What this topic covers
What this topic covers
This page covers membrane structure (fluid-mosaic model), all five modes of transport across membranes, and how cells are adapted for rapid transport. The specific spec points are flagged at the start of each section below.
Cell membranes form a partially permeable barrier between the inside and outside of cells (and around organelles). Substances cross them in five main ways, covered in detail below:
- Simple diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Co-transport
Fluid-Mosaic Model
What you need to know — membrane structure
What you need to know — membrane structure
- The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same.
- The arrangement and movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model.
- Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane.
The structure of cell membranes is described by the fluid-mosaic model:
- Fluid — the phospholipids can move freely (laterally) within their layer
- Mosaic — different proteins, glycoproteins, glycolipids and cholesterol are scattered throughout, like tiles in a mosaic
Components
Phospholipid bilayer
Two layers of phospholipids — hydrophilic heads point outward (facing water), hydrophobic tails point inward.
- This controls which substances can enter and leave the cell
- Lipid-soluble (non-polar) substances can pass through the bilayer directly by simple diffusion — e.g. fatty acids and monoglycerides absorbed in the ileum
Proteins
These can be embedded in one layer of the membrane or span the whole bilayer.
- Channel proteins — form pores for transporting water-soluble substances (specific ions / polar molecules) across the membrane (passive only)
- Carrier proteins — bind a molecule, then undergo a conformational change to release it on the other side (used in passive facilitated diffusion AND active transport)
- Glycoproteins (see below) are a type of membrane protein
Important — what each protein can do
- Channel proteins are passive only — they can only transport substances down a concentration gradient (no shape change, no ATP).
- Carrier proteins are versatile — they can act in passive facilitated diffusion (down the gradient, no ATP) OR in active transport (against the gradient, using ATP). The conformational change is the same; only the energy source differs.
In exam mark schemes, “active transport requires a carrier protein”. Channel proteins will be rejected.
Glycoproteins & Glycolipids
- Glycoproteins — proteins with carbohydrate chains; involved in cell recognition, and act as:
- Receptors — e.g. bind to hormones and neurotransmitters
- Cell-surface markers — e.g. antigens
- Glycolipids — lipids with carbohydrate chains; also act as receptors in cell signalling. They help maintain membrane stability and help cells attach to each other.
Cholesterol
Sits between the phospholipid tails and restricts the movement of other molecules in the membrane. This maintains membrane stability and fluidity across a range of temperatures (less fluid when hot, more fluid when cold).
Why are the hydrophobic tails arranged inward?
Why are the hydrophobic tails arranged inward?
The cell is in a watery environment, both inside (cytoplasm) and outside. The hydrophilic heads face the water; the hydrophobic tails hide away in the middle. This is what makes the bilayer stable.
Interactive (Explore)
Now that you know what’s in a membrane, see each mode in action. Switch between the 5 modes to compare how things move.
Simple Diffusion
What you need to know — simple diffusion
What you need to know — simple diffusion
Movement across membranes occurs by simple diffusion — involving the limitations imposed by the nature of the phospholipid bilayer.
The passive movement of molecules from a region of higher concentration to a region of lower concentration — down a concentration gradient.
- Molecules pass directly through the phospholipid bilayer
- No ATP needed
- Only works for: small, non-polar (lipid-soluble) molecules — e.g. O₂, CO₂, steroid hormones, and fatty acids / monoglycerides absorbed in the ileum
Why can't polar / charged molecules simply diffuse through?
Why can't polar / charged molecules simply diffuse through?
The hydrophobic tails in the middle of the bilayer repel polar and charged molecules. They need channel or carrier proteins to cross (facilitated diffusion).
What could the rate of simple diffusion depend on?
What could the rate of simple diffusion depend on?
