VCE Biology (Year 11) · Unit 1 — Cells and Systems · 11m read

Plasma Membrane and Transport

Have you ever wondered how a cell decides what gets in and what stays out? Your cells are wrapped in a super-thin membrane that works like a selective bouncer—letting helpful nutrients through while blocking harmful stuff. You'll learn how this membrane is built and the different transport mechanisms (active and passive) that move materials in and out, which is essential for keeping your cells alive and functioning properly in VCE Biology.

Learning objective: Describe the plasma membrane and mechanisms of transport.

By the end of this lesson you can:

• I describe the fluid mosaic model.
• I distinguish passive and active transport.
• I explain osmosis and diffusion.

The plasma membrane is a nightclub bouncer. It decides who gets in: small non-polar molecules slip through; others need channel doors or energy-using pumps.

Membranes and transport

FLUID MOSAIC MODEL:
• PHOSPHOLIPID BILAYER — hydrophilic heads out, hydrophobic tails in.
• PROTEINS embedded — channel, carrier, receptor, enzyme.
• CHOLESTEROL — regulates fluidity in animal cells.
• GLYCOPROTEINS and GLYCOLIPIDS — cell recognition.

SELECTIVE PERMEABILITY:
• Allows specific substances in/out.
• Determines what the cell can do.

PASSIVE TRANSPORT (no ATP):
• Down the concentration gradient (high → low).

• DIFFUSION — movement of small, nonpolar molecules (O₂, CO₂) directly across membrane.
• FACILITATED DIFFUSION — polar or large molecules via protein channels / carriers (glucose).
• OSMOSIS — water movement across semipermeable membrane from high to low water concentration.

ACTIVE TRANSPORT (requires ATP):
• Against concentration gradient (low → high).
• Via carrier proteins (e.g., sodium-potassium pump).

BULK TRANSPORT:
• ENDOCYTOSIS — cell engulfs material (phagocytosis for solids, pinocytosis for liquids).
• EXOCYTOSIS — vesicles fuse with membrane to release contents.

TONICITY (relative solute concentration):
• HYPOTONIC — less solute outside → water enters → cell swells / bursts.
• ISOTONIC — equal → no net water movement.
• HYPERTONIC — more solute outside → water leaves → cell shrivels (plasmolysis in plants).

FACTORS AFFECTING DIFFUSION:
• Temperature (higher = faster).
• Concentration gradient (steeper = faster).
• Surface area (larger = faster).
• Distance (shorter = faster).
• Molecule size (smaller = faster).

REAL WORLD:
• Lungs rely on diffusion of O₂/CO₂.
• Small intestine absorbs nutrients via active transport.
• Kidneys regulate water via osmosis.

EXAM TIP VOCABULARY:
• "Water potential" — tendency of water to move; moves from high to low.
• "Concentration gradient" — difference in concentration between two areas.

Transport categories

• Passive: No ATP, down gradient.
• Active: ATP, against gradient.
• Bulk: Vesicles for large quantities.

Quick reference

| Transport | Energy? | Example |
|---|---|---|
| Simple diffusion | No | O₂, CO₂ |
| Facilitated diffusion | No | Glucose via channels |
| Osmosis | No | Water |
| Active transport | ATP | Na⁺/K⁺ pump |
| Bulk transport | ATP | Endocytosis |

Worked example (Medium)

A red blood cell is placed in pure water. Predict what happens.

1. Tonicity. Pure water is hypotonic to RBC cytoplasm.
2. Osmosis. Water moves INTO the cell across the semipermeable membrane.
3. Result. Cell swells and can LYSE (burst) — no cell wall to resist pressure.

Answer: Cell takes in water and lyses due to osmotic pressure.

Worked example (Hard)

A plant cell is placed in a hypertonic salt solution. Describe and explain the sequence of changes that will occur at the cellular level.

1. Initial state. The plant cell is turgid with water filling the central vacuole, pressing the plasma membrane against the cell wall.
2. Osmosis begins. The hypertonic solution has more solute (less water) outside the cell. Water moves OUT of the cell by osmosis, down the water concentration gradient.
3. Plasmolysis. The vacuole shrinks. The plasma membrane pulls away from the cell wall — this is PLASMOLYSIS. The cell wall remains rigid because it is not selectively permeable.
4. Equilibrium. Water loss continues until the internal and external water potentials equalise. The cell does NOT burst because the rigid cell wall provides structural support, unlike an animal cell which would simply crenate (shrivel).

Answer: In a hypertonic solution, a plant cell undergoes plasmolysis: water leaves by osmosis, the vacuole shrinks, and the membrane pulls away from the cell wall.

Common mistakes

• Saying water moves from low to high concentration Students confuse SOLUTE concentration with WATER concentration. Water moves from where there is MORE water (low solute / hypotonic) to where there is LESS water (high solute / hypertonic). Fix: Always state: "Water moves from HIGH water concentration to LOW water concentration" or "from hypotonic to hypertonic side".
• Confusing diffusion and osmosis Osmosis is specifically the movement of WATER across a selectively permeable membrane. Diffusion is the movement of any substance down its concentration gradient. Fix: Use "osmosis" only for water across a membrane; use "diffusion" for other substances.
• Forgetting that active transport requires ATP Students sometimes describe active transport as passive because the carrier protein "helps" the substance across. Fix: If it moves AGAINST the gradient, it MUST use ATP — that is the defining feature of active transport.

Osmosis in medicine — IV drips and kidney dialysis

Understanding osmosis isn't just an exam exercise — it has critical medical applications. When patients receive intravenous (IV) fluids, the solution must be isotonic with blood (e.g., 0.9% saline, known as "normal saline"). If a hypotonic solution were infused, water would rush into red blood cells, causing them to swell and lyse — a potentially fatal situation called haemolysis. If a hypertonic solution were used, water would leave the cells, causing them to crenate (shrivel), impairing oxygen transport.

In kidney dialysis, osmosis and diffusion work together to filter waste from blood. The dialysis membrane is selectively permeable — small waste molecules (urea, excess salts) diffuse out of the blood into the dialysate solution, while blood cells and proteins are too large to pass through. The dialysate is carefully formulated to be isotonic for essential ions so that only wastes are removed.

Sports drinks exploit osmosis too — they are designed to be slightly hypotonic or isotonic so that water and electrolytes are absorbed quickly in the gut without causing osmotic stress to intestinal cells.

These examples illustrate why the VCE Biology membrane transport topic is foundational for health sciences, nursing, and medicine.

Practice questions

1. Active transport requires:

   1. Diffusion
   2. ATP
   3. Ribosomes
   4. Chloroplasts

   (1 marks)

2. Why is the plasma membrane described as "fluid mosaic"?

   (3 marks)

Summary

• Describe the fluid mosaic model.
• Distinguish passive and active transport.
• Explain osmosis and diffusion.