Cell Transport & Membranes
Middle SchoolA cell is defined by its boundary. The membrane is not a wall but a selective, dynamic gatekeeper — deciding what enters and leaves, and how the cell talks to its world. Master the membrane and you master the flow of matter through life.
The fluid mosaic model
The cell membrane is a phospholipid bilayer. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water the molecules spontaneously arrange tails-inward, heads-outward, forming a two-layer sheet — the hydrophobic effect at work. The 1972 fluid mosaic model (Singer & Nicolson) describes it as a fluid sea of lipids with a mosaic of proteins drifting within and through it. Cholesterol nestled between the lipids buffers fluidity; carbohydrate chains on the outer surface (glycoproteins, glycolipids) act as cell-recognition markers.
The membrane is selectively (partially) permeable: small non-polar molecules (O₂, CO₂) and water slip through; ions and large polar molecules (glucose, amino acids) need protein channels or carriers.
Modes of transport
| Process | Direction (gradient) | Energy (ATP)? | Protein needed? |
|---|---|---|---|
| Simple diffusion | High → low | No (passive) | No |
| Facilitated diffusion | High → low | No (passive) | Yes — channel/carrier |
| Osmosis | Water: high → low water potential | No (passive) | Aquaporins (optional) |
| Active transport | Low → high (against gradient) | Yes | Yes — pump protein |
| Endo/exocytosis | Bulk in/out via vesicles | Yes | Membrane + cytoskeleton |
Osmosis and water potential
Osmosis is the net diffusion of water across a partially permeable membrane, from a region of higher water potential to one of lower water potential. Water potential (Ψ, measured in kPa) is highest for pure water (Ψ = 0) and lowered by dissolved solutes (making it negative). Water always moves toward more negative Ψ.
Gradients: the currency of cellular life
Everything a membrane does comes down to concentration gradients. Passive transport is nature running downhill — molecules spread from crowded to empty regions by random thermal motion, no energy required, until equilibrium. Active transport is the cell doing work: like pumping water uphill into a reservoir, it spends ATP to build gradients the cell can later cash in. The sodium–potassium pump pumps 3 Na⁺ out and 2 K⁺ in per ATP, creating the electrochemical gradient that drives nerve impulses and secondary transport of glucose. A cell without active transport is a cell at equilibrium — and equilibrium is death.
Cells in solutions: hypertonic (more solute outside → water leaves → animal cell shrivels/crenates, plant cell plasmolyses); hypotonic (less solute outside → water enters → animal cell bursts/lyses, plant cell becomes turgid — safe, held by its wall); isotonic (equal → no net movement). The plant cell wall is why plants exploit turgor while animals must osmoregulate carefully.
Experiment: osmosis in potato cylinders
A classic quantitative experiment to find the water potential of potato tissue.
- Prepare. Cut several identical potato cylinders with a cork borer; blot and record the initial mass of each.
- Set up a gradient of solutions. Place cylinders in sucrose solutions of increasing concentration: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³. One cylinder per beaker; leave 20–30 minutes at constant temperature.
- Measure. Remove, blot gently (same technique each time — a key control), and record final mass.
- Calculate percentage change. % change = (final − initial) ÷ initial × 100. Using % (not raw grams) controls for cylinders starting at different masses.
- Plot & interpret. Plot % change (y) against concentration (x). Cylinders in dilute solutions gain mass (water in, positive %); in concentrated solutions they lose mass (water out, negative %).
- Find the answer. Where the line crosses the x-axis (0% change), there is no net osmosis — the external solution's water potential equals the potato cells' water potential. Read off that concentration; it gives the tissue's water potential.
Controlled variables — temperature, immersion time, blotting method, cylinder dimensions — isolate concentration as the only independent variable. Using percentage change and the x-intercept removes bias from unequal starting sizes and gives a precise, interpolated result rather than guessing between measured points.
Test yourself
Q1 Why can oxygen cross the membrane by simple diffusion but glucose cannot?
Oxygen is a small, non-polar molecule, so it dissolves in and passes freely through the hydrophobic core of the bilayer. Glucose is large and polar (many –OH groups), so the hydrophobic interior repels it. Glucose must use facilitated diffusion through specific carrier proteins (e.g. GLUT transporters), which provide a hydrophilic route across the membrane.
Q2 A red blood cell is placed in distilled water and bursts, but a plant cell in the same water does not. Explain.
Distilled water is hypotonic to both cells, so water enters by osmosis. The red blood cell has only a flexible membrane; as water floods in the cell swells and lyses (bursts). The plant cell has a rigid cellulose cell wall: as water enters, pressure builds until the wall pushes back with equal pressure potential, halting further net influx. The cell becomes turgid but intact. The wall provides the mechanical resistance that an animal cell lacks.
Q3 How does the sodium–potassium pump differ fundamentally from a facilitated-diffusion channel?
A facilitated-diffusion channel is passive: it lets ions flow down their gradient and needs no energy. The Na⁺/K⁺ pump is active: it moves ions against their gradients (3 Na⁺ out, 2 K⁺ in) and is powered by ATP hydrolysis, which changes the pump's shape to grab and release ions on opposite sides. The channel dissipates gradients; the pump builds them — creating the electrochemical potential that nerves and muscles depend on.
Q4 Predict the effect of a metabolic poison that stops ATP production on (a) diffusion of CO₂ and (b) uptake of mineral ions by a root.
(a) CO₂ diffusion is unaffected — simple diffusion is passive and needs no ATP; it continues as long as a gradient exists. (b) Mineral-ion uptake against a concentration gradient relies on active transport, which requires ATP. Without ATP the pumps stall and ion uptake falls dramatically. This is why waterlogged (oxygen-starved) soils, which limit respiration and ATP supply, impair mineral uptake in roots.
Q5 Cholesterol is described as a "fluidity buffer." Explain how one molecule can both increase and decrease membrane fluidity.
Cholesterol sits between phospholipid tails and acts against whatever extreme the temperature pushes toward. At high temperatures, when tails move freely and the membrane risks becoming too fluid and leaky, cholesterol's rigid ring structure restrains movement, reducing fluidity. At low temperatures, when tails would pack tightly and the membrane risks freezing solid, cholesterol props the tails apart, preventing tight packing and maintaining fluidity. It stabilises the membrane across a range of temperatures — a homeostatic molecule.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1A potato cylinder is placed in salty water. What happens to its mass?
- Salty water is more concentrated than the cell
- Water leaves the cell by osmosis
Example 2Oxygen is higher outside a cell than inside. Which way does it move and how?
- Particles move down the gradient
- Small gases cross freely, no energy
Now you try
Work each one out first, then tap to reveal the worked answer.