- Concentration gradient — a steeper gradient between the inside and outside of the membrane gives a faster rate of diffusion
- Surface area — a larger surface area gives a faster rate of diffusion
- Thickness of membrane / diffusion distance — a thinner membrane means a reduced diffusion distance, so a faster rate of diffusion
- Temperature — at higher temperatures particles have more kinetic energy, so the rate of diffusion is faster
Facilitated Diffusion
What you need to know — facilitated diffusion
What you need to know — facilitated diffusion
Movement across membranes occurs by facilitated diffusion — involving the roles of carrier proteins and channel proteins.
Like simple diffusion (passive, down a concentration gradient, no ATP) — but uses proteins to help polar / charged / large molecules cross.
Two types of protein:
- Channel proteins — form a water-filled pore that allows specific ions (e.g. Na⁺, K⁺) to pass through. Some channels are gated (they open and close in response to a signal — see synaptic transmission and nerve impulses for examples).
- Carrier proteins — bind a molecule, then undergo a conformational change to release it on the other side. Used for larger molecules like glucose and amino acids.
What could the rate of facilitated diffusion depend on?
What could the rate of facilitated diffusion depend on?
- Concentration gradient — a steeper gradient gives a faster rate (until saturation)
- Number of channel / carrier proteins — more proteins means a faster rate, up to a maximum where all proteins are saturated (no more spare capacity)
- Temperature — higher temperatures give a faster rate (until proteins denature)
Osmosis
What you need to know — osmosis
What you need to know — osmosis
Movement across membranes occurs by osmosis — explained in terms of water potential.
The passive movement of water across a partially permeable membrane, from a region of higher water potential to a region of lower water potential.
Osmosis underpins many other topics on this site, including water uptake by root hair cells, transpiration, tissue fluid formation, and water reabsorption in the kidney.
Water potential (Ψ)
- Pure water has the highest water potential = 0 kPa
- Adding solutes lowers water potential — values become more negative
- Water moves down a water potential gradient (from less negative to more negative)
| Solution | Water potential |
|---|---|
| Pure water | 0 kPa (highest) |
| Dilute solution | Small negative number (e.g. −100 kPa) |
| Concentrated solution | Large negative number (e.g. −2000 kPa) |
Which way does water move between two cells, one at −300 kPa and one at −800 kPa?
Which way does water move between two cells, one at −300 kPa and one at −800 kPa?
Water moves from the cell at −300 kPa (the higher, less negative water potential) into the cell at −800 kPa (the lower, more negative water potential).
What could the rate of osmosis depend on?
What could the rate of osmosis depend on?
- Water potential gradient — a steeper gradient (bigger Ψ difference) gives a faster rate
- Surface area of membrane — a larger surface area gives a faster rate
- Thickness of membrane — a thinner membrane gives a faster rate
- Number of aquaporins — more aquaporins gives a faster rate (water crosses aquaporins much faster than it crosses the bilayer directly)
- Temperature — higher temperatures give a faster rate (more kinetic energy)
Active Transport
What you need to know — active transport
What you need to know — active transport
Movement across membranes occurs by active transport — involving the role of carrier proteins and the importance of the hydrolysis of ATP.
The movement of molecules or ions against a concentration gradient — from low to high concentration — using ATP and carrier proteins.
Active transport is essential in many other topics, including mineral ion uptake by root hair cells, glucose reabsorption in the kidney, and the Na⁺/K⁺ pump in resting neurones.
Steps:
- The molecule binds to a carrier protein on one side of the membrane
- ATP is hydrolysed to ADP + Pᵢ (releasing energy)
- The carrier protein undergoes a conformational change, moving the molecule across the membrane
- The molecule is released on the other side
What could the rate of active transport depend on?
What could the rate of active transport depend on?
- Number of carrier proteins — more proteins gives a faster rate (up to saturation)
- Rate of respiration — ATP supply depends on respiration; less respiration means less ATP and therefore slower active transport
- Temperature — affects enzymes involved in respiration (and the carrier proteins themselves)
- Anything that affects ATP availability — e.g. respiratory inhibitors (cyanide), low oxygen
Note: concentration gradient is NOT a factor — active transport moves AGAINST the gradient, so making the gradient bigger doesn’t help.
