Membrane Transport


Overview of Membrane Transport

  • Cellular Control: Cells can control what moves in and out due to membrane's selective permeability.

  • Key Terms:

    • Solute: Any ion or molecule being transported across the membrane.

1. Passive Transport

  • Definition: Passive transport does not require energy for transport; the cell does not exert energy (work).

  • Mechanism: Solutes move from an area of high concentration to an area of lower concentration, described as moving "down the concentration gradient".

A. Simple Diffusion

  • Process: Solutes move directly through the lipid bilayer of the cell membrane.

  • Characteristics of Solutes: Generally, substances must be small, hydrophobic, non-polar, and lipophilic to diffusive pass:

    • Examples include:

    • Steroid hormones

    • Oxygen (O₂)

    • Carbon dioxide (CO₂)

    • Urea

B. Facilitated Diffusion

  • Process: This type of transport allows solutes to cross the membrane with the help of proteins that are specific to the solutes.

  • Types of Proteins:

    1. Channel Proteins:

    • Description: Form tunnels in the membrane and do not change shape during transport.

    • Example: Aquaporins – facilitate water movement, ion channels (Na+, K+, Ca2+, Cl channels).

    1. Carrier Proteins:

    • Description: Act like revolving doors, changing shape to move solutes across the membrane.

    • Examples include the transport of glucose, amino acids, and small molecules.

C. Osmosis

  • Definition: The movement of water across a membrane- a specific type of diffusion.

  • Mechanism: Water moves toward areas of higher solute concentration.

  • Methods of Water Movement:

    • Leakage through the phospholipid bilayer.

    • Through aquaporins.

    • Via processes like endocytosis and exocytosis.

2. Active Transport

  • Definition: Active transport requires energy usage at the time of transport; thus, the cell must perform work (primarily using ATP).

  • Mechanism: Solutes typically move from low concentration to high concentration, described as moving "up the concentration gradient".

A. Primary Active Transport

  • Example: Sodium-potassium pump (Na+/K+ pump).

    • Function: Pumps sodium (Na+) ions outside the cell and potassium (K+) ions into the cell.

    • Gradient: Both ions are moving against their concentration gradients.

    • Protein Role: Pumps or ATPases are proteins that facilitate this transport, powered by ATP.

B. Secondary Active Transport (Indirect Active Transport)

  • Mechanism: Utilizes the concentration gradient of a different solute as an energy source to transport another solute against its gradient.

C. Bulk or Vesicular Transport

  • Definition: A form of transport that requires energy to move large molecules.

  • Process Types:

    1. Exocytosis:

    • Mechanism: A vesicle moves to the plasma membrane, fuses with it, and releases its contents outside the cell.

    • Function: Increases membrane surface area while releasing useful solutes or waste.

    1. Endocytosis:

    • Mechanism: A vesicle forms from the plasma membrane and enters the cytosol, reducing membrane surface area.

    • Types of Endocytosis:

      • a. Pinocytosis:

        • Random uptake of any solute that gets trapped in the vesicle.

        • Occurrence: This process occurs in all cells.

      • b. Receptor-Mediated Endocytosis:

        • Deliberate uptake of specific solutes through receptor proteins on the membrane.

        • Occurrence: This process occurs in all cells.

      • c. Phagocytosis:

        • Known as "cell eating".

        • Specialized cellular function performed by some cells (e.g., amoebas, macrophages, neutrophils).

        • Phagosomes: Food vesicles that fuse with lysosomes for digestion.