A cube-shaped cell has sides of 5 µm. What is its surface area-to-volume ratio?
- Surface area = 6 × 5 × 5 = 150 µm2.
- Volume = 5 × 5 × 5 = 125 µm3.
- SA:V = 150 ÷ 125 = 1.2 µm−1.
Answer: 1.2 µm−1
Cell specialization
Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
A fertilized egg divides into cells that are all unspecialized at first. Every cell keeps the full set of genes, but each type switches on only some of them. This is differentiation. Gradients of signalling chemicals called morphogens tell cells in the early embryo which genes to express.
Stem cells can divide again and again and can differentiate into other cell types. In adults they survive in niches such as bone marrow and hair follicles.
Cell size is fitted to function, but it is limited. As a cell grows, its volume rises faster than its surface area, so the surface area-to-volume ratio falls and exchange across the membrane cannot keep up.
Rules to remember
- Differentiation = expressing some genes and not others; the DNA itself is not lost or changed.
- Totipotent: can form every cell type, including the placenta (cells of the very early embryo).
- Pluripotent: can form all body cell types but not the placenta (embryonic stem cells).
- Multipotent: can form a few related cell types (adult stem cells, e.g. bone marrow makes blood cells).
- Cube of side s: surface area = 6s2, volume = s3, SA:V = 6 ÷ s.
- Surface area fixes the rate of exchange; volume fixes the rate at which materials are used and wastes are made.
- The human egg is one of the largest cells (about 0.1 mm); sperm and red blood cells are very small.
Common mistake
Students say that a bigger cell has a smaller surface area. Its surface area is larger; it is the surface area-to-volume ratio that gets smaller.
Worked example
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