Ch. 6 Cell Membranes Notes

Lipid Composition: The lipid composition of biological membranes varies, including different phospholipid structures based on fatty acid chain length, degree of saturation, and types of phosphate groups.
- Phospholipids: Fundamental building blocks of membranes.
  - Contains hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
Cholesterol's Role:
- Proportion in Animal Cells: Can compose up to 25% of animal cell membranes.
- Membrane Fluidity:
  - Decreases fluidity at higher temperatures, making the membrane more viscous.
  - Prevents freezing and maintains fluidity at lower temperatures.
Protein Content: Varies according to the function of the membrane.
- Peripheral Membrane Proteins:
  - Do not penetrate the lipid bilayer; found on the membrane's surface.
- Integral Membrane Proteins:
  - Have both hydrophobic and hydrophilic domains; can extend entirely or partially through the bilayer.
- Fluid Mosaic Model: Integral and peripheral proteins move freely, contributing to the dynamic nature of the membrane.

Cell Recognition and Adhesion: Proteins, primarily glycoproteins, play key roles in cell adhesion and recognition.
- Glycoproteins: Carbohydrates combined with proteins, involved in recognition and binding.
- Types of Binding:
  - Homotypic Binding: Cells of the same type bond to each other.
  - Heterotypic Binding: Different cell types bond together.
Cell Junctions: Specialized structures for holding cells together:
- Tight Junctions:
  - Impermeable junctions that prevent materials from passing through the intercellular space. E.g., lining of the digestive tract.
- Desmosomes:
  - Anchorage junctions, resembling Velcro, binding adjacent cells. E.g., found in skin and heart muscle.
- Gap Junctions:
  - Allow intercellular communication and facilitate the passage of small molecules and ions between cells. E.g., found in heart and smooth muscle tissues.

Selective Permeability: Some substances can pass through the membrane while others cannot.
Passive Transport: Movement that does not require energy; occurs along a concentration gradient.
Diffusion: Molecules move from areas of higher concentration to lower concentration until equilibrium is reached.
Types of Transport:
- Simple Diffusion: Small nonpolar molecules can easily pass through the lipid bilayer.
- Facilitated Diffusion: Requires assistance via membrane proteins:
  - Channel Proteins: Form channels through which specific ions or molecules can pass.
  - Carrier Proteins: Bind to substances to facilitate their passage across the membrane.
Osmosis: Special case of diffusion, specifically for water across a semi-permeable membrane, influenced by solute concentration.
- Hypertonic: Higher solute concentration outside the cell; cells lose water and may shrivel.
- Isotonic: Equal solute concentrations; no net movement, but water moves in both directions.
- Hypotonic: Lower solute concentration outside; cells swell and may burst as water moves in.
Factors Affecting Diffusion: Movement rate depends on the size of the substance, temperature, concentration gradient, and membrane permeability.

Active Transport: Movement of substances against their concentration gradient; requires energy, usually from ATP.
Transport Protein Types:
- Uniporter: Moves a single substance in one direction.
- Symporter: Moves two substances in the same direction.
- Antiporter: Moves two substances in opposing directions.
Primary Active Transport: Involves direct hydrolysis of ATP. E.g., Sodium-Potassium Pump.
- Functioning of Na+-K+ Pump:
  - Pumps Na+ out of the cell and K+ into the cell against their gradients, utilizing ATP energy.
Secondary Active Transport: Derived from the ion concentration gradient established by primary active transport; energy can be