Gas Exchange

Question 1

Where does gas exchange occur?

Answer 1

Occurs over a gas exchange surface – a boundary between the outside environment and the internal environment of an organism.

Question 2

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

Answer 2

1.They have a large surface area

2.They are often thin just one layer of epithelial cells this provides a short diffusion pathway across the gas exchange surface.

Question 3

Gas exchange in single-celled organisms

Answer 3

absorb and release gases by diffusion through their cell-surface membranes.

relatively large surface area, thin surface, and a short diffusion pathway – so there is no need for a specialised gas exchange system.

Question 4

Gas exchange in fish

Answer 4

Lower concentration of oxygen in water than in the air. So, fish have special adaptions to get enough of it.

The gas exchange surface is the gills.

Question 5

Structure of gills:

Answer 5

-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.

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

Question 6

Blood flows in the gills:

Answer 6

Blood flows through the lamellae in one direction and water flows over them in the opposite direction. This is called a counter-current system.

Question 7

The reason for counter-current system

Answer 7

The 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 8

Gas exchange in dicotyledonous plants

Answer 8

Main gas exchange surface is the surface of the mesophyll cells in the leaf.

Gases move in and out through special pores in the epidermis (mostly the lower epidermis) called stomata.

Guard cells control the opening and closing the stomata.

No active ventilation is required as the thinness of the plant tissues and the presence of stomata helps to create a short diffusion pathway

Question 9

Structure of a leaf:

Answer 9

Waterproof cuticle

Upper epidermis - layer of tightly packed cells

Palisade mesophyll layer - layer of elongated cells containing chloroplasts.

Spongy mesophyll layer - layer of cells that contains an extensive network of air spaces.

Stomata - pores (usually) on the underside of the leaf which allow air to enter.

Guard cells - pairs of cells that control the opening and closing of the stomata.

Lower epidermis - layer of tightly packed cells

Question 10

Gas exchange in insects

Answer 10

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

Air moves into the tracheae through pores on the surface called spiracles.

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 11

Control of water loss

Answer 11

If insects are losing too much water, 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 12

Control of water loss

Answer 12

If insects are losing too much water, 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 13

What happens to plant stomata during the day

Answer 13

Plants’ stomata are 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 14

Stomata skunk in pits to trap water vapour, reducing the concentration gradient of water between the leaf and the air……..

Answer 14

This reduces evaporation of water from the leaf.

Question 15

A layer of ‘hairs’ on the epidermis…….

Answer 15

to trap water vapour round the stomata.

Question 16

Curled leaves with the stomata inside…..

Answer 16

Protecting them from the wind (windy conditions increase the rate of diffusion and evaporation).

Question 17

A reduced number of stomata………

Answer 17

so there are fewer places for water to escape.

Question 18

Thicker waxy, waterproof cuticles on leaves and stems……

Answer 18

To reduce evaporation.

Question 19

Gas exchange in humans: Passage of Air

Answer 19

• Nose / mouth
• Trachea (windpipe)
• Bronchi
• Bronchioles
• Alveoli

Question 20

Structure of the gas exchange system

Answer 20

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.

Bronchioles end in small ‘air sacs’ called alveoli. This where gases are exchanged. The rib cage, intercoastal muscles and diaphragm all work together to move air in and out.

Question 21

Intercoastal muscles

Answer 21

Found between the ribs.

There are actually two layers: external and internal intercoastal muscles.

Question 22

Ventilation

Answer 22

Consists of inspiration and expiration.

It’s controlled by the movements of the diaphragm, internal and external intercoastal muscles and ribcage.

Question 23

What happens during inspiration?

Answer 23

The external intercoastal and diaphragm muscle contract. This causes the ribcage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thoracic cavity.

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.

Is an active process – it requires energy.

Question 24

What happens during expiration?

Answer 24

The external intercoastal and diaphragm muscles relax.

Ribcage moves downwards and inwards, and the diaphragm curves upwards again.

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.

Normal expiration is a passive process – it doesn’t require energy.

Question 25

What happens during forced expiration?

Answer 25

The external intercoastal muscles relax and internal intercoastal muscles contract, pulling the ribcage further down and in.

The movement of the two sets of intercoastal muscles is said to be antagonistic (opposing).

Question 26

Alveoli

Answer 26

Lungs contain millions of microscopic air sacs where gas exchange occurs – called alveoli.

The alveoli are surrounded by a network of capillaries

Question 27

What is the structure of alveoli?

Answer 27

Wall of each alveolus is made from a single layer of thin, flat cells called alveolar epithelium.

Walls of the capillaries are made from capillary endothelium.

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

Question 28

The alveoli have a lining of thin and squamous epithelium, that allows for gas exchange……

Answer 28

The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases

Question 29

Each alveolus is surrounded by an extensive network of capillaries…….

Answer 29

Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from alveoli and into the capillaries to be carried around the body.

Question 30

The reason for extensive capillary network……

Answer 30

The walls of the capillaries are only one cell thick, and these cells are flattened, keeping the diffusion distance for gases short.

The constant flow of blood through the capillaries means that oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them.

This maintains the concentration gradient necessary for gas exchange to occur.

Question 31

Movement of oxygen and carbon dioxide through the gas exchange system

Answer 31

Air (containing oxygen) moves down the trachea, bronchi, and bronchioles into the alveoli.
This movement happens down a pressure gradient.

O2 then moves into the blood where it can be transported round the body – this movement happens down a diffusion gradient.

CO2 moves down its own diffusion and pressure gradients, but in the opposite direction to oxygen, so that it can be breathed out.

Question 32

What happens during gas exchange in the alveoli?

Answer 32

O2 diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium, and into a compound called haemoglobin in the blood.

CO2 diffuses into the alveoli from the blood.

Question 33

A thin exchange surface -

Answer 33

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

Question 34

A large surface area –

Answer 34

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

Question 35

How is concentration gradient maintained?

Answer 35

Constantly maintained by the flow of blood and ventilation.