Cell Membrane Overview and Functions

Cell Membrane

• Surrounds entire cell and organelles
• Fluid in nature, molecules move
• Phospholipid bilayer:
  • Head - polar/hydrophilic
  • Tail - nonpolar/hydrophobic
• Proteins:
  • Integral - carrier and channel proteins
  • Peripheral - receptors and antigens

Functions of Cell Membrane

• Semi-permeable barrier (selective)
  • Maintains differences in ICF and ECF composition and fluid composition in organelles
  • Protects cells from toxic substances
  • Excretes waste products
  • Transports nutrients
• Receives signals from outside:
  • Chemical signals
  • Electrical signals
• Site for attachment to neighboring cells

Transport Through Cell Membrane

• Essential for cellular life
• Necessary for exchange for cell function
• Involves incorporation of biological molecules and discharge of waste products
• Movement of particles (solute) across a membranous barrier
• Dependent on membrane permeability, transmembrane solute concentration, and solute size/charge
• Three mechanisms: passive, facilitated, and active transport
• Some mechanisms require energy and a transmembrane protein, while others do not

Transport Mechanisms

• Passive:

  • Simple diffusion
  • Facilitated diffusion
  • Filtration
  • Osmosis
  • Dialysis

• Active:

  • Primary active transport
  • Secondary active transport
  • Endocytosis/Exocytosis

Methods of Transport

• Passive:
  • Diffusion:
    • Simple (lipid bilayer, protein channels)
  • Osmosis
  • Filtration
  • Dialysis
• Active
  • Leaky channels
  • Voltage gated channels
  • Ligand gated channels

Simple Diffusion

• Movement of molecules from higher to lower concentration until equilibrium is reached.
• Can occur through lipid bilayer or protein channels.
• Factors affecting rate: lipid solubility, molecular size/weight, temperature, membrane thickness, surface area, concentration gradient, pressure gradient, and electrical gradient.

Diffusion Through Lipid Bilayer

• Lipid-soluble substances (O2, CO2, alcohols, steroids)
• Lipid-insoluble substances (water, urea, sugar) diffuse through spaces between lipids.
• Electrolytes are generally impermeable due to charge and size.

Protein Channels

• Open/leaky channels (Na+, K+)
• Gated channels: open under specific conditions (ligand-gated, voltage-gated, etc.).
• Na+, K+, Ca++ channels.
• Channel mutations can cause channelopathies (e.g., muscle and brain disorders).

Factors Affecting Rate of Diffusion

• Lipid solubility
• Molecular size and weight
• Temperature
• Thickness of the membrane
• Surface area
• Concentration gradient
• Pressure gradient
• Electrical gradient

Gibbs-Donnan Equilibrium

• Explains concentration differences in diffusible ions across a semipermeable membrane when one compartment contains nondiffusible ions.
  • Proteins are non-diffusible anions.
  • Concentration of Na+ is higher in compartment A compared to compartment B.

Osmosis & Osmotic Pressure

• Water movement across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.
• Osmotic pressure is the minimum pressure required to prevent osmosis.
• Depends on the total number of particles of solute, not the type of particle
• Osmoles (mOsmols) express the concentration of osmotically active particles (e.g. total number of particles in 1 gram molecular weight of a molecule dissolved 1 kg of water

Applied Solutions

• Isotonic: Fluids have the same osmolarity as plasma (290 mOsmols); red blood cells neither shrink nor swell in isotonic solutions. Ex. 0.9% NaCl, 5% glucose solutions.
• Hypotonic: RBC swell and hemolysis (bursting) may occur.
• Hypertonic: RBC shrink due to water moving out.

Filtration in Capillaries

• Fluid and solute movement through a membrane due to pressure difference
  • Arterial end: Filtration pressure > osmotic pressure --> fluid moves out.
  • Mid-capillary: Pressures are balanced.
  • Venous end: Osmotic pressure > filtration pressure --> fluid reenters.

Dialysis

• Separation of larger from smaller particles to remove waste products in cases of renal failure.

Active Transport

• Primary active transport
  • Requires energy (ATP)
  • Examples: Na+-K+ pump, Ca++ pump, H+-K+ pump
  • Carrier protein has ATPase activity, which, when activated, hydrolyzes ATP, releasing energy that causes a conformational change in the carrier protein.
• Secondary active transport
  • Uses the electrochemical gradient established by primary active transport
• Endocytosis
  • Pinocytosis - all cells
  • Receptor-mediated endocytosis- Highly selective to transport specific large molecules, requiring energy and calcium. E.g., low-density lipoproteins, viruses
  • Phagocytosis - specialized cells, internalizing large multimolecular particles like bacteria. Forms pseudopodia to engulf and bring material into the cell.

Peculiarities of Active Transport

• Carrier-mediated transport
• Rapid rate of transport
• Against electrochemical gradient (uphill)
• Expenditure of energy by ATPase activity of the carrier protein
• Carrier protein shows specificity, saturation, competitive inhibition, and blocking.

Active Transport, Summary

• Na+-K+ pump (example) - ATP hydrolysis - conformational change- movement of ions.
• Regulators: hormones, conc. of ions

Types of Transporters

• Uniporters - Transport one molecule in one direction
• Symporters - Transport two molecules in the same direction
• Antiporters - Transport two molecules in opposite directions