Membrane structure, diffusion and osmosis
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The cell's gatekeeper
- The fluid mosaic model
- Other components
Read the transcript of the teaching part
Last time
Before we start: last time, in Active transport, bulk transport and cell signalling, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The cell's gatekeeper
Every cell is wrapped in a membrane only seven nanometres thick. It must keep the cell contents in, and let the right substances in and out. Some molecules slip through easily, some need a helper, and some are blocked. Structure explains each case.
The fluid mosaic model
The membrane is a phospholipid bilayer. The hydrophilic heads face the watery outside and inside, and the hydrophobic tails face each other in the middle. Proteins are scattered through it like tiles in a mosaic. Some span the whole bilayer, and some sit on one side only. It is called fluid because the phospholipids and many proteins can move sideways. It is called a mosaic because of the pattern of proteins.
Other components
Cholesterol sits between the phospholipid tails in animal membranes. It makes the membrane less fluid at high temperature, and keeps it flexible at low temperature. Glycoproteins and glycolipids have carbohydrate chains on the outer surface. They act as receptors and as markers for cell recognition. Channel and carrier proteins let polar substances cross. Receptor proteins bind hormones and other signals.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Cell surface membrane structure · Diffusion and facilitated diffusion · Water potential and osmosis · Cell surface membrane structure.
The whole chapter: Cell Membranes and Transport, Cell Membranes and Transport.
Chapter 4: Cell membranes and transport
- Membrane structure, diffusion and osmosis 6 min
- Active transport, bulk transport and cell signalling 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.