Cell Membranes: Structure, Fluidity, and Permeability

Cell Membranes

Overview of Cell Membranes

Cell membranes are the boundaries that separate the internal environment of the cell from the external environment. They play a crucial role in maintaining homeostasis and mediating communication and transport between the cell and its surroundings.

• Selective permeability: Cell membranes allow certain substances to pass while restricting others.

• Fluid mosaic model: Describes the structure of cell membranes as a mosaic of lipids and proteins that can move laterally within the layer.

Membrane Structure

Composition of Cell Membranes

The cell membrane is primarily composed of a double layer of phospholipids, with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer.

• Proteins: Integral and peripheral proteins serve various functions such as transport, signaling, and structural support.

• Cholesterol: Interspersed within the phospholipid bilayer, modulating membrane fluidity.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), important for cell recognition.

Fluidity of Membranes

Membrane fluidity refers to the viscosity of the lipid bilayer, affecting the movement of proteins and lipids within the membrane.

• Factors affecting fluidity:

  • Fatty acid saturation: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease fluidity.

  • Temperature: Higher temperatures increase fluidity; lower temperatures decrease fluidity.

  • Cholesterol: Acts as a fluidity buffer, preventing membranes from becoming too rigid or too fluid.

• Adaptations: Organisms can adjust membrane lipid composition in response to temperature changes.

Example: Bacteria in cold environments increase unsaturated fatty acids in their membranes to maintain fluidity.

Membrane Proteins

Types and Functions of Membrane Proteins

Membrane proteins are essential for various cellular processes.

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Functions:

  • Transport of molecules

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to cytoskeleton and extracellular matrix

Glycoproteins and Glycolipids

Carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids) play a key role in cell recognition and communication.

• Glycoproteins: Important for immune response and cell signaling.

• Glycolipids: Contribute to membrane stability and cell recognition.

Membrane Orientation

Asymmetry of Membranes

Membranes have distinct inside and outside faces, with specific proteins and lipids oriented accordingly. This asymmetry is established during membrane synthesis in the endoplasmic reticulum and Golgi apparatus.

• Example: Glycoproteins are typically found on the extracellular surface of the plasma membrane.

Selective Permeability of Membranes

Concept of Selective Permeability

Cell membranes are selectively permeable, allowing some substances to cross more easily than others.

• Permeable to: Small, nonpolar molecules (e.g., O2, CO2), and some small polar molecules (e.g., H2O).

• Impermeable to: Large polar molecules and ions (e.g., glucose, Na+, K+).

Transport Across Membranes

Transport can be passive (no energy required) or active (requires energy).

• Passive transport: Movement of substances down their concentration gradient without energy input.

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

Types of Passive Transport

• Simple diffusion: Direct movement of molecules across the membrane.

• Facilitated diffusion: Movement of molecules via transport proteins (channels or carriers).

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

Equation for Diffusion Rate (Fick's Law):

$J = -D \frac{dC}{dx}$

Where:

• J = flux (rate of diffusion)

• D = diffusion coefficient

• \frac{dC}{dx} = concentration gradient

Table: Comparison of Transport Mechanisms

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 crossing membrane
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Summary

• Cell membranes are dynamic structures composed of lipids, proteins, and carbohydrates.

• The fluid mosaic model explains the lateral movement and diverse composition of membranes.

• Membrane fluidity is regulated by lipid composition, temperature, and cholesterol.

• Selective permeability allows cells to control their internal environment.

• Transport across membranes can be passive or active, depending on the substance and cellular needs.