Co-Transport
What you need to know — co-transport
What you need to know — co-transport
Movement across membranes occurs by co-transport — illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum.
A type of facilitated diffusion where two molecules are transported across a membrane at the same time, using the same carrier protein.
Worked example — glucose absorption in the ileum (small intestine):
- Sodium-potassium pump (active transport) on the basal membrane (blood side) actively pumps Na⁺ out of the epithelial cell into the blood (using ATP)
- This creates a low Na⁺ concentration inside the epithelial cell
- Na⁺ from the ileum lumen then moves into the cell by facilitated diffusion, bringing glucose with it through a co-transporter protein (against the glucose gradient)
- Glucose builds up inside the cell, then moves into the blood by facilitated diffusion
The same Na⁺/glucose co-transporter is also used in glucose reabsorption in the proximal convoluted tubule of the kidney.
What could the rate of co-transport depend on?
What could the rate of co-transport depend on?
- Na⁺ concentration gradient — a bigger gradient (lots of Na⁺ in the lumen, very little in the cell) gives a faster rate
- Number of co-transporter proteins on the apical membrane — more proteins gives a faster rate
- Activity of the Na⁺/K⁺ pump — if the pump slows, the Na⁺ concentration inside the cell builds up, the gradient collapses, and co-transport slows down too
- ATP supply (indirectly, via the pump) — anything that reduces respiration (low oxygen, respiratory inhibitors) will eventually stop co-transport
- Surface area — e.g. microvilli increase the number of co-transporters available
Adaptations for Rapid Transport
What you need to know — cell adaptations & rate of transport
What you need to know — cell adaptations & rate of transport
- Cells may be adapted for rapid transport across their internal or external membranes by:
- an increase in surface area, or
- an increase in the number of protein channels and carrier molecules in their membranes
- You should be able to:
- explain the adaptations of specialised cells in relation to the rate of transport
- explain how surface area, number of channel / carrier proteins, and differences in gradients of concentration or water potential affect the rate of movement across cell membranes
Cells specialised for transport (e.g. ileum epithelial cells, root hair cells) maximise their rate by:
- Increased surface area — e.g. microvilli on ileum cells; long projections on root hair cells
- More channel / carrier proteins in the membrane
- More mitochondria — to supply ATP for active transport
For the rate factors specific to each transport mode, see the “What could the rate of … depend on?” callout under each section above.
Required Practical 3
Water Potential of Plant Tissue (Potatoes)
Aim: Use a dilution series of sucrose solutions to find the water potential of plant tissue (e.g. potato).
Method:
- Make a dilution series of sucrose solutions (the independent variable) — e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³
- Cut potato cylinders of identical mass / length / surface area (control variables)
- Place one in each sucrose solution; leave for a set time at a set temperature (control variables)
- Re-weigh — calculate % change in mass (the dependent variable)
- Plot % change in mass (y-axis) vs sucrose concentration (x-axis)
- The x-intercept (where % change = 0) gives the sucrose concentration with the same water potential as the potato cells.
Why % change in mass, not absolute change?
Cylinders may have slightly different starting masses. Using percentage change standardises across all samples so they can be compared fairly.
Formula: % change = (final mass − initial mass) ÷ initial mass × 100
Example results
A student cut six potato cylinders of starting mass 2.50 g and left them in different sucrose solutions for 30 minutes:
| Sucrose conc. (mol dm⁻³) | Initial mass (g) | Final mass (g) | Change (g) | % change |
|---|---|---|---|---|
| 0.0 | 2.50 | 2.81 | +0.31 | +12.4 |
| 0.2 | 2.50 | 2.63 | +0.13 | +5.2 |
| 0.4 | 2.50 | 2.42 | −0.08 | −3.2 |
| 0.6 | 2.50 | 2.25 | −0.25 | −10.0 |
| 0.8 | 2.50 | 2.13 | −0.37 | −14.8 |
| 1.0 | 2.50 | 2.05 | −0.45 | −18.0 |
What’s happening:
- In dilute sucrose (0.0, 0.2) the cells have a lower water potential than the solution, so water enters the cells by osmosis and mass increases.
