Membrane structure: core ideas
Cell membranes are based on a phospholipid bilayer made from amphipathic lipids: hydrophilic phosphate heads face water, hydrophobic fatty acid tails face inward.
In water, phospholipids self-assemble into a continuous bilayer.
The hydrophobic core acts as a barrier to ions, polar molecules, and large molecules.
Small non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂) can cross directly by simple diffusion.
Membranes separate aqueous environments, helping cells maintain different internal conditions from their surroundings.
This diagram shows the fluid mosaic model: a phospholipid bilayer with integral proteins, peripheral proteins, glycoproteins, glycolipids, and cholesterol. It is useful for labelling overall membrane structure and identifying hydrophilic versus hydrophobic regions. Source
Membrane proteins and membrane carbohydrates
Integral proteins are embedded in one or both layers of the membrane.
Peripheral proteins are attached to the inner or outer surface of the membrane.
Membrane proteins have diverse structures and functions, including transport, receptors, enzymatic roles, and cell recognition.
Glycoproteins = proteins with attached carbohydrate chains.
Glycolipids = lipids with attached carbohydrate chains.
Carbohydrate chains are found on the extracellular side of membranes.
Glycoproteins and glycolipids are important for cell adhesion and cell recognition.
Transport across membranes: simple diffusion, osmosis, facilitated diffusion, active transport
Simple diffusion = passive movement of particles from higher to lower concentration directly through the bilayer.
Example: O₂ and CO₂ moving between phospholipids.
Osmosis = movement of water molecules across a partially permeable membrane.
Osmosis depends on:
random movement of particles
a membrane that is permeable to water but not to solute
a difference in solute concentration across the membrane
Aquaporins are channel proteins that increase the rate of water movement.
Facilitated diffusion uses channel proteins to allow specific particles to move down their concentration gradient.
Channel proteins make membranes selectively permeable because only certain ions or molecules can pass when channels are open.
Active transport uses ATP and pump proteins to move substances against a concentration gradient.
The image shows osmosis across a semipermeable membrane, with water moving toward the side with higher solute concentration. It is helpful for explaining that osmosis is about water potential differences caused by solute concentration differences. Source
Selective permeability
Membranes are selectively permeable, not freely permeable.
Simple diffusion is not selective: it mainly depends on particle size and whether the particle is hydrophobic or hydrophilic.
Facilitated diffusion is selective because channel proteins allow only certain particles through.
Active transport is selective because pump proteins bind and move specific particles.
Therefore, protein composition is a major determinant of what crosses a membrane.
Fluid mosaic model
The membrane is described by the fluid mosaic model.
Fluid = phospholipids and many proteins can move laterally within the membrane.
Mosaic = the membrane contains a mixture of phospholipids, cholesterol, integral proteins, peripheral proteins, glycoproteins, and glycolipids.
In an exam diagram, show:
phospholipid bilayer
integral proteins
peripheral proteins
glycoproteins
cholesterol
hydrophilic heads and hydrophobic tails
Checklist: can you do this?
Draw and label a 2D fluid mosaic model membrane.
Distinguish between simple diffusion, facilitated diffusion, osmosis, and active transport.
Explain why O₂/CO₂ cross easily but ions do not.
Interpret membrane transport questions using concentration gradients and ATP use.
Identify integral proteins, peripheral proteins, glycoproteins, glycolipids, and cholesterol in diagrams.
HL only: membrane fluidity
Membrane fluidity depends partly on the fatty acid composition of phospholipids.
Unsaturated fatty acids have double bonds that create kinks, preventing tight packing and increasing fluidity.
Saturated fatty acids pack more tightly, making membranes less fluid and more stable at higher temperatures.
Cells can adjust membrane composition as an adaptation to habitat.
Cold-adapted organisms commonly have a higher proportion of unsaturated fatty acids to keep membranes functional at low temperature.
HL only: cholesterol in animal membranes
Cholesterol fits between phospholipids in the membrane.
It acts as a fluidity modulator.
