1.10 Exchanging Materials

A free GCSE Biology lesson for AQA (8461) from OBA School: surface area to volume ratio, and how exchange surfaces are adapted. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Calculate the surface area, volume and surface area : volume ratio of simple shapes
• Explain why single-celled organisms can rely on diffusion but large organisms cannot
• Describe the features that make an exchange surface effective
• Apply these features to the lungs, small intestine, fish gills, roots and leaves

Key words

• Surface area (SA): The total area of all the outside faces of an object.
• Volume (V): The amount of space an object takes up.
• SA : V ratio: Surface area divided by volume. Written as a number to 1, e.g. 3 : 1.
• Exchange surface: A specialised surface where substances pass into or out of an organism, such as the alveoli or villi.
• Ventilation: Moving air or water over a gas-exchange surface, e.g. breathing.

Surface area to volume ratio

An organism takes in substances such as oxygen across its surface, but every part of its volume needs them. So what matters is how much surface it has for each unit of volume.

For a cube of side s: surface area = 6 × s² volume = s³

For a cuboid l × w × h: surface area = 2(lw + lh + wh) volume = l × w × h

SA : V ratio = surface area ÷ volume, written as “answer : 1”.

Worked example: a cuboid

A flat leaf-shaped piece of tissue is 4 cm × 2 cm × 1 cm.
SA = 2(4×2 + 4×1 + 2×1) = 2(8 + 4 + 2) = 28 cm²
V = 4 × 2 × 1 = 8 cm³
SA : V = 28 ÷ 8 = 3.5

Answer: 3.5 : 1, higher than a 2 cm cube (3 : 1) with the same volume, because it is flatter

Why big organisms need exchange surfaces

A single-celled organism, such as an amoeba, has a large SA : V ratio and a very short distance from its surface to its centre. Diffusion across its cell membrane supplies all the oxygen and food it needs and removes its waste.

In a large multicellular organism the SA : V ratio is small and many cells are far from the surface. Diffusion over these distances would be far too slow. So large organisms have:

• specialised exchange surfaces (lungs, gills, villi, roots, leaves) with a large area, and
• transport systems (blood and circulatory system in animals; xylem and phloem in plants) to carry substances to and from every cell.

Exam tip: A bigger animal has a bigger surface area, but a smaller surface area compared with its volume. Always talk about the ratio.

Features of an effective exchange surface

• Large surface area; How it increases the rate of exchange more particles can cross at the same time
• Thin membrane / short diffusion path; How it increases the rate of exchange particles have less distance to travel
• Efficient blood supply (animals); How it increases the rate of exchange carries substances away (or brings them), keeping the concentration gradient steep
• Ventilated (gas exchange in animals); How it increases the rate of exchange breathing brings fresh air or water, keeping the gradient steep

Exchange surfaces in animals and plants

• Lungs; Exchange surface alveoli; Key adaptations millions of tiny air sacs give a huge area; walls one cell thick; rich capillary network; breathing ventilates them
• Small intestine; Exchange surface villi (and microvilli); Key adaptations folds give a huge area; thin lining; good blood supply carries digested food away
• Fish gills; Exchange surface gill filaments with thin plates (lamellae); Key adaptations large area; thin; good blood supply; water flows over constantly
• Roots; Exchange surface root hair cells; Key adaptations long projections give a large area for water and ion uptake
• Leaves; Exchange surface flat leaves, air spaces, stomata; Key adaptations flat shape and internal air spaces give a large area; stomata let gases in and out

Common mistakes

• Saying large organisms have a “smaller surface area”. They have a larger area but a smaller surface area to volume ratio.
• Using 4 faces instead of 6 when finding the surface area of a cube.
• Writing the ratio upside down (V : SA). It is always SA : V.
• Giving “large surface area” for blood supply. A good blood supply keeps the concentration gradient steep.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the surface area of a cube with sides of 3 cm?

Answer: 54

Working: 6 × 3 × 3 = 54 cm².

Question 2. As an organism gets larger, what happens to its surface area to volume ratio? (Choose from: It increases, It decreases, It stays the same, It doubles.)

Answer: It decreases

Working: Volume increases faster than surface area, so the SA : V ratio decreases.

Question 3. True or false? A single-celled organism can get all the oxygen it needs by diffusion across its surface.

Answer: True

Working: True. It has a large SA : V ratio and a short diffusion distance.

Related lessons

• Previous lesson: 1.9 Active Transport
• Next lesson: 1.11 Required Practicals: Cell Structure and Transport
• In this chapter: 1.1 The World of the Microscope
• In this chapter: 1.2 Animal and Plant Cells
• In this chapter: 1.3 Eukaryotic and Prokaryotic Cells

Continue in the full course

This lesson is section 1.10 (AGCBH0110) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.