B2.1 Membranes and membrane transport | IB DP Biology

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Every cell is separated from its surroundings by a membrane about seven nanometres thick, and almost everything that enters or leaves has to get past it. This topic answers two questions: how lipids and proteins assemble into a membrane, and what decides whether a particular substance can cross it. The answer to the second is nearly always the same — size, charge, and whether there is a protein to help.

🎯What you need to be able to do

Explain why lipid bilayers form and why they are barriers to ions and polar molecules.
Explain simple diffusion, osmosis (and the role of aquaporins), facilitated diffusion through channels, and active transport by pumps.
Distinguish integral and peripheral proteins, and explain what makes membrane permeability selective.
Outline the structure and roles of glycoproteins and glycolipids.
Draw and label the fluid mosaic model, showing hydrophobic and hydrophilic regions.
AHL Relate fatty acid composition and cholesterol to membrane fluidity, and explain endocytosis and exocytosis.
AHL Explain gated ion channels, the sodium–potassium pump, sodium-dependent glucose cotransporters, and cell adhesion molecules.

📚The biology

Lipid bilayers

Phospholipids are amphipathic, with a hydrophilic phosphate head and hydrophobic fatty acid tails (B1.1). In water they naturally form continuous sheet-like bilayers: tails inward, heads outward. Because any exposed edge would bring tails into contact with water, bilayers tend to close up on themselves into continuous surfaces, and they reseal if punctured. This lipid bilayer is the basic structure of every cell membrane.

The bilayer as a barrier

The core of the membrane consists of hydrophobic hydrocarbon chains. Particles that are hydrophilic — ions and polar molecules — cannot easily dissolve in this core, and large molecules cannot fit between the phospholipids. The membrane therefore has low permeability to them, and acts as an effective barrier between two aqueous solutions: the cytoplasm and the fluid outside.

Simple diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It needs no energy from the cell.

In simple diffusion, particles pass directly between the phospholipids. Only small and non-polar particles can do this. Oxygen and carbon dioxide are the standard examples: oxygen diffuses into a cell because it is used in respiration, keeping its concentration inside low; carbon dioxide diffuses out because respiration keeps its concentration inside high.

Membrane proteins

Membranes contain proteins with a great diversity of structures, locations and functions: transport, receptors, enzymes, cell recognition and adhesion. By location they are of two types:

Integral proteins
Embedded in one or both layers of the bilayer; many span it completely (transmembrane). They have hydrophobic regions in contact with the tails. Channels and pumps are integral.

Peripheral proteins
Attached to one surface of the bilayer, often bound to integral proteins or to phospholipid heads. They do not enter the hydrophobic core, and can be removed more easily.

Osmosis and aquaporins

Osmosis is the net movement of water across a partially permeable membrane from a solution of lower solute concentration to one of higher solute concentration. The explanation:

water molecules move randomly and constantly cross the membrane in both directions;
the membrane is impermeable to the solutes, so they cannot move to even out the difference;
solute particles attract water molecules and reduce the number free to move, so where the solute concentration is higher, fewer water molecules cross the membrane per second; more cross from the side with lower solute concentration, giving a net flow in that direction.

Water is polar, and some does squeeze between phospholipids, but only slowly. Many cells have aquaporins, channel proteins that allow water molecules through in single file very rapidly, while blocking ions. Cells that must move large volumes of water — in the kidney collecting ducts, or in plant roots — have many (D3.3). Osmosis in cells is treated in detail in D2.3.

Facilitated diffusion through channel proteins

Facilitated diffusion is diffusion through a membrane protein, down the concentration gradient, without energy from the cell. Channel proteins are integral proteins with a pore through the middle. The pore’s diameter and the charges lining it mean that only one specific type of ion can pass — a potassium channel lets K⁺ through but not Na⁺. Many channels can open and close, so the membrane is permeable to that ion only when the channel is open. This makes the membrane selectively permeable.

Active transport by pump proteins

Active transport moves particles across a membrane against a concentration gradient, from low to high concentration. This needs energy, supplied by ATP. Pump proteins bind a specific particle on one side, use energy from ATP hydrolysis to change shape, and release the particle on the other side. Because each pump is specific, active transport is selective too. Examples: uptake of mineral ions by root hair cells, and the sodium–potassium pump in neurons.

What makes permeability selective

Simple diffusion — not selective
Depends only on the size of the particle and whether it is hydrophobic or hydrophilic. Anything small and non-polar gets through; the cell cannot choose.

Facilitated diffusion and active transport — selective
Depend on specific proteins. The cell controls which substances cross by which channels and pumps it makes, and by opening or closing them.

Glycoproteins and glycolipids

Glycoproteins are membrane proteins with carbohydrate chains attached; glycolipids are lipids with carbohydrate chains attached. In both cases the carbohydrate is always on the extracellular side of the plasma membrane, forming a sugary coat on the cell surface (the glycocalyx). They have two main roles:

Cell recognition — the carbohydrate chains act as markers identifying the cell type and marking it as “self”, as with the ABO antigens (B1.1), and allowing the immune system to recognize foreign cells.
Cell adhesion — binding to neighbouring cells or to the extracellular matrix.

