Midterm 1

  1. Fluid Mosaic Model: The cell membrane is composed of a phospholipid bilayer embedded with various proteins, cholesterol molecules, and other biomolecules. This model describes the dynamic nature of the membrane, where proteins and lipids can move laterally within the bilayer.

  2. Hydrophilic and Hydrophobic Regions: Phospholipids, the primary components of the membrane, have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This arrangement forms a barrier that controls the passage of substances into and out of the cell.

  3. Concentration Gradients and Permeability: Concentration gradients refer to the difference in concentration of a substance between two regions. Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. Permeability refers to the ability of substances to pass through the membrane. Small, non-polar molecules (like oxygen and carbon dioxide) can diffuse freely through the lipid bilayer, while larger molecules and ions require specific transport mechanisms.

  4. Brownian Motion and Diffusion: Brownian motion describes the random movement of particles suspended in a fluid. Diffusion is the result of Brownian motion and drives the movement of molecules across a membrane.

  5. Osmosis: Osmosis is the diffusion of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process is crucial for maintaining cell hydration and regulating internal solute concentrations.

  6. Crenation and Lysis: Crenation occurs when a cell shrinks due to water leaving the cell by osmosis, typically in a hypertonic environment. Lysis, on the other hand, is the bursting of a cell due to the influx of water, usually in a hypotonic environment.

  7. Transport Mechanisms:

    • Simple Diffusion: Small, non-polar molecules move directly through the lipid bilayer.

    • Facilitated Diffusion: Larger or polar molecules use protein channels or carriers to facilitate their movement across the membrane.

    • Active Transport: Requires energy (ATP) to move molecules against their concentration gradient, usually through protein pumps.

    • Secondary Active Transport: Uses the energy stored in an electrochemical gradient to drive the movement of molecules.

    • Filtration: Involves the movement of substances across a membrane due to pressure differences.

    • Phagocytosis and Pinocytosis: These are forms of endocytosis where cells engulf large particles (phagocytosis) or fluid droplets (pinocytosis) by forming vesicles.

    • Endocytosis and Exocytosis: Processes by which cells internalize (endocytosis) or expel (exocytosis) large molecules or particles by forming membrane-bound vesicles.