AHL: Membrane structure and transport
IB Biology HLΒ· B2.6 Membranes (AHL)Β· 25 min read
1. Membrane Structure: The Fluid Mosaic Modelβ β ββββ± 8 min
Fluid Mosaic Model
The currently accepted model of cell membrane structure, describing a phospholipid bilayer with scattered, embedded and associated proteins that can move laterally within the layer.
Example:
The term 'fluid' refers to lateral movement of components, while 'mosaic' refers to the mixed arrangement of lipids, proteins and carbohydrates.
All biological membranes share the same core structure: a double layer of phospholipids with hydrophobic tails oriented inward (away from water) and hydrophilic heads oriented outward toward the aqueous environment on both sides of the membrane. The amphipathic nature of phospholipids drives this spontaneous arrangement.
Explain why phospholipids naturally form bilayers in aqueous environments.
- 1
Phospholipids are amphipathic, meaning they have two chemically distinct regions.
- 2
The polar phosphate head is hydrophilic (water-loving) and interacts favourably with water on both sides of the membrane.
- 3
The non-polar fatty acid tails are hydrophobic (water-fearing) and avoid interaction with water by clustering together in the interior of the bilayer.
- 4
This arrangement is the most energetically stable, so bilayers form spontaneously.
2. Membrane Components and Their Functionsβ β ββββ± 7 min
Most of the specific functions of membranes are carried out by proteins and carbohydrates embedded in or bound to the phospholipid bilayer. Proteins are classified based on their position relative to the bilayer.
Integral Membrane Proteins
Proteins embedded within the phospholipid bilayer, with hydrophobic regions that interact with the hydrophobic core of the bilayer. Transmembrane integral proteins span the entire bilayer, with domains exposed on both sides.
Example:
Ion channels and hormone receptors are common transmembrane integral proteins.
Transport of specific substances across the membrane
Enzymatic catalysis of reactions at the membrane surface
Cell-cell recognition via attached carbohydrate groups
Signal transduction of extracellular signals to the cell interior
Intercellular joining between adjacent cells
Attachment to the cytoskeleton and extracellular matrix
A researcher labels membrane proteins with a large hydrophilic fluorescent marker that cannot cross the phospholipid bilayer. The marker binds to the test protein when applied to both the outside and inside of the cell. What classification is this protein? Explain.
- 1
Recall that membrane proteins are classified by their position relative to the phospholipid bilayer.
- 2
The marker can bind to the protein on both sides of the membrane, meaning the protein is exposed to both the extracellular and intracellular environments.
- 3
Only transmembrane integral proteins span the entire bilayer, exposing regions to both sides of the membrane.
- 4
Conclusion: This is a transmembrane integral membrane protein.
3. Passive Transport Across Membranesβ β β βββ± 8 min
Passive transport is the movement of substances across a membrane down their concentration gradient, so it does not require input of cellular energy (ATP). The three main types are simple diffusion, facilitated diffusion, and osmosis.
Osmosis
The passive net movement of water molecules across a selectively permeable membrane, from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential).
Water is polar, so it diffuses slowly through the hydrophobic core of the phospholipid bilayer. Most rapid water movement across membranes occurs through specialized channel proteins called aquaporins.
Predict the effect of placing a fully turgid plant cell into a 1M sucrose solution, and explain the outcome.
- 1
A 1M sucrose solution has a much higher solute concentration (lower water potential) than the plant cell cytoplasm.
- 2
By osmosis, net water movement will be out of the cell, from higher water potential (cytoplasm) to lower water potential (external solution).
- 3
As water leaves, the volume of the cytoplasm decreases. The rigid plant cell wall does not shrink, so the cell membrane pulls away from the cell wall.
- 4
This outcome is called plasmolysis; the cell is described as plasmolysed.
Test your understanding of tonicity:
What happens to an animal cell placed in a hypotonic solution?
A: Shrivel
B: Lyse (burst)
C: Stay the same size
D: Become turgid
Reveal answer
B βAnimal cells do not have a rigid cell wall. In a hypotonic solution, water enters the cell by osmosis, causing it to swell and eventually burst. Plant cells become turgid in hypotonic conditions, thanks to their cell wall.
