Topic 3 — A: Exchange and Transport Systems

Question 1

why do plants and animals have adaptations

Answer 1

for gas exchange as they are large organisms

Question 2

how does gas exchange occur?

Answer 2

gas exchange surface — a boundary between the outside environment and the internal environment of an organism.

Question 3

how do organisms use oxygen and carbon dioxide?

Answer 3

Organisms need oxygen and carbon dioxide to diffuse across gas exchange surfaces as quickly as possible.

Question 4

Most gas exchange surfaces have two things in common that increase the rate of diffusion:

Answer 4

1. They have a large surface area.
2. They’re thin (often just one layer of epithelial cells) — this provides a short diffusion pathway across the gas exchange surface.

Question 5

organisms concentration gradient:

Answer 5

maintains a steep concentration gradient of gases across the exchange surface, which increases the rate of diffusion.

Question 6

Gas exchange in single-celled organisms

Answer 6

Single-celled organisms absorb and release gases by diffusion through their cell-surface membranes. They have a relatively large surface area, a thin surface and a short diffusion pathway (oxygen can take part in biochemical reactions as soon as it diffuses into the cell) — so there’s no need for a specialised gas exchange system.

Question 7

Gas exchange in fish

Answer 7

There’s a lower concentration of oxygen in water than in air. So fish have special adaptations to get enough of it. In a fish, the gas exchange surface is the gills.

Question 8

Structure of gills

Answer 8

Water, containing oxygen, enters the fish through its mouth and passes out through the gills. Each gill is made of lots of thin plates called gill filaments, which give a large surface area for exchange of gases (and so increase the rate of diffusion). The gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more

lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and the blood.

Question 9

what is counter-current system?

Answer 9

In the gills of a fish, blood flows through the lamellae in one direction and water flows over them in the opposite direction

Question 10

water in the counter current system:

Answer 10

water with a relatively high oxygen concentration always flows next to blood with a lower concentration of oxygen.

means that a steep concentration gradient is maintained between the water and the blood — so as much oxygen as possible diffuses from the water into the blood.

Question 11

what do plants need for photosynthesis?

Answer 11

CO2, O2

which produces O2 as a waste gas. They need O2 for respiration, which produces CO2 as a waste gas.

Question 12

main gas exchange surface in plants:

Answer 12

the surface of the mesophyll cells in the leaf.

Question 13

how are mesophyll cells adapted?

Answer 13

large surface area.

Question 14

where are mesophyll cells?

Answer 14

inside the leaf.

Question 15

gases in the leaf:

Answer 15

move in and out through special pores in the epidermis (mostly the lower epidermis) called stomata (singular = stoma). The stomata can open to allow exchange of gases, and close if the plant is losing too much water.

Question 16

what do guard cells do?

Answer 16

control the opening and closing of stomata.

Question 17

parts of the leaf:

Answer 17

• waxy cuticle
• upper epidermis cell
• palisade mesophyll cell
• xylem and phloem
• air space
• spongy mesophyll cells
• waxy cuticle
• lower epidermis
• guard cell
• stoma
• lower epidermis

Question 18

what are air pipes in insects called?

Answer 18

Terrestrial insects have microscopic air-filled pipes called tracheae which they use for gas exchange.

Question 19

process of air movement in insects:

Answer 19

Oxygen travels down the concentration gradient towards the cells. The tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells. This means that oxygen diffuses directly into the respiring cells — the insect’s circulatory system doesn’t transport O2. Carbon dioxide from the cells moves down
its own concentration gradient towards the spiracles to be released into the atmosphere. Insects use rhythmic abdominal movements to move air in and out of the spiracles.

Question 20

what happens if insects lose too much water?

Answer 20

they close their spiracles using muscles. They also have a waterproof, waxy cuticle all over their body and tiny hairs around their spiracles, both of which reduce evaporation.

Question 21

stomata in plants:

Answer 21

usually kept open during the day to allow gaseous exchange. Water enters the guard cells, making them turgid, which opens the stomatal pore. If the plant starts to get dehydrated, the guard cells lose water and become flaccid, which closes the pore.

