Unit 2 Transport across the cell membrane
Key unit competence: Validate physiological processes by which materials move in and out of cells and their significance to the life of organisms.
Key ideas
- Substances cross membranes by passive processes (no ATP from the cell; down a gradient): simple diffusion, facilitated diffusion and osmosis; and by active processes (ATP needed): active transport, endocytosis and exocytosis.
- Diffusion is the net movement of molecules from a region of higher to lower concentration. Fick's law: rate ∝ (surface area × concentration difference) ÷ thickness of the exchange surface. Rate also increases with temperature, decreases with molecule size; lipid-soluble and small molecules (O₂, CO₂, steroids) cross the phospholipid bilayer directly.
- Facilitated diffusion uses channel proteins (water-filled pores for ions, e.g. gated Na⁺ channels; aquaporins for water) or carrier proteins that change shape (e.g. glucose transporters). It is passive and specific, and its rate levels off when all proteins are in use (saturation).
- Water potential (ψ, psi) is the tendency of water to move; pure water has ψ = 0 kPa; solutions have negative values. Water moves by osmosis from higher (less negative) ψ to lower (more negative) ψ across a partially permeable membrane. In plant cells ψ = ψs + ψp (solute potential, always negative, + pressure potential, usually positive).
- Plant cells in a solution of higher ψ gain water and become turgid (ψp rises until ψ cell = ψ solution); in lower ψ they lose water, become flaccid and finally plasmolysed (membrane pulls from the wall; incipient plasmolysis when 50% of cells are plasmolysed, used to estimate ψs). Animal cells have no wall: red cells haemolyse in pure water and crenate in concentrated solutions, so body fluids are kept isotonic by osmoregulation.
- Active transport moves substances against a concentration gradient using carrier proteins (pumps) and energy from ATP. Primary active transport: ATP is used directly, e.g. the sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP, maintaining the resting potential and cell volume. Secondary active transport (co-transport): the Na⁺ gradient made by the pump drives another substance, e.g. Na⁺-glucose co-transport in ileum and kidney cells.
- Factors affecting active transport: anything affecting respiration (oxygen supply, temperature, glucose supply, respiratory inhibitors such as cyanide), and the number of carrier proteins. Cells doing much active transport have many mitochondria and microvilli.
- Significance of active transport: mineral ion uptake by root hairs; absorption of glucose and amino acids in the ileum; selective reabsorption in kidney tubules; nerve impulses (Na⁺/K⁺ pump); loading sucrose into phloem; secretion of acid by stomach cells.
- Bulk transport: endocytosis (membrane folds in to form a vesicle): phagocytosis (solids; neutrophils and macrophages engulfing bacteria) and pinocytosis (liquids); receptor-mediated endocytosis (e.g. cholesterol uptake). Exocytosis: vesicles fuse with the membrane to release contents (enzymes from pancreas, hormones, neurotransmitters, mucus). Both need ATP.
Common misconceptions
- "Water moves from low to high water potential." No: water moves from higher (less negative) to lower (more negative) water potential.
- "Facilitated diffusion requires ATP because it uses proteins." No: it uses proteins but is passive, down the gradient.
- "The sodium-potassium pump moves equal numbers of ions." No: it moves 3 Na⁺ out for every 2 K⁺ in.
- "Plasmolysis bursts plant cells." No: plasmolysis is shrinkage of the protoplast away from the wall; bursting (lysis) happens in animal cells in hypotonic solutions.
Worked examples
Example 1. A plant cell has ψs = −800 kPa and ψp = +300 kPa. It is placed in a solution of ψ = −700 kPa. Will water enter or leave the cell? Explain.
Solution
Cell ψ = ψs + ψp = −800 + 300 = −500 kPa. The solution (−700 kPa) has a lower (more negative) water potential than the cell, so water leaves the cell by osmosis; the cell becomes less turgid until the potentials are equal.
Example 2. Explain how glucose is absorbed into ileum cells against its concentration gradient.
Solution
The Na⁺/K⁺ pump in the basal membrane uses ATP to pump Na⁺ out of the cell, keeping Na⁺ inside low. Na⁺ then diffuses into the cell from the gut through a co-transporter protein that carries glucose with it, even against glucose's gradient (secondary active transport). Glucose leaves the cell into the blood by facilitated diffusion.
Practice questions
Distinguish between simple diffusion, facilitated diffusion and active transport.