The fluid mosaic model

The fluid mosaic model describes the membrane as a fluid phospholipid bilayer in which proteins are scattered like the pieces of a mosaic. “Fluid” because phospholipids and many proteins can move sideways within their layer.

When you draw it in two dimensions, include:

a phospholipid bilayer, heads (circles) facing out on both sides, tails (two lines each) facing in;
integral proteins passing into or through the bilayer, and peripheral proteins on one surface;
a glycoprotein with a branched carbohydrate chain on the outer surface;
cholesterol molecules between the phospholipid tails;
labels indicating the hydrophilic regions (heads, and protein surfaces facing water) and the hydrophobic region (the core of tails).

Fatty acids and fluidity AHL

A membrane must be fluid enough to be flexible, but not so fluid that it loses its strength. The fatty acid composition of the phospholipids controls this:

Unsaturated fatty acids have kinked tails that do not pack closely and have lower melting points, so they keep the membrane fluid and flexible at the temperatures the cell experiences.
Saturated fatty acids pack closely and have higher melting points, so they make the membrane stronger and less fluid, especially at higher temperatures.

Organisms adapt membrane composition to their habitat. Animals living in cold environments — for example fish of polar seas, or the legs of reindeer which are much colder than the body core — have a higher proportion of unsaturated fatty acids in their membranes, which keeps them fluid in the cold. Organisms from hot environments tend to have more saturated fatty acids.

Cholesterol and fluidity AHL

In animal cell membranes, cholesterol molecules sit between the phospholipids, with the small hydrophilic –OH group next to the phosphate heads and the rigid ring structure among the fatty acid tails. Cholesterol acts as a modulator of fluidity, working in both directions:

At higher temperatures
it restricts the movement of the fatty acid tails, reducing fluidity and stabilizing the membrane.

At lower temperatures
it prevents the tails packing tightly together, preventing stiffening.

Vesicles: endocytosis and exocytosis AHL

Because the membrane is fluid, it can bend, pinch off and fuse. This allows large quantities of material to be moved in or out of the cell in vesicles, using energy from ATP.

Endocytosis
Part of the plasma membrane folds inwards around material outside the cell and pinches off as a vesicle inside. Examples: phagocytosis of bacteria by white blood cells (C3.2); uptake of cholesterol-carrying particles by body cells.

Exocytosis
A vesicle moves to the plasma membrane and fuses with it, releasing its contents outside. Examples: secretion of digestive enzymes from pancreatic cells; release of neurotransmitters at synapses (C2.2).

In both, the total area of membrane is conserved: membrane taken in by endocytosis is balanced by membrane added by exocytosis.

Gated ion channels in neurons AHL

Gated channels open or close in response to a stimulus.

Neurotransmitter-gated channels open when a specific neurotransmitter binds. The nicotinic acetylcholine receptor is a channel that opens when acetylcholine binds, letting positive ions (mainly Na⁺) into the cell.
Voltage-gated channels open or close in response to changes in the voltage across the membrane. Voltage-gated sodium and potassium channels produce the action potential (C2.2).

The sodium–potassium pump AHL

The sodium–potassium pump is an exchange transporter: it moves two different ions in opposite directions, both against their concentration gradients. Each cycle:

Three Na⁺ ions from the cytoplasm bind to the pump.
ATP is hydrolysed and a phosphate group attaches to the pump, changing its shape so that it opens to the outside and releases the 3 Na⁺ outside.
Two K⁺ ions from outside bind.
The phosphate is released, the pump returns to its original shape, and the 2 K⁺ are released inside.

Because three positive charges go out for every two that come in, the pump makes the inside of the cell more negative. The concentration gradients it establishes are the basis of the membrane potential of neurons (C2.2).

Sodium-dependent glucose cotransporters AHL

A cotransporter moves two substances together. The sodium-dependent glucose cotransporter carries a Na⁺ ion and a glucose molecule into the cell at the same time. Sodium moves down its concentration gradient, and the energy of that movement is used to carry glucose against its gradient.

This is indirect active transport: the cotransporter uses no ATP itself, but the sodium gradient it depends on is maintained by sodium–potassium pumps, which do use ATP. It is used where glucose must be absorbed completely even when its concentration outside is low:

in absorption of glucose from digested food by epithelial cells of the small intestine;
in reabsorption of glucose from the filtrate by cells of the proximal convoluted tubule in the nephron, so that no glucose is lost in urine (D3.3).

Cell adhesion AHL

In multicellular organisms, cells stick together to form tissues. This depends on cell-adhesion molecules (CAMs), proteins in the plasma membrane that bind to CAMs on neighbouring cells or to the extracellular matrix. Different forms of CAM are used for different types of junction between cells — some seal cells tightly together, others anchor them strongly, others allow communication between them.

✏️Worked example

(a) In an electron micrograph at a magnification of ×500 000, a plasma membrane appears as two dark lines separated by a light band. The total width of the image is 3.5 mm. Calculate the actual thickness of the membrane in nanometres.
(b) Predict, with reasons, which of the following cross a phospholipid bilayer by simple diffusion: oxygen, sodium ions, glucose, steroid hormones.
(c) AHL A neuron’s sodium–potassium pumps hydrolyse 150 ATP molecules in a given time. Calculate the numbers of Na⁺ and K⁺ ions moved, and the net number of positive charges moved out of the cell.

