Key Idea: Every cell is wrapped in a cell membrane — a phospholipid bilayer studded with proteins, described as a fluid mosaic. Its job is to be selectively permeable: to let some things across and keep others out. The whole topic then answers one question — how does a particular substance get across? Two things decide the route: how big and how polar the particle is, and whether it travels with or against its concentration gradient. Going with the gradient costs the cell no energy (passive: simple diffusion, osmosis, facilitated diffusion). Going against it, or moving bulk cargo, costs ATP (active transport, endo/exocytosis). This topic is a regular on Paper 1A (identify components / classify a transport process), Paper 1B and Paper 3 data (osmosis mass changes, saturation curves, dialysis tubing) and Paper 2 (explain bilayer formation, describe transport processes, contrast passive and active).
🧱 Membrane structure & the fluid mosaic model
The membrane is built from phospholipids, each of which is amphipathic — it has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty-acid tails. There is water on both sides of a membrane (outside the cell and in the cytoplasm). The heads are drawn to that water, so they face outward on both surfaces; the tails are repelled by it, so they are pushed inward and meet in the middle. The result is two rows — a bilayer — that forms spontaneously, with a water-free core. Scattered through the bilayer are proteins (channel, carrier, peripheral), glycoproteins (cell recognition) and cholesterol (steadies fluidity). Because parts can drift (fluid) and many different molecules are dotted through it (mosaic), this is the fluid mosaic model — which replaced the older Davson–Danielli model once electron microscopy showed proteins embedded within the bilayer, not just coating it.
The fluid mosaic model: a phospholipid bilayer with hydrophilic heads facing the water on both surfaces and hydrophobic tails tucked into the core, studded with channel and carrier proteins, a glycoprotein (cell recognition) and cholesterol — all dotted through it like tiles in a mosaic.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Membrane component | Hydrophilic or hydrophobic? | What it does |
|---|---|---|
| Phosphate heads | hydrophilic | face the water on both surfaces |
| Fatty-acid tails | hydrophobic | form the core; block large/polar molecules |
| Channel & carrier proteins | span the bilayer | transport ions and polar molecules across |
| Glycoprotein | carbohydrate on the outer surface | cell recognition / 'identity tag' |
| Cholesterol | wedged between phospholipids | stabilises fluidity, reduces leakiness |
Heads love water, tails hate it. Heads point out to the water; tails hide in the core. Fluid = parts drift sideways; mosaic = many different molecules scattered through the bilayer like tiles.
💨 Simple diffusion & osmosis
Some substances cross without the cell spending any energy — this is passive transport, and the particles move down their concentration gradient (from where there are more of them to where there are fewer), all on their own. Simple diffusion moves small, non-polar molecules (such as oxygen and carbon dioxide) and lipid-soluble molecules (such as steroid hormones) straight through the bilayer — they are not repelled by the hydrophobic core, so they slip across without a protein. Osmosis is the special case for water: the net movement of water across a partially permeable membrane, from a more dilute solution (higher water potential) to a more concentrated one (lower water potential). Aquaporins (water-channel proteins) speed it up, but osmosis stays passive. The steeper the gradient, the faster the diffusion.
Osmosis decides what happens to a cell: in a hypotonic (more dilute) solution water enters and the cell swells; in an isotonic solution there is no net movement; in a hypertonic (more concentrated) solution water leaves and the cell shrinks.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Simple diffusion | Osmosis |
|---|---|---|
| What moves | small / non-polar molecules (O₂, CO₂, steroids) | water only |
| Across what | straight through the phospholipid bilayer | a partially permeable membrane |
| Direction | high → low concentration | dilute → concentrated (high → low water potential) |
| Energy | passive (no ATP) | passive (no ATP); aquaporins speed it up |
Cell mass rising = water entering → the outside is more dilute (hypotonic); cell mass falling = water leaving → the outside is more concentrated (hypertonic); no change = isotonic. And: osmosis is just the diffusion of water — toward the concentrated side, to even things out.
