Movement Across Membranes
Cells must take in oxygen, glucose, ions and water, and get rid of carbon dioxide and other wastes, all across a membrane that is a barrier to most water-soluble substances. There are six ways in which substances cross: simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. The exam asks you to describe and explain each process, to decide from data which process is taking place, and to investigate diffusion with Visking tubing and agar. The key distinctions are whether the process is passive or uses energy from ATP, whether it is down or against a concentration gradient, and whether it needs membrane proteins.
Simple diffusion
Molecules and ions in a liquid or gas are in constant, random motion. If there is a region where a substance is more concentrated, random movement will, on average, carry more particles out of that region than into it, until the concentration is the same everywhere.
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, as a result of their random movement. It is a passive process: it does not use energy from ATP.
Particles still move in both directions; "net" means more move one way than the other. When the concentrations are equal (equilibrium), random movement continues, but there is no net movement.
Simple diffusion across a membrane happens when molecules pass straight through the phospholipid bilayer. This is possible only for:
- small, non-polar molecules, such as oxygen and carbon dioxide;
- lipid-soluble molecules, such as steroid hormones, alcohol and some vitamins (A, D, E, K);
- to a small extent, very small polar molecules such as water and urea, which can slip between phospholipids.
Ions and large polar molecules (glucose, amino acids) cannot cross the hydrophobic core by simple diffusion.
Factors affecting the rate of diffusion
The rate of diffusion across a surface increases with:
- a steeper concentration gradient (larger difference in concentration);
- a larger surface area across which diffusion occurs;
- a shorter diffusion distance (thinner surface);
- a higher temperature (more kinetic energy, faster random movement);
- for membranes: smaller molecules, and non-polar / lipid-soluble molecules (which dissolve in the bilayer).
Gas exchange surfaces show all of these: alveoli have an enormous surface area, walls one cell thick, and a steep gradient maintained by ventilation and blood flow.
The relationship can be summarised as rate of diffusion . You do not need to quote Fick's law, but it is a useful way to remember the factors.
Facilitated diffusion
Ions and polar molecules can diffuse across a membrane only with the help of membrane proteins.
Facilitated diffusion is the diffusion of ions or polar molecules through a membrane via specific channel proteins or carrier proteins, down a concentration gradient. It is passive: no ATP is used.
- Channel proteins form water-filled, hydrophilic pores. Each is specific to particular ions or molecules, because of the size of the pore and the charges lining it. Many are gated: they open only when stimulated (e.g. voltage-gated channels in neurones). Aquaporins are channel proteins for water.
- Carrier proteins have a binding site complementary to a specific molecule, such as glucose. The molecule binds, the carrier changes shape, and the molecule is released on the other side. Glucose enters most cells by facilitated diffusion through carrier proteins.
The rate of facilitated diffusion depends on the concentration gradient and on the number of channel or carrier proteins in the membrane. When all the proteins are working at their maximum rate, a steeper gradient cannot increase the rate further.
(Horizontal axis: concentration difference across the membrane; vertical axis: rate of uptake. The straight line is simple diffusion: the rate is proportional to the concentration difference. The curve that levels off is facilitated diffusion: at high concentration differences the rate is limited by the number of channel or carrier proteins, which are all in use.)
Osmosis
Osmosis is the diffusion of water molecules across a partially permeable membrane, from a region of higher water potential to a region of lower water potential. It is passive. It is important enough to have its own note: Osmosis and water potential.
Active transport
Sometimes cells need to take in substances that are already more concentrated inside the cell, or remove substances that are more concentrated outside. This means moving them against their concentration gradient, which needs energy.
Active transport is the movement of molecules or ions across a membrane against their concentration gradient (from lower to higher concentration), through carrier proteins (sometimes called pumps), using energy from the hydrolysis of ATP produced by respiration.
How it works:
- The ion or molecule binds to a specific site on the carrier protein (on the side where it is less concentrated).
- ATP binds to the carrier and is hydrolysed to ADP and phosphate, releasing energy (the phosphate may attach to the carrier).
- This causes the carrier protein to change shape, carrying the ion or molecule across the membrane.
- The ion or molecule is released on the other side, where it is more concentrated.
- The phosphate is released and the carrier returns to its original shape.
The best-known example is the sodium–potassium pump ( pump), present in the cell surface membrane of almost all animal cells. For each ATP molecule hydrolysed, it moves three ions out of the cell and two ions in, both against their concentration gradients. This maintains the gradients that neurones use to send impulses and that many cells use to drive the uptake of other substances.
