Section 7.2: Plasma Membrane

Overview

• Focus of this section: Plasma membrane
• Importance: Maintains balance within cells
• Main topics covered:
  • Maintaining a balance
  • Structure of the plasma membrane

Maintaining a Balance

• Plasma membrane: Boundary between the cell and its environment
  • Separates inside from outside
  • Controls entry of nutrients and exit of waste
• Homeostasis: Maintaining a consistent environment
  • Keeps necessary substances in and harmful substances out
  • Like walls of a house regulating who comes in

Selective Permeability

• Definition: Allows some molecules to enter while keeping others out
  • Permeable: Allows passage
  • Selective: Choosy about what passes through
• Examples:
  • Water can easily enter and exit
  • Larger molecules use channels to cross the membrane

Structure of the Plasma Membrane

• Composition: Phospholipid bilayer
  • Lipids: Fats and oils
  • Phospholipids: Lipids with a phosphate group
    • Two fatty acid tails + phosphate group
    • Forms a sandwich-like structure
  • Polar heads: Mixes with water (outer ends)
  • Non-polar tails: Does not mix with water (center)

Fluid Mosaic Model

• Fluid: Phospholipids move around within the membrane
• Mosaic: Combination of diverse proteins and lipids
• **Components: **
  • Cholesterol: Stabilizes membrane, prevents phospholipids from sticking
    • Excess cholesterol can harden membranes, block arteries
  • Transport proteins: Form channels for molecules to move in and out
  • External proteins and carbohydrates: Help with cell identification
  • Internal proteins: Provide flexibility, attach to the cell’s internal structure

Visualization

• Extracellular fluid: Outside the cell
• Intracellular fluid: Inside the cell (cytoplasm)
• Plasma membrane structure:
  • Transport proteins
  • Cholesterol
  • External proteins and carbohydrates for identification
  • Internal proteins

Review

• Covered balance maintenance, plasma membrane, and its structure
• Discussed the fluid mosaic model
• End of section