Metabolism and Cell Membranes

Every cell membrane shows selective permeability. This means that some substances but not others can cross them in certain ways, at certain times. Gases, nonpolar molecules, and even water can move quite readily through the lipid bilayer.

In contrast, glucose and other large polar molecules must cross the membrane through transport proteins.

Passive Transport: In passive transport, solutes pass through the cell membrane with assistance from transport proteins in accordance with the concentration gradient. Transport proteins change shape to move substances into and out of the cell.

This animation (Audio - Important) illustrates passive transport.

This animation (Audio - Important) also illustrates passive transport.

Active Transport: In active transport, solutes can move against concentration gradients with assistance from transport proteins that can change their shape with energy supplied by ATP. The sodium-potassium pump is a major cotransport system in that it can set up concentration gradients that can in turn drive other transport activities.

These three animations (Audio - Important) illustrate active transport
active transport version 1,
active transport version 2,
active transport version 3.

REVIEW: _____ can readily diffuse across a lipid bilayer.
a. Glucose
b. Oxygen
c. Carbon dioxide
d. b and c

REVIEW: Sodium ions cross a membrane through transport proteins that receive an energy boost. This is an example of _____ .

REVIEW: Movement of a molecule against a concentration gradient is

REVIEW: The sodium-potassium pump
a. is a form of primary active transport.
b. pumps 3 Na+ out of a cell for every 2K+ that it pumps in.
c. uses ATP.
d. is a form of secondary active transport.
e. is described accurately in the first three answers.