Movement of Substances Across the Plasma Membrane in SPM Biology
The plasma membrane is selectively permeable: it lets some substances through and blocks others. How a substance crosses depends on its size, charge, whether it is lipid-soluble, and the concentration gradient. SPM Biology tests your understanding of four transport mechanisms and the experiments that demonstrate them.
The Fluid Mosaic Model
The membrane is not a solid wall but a phospholipid bilayer with proteins embedded in it. Phospholipids have a hydrophilic head and hydrophobic tail, so they arrange themselves into two layers with tails pointing inward. Cholesterol stabilises the bilayer, while transport proteins form channels and pumps. Small non-polar molecules like oxygen and carbon dioxide slip straight through; larger or charged particles need help.
The Four Transport Mechanisms
| Mechanism | Direction | Energy | Example |
|---|---|---|---|
| Simple diffusion | High to low | None (passive) | O₂ entering cells |
| Osmosis | Water: high water potential to low | None (passive) | Root cells absorbing water |
| Facilitated diffusion | High to low via protein | None (passive) | Glucose entering intestinal cells |
| Active transport | Low to high (against gradient) | Requires ATP | Mineral ions into root hairs |
Memorise this table. The distinction examiners test most often is passive versus active: anything moving down a concentration gradient without energy is passive; anything moving against a gradient needs ATP. If a question says the cell used oxygen and produced more carbon dioxide, it hints at respiration powering active transport.
Osmosis in Detail
Osmosis is the net movement of water from a region of high water potential to a region of low water potential across a partially permeable membrane. The classic experiment uses a Visking tubing bag containing sugar solution suspended in distilled water. Over time, water enters the bag and the level in the capillary tube rises, demonstrating osmosis.
Apply this to animal and plant cells differently. An animal cell in a hypertonic solution (lower water potential than the cell) loses water and shrinks — crenation. In a hypotonic solution it gains water and may burst — haemolysis. Plant cells behave differently because of the rigid cell wall: in a hypotonic solution they become turgid (firm) rather than bursting, and in a hypertonic solution the cytoplasm pulls away from the wall — plasmolysis.
Why Turgor Matters
Turgor pressure gives non-woody plants their support. A wilted plant has lost turgor because its cells are in a hypertonic environment. When you water it, water re-enters by osmosis, the vacuole swells, the cytoplasm presses against the cell wall, and the plant stands upright again. This everyday observation is osmosis in action and makes a memorable exam example.
Facilitated Diffusion and Active Transport
Facilitated diffusion moves substances down their gradient but through protein channels or carriers, which is necessary for polar molecules like glucose and ions. Active transport goes the other way: it moves substances against their gradient using energy from ATP. The sodium-potassium pump in nerve cells is the textbook example — it continuously moves sodium out and potassium in to maintain the resting potential.
Experiments You Must Know
- Visking tubing with sugar solution — demonstrates osmosis.
- Potato strips in different salt concentrations — measure mass change to find isotonic point.
- Red blood cells in distilled water vs salt solution — shows haemolysis and crenation.
- Yeast cells taking up methylene blue — illustrates active transport if oxygen is present.
Exam Technique
When describing transport, always state four things: the substance moved, the direction, the mechanism, and the reason (concentration gradient or ATP). Vague answers like 'water moves in' lose marks; precise answers like 'water moves by osmosis from the hypotonic surrounding solution into the cell cytoplasm which has a lower water potential' earn full marks.