Structure of Plasma Membranes

STRUCTURE OF PLASMA MEMBRANES

• Highly Organized Structure

  • Composed of phospholipids arranged in a bilayer.

  • Globular proteins are inserted into the lipid bilayer.

  • Fluid mosaic model depicts proteins floating in or on the fluid lipid bilayer, similar to boats on a pond.

COMPONENTS OF CELLULAR MEMBRANES

• Four Major Components of Membranes

  • Phospholipid Bilayer:

  • Flexible matrix, serves as a barrier to permeability.

  • Transmembrane Proteins:

  • Integral membrane proteins that span the bilayer.

  • Interior Protein Network:

  • Peripheral membrane proteins that provide structural support.

  • Cell Surface Markers:

  • Glycoproteins and glycolipids that serve in cell recognition and signaling.

PHOSPHOLIPIDS

• Structural Components

  • Comprised of:

  • Glycerol: A 3-carbon polyalcohol.

  • Fatty Acids:

    • Two fatty acids attached to the glycerol, which are nonpolar and hydrophobic (water-fearing).

  • Phosphate Group:

    • Attached to the glycerol, making it polar and hydrophilic (water-loving).

  • Self-Organization:

  • Phospholipids spontaneously form a bilayer with fatty acids on the inside and phosphate groups on both surfaces.

• Molecular Example

  • The phospholipid molecule displays:

  • Hydrophilic Head: Composed of the phosphate group attached to glycerol.

  • Hydrophobic Tails: Each tail consists of a long hydrocarbon chain with saturated or unsaturated fatty acids.

MEMBRANE PROTEINS

• Functions of Membrane Proteins:

  • Include transporters, enzymes, cell-surface receptors, cell-surface identity markers, cell-to-cell adhesion proteins, and attachments to the cytoskeleton.

• Integral Membrane Proteins:

  • Span the lipid bilayer (transmembrane proteins).

  • Nonpolar regions are embedded in the bilayer; polar regions protrude from both sides.

  • Transmembrane Domain:

  • Spans the bilayer, often composed of hydrophobic amino acids arranged in alpha helices.

PASSIVE TRANSPORT

• Characteristics of Passive Transport

  • Involves movement of molecules through the membrane without energy input.

  • Movement occurs in response to a concentration gradient, from high concentration to low concentration until equilibrium is reached.

• Types of Diffusion:

  • Simple Diffusion: Movement occurs directly through the phospholipid bilayer.

  • Facilitated Transport: Substances cross the membrane with the aid of transport proteins (channel proteins or carrier proteins).

• Osmosis:

  • Defined as the net diffusion of water across a membrane toward a higher solute concentration.

OSMOTIC CONCENTRATION

• Terms:

  • Hypertonic Solution: Higher solute concentration than the reference solution.

  • Hypotonic Solution: Lower solute concentration than the reference solution.

  • Isotonic Solution: Equal solute concentration to the reference solution.

• Aquaporins: Specialized channels that facilitate water movement across the membrane.

• Osmotic Pressure:

  • The force needed to stop osmotic flow.

  • Cells in hypotonic solutions may gain water and swell, increasing pressure; if the membrane is strong, it can withstand this pressure.

• Cellular Adaptations:

  • Plant cells maintain turgor pressure against the cell wall due to osmotic pressure, while animal cells require isotonic environments to avoid bursting.

ACTIVE TRANSPORT

• Definition:

  • Active transport requires energy (ATP) to move substances against their concentration gradient, from low to high concentration.

• Carrier Proteins:

  • Types include uniporters (move one molecule), symporters (move two molecules in the same direction), and antiporters (move two molecules in opposite directions).

• Sodium-Potassium Pump (Na+/K+ Pump):

  • A critical example of primary active transport that uses ATP directly.

  • The pump moves 3 Na+ out of the cell and 2 K+ into the cell against their gradients, altering the carrier protein's conformation to facilitate transport.

• Coupled Transport:

  • Secondary active transport that indirectly uses ATP.

  • Utilizes the energy from the diffusion of one molecule (e.g., Na+) to facilitate the transport of another molecule (e.g., glucose) against its gradient.

BULK TRANSPORT

• Endocytosis: Movement of substances into the cell.

  • Phagocytosis: Cell engulfs particulate matter.

  • Pinocytosis: Cell takes in fluid.

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to a receptor.

• Exocytosis: Movement of substances out of the cell, which also requires energy.

• Visual Representation:

  • Diagrams (Figures) illustrate processes such as phagocytosis, pinocytosis, and receptor-mediated endocytosis, visually representing how substances are ingested or expelled by cells.