Membrane Structure and Function

Overview

The cell membrane is a dynamic structure that regulates the movement of substances into and out of the cell, maintains cellular integrity, and facilitates communication and recognition. This chapter covers the fluid mosaic model, membrane permeability, types of membrane transport, and the roles of membrane proteins and carbohydrates.

Fluid Mosaic Model of the Membrane

The fluid mosaic model describes the cell membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. This model explains the flexibility and varied functions of the membrane.

• Phospholipids: Amphipathic molecules with a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails. This arrangement forms a bilayer, creating a semi-permeable barrier.

• Membrane fluidity: Maintained by hydrophobic interactions, which are weaker than covalent bonds. Lipids and proteins can move laterally within the membrane, allowing rapid movement and flexibility.

• Factors affecting fluidity:

  • Decreasing temperature: Membranes solidify at lower temperatures, depending on lipid composition.

  • Unsaturated hydrocarbon chains: Increase fluidity due to kinks that prevent tight packing.

  • Cholesterol: Acts as a fluidity buffer, restraining movement at high temperatures and preventing solidification at low temperatures.

  • Saturated hydrocarbon tails: Decrease fluidity by allowing tight packing.

Membrane Proteins

Proteins are integral to membrane function and are classified as:

• Integral proteins: Penetrate the hydrophobic interior of the lipid bilayer.

• Peripheral proteins: Loosely bound to the surface of the membrane, often attached to exposed parts of integral proteins.

Major functions of membrane proteins include:

Membrane Carbohydrates

Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) are important for cell-cell recognition and sorting of cells into tissues and organs.

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: Carbohydrates covalently bonded to proteins.

Labeling Membrane Components

Key components of an animal cell membrane include:

• Glycolipid: Cell recognition

• Glycoprotein: Cell recognition

• Integral protein: Transport, cell recognition, signal transduction

• Peripheral protein: Cell recognition, signal transduction

• Cholesterol: Fluidity buffer

• Phospholipid: Forms bilayer boundaries

• ECM fibers: Support and interact with animal cells

• Cytoskeleton microfilaments: Mechanical support, transport, signaling

• Integrins: Connect cytoskeleton to extracellular matrix

Membrane Transport

Types of Membrane Transport

Transport across membranes can be passive or active, depending on energy requirements.

• Passive transport: Movement of small molecules without energy input. Includes diffusion and facilitated diffusion.

• Active transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

• Bulk transport: Movement of large molecules via processes like endocytosis and exocytosis.

Selective Permeability

The structure of the membrane allows selective permeability, meaning only certain substances can pass through easily.

• Channel proteins: Provide hydrophilic channels for molecules or ions to pass through.

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

Diffusion, Osmosis, and Facilitated Diffusion

These processes describe the movement of substances across membranes:

• Diffusion: Movement of molecules from high to low concentration until equilibrium is reached.

• Concentration gradient: Difference in concentration across a space.

• Passive transport: Diffusion across a selectively permeable membrane without energy input.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmotic Solutions and Cell Response

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; cell loses water and shrivels.

• Hypotonic: Lower solute concentration outside the cell; cell gains water and may burst (animal cells) or become turgid (plant cells).

• Turgid: Very firm; plant cells in hypotonic solution.

• Flaccid: Limp; plant cells in isotonic solution.

• Plasmolysis: Plant cell shrivels and plasma membrane pulls away from the wall in hypertonic solution.

Example: Red Blood Cells and Plant Cells in Hypotonic Solution

• Red blood cells burst in hypotonic solution due to lack of cell wall.

• Plant cells become turgid and do not burst because the cell wall provides structural support.

Key Equations

• Diffusion rate (Fick's Law): $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmotic pressure: $\Pi = iMRT$ where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Additional info:

• Membrane proteins and carbohydrates play crucial roles in cell signaling, recognition, and structural integrity.

• Bulk transport includes endocytosis (import) and exocytosis (export) of large molecules.