- In concentrated sucrose (0.4 onwards) the cells have a higher water potential than the solution, so water leaves the cells by osmosis and mass decreases.
- Somewhere between 0.2 and 0.4 the cells and the solution have the same water potential, so there’s no net water movement and no mass change.
Example graph
Reading the graph:
The x-intercept (where % change = 0) is approximately 0.32 mol dm⁻³. At this concentration there’s no net water movement, so the potato cells must have the same water potential as 0.32 mol dm⁻³ sucrose.
Why blot the cylinders dry before weighing?
Why blot the cylinders dry before weighing?
Excess surface water would add mass that isn’t from osmosis, making the final reading too high and skewing the % change. Blot all cylinders the same way (with the same paper towel, same number of dabs) so it’s a controlled step.
Required Practical 4
Membrane Permeability
Aim: Investigate the effect of a named variable (e.g. temperature, ethanol concentration, pH) on the permeability of cell-surface membranes.
Standard method (using beetroot):
- Cut equal-sized beetroot cylinders of equal mass / surface area (control variables); rinse thoroughly to remove pigment from the cut surfaces (otherwise free pigment will inflate the readings)
- Place one cylinder into each of several water baths at different temperatures (the independent variable) — or alternatively, different ethanol concentrations / pH
- Leave for a fixed time (control variable)
- Measure the intensity of red pigment in the surrounding solution using a colorimeter (the dependent variable), or visually rank by eye if no colorimeter
- Higher absorbance = more pigment released = greater membrane damage = higher permeability
Why beetroot? It contains a red pigment (betalain) in its vacuoles. If the cell-surface and tonoplast membranes are damaged, the pigment leaks out — making the surrounding solution darker red.
At higher temperature (or higher ethanol concentration), more pigment is released because:
- Phospholipids gain kinetic energy and move more, so gaps form in the bilayer
- Membrane proteins denature (loss of tertiary structure), so channels and carriers no longer fit and the bilayer is disrupted further
- The membrane becomes more permeable overall
Example results
A student tested beetroot cylinders at four temperatures and measured absorbance of the surrounding solution:
| Temperature (°C) | Absorbance |
|---|---|
| 0 | 0.05 |
| 20 | 0.10 |
| 40 | 0.20 |
| 50 | 0.55 |
Reading the graph:
- 0-40°C — small, gradual rise as phospholipids gain a bit of kinetic energy.
- 40-50°C — much steeper rise. Membrane proteins denature and the phospholipid bilayer becomes more fluid / disrupted, so the membrane becomes more permeable and pigment escapes faster.
What would happen if you kept heating above 50°C — would absorbance fall like an enzyme curve?
What would happen if you kept heating above 50°C — would absorbance fall like an enzyme curve?
No. With an enzyme, denaturation stops the reaction, so activity drops past the optimum. With a membrane, denaturation makes it leakier, so absorbance keeps rising until all the pigment is out. Then it plateaus. It never decreases.
Why must you rinse the beetroot cylinders thoroughly before starting?
Why must you rinse the beetroot cylinders thoroughly before starting?
Cutting the beetroot damages cells on the cut surfaces, releasing pigment that’s nothing to do with the experiment. If you don’t rinse it off, the surrounding solution will start coloured already, making your absorbance readings too high and not a true measure of membrane damage caused by your variable.
Exam Question Practice
Describe the processes of facilitated diffusion and active transport.
- Facilitated diffusion
- Active transport
Hint
What types of molecules need help crossing? What direction relative to the gradient? What provides the energy (if any)? What type of protein is involved in each?