Show answer
Simple diffusion: passive, down the gradient, through the bilayer, no proteins. Facilitated diffusion: passive, down the gradient, through channel or carrier proteins, specific, saturable. Active transport: against the gradient, carrier proteins (pumps), needs ATP.
Common mistake: Saying facilitated diffusion uses ATP.
State Fick's law and use it to explain one adaptation of alveoli.
Show answer
Rate of diffusion ∝ surface area × concentration difference ÷ thickness of membrane. Alveoli have a huge surface area, thin walls (one cell) and ventilation plus blood flow to keep a large concentration difference, maximising diffusion.
Common mistake: Leaving out the thickness factor.
Define water potential and explain the terms solute potential and pressure potential.
Show answer
Water potential is the tendency of water to leave a system (kPa; pure water = 0). Solute potential: lowering of ψ by dissolved solutes (always negative). Pressure potential: increase in ψ due to pressure, e.g. the wall pushing on the protoplast (usually positive). ψ = ψs + ψp.
Common mistake: Giving solutes a positive potential.
Describe what happens to a plant cell placed in a solution of lower water potential.
Show answer
Water leaves the vacuole by osmosis; pressure potential falls; the cell becomes flaccid; then the protoplast shrinks away from the cell wall (plasmolysis); the space between fills with the external solution.
Common mistake: Saying the cell wall shrinks.
Explain how the sodium-potassium pump works and its importance.
Show answer
A carrier protein uses one ATP to pump 3 Na⁺ out and 2 K⁺ in against their gradients; it maintains the resting membrane potential, controls cell volume, and creates the Na⁺ gradient used in co-transport.
Common mistake: Saying it moves ions down their gradients.
State four factors that affect the rate of active transport.
Show answer
Oxygen concentration; temperature; glucose (respiratory substrate) availability; respiratory inhibitors (cyanide); number of carrier proteins; pH.
Common mistake: Including concentration gradient as essential.
Distinguish between phagocytosis, pinocytosis and exocytosis.
Show answer
Phagocytosis: intake of solid particles in vesicles (bacteria by neutrophils). Pinocytosis: intake of fluid droplets. Exocytosis: release of materials from vesicles fusing with the membrane (enzymes, hormones).
Common mistake: Mixing up the directions.
Describe how to find the water potential of potato tissue.
Show answer
Cut equal potato cylinders; blot and weigh; place in a range of sucrose solutions of known ψ; after some hours blot and reweigh; calculate % change in mass; plot against ψ; the ψ where the line crosses zero change equals the tissue's water potential.
Common mistake: Not blotting the cylinders before weighing.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Engulfing of bacteria by neutrophils is: a. higher to lower water potential 2. Water moves by osmosis from: b. simple diffusion through the bilayer 3. Small lipid-soluble molecules cross the membrane by: c. phagocytosis 4. The point at which 50% of plant cells are plasmolysed is: d. incipient plasmolysis Show answer
1 → c (phagocytosis) · 2 → a (higher to lower water potential) · 3 → b (simple diffusion through the bilayer) · 4 → d (incipient plasmolysis)
- A type of endocytosis.
- From less negative to more negative.
- They dissolve in the lipid layer.
- Used to estimate solute potential.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
aquaporins · active transport · osmosis · exocytosis- Release of insulin from pancreatic cells occurs by .
- Water channels in membranes are called .
- Mineral ions are absorbed by root hair cells mainly by .
Show answer
1: exocytosis · 2: aquaporins · 3: active transport
- Vesicles fuse with the membrane.
- They speed up osmosis.
- Ions are more concentrated inside.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Using an ion gradient made by a pump to move another substance is: a. shrink and become crenated 2. Solute potential is always: b. secondary active transport (co-transport) 3. Red blood cells in concentrated salt solution: c. 3 Na⁺ out and 2 K⁺ in per ATP 4. The sodium-potassium pump moves: d. negative Show answer
1 → b (secondary active transport (co-transport)) · 2 → d (negative) · 3 → a (shrink and become crenated) · 4 → c (3 Na⁺ out and 2 K⁺ in per ATP)
- Example: sodium-glucose co-transport.
- Solutes lower water potential.
- Water leaves by osmosis.
- It creates the resting potential.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
vesicles from the Golgi · channel or carrier proteins without ATP · mitochondria · 0 kPa- Facilitated diffusion uses .
- Cells specialised for active transport usually have many .
- The water potential of pure water at atmospheric pressure is .
Show answer
1: channel or carrier proteins without ATP · 2: mitochondria · 3: 0 kPa
- It is passive.
- They supply ATP.
- Solutes make it negative.