🚪 Facilitated diffusion: channel & carrier proteins
Ions (like Na⁺ and K⁺) and large polar molecules (like glucose) are charged or water-loving, so the hydrophobic core blocks them — they cannot slip through the bilayer like oxygen can. Facilitated diffusion gives them a route: they cross through a transport protein while still moving down the concentration gradient, so no ATP is used — it is passive, just like simple diffusion. The only difference is the route (through a protein, not the bilayer). A channel protein is an open, water-filled pore the particle slips straight through (fast; ions, water via aquaporins). A carrier protein binds the molecule and changes shape to ferry it across (slower; sugars like glucose and fructose).
| Feature | Channel protein | Carrier protein |
|---|---|---|
| How it works | an open hydrophilic pore the particle passes through | binds the molecule, then changes shape to move it across |
| Speed | faster (the route is open) | slower (must change shape each time) |
| Typical passengers | ions (Na⁺, K⁺) and water (aquaporins) | larger molecules (glucose, fructose) |
| Direction & energy | down the gradient · no ATP (passive) | down the gradient · no ATP (passive) |
On a rate-vs-concentration graph, simple diffusion keeps rising in a straight line, but facilitated diffusion levels off — because the transport proteins become saturated (every channel/carrier is occupied), so the membrane is at its maximum rate. Channel = an open channel; carrier = it carries by changing shape — both: down the gradient, no ATP.
⬆️ Active transport & the sodium-potassium pump
Sometimes a cell needs to move a substance the wrong way — against its concentration gradient, from low to high. That cannot happen on its own. Active transport does this: a pump protein uses energy from ATP to force the particle against its gradient. Two words give it away — 'against' the gradient and 'uses ATP'; passive transport does neither. The classic example is the sodium-potassium pump (Na⁺/K⁺ pump). Each cycle uses one ATP to push 3 Na⁺ out of the cell and 2 K⁺ in, both against their gradients. Because ions constantly leak back, the pump must run continuously to maintain the cell's high-K⁺ / low-Na⁺ interior — this is why an ion-concentration table shows steep differences that diffusion alone would erase.
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction | down the gradient (high → low) | against the gradient (low → high) |
| Energy (ATP) | none used | uses ATP |
| Protein | none / channel / carrier | a pump protein |
| Effect on gradients | evens them out | builds and maintains them |
Active = Against + ATP — both 'A' words go together. And for the pump: '3 out, 2 in' — three sodium out, two potassium in, per ATP. No ATP → the pump stops → the gradients slowly equalise.
📥 Selective permeability, modelling & bulk transport
Pulling it together: the membrane is selectively (partially) permeable — it lets some substances cross but not others, mainly by size and polarity. Small non-polar molecules cross freely; large molecules (starch, proteins) cannot cross the bilayer at all. Because real membranes are hard to study, biologists use a model: dialysis (Visking) tubing behaves like a partially permeable membrane. Fill it with a glucose-and-starch mixture, sit it in water, and glucose passes out through the pores (it is small) while starch stays in (it is too large) — exactly how a real membrane absorbs small digested molecules but holds back large ones. Material too big to cross the bilayer is moved in vesicles — bulk transport, which uses ATP. Endocytosis brings material in (the membrane folds inwards and pinches off a vesicle — e.g. a white blood cell engulfing a bacterium); exocytosis sends material out (a vesicle fuses with the membrane — e.g. a gland cell secreting an enzyme).
The four ways substances cross a membrane side by side: simple diffusion (small molecules through the bilayer), facilitated diffusion (ions and large polar molecules through a protein) — both passive, down the gradient — then active transport (a pump works against the gradient) and bulk transport (vesicles), both of which use ATP.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Type of particle | Can it cross? | How it crosses (if it can) |
|---|---|---|
| Small, non-polar (O₂, CO₂) | yes — freely | simple diffusion through the bilayer |
| Water | yes | osmosis, helped by aquaporins |
| Ions & small polar (glucose) | only with help | facilitated diffusion or active transport (proteins) |
| Large molecules (starch, proteins) | not through the bilayer | bulk transport in vesicles (uses ATP) |
Endo = into the cell ('enter'); exo = exit the cell — and both are active (use ATP). In the dialysis-tubing model, always explain the result by molecule size fitting (or not) through the pores — not just 'glucose left and starch stayed'.
