Biology: Plasma Membranes Overview

Plasma Membranes

• Plasma membranes are approximately 8nm thick (8000 times the thickness of paper).
• They control the passage of substances into and out of cells.
• They exhibit selectivity in chemical exchange, a fundamental characteristic of living cells.

Membrane Structure

• Membranes are primarily composed of phospholipids and proteins, with carbohydrates also playing a role.
• Phospholipids are the most abundant molecules, forming a bilayer.
• Phospholipids are amphipathic, possessing hydrophilic heads and hydrophobic tails.
• Proteins are also amphipathic, with both hydrophilic and hydrophobic regions.

Fluid Mosaic Model

• Membranes are fluid structures with a mosaic of embedded proteins within (or attached to) a phospholipid bilayer.
• Proteins and phospholipids are not rigidly fixed in place, but the tails of the phospholipids are held together by hydrophobic interactions.

Membrane Protein Diversity

• Membrane proteins are categorized into integral and peripheral proteins.
• Integral proteins penetrate the hydrophobic interior of the membrane, often traversing the entire membrane (transmembrane proteins).
• Peripheral proteins are not embedded within the membrane but are loosely associated with the hydrophilic portions of integral proteins or other membrane surfaces.

Membrane Transport

• Biological membranes are selectively permeable, meaning they regulate the movement of substances across their boundaries.
• Nonpolar molecules (e.g., hydrocarbons, O2, CO2) readily cross the lipid bilayer.
• Polar molecules (e.g., water) and ions diffuse more slowly through the membrane or via membrane proteins.

Passive Transport

• Passive transport does not require energy input from the cell.
• It follows the concentration gradient, moving substances from high to low concentration.
• Simple diffusion - molecules move across a membrane without assistance.
• Facilitated diffusion - molecules move across a membrane through proteins.

Active Transport

• Active transport requires energy input from the cell.
• It moves solutes against their concentration gradient (from low to high concentration).
• The most common energy source is ATP.
• Includes several types: primary, secondary, bulk (exocytosis, endocytosis).

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• It moves water from high to low water concentration.
• Key terms: isotonic, hypertonic, hypotonic solutions. Cell response to different solutions vary depending on if the cell has or does not have a cell wall

Types of Transmembrane Proteins

• Channels – allow molecules to pass through by providing a tunnel.
• Carriers – bind to molecules and change shape, facilitating their movement across the membrane.

Other Forms of Bulk Transport

• Exocytosis – materials are released from the cell in vesicles.
• Endocytosis – materials are taken into the cell in vesicles. There are three types:
  • Phagocytosis (“cell eating”) – solids
  • Pinocytosis (“cell drinking”) – liquids
  • Receptor-mediated endocytosis – specific molecules.

Sodium-Potassium Pump

• Active transport mechanism that moves Na+ out of the cell and K+ into the cell (against their concentration gradients).
• Crucial role in maintaining cell volume and nerve impulse transmission