Mark Scheme
- (Movement of) polar / charged molecules (1 mark)
- (Facilitated diffusion) movement down a concentration gradient via carrier / channel protein (1 mark)
- (Active transport) movement against a concentration gradient via carrier protein using ATP (1 mark)
Tips from examiner reports
- Give enough detail — basic definitions alone may not score full marks
- For active transport, mention carrier proteins specifically (not just “proteins” or channel proteins)
- For facilitated diffusion, specify channel or carrier — “transport proteins” is too vague
- Facilitated diffusion is passive — don’t say it requires ATP
- Remember that charged molecules (ions) can also move by these processes
A scientist used apparatus to investigate osmosis. A bladder containing molasses was sealed inside a tube and placed in a beaker of water. The bladder is partially permeable.
Use your understanding of osmosis to explain why the air pressure in the tube increased.
(3 marks)Hint
Compare the water potential of the two solutions. Where does water move, and what effect does this have on the volume inside?
Mark Scheme
- Molasses has a lower water potential OR water has higher water potential (1 mark)
- Water moves in across the partially permeable bladder (1 mark)
- Increased solution volume OR decreased air volume (1 mark)
Comments from mark scheme
- Accept “more negative” for “lower” or “less negative” for “higher”
- Accept “semi-permeable” for partially permeable
Tips from examiner reports
- Use the term water potential, NOT “water concentration”, when explaining osmosis
- Apply osmosis principles to the specific context — don’t just write a generic definition
- Be clear about whether it’s the air or the solution that changed in volume
- Identify the partially permeable membrane and state the direction of water movement
The movement of Na⁺ out of the cell allows the absorption of glucose into the cell lining the ileum.
Explain how.
(2 marks)Hint
Think about the sequence: sodium-potassium pump creates the Na⁺ gradient, which then drives co-transport. What does the gradient allow to happen?
Mark Scheme
- Maintains a concentration gradient for Na⁺ (from ileum into cell) (1 mark)
- Na⁺ moving in by co-transport / facilitated diffusion brings glucose with it (1 mark)
Comments from mark scheme
- Accept “maintains a lower concentration of Na⁺ inside the cell compared with outside”
- Accept “co-transporter” for “co-transport”
Tips from examiner reports
- The sodium-potassium pump creates the gradient, but the actual glucose absorption happens via the co-transporter protein
- Glucose is carried with Na⁺ — same protein, simultaneous movement
- This is why glucose can move against its own concentration gradient — the energy comes indirectly from the Na⁺ gradient
A student investigated extracting anthocyanin pigment from crushed blueberries using three different solvents. She kept constant:
- the mass of fresh blueberries
- the volume of extraction solvent
- the time for the mixture to stand
Name two other variables the student should have kept constant during this investigation.
(2 marks)Hint
Think about ANYTHING that could affect how much pigment is released. What could differ between trials if you didn’t control it?
Mark Scheme
Any two of:
- Temperature (1 mark)
- Agitation / mixing / stirring (1 mark)
- Source / age / type of blueberries (1 mark)
- Crushing of the blueberries (extent / method) (1 mark)
- Rinsing of the blueberries before mixing (1 mark)
- Concentration of ethanol / acid in the solvent (1 mark)
Comments from mark scheme
- Do NOT accept “pH” (already controlled by the solvent recipe)
- Accept “filtering method”
Tips from examiner reports
- For control variables, think about ALL inputs that could affect the outcome (not just the obvious ones)
- “Temperature” is a common miss — it always affects rate of diffusion/extraction
A student soaked crushed blueberries in three solvents for 1 hour, then filtered them and measured the light absorbance of the filtrate.
Results:
- Water gave the lowest absorbance
- Ethanol (E) gave higher absorbance
- Ethanol + acid (F) gave the highest absorbance of all
Use your knowledge of membrane structure to explain the results.
(4 marks)Hint
What does ethanol do to phospholipids? What does acid do to proteins? Link both to the membrane structure and to pigment release.
