Cell Membrane Structure and Function

Cell Membrane

• Also known as the plasma membrane
• Found in all cells
• Semipermeable and selective; controls what enters and exits the cell
• Functions:
  • Controls what enters and exits the cell
  • Maintains homeostasis (internal balance)
  • Provides protection and support for the cell
  • Allows for recognition and communication among cells

Structure of the Cell Membrane

• Fluid and flexible (Fluid Mosaic Model)
• Lipid bilayer: Two layers of phospholipids
  • Phosphate head is polar (hydrophilic, "likes water")
  • Fatty acid tails are non-polar (hydrophobic, "hates water")
• Interstitial fluid (extracellular) is outside the cell
• Cytosol (intracellular) is inside the cell

Components of the Cell Membrane

• Transport Proteins: Help move large or charged molecules across the membrane (e.g., glucose, K+, Na+, Cl−).
  • Positive or negative charge (+/−) indicates need for transport protein
• Glycolipids and Glycoproteins: Cell recognition (like nametags); have attached carbohydrate chains.
• Cholesterol: Helps stabilize the membrane.

Membrane Proteins

• Integral: Embedded within the membrane
  • Transmembrane: Integral protein that passes completely through the membrane
• Peripheral: Attached to one side of the membrane
• Lipid-anchored: Attached to the membrane through a lipid

Passive Transport: Three Types

• Simple Diffusion: Diffusion directly across the cell membrane (e.g., oxygen, carbon dioxide).
• Facilitated Diffusion: Diffusion with the help of transport proteins (e.g., glucose, ions).
• Osmosis: Diffusion of only water, following higher solute concentration

Passive Transport: Diffusion

• Random movement of particles from high to low concentration.
• Continues until equilibrium is reached.
• Molecules continue to move in equal and opposite directions.

Passive Transport: Facilitated Diffusion

• Specific particles diffuse through transport proteins in the membrane.
• Transport proteins "select" only certain molecules for passage.
• Transports larger or charged molecules.

Passive Transport: Osmosis

• Water diffuses from low to high solute concentration.
• Plants prefer hypotonic solutions.
• Human cells prefer isotonic solutions.
• Hypotonic: Cell swells (may explode)
• Isotonic: Cell functions normally.
• Hypertonic: Cell shrinks (crenates)

Key Terms in Osmosis

• Solution: Mixture of solute and solvent.
• Solvent: Dissolving material (e.g., water).
• Solute: Dissolved material (e.g., sugar).
• Concentration: Amount of solute per amount of solvent.
• Concentration gradient: Difference in concentration across a membrane.
• Equilibrium: Equal concentration on both sides of a membrane.
• Net movement: Direction of majority particle movement.

Active Transport

• Requires energy (ATP)
• Moves particles against the concentration gradient (low to high concentration)

Two Types of Active Transport

• Protein Pumps:
  • Primary Active Transport: Directly uses ATP to move molecules across a membrane against their concentration gradient (e.g., sodium-potassium pump).
    • 3 Na+ out, 2 K+ in important for muscle and brain function
  • Secondary Active Transport: Indirectly uses energy from primary active transport to move molecules across a membrane against their electrochemical gradient.
• Bulk Transport: Large amounts of material move into or out of the cell.
  • Endocytosis: Moving material into the cell
    • Phagocytosis ("cell eating")
    • Pinocytosis ("cell drinking")
  • Exocytosis: Moving material out of the cell

Bulk Transport: Endocytosis

• Taking material into the cell.
• Phagocytosis: "Cell eating" (e.g., immune system cells engulfing bacteria)
• Pinocytosis: "Cell drinking" (e.g., absorbing vitamins and fluids in the intestines)

Bulk Transport: Exocytosis

• Moving material out of the cell.
• Examples: Expelling wastes or secreting hormones/neurotransmitters.