1.3Cell Structure and Molecular Biology
Membranes and transport — EST 2 Biology Revision Notes & Topical Questions
25 min read 40 parts Lesson 3 of 11 2 quizzes
SAT Biology membranes and transport: explain membrane structure and predict how substances move by passive and active processes. The plasma membrane is a selectively permeable phospholipid bilayer.
- fluid mosaic
- Passive transport
- simple diffusion
- facilitated diffusion
- Active transport
- sodium-potassium pump
Section 1 of 40Fluid mosaic membrane
Section 2 of 40Diffusion and facilitated diffusion
Section 3 of 40Osmosis and tonicity
Section 4 of 40Active transport and pumps
Section 5 of 40Bulk membrane transport
Section 6 of 40Active transport and the sodium-potassium pump
Active transport is the movement of molecules or ions against a concentration gradient, from low to high concentration, using energy from respiration in the form of ATP. It always occurs across a membrane and involves carrier proteins, which use ATP and change shape to move the substance across. Active transport allows the establishment and maintenance of concentration gradients.
The Na+/K+ ATPase is a sodium-potassium pump. It uses ATP to pump sodium and potassium ions in opposite directions across the membranes of nerve cells, establishing and maintaining the gradients that contribute to the membrane potential of neurones. After an impulse, the pump restores the ion distribution so the neurone can return to its resting state.
Section 7 of 40Endocytosis and exocytosis
Endocytosis and exocytosis move large substances, or large amounts of substances, into and out of cells; they can even move whole cells or parts of cells.
Section 8 of 40Facilitated diffusion compared
Facilitated diffusion is the movement of large polar molecules and ions across membranes through transport proteins (channel or carrier proteins), down a concentration gradient and with no energy input. The proteins are highly specific, carrying only one type of substance. Carrier proteins change shape to move a substance from one side to the other.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down the gradient | Down the gradient | Against the gradient |
| Energy (ATP) | No | No | Yes |
| Protein needed | No | Yes (channel or carrier) | Yes (carrier pump) |
| Example | O2, CO2 | Glucose, ions | Na+/K+ pump |
Do not confuse facilitated diffusion with active transport: both can use carrier proteins, but facilitated diffusion moves down a gradient and needs no ATP.
Section 9 of 40Tonicity and water potential
Tonicity is relativeit describes the external solution compared with the cell contents.
Water moves into or out of cells depending on the water potential (Ψ) of the surroundings, from high to low water potential. Ψ = Ψs + Ψp. Solute potential (Ψs) is the effect of solutes, and is always zero or negative; pressure potential (Ψp) is the physical pressure on the solution.
- and it can be negative in xylem vessels,
- where water is under tension.
Section 10 of 40Worked examples
Distinguish between facilitated diffusion and active transport. [3]3 marks
Facilitated diffusion moves down a gradient without ATP through channel or carrier proteins; active transport moves against a gradient and needs ATP.
State the role of the Na+/K+ ATPase in nerve cells. [2]2 marks
Uses ATP to pump Na+ and K+ in opposite directions, maintaining gradients and membrane potential.
Why do exocytosis and endocytosis need energy? [1]1 mark
Moving and fusing vesicles requires ATP, so they are active transport.
Describe what happens to an animal cell placed in a hypertonic solution. [2]2 marks
Water leaves by osmosis and the cell shrinks.
Section 11 of 40Calculating water potential
Water moves into or out of cells depending on the water potential (Ψ) of the surroundings, from high to low water potential. Ψ = Ψs + Ψp. Solute potential (Ψs) is the effect of solutes, and is always zero or negative; pressure potential (Ψp) is the physical pressure on the solution.
- and it can be negative in xylem vessels,
- where water is under tension.
Worked example
- find the water potential of a 0.5 mol/dm3 NaCl solution at 20 °C in an open beaker.
- Ψs = −iCRT.
Section 12 of 40Membrane structure and the fluid mosaic model
Membranes surround all living cells and form many internal organelles. Phospholipids have a polar, hydrophilic phosphate head and two nonpolar fatty acid tails, so in water they form a bilayer with heads outward and hydrophobic tails inside. Proteins are embedded in the bilayer; their hydrophilic regions (charged and polar side groups) are exposed to the aqueous cytosol or folded inside, while hydrophobic regions (nonpolar side groups) sit within the lipid core.
The fluid mosaic model describes the plasma membrane as a framework of phospholipids with embedded proteins, steroids (cholesterol), glycoproteins and glycolipids that can move around within the layer.
