Transport Across the Cell Membrane - In Depth Notes

Overview of the Plasma Membrane

  • Definition: A selective barrier that surrounds the cell, providing structure + regulating what enters + exits.

  • Composition:

    • Consists of phospholipids, proteins, + cholesterol.

    • Phospholipid bilayer forms the fundamental structure, with cholesterol molecules integrated in.

    • Proteins are categorized as integral (spanning across the membrane) + peripheral (attached to the membrane surface), often described as "icebergs floating in a sea of phospholipids."

Characteristics of the Plasma Membrane

  • Selectively Permeable: Some substances pass through freely, while others do not.

  • Factors Affecting Permeability:

    1. Solubility in lipids.

    2. Presence of driving forces (concentration gradient).

    3. Size of the molecules.

Types of Membrane Transport

Passive Transport

  • Does not require energy; relies on concentration gradients.

  1. Diffusion through the Lipid Bilayer:

    • Lipid-soluble substances (e.g., respiratory gases, certain alcohols, + urea) can pass freely.

    • Driven by concentration gradients.

  2. Diffusion through Ion Channels:

    • Water-soluble substances (e.g., ions, small sugars, + amino acids) require integral membrane proteins to facilitate transport.

  3. Facilitated Diffusion:

    • Transport of water-soluble substances requires specific transporter proteins.

    • Solutes like glucose + fructose use specific carriers; movement rates depend on concentration gradients + transporter availability.

Active Transport

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

  1. Primary Active Transport:

    • Directly uses ATP (e.g., sodium-potassium pump).

    • Maintains low Na+ + high K+ concentrations in cells, consuming ~40% of cellular ATP.

  2. Secondary Active Transport:

    • Indirectly utilizes ATP by leveraging concentration gradients of other ions (e.g., Na+ or H+).

    • Can involve:

      • Symporters: Move two substances in the same direction.

      • Antiporters: Move two substances in opposite directions.

Membrane Transport of Complex Molecules

  1. Exocytosis:

    • Process of moving large molecules out of the cell, prevalent in secretory cells.

    • Involves the fusion of vesicles containing the secretions with the plasma membrane.

  2. Endocytosis:

    • Movement of large particles or molecules into the cell. Types include:

      • Pinocytosis: Engulfing of small particles + fluids.

      • Phagocytosis: Engulfing of larger particles.

      • Receptor-Mediated Endocytosis: Specific substances are taken in via binding to receptors in regions called caveolae.