# Revision on Topic 4 · Movement of Substances Across the Cell Membrane

F.4 HKDSE Biology · Topic IV · Lesson 2 (Revision on Topic 4)

The four transport processes compared, and how to read transport experiments.

## Figure

Two exam graphs on different axes: rate of uptake against outside concentration (left); concentration inside the cell against time (right).

## Key points

- **Fluid mosaic model (流動鑲嵌模型)**: **phospholipid bilayer** with **proteins interspersed**; phospholipids **move laterally** → **fluid**, **flexible**, **strong**.
- **Differentially permeable**: **small, non-polar** molecules (O~2~, CO~2~) cross the bilayer itself; **polar** molecules and **ions** need **channel** or **carrier proteins**.
- **Diffusion**: **net movement** of particles **down the concentration gradient**; faster with a **steeper gradient**, **higher temperature**, **smaller particles**, **shorter distance**, **larger surface area**.
- **Osmosis**: **net movement of water** across a **differentially permeable membrane** from **higher** to **lower water potential (水勢)**; passive; works across non-living **dialysis tubing**.
- **Hypotonic / isotonic / hypertonic**: water potential **higher / same / lower** than the cytoplasm's. Animal cell **bursts / no change / shrinks and wrinkles**; plant cell **turgid / no change / plasmolysed (質壁分離)**.
- **Active transport (主動轉運)**: **against the gradient** by **carrier proteins** using **energy** from respiration; **living** membrane only (glucose in the intestine, minerals into roots).
- **Phagocytosis (吞噬)**: **large particles** taken in by a **pit** or **pseudopodia**; needs **energy** (Amoeba; white blood cells).
- **Reading experiments**: uptake **stops at inside = outside** → diffusion; **goes on against the gradient** or **needs oxygen** → active transport; **rate levels off** → carrier or channel proteins **all in use**.

## Must know

| Process | Membrane needed? | Active or passive? | Energy needed? | Importance |
|---|---|---|---|---|
| **Diffusion** | No | Passive | No | Cells obtain O~2~ and nutrients, remove CO~2~ |
| **Osmosis** | **Differentially permeable** membrane | Passive | No | Water enters or leaves cells; absorbed in the small intestine and plant roots |
| **Active transport** | **Living** membrane | Active | Yes | Glucose absorbed in the small intestine; minerals from the soil into roots |
| **Phagocytosis** | **Living** membrane | Active | Yes | Unicellular organisms engulf food; white blood cells engulf microorganisms |

No oxygen or cyanide stops active transport and phagocytosis but not diffusion or osmosis; a lower temperature slows all four (less kinetic energy, less respiration).

## Common mistakes

- ✗ "Water moves only from A to B." ✓ Water molecules cross the membrane **both ways**; osmosis is the **net** movement. A rising level stops when water enters and leaves at the **same rate**.
- ✗ "Substance X enters through carrier proteins, so it is taken up by active transport." ✓ Carrier proteins also carry substances **down** the gradient by diffusion, with no energy. Active transport is shown only by uptake **against** the gradient, or uptake that **needs oxygen** and stops with cyanide.
- ✗ "Glucose solution in dialysis tubing makes the level rise, like sucrose." ✓ **Glucose is small enough to pass through** the tubing, so the concentrations inside and outside become equal and there is **no lasting rise**; sucrose cannot leave, so water keeps entering and the level **rises**.
- ✗ "An onion epidermal cell in distilled water bursts." ✓ Water enters by osmosis and the cell becomes **turgid**; the **rigid cell wall** stops it bursting. Only animal cells (no wall) burst.
- ✗ "The rate of uptake levels off, so it is simple diffusion." ✓ The rate of **simple diffusion keeps rising** with the concentration gradient; a **plateau** means the **carrier or channel proteins are all in use**.

## Quick check

1. A peeled potato cup holding 20% sucrose solution stands in distilled water. What happens to the liquid level in the cavity after four hours, and why?
2. An amoeba takes up substance B until the concentration inside equals that outside, then stops; substance A keeps entering at a constant rate even when the inside concentration is higher. Name the mechanism for each, with evidence.
3. Dialysis tubing of sucrose solution on a thistle funnel stands in distilled water. The liquid level rises, then stops. Why does it stop?
4. Root cells stop taking up chloride ions when oxygen is removed or cyanide is added. What does this show, and why?
5. 5 cm³ of liquid A is added to 1 cm³ of fresh blood; after 30 minutes the mixture is clear red. What is liquid A? Explain.

## Answers

1. It rises: the sucrose solution has a lower water potential than the distilled water, so there is a net movement of water through the living potato cells into the cavity by osmosis.
2. B: diffusion, because uptake stopped when the concentrations became equal. A: active transport, because uptake continued against the gradient, which needs energy.
3. Water crosses both ways. As water enters, the sucrose solution is diluted and the water-potential difference shrinks, until water leaves as fast as it enters: no net movement.
4. Active transport: it needs energy from respiration, so no oxygen or cyanide stops the uptake.
5. Distilled water. Its higher water potential makes water enter the red blood cells by osmosis; they swell and burst (haemolysis) and the released haemoglobin turns the mixture clear red.

## Did you know?

Diffusion is fast only over very short distances. Oxygen diffuses across a cell in a fraction of a second, but it would take hours to diffuse just 1 cm through water. This is one reason why large animals need blood to carry oxygen.