Other examples:
- root hair cells absorb mineral ions such as nitrate from soil water, where they are much less concentrated than in the cytoplasm;
- epithelial cells of the small intestine absorb the last of the glucose and amino acids from the gut lumen;
- kidney tubule cells reabsorb glucose and ions;
- companion cells load sucrose into phloem.
Because active transport depends on ATP, anything that reduces respiration reduces active transport: lack of oxygen, low temperature, lack of respiratory substrate, or respiratory inhibitors such as cyanide. Cells that carry out a lot of active transport have many mitochondria. Simple and facilitated diffusion are not affected by respiratory inhibitors.
(Horizontal axis: oxygen concentration around root tissue; vertical axis: rate of uptake of nitrate ions by active transport. As oxygen concentration increases, aerobic respiration produces more ATP and the rate of uptake increases; it levels off when the number of carrier proteins (or another factor) becomes limiting.)
Endocytosis and exocytosis
Large molecules, particles and even whole cells are too big to pass through channel or carrier proteins. They are moved in bulk, in vesicles, by changing the shape of the membrane itself. These processes use energy from ATP (to move the membrane and the vesicles along the cytoskeleton), so they are active.
- Endocytosis is the bulk movement of materials into a cell: the cell surface membrane invaginates (folds inwards) around the material and pinches off to form a vesicle (or vacuole) inside the cytoplasm.
- Exocytosis is the bulk movement of materials out of a cell: a vesicle moves to the cell surface membrane, fuses with it, and releases its contents outside the cell.
There are two types of endocytosis:
- Phagocytosis ("cell eating"): the uptake of solid material, such as bacteria taken in by phagocytes (neutrophils and macrophages). The phagocytic vacuole then fuses with lysosomes, whose hydrolytic enzymes digest the contents.
- Pinocytosis ("cell drinking"): the uptake of liquid, containing dissolved molecules, in small vesicles.
Exocytosis is used for secretion: for example, digestive enzymes from pancreatic cells, hormones such as insulin, mucus from goblet cells, and neurotransmitters at synapses. In plants, exocytosis delivers materials for building the cell wall. The vesicles are made by the Golgi body. Exocytosis adds membrane to the cell surface; endocytosis removes it.
Comparing the processes
| Process | Direction relative to concentration gradient | Energy from ATP? | Membrane proteins needed? | What moves | Example |
|---|---|---|---|---|---|
| Simple diffusion | down | no (passive) | no | small non-polar or lipid-soluble molecules | , across alveolar walls |
| Facilitated diffusion | down | no (passive) | yes: channel or carrier proteins | ions and polar molecules | glucose into red blood cells; through channels |
| Osmosis | down a water potential gradient | no (passive) | no (but aquaporins speed it up) | water | water into root hair cells |
| Active transport | against | yes | yes: carrier proteins (pumps) | specific ions and molecules | pump; mineral ions into root hairs |
| Endocytosis | not applicable (bulk) | yes | no (membrane forms vesicle) | large molecules, particles, cells, liquids | phagocytosis of bacteria |
| Exocytosis | not applicable (bulk) | yes | no (vesicle fuses with membrane) | secretions | release of enzymes, hormones, neurotransmitters |
Name the process described in each case.
(a) Carbon dioxide leaves a respiring cell. (b) A neutrophil engulfs a bacterium. (c) Root hair cells take up magnesium ions from soil water in which their concentration is lower than in the cells. (d) Glucose enters a red blood cell through a specific transport protein, down its concentration gradient. (e) Insulin is released from a β-cell in the pancreas.
Solution
(a) Simple diffusion (small non-polar molecule, down its gradient through the bilayer). (b) Endocytosis (phagocytosis). (c) Active transport (against the concentration gradient). (d) Facilitated diffusion (through a carrier protein, passive). (e) Exocytosis.
The uptake of potassium ions by carrot tissue was measured with and without cyanide, a respiratory inhibitor. Uptake without cyanide was arbitrary units per hour; with cyanide it was arbitrary units per hour.
(a) Calculate the percentage decrease in uptake caused by cyanide. (b) Explain what the results suggest about how potassium ions are taken up. (c) Suggest why uptake did not fall to zero.
Solution
(a) .
(b) Cyanide stops aerobic respiration, so ATP production falls; uptake decreases greatly, so most potassium uptake is by active transport, which needs ATP (to change the shape of carrier proteins). The uptake is probably against a concentration gradient.
(c) Some potassium ions still enter by diffusion (facilitated diffusion through channel proteins) down a concentration gradient, which does not need ATP; or some ATP is still made by anaerobic respiration; or ions diffuse into the cell walls (which are fully permeable).
