Cell Membrane Structure and Function

Phospholipids and Membrane Fluidity

  • Phospholipid Structure: Phospholipids consist of a polar head group and two fatty acid tails. The properties of the fatty acids affect membrane characteristics.
    • Saturated Fatty Acids:
    • Increasing amounts of saturated fatty acids in phospholipid tails increase the transition temperature.
    • Result: Higher rigidity in cell membranes.
    • Unsaturated Fatty Acids:
    • Increasing amounts of unsaturated fatty acids decrease the transition temperature.
    • Result: Increased fluidity of the cell membrane.

Cholesterol's Role in Membrane Stability

  • Effect of Cholesterol:
    • At high temperatures:
    • Phospholipid tails tend to spread out, leading to increased fluidity.
    • Cholesterol inserts itself between phospholipids to reduce fluidity and increase rigidity.
    • At low temperatures:
    • Phospholipid tails pack tightly, risking membrane rigidity.
    • Cholesterol prevents tight packing, maintaining fluidity.

Temperature Effects on Bacterial Membranes

  • Bacteria adapt the composition of their phospholipids based on environmental temperatures.
    • Geothermal Bacteria (high temperatures):
    • Higher levels of saturated fatty acids are present to increase rigidity and prevent too much fluidity.
    • Glacial Bacteria (low temperatures):
    • Higher levels of unsaturated fatty acids are present to prevent excessive rigidity and ensure adequate fluidity.

Membrane Permeability

  • Permeability Factors: Membrane permeability is influenced by the polarity of molecules:
    • Ions (Na ext{⁺}, Cl ext{⁻}): Very impermeable due to their charge and polar nature.
    • Water: Exceptionally permeable through osmosis despite its polarity.
    • Nonpolar molecules (e.g., tryptophan): More permeable as they can easily pass through the phospholipid bilayer.

Types of Membrane Proteins

  • Integral Membrane Proteins:
    • Span the entire membrane.
    • Function as channels or transporters for ions and molecules.
    • Cannot be removed without damaging the cell membrane.
  • Peripheral Membrane Proteins:
    • Bound to either membrane surface (inner or outer).
    • Can be easily removed without damaging the membrane.

Protein Structure and Function

  • Amino Acids in Membranes:
    • Nonpolar amino acids face the lipid bilayer's interior.
    • Polar amino acids face the channel interior, allowing polar molecules to pass.

Prostaglandin Synthesis and Inhibition by Aspirin

  • Synthesis:
    • Prostaglandins are created from arachidonic acid by the enzyme prostaglandin H2 synthase.
    • Arachidonic acid (20 carbon fatty acid) is transported through a nonpolar channel to interact with this polar enzyme in the cytosol.
  • Aspirin Mechanism:
    • Aspirin blocks this channel, preventing arachidonic acid from reaching the enzyme, thus inhibiting prostaglandin production and the inflammatory response.
    • It attaches an acetate group to a serine residue in the channel, blocking the pathway.

Phospholipid Movement and Membrane Transport Mechanisms

  • Lateral vs. Transverse Diffusion:
    • Lateral diffusion: Rapid movement of phospholipids within the same layer of the membrane.
    • Transverse diffusion (flip-flop): Much slower, requiring enzymes (flippases) to facilitate movement between layers.

Membrane Transport Types

  • Passive Transport: Movement along a concentration gradient without energy usage.
    • Types of Passive Transport:
    • Simple Diffusion: Nonpolar small molecules passing directly through the membrane (e.g., O ext{₂}, CO ext{₂}). No transporter needed.
    • Facilitated Diffusion: Polar molecules moving via protein transporters (e.g., glucose, water).
  • Active Transport: Movement against a concentration gradient, requiring energy (ATP).
    • Types of Active Transport:
    • Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na ext{⁺}/K ext{⁺} pump).
    • Secondary Active Transport: Uses the energy from primary transport to move other substances against their gradient (symporters and antiporters).

Sodium-Potassium Pump

  • Function: Pumps 3 Na ext{⁺} ions out and 2 K ext{⁺} ions into the cell, crucial for maintaining resting membrane potential.
  • Energy Usage: Requires ATP to function (primary active transport).

Calcium and Nerve Impulses

  • Sodium influx after neurotransmitter binding leads to depolarization and action potential generation, followed by Ca ext{²⁺} release to propagate the signal in neurons.

Ion Channel Specificity and Potassium Channel Mechanism

  • Potassium Channels: Allow selective passage of K ext{⁺} while blocking Na ext{⁺} based on ion size and hydration states.

Summary of Fat Metabolism

  • Energy Sources:
    • Glucose: First source of energy from food intake.
    • Glycogen: Second source once glucose is depleted.
    • Lipids: Last resort for energy, yielding 6 times more energy compared to glycogen.
    • Triglycerides: Stored in adipose tissue and broken down by lipase to release fatty acids for energy.
  • Fatty Acid Activation:
    • Fatty acids are activated before transport across mitochondrial membranes using ATP (equivalent to 2 ATP).
  • Beta Oxidation: Takes place in the mitochondrial matrix, cleaving 2 carbon units off fatty acids to form acetyl CoA for the citric acid cycle.

Regulation of Lipase Activity

  • Hormonal Control: Hormones like epinephrine and glucagon activate lipase to mobilize fat stores during energy demands, while insulin inhibits this pathway by promoting phosphodiesterase to degrade cAMP.

Importance of Caffeine in Fat Metabolism

  • Role of Caffeine: Inhibits phosphodiesterase, prolonging cAMP's action and promoting fat breakdown, making it beneficial for workouts, particularly during fasting states.