Characteristics of an Efficient Gas Exchange System

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:

Discuss the four characteristics of an efficient gas exchange system, that is relevant to the gas exchange of any taxonomic group.
Discuss the benefits and challenges that come with terrestrial and aquatic habitats, for gas exchange systems.

Glossary

Key terms and definitions.

aquatic: To live in water.

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

concentration: A measure of mass (of solute) per volume (of solution). How crowded particles are in a specific space or volume.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic 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.

large SA : V: Characteristic that increases the rate of diffusion because there are more sites for gases to enter and exit the respiratory surface.

moist respiratory surface: Characteristic that increases the rate of diffusion because gases must first dissolve before they can diffuse.

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

passive transport: Transport of a substance across a cell membrane by diffusion; energy is not required.

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: The site of gas exchange in an organism. It must be thin, moist and have a large surface area to volume ratio to maximise the rate of gas exchange.

taxonomic group: A collection of organisms classified together based on shared characteristics and evolutionary relationships. E.g. mammals, fish and insects.

terrestrial: To live on land.

thin respiratory surface: Characteristic that increases the rate of diffusion because the diffusion distance for O₂ / CO₂ is short.

What makes a gas exchange system efficient at gas exchange?

For gas exchange systems to be efficient, they must have the 4 characteristics that maximise the rate of diffusion.

The specialised respiratory surface must have a large surface area to volume ratio (SA : V).
The specialised respiratory surface must be moist.
The the specialised respiratory surface must be thin.
And there must be a large concentration gradient across the respiratory surface.

How does a large SA : V maximise the rate of diffusion?

The rate that oxygen (O₂) and carbon dioxide (CO₂) can diffuse across the specialised respiratory surface depends on the amount of surface area available. This is because if the respiratory surface has a larger surface area, there will be more sites for oxygen and carbon dioxide to enter and exit the respiratory surface.

Diffusion is more efficient, when the gas exchange system has a large SA : V. This is because with a higher SA : V, there’s a large enough respiratory surface for diffusion to take place, per volume of air or water taken in by ventilation. In other words, per volume of air or water available, there’s plenty enough surface area for gas exchange to happen.

Without a large SA : V, there would not be enough sites along the specialised respiratory surface for oxygen and carbon dioxide to diffuse across - reducing the rate of diffusion.

How does a moist respiratory surface maximise the rate of diffusion?

For gas exchange to happen, oxygen and carbon dioxide must first dissolve in water before they can diffuse across a semi-permeable membrane, and enter or exit a cell. Because of this, a specialised respiratory surface must be moist with water, and not dry.

Without a moist respiratory surface, oxygen and carbon dioxide would not be able to diffuse across the respiratory surface because there is no water to dissolve into first - stopping gas exchange.

How does a thin respiratory surface maximise the rate of diffusion?

The rate that oxygen and carbon dioxide can diffuse across a specialised respiratory surface depends on how far the molecules have to travel. So, the specialised respiratory surface has to be as thin as possible, to make the distance the molecules have to travel as short as possible. A short diffusion distance, means molecules can diffuse faster.

You must remember that diffusion is a form of passive transport - it is a relatively slow process. Diffusion is made even slower if the distance a molecule has to travel is very far. Keep this in mind when you're reading about the circulatory system's effect on animal size.

Without a thin membrane, it would take too long for oxygen and carbon dioxide to diffuse across the specialised respiratory surface - reducing the rate of diffusion.

How does a large concentration gradient maximise the rate of diffusion?

The rate that oxygen and carbon dioxide can diffuse across a specialised respiratory surface depends on how large/steep the concentration gradient is across the membrane. For a concentration gradient to be large/steep, there must be a very high concentration of molecules on one side of the respiratory surface, and a very low concentration of the same molecule on the other side.

This is because diffusion is solely driven by a concentration gradient. The larger the concentration gradient, the higher the rate of diffusion. But without a concentration gradient (i.e. if both sides had equal concentrations), then there would be nothing driving diffusion across the specialised respiratory surface - stopping gas exchange.

All animals need to maximise the rate of diffusion.

Humans, snapper, and crickets need the same thing - they need an efficient gas exchange system, but they all go about getting this in different ways.

Different taxonomic groups have different adaptations to meet these requirements for an efficient gas exchange system.

Different taxonomic groups have different adaptations for increasing SA : V, keeping the respiratory surface moist and thin, and for maintaining a large concentration gradient across the specialised respiratory surface.

How to consider ecological niche when comparing the gas exchange system adaptations of different taxonomic groups?

Humans, snapper, and crickets have different ecological niches:

Snapper are aquatic, oxygen from water.
Humans and crickets are terrestrial, oxygen from air.

This means that their gas exchange systems have evolved different adaptations to be able to survive and thrive in their ecological niche.

(Ecological niche is a organism's 'way of life', which includes how an organism is adapted to use the resources available in the habitat it lives to survive.)

There are several aspects of an animal's’ ecological niche that you must consider when comparing the gas system adaptations of different animals:

Habitat / source of oxygen (see below)
Problems that come with that source of oxygen (see below)
Size of animal
- Crickets and insects in general are small --> Don't need circulatory system
- Snapper and humans are relatively large --> Need a circulatory system

More on Characteristics of an Efficienct Gas Exchange System

📕 article 📸 image 🎮 interactive 🎦 video