The rate at which a type of cell absorbs substance P was measured at different external concentrations of P. The rate increased in proportion to concentration at first, then levelled off at higher concentrations, even though the concentration inside the cells remained lower than outside. A respiratory inhibitor had no effect on the rate.
Identify the method of uptake and explain the evidence.
Solution
Facilitated diffusion.
- Uptake is down a concentration gradient (inside concentration lower than outside).
- A respiratory inhibitor has no effect, so it does not need ATP: it is passive.
- The rate levels off at high concentrations, so it is limited by the number of channel or carrier proteins: when all are in use, a steeper gradient cannot increase the rate. (Simple diffusion would give a rate that keeps increasing in proportion to the gradient.)
Discs of beetroot tissue were placed in a solution of potassium chloride. One batch was kept in aerated solution and the other in solution bubbled with nitrogen. The total potassium in the tissue was measured.
| Time / min | 0 | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|---|
| K⁺ uptake, aerated / µmol g⁻¹ | 0 | 3.0 | 4.2 | 5.4 | 6.6 | 7.8 | 9.0 |
| K⁺ uptake, nitrogen / µmol g⁻¹ | 0 | 2.6 | 2.8 | 2.9 | 3.0 | 3.0 | 3.0 |
(a) Calculate the rate of uptake by the aerated tissue between 10 and 60 minutes. (b) Explain the differences between the two results.
Solution
(a) Rate .
(b)
- In the first 10 minutes uptake is rapid and similar in both (3.0 and 2.6 µmol g⁻¹), so it does not depend on oxygen;
- this initial uptake is passive: diffusion into the cell walls / facilitated diffusion down a concentration gradient.
- After 10 minutes, uptake continues at a steady rate (0.12 µmol g⁻¹ min⁻¹) in aerated tissue, but almost stops in nitrogen (levels off at 3.0).
- Oxygen is needed for aerobic respiration, which produces ATP;
- ATP is needed for active transport of through carrier proteins, against the concentration gradient (once the cells' concentration exceeds the solution's);
- without oxygen little ATP is made, so active transport stops; the plateau shows that passive uptake has reached equilibrium.
Investigating diffusion with Visking (dialysis) tubing
Visking tubing is a partially permeable artificial membrane. It has tiny pores that let water and small molecules (glucose, iodine) pass through but not large molecules (starch, proteins). It behaves like a cell membrane in that respect, although it has no proteins.
- Method: soak a length of Visking tubing in water to soften it, tie one end, and fill it with a mixture of starch and glucose solutions. Tie the other end, rinse the outside, and place the tube in a boiling tube of distilled water at a constant temperature. At intervals, test samples of the surrounding water for glucose (Benedict's test or glucose test strip) and for starch (iodine solution).
- Results: glucose is detected outside the tubing; starch is not.
- Explanation: glucose molecules are small enough to pass through the pores and diffuse down their concentration gradient into the water; starch molecules are too large.
- Variation: put starch inside and iodine solution outside. The contents of the tubing turn blue-black as iodine diffuses in, while the outside stays orange-brown.
- Variables to investigate: concentration gradient (glucose concentration inside), temperature, surface area (length of tubing). The dependent variable could be time for glucose to be first detected outside, or the glucose concentration outside after a fixed time (semi-quantitative Benedict's or a colorimeter).
- Controls and standardisation: same length and type of tubing, volumes of solutions, temperature; rinse outside of tubing before starting so any spilt solution does not give a false positive.
- Diffusion is net movement down a concentration gradient. Do not say molecules "want" to move or "move to fill space".
- Facilitated diffusion is passive. It does not use ATP, even though it uses proteins. Only active transport (and bulk transport) uses ATP.
- Active transport uses carrier proteins, not channel proteins.
- Do not write "against the concentration gradient" for diffusion, or "down" for active transport. Learn the direction with each definition.
- "Energy" alone is vague: write "energy from ATP (produced by respiration)".
- Phagocytosis is a type of endocytosis, not a separate process.
- Learn the syllabus definitions of diffusion and active transport word for word: examiners look for "net movement", "down/against a concentration gradient", "random movement", "carrier proteins", "ATP".
- In data questions, look for these clues: rate affected by oxygen, cyanide or temperature beyond normal diffusion effects → active transport; rate levels off with increasing concentration → protein-mediated (facilitated or active); uptake continues against a gradient → active transport.
- "Suggest why the cell has many mitochondria" → to make ATP for active transport (or exocytosis).
