lecture recording on 12 March 2025 at 13.14.55 PM

Phospholipids and Plasma Membrane

• Composition of Plasma Membrane:

  • Made up of phospholipids with:

    • Polar head (hydrophilic)

    • Nonpolar tails (hydrophobic)

  • Tails can be either saturated or unsaturated fatty acids.

• Selectivity of Membrane:

  • Most membranes are impermeable to large molecules.

  • Small molecules (e.g., oxygen, CO2) can diffuse easily across, while larger molecules (e.g., water) require transport proteins for movement across the plasma membrane.

Transport Proteins

• Function:

  • Assist in the transport of substances across the plasma membrane.

  • Essential for moving both small and larger molecules.

• Amino Acid Properties:

  • Transport proteins are composed of amino acids that may be polar or nonpolar.

  • Amino acids spanning the hydrophobic region of the membrane must be nonpolar.

  • Amino acids that are outside the membrane (exposed to the aqueous environment) must be polar.

Passive vs. Active Transport

• Passive Transport:

  • No energy required; molecules move by diffusion from high to low concentration.

  • Example: Simple diffusion, facilitated diffusion through transport proteins.

• Active Transport:

  • Requires energy (ATP) to move substances against their concentration gradient (low to high concentration).

  • Example: Sodium-potassium pump.

Vesicle Transport

• Vesicle Formation:

  • Proteins synthesized on ribosomes are packaged into vesicles after processing in the Golgi apparatus.

  • Vesicles may excrete proteins from the cell or engulf substances via phagocytosis (cell eating).

Aquaporins

• Function:

  • Aquaporins are specific transport proteins facilitating passive transport of water across the membrane, allowing water molecules to move from areas of high concentration to low concentration.

Membrane Fluidity

• Importance of Membrane Fluidity:

  • Membrane fluidity is crucial for proper function; it allows movement of proteins and lipids within the bilayer.

  • Fluidity is influenced by:

    • Temperature: As temperature decreases, membranes can transition from fluid to solid states.

    • Types of Fats: Unsaturated fats increase fluidity due to their kinked structure preventing tight packing, while saturated fats are solid and promote tighter packing.

• Role of Cholesterol:

  • Cholesterol modulates membrane fluidity:

    • At high temperatures, it can decrease fluidity.

    • At low temperatures, it can maintain fluidity by preventing phospholipid packing.

Adaptations in Cold Environments

• Frog Adaptation:

  • Frogs have membranes rich in unsaturated fats and cholesterol preventing cell damage during freezing.

  • Research into genetic modification for similar adaptations in humans is ongoing.

• Fish Adaptations:

  • Ice fish possess transparent blood due to mutations in hemoglobin; adapted to cold, oxygen-rich waters with high cholesterol content in membranes to prevent freezing.

Membrane Proteins

• Types of Membrane Proteins:

  • Integral Proteins: Span the membrane, usually containing nonpolar amino acids in the hydrophobic regions.

  • Peripheral Proteins: Bound to the membrane's surface, containing polar amino acids.

• Functions of Membrane Proteins:

  • Facilitate enzymatic reactions, signal transduction, cell recognition, and attachment to the cytoskeleton.

Selective Permeability

• Mechanisms of Selective Permeability:

  • The hydrophobic core of the membrane permits the passage of small, nonpolar molecules.

  • Larger or polar molecules require specific transport proteins to cross the membrane.

  • Example: Glucose cannot pass freely and needs transport proteins due to its size and polarity.

Clinical Relevance of Diffusion and Transport

• Understanding Transport Mechanisms:

  • Key to treating diseases like diabetes; knowledge of osmosis and diffusion is critical for medical professionals.

  • Medications must be designed considering their polarity and size for absorption across cell membranes.