Mark Scheme
- Higher absorbance indicates more anthocyanin (pigment) in the solution (1 mark)
- More membrane damage / permeability leads to more anthocyanin release (1 mark)
- Ethanol dissolves phospholipids in the bilayer — so E and F give more release than water alone (1 mark)
- Acid denatures membrane proteins (loss of tertiary structure) — so F gives more release than E (1 mark)
Comments from mark scheme
- Accept “pigment” for “anthocyanin”
- Accept “most” for “more”
- For protein denaturation, accept “change in tertiary structure” or “breaks hydrogen / ionic bonds”
Tips from examiner reports
- Practical experience was poorly demonstrated nationally — students struggled to explain WHY ethanol disrupts membranes (it dissolves phospholipids)
- Acid affects the PROTEINS in the membrane (denatures them), not just the phospholipids
- Always link the observation (absorbance) to the biology (membrane structure damage)
A student wanted to do the same anthocyanin investigation but did not have a colorimeter.
Describe a method this student could use to prepare colour standards and use them to give data for the total anthocyanin extracted.
(3 marks)Hint
You need a way to measure colour intensity by eye. How could you create known reference colours to compare against?
Mark Scheme
- Use a known concentration of blueberry juice / extract (1 mark)
- Prepare a dilution series of that standard (e.g. serial dilutions) (1 mark)
- Compare each sample to the colour standards and assign a score / value / concentration (1 mark)
Comments from mark scheme
- Accept “serial dilutions”
- Dilution can be of either the pigment OR the solvent
- For “colour standards” accept “dilutions”
Tips from examiner reports
- Common wrong answers: “look up values in a book”, “draw a calibration curve from no standard” — both scored zero
- The key idea is a dilution series of a known starting concentration so each colour in your range corresponds to a known concentration
The higher rate of transpiration at high tide shows that the mangrove tree is absorbing water from the sea water surrounding its roots.
Describe an experiment you could do to investigate whether the mangrove root cells have a lower water potential than sea water.
You are given:
- a piece of fresh mangrove root
- sea water
- access to laboratory equipment
Hint
Same logic as Practical 3, but with one solution (sea water) instead of a dilution series. Think about what to measure, how long for, and how to interpret the result.
Mark Scheme
- Record mass / length of the root piece before and after (1 mark)
- Place in sea water for a specified / equal time (1 mark)
- Method to remove surface water before reweighing (e.g. blot with tissue) (1 mark)
- Increase in mass / length shows water has been absorbed by osmosis, so the cells have a lower water potential than sea water (1 mark)
Comments from mark scheme
- Accept “weight” for “mass”, “diameter” for “length”
- Reject “size” once (then allow ECF)
- Accept “blot dry / tissue paper” for surface-water removal
- Microscope-based alternative also valid (turgid vs flaccid cells)
Tips from examiner reports
- Always describe mass change, not absolute mass
- Include a step to dry tissue before reweighing — otherwise mass is inflated by surface water
- Say “water moves by osmosis” — NOT “seawater moves by osmosis”
- A potometer measures transpiration, not osmotic mass change — don’t suggest one here
C. difficile bacteria release toxins that damage the cells lining the ileum, causing them to lose their microvilli.
Explain why the damage to the cells lining the ileum reduces absorption of the products of digestion, AND why this reduces absorption of water.
(3 marks)Hint
Water moves by OSMOSIS, not diffusion. Think about water potential: where is it higher and lower after damage?
Mark Scheme
- Reduced surface area OR fewer co-transport / carrier / channel proteins (1 mark)
- Decreases water potential in the ileum OR increases water potential in the cells (1 mark)
- (So) water moves out of cells into the ileum by osmosis OR less water moves into cells (1 mark)
Comments from mark scheme
- Ignore references to diffusion / facilitated diffusion / active transport for water
- Accept “gut” for “ileum”
- Accept Ψ for water potential (but ignore “WP”)
Tips from examiner reports
- Loss of microvilli causes less surface area, which causes less absorption (this IS a sequence — arrows fine in your own answer)
- Less glucose / amino acid absorption means the water potential in the ileum stays low (lots of solutes) and the water potential in the cells stays high, so water moves the wrong way (out of cells, into the ileum)
- Water always moves by osmosis, never by “diffusion” in this context
Comments from mark scheme