Transport Across the Cell Membrane
Senior 2 · Biology
Transport Across the Cell Membrane
Lesson 1 of 5
16 min
Let's begin
Every cell in your body must take in oxygen, glucose and water, and get rid of carbon dioxide and wastes, all while keeping harmful substances out. The gatekeeper that controls this traffic is the cell membrane (also called the plasma membrane), a layer so thin that ten thousand of them stacked together would be thinner than a sheet of paper.
Understanding the membrane explains why a salty snack makes you thirsty, how hospital drips are prepared, and how roots absorb water from the soil.
In 1972 the scientists S. J. Singer and Garth Nicolson described the membrane with the fluid mosaic model:
The main parts are:
| Substance | Passes easily? | Why |
|---|---|---|
| Oxygen and carbon dioxide | yes | small molecules that dissolve in the oily middle layer |
| Water | yes | very small; also passes through special channel proteins |
| Glucose, amino acids | only through proteins | too large and water-loving to cross the oily layer alone |
| Ions (Na⁺, K⁺, Cl⁻) | only through proteins | their electric charge is repelled by the oily layer |
| Starch, proteins | no | far too large; can enter only by endocytosis |
Question: A plant cell has both a cell wall and a cell membrane. Which of them decides what enters the cell? Explain.
Solution: The cell membrane. The cellulose cell wall is fully permeable: water and dissolved substances pass straight through its gaps. Its job is support and protection. The membrane is partially permeable, so it is the barrier that selects what enters and leaves.
Substances cross membranes in two broad ways, which the next lessons explain:
Tip
Memory hook: think of the membrane as a sandwich. The watery "bread" is the heads, the oily "filling" is the tails, and the proteins are toothpicks pushed through it, some of them hollow.
Exam technique: use the exact phrase partially permeable (or selectively permeable). "Semi-permeable" is accepted by some examiners but not others.
Name the two main kinds of molecule that make up the cell membrane.
Solution: phospholipids (arranged in a bilayer) and proteins.
Explain why the membrane is described as a "fluid mosaic".
Solution: It is fluid because its phospholipids and proteins can move sideways within the layer. It is a mosaic because it is made of many different molecules fitted together, like the tiles of a mosaic.
Why can oxygen cross the membrane easily while sodium ions cannot?
Solution: Oxygen is small and uncharged, so it dissolves in the oily, hydrophobic middle of the bilayer and passes through. Sodium ions are charged, so the hydrophobic layer repels them, and they can cross only through channel or carrier proteins.
State one function of the carbohydrate chains on the outer surface.
Solution: Cell recognition: they let cells identify each other, and let the immune system distinguish the body's own cells from foreign ones.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