Section 13 of 40Selective permeability and cell walls
The hydrophobic interior makes membranes selectively permeable, controlling which substances cross between the cell and its surroundings.
| Substance | How it crosses |
|---|---|
| Small nonpolar molecules (N2, O2, CO2) | Pass freely through the bilayer |
| Small polar molecules (H2O, NH3) | Pass between phospholipids in small amounts; water mainly uses aquaporins |
| Large polar molecules and ions | Need transport proteins (facilitated diffusion or active transport) |
| Macromolecules | Endocytosis and exocytosis |
Selective permeability allows concentration gradients to form: a membrane may block a substance, giving a higher concentration on the supply side, or contain proteins that pump a substance across so a high concentration builds up on one side.
Bacteria, archaea, fungi and plant cells also have a cell wall, which gives rigidity, is freely permeable to small molecules and can act as a barrier to some substances.
Section 14 of 40The central vacuole
The central vacuole of plant cells contributes to cell homeostasis. It stores water, and a full vacuole maintains the pressure potential and gives structural support. Ion channels in its membrane open and close to regulate the ions in the cytoplasm.
Remember that solute potential is always negative or zero, that adding solute makes the water potential more negative, and that water moves from a less negative to a more negative water potential.
Section 15 of 40Worked examples
Calculate the solute potential of 0.5 mol/dm3 NaCl at 20 °C (R = 0.0831). [4]4 marks
i = 2, T = 293 K; Ψs = −2 × 0.5 × 0.0831 × 293 = −24.3 bars.
Why does water potential equal solute potential in an open beaker? [1]1 mark
Pressure potential is zero.
Explain why oxygen crosses membranes easily but ions do not. [2]2 marks
Oxygen is small and nonpolar, so dissolves in the hydrophobic core; ions are charged and need transport proteins.
Describe the fluid mosaic model. [2]2 marks
A phospholipid framework with embedded proteins and other molecules that can move within the layer.
Section 16 of 40Passive and active transport
Simple diffusion is passive transport: molecules move from high to low concentration with no direct input of metabolic energy. Active transport requires direct energy input to move molecules from low to high concentration. The mechanism a substance uses depends on its size, polarity and charge, and on the concentration gradient.
The hydrophobic interior makes membranes selectively permeable, controlling which substances cross between the cell and its surroundings.
| Substance | How it crosses |
|---|---|
| Small nonpolar molecules (N2, O2, CO2) | Pass freely through the bilayer |
| Small polar molecules (H2O, NH3) | Pass between phospholipids in small amounts; water mainly uses aquaporins |
| Large polar molecules and ions | Need transport proteins (facilitated diffusion or active transport) |
| Macromolecules | Endocytosis and exocytosis |
Selective permeability allows concentration gradients to form: a membrane may block a substance, giving a higher concentration on the supply side, or contain proteins that pump a substance across so a high concentration builds up on one side.
Bacteria, archaea, fungi and plant cells also have a cell wall, which gives rigidity, is freely permeable to small molecules and can act as a barrier to some substances.
Section 17 of 40Active transport and the sodium-potassium pump
Active transport is the movement of molecules or ions against a concentration gradient, from low to high concentration, using energy from respiration in the form of ATP. It always occurs across a membrane and involves carrier proteins, which use ATP and change shape to move the substance across. Active transport allows the establishment and maintenance of concentration gradients.
The Na+/K+ ATPase is a sodium-potassium pump. It uses ATP to pump sodium and potassium ions in opposite directions across the membranes of nerve cells, establishing and maintaining the gradients that contribute to the membrane potential of neurones. After an impulse, the pump restores the ion distribution so the neurone can return to its resting state.
Section 18 of 40Facilitated diffusion
Facilitated diffusion is the movement of large polar molecules and ions across membranes through transport proteins (channel or carrier proteins), down a concentration gradient and with no energy input. The proteins are highly specific, carrying only one type of substance. Carrier proteins change shape to move a substance from one side to the other.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down the gradient | Down the gradient | Against the gradient |
| Energy (ATP) | No | No | Yes |
| Protein needed | No | Yes (channel or carrier) | Yes (carrier pump) |
| Example | O2, CO2 | Glucose, ions | Na+/K+ pump |
Do not confuse facilitated diffusion with active transport: both can use carrier proteins, but facilitated diffusion moves down a gradient and needs no ATP.
Section 19 of 40Endocytosis and exocytosis, tonicity
Endocytosis and exocytosis move large substances, or large amounts of substances, into and out of cells; they can even move whole cells or parts of cells.
Tonicity is relativeit describes the external solution compared with the cell contents.
Section 20 of 40Worked examples
Explain how a membrane can build up a high concentration of a substance inside a cell. [2]2 marks
Carrier proteins actively pump it across using ATP.
Give two examples of molecules that cross by simple diffusion. [2]2 marks
Oxygen and carbon dioxide.
Why does water rely mainly on aquaporins? [2]2 marks
It is polar, so only small amounts pass between the phospholipids.
Compare facilitated diffusion and simple diffusion. [2]2 marks
Both are passive, down the gradient; facilitated diffusion needs transport proteins.
