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

What to Know

Small nonpolar molecules can cross the bilayer more easily than ions or large polar molecules.
Transport proteins provide specific routes for substances that cannot pass directly through lipids.
Active transport uses energy to move substances against gradients, while passive transport follows gradients.
Always connect this topic back to the larger unit: Cells.

Detailed Notes

Plasma membrane and membrane transport belongs to Unit 2: Cells. The plasma membrane is a selective boundary whose structure controls which materials cross and how they move. Treat the topic as a relationship between structure, process, evidence, and biological function instead of memorizing an isolated definition.

Key relationships: Small nonpolar molecules can cross the bilayer more easily than ions or large polar molecules. Transport proteins provide specific routes for substances that cannot pass directly through lipids. Active transport uses energy to move substances against gradients, while passive transport follows gradients.

Model to use: Draw a membrane with phospholipids, proteins, and concentration gradients to predict the direction and method of transport. A clear model should label the parts, show the direction of movement or information flow when relevant, and connect the diagram back to the biological outcome.

AP-style move: For membrane transport questions, identify the substance, its charge or polarity, the gradient, and whether energy is required. Strong answers name the concept, apply it to the scenario, and explain the reasoning in complete sentences.

Key Vocabulary

Fluid mosaic model

A model describing membranes as moving phospholipids with embedded proteins.

Selective permeability

The ability of a membrane to allow some substances through more easily than others.

Channel protein

A membrane protein that forms a passage for specific ions or molecules.

Carrier protein

A membrane protein that changes shape to move a substance across the membrane.

Electrochemical gradient

A combined concentration and charge difference across a membrane.

Bulk transport

Movement of large materials into or out of a cell using vesicles.

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.

Official Alignment

Reference and Attribution

Biology concept reference: Biology for AP Courses by OpenStax, Rice University. Access for free at OpenStax. Licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License. EduCompanion used this source as a conceptual reference and rewrote the study notes in original wording.