Right now, every cell in your body is surrounded by a membrane less than 10 nm thick — deciding, molecule by molecule, what crosses and what doesn't. This lesson explains how.
When onion skin cells are mounted in distilled water vs. salt solution, something dramatic happens to the plasma membrane — visible under a microscope.
Membrane pulls away from the cell wall in hypertonic solution — direct evidence of a selectively permeable membrane.
Watch them fall, react to water, and arrange themselves. The structure that forms depends on how many molecules there are.
With only 5 molecules, they form a monolayer at the water surface — hydrophilic heads anchor in the water, hydrophobic tails splay upward into the air.
Proteins, cholesterol, and carbohydrates are embedded in and on the bilayer — described by the Fluid Mosaic Model (Singer & Nicolson, 1972).
The Fluid Mosaic Model — integral proteins, channel proteins, cholesterol, glycoproteins, and peripheral proteins embedded in the phospholipid bilayer.
Channel & carrier proteins move ions and polar molecules (facilitated diffusion & active transport).
Glycoproteins act as molecular ID tags — how immune cells identify "self" vs. "foreign."
Anchoring enzymes to membranes localises and coordinates metabolic pathways.
Bind hormones and chemical signals; relay messages into the cell (signal transduction).
Connect adjacent cells together (tight junctions, gap junctions, desmosomes).
Link membrane to the cytoskeleton and extracellular matrix for structural support.
Cholesterol is amphipathic — its OH group is hydrophilic; its rigid steroid rings and hydrocarbon tail are hydrophobic. It wedges between phospholipids.
Its key function is to buffer fluidity — preventing the membrane from becoming too rigid in the cold or too loose in heat.
| Condition | Effect | Mechanism |
|---|---|---|
| Cold temperature | ↑ Increases fluidity | Disrupts tight packing; prevents membrane from solidifying. |
| High temperature | ↓ Decreases fluidity | Restrains phospholipid movement; prevents excessive looseness. |
Cholesterol is amphipathic: OH group anchors near the bilayer surface; rigid rings stabilise neighbouring tails
The degree of saturation of fatty acid tails directly affects how tightly they pack — and therefore how fluid the membrane is. Explore it below.
Saturated tails pack tightly in straight rows — the membrane is relatively viscous.
The Channichthyidae live in the Southern Ocean near Antarctica — near-freezing water, year-round. What would you expect in the lipid composition of their membranes?
What would you most likely find in a crocodile ice fish membrane — compared to a warm-water fish?
More unsaturated (kinked) tails. At near-freezing temperatures, saturated tails would pack so tightly the membrane solidifies. Kinked unsaturated tails prevent tight packing, keeping the membrane fluid.
Same chemistry as why olive oil (unsaturated) stays liquid in the fridge while butter (saturated) goes solid.
IB exam tip: A question asking you to explain this adaptation needs both the mechanism (kinks prevent packing) and the consequence (membrane stays fluid at low temperature). A question asking you to predict only needs the outcome.
The hydrophobic core is the barrier. The rule: like dissolves like. Nonpolar molecules slip through; polar and charged molecules need help.
Small nonpolar molecules cross freely. Water crosses slowly. Large polar molecules and ions need transport proteins or cannot cross.
IB Biology Paper 2 questions are marked using command terms. Read the prompt carefully — the command term tells you exactly how much depth is needed. Write your answer, then reveal the mark scheme.
The Channichthyidae (crocodile icefish) live year-round in the Southern Ocean at temperatures of approximately −2°C to 2°C. Researchers found that their cell membranes have an unusually high proportion of unsaturated fatty acids compared to tropical fish at 28°C.
OUTLINE = brief account of the main points. Award 1 mark per point, max 3.
EXPLAIN = give a reason + mechanism. Each mark requires both what and why.
PREDICT = give an expected result. JUSTIFY = give valid reasons. One mark each.
STATE = give a specific fact. No explanation needed.
A scientist treats a cell with a drug that destroys all glycoproteins on the plasma membrane. She then exposes the treated cell to immune cells from the same organism.
EXPLAIN = reason + mechanism required. Award 1 mark per valid explained point, max 6. Accept any six from the following.
Accept: intrinsic = integral; extrinsic = peripheral. Do not accept answers that only name a protein type without explaining its function.
Fill in the blanks. Stuck? Tap Reveal answers.
The plasma membrane is described by the mosaic model. Phospholipids are — their heads are hydrophilic and tails are hydrophobic. buffers fluidity across temperatures. Small molecules cross the membrane freely, while ions require to pass through.
The bilayer forms spontaneously — no energy required. Hydrophobic interactions drive the tails inward; heads face the water on both sides.
"Fluid" = phospholipids and proteins drift laterally. "Mosaic" = diverse embedded proteins scattered like tiles. Proposed by Singer & Nicolson, 1972.
Amphipathic steroid that buffers membrane fluidity — increases it at cold temperatures (prevents freezing), decreases it at high temperatures (prevents excess looseness).
Small nonpolar molecules cross freely. Polar molecules cross slowly. Ions & large polar molecules need channel or carrier proteins. The membrane is selective, not impermeable.
"The membrane is not a wall. It's a living, fluid, selective boundary that every cell depends on."