(a) Convert to nanometres: 3.5 mm = 3.5 × 10⁶ nm.

\[ \text{actual size} = \frac{3.5 \times 10^{6}\ \mathrm{nm}}{500\,000} = 7\ \mathrm{nm} \]

(b)

Oxygen — yes. Small and non-polar, so it dissolves in the hydrophobic core.
Sodium ions — no. Charged, so they cannot pass through the hydrophobic core; they need channels or pumps.
Glucose — no (or only negligibly). Polar and relatively large; it crosses by facilitated diffusion or cotransport.
Steroid hormones — yes. Although fairly large, they are non-polar lipids and dissolve in the bilayer.

(c) Each ATP moves 3 Na⁺ out and 2 K⁺ in:

\[ \mathrm{Na^{+}}: 150 \times 3 = 450\ \text{out} \qquad \mathrm{K^{+}}: 150 \times 2 = 300\ \text{in} \]

Net positive charges moved out: 450 − 300 = 150 — one for every ATP used. That net outward movement of positive charge helps make the inside of the cell negative.

Check it. Biological membranes are about 7–10 nm thick, so 7 nm is right; an answer in micrometres or millimetres signals a unit error. In (b), the pattern should be: non-polar crosses, charged and polar do not — size alone does not decide it, as the steroid shows.

“Small molecules diffuse through the membrane.” This is only half the rule, and it gives the wrong answer for ions, which are tiny but cannot cross. The deciding property is polarity: the core of the membrane is hydrophobic. Always mention both size and polarity when you explain simple diffusion.

📝Practise

Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.

1. Distinguish between simple diffusion, facilitated diffusion and active transport.

Simple diffusion: net movement down a concentration gradient, directly between phospholipids, no membrane protein, no ATP; only for small, non-polar particles such as O₂ and CO₂. Facilitated diffusion: net movement down a concentration gradient through a specific membrane protein (such as a channel), no ATP; allows ions and polar molecules to cross. Active transport: movement against a concentration gradient through a specific pump protein, requiring energy from ATP.

2. Explain why the phospholipid bilayer is an effective barrier to ions.

The core of the bilayer is made of the hydrophobic hydrocarbon tails of phospholipids. Ions are charged and therefore hydrophilic: they are surrounded by water molecules and cannot dissolve in, or pass through, a non-polar region. The membrane therefore has very low permeability to ions, which can only cross where there are specific channel or pump proteins.

3. Explain, in terms of particle movement, why water enters a cell placed in a solution with a lower solute concentration than its cytoplasm.

Water molecules move randomly and cross the membrane in both directions. The membrane is impermeable to the solutes, which cannot move to even out the concentration difference. Solute particles attract water molecules, so where the solute concentration is higher (in the cytoplasm) fewer water molecules are free to cross per unit time. More water molecules therefore cross from the outside solution (lower solute concentration) into the cell than cross out, giving a net movement of water into the cell by osmosis, speeded up by aquaporins if present.

4. Describe how to draw the fluid mosaic model, and state where carbohydrates are found in it.

Draw a phospholipid bilayer: two rows of heads (circles) facing outwards, with the tails pointing inwards. Show integral proteins embedded in or spanning the bilayer, and peripheral proteins attached to one surface. Show cholesterol between phospholipid tails. Show a glycoprotein with a carbohydrate chain. Label the hydrophilic heads and the hydrophobic core. Carbohydrates (in glycoproteins and glycolipids) are found only on the extracellular (outer) side of the membrane.

5. AHL Explain how cholesterol affects membrane fluidity at high and at low temperatures.

Cholesterol molecules are positioned between phospholipids, with their –OH group near the heads and their rigid rings among the fatty acid tails. At high temperatures they restrict the movement of the tails, reducing fluidity and stabilizing the membrane so it does not become too fluid. At low temperatures they prevent the tails packing closely together, stopping the membrane from becoming too rigid. Cholesterol therefore modulates fluidity, keeping it within a suitable range.

6. AHL Explain how glucose is absorbed into epithelial cells of the small intestine even when the concentration of glucose inside the cells is higher than in the gut.

Absorption uses sodium-dependent glucose cotransporters. Sodium–potassium pumps on the other side of the epithelial cell use ATP to pump Na⁺ out, keeping the Na⁺ concentration inside low. Na⁺ therefore diffuses down its concentration gradient from the gut into the cell through the cotransporter, and the cotransporter uses this movement to carry glucose into the cell against its concentration gradient. This is indirect active transport: ATP is used by the pump, not by the cotransporter itself.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

PhET Interactive Simulations (University of Colorado) — Membrane Channels, showing how channel proteins allow facilitated diffusion.
RCSB Protein Data Bank, Molecule of the Month — the sodium–potassium pump and aquaporins, with 3D structures.
HHMI BioInteractive — animations of membrane transport, including cotransport in the intestine.