air spaces of the spongy mesophyll
   - Facilitates diffusion within the leaf.
4. Stomata (singular stoma) are pores in the leaf surface that allow gases to diffuse in and out of the leaf easily.
   - The opening and closure of a stoma is controlled by two guard cells that can change shape.
   - Depending on the degree of turgor (and shape) of the guard cells, a stomatal aperture (pore) may be present or not.
   - Stomata are open when the guard cells are turgid and closed when they are not.

Question 189

Guard cells, unlike other epidermal cells, contain …

Answer 189

Chloroplasts

Question 190

Stomata are open when the guard cells are …

Answer 190

Turgid

Question 191

When the guard cells are turgid, the stomata are …

Answer 191

Open

Question 192

Stomata are closed when the guard cells are …

Answer 192

Flaccid

Question 194

Most water loss from leaves (transpiration) is during the …

Answer 194

Day, when it is usually warmer

Question 195

Most water loss from leaves (transpiration) is during the day, when it is usually warmer. Why can’t the stomata close to reduce water loss?

Answer 195

The stomata are unable to close then without significantly compromising the rate of photosynthesis (and growth).

Question 196

What is the gas exchange surface in mammals?

Answer 196

The alveolar wall

Question 197

Apart from the thin alveolar and capillary walls, how else is the diffusion distance reduced?

Answer 197

The red blood cells have to squeeze their way through the narrow capillaries, ensuring that they are in contact with the endothelial wall, which further reduces the diffusion pathway.

The capillaries sit tight against the alveolar walls, reducing the diffusion pathway.

Question 198

Mammals have a _________ ______ respiratory system

Answer 198

Pulmonary (lung)

Question 199

Alveoli have a rich blood supply. Describe the structure of the capillary network surrounding each alveolus.

Answer 199

The contact surface area between the alveoli and the blood is maximised by the degree of proliferation of capillaries around each alveolus, to the extent that there is almost a continuous layer of blood surrounding each alveolus

Question 200

The diagram on page 127 of the textbook shows the structure of the respiratory tree (system) of a mammalian (human). What is the main function of the structures present (other than the alveoli)?

Answer 200

The other structures (apart from alveoli) shown in the diagram are concerned with ensuring that the alveoli are well ventilated.

Question 201

When the guard cells are flaccid the stomata are …

Answer 201

Closed

Question 202

The lungs are naturally …

Answer 202

Elastic

Question 203

The natural elasticity of the lungs produces __ ______ _____ during exhalation

Answer 203

An elastic recoil

Question 204

The natural elasticity of the lungs produces an elastic recoil during exhalation which …

Answer 204

Helps force air out of the lungs.

Question 205

Often by the time emphysema is diagnosed, the individual’s lungs are …

Answer 205

Irreversibly damaged

Question 206

Past Paper Question - June 2018 AS2 Q1
Q1 A variety of substances are required by living organisms in order to function.

Identify the substances described below:

• reduces surface tension in the moisture layer in the alveoli.
_________________________________________________________________

• directly converts fibrinogen to fibrin. _________________________________________________________________

• waterproof component of the Casparian strip in the endodermis. _________________________________________________________________

• an oxygen store in red muscle. _________________________________________________________________

• carbohydrate transported by the phloem.
_________________________________________________________________ [5]

Answer 206

Surfactant;

thrombin;
suberin;
myoglobin;
sucrose; [5]

Question 207

Past Paper Question - June 2018 AS2 Q3 a)
Q3 Fick’s Law shows how three factors affect the rate of diffusion:

diffusion rate ∝ surface area × difference in concentration
——————————————————
thickness of membrane

a) Compare and contrast one factor from Fick’s Law as it relates to the gas exchange organs in plants and mammals. In your answer make reference to the structures involved.