Question 22

what are xerophytes?

Answer 22

Some plants are specially adapted for life in warm, dry or windy habitats, where water loss is a problem.

Question 23

adaptations of xerophytic cells:

Answer 23

• Stomata sunk in pits
• layer of hairs
• Curled leaves with the stomata inside
• reduced number of stomata
• Thicker waxy, waterproof cuticles

Question 24

what do stomata sunk in pits do?

Answer 24

trap water vapour, reducing the concentration gradient of water between the leaf and the air. This reduces evaporation of water from the leaf.

Question 25

what do layer of ‘hairs’ on the epidermis do?

Answer 25

trap water vapour round the stomata.

Question 26

what do curled leaves with the stomata inside do?

Answer 26

protect plants from wind (windy conditions increase the rate of diffusion and evaporation).

Question 27

what does a reduced number of stomata do?

Answer 27

there are fewer places for water to escape.

Question 28

what does a thicker waxy, waterproof cuticle do?

Answer 28

reduce evaporation

Question 29

why do humans breathe?

Answer 29

Humans need to get oxygen into the blood (for respiration) and they need to get rid of carbon dioxide (made by respiring cells).

Question 30

parts of human gas exchange system:

Answer 30

• trachea
• ribcage
• intercostal muscles
• bronchus
• bronchiole
• lung
• diaphragm
• alveoli

Question 31

process after breathing in:

Answer 31

As you breathe in, air enters the trachea (windpipe). The trachea splits into two bronchi — one bronchus leading to each lung. Each bronchus then branches off into smaller tubes called bronchioles. The bronchioles end in small ‘air sacs’ called alveoli. This is where gases are exchanged (see next page). The ribcage, intercostal muscles and diaphragm all work together to move air in and out.

Question 32

where are intercostal muscles found?

Answer 32

found between the ribs

Question 33

how many layers of intercostal muscles are there?

Answer 33

three

Question 34

layers of intercostal muscles:

Answer 34

internal and external intercostal muscles.

Question 35

where are the internal intercostal muscles?

Answer 35

on the inside of the external intercostal muscles.

Question 36

what does ventilation consist of?

Answer 36

inspiration (breathing in) and expiration (breathing out)

Question 37

what is ventilation controlled by?

Answer 37

controlled by the movements of the diaphragm, internal and external intercostal muscles and ribcage.

Question 38

what contracts during inspiration?

Answer 38

external intercostal and diaphragm muscles contract.

Question 39

what happens after the external intercostal and diaphragm muscles contract?

Answer 39

causes the ribcage to move upwards and outwards and the diaphragm
to flatten, increasing the volume of the thoracic cavity (the space where the lungs are). As the volume of the thoracic cavity increases, the lung pressure decreases to below atmospheric pressure. Air will always flow from an area of higher pressure to an area of lower pressure (i.e. down a pressure gradient) so air flows down the trachea and into the lungs.

Question 40

what type of process is inspiration?

Answer 40

active process — it requires energy.

Question 41

thorax when air goes in:

Answer 41

thorax volume increases, air pressure decreases
- external intercostal muscles contract causing ribs to move upward and outwards
- diaphragm contracts causing it to move downwards and flatten

Question 42

expiration process:

Answer 42

external intercostal and diaphragm muscles relax. The ribcage moves downwards and inwards, and the diaphragm curves upwards again (so it becomes dome-shaped). The volume of the thoracic cavity decreases, causing the air pressure to increase to above atmospheric pressure. Air is forced down the pressure gradient and out of the lungs.

Question 43

what type of process is normal expiration?

Answer 43

passive process

Question 44

forced expiration:

Answer 44

the external intercostal muscles relax and internal intercostal muscles contract, pulling the ribcage further down and in. During this time, the movement of the two sets of intercostal muscles is said to be antagonistic (opposing)

Question 45

Alveoli and gas exchange:

Answer 45

Lungs contain millions of microscopic air sacs where gas exchange occurs — called alveoli. The alveoli
are surrounded by a network of capillaries

Question 46

Alveoli structure:

Answer 46

The wall of each alveolus is made from a single layer of thin, flat cells called alveolar epithelium. The walls of the capillaries are made from capillary endothelium

Question 47

what do walls of the alveoli contain?

