Unit 6: Cell Transport

  • Fluid Mosaic Model of the Plasma Membrane

    • The cell membrane is made of a double layer (bilayer) of phospholipids that includes transport proteins and cholesterol molecules.

    • The polarity and orientation of membrane components are crucial to its properties.

  • The cell membrane separates the interior of the cell from the external environment.

  • The cell membrane regulates what enters and exits the cell to maintain homeostasis.

    • Homeostasis: The biological balance that cells (and organisms) must maintain for survival.

  • The cell membrane is semipermeable:

    • Very small molecules, such as water, can pass directly through the membrane.

    • Larger molecules, like sugar, must pass through "protein gates" (transport proteins).

2. Active Transport

  • Active Transport: Movement against the concentration gradient using energy (ATP).

    • Types of Active Transport:

    • Protein Pump: A mechanism that moves ions across the membrane against the concentration gradient.

    • Exocytosis: The process where materials are expelled from the cell.

    • Endocytosis: The process of taking substances into the cell.

      • Types of Endocytosis:

      1. Phagocytosis: The engulfing of solid particles by the cell.

      2. Pinocytosis: The engulfing of liquid substances.

3. Passive Transport

  • Passive Transport: Movement along the concentration gradient without energy.

    • Types of Passive Transport:

    • Simple Diffusion: Movement of very small particles dissolved in water from an area of higher concentration to an area of lower concentration along the concentration gradient.

    • Facilitated Diffusion: Movement of molecules across the membrane through protein channels without energy use.

    • Osmosis: The movement of water through a semipermeable membrane from an area of higher water concentration to an area of lower water concentration along the concentration gradient.

4. Diffusion and Cell Size

  • Analyze why cell size is limited by diffusion:

    • If a cell is too large, nutrients cannot diffuse quickly enough throughout the entire volume of the cell.

    • When the volume of a cell becomes too large relative to its surface area, diffusion rates decrease.

    • Smaller cells have a higher surface area to volume ratio (SA:V), enabling quicker diffusion across the cell.

  • Tonicity: Refers to the amount of solute (NaCl, sugar, iodine, protein, etc.) in a solution.

    • Water: Known as the universal solvent.

  • Hypertonic Solution: Has a higher solute concentration than the surrounding fluid; water molecules move out of the cell, causing it to shrivel.

  • Hypotonic Solution: Has a lower solute concentration than the surrounding fluid; water molecules move into the cell, causing it to expand and potentially lyse (burst).

  • Isotonic Solution: Has the same solute concentration as the surrounding fluid; there is no net movement of water molecules, and cell size remains constant.

    • Terminology:

    • "hyper" = more solute

    • "hypo" = less solute

    • "iso" = same solute