Transport proteins in cell membranes allow specific substances to pass through either through passive or active transport. Passive transport involves facilitated diffusion, where solutes bind to transport proteins and diffuse across membranes down their concentration gradients without energy. Active transport pumps solutes against their gradients, such as calcium and sodium-potassium pumps which use energy from ATP to move ions across membranes.

1. © Cengage Learning 2015
5.7 How Do Ions and Charged Molecules
Cross Cell Membranes?
• Transport proteins allow only specific substance to cross
the membrane
• Passive transport: solutes move through membrane;
requires no energy
– Example: facilitated diffusion - solute binds to transport protein
and moves across membrane with its concentration gradient

2. © Cengage Learning 2015
How Do Ions and Charged Molecules
Cross Cell Membranes? (cont’d.)
• Active transport: transport protein pumps a solute against
its concentration gradient; requires energy
– Examples:
• Calcium pumps
• Sodium–potassium pumps

3. © Cengage Learning 2015
How Do Ions and Charged Molecules
Cross Cell Membranes? (cont’d.)
Cytoplasm
Extracellular
Fluid
Figure 5.28 The sodium–potassium pump. This transport protein (gray)
actively
transports sodium ions (Na+) from cytoplasm to extracellular fluid, and
potassium ions (K+) in the other direction. The transfer of a phosphate
group ( P ) from ATP provides energy required for transporting the ions
against their concentration gradient.