membrane

MEMBRANE AND MEMBRANE TRANSPORT

B2.1.1 Lipid Bilayers as the Basis of Cell Membranes

  • Definition of Membranes:

    • Membranes serve as essential components of cells.

    • The plasma membrane creates a boundary between the cell and its surrounding environment.

    • Inside eukaryotic cells, membranes compartmentalize the cytoplasm into various compartments.

    • Basic Structure:

    • Consists of a bilayer of phospholipids and other amphipathic molecules.

    • The membrane is approximately 10 nanometres or less in thickness and regulates the passage of substances.

B2.1.2 Lipid Bilayers as Barriers

  • Components of Phospholipid Molecules:

    • Comprises a phosphate head and two hydrocarbon tails.

    • Properties of Tails:

    • Tails are hydrophobic, leading to interactions that form the core of biological membranes.

    • Permeability:

    • The membrane core exhibits low permeability to hydrophilic particles, including ions (both positively and negatively charged) and polar molecules such as glucose.

Factors Influencing Membrane Permeability

  • Molecular Size:

    • Molecular size is a key factor affecting permeability rates.

    • Trend in Permeability:

    • Larger molecules generally have lower permeability.

    • Example: Water molecules, slightly larger than oxygen atoms, can traverse membranes more easily than larger molecules like glycogen or proteins.

B2.1.9 Structure and Function of Glycoproteins and Glycolipids

  • Glycoproteins:

    • Are conjugated proteins with a carbohydrate component that protrude into the extracellular environment of cells.

  • Glycolipids:

    • Composed of carbohydrates linked to lipids, usually consisting of a monosaccharide or a short chain (2-4 sugar units).

  • Functions:

    • Essential for cell recognition, they play a role in the immune system by differentiating self from non-self cells, enabling the detection and destruction of pathogens and foreign tissues.

    • Together, glycoproteins and glycolipids comprise a glycocalyx, which helps bind adjacent cells while preventing tissue disintegration.

Types of Membrane Proteins

  1. Transport Proteins: Control movement of substances through the membrane.

  2. Receptor Proteins: Bind to signaling molecules like hormones.

  3. Enzyme Proteins: Catalyze biochemical reactions.

  4. Structural Proteins: Provide support and integrity to the membrane.

  5. Identity Proteins: Share identifiers that distinguish one cell from another.

B2.1.4 Integral and Peripheral Proteins in Membranes

  • Integral Proteins:

    • Have hydrophobic regions and are typically embedded in the membrane, some cross through (transmembrane proteins).

  • Peripheral Proteins:

    • Are hydrophilic on their surfaces and are attached to the surface of integral proteins; attachments are often reversible.

Selectivity in Membrane Permeability

  • Selectively Permeable Membranes:

    • Membranes allow specific substances to pass while restricting others.

    • Exhibits properties of being semi-permeable.

    • Small, electrically neutral molecules pass more easily than charged particles.

Passive Transport vs. Active Transport

  • Passive Transport:

    • Molecules move from high to low concentration without energy expenditure.

    • Types of Passive Transport:

    1. Simple Diffusion: Movement of small non-polar molecules (e.g., O₂).

    2. Facilitated Diffusion: Requires transport proteins for ions and polar molecules. Fast and specific, does not use ATP.

  • Active Transport:

    • Moves substances against their concentration gradient, utilizing energy (usually ATP).

    • Examples of Active Transport: sodium-potassium pump, sodium-dependent glucose co-transporters.

Osmosis

  • Definition: The diffusion of water through a semi-permeable membrane from a region of lower solute concentration to a higher solute concentration.

  • Role of Aquaporins:

    • Some cells possess aquaporins, specialized channels that greatly facilitate water passage, especially in kidney and root hair cells.

B2.1.16 Sodium-Potassium Pump and Membrane Potential

  • Sodium-Potassium Pump:

    • Essential for generating charge imbalances across neuron membranes, which are critical for generating nerve impulses. Pumps 3 Na⁺ out and 2 K⁺ in using ATP.

  • Membrane Potential: Achieved through ion concentration differences maintained by pumps and channels.

Gated Ion Channels and Nerve Impulses

  • Gated Ion Channels: Allow regulated ion flow across membranes; crucial for neuron function (e.g., voltage-gated Na⁺ and K⁺ channels).

  • Actions during a nerve impulse:

    • Resting State: Channels closed, maintaining resting potential.

    • Depolarization: Na⁺ influx raises the membrane potential; channels open at threshold potential.

    • Repolarization: K⁺ efflux restores negative charge inside the neuron.

Endocytosis and Exocytosis

  • Endocytosis: Process for importing bulky materials (e.g., nutrients).

    • Phagocytosis: Engulfing large particles or cells.

    • Pinocytosis: Endocytosis of liquids and small particles.

  • Exocytosis: Expels materials from the cell using vesicles that fuse with the plasma membrane.

Summary of Membrane Properties

  • Membrane thickness: approximately 10 nm.

  • Composed of lipids, proteins, and carbohydrates.

  • Selectively permeable due to lipid bilayer properties utilizing amphipathic phospholipids.

Conclusion

  • Membranes and their models, transport mechanisms, and compositions elucidate the intricate relationship between structure and function in cellular biology. The fluid mosaic model and integral functionalities vital in maintaining cellular homeostasis and transport dynamics emphasize the unified role of membranes in life processes.