How does the structure of the membrane determine which substances can cross it?
Topic 2.5 Membrane Permeability: explain how the structure of biological membranes influences selective permeability.
A focused answer to AP Biology Topic 2.5, covering selective permeability, why the phospholipid bilayer blocks polar and charged substances, the factors affecting permeability, and the role of transport proteins.
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What this topic is asking
The College Board (Topic 2.5) wants you to explain how the structure of the membrane gives rise to selective permeability: which substances cross freely, which need help, and why. The key is the hydrophobic core of the phospholipid bilayer.
What selective permeability means
The middle of the bilayer is made of nonpolar fatty-acid tails, so it repels polar and charged substances. This single structural fact explains most permeability patterns.
Who crosses and who does not
Factors affecting permeability
Permeability depends on both the substance and the membrane:
- Size: smaller molecules cross more readily.
- Polarity and charge: nonpolar crosses easily; polar slowly; charged not at all without proteins.
- Lipid composition: more unsaturated fatty acids increase fluidity and permeability; cholesterol buffers it.
- Temperature: higher temperature increases fluidity and the rate of diffusion, up to the point where the membrane breaks down.
Permeability and homeostasis
Selective permeability is what makes a controlled internal environment possible. If the membrane let everything through, the cell could not maintain concentrations of ions, nutrients and signalling molecules different from its surroundings, and homeostasis would be impossible. By admitting some substances freely (such as oxygen and carbon dioxide for respiration) while restricting others (ions, glucose) to regulated protein-mediated routes, the cell controls its composition. This selectivity underlies the resting membrane potential in nerve cells, the maintenance of ion gradients, and the uptake of specific nutrients. Permeability is therefore not a fixed property but a regulated one: cells adjust the proteins in their membranes and the lipid composition to change what crosses and how fast, tuning permeability to their needs and conditions.
How the membrane itself tunes permeability
Permeability is not only about the substance; the membrane's own composition sets how easily anything crosses, and the College Board expects you to connect fluidity to permeability. The bilayer behaves like a two-dimensional fluid, and its fluidity depends on its lipids. Unsaturated fatty-acid tails contain double bonds that put kinks in the chain, preventing the tails from packing tightly, so a membrane rich in unsaturated lipids is more fluid and more permeable. Saturated tails are straight, pack closely, and make a stiffer, less permeable membrane.
Cholesterol acts as a fluidity buffer: at high temperature it restrains phospholipid movement, reducing fluidity and permeability; at low temperature it spaces the phospholipids apart and stops them packing into a rigid gel. Temperature itself raises fluidity as it increases, speeding diffusion, but past a point the bilayer loses integrity and becomes leaky. Many organisms adjust the proportion of unsaturated lipids when the temperature changes, keeping permeability within a useful range.
Why proteins are needed
Because the bilayer blocks polar and charged substances, cells embed transport proteins (channels and carriers) that provide a hydrophilic path or binding site, letting specific substances cross while preserving the barrier. This sets up the next topics: passive transport, facilitated diffusion and active transport.
Try this
Q1. Identify which of the following crosses the bilayer most easily and explain: oxygen, glucose, or a sodium ion. [2 points]
- Cue. Oxygen; it is small and nonpolar, so it dissolves in and diffuses through the hydrophobic core, while glucose (large polar) and sodium (charged) cannot.
Q2. Explain why increasing the proportion of unsaturated fatty acids tends to increase membrane permeability. [2 points]
- Cue. Unsaturated tails have kinks that prevent tight packing, raising fluidity, so substances diffuse across more readily.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 20204 marksSection II (short FRQ). Oxygen and carbon dioxide cross the plasma membrane easily, but glucose and sodium ions do not cross the bilayer without help. Explain these observations using the structure of the membrane.Show worked answer →
A 4-point concept-explanation FRQ on selective permeability.
Point 1 (bilayer core): the interior of the bilayer is hydrophobic (nonpolar fatty-acid tails).
Point 2 (small nonpolar pass): oxygen and carbon dioxide are small and nonpolar, so they dissolve in and diffuse through the hydrophobic core freely.
Point 3 (polar blocked): glucose is large and polar, so it cannot pass through the hydrophobic core.
Point 4 (charged blocked): sodium ions are charged, so they are repelled by the hydrophobic interior and require transport proteins to cross.
Markers reward linking each substance's size and polarity to its ability (or inability) to cross the hydrophobic core.
AP 20233 marksSection I-style data question rewritten as a short FRQ. The permeability of an artificial bilayer (relative units) was: O2 = 100, water = 30, glucose = 0.5, Na+ = 0.001. (a) Calculate how many times more permeable the bilayer is to oxygen than to glucose. (b) Explain the trend in the data.Show worked answer →
A 3-point quantitative and concept FRQ.
(a) Calculate (1 point): times more permeable to oxygen than to glucose.
(b) Explain (1 point): permeability falls as molecules become larger, more polar or charged, because the hydrophobic core blocks polar and charged substances; (1 point) oxygen (small, nonpolar) crosses easily, water (small, polar) crosses slowly, and glucose and especially sodium ions are blocked.
Markers reward the correct factor and a trend explanation based on size and polarity relative to the hydrophobic core.
Related dot points
- Topic 2.4 Plasma Membranes: describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.
A focused answer to AP Biology Topic 2.4, covering the fluid-mosaic model, the phospholipid bilayer, membrane proteins, cholesterol and carbohydrates, and how each component maintains the cell's internal environment.
- Topic 2.6 Membrane Transport: describe the mechanisms that organisms use to transport large and small molecules across the membrane and the energy requirements of passive and active transport.
A focused answer to AP Biology Topic 2.6, covering passive transport (diffusion and osmosis) versus active transport, the role of concentration gradients and ATP, and bulk transport by endocytosis and exocytosis.
- Topic 2.7 Facilitated Diffusion: explain how the structure of channel and carrier proteins allows the facilitated diffusion of polar molecules and ions across a membrane.
A focused answer to AP Biology Topic 2.7, covering facilitated diffusion through channel and carrier proteins, aquaporins, why it is passive, and how it differs from simple diffusion and active transport.
- Topic 2.9 Mechanisms of Transport: explain how active transport and bulk transport move ions and large molecules across membranes and establish electrochemical gradients.
A focused answer to AP Biology Topic 2.9, covering active transport, the sodium-potassium pump, electrochemical gradients, secondary active transport, and bulk transport by endocytosis and exocytosis.
- Topic 2.8 Tonicity and Osmoregulation: explain how concentration gradients of water and solutes affect the movement of water into and out of cells, and how organisms regulate their water balance.
A focused answer to AP Biology Topic 2.8, covering hypotonic, hypertonic and isotonic solutions, osmosis, water potential, and how cells and organisms osmoregulate, with full worked water-potential calculations.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)