Answer 47

protein called elastin. Elastin is elastic — it helps the alveoli to return (recoil) to their normal shape after inhaling and exhaling air.

Question 48

Movement of oxygen and carbon dioxide through the gas exchange system:

Answer 48

Air (containing oxygen) moves down the trachea, bronchi and bronchioles into the alveoli. This movement happens down a pressure gradient. Oxygen then moves into the blood where it can be transported round the body — this movement happens down a diffusion gradient.
Carbon dioxide moves down its own diffusion and pressure gradients, but in the opposite direction to oxygen, so that it can be breathed out.

Question 49

Gas exchange in the alveoli:

Answer 49

Oxygen (O2) diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium, and into a compound called haemoglobin in the blood. Carbon dioxide (CO2) diffuses into the alveoli from the blood

Question 50

how oxygen moves through the gas exchange system from when it is first inhaled to reaching the blood:

Answer 50

trachea → bronchi → bronchioles → alveoli → alveolar epithelium → capillary endothelium → blood

Question 51

Factors affecting the rate of diffusion:

Answer 51

A thin exchange surface

A large surface area

steep concentration gradient

Question 52

how does thin exchange surface affect diffusion rate?

Answer 52

the alveolar epithelium is only one cell thick. This means there’s a short diffusion pathway (which speeds up diffusion).

Question 53

how does a large surface area affect diffusion rate in alveoli?

Answer 53

there are millions of alveoli.
This means there’s a large surface area for gas exchange.

Question 54

how does a steep concentration gradient of oxygen and carbon dioxide affect diffusion rate in alveoli?

Answer 54

between the alveoli and the capillaries, which increases the rate of diffusion. This is constantly maintained by the flow of blood and ventilation

Question 55

what does lung disease affect?

Answer 55

ventilation (breathing) and gas exchange in the lungs

Question 56

Tidal volume definition:

Answer 56

the volume of air in each breath
— it’s usually between 0.4 dm3 and 0.5 dm3 for adults.

Question 57

Ventilation rate definition:

Answer 57

the number of breaths per minute. For a healthy person at rest it’s about 15 breaths.

Question 58

Forced expiratory volume1 definition:

Answer 58

(FEV1) is the maximum volume of air that can be breathed out in 1 second.

Question 59

Forced vital capacity (FVC) definition:

Answer 59

the maximum volume of air it is possible to breathe forcefully out of the lungs after a really deep breath in.

Question 60

example of lung disease affecting lung function:

Answer 60

Pulmonary tuberculosis (TB) is a lung disease caused by bacteria. When someone becomes infected with tuberculosis bacteria, immune system cells build a wall around the bacteria in the lungs. This forms small, hard lumps known as tubercles (see Figure 1). Infected tissue within the tubercles dies and the gaseous exchange surface is damaged, so tidal volume is decreased. TB also causes fibrosis (see below), which further reduces the tidal volume.
A reduced tidal volume means less air can be inhaled with each breath. In order to take in enough oxygen, patients have to breathe faster, i.e. ventilation rate is increased. Common symptoms include a persistent cough, coughing up blood and mucus, chest pains, shortness of breath and fatigue.

Question 61

fibrosis affecting lungs:

Answer 61

Fibrosis is the formation of scar tissue in the lungs (see Figure 2). This can be the result of an infection or exposure to substances like asbestos or dust. Scar tissue is thicker and less elastic than normal lung tissue. This means that the lungs are less able to expand and so can’t hold as much air as normal — tidal volume is reduced, and so is FVC (i.e. a smaller volume of air can be forcefully breathed out). There’s a
reduction in the rate of gaseous exchange — diffusion is slower across a
thicker scarred membrane. Fibrosis sufferers have a
faster ventilation rate than normal — to get enough air into their lungs to oxygenate their blood. Symptoms of fibrosis include shortness of breath, a dry cough, chest pain, fatigue and weakness.

