Study Unit 2 lecture notes

Cell Membranes Unit Notes

Amphipathic Molecules

• Definition: Molecules that have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts.

• Phospholipid Structure:

  • Hydrophilic "Head":

    • Contains a phosphate functional group with a negative charge.

    • Attracts water, facilitating interaction in aqueous environments.

  • Hydrophobic "Tail":

    • Composed of two fatty acid chains (hydrophobic hydrocarbon tails).

    • Avoids water and aggregates with other hydrophobic substances.

Example: Phosphatidylcholine

• Contains choline and phosphate as a hydrophilic head.

• Forms a bilayer in an aqueous environment where the tails face inward.

Fluid Mosaic Model

• Concept:

  • Membranes are composed of a phospholipid bilayer with proteins embedded, allowing fluid movement.

• Fluidity:

  • Membrane components can move freely within the layer, contributing to its dynamic and adaptable nature.

• Mosaic:

  • Made up of many discrete components.

Membrane Components

• Carbohydrates: Located on the outer membrane, forming glycoproteins and glycolipids, serving in cell recognition.

• Cholesterol: Impacts membrane fluidity by reducing movement at high concentrations and preventing packing in low concentrations.

Membrane Fluidity Factors

• Fatty Acid Chain Length:

  • Shorter fatty acid chains result in increased membrane fluidity. This is because shorter chains have less surface area, reducing van der Waals interactions between neighbouring lipid molecules, allowing for more movement.

  • Conversely, longer fatty acid chains lead to decreased fluidity due to increased surface area contributing to stronger interactions and tighter packing of lipids, making the membrane more rigid.

• Degree of Saturation:

  • Saturated Fatty Acids: These have no double bonds between carbon atoms, allowing them to pack closely together, which reduces the space within the membrane and decreases fluidity.

  • Unsaturated Fatty Acids: These contain one or more double bonds, creating kinks in their structure. This prevents them from packing tightly, thus increasing the fluidity of the membrane.

• Cholesterol plays a critical role in maintaining the fluidity of cell membranes.

• At lower concentrations: Cholesterol helps maintain membrane fluidity by preventing fatty acid chains from packing too closely together, which promotes movement and flexibility within the membrane.

• At higher concentrations: Cholesterol can decrease membrane fluidity by stabilizing the membrane structure. It limits the movement of phospholipids, contributing to a more rigid membrane while still supporting its integrity under various conditions.

• Temperature:

  • Membrane fluidity increases with temperature. As the temperature rises, the kinetic energy of the lipid molecules also increases, leading to greater movement and flexibility of the membrane. This enhancement in fluidity is crucial for maintaining proper membrane function and integrity under varying physiological conditions.

Membrane Proteins

• Types:

  • Integral Proteins: Span the membrane and interact with both aqueous environments inside and outside the cell (stretches of amino acids with hydrophobic and hydrophilic side chains).

  • Transmembrane Proteins: A type of integral protein that extends across both sides of the lipid bilayer, allowing for the transport of specific molecules, such as ions and larger substrates, through the cell membrane.

  • Integrin: A type of transmembrane protein that plays a crucial role in cell adhesion, signalling, and has binding sites for the extracellular matrix and the cell's cytoskeleton.

  • Membrane Proteins: move around in the membrane, some are confines to a specific region

  • Peripheral Proteins: Noncovalently attached to the outer or inner surfaces without embedding into the membrane, having polar/charged groups.

  • Anchored Proteins: Covalently bonded to lipids, integrating into the membrane.

Functionality

• Membrane proteins involved in various functions:

  • Channel formation for specific ion transport.

  • Transmembrane proteins for communication and signalling across the membrane.

Membrane Carbohydrates

• Location: The outer surface of the cell membrane

• Role: Carbohydrates found on the membrane surface contribute to cell and molecule recognition and interaction.

• Types:

  • Glycoproteins: Carbohydrate covalently bonded to a lipid. Function: They play a crucial role in cell signalling and communication by acting as receptors or ligands that initiate cellular responses.

  • Glycolipids: 1 or more short carbohydrate chain covalently bonged to a protein - oligosaccharides. Function: They play a crucial role in immunity, signalling, and cell adhesion by facilitating communication between cells.

  • Proteoglycans: Heavily glycosylated proteins aiding in cell interaction. Function: Cell recognition and adhesion

Cell Junctions

• Types:

  • Tight Junctions: Prevent substances from passing between cells; maintain boundary integrity. Restrict the migration of membrane proteins over the cell surface from one face to another.

  • Desmosomes: Link intermediary filaments, providing mechanical support while allowing movement around them int eh extracellular matrix.

  • Gap Junctions: Channels between adjacent cells, allowing substances to pass between cells (e.g., in cardiac muscle for synchronized contraction).

Membrane Selective Permeability

• Definition: Biological membranes selectively allow certain substances to enter or exit cells.

• Transport Mechanisms:

  • Passive Transport: No energy required; includes diffusion (simple and facilitated).

  • Active Transport: Energy required; moving substances against their concentration gradient.

Diffusion & Osmosis

• Diffusion: Movement of particles from areas of high concentration to low concentration.

• Factors Influencing Diffusion:

  • Size of Molecules: Smaller molecules diffuse faster than larger ones.

  • Concentration Gradient: A steeper gradient results in faster diffusion.

  • Density of the Solution: Higher density solutions slow down diffusion.

  • Temperature: Higher temperatures increase molecular kinetic energy, resulting in faster diffusion.

  • Surface Area: Larger surface areas facilitate faster diffusion rates by providing more space for molecules to pass through.

• Osmosis: Specific to water movement from low solute concentration toward high solute concentration.

Active Transport

• Types:

  • Uniporter: Moves a single substance in one direction.

  • Simporter: Moves two substances in the same direction (e.g., Na+ and glucose).

  • Antiporter: Moves two substances in opposite directions (e.g., Na+/K+ pump).

Na+/K+ Pump

• Function: Uses ATP to transport Na+ out and K+ into cells, maintaining membrane potential and ion balance.

Endocytosis and Exocytosis

• Mechanisms for moving large molecules across the membrane via vesicles.

  • Endocytosis: Engulfing materials into the cell.

    • Types include phagocytosis (solids), pinocytosis (liquids), and receptor-mediated endocytosis (specific).

  • Exocytosis: Secretion of biosynthetic products from the cell (e.g., neurotransmitters).

• Vesicle Fusion: Vesicles fuse with the membrane to release or incorporate content.