Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- The plasma membrane separates the cytosol from the extracellular fluid and keeps the two different.
- Its framework is a phospholipid bilayer. Polar, hydrophilic phosphate heads face the watery fluid inside and outside the cell. Nonpolar, hydrophobic fatty acid tails face each other in the middle.
- The bilayer forms by itself: it is the most stable arrangement in water. No energy from the cell is needed.
- Membrane proteins can be hydrophilic, hydrophobic or both. Their hydrophobic regions (nonpolar R groups) form the surface that touches the tails. Their hydrophilic regions (polar and charged R groups) face the cytosol or outside fluid, or lie inside the protein, for example lining a pore.
- Integral proteins pass through the bilayer; peripheral proteins sit on one surface.
- Fluid mosaic model: phospholipids with proteins, steroids such as cholesterol (vertebrate animals), glycoproteins and glycolipids set into them. All can move sideways within the membrane.
Key relationships
| Component | Where it sits | Job |
|---|---|---|
| phospholipids | two layers, heads out, tails in | barrier: the hydrophobic interior blocks ions and polar molecules |
| integral proteins | through the bilayer | channels, carriers, receptors |
| peripheral proteins | on one face | enzymes, signalling, links to the cytoskeleton |
| cholesterol (vertebrate animals) | among the tails | keeps the membrane stable as temperature changes |
| glycoproteins, glycolipids | carbohydrate chains on the outer face | cell recognition and adhesion |
| Calculation | Use |
|---|---|
| helix length = number of amino acids × 0.15 nm | about 20 or more nonpolar amino acids are needed to cross a 3 to 4 nm hydrophobic interior |
Assumptions behind diagrams and numbers
- Membrane diagrams are not to scale and show a moving structure frozen at one instant.
- The 0.15 nm rule assumes the segment is an α-helix running straight across the membrane.
- A single experiment (one run, no repeats) can support an idea about fluidity but cannot settle it.
Mistakes to avoid
- Drawing tails facing the water or a single layer of phospholipids.
- Saying a whole membrane protein is hydrophobic.
- Putting carbohydrate chains on the cytosol side.
- Calling a protein integral when it only touches one surface.
- Saying “fluid” means the membrane has no fixed shape; it means components drift sideways.
- Saying cells must spend energy to keep the bilayer together.
Quick self-check
- Which part of a phospholipid faces the cytosol? (The polar phosphate head of the inner layer.)
- How long is a helix of 18 amino acids? (18 × 0.15 = 2.7 nm, a little short of a 3 to 4 nm interior.)
- How can you tell the outside face of a membrane in a diagram? (The carbohydrate chains of glycoproteins and glycolipids point outwards.)
- What does a lack of recovery after photobleaching in fixed cells show? (It is a control: when movement is blocked, there is no recovery, so recovery in living cells is caused by movement.)
Next: practice questions.