Cell Membrane and Transport Mechanisms

Eukaryotic Cell Membranes

• Internal Membranes:
  • Enclose organelles; very thin (~5nm width) and flexible.
  • Dynamic; can allow substances to cross or restrict movement.

General Functions of Cell Membrane

• Barrier:
  • Regulates what goes in and out of the cell/organelle.
• Transport:
  • Mediates communication, attachment between cells, and plays a role in cell crawling.
• Main Components:
  • Phospholipids: Basic structural unit.
  • Sterols/Cholesterol: Modulate fluidity.
  • Proteins: Various functions including glycoproteins and glycolipids.

Structure of Cell Membrane

• Phospholipid Bilayer:
  • Composed mainly of phospholipids; hydrophilic heads and hydrophobic tails.
  • Unsaturated vs. Saturated Fatty Acids:
  • Saturated: No double bonds; straight chains.
  • Unsaturated: One or more double bonds; kinks in the chain.

Membrane Proteins

• Types of Membrane Proteins:
  • Integral Proteins: Embedded within the membrane; tightly associated.
  • Example: alpha helices and beta barrels.
  • Peripheral Proteins: Loosely associated; can move laterally.
• Fluid Mosaic Model:
  • Describes the membrane as a fluid structure with various proteins floating in or on the fluid lipid bilayer.

Factors Influencing Fluidity

• Temperature:
  • High temperature increases fluidity; low temperature decreases fluidity.
• Fatty Acid Saturation:
  • Unsaturated fatty acids increase fluidity due to kinks.
• Cholesterol:
  • Regulates membrane fluidity; maintains optimal fluidity at varying temperatures.

Membrane Composition

• Membranes contain various sugars, proteins, and lipids.
• Cell Cortex:
  • A network of proteins that interacts with the membrane, reinforcing it and helping with cell shape and function.

Transport Across Cell Membranes

• Transport Proteins: Allow substances to cross the membrane.
  • Channel Proteins:
  • Form continuous channels for fast transport; specific for size and charge.
  • Carrier Proteins:
  • Bind specific substances, change shape, and release them on the other side; involve slower transport.
  • Pumps:
  • Active transport mechanisms that require ATP to move substances against the concentration gradient.

Diffusion and Transport Types

• Simple Diffusion:
  • Small nonpolar molecules (O2, CO2) move from high to low concentration directly through the membrane.
• Facilitated Diffusion:
  • Molecules move down their concentration gradient through transport proteins (helped by channel or carrier proteins).

Active Transport

• ATP-Powered Pumps:
  • Move substances from lower to higher concentration using energy, such as the sodium-potassium pump (Na+/K+ pump).
  • Critical for maintaining cellular electrochemical gradients.
  • Example:
  • Na+/K+ pump: Pumps 3 Na+ out and 2 K+ in.
• Osmosis:
  • Movement of water across membranes to balance solute concentrations, crucial for cell function and integrity.

Additional Pumps

• Calcium Pumps:
  • Maintain high concentrations of Ca2+ outside cells, crucial for signaling processes.
• Proton Pumps:
  • Help maintain pH and electrochemical gradients within cells and organelles.