B1 - Surface Area : Volume

What is surface area to volume ratio?

The ratio of the total surface area of an organism compared to its volume.

How does surface area to volume ratio change as an organism gets larger?

The surface area to volume ratio decreases.

What is the surface area to volume ratio of a cube with 1mm sides?

6:1

What is the surface area to volume ratio of a cube with 2mm sides?

3:1

What is the surface area to volume ratio of a cube with 3mm sides?

2:1

Why is surface area to volume ratio important for diffusion?

It affects how quickly substances like oxygen can diffuse into cells.

Why can single

celled organisms rely on diffusion alone? - Because they have a high surface area to volume ratio.

Give an example of a single

celled organism with a high surface area to volume ratio. - Amoeba

Why can't multicellular organisms rely on diffusion alone?

Their surface area is too small for their large volume, so diffusion is too slow to reach all cells.

What problems do large multicellular organisms face with diffusion?

Inner cells are too far from the surface to receive enough substances by diffusion.

How do multicellular organisms overcome the limitations of diffusion?

They have specialised exchange surfaces and transport systems.

What is the purpose of an exchange surface?

To increase the rate of diffusion of substances in and out of the body.

Give two examples of exchange surfaces in animals.

Lungs and gills

What is the function of the lungs in mammals?

To exchange oxygen and carbon dioxide between the air and the blood.

What is the function of gills in fish?

To exchange oxygen and carbon dioxide between water and the blood.

Where do fish get oxygen from?

From oxygen dissolved in water.

How does water flow through a fish for gas exchange?

Water enters the mouth, flows over the gills, and exits through the gill openings.

What structures in fish are responsible for gas exchange?

Gill filaments

What happens at the gill filaments in fish?

Oxygen diffuses from the water into the blood, and carbon dioxide diffuses out.

How does oxygen enter a fish's bloodstream?

By diffusion from the water across the gill filaments.

What are three adaptations of gill filaments for gas exchange?

Large surface area, thin membrane, and good blood supply.

How does a large surface area help gas exchange in fish gills?

It increases the amount of oxygen that can diffuse in.

How does a thin membrane help diffusion in gills?

It provides a short diffusion pathway.

Why is an efficient blood supply important for gas exchange in gills?

It maintains a steep concentration gradient by removing oxygenated blood quickly.

Why is maintaining a steep concentration gradient important?

It ensures that diffusion continues efficiently.

Do fish gills use active transport or diffusion for gas exchange?

Diffusion

Why do fish need a specialised gas exchange surface like gills?

Because they are multicellular and cannot rely on diffusion through their body surface.

How does blood flow in the gill filaments support efficient gas exchange?

It moves deoxygenated blood in, and takes oxygenated blood away to maintain the gradient.

Why is surface area to volume ratio lower in large animals?

Because their volume increases faster than their surface area as they grow.

What happens if cells in the centre of a large organism can't get enough oxygen?

The organism cannot survive without specialised exchange and transport systems.

What is the key idea about SA:V ratio in relation to cell function?

High SA:V allows efficient exchange; low SA:V requires adaptations.