2.5 The Cell Membrane
Notes
The cell membrane controls what enters and leaves the cell. It is selectively permeable.
- It is made of a phospholipid bilayer: hydrophilic (water-loving) heads face outward and hydrophobic (water-fearing) tails face inward.
- Proteins in the membrane act as channels, pumps, receptors and ID tags.
- Cholesterol helps keep animal membranes stable.
- The fluid mosaic model describes the membrane as a flexible layer with proteins moving within it.
- Small, nonpolar molecules (like $\text{O}_2$ and $\text{CO}_2$) pass through easily; ions and large molecules need proteins.
Practice
Why do phospholipid tails point toward the inside of the bilayer?
The water-fearing tails hide from the watery cytoplasm and surroundings.
Which substance can most easily pass directly through the phospholipid bilayer?
Small nonpolar molecules slip between the phospholipids.
"Selectively permeable" means the membrane:
The membrane controls which materials cross.
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