B2.3C Specialized cells: pneumocytes, muscle fibres and gametes (HL)

Cells Reshape Themselves to Trade Faster Across Their Membranes

A high surface area-to-volume ratio lets a cell move materials in and out fast enough to keep up with what its inside is doing.

1. When a cell needs a high rate of exchange or reaction, it cannot just shrink, so it evolves structural adaptations that add surface area without adding much volume.
2. Each adaptation below is tied to a specific job, and in every case the extra surface area is what makes that job possible.

External Projections: Reaching Out for Absorption

1. Some cells push out projections so that more of their membrane touches the material they need to absorb.
2. Microvilli
   1. Microvilli are tiny finger-like projections of the cell membrane that greatly increase the surface area of the cell.
   2. They give more room for transport proteins and channels, so materials cross the membrane faster.
   3. They cover the epithelial cells lining the small intestine and the proximal tubule of the nephron, both sites of heavy absorption.
3. Root Hair Cells
   1. A root hair cell grows one long, thin projection out into the spaces between soil particles.
   2. This projection raises the surface area for uptake of water by osmosis and minerals by active transport, such as nitrate, phosphate, and potassium.