Question 62

asthma affecting lungs:

Answer 62

Asthma is a respiratory condition where the airways become inflamed and irritated. The causes vary from case to case but it’s usually because of an allergic reaction to substances such as pollen and dust.
During an asthma attack, the smooth muscle lining the bronchioles contracts and a large amount of mucus is produced (see Figure 3). This causes constriction of the airways, making it difficult for the sufferer to breathe properly. Air flow in and out of the lungs is severely reduced, so less oxygen enters the alveoli and moves into the blood. Reduced air flow means that FEV1 is severely reduced (i.e. less air
can be breathed out in 1 second).
Symptoms include wheezing, a tight chest and shortness of breath. During an attack the symptoms come on very suddenly. They can be relieved by drugs (often in inhalers) which cause the muscle in the bronchioles to relax, opening up the airways.

Question 63

healthy bronchiole description:

Answer 63

• smooth muscle
• epithelial tissue
• airway

Question 64

unhealthy bronchiole description:

Answer 64

• smooth muscle contacts
• airway constricts
• mucus is produced

Question 65

emphysema affecting lungs:

Answer 65

Emphysema is a lung disease caused by smoking or long-term exposure to air pollution — foreign particles in the smoke (or air) become trapped in the alveoli. This causes inflammation, which attracts phagocytes to the area. The phagocytes produce an enzyme that breaks down elastin (a protein found in the walls of the alveoli).
Figure 4: A lung section showing a constricted bronchiole (circled).
Elastin is elastic — it helps the alveoli to return to their normal shape after inhaling and exhaling air. Loss of elastin means the alveoli can’t recoil to expel air as well (it remains trapped in
the alveoli). It also leads to destruction
of the alveoli walls, which reduces the surface area of the alveoli (see Figure 5), so the rate of gaseous exchange decreases.
Symptoms of emphysema include
shortness of breath and wheezing. People
with emphysema have an increased
ventilation rate as they try to increase the
amount of air (containing oxygen) reaching their lungs

Question 66

The effect of lung diseases on gas exchange:

Answer 66

TB, fibrosis, asthma and emphysema all reduce the rate of gas exchange in the alveoli. Less oxygen is able to diffuse into the bloodstream, the body cells receive less oxygen and the rate of aerobic respiration is reduced. This means less energy is released and sufferers often feel tired and weak.

Question 67

graph for lung disease example:

Answer 67

FEV1 and FVC are much lower than normal for someone with an obstructive lung disease because obstructive diseases make it difficult to breathe out. For example, after an asthma attack, the bronchioles are constricted and full of mucus — this narrows the airways, reducing air flow out of the lungs and leading to a large drop in FEV1. Because it’s harder to breathe out, it may also take longer for someone with an obstructive disease to forcibly breathe out all the air in their lungs (i.e. reach their FVC).

Question 68

percentage change formula:

Answer 68

percentage change = final value - original value / original value x 100

Question 69

what is a spirometer?

Answer 69

tests to investigate lung function and diagnose lung diseases.
- machine used to measure the volume of air breathed in and out.

Question 70

what are risk factors?

Answer 70

factors that will increase a person’s chance of getting that disease.

Question 71

example of risk factor:

Answer 71

(smoking is a risk factor for lung cancer).

Question 72

what is a correlation?

Answer 72

a link between two things
- doesn’t always mean that one thing causes the other.

Question 73

describing data example:

Answer 73

Figure 1 shows that the number of adult males in Great Britain who smoke decreased between 1990 and 2012. Figure 2 shows that the male lung cancer mortality (death) rate decreased between 1990 and 2012 in the United Kingdom.

Question 74

drawing conclusions example

Answer 74

You need to be careful what you say here. There’s a correlation (link) between the number of males who smoked and the mortality rate for male lung cancer. But you can’t say that one caused the other. There could
be other reasons for the trend, e.g. deaths due to lung cancer may have decreased because less asbestos was being used in homes (not because fewer people were smoking).

