facilitated diffusion

Facilitated Diffusion

  • Definition: Facilitated diffusion is a type of passive transport that requires membrane proteins to assist in the movement of molecules.

    • Concept: Movement is from areas of higher concentration to lower concentration (down the concentration gradient) without the need for additional ATP (cell energy).

    • Energy Source: The energy comes from molecular motion of the substances being transported.

Proteins Involved

  • Two main types of membrane proteins used in facilitated diffusion:

    1. Channel Proteins

      • Description: Form a tunnel-like structure that allows specific molecules or ions to pass through the membrane.

      • Example: Aquaporins allow the passage of water molecules and are present in all cell membranes.

    2. Carrier Proteins

      • Description: Act like a swinging door to transport specific molecules by changing shape upon binding.

      • Example: Glucose binds to a carrier protein, causing it to change shape and transport glucose into or out of the cell.

Unique Channels

  • Gated Channels: Channels that can open or close in response to different stimuli:

    • Voltage-Gated Channels: Open/close in response to electrical charge changes.

      • Example: Sodium-potassium channels in neurons.

    • Ligand-Gated Channels: Open/close in response to the binding of a ligand (like a neurotransmitter).

    • Mechanoreceptors: Respond to mechanical pressure or distortion and are involved in sensory processes such as touch and temperature regulation.

Active Transport

  • Role: Active transport moves substances against their concentration gradient, requiring energy (ATP) to concentrate substances within cells.

    • Examples: Sodium-potassium pumps and proton pumps are crucial for maintaining concentration gradients essential for various cellular functions.

    • Cotransport: The combined transport of two substances, where the movement of one substance down its gradient drives the movement of another against its gradient.

      • Types:

        • Symporter: Both substances move in the same direction.

        • Antiporter: Substances move in opposite directions.

Bulk Transport

  • Transmembrane Processes: Both exocytosis and endocytosis are forms of active transport involving vesicle formation to move larger molecules across the cell membrane:

    • Exocytosis: Process of expelling materials from the cell.

    • Endocytosis: Process of engulfing substances into the cell.

  • Thistle Tube (Osmometer): A device used to demonstrate osmosis:

    • Setup: A beaker with pure water, inserting a thistle tube containing a sugar solution.

    • Mechanism: Water moves from higher (pure water) to lower water potential (sugar solution) until equilibrium is reached.

    • Water Potential: Calculated based on solute concentration. Pure water has a water potential of 0; solute solutions have negative water potential.

      • Outcome: Water will rise in the thistle tube until the two water potentials equalize, achieving dynamic equilibrium.

Solute Potential and Osmosis in Cells

  • Carrot Cell Experiment: Shows effects of hypertonic and hypotonic solutions on plant cells:

    • Hypertonic: Carrots in salt solution lose water and become limp.

    • Hypotonic: Carrots in freshwater gain water and become stiff and hard.

  • Dialysis Bag Experiment: Demonstrates osmotic behavior:

    • Concept: Bags filled with different concentrations of sucrose placed in a uniform sucrose solution to observe mass gain or loss.

    • Results: Indicate osmotic gradients and resulting mass changes in the bags indicating water movement based on solute concentration.