4. Active Transport and Bulk Transportβ β β βββ± 10 min
Active transport moves substances across the membrane against their concentration gradient, from an area of low concentration to an area of high concentration. This process is not spontaneous, so it requires energy input, usually from ATP, and specific transporter proteins.
Bulk transport is used for large molecules (like proteins) or large volumes of substance that cannot cross the membrane via transporter proteins. It occurs via formation of membrane vesicles, and also requires energy. Endocytosis brings substances into the cell, while exocytosis releases substances out of the cell.
The sodium-potassium pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell, against their respective concentration gradients. Explain why this process requires ATP.
- 1
Moving any substance against its concentration gradient requires an input of free energy, as it opposes the spontaneous direction of diffusion.
- 2
ATP donates a phosphate group to the pump protein, causing a conformational change that moves the bound ions across the membrane.
- 3
After releasing the ions on the opposite side of the membrane, the phosphate group is released, and the pump returns to its original conformation to bind new ions.
- 4
Without ATP, the required conformational change cannot occur, so the pump cannot function to move ions against their gradient.
5. Common Pitfalls
Wrong move:
Stating that cholesterol always increases membrane fluidity
Why:
Cholesterol has a dual, temperature-dependent effect on fluidity, not a single universal effect
Correct move:
Explain that cholesterol reduces fluidity at high temperatures and increases fluidity at low temperatures, maintaining membrane stability.
Wrong move:
Defining osmosis as movement of solute across a membrane
Why:
Osmosis specifically describes net movement of water, not solute
Correct move:
Define osmosis as net movement of water across a selectively permeable membrane down a water potential gradient.
Wrong move:
Claiming all cell membranes contain cholesterol
Why:
Cholesterol is only present in animal cell membranes, not plant or prokaryotic membranes
Correct move:
Specify that cholesterol is a component of animal cell membranes that regulates fluidity.
Wrong move:
Thinking endocytosis is active only because it moves substances against their gradient
Why:
Endocytosis is classified as active because it requires ATP energy, not because of the cargo concentration gradient
Correct move:
Classify bulk transport (endocytosis/exocytosis) as active transport because it requires energy input for vesicle formation and movement.
Wrong move:
Confusing facilitated diffusion with active transport
Why:
Both use membrane proteins, but facilitated diffusion moves down the gradient and does not need energy
Correct move:
Remember: facilitated diffusion is passive, only active transport requires energy to move against the gradient.
6. Quick Reference Cheatsheet
Component/Process | Key Feature | Energy Required |
|---|---|---|
Fluid mosaic model | Phospholipid bilayer with mobile proteins | N/A |
Integral protein | Embedded in hydrophobic bilayer core | N/A |
Peripheral protein | Bound to membrane surface | N/A |
Simple diffusion | Small non-polar molecules down gradient | No |
Facilitated diffusion | Polar/charged molecules down gradient via proteins | No |
Osmosis | Net water movement down water potential gradient | No |
Primary active transport | Molecules against gradient via pumps | Yes (ATP) |
Endocytosis | Bulk transport into cell via vesicles | Yes |
Exocytosis | Bulk transport out of cell via vesicles | Yes |
7. Frequently Asked
What is the difference between integral and peripheral membrane proteins?
Integral proteins are embedded within the phospholipid bilayer (often spanning the entire membrane as transmembrane proteins), while peripheral proteins are only bound to the surface of the membrane, not penetrating the hydrophobic core.
Why is cholesterol important in animal cell membranes?
Cholesterol modulates membrane fluidity across temperature ranges: it reduces excess fluidity at high temperatures by restricting phospholipid movement, and prevents solidification at low temperatures by disrupting tight packing of phospholipid tails.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· Paper 1
Membrane protein function multiple choice
- 2024 Β· Paper 2
Osmosis experiment data analysis
- 2023 Β· Paper 1
Active transport vs diffusion comparison
Going deeper
What's Next
Understanding membrane structure and transport is foundational for almost all further topics in IB Biology. Membrane transport is critical for understanding cell signaling, whole-body homeostasis, and osmoregulation, while the formation of proton gradients across membranes is core to both cellular respiration and photosynthesis. The fluid mosaic model also perfectly illustrates the core IB theme of form follows function: the structure of the membrane directly enables its roles as a selective barrier and communication interface between the cell and its environment. Build on this knowledge with the linked topics below to connect membrane function to broader cellular processes.