Explain why a cell with a higher surface area to volume ratio is able to rely on diffusion alone to meet its metabolic demands, whereas a larger cell cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Unicellular organisms such as Amoeba proteus can exchange substances with their environment entirely by diffusion across their cell surface membrane. Multicellular organisms require specialised exchange surfaces and transport systems to supply their cells.
Model answer (4 marks)
A cell with a high surface area to volume ratio has a large membrane area relative to its cytoplasmic volume, so more molecules can enter or leave per unit time.
Because the cell is small, the distance from the membrane to the centre is short; diffusion therefore reaches all parts of the cytoplasm quickly.
In a larger cell the volume increases faster than the surface area, so the ratio falls. The membrane area available for diffusion is no longer sufficient to supply the larger volume of cytoplasm, and the rate of diffusion cannot meet the metabolic demands.
Thus small cells can rely on diffusion alone, whereas larger cells need specialised transport systems.
Because the cell is small, the distance from the membrane to the centre is short; diffusion therefore reaches all parts of the cytoplasm quickly.
In a larger cell the volume increases faster than the surface area, so the ratio falls. The membrane area available for diffusion is no longer sufficient to supply the larger volume of cytoplasm, and the rate of diffusion cannot meet the metabolic demands.
Thus small cells can rely on diffusion alone, whereas larger cells need specialised transport systems.
Examiner tips
- Use the key terms ‘surface area to volume ratio’, ‘diffusion distance’, and ‘metabolic demands’.
- Explain the relationship between size, ratio and diffusion rate in two or three sentences.
Mark scheme (4 marks)
- A higher surface area to volume ratio means the surface area available for diffusion is large relative to the volume of cytoplasm that needs to be supplied.
- Diffusion distance from the surface membrane to the centre of the cell is short in a small cell, so substances reach all parts of the cytoplasm quickly.
- In a larger cell, the volume increases proportionally more than surface area, so the surface area to volume ratio decreases.
- The rate of diffusion across the membrane becomes insufficient to supply the metabolic demands of the larger volume of cytoplasm, so diffusion alone cannot meet the cell's requirements.
Key terms in this question
surface area to volume ratio · diffusion · metabolic demands
Related
- Revision notes: Cell structure →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cell structure questions
- A student observes two types of cells under an electron microscope: a mesophyll …
- A student examined two types of cell under an electron microscope and noted that…
- Explain why a cell with a large number of mitochondria and an extensively folded…
- Explain why the resolution of a transmission electron microscope (TEM) is far gr…
- Explain why the resolution of a transmission electron microscope is greater than…
- Explain why the resolution of an electron microscope is greater than that of a l…