Educerie · AP · Biology
AP Biology 2.5 Membrane Transport
In one breath
- Selective permeability lets cells build concentration gradients.
- Passive transport is the net movement of molecules down a gradient, from high to low concentration, with no direct input of metabolic energy: simple diffusion, facilitated diffusion and osmosis.
- Active transport needs a direct input of energy, usually ATP, and a membrane pump, and can move molecules up a gradient, from low to high.
- Large molecules or large amounts move in vesicles: endocytosis folds the membrane inward to bring material in, exocytosis fuses vesicles with the membrane to release it, and both need energy.
- In data, movement against a gradient or a rate that collapses without ATP means active transport.
What you must be able to do
- Describe how selective permeability lets concentration gradients form, and how passive and active transport differ in direction and in energyConcentrations inside and outside a cell: "Identify the type of transport and justify"
- Describe endocytosis and exocytosis, and explain why they need energyA diagram of a vesicle forming at the membrane: "Describe the process shown"
- Propose a new investigation from the evidence in an experiment"Propose a follow-up experiment to test whether uptake depends on ATP"
Once you have read the note, tick off each one you can do in Check your understanding.
In the exam. Section I multiple choice gives concentrations inside and outside a cell and asks which way a substance moves and whether energy is needed, gives uptake data with and without a metabolic poison, or shows a model of vesicles fusing with or budding from the membrane. In Section II it feeds Interpreting and evaluating experimental results (decide from the data whether transport is active or passive), Scientific investigation (identify variables and controls, or propose a follow-up experiment), and Conceptual analysis (explain how a cell secretes a large protein).
1Concentration gradients
A concentration gradient is a difference in the concentration of a substance between two regions. Across a membrane, it means more of the substance on one side than the other.
Gradients exist only because the membrane is selectively permeable. If ions could cross the bilayer freely, they would spread evenly and every gradient would vanish. Because the core blocks them, the cell can hold, say, K⁺ at a high concentration inside and Na⁺ at a high concentration outside.
Molecules are always moving at random. Where there are more of them, more move away by chance than move in. The result is a net movement from high concentration to low, down the gradient, until the concentrations are equal. At that point molecules still move both ways, but the numbers balance: dynamic equilibrium.
2Passive transport: downhill, no energy spent
Passive transport is the net movement of molecules from a region of higher concentration to a region of lower concentration, without the direct input of metabolic energy. The energy comes from the random motion of the molecules themselves; the gradient is what drives it.
There are three kinds, each with its own topic:
- Simple diffusion through the bilayer, for small nonpolar molecules such as O₂ and CO₂ (2.4).
- Facilitated diffusion through channel or carrier proteins, for ions and large polar molecules (2.6).
- Osmosis, the diffusion of water across a membrane (2.7).
All three run down a gradient, and all three stop (at net zero) when the gradient is gone.
3Active transport: uphill, energy spent
Active transport requires the direct input of energy, usually by breaking down ATP, to move molecules. In many cases it moves them from a region of lower concentration to a region of higher concentration, against their gradient. That is something passive transport can never do.
Active transport always uses a membrane protein, a pump, which uses the energy to change shape and carry the molecule across (2.8 describes the best-known pump). Cells use it to take in scarce nutrients and ions, to push out waste, and to build up the gradients that passive transport later runs down.
Down the gradient is passive and costs no ATP; up the gradient is active and costs ATP.
How to tell from data: stop ATP production (with a metabolic poison, or by removing oxygen) and see what happens. Active transport slows or stops. Passive transport carries on.
4Endocytosis and exocytosis: moving material in bulk
Some things are too big for any channel or pump: proteins, polysaccharides, whole bacteria, or simply a large amount of material at once. Cells move these in vesicles, small sacs of membrane. Both processes need energy, because the cell must reshape its membrane and move the vesicles.
- Endocytosis brings material in. The plasma membrane folds inward around the material, the fold deepens into a pocket, and its neck pinches off to form a new small vesicle inside the cell, holding material from outside. White blood cells engulf bacteria this way.
- Exocytosis sends material out. A vesicle inside the cell moves to the plasma membrane, its membrane fuses with the plasma membrane, and its contents are released outside. Pancreatic cells secrete digestive enzymes and the hormone insulin this way, and nerve cells release neurotransmitters this way.
Figure 1 shows both. In (a), follow the membrane from a shallow dip, to a deep pocket, to a closed vesicle. In (b), the same steps run in reverse: a vesicle meets the membrane, opens, and its contents end up outside.
Exocytosis links to 2.1: the secretory vesicles that the Golgi packages are the ones that fuse with the plasma membrane.
5Reading an experiment, and proposing the next one
An invented experiment: root tissue is placed in a solution of potassium ions, and K⁺ uptake is measured under three conditions.
| Condition | K⁺ inside cells at start / mmol L⁻¹ | K⁺ in solution / mmol L⁻¹ | Rate of K⁺ uptake / µmol g⁻¹ h⁻¹ |
|---|---|---|---|
| Air bubbled through, 25 °C | 80 | 2 | 6.4 |
| Nitrogen bubbled through (no oxygen), 25 °C | 80 | 2 | 0.5 |
| Air bubbled through, 25 °C, metabolic poison added | 80 | 2 | 0.4 |
Two pieces of evidence point to active transport. K⁺ is entering cells that already contain 40 times more K⁺ than the solution, so it is moving against its gradient. And when respiration is stopped, by removing oxygen or by the poison, uptake almost stops, so it depends on energy from respiration.
AP often asks you to propose a new investigation from evidence like this. A good proposal names one thing to change, what to measure, and what to keep the same. For example: "To test whether uptake depends on the amount of ATP available, measure K⁺ uptake at several concentrations of the poison, keeping temperature, K⁺ concentration and tissue mass the same; if uptake is active, it should fall as poison concentration rises." Or: "To test whether uptake is carried by a protein, measure uptake at increasing external K⁺ concentrations; protein-carried transport should level off once all the proteins are in use."
Practice questions
5 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.