# 4.1 Cell Membrane · 4.2 Movement of Substances Across the Cell Membrane

F.4 HKDSE Biology · Topic IV · Lesson 1 (4.1–4.2)

How the fluid mosaic membrane is built, and the four ways substances cross it: diffusion, osmosis, active transport and phagocytosis.

## Figure

A plant cell in three solutions: water moves by osmosis from higher to lower water potential.

## Key points

- **Fluid mosaic model (流動鑲嵌模型)**: a **phospholipid bilayer** with **proteins** interspersed in a mosaic pattern; phospholipids and some proteins **move laterally**, so the membrane is fluid.
- **Phospholipid** = glycerol + phosphate group + two fatty acids; **hydrophilic heads** face the water, **hydrophobic tails** point inwards.
- **Channel proteins (通道蛋白)** provide channels (ions); **carrier proteins (載體蛋白)** bind and carry substances; **receptor proteins (受體蛋白)** bind hormones; **antigens** for cell recognition; **cholesterol** stabilises it.
- **Differentially permeable**: small **non-polar** molecules (O~2~, CO~2~) cross freely; **polar** molecules and **ions** need channel or carrier proteins; **large** molecules cannot. Heat and organic solvents raise permeability; boiling: **fully permeable**.
- **Diffusion**: **net** movement of particles **down the concentration gradient**; **passive**. Faster with a steeper gradient, larger surface area, shorter distance, higher temperature.
- **Water potential**: pure water = **0 kPa**, the highest; every solution is **negative**; the more concentrated, the **lower** the water potential.
- **Osmosis**: **net** movement of water from **higher** to **lower water potential** across a **differentially permeable membrane**; passive; no **net** movement at equilibrium.
- **Active transport**: **carrier proteins** move substances **against** the gradient using **energy from respiration**; living cells only (slowed by low temperature, low O~2~, cyanide).
- **Phagocytosis**: the membrane folds in or extends **pseudopodia** to enclose **large solid particles** in a **vacuole**; needs **energy**. E.g. Amoeba, white blood cells.

## Must know

| Process | Energy from respiration? | Membrane protein? | Direction | Examples |
|---|---|---|---|---|
| **Diffusion** | No (passive) | No for O~2~, CO~2~; channel or carrier proteins for polar substances | Down the concentration gradient, until equilibrium | Gas exchange in the lungs; absorption of digested food |
| **Osmosis** | No (passive) | No (water crosses the membrane) | From higher to lower water potential | Water uptake by root hairs and the intestine |
| **Active transport** | Yes | Carrier proteins | Against the concentration gradient | Minerals into roots; glucose and amino acids into small-intestine cells |
| **Phagocytosis** | Yes | No carrier protein (membrane folds in or extends pseudopodia) | Large solid particles or whole cells taken in | Amoeba feeding; white blood cells engulfing bacteria |

Animal cell (no cell wall): **swells and may burst** in a hypotonic solution, **no change** in an isotonic one, **shrinks and becomes wrinkled** in a hypertonic one.

## Common mistakes

- ✗ "The concentrated sucrose solution has a higher water potential than the cell sap, so water left the cells." ✓ A more concentrated solution has a **lower** water potential (more negative); water moves from the higher water potential (cell sap) to the lower one (solution).
- ✗ "At equilibrium, water stops crossing the membrane." ✓ Osmosis is a **net** movement: water molecules cross in both directions all the time; at equilibrium the two flows are equal, so there is no **net** movement.
- ✗ "Animal cells plasmolyse in a concentrated solution." ✓ Only cells with a **cell wall** (e.g. plant cells) plasmolyse, when the membrane pulls away from the wall; animal cells just **shrink and wrinkle**.
- ✗ "Glucose entering a cell through a carrier protein is active transport." ✓ Through a channel or carrier protein **down** the gradient it is still **diffusion** (no energy from respiration); it is active transport only when moved **against** the gradient.
- ✗ "Boiled potato strips still gain water by osmosis in distilled water." ✓ Boiling destroys the cell membrane, making it **fully permeable**; osmosis needs a **living**, differentially permeable membrane, so boiled strips no longer gain or lose water by osmosis.

## Quick check

1. A red blood cell is put in distilled water and a leaf epidermal cell in concentrated sucrose solution. What happens to each cell, and why?
2. Which two of diffusion, osmosis, active transport and phagocytosis need energy from respiration? Give one example of each.
3. Oxygen diffuses straight through the phospholipid bilayer but glucose cannot. Explain with reference to the structure of the membrane.
4. Dialysis tubing (impermeable to sucrose) filled with 10% sucrose solution is placed in distilled water. In which direction is the net movement of water, and how does the water potential inside the tubing change with time?
5. Potato strips of equal length are put into 0.2 M, 0.4 M, 0.6 M and 0.8 M sucrose solutions; their lengths change by +3, +1, −1 and −2 mm. Between which two concentrations is the solution isotonic to the potato cells? Explain.

## Answers

1. Red blood cell: distilled water has a higher water potential, water enters by osmosis, the cell swells and may burst (no cell wall). Leaf cell: the solution has a lower water potential, water leaves by osmosis, the cell becomes flaccid and plasmolysed.
2. Active transport (e.g. mineral uptake by roots) and phagocytosis (e.g. white blood cells engulfing bacteria).
3. The core of the bilayer is hydrophobic. O~2~ is small and non-polar, so it dissolves in the bilayer and crosses; glucose is polar, is repelled by the bilayer and needs a channel or carrier protein.
4. Net movement into the tubing (distilled water has the higher water potential, 0 kPa). The water potential inside rises (becomes less negative) as water enters, then levels off when it equals that outside and there is no net movement.
5. Between 0.4 M and 0.6 M: the length change passes through zero there, so there is no net movement of water, meaning the cell sap and the solution have the same water potential.

## Did you know?

A 0.9% sodium chloride solution ("normal saline") is isotonic to human blood, which is why it is used in hospital drips: pure water injected into a vein would make red blood cells swell and burst.