Recording-2025-03-26T14:41:40.401Z

Chapter 7: Membrane Structure and Function

Plasma Membrane Overview

• The plasma membrane is selectively permeable:

  • Allows certain substances to pass through easily, such as water.

  • Some substances have difficulty crossing, while others cannot cross at all without assistance.

Aquaporins

• Aquaporins are specialized proteins that facilitate the passage of water through the plasma membrane.

• Essential for maintaining cellular water balance as simple diffusion via osmosis is insufficient for cellular needs.

Fluid Mosaic Model

• The cell membrane is described by the fluid mosaic model:

  • Composed primarily of phospholipids, which have:

    • A polar glycerol head (hydrophilic) and two nonpolar fatty acid tails (hydrophobic).

  • Phospholipid bilayers spontaneously form in aqueous environments to protect their hydrophobic tails from water.

Structure of Phospholipids

• Phospholipids are structurally similar, having:

  • A glycerol backbone with two fatty acid chains and a phosphate group attached to a choline.

  • When upset by water, they arrange into bilayers with hydrophobic tails shielded inward and heads facing outward towards the aqueous environments.

Fluidity and Mobility of Membranes

• Fluidity refers to the dynamic nature of the membrane, where molecules are not static and can move laterally. This movement is essential for cellular function and occurs approximately 10^7 times per second.

• Factors influencing membrane fluidity:

  • Temperatures: Higher temperature can cause membranes to become more fluid, while lower temperatures can make them more viscous.

  • Cholesterol: Maintains fluidity by preventing fatty acid tails from packing too closely together during temperature fluctuations.

Membrane Proteins

• Membrane proteins can be categorized as:

  • Peripheral proteins: Located on the outer surface of the membrane; often involved in signaling and structural functions.

  • Integral proteins: Span the membrane, with hydrophobic regions interacting with the fatty acid tails of phospholipids. May function as channels, transporters, or receptors.

  • Transmembrane proteins: A type of integral protein that spans both layers of the membrane.

Cell Functions and Interactions

• Proteins within the membrane are involved in various functions:

  • Transport: Facilitate movement in/out of the cell.

  • Signal transduction: Relay signals from outside to inside the cell.

  • Cell-cell recognition: Glycoproteins and glycolipids serve as markers for cell identity.

  • Attachment: Connect the cell to the cytoskeleton and extracellular matrix.

Transport Mechanisms

• Passive Transport: No energy required; substances move down their concentration gradient (e.g., through diffusion).

• Active Transport: Requires energy (typically ATP) to move substances against their concentration gradient.

• Osmosis: Special case of diffusion concerning water movement across a semi-permeable membrane, often from an area of lower solute concentration to higher concentration, aiming for equilibrium.

Tonicity and Cellular Response

• Tonicity defines how a solution affects cell volume:

  • Isotonic: Equal solute concentration across the membrane; cell maintains shape.

  • Hypertonic: Higher solute concentration outside; water moves out, causing cell shrinkage.

  • Hypotonic: Lower solute concentration outside; water moves into cells, which may lead to potential lysis unless the cell has a wall (e.g., plant cells benefit from turgor pressure in hypotonic environments).

Summary

• The plasma membrane's structure maintains selective permeability and fluidity, essential for cellular homeostasis. Understanding the roles of phospholipids and proteins offers insights into how cells interact with their environments and transport necessary substances.