✍️ Worked examples
IB-style question — how a bilayer forms
Explain how the amphipathic nature of phospholipids allows them to form the bilayer of a cell membrane. [4]
How to score all four marks:
Define amphipathic. Each phospholipid has a hydrophilic (water-loving) head and hydrophobic (water-hating) tails.
Place the heads. There is water on both sides of the membrane, so the heads face outward toward the water on each surface.
Place the tails. The hydrophobic tails are repelled by water, so they point inward, away from it, and meet in the middle.
State the outcome. This gives two rows of phospholipids — a bilayer — that forms spontaneously, with a hydrophobic core between two hydrophilic surfaces. (1 mark each: amphipathic; heads to the water; tails away from water; bilayer forms.)
Phospholipids are amphipathic; with water on both sides the hydrophilic heads face outward and the hydrophobic tails point inward, so two rows line up spontaneously into a bilayer with a hydrophobic core.
IB-style question — read the net direction of water movement [data]
A cell is placed in a salt solution and its mass steadily decreases over four minutes. State the net direction of water movement and explain what this tells you about the solution. [3]
How to score all three marks:
Read the trend. The cell's mass decreases, so it is losing water — water moves out of the cell.
Name the process. This net movement of water out of the cell is osmosis, down the water-potential gradient.
Explain it. Water moves toward the more concentrated solution, so the outside is more concentrated (hypertonic) — lower water potential than the cell. (Mark 1: water out. Mark 2: by osmosis. Mark 3: outside is more concentrated / hypertonic.)
Net water movement is OUT of the cell (mass falls), by osmosis, because the outside solution is more concentrated (hypertonic / lower water potential) than the cell.
IB-style question — describe facilitated diffusion
Describe how a polar molecule such as glucose crosses the cell membrane by facilitated diffusion. [4]
How to score all four marks:
Why a protein is needed. Glucose is large and polar, so it cannot pass through the hydrophobic core of the bilayer.
Name the route. It crosses through a transport protein — a carrier protein (or channel protein) specific to it.
State the direction. It moves down the concentration gradient, from higher to lower glucose concentration.
State it is passive. No ATP is used, so it is passive transport. (1 mark each: needs a protein because polar/large; through a channel/carrier protein; down the gradient; passive / no ATP.)
Glucose is too large and polar to cross the bilayer, so it passes through a channel/carrier protein, moving down its concentration gradient without using ATP — facilitated diffusion is passive.
IB-style question — maintaining ion gradients
A table shows a nerve cell has much higher potassium and much lower sodium inside than the fluid outside. Explain how the cell maintains these differences. [3]
How to score all three marks:
Name the process. The differences are maintained by active transport, carried out by the sodium-potassium pump in the membrane.
Direction and energy. The pump moves Na⁺ out and K⁺ in, both against their gradients, using energy from ATP.
Why it keeps running. Ions constantly leak back, so the pump runs continuously to replace them and maintain the differences. (Mark 1: active transport / Na⁺/K⁺ pump. Mark 2: against the gradient using ATP. Mark 3: counteracts leakage / runs continuously.)
Active transport by the Na⁺/K⁺ pump moves Na⁺ out and K⁺ in against their gradients using ATP; it runs continuously to replace ions that leak back, so the steep differences are maintained.
IB-style question — reason from dialysis-tubing data [data]
Dialysis tubing filled with glucose and starch is placed in distilled water. After 30 minutes glucose is found in the water but starch is not. Explain how this models the selective permeability of a cell membrane. [4]
How to score all four marks:
Name the property. The tubing is partially (selectively) permeable — it lets some molecules through but not others, like a real membrane.
Explain the glucose. Glucose is small enough to pass through the pores, so it diffuses out into the water (down its gradient).
Explain the starch. Starch is too large to fit through the pores, so it cannot cross and stays inside.
Link to a real membrane. This mirrors a cell / intestinal membrane absorbing small digested products (glucose) while holding back large undigested molecules (starch). (1 mark each: partially permeable; glucose small → out; starch large → in; link to selective absorption.)
