Biology Chapter 5: Cellular Membranes

Membrane Structure

• Phospholipids are arranged in a bilayer with globular proteins inserted in the lipid bilayer, forming a fluid mosaic model
• The fluid mosaic model describes proteins floating in or on the fluid lipid bilayer, like boats on a pond

Cellular Membrane Components

• 4 main components of cellular membranes:
  • Phospholipid bilayer (flexible matrix, barrier to permeability)
  • Transmembrane proteins (integral membrane proteins)
  • Interior protein network (peripheral membrane proteins)
  • Cell surface markers (glycoproteins and glycolipids)

Phospholipid Structure

• Consists of:
  • Glycerol (a 3-carbon polyalcohol)
  • 2 fatty acids attached to the glycerol (nonpolar and hydrophobic)
  • Phosphate group attached to the glycerol (polar and hydrophilic)
• Spontaneously forms a bilayer with fatty acids on the inside and phosphate groups on both surfaces

Membrane Fluidity

• Bilayers are fluid due to hydrogen bonding between water molecules, allowing individual phospholipids and unanchored proteins to move through the membrane
• Environmental influences on membrane fluidity:
  • Saturated fatty acids make the membrane less fluid than unsaturated fatty acids
  • Warm temperatures make the membrane more fluid than cold temperatures
  • Sterols like cholesterol can increase or decrease membrane fluidity depending on the temperature

Membrane Proteins

• Various functions:
  • Transporters
  • Enzymes
  • Cell-surface receptors
  • Cell-surface identity markers
  • Cell-to-cell adhesion proteins
  • Attachments to the cytoskeleton
• Structure relates to function:
  • Diverse functions arise from diverse structures of membrane proteins
  • Common structural features related to their role as membrane proteins
• Types of membrane proteins:
  • Peripheral proteins (anchoring molecules attach membrane protein to surface)
  • Integral membrane proteins (span the lipid bilayer, with nonpolar regions embedded in the interior and polar regions protruding from both sides)

Passive Transport

• Movement of molecules through the membrane without energy input
• Diffusion: movement of molecules from high to low concentration
• Diffusion will continue until concentration is equal in all regions
• Factors affecting permeability:
  • Hydrophobic interior of the membrane repels polar molecules but not nonpolar molecules
  • Limited permeability to small polar molecules
  • Very limited permeability to larger polar molecules and ions
• Facilitated diffusion:
  • Molecules that cannot cross the membrane easily move through proteins
  • Channel proteins: hydrophilic channel when open
  • Carrier proteins: bind specifically to molecules they assist

Osmosis

• Net diffusion of water across a membrane toward a higher solute concentration
• Osmotic concentration:
  • Hypertonic solution: higher solute concentration
  • Hypotonic solution: lower solute concentration
  • Isotonic solution: same osmotic concentration as the cell
• Osmotic pressure:
  • Force needed to stop osmotic flow
  • Cell in a hypotonic solution gains water, causing cell to swell and creating pressure

Active Transport

• Requires energy input (ATP) to move substances from low to high concentration
• Uses highly selective carrier proteins
• Types of carrier proteins:
  • Uniporters: move one molecule at a time
  • Symporters: move two molecules in the same direction
  • Antiporters: move two molecules in opposite directions
• Sodium-potassium (Na+-K+) pump: direct use of ATP for active transport, moving 3 Na+ out of the cell and 2 K+ into the cell against their concentration gradient

Bulk Transport

• Endocytosis:
  • Movement of substances into the cell
  • Phagosytosis: cell takes in particulate matter
  • Pinocytosis: cell takes in only fluid
  • Receptor-mediated endocytosis: specific molecules are taken in after binding to a receptor
• Exocytosis:
  • Movement of substances out of the cell
  • Requires energy