Module 3: Section 1 - Exchange and Transport

Question 1

How do you calculate surface area to volume ratio?

Answer 1

Divide the SA by the volume

Question 2

Who has the biggest SA : volume ratio: a mouse or a hippo?

Answer 2

A mouse has a bigger SA relative to its volume than a hippo

Question 3

An organism needs to supply every one of its cells with substances like glucose and oxygen. It also needs to remove waste products from every cell to avoid damaging itself. How does a single-celled organism do this?

Answer 3

1) in single-celled organisms, these substances can diffuse directly into (or out of) the cell across the cell surface membrane. The diffusion rate is quick because of the small distances the substances have to travel

Question 4

In multi-cellular animals, diffusion across the outer membrane is too slow - give three reasons why?

NB: This means that rather than using a straight-forward diffusion to absorb and excrete substances, multicellular animals need specialised exchange surfaces - like alveoli in the lungs

Answer 4

1) some cells are deep within the body - there’s a big distance between them and the outside environment
2) larger animals have a low surface area to volume ratio - it’s difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface
3) multicellular organisms have a higher metabolic rate than single-celled organisms, so they use up oxygen and glucose faster

Question 5

Most exchange surfaces have a large surface area to improve their efficiency. Explain the example of root hair cells please

Answer 5

1) the cells on plant roots grow into long ‘hairs’ which stick out into the soil. Each branch of a root will be covered in millions of these microscopic hairs
2) this gives the roots a large surface area, which helps to increase the rate of absorption of water (by osmosis) and mineral ions (by active transport) from the soil

Question 6

Most exchange surfaces are thin to improve their efficiency. Explain the example of the alveoli please

Answer 6

1) the alveoli are the gas exchange surface in the lungs
2) each alveolus is made from a single layer of thin, flat cells called the alveolar epithelium
3) O2 diffuses out of the alveolar space into the blood. CO2 diffuses in the opposite direction
4) the thin alveolar epithelium helps to decrease the distance over which O2 and CO2 diffusion takes place, which increases the rate of diffusion

Question 7

Most exchange surfaces are have good ventilation to improve their efficiency. Explain the example of the alveoli and the fish gills please

Answer 7

Example 1 - Alveoli
1) the alveoli are surrounded by a large capillary network, giving each alveolus its own blood supply. The blood constantly takes oxygen away from the alveoli, and brings more carbon dioxide

2) the lungs are also ventilated so the air in each alveolus is constantly replaced
3) these features help to maintain the concentration gradients of O2 and CO2

Example 2 - Fish Gills
1) the gills are the gas exchange surface in fish. In the gills, O2 and CO2 are exchanged between the fish’s blood and the surrounding water

2) fish gills contain a large network of capillaries - this keeps them well-supplied with blood. They’re also well-ventilated - fresh water constantly passes over them. These features help to maintain a concentration gradient of O2 - increasing the rate at which O2 diffuses into the blood

Question 8

Explain in 5 steps what happens as you breathe in

Answer 8

1) as you breathe in, air enters the trachea (windpipe)
2) the trachea splits into two bronchi - one bronchus leading to each lung
3) each bronchus then branches off into smaller tubes called bronchioles
4) the bronchioles end in small ‘air sacs’ called alveoli where gases are exchanged
5) the ribcage, intercostal muscles and diaphragm all work together to move in and out

Question 9

What is the purpose of goblet cells?

Answer 9

Goblet cells (lining the airways) secrete mucus. The mucus traps microorganisms and dust particles in the inhaled air, stopping them from reaching the alveoli

Question 10

What is the purpose of cilia?

Answer 10

Cilia (on the surface of cells lining the airways) beat the mucus. This moves the mucus (plus the trapped microorganisms and dust) upward away from the alveoli towards the throat, where it’s swallowed. This helps prevent lung infections

Question 11

What is the purpose of elastic fibres?

Answer 11

Elastic fibres in the walls of the trachea, bronchi, bronchioles and alveoli help the process of breathing out. On breathing in, the lungs inflate and the elastic fibres are stretched. Then, the fibres recoil to help push the air out when relaxing

Question 12

What is the purpose of smooth muscle?

Answer 12

Smooth muscle in the walls of the trachea, bronchi and bronchioles allows their diameter to be controlled. During exercise the smooth muscle relaxes, making the tubes wider. This means there’s less resistance to airflow and air can move in and out of the lungs more easily

Question 13

What is the purpose of rings of cartilage?

Answer 13

Rings of cartilage in the walls of the trachea and bronchi provide support. It’s strong but flexible - it stops the trachea and bronchi collapsing when you breathe in and the pressure drops

Question 14

Talk me through the five steps of inspiration please

Answer 14

1) the external intercostal and diaphragm muscles contract
2) this causes the ribcage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thorax (the space where the lungs are)
3) as the volume of the thorax increases the lung pressure decreases (to below atmospheric pressure)
4) this causes air to flow into the lungs
5) inspiration is an active process - it requires energy

Question 15

Talk me through the six steps of expiration please

Answer 15

1) the external intercostal and diaphragm muscles relax
2) the ribcage moves downwards and inwards and the diaphragm becomes curved again
3) the thorax volume decreases, causing the air pressure to increase (to above atmospheric pressure)
4) air is forced out of the lungs
5) normal expiration is a passive process - it doesn’t require energy
6) expiration can be forced though. During forced expiration, the internal intercostal muscles contract, to pull the ribcage down and in

Question 16

Define tidal volume

Answer 16

Tidal volume - the volume of air in each breath - usually about 0.4 dm cubed

Question 17

Define vital capacity

Answer 17

vital capacity - the maximum volume of air that can be breathed in or out

Question 18

Define breathing rate

Answer 18

breathing rate - how many breaths are taken - usually in a minute

Question 19

Define oxygen consumption

Answer 19

oxygen consumption/uptake - the rate at which an organism uses up oxygen (e.g. the number of dm cubed used per minute)

Question 20

A spirometer is a machine that can give readings of tidal volume, vital capacity, breathing rate and oxygen uptake. How do we use a spirometer and how does it work?

