AP Biology Unit 2 membrane transport: a complete overview of the plasma membrane, permeability, passive and active transport, and osmoregulation

What membrane transport actually demands

Membrane transport is the heart of AP Biology Unit 2 and a favorite source of both multiple-choice sets and free-response questions, because it combines structure-to-function reasoning with quantitative skills (water potential, surface-area-to-volume ratio, and reading rate graphs). The examiners want you to explain why the membrane lets some things across and not others, to classify any transport event as passive or active, and to predict and calculate water movement.

This guide ties together the matching dot-point pages, each of which has its own practice questions: plasma membranes, membrane permeability, membrane transport, facilitated diffusion, tonicity and osmoregulation, and mechanisms of transport.

The plasma membrane

The plasma membrane is a fluid-mosaic: a phospholipid bilayer studded with proteins, cholesterol and surface carbohydrates. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails, so in water the heads face out to the aqueous interior and exterior while the tails turn inward. This makes the membrane selectively permeable. Embedded transport proteins move specific substances; receptor proteins receive signals; cholesterol buffers fluidity; and carbohydrates allow recognition. Fluidity must stay in a working range, and organisms adjust their lipids (saturation, cholesterol) to maintain it across temperatures.

Selective permeability

The hydrophobic core does the selecting:

• Small nonpolar molecules (oxygen, carbon dioxide) cross the bilayer freely.
• Small polar molecules (water) cross slowly, and quickly through aquaporins.
• Large polar molecules (glucose) and all ions cannot cross unaided and need transport proteins.

Permeability also depends on the membrane: more unsaturated lipids and higher temperature raise fluidity and permeability, while cholesterol buffers it.

Passive transport

Passive transport moves substances down their gradient with no cellular energy:

• Simple diffusion of small nonpolar molecules across the bilayer.
• Osmosis, the diffusion of water across a selectively permeable membrane.
• Facilitated diffusion of polar molecules and ions through channel proteins (pores, including aquaporins) or carrier proteins (bind, change shape, release).

Facilitated diffusion is limited by the number of transport proteins, so its rate plateaus at high concentration (saturation), unlike simple diffusion.

Tonicity and water potential

Tonicity compares external and internal solute concentration. In a hypotonic solution water enters (animal cells may burst, plant cells become turgid); in a hypertonic solution water leaves (animal cells crenate, plant cells plasmolyse); in an isotonic solution there is no net movement. Predict the direction with water potential:

Water moves toward lower (more negative) water potential. The constants ($R = 0.0831$ liter bar per mole kelvin) and the formula are on the AP equations sheet. Organisms osmoregulate using contractile vacuoles, kidneys, and the active transport of solutes.

Active and bulk transport

Active transport moves substances against their gradient using ATP. The sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP, building an electrochemical gradient (both a charge and a concentration difference) that powers nerve impulses and secondary active transport (for example sodium-driven glucose uptake). Bulk transport moves large material in vesicles: endocytosis into the cell and exocytosis out, both requiring energy.

How membrane transport is examined

• Classify and compare. Decide whether a transport event is passive or active from its direction relative to the gradient and its energy use.
• Calculations. Water potential and solute potential, surface-area-to-volume ratio, and fold or percentage changes from data.
• Graph interpretation. Recognizing saturation (facilitated diffusion) versus a continuing rise (simple diffusion).
• Application. Predicting what happens to a cell in a given solution, and explaining osmoregulation.

Check your knowledge

A mix of recall, calculation and application questions covering Unit 2 membrane transport. Attempt them under timed conditions, then check against the solutions.

1. State the two parts of a phospholipid and where each faces in the membrane. (2 marks)
2. Identify which crosses the bilayer most easily: oxygen, glucose or a chloride ion, and explain. (2 marks)
3. Define facilitated diffusion and state whether it uses ATP. (2 marks)
4. A cell is placed in a hypertonic solution. Identify the direction of net water movement and the effect on an animal cell. (2 marks)
5. Calculate the solute potential of a $0.2\ M$ non-ionizing solution at $300\ K$ ($i = 1$, $R = 0.0831$). (2 marks)
6. State how many sodium and potassium ions the sodium-potassium pump moves per ATP and in which directions. (2 marks)
7. Explain why the rate of facilitated diffusion plateaus at high solute concentration. (2 marks)
8. Identify the type of transport that brings a large particle into the cell in a vesicle. (1 mark)