How does SA:V change as organisms get larger?

It decreases, because volume increases faster than surface area.

Why can small organisms exchange gases by diffusion across their surface alone?

They have a large SA:V and a short diffusion distance, so the surface supplies all cells.

Why do large organisms need a mass transport system?

Their SA:V is small and diffusion distances are long, so diffusion alone is too slow to supply all the cells.

What does a mass transport system do?

Moves substances in bulk between the exchange surfaces and the cells.

Why do large organisms need specialised gas exchange surfaces?

The SA:V of the whole body is too small, so a surface with a large area is needed to supply enough oxygen.

Give three features of an efficient exchange surface.

Large surface area, thin (short diffusion distance), steep concentration gradient (ventilation and blood supply).

Why do flatworms not need a circulatory system?

They are thin and flat, so they have a large SA:V and every cell is close to the surface.

What three factors affect the rate of diffusion?

Surface area, concentration gradient and diffusion distance.

Surface area to volume ratioEdexcel A-Level Biology B: Flashcards

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What is surface area to volume ratio?

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What is surface area to volume ratio?: The surface area of an organism divided by its volume.
How does SA:V change as organisms get larger?: It decreases, because volume increases faster than surface area.
What is SA:V for a cube of side s?: 6s² / s³ = 6/s.
What is SA:V for a sphere of radius r?: 4πr² / (4/3 πr³) = 3/r.
Why can small organisms exchange gases by diffusion across their surface alone?: They have a large SA:V and a short diffusion distance, so the surface supplies all cells.
Why do large organisms need a mass transport system?: Their SA:V is small and diffusion distances are long, so diffusion alone is too slow to supply all the cells.
What does a mass transport system do?: Moves substances in bulk between the exchange surfaces and the cells.
Why do large organisms need specialised gas exchange surfaces?: The SA:V of the whole body is too small, so a surface with a large area is needed to supply enough oxygen.
Give three features of an efficient exchange surface.: Large surface area, thin (short diffusion distance), steep concentration gradient (ventilation and blood supply).
Why do flatworms not need a circulatory system?: They are thin and flat, so they have a large SA:V and every cell is close to the surface.
What three factors affect the rate of diffusion?: Surface area, concentration gradient and diffusion distance.
What is the SA:V of a 2 mm cube?: 24 mm² / 8 mm³ = 3 mm⁻¹.

Exam questions on Surface area to volume ratio

A student made cubes of agar jelly containing a pH indicator, with sides of 2 mm, 4 mm and 8 mm, to model cells of different sizes. She placed them in dilute acid and timed how long the acid took to change the colour of the whole cube. (Surface area of a cube = 6 x side squared; volume = side cubed.)
The 8 mm cube took much longer to change colour completely than the 2 mm cube. Explain this result.2 marks
The planarian is a flatworm about 15 mm long, 3 mm wide and less than 0.5 mm thick. It has no lungs, gills or blood. Its cells use oxygen from the water around it, which diffuses in across its body surface. A human, by contrast, is about 1.7 m tall and has lungs and a circulatory system.
Explain how the shape of the planarian allows it to obtain enough oxygen without a circulatory system.2 marks
A spherical bacterium has a diameter of 2 µm, and a spherical human cell has a diameter of 20 µm. A human adult is a large multicellular organism made of many cells of this size. (Surface area of a sphere = 4πr²; volume of a sphere = 4/3 πr³.)
Calculate the surface area to volume ratio of each cell and state how many times greater it is for the bacterium than for the human cell.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).