Anatomy and Physiology / Unit 2: Cells and Tissues
2.1 Plasma Membrane and Transport
Notes
The plasma membrane is a phospholipid bilayer with embedded proteins and cholesterol. It is selectively permeable.
- Phospholipid heads are hydrophilic (face water); tails are hydrophobic (face inward).
- Passive transport needs no ATP: simple diffusion (high to low concentration), facilitated diffusion (through carrier or channel proteins), osmosis (diffusion of water across a membrane), and filtration (pushed by hydrostatic pressure, as in the kidney).
- Active transport uses ATP to move solutes against their gradient (for example the sodium-potassium pump). Vesicular transport includes endocytosis and exocytosis.
- Tonicity: in an isotonic solution cells keep their shape; in a hypotonic solution water enters and cells swell (may burst); in a hypertonic solution water leaves and cells shrink (crenate).
Practice
Oxygen moves from the blood into tissue cells by simple diffusion. This happens because:
Diffusion is passive movement down a concentration gradient.
Red blood cells are placed in pure water. What is most likely to happen?
Pure water is hypotonic to the cell interior, so water moves in by osmosis.
Which process requires ATP?
Active transport moves ions against their gradients and requires ATP.
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