Cell Membrane Structure and Function

Study Notes

A very thin, elastic, semipermeable membrane (allowing some substances to pass through and preventing others) that surrounds the cell.
Thickness: About 7-9.0 nm (70 - 90 Ao = Angstrom = 10-10 of meter).
Separates the cytoplasm from the extracellular fluid (ECF).
Maintains the cell's internal environment.
Transports macromolecules into and out of the cell.
Controls the distribution of ions (e.g., Na+, K+) in both the intracellular fluid (ICF) and ECF.
Contains receptors for hormones and transmitter substances.
Generates transmembrane membrane potentials.

Lipids (42%): Form the basic structure or backbone of the cell membrane.
- Phospholipids
- Cholesterol
- Glycolipids
Proteins (55%):
- Integral (intrinsic) Proteins: Bind to the hydrophobic center of lipids.
  - Transmembrane proteins: Span the entire bilayer and act as:
    - Channels: For diffusion of small ions.
    - Carriers: Transport substances (e.g., glucose).
    - Pumps: Actively transport ions.
    - Receptors: Initiate intracellular reactions when activated.
  - Present only on one side of the membrane: Act as enzymes (e.g., adenyl cyclase, which forms cyclic AMP from ATP).
- Peripheral (extrinsic) Proteins: Bind to the hydrophilic polar heads of the lipids or to the integral proteins.
  - Bind to the ICF surface of the membrane and contribute to the cytoskeleton.
  - Bind to the ECF surface of the membrane and contribute to the glycocalyx or cell coat.
Carbohydrates (3%): Contribute to the glycocalyx or cell coat.

Protein channels that allow passage of ions (e.g., Na+ ions) through the cell membrane.
Types:
- Leak ion channels: Always open.
- Chemical-gated ion channels: Open when a chemical substance binds to its receptor.
- Voltage-gated ion channels: Open by changes in the cell membrane potential.

The movement of substances across the cell membrane down its electrochemical gradient due to the continuous random motion of its particles.
Types:
- Simple diffusion: Movement of substances across the cell membrane down its electrochemical gradient by simple movement without the necessity of binding with carrier proteins.
- Facilitated diffusion: Movement of substances across the cell membrane down its electrochemical gradient, which needs the presence of carrier proteins.
- Osmosis: The passive flow of water across a semi-permeable membrane down a concentration gradient of water.

The movement of substances across the cell membrane against its electrochemical gradient.
Requires energy.
Uses carrier proteins.

The cell membrane provides structure and controls the movement of substances into and out of the cell.
Ion channels are important for regulating ion movement across the cell membrane.
Membrane transport can be passive (diffusion) or active (active transport and vesicular transport) and involves the use of carrier proteins.
Vesicular transport is important for the movement of large molecules and particles.