Cell Membrane: Transport and Specialised Functions

Cell Membrane Functions

  • Chemical Messenger

    • Breakdown products, ions, CAM of another cell.

    • A receptor that binds to chemical messengers such as hormones sent by other cells, terminating their effect.

  • Enzyme

    • An enzyme that breaks down a chemical messenger and terminates its effect.

  • Ion Channel

    • A channel protein that is constantly open, allowing ions to pass into and out of the cell.

  • Gated Ion Channel

    • A channel that opens and closes to allow ions through only at certain times.

  • Cell-Identity Marker

    • A glycoprotein acting as a marker that distinguishes the body's own cells from foreign cells.

  • Cell-Adhesion Molecule (CAM)

    • A molecule that binds one cell to another.

Membrane Structure

  • Phospholipid Bilayer

    • Semipermeable membrane where small molecules can diffuse through the lipid bilayer according to their concentration gradients.

    • Electrically charged and polar molecules cannot diffuse easily; they often require specific proteins to help them cross the membrane.

Selective Permeability of Membrane

  • The cell membrane's selective permeability means it regulates the exchange of materials with the environment.

  • Membrane enables controlled molecular traffic via various mechanisms.

Osmosis

  • Defined as the diffusion of water across a selectively permeable membrane.

    • Water moves from areas of lower solute concentration to areas of higher solute concentration.

Types of Transport

  • Passive Transport

    • Requires no energy input and typically involves diffusion.

  • Active Transport

    • Requires energy—usually in the form of ATP—to move substances against their concentration gradients.

Types of Passive Transport

  • Simple Diffusion

    • A form of passive transport allowing solutes to diffuse directly through the cell membrane if moving along their concentration gradient.

  • Facilitated Diffusion

    • Requires specialized proteins (channels and carriers) to transport substances across the membrane more effectively than simple diffusion.

Aquaporins

  • Specialized channel proteins facilitating the single-file movement of water molecules across cell membranes.

    • Consist of four subunits, each having a transmembrane 6-helix bundle.

Details of Active Transport

  • Active Transport Types

    • Uniporter: Moves one substance in one direction.

    • Symporter: Moves two substances in the same direction.

    • Antiporter: Moves two substances in opposite directions.

  • Directional Transport: Active transport is directional, either inside-out or outside-in.

  • Energy Dependence: This process can be directly energized by ATP hydrolysis (primary active transport) or indirectly by concentration gradients (secondary active transport).

Ion Channels

  • Definition: Channel proteins featuring hydrophilic pores predominantly gated; can open or close to allow ions to pass.

    • Opening mechanism includes stimuli from chemical signals (ligand-gated) or electrical charge changes (voltage-gated).

Overview of Membrane Transport Mechanisms

  • Osmosis, Diffusion: General transport methods across cell membranes.

  • Channel Transport: Includes gated and non-gated methods for ion and small molecule movement.

  • Active Transport Methods:

    • Uses protein pumps like ATPases to move ions against their gradient.

Endocytosis and Exocytosis

  • Endocytosis: Process of bulk transport into the cell (from outside to inside).

  • Exocytosis: Process of expelling material from the cell (from inside to outside).

  • Typical examples include neurotransmitter release and hormone secretion (e.g., insulin).

Types of Endocytosis

  1. Phagocytosis: Engulfing particles into a vacuole that fuses with a lysosome for digestion.

  2. Pinocytosis: Uptake of extracellular fluid along with dissolved molecules into small vesicles.

  3. Receptor-Mediated Endocytosis: Cell forms a vesicle after specific ligand binding to receptors.

Receptors and Signal Transduction

  • Receptors: Specialized membrane proteins that engage with specific ligands (molecules) to send messages that activate responses inside the cell.

Types of Receptors

  • All receptors bind extracellular ligands and transmit signals, but they can differ in structure and function:

    • Ion Channel Receptors: For example, the Acetylcholine receptor (AChR) that opens upon ligand binding, allowing Na+ influx triggering muscle contractions.

    • Enzyme-Linked Receptors: Such as insulin receptors, which induce conformational changes triggering intracellular signaling cascades through phosphorylation mechanisms.

    • G Protein-Coupled Receptors (GPCRs): They activate G proteins upon ligand binding, which then influence internal cellular pathways.

Importance of Membranes in Cellular Interactions

  • Cell Adhesion Molecules (CAM): Membrane proteins mediating cell interactions with other cells or the extracellular matrix.

  • Integral to immune responses, such as those mediated by the Major Histocompatibility Complexes (MHC).

Membrane Dysfunction and Related Diseases

  • Cystic Fibrosis: Caused by mutations in the CFTR channel affecting ion transport.

  • Alzheimer’s Disease: Involves amyloid-beta plaques that disrupt cell membrane integrity, affecting cellular function.