Big picture
The plasma membrane is a selectively permeable boundary. Its phospholipid bilayer separates the cell from its environment while proteins control communication, recognition, and the movement of materials.
Core concepts
Membrane structure
- Phospholipids have hydrophilic heads and hydrophobic tails.
- Cholesterol helps stabilize membrane fluidity.
- Membrane proteins act as channels, carriers, receptors, enzymes, and anchors.
Passive transport
Passive transport moves substances down their concentration gradient without direct cellular energy.
- Simple diffusion: small nonpolar molecules cross the bilayer directly.
- Facilitated diffusion: channels or carriers help polar molecules and ions cross.
- Osmosis: water moves across a selectively permeable membrane toward the side with more dissolved solute.
Active transport
Active transport moves substances against a gradient and requires energy. Primary active transport uses ATP directly, while secondary active transport uses energy stored in another ion gradient.
Knowledge check
- Why can oxygen cross the membrane more easily than sodium ions?
- What happens to an animal cell in a hypotonic solution?
- How does the sodium-potassium pump differ from a channel protein?
Quick review plan
Draw one membrane, label its major parts, and add one example each of simple diffusion, facilitated diffusion, osmosis, and active transport.