Compare _______________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________

Contrast ________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________ [2]

Answer 207

Q3 a) Any one pair:

• both have a large surface area;
• in plants provided by the spongy mesophyll of the leaf, in mammals millions of lobed alveoli

or

• both have large concentration gradients;
• maintained in plants by respiration/photosynthesis in mesophyll layer
and in mammals by close blood supply/ventilation

or

• both have thin membranes (to provide a short diffusion pathway);
• for example one layer of squamous epithelium in alveoli/plants a few
cell layers in leaf [2]

Question 208

Past Paper Question - June 2018 AS2 Q3 b)i)
Q3 b) Below is a partially drawn graph showing the net carbon dioxide intake by a plant over the first 12 hours of a 24-hour period.

i) Complete the graph to show the expected carbon dioxide intake for 12-24 hours. [2]

(Go do this past paper question)

Answer 208

Q3 b)i) Graph drawn in an approximate mirror image of first 12 hours;
reaching compensation point at approximately 18 hours and dropping
to approximately same point below the x-axis; [2]

Question 209

Past Paper Question - June 2018 AS2 Q3 b)ii)
Q3 b) Below is a partially drawn graph showing the net carbon dioxide intake by a plant over the first 12 hours of a 24-hour period.

ii) Describe and explain the carbon dioxide intake shown for 0-12 hours. [4]

(Go do this past paper question)

Answer 209

Q3 b)ii) Any two pairs:

• From 0–4 hours (0–6 hours) carbon dioxide is given out (negative
intake) by the plant;
• the plant is in darkness/low light so only respiration occurring/
respiration exceeds photosynthesis;

or

• At 6 hours CO2 intake is 0/no net intake or output of CO2;
• where rates of photosynthesis and respiration are equal;

or

• From 6–12 hours, CO2 intake increases;
• light intensity increases, so photosynthesis exceeds respiration; [2]

Question 210

Past Paper Question - June 2018 AS2 Q3 b)iii)
Q3 b) Below is a partially drawn graph showing the net carbon dioxide intake by a plant over the first 12 hours of a 24-hour period.

i) Complete the graph to show the expected carbon dioxide intake for 12-24 hours. [2]
iii) Shade on the graph the area which represents a net gain in glucose. Explain your answer. [2]

(Go do this past paper question)

Answer 210

Q3 b)i) Graph drawn in an approximate mirror image of first 12 hours;
reaching compensation point at approximately 18 hours and dropping
to approximately same point below the x-axis; [2]

iii) Shade part of graph above dotted line and under solid line; production of glucose in photosynthesis must exceed the loss of glucose in respiration; [2]

Question 211

Past Paper Question - June 2018 AS2 Q3 b)ii)
Q3 b) Below is a partially drawn graph showing the net carbon dioxide intake by a plant over the first 12 hours of a 24-hour period.

ii) Describe and explain the carbon dioxide intake shown for 0-12 hours. [4]

(Go do this past paper question)

Answer 211

Q3 b)ii) Any two pairs:

• From 0–4 hours (0–6 hours) carbon dioxide is given out (negative
intake) by the plant;
• the plant is in darkness/low light so only respiration occurring/
respiration exceeds photosynthesis;

or

• At 6 hours CO2 intake is 0/no net intake or output of CO2;
• where rates of photosynthesis and respiration are equal;

or

• From 6–12 hours, CO2 intake increases;
• light intensity increases, so photosynthesis exceeds respiration; [2]

Question 212

Past Paper Question - June 2018 AS2 Q3 b)iii)
Q3 b) Below is a partially drawn graph showing the net carbon dioxide intake by a plant over the first 12 hours of a 24-hour period.

i) Complete the graph to show the expected carbon dioxide intake for 12-24 hours. [2]
iii) Shade on the graph the area which represents a net gain in glucose. Explain your answer. [2]

(Go do this past paper question)

Answer 212

Q3 b)i) Graph drawn in an approximate mirror image of first 12 hours;
reaching compensation point at approximately 18 hours and dropping
to approximately same point below the x-axis; [2]

iii) Shade part of graph above dotted line and under solid line; production of glucose in photosynthesis must exceed the loss of glucose in respiration; [2]