7 Membranes Lecture Slides

Chapter Overview

• Topic: Membrane Structure and Function

Membrane Traffic Regulation

Movement Across the Plasma Membrane

• Small molecules can pass through the plasma membrane via two mechanisms:

  • Passive transport: No energy required.

    • May involve transport proteins.

  • Active transport: Requires energy and a transport protein.

  • Bulk transport: Involves large molecules using either exocytosis or endocytosis.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Structure and Composition

• Main Components:

  • Lipids, proteins, and carbohydrates.

  • Phospholipids form the basic bilayer of membranes.

• Phospholipids:

  • Amphipathic nature: hydrophobic tails and hydrophilic heads.

• Membrane Proteins:

  • Most proteins are amphipathic, allowing them to interact with both aqueous environments and the lipid bilayer.

• Fluid Mosaic Model:

  • Membranes are a mosaic of proteins in a fluid phospholipid bilayer, with proteins occasionally clustering for specific functions.

Membrane Fluidity

Characteristics

• Membranes are held together by hydrophobic interactions.

• Lipids can move laterally and occasionally flip across layers.

• As temperature changes:

  • Membranes transition from fluid to solid states based on lipid composition:

    • Unsaturated fatty acids maintain fluidity, while saturated fatty acids create a more solid membrane.

• Cholesterol's Role:

  • At high temperatures, cholesterol reduces movement of phospholipids.

  • At low temperatures, it helps prevent tight packing, thus maintaining fluidity.

Evolutionary Adaptations

• Membranes adapt their lipid composition in response to temperature changes in the environment,

  • Example: Fish in cold waters have more unsaturated fatty acids.

• Species may modify lipid composition to prevent membrane solidification during temperature fluctuations.

Membrane Proteins and Functions

Types of Membrane Proteins

• Two Main Types:

  • Peripheral proteins: Loosely bound to the surface.

  • Integral proteins: Span the membrane, including transmembrane proteins with hydrophobic regions that interact with the lipid bilayer.

Functions of Membrane Proteins

• Functions include:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix.

Cell Recognition and Membrane Carbohydrates

• Cells recognize each other through surface molecules like glycolipids and glycoproteins, which have attached carbohydrate chains.

• These structures assist in cell identification and communication.

Selective Permeability of Membranes

• Permeability is determined by:

  • Lipid bilayer's hydrophobic core that allows nonpolar molecules to pass while impeding polar molecules.

• Transport Proteins play a crucial role in the movement of hydrophilic substances across the membrane:

  • Channel Proteins: Provide corridors for specific molecules.

  • Carrier Proteins: Bind to molecules and change shape to assist transport.

Passive Transport: Diffusion

• Diffusion: Movement of particles from areas of higher concentration to lower concentration.

• Concentration Gradient:

• Water balance and tonicity affect cell survival and functioning.

Osmosis

• Osmosis: Diffusion of water across a selectively permeable membrane, moving from areas of lower solute concentration to higher solute concentration until equilibrium.

Tonicity and Water Balance

• Types of Tonicity:

  • Isotonic: Equal solute concentration

  • Hypertonic: Higher concentration outside the cell causing water loss and potential cell death.

  • Hypotonic: Lower concentration outside, leading to water influx that can cause cells to swell or burst.

Active Transport

• Definition: Movement of substances against their concentration gradient, requiring energy input (ATP).

• Examples:

  • Sodium-potassium pump maintains essential ion concentrations across membranes.

Cotransport

• Active transport can indirectly drive the transport of other solutes via cotransporters, coupling the movement of molecules down their concentration gradient to the active transport of another substance.

• occurs when active transport of a solute indirectly drives transport of other substances.

Bulk Transport Mechanisms

• Exocytosis: Transport vesicles fuse with the membrane to release contents outside the cell.

  • Many secretory cells use this to export their products (ex: cells in the pancreas secrete insulin by exocytosis).

• Endocytosis: Process of taking in large molecules via vesicle formation. Types include:

  • Phagocytosis: Engulfing large particles.

  • Pinocytosis: Uptake of extracellular fluid and solutes.

  • Receptor-mediated endocytosis: Involves receptors that trigger vesicle formation in response to specific solute binding.