2.4Cell Structure
Cell Membranes and Transport — AQA A Level Biology 7402 Revision Notes & Topical Questions
15 min read 21 parts Lesson 23 of 47 Unit quiz
Membrane Structure and Diffusion — AQA A LEVEL BIOLOGY 7402 notes: Describe the fluid mosaic model, factors affecting membrane fluidity and the factors affecting diffusion.The basic structure of all cell membranes is the same.
- phospholipid bilayer
- Intrinsic (integral) proteins
- extrinsic proteins
- Cholesterol
- Glycolipids and glycoproteins
- less fluid
Section 1 of 21The fluid mosaic model
The basic structure of all cell membranes is the same.
Section 2 of 21Components of the membrane
Each phospholipid has two regions
- a phosphate head that is polar (hydrophilic) and soluble in water, and two fatty acid tails that are non-polar (hydrophobic) and insoluble in water.
- The bilayer arranges itself so the hydrophobic tails face inward, forming a hydrophobic core, while the hydrophilic heads face outward towards the aqueous environment.
- The structure of each protein determines its position.
- It stabilises the membrane at higher temperatures by stopping it becoming too fluid, makes it less permeable to small charged particles (such as ions),
- and strengthens it so the cell does not burst.
- where they act as receptor molecules and bind certain substances at the cell's surface.
Section 3 of 21Membrane fluidity
Membranes become less fluid when there is a greater proportion of saturated fatty acid chains (the chains pack tightly and have many intermolecular forces between them) or a lower temperature (molecules have less energy and pack more closely). They become more fluid with more unsaturated fatty acids (kinks prevent tight packing) and at higher temperatures.
Section 4 of 21Diffusion
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the bilayer. Factors affecting the rate:
| Factor | Effect |
|---|---|
| Concentration gradient | A steeper gradient increases the net rate of diffusion |
| Temperature | Higher temperatures give molecules more kinetic energy so they move faster, giving a higher rate |
| Surface area | A larger surface area allows more molecules to diffuse at once |
| Diffusion distance | A thinner exchange surface shortens the distance and increases the rate |
Section 5 of 21Worked examples
Explain why the cell surface membrane is described as fluid mosaic. [2]2 marks
Phospholipids and some proteins move within the layer, so the structure is fluid. Proteins of different shapes and sizes are embedded randomly, giving a mosaic appearance.
Describe the roles of cholesterol in a membrane. [2]2 marks
It packs between phospholipid tails, restricting their movement so the membrane is less fluid at higher temperatures. It also reduces the permeability of the membrane to ions.
Why are small, non-polar molecules able to diffuse through a membrane but ions cannot? [2]2 marks
Non-polar molecules dissolve in the hydrophobic core of the bilayer. Ions are charged and are repelled by the hydrophobic tails, so they need channel or carrier proteins.
Section 6 of 21Facilitated diffusion
Movement from high to low concentration is diffusion. If it needs the help of a protein, for example because the molecule is charged and cannot pass directly through the phospholipid bilayer, it is facilitated diffusion. It is passive and needs no ATP.
- Most channel proteins are gatedpart of the protein on the inside surface can move to close or open the pore, controlling the exchange of ions.
Section 7 of 21Osmosis and water potential
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. The more negative the water potential, the lower it is and the further it is from pure water (0 kPa).
Plant cells have a cell wall.
- so the effects are more severe: in a hypertonic solution they shrink and shrivel;
- in a hypotonic solution they swell and may burst (cytolysis).
Section 8 of 21Required practical: water potential of potato
This investigates how immersing plant tissue in solutions of different water potentials affects it, and the results estimate the water potential of the tissue. Cylinders of potato are placed in sucrose solutions of a range of concentrations (at least 5 different concentrations).
A positive change means the potato gained water by osmosis, so the solution had a higher water potential; cells became turgid and the potato felt firm.
Plot a calibration curve of percentage change against sucrose concentration; the concentration where the line crosses zero change is where the water potential of the solution equals that of the tissue.
Section 9 of 21Presenting data
Decide what type of data you have, then choose the appropriate graph. Plot the independent variable on the x-axis and the dependent variable on the y-axis, use appropriate linear scales and choose scales that let all data points fit within the graph area.
Section 10 of 21Worked examples
Calculate the percentage change in mass for a potato cylinder with initial mass 3.20 g and final mass 2.88 g. [2]2 marks
Change = −0.32 g. −0.32 ÷ 3.20 × 100 = −10%.
Explain how the water potential of the potato tissue is found from the graph. [2]2 marks
Read the sucrose concentration where percentage change is zero. At this point there is no net osmosis, so the water potential of the solution equals that of the tissue.
Explain why an animal cell placed in pure water bursts but a plant cell does not. [3]3 marks
Water enters by osmosis because water potential is higher outside. The plant cell wall resists expansion and the cell becomes turgid. The animal cell has no wall so it swells and bursts (cytolysis).
Section 11 of 21Active transport
Active transport is the movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration (against the gradient), using energy from respiration (ATP).
