AP Biology Unit 2 Topic 4: Plasma membrane and membrane transport

What to Know

Identify the system, surroundings, and scale before explaining a process.
Use diagrams, graphs, and tables as evidence rather than decoration.
For quantitative questions, keep units visible from the setup through the final answer.
Always connect this topic back to the larger unit: Cells.

Detailed Notes

Plasma membrane and membrane transport belongs to Cells, so study it as part of a larger scientific system rather than as a stand-alone fact. Start by identifying what is being described, what is changing, and what evidence would let you defend a claim.

In AP Biology, strong answers usually connect a visible pattern to an underlying mechanism. That means explaining not only what happens, but why it happens at the particle, organism, environmental, or system level.

For AP-style questions, expect this topic to appear with graphs, diagrams, data tables, experiments, or written scenarios. Your job is to describe the evidence, apply the correct concept, and explain the reasoning that connects them.

Key Vocabulary

Phospholipid bilayer

A double layer of phospholipids that forms the basic structure of cell membranes.

Selective permeability

The property that allows some substances to cross a membrane more easily than others.

Passive transport

Movement across a membrane that does not require cellular energy.

Active transport

Movement across a membrane that requires energy, often against a concentration gradient.

Transport protein

A membrane protein that helps move specific substances across the membrane.

Quick Practice

How would you explain Plasma membrane and membrane transport in one or two AP-style sentences?

Name the concept, apply it to a specific example or source, and explain the reasoning that connects the evidence to your answer.