At high temperatures, cholesterol helps stabilize membranes and prevents them becoming too fluid.
At low temperatures, cholesterol prevents phospholipids packing too tightly, so the membrane does not become too stiff.
Cholesterol is therefore important in maintaining membrane stability across temperature changes.
HL only: vesicles, endocytosis and exocytosis
Membrane fluidity allows membranes to bend, fuse, and pinch off to form vesicles.
Endocytosis = taking materials into the cell by forming vesicles from the plasma membrane.
Example: uptake of large particles or fluids.
Exocytosis = vesicles fuse with the plasma membrane to release substances out of the cell.
Example: secretion of proteins or neurotransmitters.
Expect exam questions linking membrane fluidity to vesicle formation and membrane fusion.
The image shows the plasma membrane invaginating and pinching off to form a vesicle during endocytosis. It is useful for connecting membrane fluidity with vesicle formation and bulk transport. Source
HL only: gated ion channels and neurons
Gated ion channels open or close in response to a stimulus.
Neurotransmitter-gated ion channels open when a signalling molecule binds.
Example: nicotinic acetylcholine receptor.
Voltage-gated channels open or close in response to changes in membrane potential.
Examples: voltage-gated sodium channels and voltage-gated potassium channels.
These channels are essential for neuronal signalling.
HL only: sodium–potassium pump and membrane potential
The sodium–potassium pump is an exchange transporter.
It uses ATP to move Na⁺ out of the cell and K⁺ into the cell.
This is active transport because ions move against their gradients.
The pump helps generate and maintain membrane potential.
This is especially important in neurons.
This figure compares a uniporter, symporter, and antiporter, making it useful for understanding why the sodium–potassium pump is an exchange transporter (antiporter). Use it to connect ATP-driven pumping with selective transport across membranes. Source
HL only: sodium-dependent glucose cotransport
Sodium-dependent glucose cotransporters are examples of indirect active transport (secondary active transport).
They use the Na⁺ gradient created by the sodium–potassium pump.
As Na⁺ moves down its gradient, glucose is transported with it.
This is important in glucose absorption in the small intestine.
It is also important in glucose reabsorption in the nephron.
Key exam idea: ATP is used indirectly, because the cotransporter depends on the Na⁺ gradient generated by an ATP-powered pump.
It illustrates Na⁺ and glucose entering together through a symporter, powered by the sodium gradient rather than direct ATP hydrolysis at the cotransporter itself. This is a clear visual for secondary active transport. Source
HL only: cell adhesion
Cells form tissues partly by cell adhesion.
Cell-adhesion molecules (CAMs) help cells attach to each other.
Different forms of CAMs are used in different cell–cell junctions.
Detailed names of junction types are not required here.
Focus on the idea that membrane molecules are essential for building and maintaining tissues.
Exam traps and high-yield distinctions
Osmosis is only about water; movement of solutes is not osmosis.
Facilitated diffusion is passive: it uses a protein, but no ATP.
Active transport always needs energy, directly or indirectly.
Simple diffusion does not involve transport proteins.
Selective permeability mainly depends on membrane proteins, not just the phospholipid bilayer.
Glycoproteins/glycolipids are for recognition/adhesion, not bulk ATP production.
Cholesterol is a fluidity regulator, not a phospholipid.
One-minute exam summary
Membranes are phospholipid bilayers with embedded proteins and surface carbohydrates.
The bilayer’s hydrophobic core blocks ions, polar molecules, and large molecules.
Simple diffusion moves small non-polar molecules across the bilayer.
Osmosis is water movement, often via aquaporins.
Facilitated diffusion uses channel proteins; active transport uses ATP-powered pumps.
The fluid mosaic model explains membrane structure.
HL: fluidity depends on fatty acid saturation and cholesterol; membranes form vesicles by endocytosis/exocytosis; gated ion channels, Na⁺/K⁺ pumps, Na⁺-glucose cotransporters, and CAMs add transport and tissue-level detail.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.