Overview of Gas Exchange

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.3 Plant & Animal Adaptations

Learning Intentions

By the end of this topic, you should be able to:

Explain why animals need oxygen and where they obtain this oxygen from their habitat.
Explain how animals obtain oxygen from their habitat through ventilation then gas exchange.
Explain how gas exchange takes place through diffusion of gases across a specialised respiratory surface.

Glossary

Key terms and definitions.

aerobic cellular respiration: The process by which cells break down glucose using oxygen to produce energy, and water and carbon dioxide as by-products.

aquatic: To live in water.

breathing / ventilation: Movement of air/water in and out of the gas exchange system.

carbon dioxide (CO₂): Molecule produced during aerobic cellular respiration; makes up 0.04% of air.

concentration gradient: A difference in the concentration of a substance between two areas. It is the sole driver of diffusion.

cricket: Example of an insect with a tracheal system for gas exchange.

diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.

ecological niche: the functional position of an organism in its environment (the environment being the habitat and the resources available in this habitat).

gas exchange: The process of obtaining oxygen from the environment and releasing carbon dioxide.

human: Example of a mammal with a lung system for gas exchange.

oxygen (O₂): Molecule needed for aerobic cellular respiration; makes up 21% of air and 1% of water.

semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

snapper: Example of a fish with a gill system for gas exchange.

specialised respiratory surface: A thin, moist structure in an organism where gas exchange occurs. It has a large surface area to volume ratio to maximise the rate of gas exchange.

terrestrial: To live on land.

Why do animals need oxygen?

All animals need a large amount of energy (also known as ATP) to be able to carry out the activities they do to look for food, migrate, escape predators, reproduce, and so on.

For example, muscle cells need a large amount of energy to be able to contract and relax, which gives animals the ability to escape predators. The large amount of energy needed is produced inside animal cells through a process called aerobic cellular respiration. Without aerobic respiration, animals would not be able to survive in their ecological niche.

It just so happens that one of the molecules absolutely needed for aerobic respiration is oxygen (O₂). Without oxygen, aerobic respiration cannot happen, which means the animal would not get the energy it needs to survive in its ecological niche. So, animals absolutely need oxygen for aerobic respiration to make energy.

Where do animals get their oxygen from?

An animal's source of oxygen depends on their ecological niche. (Remember that ecological niche is the "functional position of an organism in its environment" - the environment being the habitat, and the resources available in this habitat). So the source of oxygen depends on where the animal lives (land or sea) and what oxygen resources are available in this habitat (oxygen in air or dissolved oxygen in water).

Humans (like all mammals) and crickets (like all insects) are terrestrial animals, meaning they live on land and get their oxygen from the oxygen available in air. Whereas snapper (like all fish) are aquatic animals, meaning they live in bodies of water, and get their oxygen from the dissolved oxygen available in water. Dissolved oxygen is oxygen that has dissolved from the atmosphere, into the ocean.

An overview of gas exchange, and the different adaptations different animals have.

How do animals get oxygen from air or water?

Gas Exchange

All animals need to absorb oxygen (O₂) from air or water, and release carbon dioxide (CO₂) back into air or water through a process called gas exchange. Gas exchange is the process where different gases (oxygen and carbon dioxide) get exchanged/transferred/swapped in opposite directions.

Gas exchange is the process that supplies the organism with oxygen, as oxygen goes from the outside of the body/cell to the inside of the body/cell.
Gas exchange is also the process that removes carbon dioxide from the organism, as carbon dioxide goes from the inside of the body/cell to the outside of the body/cell.

Diffusion

Oxygen and carbon dioxide get exchanged in opposite directions, through the process of diffusion.

Diffusion is the passive movement of molecules from an area of high concentration to low concentration.

For diffusion to occur, there must be a specialised respiratory surface that is semi-permeable. There must also be a concentration gradient across this specialised respiratory surface to drive the process of diffusion. In other words, the concentration of oxygen must be higher on one side of the membrane, and lower on the other side of the membrane.

Oxygen moves across the semi-permeable membrane from an area of high concentration outside of the body, to an area of low concentration inside of the body.
Carbon dioxide moves across the semi-permeable membrane from an area of high concentration inside of the body, to an area of low concentration outside of the body.

Links breathing, gas exchange, diffusion, and aerobic cellular respiration.

How do animals take in air/water into their specialised respiratory surface?

Ventilation

Before gas exchange and diffusion can happen, air/water first needs to get from the outside of the body, to the specialised respiratory surface which is located inside of the body. So animals must first take in the air/water from their surroundings, through a process called ventilation.

Ventilation is the flow of air or water in and out of the gas exchange system, to increase the rate of gas exchange.

I like to use the analogy of getting a bad smell out of a room. When you want to get a bad smell out of the room, you open windows and get a fan going, to get some air flowing through the room. The open window lets new air in and allows old smelly air to escape, while the fan circulates the air faster.

So animals like humans (mammal), crickets (insect), and snapper (fish) ventilate to increase the flow of new air/water in and old air/water out, to increase the rate of gas exchange.