- In Visking tubing questions, explain results in terms of molecule size relative to pore size, and concentration gradients.
- Diffusion: net movement of molecules or ions from higher to lower concentration, by random movement; passive.
- Simple diffusion through the bilayer: small non-polar or lipid-soluble molecules (O₂, CO₂).
- Rate increases with steeper gradient, larger surface area, shorter distance, higher temperature.
- Facilitated diffusion: ions and polar molecules through specific channel or carrier proteins, down a gradient; passive; rate limited by number of proteins.
- Active transport: against a concentration gradient, through carrier proteins, using ATP from respiration; e.g. Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in).
- Endocytosis (phagocytosis of solids, pinocytosis of liquids) and exocytosis (secretion) move bulk materials in vesicles and use ATP.
- Visking tubing is partially permeable: glucose passes through, starch does not.
Practice questions
- Define diffusion.
- State three factors that increase the rate of diffusion across a cell surface membrane.
- Explain why oxygen can cross the cell surface membrane by simple diffusion but glucose cannot. (3 marks)
- State two similarities and two differences between facilitated diffusion and active transport.
- Describe the process of active transport. (4 marks)
- Explain why cells in the lining of the small intestine have many mitochondria.
- Distinguish between phagocytosis and exocytosis.
- A length of Visking tubing containing starch solution was placed in a beaker of iodine solution. After 20 minutes the contents of the tubing were blue-black and the solution outside was still orange-brown. Explain these observations. (3 marks)
- The concentration of inside a cell is about and outside about . (a) Calculate how many times more concentrated is outside. (b) Name the process that maintains this difference and explain why it stops if the cell is deprived of oxygen.
- A scientist measured the rate of glucose uptake by isolated cells at increasing glucose concentrations, both with and without a respiratory inhibitor. Without inhibitor the rate levelled off at 50 units; with inhibitor the rate also levelled off at 50 units. In both, uptake occurred only while the external concentration was higher than the internal. A second substance, Q, was taken up only without inhibitor, and continued after the internal concentration exceeded the external. Identify the mechanism for glucose and for Q, explaining your reasoning. (6 marks)
Answers
- The net movement of molecules or ions from a region of higher concentration to a region of lower concentration (down a concentration gradient), as a result of their random movement; passive.
- Any three: steeper concentration gradient; larger surface area; thinner membrane / shorter diffusion distance; higher temperature; smaller or more lipid-soluble molecules.
- Oxygen is small and non-polar, so it can dissolve in / pass through the hydrophobic core of the phospholipid bilayer. Glucose is polar (hydrophilic) and relatively large, so it cannot pass through the hydrophobic core; it needs a carrier protein (facilitated diffusion).
- Similarities: both use specific membrane proteins (carrier proteins); both are selective/specific; both can be limited by the number of proteins. Differences: facilitated diffusion is passive, active transport uses ATP; facilitated diffusion is down a concentration gradient, active transport against; facilitated diffusion can use channel proteins, active transport only carrier proteins.
- The molecule/ion binds to a specific carrier protein; ATP is hydrolysed (to ADP + Pi), releasing energy; the carrier protein changes shape; the molecule/ion is moved across the membrane against its concentration gradient and released; the carrier returns to its original shape.
- They absorb glucose and amino acids by active transport (against the concentration gradient), which needs ATP; mitochondria carry out aerobic respiration to make that ATP.
- Phagocytosis is the bulk uptake of solid material into a cell by endocytosis: the membrane folds around the material and forms a vesicle (phagocytic vacuole). Exocytosis is the bulk release of materials from a cell: a vesicle fuses with the cell surface membrane and releases its contents outside.
- Visking tubing is partially permeable; iodine molecules/ions are small enough to pass through its pores and diffuse into the tubing down their concentration gradient, where they react with starch to give blue-black; starch molecules are too large to pass out, so no starch reaches the outside to react with iodine.
- (a) times (about 12 times). (b) Active transport by the sodium–potassium pump (carrier proteins pumping 3 out and 2 in per ATP). Without oxygen, aerobic respiration stops, little ATP is produced, so the pump cannot change shape to move against its gradient; then diffuses back in and the difference falls.
- Glucose: facilitated diffusion. Not affected by respiratory inhibitor, so no ATP needed (passive); uptake only down its concentration gradient; rate levels off because limited by number of carrier proteins (all in use). Q: active transport. Uptake stops with respiratory inhibitor, so depends on ATP from respiration; uptake continues against the concentration gradient (when internal concentration exceeds external), which only active transport can achieve, using carrier proteins.