Cell Membrane and Transport Study Guide

Cells and Transport Teaching Resources · High school biology; AP extensions where noted.

This guide connects membrane structure to transport. Use it after cell parts and before an osmosis investigation. Students need to distinguish solute from solvent and understand a concentration gradient. Materials: notebook, pencil, and a teacher-selected membrane diagram.

Learning objectives

Use molecule properties and gradients to explain membrane crossing.
Distinguish passive transport, active transport, and vesicle transport.
Predict water movement while stating permeability assumptions.

A decision process

Identify what moves: water, an ion, a small nonpolar molecule, or a large cargo.
State whether the membrane permits it to cross and whether a suitable protein is present.
Compare conditions on each side. For ions, consider electrical as well as concentration differences.
Identify whether energy input or coupling to another gradient is needed.

Compare the mechanisms

Simple diffusion: small nonpolar molecules such as oxygen can cross the lipid region down their concentration gradient.
Facilitated diffusion: a channel or carrier permits passive movement down the relevant gradient. A protein’s presence does not by itself mean ATP is used.
Osmosis: net water movement across a selectively permeable membrane, governed by water potential. In simple cases with equal pressure and nonpenetrating solutes, net water movement is toward the side with higher effective solute concentration.
Primary active transport: a pump uses an energy source directly. The animal sodium–potassium ATPase uses ATP to move three sodium ions out for every two potassium ions in.
Secondary active transport: movement down one electrochemical gradient drives another substance uphill. It need not consume ATP directly at the cotransporter.
Endocytosis and exocytosis: membrane vesicles bring material in or release it out. These are energy-requiring cellular processes rather than diffusion through a channel.

Practice with stated assumptions

Oxygen concentration is higher outside a cell, and oxygen can cross its membrane. Predict its initial net movement and mechanism.
A membrane is impermeable to sodium without proteins. An open sodium channel is added. What information is still needed to predict sodium movement?
A cell contains 0.20 M nonpenetrating solute and is placed in 0.10 M nonpenetrating solute at the same initial pressure. Predict initial net water movement, assuming the cell is water-permeable.
A transporter moves one substance uphill while another moves downhill. Explain why the first substance’s transport is active even if the transporter does not hydrolyze ATP.
A cell releases a packaged protein. Explain the role of a vesicle and the plasma membrane.

Feedback on these original prompts

1: oxygen initially moves net inward by simple diffusion. 2: both sodium concentrations and membrane voltage are needed; concentration alone is insufficient for an ion. 3: water initially moves net into the cell. Its outside solution is hypotonic relative to the cell under these assumptions. 4: the downhill gradient supplies energy; another process must maintain that gradient. 5: a vesicle fuses with the plasma membrane and releases its contents by exocytosis.

Teaching notes

At equilibrium, individual molecules keep moving even when net movement is zero. Isotonic describes effects on cell volume and does not mean water is stationary. Plant walls resist expansion but do not replace the membrane’s selective role. For AP, use water potential and electrochemical gradients explicitly; for an introductory lesson, label the simplifying assumptions instead of implying they hold universally.