IB Biology · Theme B: Form and function · SL and HL
B2.1 Membranes and membrane transport
A one-page summary of B2.1 Membranes and membrane transport, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do molecules of lipid and protein assemble into biological membranes?
What determines whether a substance can pass through a biological membrane?
What B2.1 covers
The lipid bilayer and its proteins
- B2.1.1Lipid bilayers
- B2.1.2The bilayer is a barrier
- B2.1.3Simple diffusion across membranes
- B2.1.4Integral and peripheral proteins
Moving across membranes
- B2.1.5Osmosis
- B2.1.5Aquaporins
- B2.1.6Channel proteins
- B2.1.7Pump proteins
- B2.1.8Selective permeability
The membrane as a whole
- B2.1.9Glycoproteins and glycolipids
- B2.1.10The fluid mosaic model
Fluidity and specialized transport
- B2.1.11 · HLFatty acids and fluidity
- B2.1.12 · HLCholesterol and fluidity
- B2.1.13 · HLEndocytosis
- B2.1.13 · HLExocytosis
- B2.1.14 · HLGated ion channels in neurons
- B2.1.15 · HLThe sodium–potassium pump
- B2.1.16 · HLSodium-dependent glucose cotransport
- B2.1.17 · HLCells adhere to form tissues
B2.1 Membranes and membrane transport: summary
Bilayer
- Amphipathic phospholipids form bilayers in water.
- Hydrophobic core: a barrier to ions, polar and large molecules.
Passive transport
- Simple diffusion: O₂, CO₂ between phospholipids.
- Osmosis (aquaporins speed it up); channels for facilitated diffusion.
Proteins and selectivity
- Integral (embedded) and peripheral (on the surface).
- Channels and pumps are selective; simple diffusion is not.
Surface and model
- Glycoproteins and glycolipids: carbohydrate outside; recognition and adhesion.
- Fluid mosaic model: draw it with all five components.
HL · Fluidity and vesicles
- Unsaturated: fluid when cold; saturated: strong when hot; cholesterol modulates.
- Fluidity allows endocytosis and exocytosis.
HL · Specialized transport
- Gated channels (ACh receptor; voltage-gated Na⁺, K⁺); Na⁺/K⁺ pump: 3 out, 2 in.
- Na⁺-glucose cotransport (gut, nephron); CAMs make tissues.
Key terms
- Amphipathic
- having a hydrophilic part and a hydrophobic part.
- Phospholipid bilayer
- two layers of phospholipids, tails inwards.
- Simple diffusion
- net movement down a gradient, directly through the bilayer.
- Integral protein
- a protein embedded in one or both lipid layers.
- Peripheral protein
- a protein attached to one surface of the membrane.
- Osmosis
- net movement of water to a higher solute concentration.
- Aquaporin
- a channel protein that lets water cross rapidly.
- Facilitated diffusion
- passive movement through a channel protein.
- Active transport
- movement by a pump, using ATP, against a gradient.
- Glycoprotein
- a membrane protein with carbohydrate attached.
- Membrane fluidity
- how freely the lipids and proteins of a membrane move.
- Cholesterol
- a lipid that modulates the fluidity of animal membranes.
- Endocytosis
- taking material in by forming a vesicle from the membrane.
- Exocytosis
- releasing material when a vesicle fuses with the membrane.
- Gated channel
- a channel that opens only in response to a signal.
- Exchange transporter
- a pump that moves two ions in opposite directions.
- Cotransporter
- a protein that moves two substances together, one down its gradient.
- CAM
- cell-adhesion molecule: a membrane protein that binds cells together.
Sample exam questions
Three of the 52 multiple-choice questions for B2.1. Try each one before opening the answer.
Question 1. What does the fluid mosaic model describe?
- A rigid layer of protein around a lipid core
- A bilayer of lipids with proteins scattered in it
- A single layer of phospholipids with heads facing out
- A layer of cellulose with embedded proteins
Show the answer
Answer: B. Proteins are scattered like the tiles of a mosaic in a phospholipid bilayer whose molecules move sideways.
Question 2. A plant cell is placed in a concentrated salt solution. What happens?
- Water enters and the cell bursts open.
- Water leaves and the membrane pulls away.
- Salt enters and the cell swells.
- Nothing happens because of the wall.
Show the answer
Answer: B. Water leaves by osmosis, so the cytoplasm shrinks and the membrane pulls away from the wall: plasmolysis.
Question 3. Why is a phospholipid described as amphipathic?
- It has two fatty acid tails.
- It contains both phosphorus and nitrogen.
- It can form ionic bonds.
- It has hydrophilic and hydrophobic parts.
Show the answer
Answer: D. A phospholipid has a hydrophilic phosphate head and two hydrophobic hydrocarbon tails.
Linking questions
Questions that connect B2.1 to other parts of the course, the kind that come up in Paper 2.
- Phospholipids are the structural foundation of cell membranes. How do the properties of phospholipids, introduced in B1.1, enable the spontaneous formation of bilayers in aqueous environments? (see B1.1)
- Membrane proteins are synthesised by ribosomes on the rough endoplasmic reticulum. How does the endomembrane system (B2.2) deliver these proteins to their correct membrane destinations? (see B2.2)
- The sodium-potassium pump maintains resting membrane potential, which is essential for nerve impulse transmission (C2.2). Explain how the pump's unequal transport ratio (3 Na⁺ out : 2 K⁺ in) contributes to the inside-negative membrane potential. (see C2.2)
- Water potential in plant tissues (D2.3) depends on both solute concentration and pressure. How does osmosis, driven by differences in water potential, explain the movement of water from soil into root hair cells and then through the root to the xylem? (see D2.3)
Practise B2.1
Study notes, every question and full markschemes for B2.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.