Section 21 of 40ATP: the energy currency
ATP stores energy that is immediately available for cell activities such as muscle contraction, nerve impulses and active transport.
Section 22 of 40Active transport in detail
Active transport is the movement of molecules or ions against a concentration gradient, from low to high concentration, using energy from respiration in the form of ATP. It always occurs across a membrane and involves carrier proteins, which use ATP and change shape to move the substance across. Active transport allows the establishment and maintenance of concentration gradients.
The Na+/K+ ATPase is a sodium-potassium pump. It uses ATP to pump sodium and potassium ions in opposite directions across the membranes of nerve cells, establishing and maintaining the gradients that contribute to the membrane potential of neurones. After an impulse, the pump restores the ion distribution so the neurone can return to its resting state.
Section 23 of 40Comparison of transport mechanisms
Facilitated diffusion is the movement of large polar molecules and ions across membranes through transport proteins (channel or carrier proteins), down a concentration gradient and with no energy input. The proteins are highly specific, carrying only one type of substance. Carrier proteins change shape to move a substance from one side to the other.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down the gradient | Down the gradient | Against the gradient |
| Energy (ATP) | No | No | Yes |
| Protein needed | No | Yes (channel or carrier) | Yes (carrier pump) |
| Example | O2, CO2 | Glucose, ions | Na+/K+ pump |
Do not confuse facilitated diffusion with active transport: both can use carrier proteins, but facilitated diffusion moves down a gradient and needs no ATP.
Section 24 of 40Vesicle transport
Endocytosis and exocytosis move large substances, or large amounts of substances, into and out of cells; they can even move whole cells or parts of cells.
Section 25 of 40Linking ATP to cell activities
ATP drives many processes besides active transport: muscle contraction, the synthesis of macromolecules, and nerve impulses.
Section 26 of 40Exam technique
For active transport questions, include four points: movement against the concentration gradient, a carrier protein, ATP (from respiration) as the energy source, and an example such as the Na+/K+ pump or uptake of mineral ions by root hairs. For a comparison, state whether each process needs energy and whether it moves substances down or against a gradient.
Section 27 of 40Worked examples
Describe how ATP releases energy. [2]2 marks
Its terminal phosphate is removed, forming ADP and releasing energy.
Explain why the sodium-potassium pump is vital to neurones. [3]3 marks
It restores the ion distribution after an impulse so the neurone returns to its resting state.
Why is active transport sensitive to respiratory poisons? [2]2 marks
ATP supply falls, so pumps stop working.
State one difference between active transport and diffusion. [1]1 mark
Active transport needs ATP and moves against a gradient.
Section 28 of 40Selective permeability
The hydrophobic interior makes membranes selectively permeable, controlling which substances cross between the cell and its surroundings.
| Substance | How it crosses |
|---|---|
| Small nonpolar molecules (N2, O2, CO2) | Pass freely through the bilayer |
| Small polar molecules (H2O, NH3) | Pass between phospholipids in small amounts; water mainly uses aquaporins |
| Large polar molecules and ions | Need transport proteins (facilitated diffusion or active transport) |
| Macromolecules | Endocytosis and exocytosis |
Selective permeability allows concentration gradients to form: a membrane may block a substance, giving a higher concentration on the supply side, or contain proteins that pump a substance across so a high concentration builds up on one side.
Bacteria, archaea, fungi and plant cells also have a cell wall, which gives rigidity, is freely permeable to small molecules and can act as a barrier to some substances.
Section 29 of 40Passive and active transport
Simple diffusion is passive transport: molecules move from high to low concentration with no direct input of metabolic energy. Active transport requires direct energy input to move molecules from low to high concentration. The mechanism a substance uses depends on its size, polarity and charge, and on the concentration gradient.
Active transport is the movement of molecules or ions against a concentration gradient, from low to high concentration, using energy from respiration in the form of ATP. It always occurs across a membrane and involves carrier proteins, which use ATP and change shape to move the substance across. Active transport allows the establishment and maintenance of concentration gradients.
The Na+/K+ ATPase is a sodium-potassium pump. It uses ATP to pump sodium and potassium ions in opposite directions across the membranes of nerve cells, establishing and maintaining the gradients that contribute to the membrane potential of neurones. After an impulse, the pump restores the ion distribution so the neurone can return to its resting state.
Section 30 of 40Facilitated diffusion
Facilitated diffusion is the movement of large polar molecules and ions across membranes through transport proteins (channel or carrier proteins), down a concentration gradient and with no energy input. The proteins are highly specific, carrying only one type of substance. Carrier proteins change shape to move a substance from one side to the other.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down the gradient | Down the gradient | Against the gradient |
| Energy (ATP) | No | No | Yes |
| Protein needed | No | Yes (channel or carrier) | Yes (carrier pump) |
| Example | O2, CO2 | Glucose, ions | Na+/K+ pump |
Do not confuse facilitated diffusion with active transport: both can use carrier proteins, but facilitated diffusion moves down a gradient and needs no ATP.