Question 75

Other points to consider with interpreting correlations example:

Answer 75

Figure 2 shows mortality (death) rates. The rate of cases of lung cancer may have been increasing but medical advances may mean more people were surviving (so only mortality was decreasing, not the number of people developing the disease).

Question 76

describing graphs data example:

Answer 76

Figure 3 shows that the number of new cases of asthma in the UK fell between 1996 and 2000, from 87 to 62 per 100 000 people.
Figure 4 shows that the emissions of sulfur dioxide in the UK fell between 1996 and 2000, from 2 to 1.2 million tonnes.

Question 77

drawing conclusions graph example:

Answer 77

Be careful what you say when drawing conclusions. Here there’s a link between the number of new cases of asthma and emissions of sulfur dioxide in the UK — the rate of new cases of asthma has fallen as sulfur dioxide emissions have fallen. You can’t say that one causes the other though because there could be other reasons for the trend, e.g. the number of
new cases of asthma could be falling due to the decrease in the number of people smoking. You can’t say the reduction in asthma cases is linked to
a reduction in air pollution (in general) either as only sulfur dioxide levels were studied

Question 78

Other points to consider in graph questions

Answer 78

Figure 3 shows new cases of asthma. The rate of new cases may be decreasing but existing cases may be becoming more severe. The emissions were for the whole of the UK but air pollution varies from area to area,
e.g. cities tend to be more polluted. The asthma data doesn’t take into account any other factors that may increase the risk of developing asthma, e.g. allergies, smoking, etc.

Question 79

evaluation example of data:

Answer 79

Advertising of tobacco products
Medical studies in the 1950s and 1960s documented the link between smoking and various forms of cancer, particularly lung cancer. The evidence prompted the first voluntary agreement between the UK government and tobacco companies in 1971, which stated that tobacco products and adverts should carry a health warning label.
However, despite further evidence for smoking-related health risks, it was not until 2003 that bans on advertising of tobacco-based products began to replace the voluntary agreements. As of October 2008, picture health warnings were made compulsory on all UK boxes of cigarettes after studies suggested they were more effective than written warnings alone.
Passive smoking
During the 1980s and 1990s a number of reports were published linking lung-cancer (and other diseases) in non-smokers to smoke that they had been passively exposed to. In 1997, the government initiated a voluntary agreement for workplaces, pubs and restaurants to increase provision of smoke-free areas for non-smokers. However, this had a limited impact.
During the early 2000s, evidence for the effects of passive smoking continued to grow and public support for smoke-free areas increased. In 2002, the British Medical Association called for a ban on smoking in public places. Finally, in 2007, workplaces and public areas such as pubs and restaurants were made smoke-free by law. Increasing concern about the impact of passive smoking on children has recently led to a ban on smoking in cars carrying under-18s, which will come into force in October 2015.

Question 80

example of restrictions on sources of air pollution:

Answer 80

In response to studies connecting air pollution to various diseases and as part of the Clean Air Programme for Europe, the EU adopted the National Emission Ceilings Directive in 2001. This set upper limits on the total emissions of four major pollutants in the atmosphere, to be achieved by 2010. The four pollutants covered were sulfur dioxides, nitrogen oxides, non-methane volatile organic compounds and ammonia. However, twelve member states failed to meet their emissions targets for at least one pollutant in 2010.
Following a review of progress in 2011 and further scientific evidence, e.g. on the effects of particulate matter on lung function, the directive has been revised. New limits are being agreed on for 2020 with tougher enforcement. Particulate matter has now been included in the emissions targets, along with methane.
Clean Power for Transport
The EU also introduced the Clean Power for Transport package to promote cleaner fuels for vehicles and all new cars are required to comply with Euro Standards on emissions, which are tested during the car’s annual MOT.
The UK government also taxes car owners according to their car’s emissions with discounts available for cars that have less-polluting fuels.