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Unlock This CourseEvery cell needs to take in useful substances, such as oxygen, and get rid of waste. These substances pass across the cell surface. The bigger the surface compared with the amount of living material inside, the easier it is to get enough in and out.
To compare this, we use the surface area to volume ratio. It tells you how much surface there is for every unit of volume.
Key terms:
In exams you will use cubes to stand in for cells or organisms. A cube has six identical square faces, so:
A cube has sides of 2 cm.
Surface area = 6 × (2 × 2) = 24 cm2
Volume = 2 × 2 × 2 = 8 cm3
Ratio = 24 ÷ 8 = 3, so the surface area to volume ratio is 3:1.
Now compare a cube with sides of 1 cm. Its surface area is 6 cm2 and its volume is 1 cm3, so its ratio is 6:1.
The cube that is twice as wide has a smaller ratio. This is the key pattern: as an object gets bigger, its surface area to volume ratio gets smaller. The volume grows much faster than the surface area.
Cube A has sides of 3 cm and cube B has sides of 6 cm.
Cube A: surface area = 6 × 9 = 54 cm2, volume = 27 cm3, ratio = 2:1.
Cube B: surface area = 6 × 36 = 216 cm2, volume = 216 cm3, ratio = 1:1.
Cube A has the larger ratio. Its ratio is twice that of cube B.
It has a relatively large surface area to volume ratio. Every part of the cell is close to the surface. This allows enough molecules to be transported into and out of the cell to meet the needs of the organism. Diffusion across the cell membrane is enough.
It is much bigger, so its surface area to volume ratio is small. Many cells are deep inside the body, far from the outside. Diffusion across the outer surface alone is far too slow to reach them.
This is why multicellular organisms need two things:
In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This allows enough molecules to be transported into and out of cells for the organism's needs. The effectiveness of an exchange surface is increased by:
More room for substances to pass through at once.
This gives a short diffusion path, so substances have only a tiny distance to travel.
Blood carries substances away and brings more, which keeps the concentration gradient steep.
Moving air or water over the surface keeps fresh supplies arriving.
Each of these has a large surface area and a thin membrane. Animal exchange surfaces also have a good blood supply.
The small intestine absorbs the products of digestion into the blood. Its inside wall is covered in tiny folds called villi, which give a very large surface area. The wall is thin, so the diffusion path is short. A rich blood supply carries the absorbed food away.
The lungs exchange oxygen and carbon dioxide. They contain millions of tiny air sacs called alveoli, which give a large surface area. The walls of the alveoli are very thin and are surrounded by blood vessels, so there is an efficient blood supply. Breathing ventilates the lungs, which keeps a high concentration of oxygen in the air sacs.
Gills are made of many thin plates, which give a large surface area. The plates are thin and full of blood. A fish moves water over its gills to ventilate them, so there is always fresh oxygen-rich water in contact with them.
Roots take in water and mineral ions from the soil. Root hairs give the root a large surface area, and each root hair has a thin surface, so the distance for substances to travel is short.
Leaves exchange gases with the air. A leaf is broad and flat, which gives a large surface area, and it is thin, so gases only have a short distance to travel inside. Gases move in and out through tiny holes in the leaf surface. Plants have no blood, so they do not have a blood supply or ventilation.
Saying that a bigger organism has a bigger surface area to volume ratio. It is the other way round: bigger objects have a smaller ratio. Another mistake is to say that large organisms need exchange surfaces "because they are big" without linking it to the ratio. Always say that the ratio is small, so diffusion across the outside is too slow to supply every cell. Finally, do not say that gills or lungs "pull" oxygen in. Oxygen moves by diffusion.
A scientist models cells using cubes. Cube X has sides of 2 cm. Cube Y has sides of 4 cm.
(a) Calculate the surface area to volume ratio of cube Y. Show your working. [3 marks]
(b) Cube X has the larger ratio. Explain why a large ratio is useful for a single-celled organism. [2 marks]
(c) Explain why a large multicellular organism needs a transport system. [2 marks]
(d) Describe two features of the lungs that make them an effective exchange surface. [2 marks]
(a) Surface area = 6 × 4 × 4 = 96 cm2 (1). Volume = 4 × 4 × 4 = 64 cm3 (1). Ratio = 96 ÷ 64 = 1.5:1 (1).
(b) A large ratio means enough molecules can be transported into and out of the cell (1) to meet the needs of the organism (1).
(c) A large organism has a small surface area to volume ratio (1), so diffusion across the outside is too slow to reach all the cells, so a transport system carries substances to and from them (1).
(d) Any two from: large surface area because of the many alveoli (1); very thin walls, so a short diffusion path (1); efficient blood supply (1); ventilated by breathing (1).
In part (a), write out the surface area, the volume and the division on separate lines. Even if you slip up on the final answer, you can still pick up the working marks.