Section 31 of 40Endocytosis and exocytosis
Endocytosis and exocytosis move large substances, or large amounts of substances, into and out of cells; they can even move whole cells or parts of cells.
Section 32 of 40Worked examples
Explain why polar molecules cross membranes slowly. [2]2 marks
The bilayer interior is hydrophobic, so they need transport proteins.
Name the protein that lets water cross quickly. [1]1 mark
Aquaporin.
Give one function of a cell wall. [1]1 mark
Structural rigidity.
Explain how a concentration gradient can be maintained across a membrane. [2]2 marks
The membrane blocks passage or proteins pump the substance.
Section 33 of 40Tonicity
Tonicity is relativeit describes the external solution compared with the cell contents.
Section 34 of 40Water potential
Water moves into or out of cells depending on the water potential (Ψ) of the surroundings, from high to low water potential. Ψ = Ψs + Ψp. Solute potential (Ψs) is the effect of solutes, and is always zero or negative; pressure potential (Ψp) is the physical pressure on the solution.
- and it can be negative in xylem vessels,
- where water is under tension.
Worked example
- find the water potential of a 0.5 mol/dm3 NaCl solution at 20 °C in an open beaker.
- Ψs = −iCRT.
Section 35 of 40Investigating osmosis
Osmosis practical. Cut potato cylinders of equal length, blot and weigh them.
- where the solute potential of the solution equals that of the potato tissue.
Section 36 of 40Applying the ideas
To predict direction, compare the water potential of the cell with that of the solution: water moves from higher (less negative) to lower (more negative).
When describing an experiment, state the independent variable (concentration), the dependent variable (percentage change in mass), the controlled variables and how reliability is improved by repeating and calculating a mean. When interpreting results, link the direction of mass change to the water potential of the solution relative to the tissue.
Section 37 of 40More on tonicity in living cells
Red blood cells in distilled water take in water and burst (lysis); in concentrated salt solution they shrink and become crenated.
- which provides support for non-woody plants;
- if they lose water they become flaccid and then plasmolysed, and the plant wilts.
Section 38 of 40Worked examples
Predict water movement for cell Ψ = −5 bar in solution Ψ = −8 bar. [2]2 marks
Water leaves the cell, moving to the more negative Ψ.
Describe the effect on a plant cell in a hypotonic solution. [2]2 marks
Water enters; the cell becomes turgid and the cell wall prevents bursting.
State two controlled variables in the potato experiment. [2]2 marks
Size of cylinders, volume of solution, temperature, time.
How is the isotonic point found? [1]1 mark
Where the graph of percentage mass change crosses zero.
Section 39 of 40Key terms
| Term | Meaning |
|---|---|
| Active transport | Movement against a gradient using ATP. |
| Facilitated diffusion | Passive movement through transport proteins. |
| Hypotonic | Having a lower solute concentration than the cell. |
| Hypertonic | Having a higher solute concentration than the cell. |
| Water potential | The tendency of water to move; Ψ = Ψs + Ψp. |
| Exocytosis | Release of materials from vesicles at the cell surface. |
| Term | Meaning |
|---|---|
| Solute potential | The effect of solutes on water potential (always ≤ 0). |
| Pressure potential | Physical pressure on a solution. |
| Fluid mosaic model | Model of the membrane as a flexible lipid layer with embedded proteins. |
| Aquaporin | A channel protein for water. |
| Selectively permeable | Allowing only some substances through. |
| Term | Meaning |
|---|---|
| Passive transport | Movement down a gradient without energy input. |
| Carrier protein | A protein that changes shape to move a substance across a membrane. |
| Aquaporin | A channel protein that lets water cross. |
| Concentration gradient | A difference in concentration between two regions. |
| Term | Meaning |
|---|---|
| ATP | The immediate energy currency of cells. |
| ADP | Adenosine diphosphate, formed when ATP loses a phosphate. |
| Sodium-potassium pump | A carrier protein using ATP to move Na+ and K+. |
| Membrane potential | Difference in electrical charge across a membrane. |
| Term | Meaning |
|---|---|
| Selective permeability | Allowing some substances across but not others. |
| Aquaporin | Channel protein for water. |
| Cell wall | Rigid layer outside the membrane in plants, fungi and bacteria. |
| Gradient | A difference in concentration. |
| Term | Meaning |
|---|---|
| Osmosis | Net movement of water across a partially permeable membrane from high to low water potential. |
| Plasmolysis | Shrinking of the protoplast away from the cell wall. |
| Turgid | Swollen and firm with water. |
| Isotonic | Having the same solute concentration. |
Section 40 of 40Exam technique
Marked practiceQuizzes for this lesson
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