Cell transport

Fluid Mosaic Model

  • Cell Membrane Composition:

    • Comprised mainly of phospholipids, proteins, and carbohydrates.

    • Represents a dynamic structure with molecules in constant flux, allowing for flexibility and functionality.

Lipids

  • Phospholipids:

    • Characteristics: Flexible and semipermeable.

    • Function: Acts as the gatekeeper of the cell membrane.

  • Glycolipids:

    • Purpose: Provides stability, recognition, and adhesion functions.

  • Cholesterol:

    • Role: Adds stability and rigidity to the membrane.

Proteins

  • Types of Membrane Proteins:

    • Peripheral Proteins:

      • Located on one side of the membrane.

    • Integral Proteins:

      • Spans both phospholipid layers.

  • Channel Proteins:

    • Features: Contains open pores allowing specific sizes/charges to pass.

  • Carrier Proteins:

    • Mechanism: Molecules must bind to proteins to cross the membrane.

  • Glycoproteins:

    • Function: Serve as receptors and facilitate signaling.

Diffusion

  • Overview:

    • Energy Requirement: None (passive transport).

    • Process: Movement of molecules from high concentration to low concentration.

    • Concentration Gradient:

      • The natural flow in which diffusion occurs.

Facilitated Diffusion

  • Description:

    • Utilizes membrane proteins to assist in molecule transport across the membrane.

    • Energy Requirement: None.

    • Direction: High to low concentration (along the concentration gradient).

Osmosis

  • Definition:

    • Movement of water (solvent) across a semipermeable membrane.

    • Energy Requirement: None.

  • Mechanism:

    • Water moves from an area of high concentration to low concentration of water.

    • Occurs due to uneven solute concentration.

Tonicity

  • Concept:

    • Refers to the ability of an extracellular solution to affect water movement into or out of a cell via osmosis.

Active Transport

  • Essentials:

    • Energy Requirement: Yes (active transport).

    • Direction: Moves molecules from low to high concentration (against the concentration gradient).

    • Mechanism: Requires special membrane proteins, known as solute pumps.

    • Examples: Mainly involves ions such as Na+, K+, and Ca2+; crucial for maintaining ionic gradients in neurons (-50 to -70 mV).

Primary Active Transport vs Secondary Active Transport

  • Differentiation by Energy Source:

    • Primary Active Transport:

      • Energy sourced directly from ATP.

      • Example: Na+/K+ pump, which pushes ions against their concentration gradient.

    • Secondary Active Transport:

      • Utilizes energy stored in the ionic concentration gradient from primary active transport.

      • Mechanism: Na+ can perform 'work' to drag other substances (e.g., glucose) across the membrane.

Vesicular Transport

  • Endocytosis:

    • Process of bringing molecules into the cell through membrane movement.

  • Exocytosis:

    • Process of pushing molecules out of the cell due to membrane movement.

Examples of Transport Mechanisms

  • Exocytosis Examples:

    • Creating new membrane receptors.

    • Exporting cellular products.

  • Endocytosis Examples:

    • Engulfing and destroying substances (e.g., white blood cells consuming bacteria).

    • Receptor-mediated endocytosis, where cells ingest proteins, hormones, and viruses.