Section 12 of 21Co-transport in the ileum
Co-transport is the coupled movement of two substances across a membrane via a carrier protein: one moves down its concentration gradient, allowing the other to move against its gradient.
Section 13 of 21Adaptations for rapid transport
Rates of transport depend on the type of transport (diffusion, facilitated diffusion, active transport) and on factors including surface area, numbers of transport proteins, and ATP supply. Some cells are specialised to allow rapid transport across their internal or external membranes for absorption, secretion or gas exchange.
| Feature | Benefit | Example |
|---|---|---|
| Folded membranes (microvilli, long hair-like projections) | Large surface area | Ileum epithelium; root hair cells absorbing water and mineral ions |
| Many mitochondria | More ATP for active transport against a gradient | Root hair cells absorbing ions from soil |
| Many carrier or channel proteins | Faster transport | Ileum cells |
| Many aquaporins | Rapid water movement by facilitated diffusion | Cells in the collecting duct of the kidney |
In the kidney cells described, water is transported via facilitated diffusion through channel proteins. Remember, water can also diffuse through the membrane directly, even though it is polar, because it is a small molecule.
Section 14 of 21Worked examples
Explain how glucose is absorbed from the ileum into epithelial cells against a concentration gradient. [4]4 marks
Sodium ions are actively transported out of the cell, lowering the internal Na+ concentration. Na+ then moves into the cell down its gradient through a co-transporter, bringing glucose with it. Glucose accumulates in the cell at a higher concentration than in the lumen.
Explain why root hair cells contain many mitochondria. [2]2 marks
Mitochondria supply ATP by aerobic respiration, which active transport of mineral ions into the cell requires.
Suggest why cyanide reduces ion uptake. [2]2 marks
Cyanide inhibits respiration, so less ATP is available and active transport stops.
Section 15 of 21Principle
The permeability of cell membranes is affected by conditions such as temperature and the concentration of organic solvents (alcohol or acetone). Beetroot is used because its cells contain a red pigment (betalain) in the vacuole. If the membranes become more permeable, the pigment leaks out into the surrounding water and the amount of colour can be measured.
Section 16 of 21Method
- The more light absorbed, the more pigment present, so the greater the permeability.
Cuvettes hold the liquid in a colourimeter. They may differ very slightly in thickness; a thicker or scratched cuvette absorbs slightly more light, so use the same cuvette or blank each time and zero the colourimeter with water.
Section 17 of 21Explaining results
At low temperatures there is little pigment leakage because the membrane is rigid and phospholipids are tightly packed. As temperature rises, phospholipids gain kinetic energy, the bilayer becomes more fluid, and carrier and channel proteins may denature at high temperatures (usually above about 45 °C) leaving gaps, so permeability and pigment loss increase sharply.
Higher concentrations of solvents, such as alcohol or acetone, increase permeability because they dissolve the lipids in the membrane, causing it to lose its structure.
Section 18 of 21Variables and improvements
Section 19 of 21Worked examples
Explain why the beetroot must be rinsed before the investigation. [2]2 marks
Cut cells release pigment, which would raise the absorbance in every tube and so give false results.
Explain why higher temperatures lead to a darker solution. [3]3 marks
Phospholipids gain kinetic energy and move apart, making the membrane more fluid and permeable. Proteins in the membrane denature, creating gaps, and more pigment diffuses out.
Suggest why a colourimeter is better than judging the colour by eye. [1]1 mark
It gives quantitative, objective data that can be compared reliably.
Section 20 of 21Key terms
| Term | Meaning |
|---|---|
| Phospholipid bilayer | Two layers of phospholipids with hydrophobic tails inwards |
| Hydrophilic | Attracted to water |
| Hydrophobic | Repelled by water |
| Cholesterol | Lipid that regulates membrane fluidity |
| Glycoprotein | Protein with a carbohydrate chain, often a receptor or antigen |
| Diffusion | Net movement down a concentration gradient |
| Term | Meaning |
|---|---|
| Facilitated diffusion | Passive movement via a channel or carrier protein |
| Osmosis | Net movement of water from higher to lower water potential across a partially permeable membrane |
| Water potential | Measure of tendency of water to move; pure water is 0 |
| Plasmolysis | Protoplast shrinking away from the cell wall |
| Turgid | Plant cell swollen with water, firm |
| Flaccid | Plant cell that has lost water, floppy |
| Term | Meaning |
|---|---|
| Active transport | Movement against a concentration gradient using ATP and a carrier protein |
| Co-transport | Coupled movement of two substances via a carrier protein |
| Aquaporin | Channel protein allowing rapid water movement |
| Carrier protein | Protein that changes shape to move molecules across a membrane |
| Microvilli | Folds in the cell surface membrane that increase surface area |
| Term | Meaning |
|---|---|
| Permeability | How easily substances can pass through a membrane |
| Colourimeter | Instrument that measures light absorbance of a solution |
| Cuvette | Small container for a colourimeter sample |
| Denature | Permanent change in the shape of a protein |
| Solvent | Substance, such as alcohol, that dissolves other substances |
Section 21 of 21Exam technique
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