The tubing is partially permeable: glucose is small enough to diffuse out through the pores, but starch is too large to pass and stays inside — exactly how a selectively permeable membrane lets small molecules cross while blocking large ones.
✅ Quick self-check
Tap each card to check yourself.
Why does a phospholipid bilayer form, and why is it a 'fluid mosaic'? Phospholipids are amphipathic; with water on both sides the hydrophilic heads face outward and the hydrophobic tails point inward, so two rows form spontaneously. 'Fluid' = parts drift sideways; 'mosaic' = proteins, glycoproteins and cholesterol are scattered through it like tiles.
What's the difference between simple diffusion and osmosis? Simple diffusion is the passive movement of small / non-polar molecules straight through the bilayer, down their concentration gradient. Osmosis is the passive movement of WATER across a partially permeable membrane, from a more dilute solution (higher water potential) to a more concentrated one.
Why do glucose and ions need facilitated diffusion, and is it active or passive? They are large or charged/polar, so the hydrophobic core blocks them — they need a channel or carrier protein. It is still passive: they move down the gradient and use no ATP; the protein only provides a route.
What makes active transport different, and what does the Na⁺/K⁺ pump do? Active transport moves a substance AGAINST its gradient using ATP and a pump protein. The sodium-potassium pump uses one ATP to push 3 Na⁺ out and 2 K⁺ in, running continuously to maintain the cell's high-K⁺ / low-Na⁺ interior.
What does 'selectively permeable' mean, and why does starch stay in dialysis tubing? The membrane lets some substances cross but blocks others, mainly by size and polarity. In the dialysis-tubing model, glucose is small enough to pass through the pores and diffuses out, but starch is too large to fit, so it stays inside.
What is bulk transport, and how do endocytosis and exocytosis differ? Moving large material in vesicles, which uses ATP (active). Endocytosis brings material INTO the cell — the membrane folds inwards and pinches off a vesicle. Exocytosis sends material OUT — a vesicle fuses with the membrane and releases its contents.
Exam Tips
- Trace membrane structure to one fact: phospholipids are amphipathic, so with water on both sides heads go out and tails go in — a bilayer forms spontaneously.
- Hydrophilic = water-loving (heads, outward); hydrophobic = water-hating (tails, inward). Swapping these is the most common error.
- Non-polar / lipid-soluble molecules (O₂, CO₂, steroid hormones) cross the bilayer directly; charged or large ones need a protein.
- For osmosis data: mass rising = water in (hypotonic outside); mass falling = water out (hypertonic outside). Always link direction to which solution is more concentrated.
- For ANY facilitated-diffusion answer, include 'down the concentration gradient' AND 'no ATP / passive' — these are the marks that separate it from active transport.
- A rate graph that LEVELS OFF means the transport proteins are saturated — the classic data clue for facilitated (not simple) diffusion.
- Active = AGAINST the gradient + uses ATP. The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP, running continuously to MAINTAIN the gradients.
- In dialysis-tubing data, explain the result by molecule SIZE fitting (or not) through the pores — don't just restate that glucose left and starch stayed.
- Bulk transport ALWAYS uses ATP — never call endocytosis or exocytosis passive. Endo = into the cell, exo = exit the cell.
Key Idea: The membrane is a phospholipid bilayer — amphipathic phospholipids with hydrophilic heads out and hydrophobic tails in — studded with proteins, glycoproteins and cholesterol: a fluid mosaic (which replaced Davson–Danielli). It is selectively permeable. Substances cross by route and gradient: simple diffusion (small / non-polar, through the bilayer) and osmosis (water, dilute → concentrated) are passive; facilitated diffusion carries ions and large polar molecules through channel or carrier proteins, still down the gradient with no ATP. Going against the gradient needs active transport — a pump protein using ATP (the Na⁺/K⁺ pump: 3 out, 2 in, maintaining gradients). Large cargo too big for the bilayer moves by bulk transport in vesicles (endocytosis in, exocytosis out — both use ATP), modelled neatly by dialysis tubing where small glucose passes out but large starch stays in.