Answer 20

1) a spirometer has an oxygen-filled chamber with a movable lid
2) the person breathes through a tube connected to the oxygen chamber
3) as the person breathes in and out, the lid of the chamber moves up and down
4) these movements can be recorded by a pen attached to the lid of the chamber - this writes on a rotating drum, creating a spirometer trace. Or the spirometer can be hooked up to a motion sensor - this will use the movements to produce electronic signals, which are picked up by a data logger
5) the soda lime in the tube the subject breathes into absorbs carbon dioxide

Question 21

The total volume of gas in the chamber of a spirometer decreases over time - why is this?

Answer 21

The total volume of gas in the chamber decreases over time. This is because the air that’s breathed out is a mixture of oxygen and carbon dioxide. The carbon dioxide is absorbed by the soda lime - so there’s only oxygen in the chamber which the subject inhales from. As this oxygen gets used up by respiration, the total volume decreases

Question 22

There’s a lower concentration of oxygen in water than in air. So fish have special adaptions to get rid of it. Explain how gills are adapted please

Answer 22

1) water, containing oxygen, enters the fish through its mouth and passes out through the gills
2) each gill is made of lots of thin branches called gill filaments or primary lamellae, which give a big surface area for exchange of gases. The gill filaments are covered in lots of tiny structures called gill plates or secondary lamellae, which increase the surface area even more. Each gill is supported by a gill arch
3) the gill plates have lots of blood capillaries and a thin surface layer of cells to speed up diffusion
4) blood flows through the gill plates in one direction and water flows over in the opposite direction. This is called a counter-current system. It maintains a large concentration gradient between the water and the blood. The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood

Question 23

Fish use a counter-current system for gas exchange - please explain how this works

Answer 23

1) blood flows through the gill plates in one direction and water flows over in the opposite direction. This is called a counter-current system. It maintains a large concentration gradient between the water and the blood. The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood

Question 24

How are fish gills ventilated in bony fish?

Answer 24

1) the fish opens its mouth, which lowers the floor of the buccal cavity (the space inside the mouth). The volume of the buccal cavity increases, decreasing the pressure inside the cavity. Water is then sucked in to the cavity
2) when the fish closes its mouth, the floor of the buccal cavity is raised again. The volume inside the cavity decreases, the pressure increases, and water is forced out of the cavity across the gill filaments
3) each gill is covered by a bony flap called the operculum (which protect the gill). The increase in pressure forces the operculum on each side of the head to open, allowing water to leave the gills

Question 25

Give the five steps in which you would dissect a fish gill please

Answer 25

1) firstly, fish dissection is messy so make sure you’re wearing an apron and gloves
2) place your chosen fish (perch or salmon works well) in a dissection tray or on a cutting board
3) push back the operculum and use scissors to carefully remove the gills. Cut each gill arch through the bone at the top and bottom
4) if you look closely, you should be able to see the gill filaments
5) finish off by drawing the gill and labelling it

Question 26

How do insects use tracheae to exchange gases?

Answer 26

1) insects have microscopic air-filled pipes called tracheae which they use for gas exchange
2) air moves into the tracheae through pores on the insect’s surface called spiracles
3) oxygen travels down the concentration gradient towards the cells. Carbon dioxide from the cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere
4) the tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells. The tracheoles also contain fluid, which oxygen dissolves in

Question 27

What do insects use rhythmic abdominal movements for?

Answer 27

Insects use rhythmic abdominal movements to change the volume of their bodies and move air in and out of the spiracles. When larger insects are flying, they use their wing movements to pump their thoraxes too

Question 28

How would you dissect the gaseous exchange system in insects?

NB: big insects like grasshoppers or cockroaches are usually best for dissecting because they’re easier to handle. For dissection, you’ll need to use an insect that’s been humanely killed fairly recently

Answer 28

1) first fix the insect to a dissecting board. You can put dissecting pins through its legs to hold it in place
2) to examine the tracheae, you’ll need to carefully cut and remove a piece of exoskeleton (the insect’s hard outer shell) from along the length of the insect’s abdomen
3) use a syringe to fill the abdomen with saline solution. You should be able to see a network of very thin silvery-grey tubes - these are the tracheae. They look silver because they’re filled with air
4) you can examine the tracheae under a light microscope using a wet mount slide. Again, the tracheae will appear silver or grey. You should also be able to see rings of chitin in the walls of the tracheae - these are there for support (like rings of cartilage in a human trachea)

Question 29

Take a break

Answer 29

go running / have a cup of tea / have a shower

relax!