Cell Membrane Structure and Function: Life at the Edge

Life at the Edge: The Plasma Membrane

Overview of the Plasma Membrane

The plasma membrane defines the boundary of the cell, separating its internal environment from the external surroundings. This selective barrier is essential for maintaining homeostasis and regulating the movement of substances in and out of the cell.

• Selective permeability: The plasma membrane allows some substances to cross more easily than others, which is fundamental to life.

• Communication: The membrane plays a key role in cell signaling and communication, enabling cells to respond to their environment and interact with other cells.

• Structure: The membrane is primarily composed of lipids and proteins, with carbohydrates also present.

Example: Figure 5.1 illustrates how brain cells communicate with each other via the plasma membrane.

Membrane Structure: Lipids and Proteins

Lipid Bilayer and Amphipathic Molecules

The plasma membrane consists of a phospholipid bilayer with embedded proteins. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Phospholipid bilayer: Forms the fundamental structure of the membrane, with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins: Integral and peripheral proteins are interspersed throughout the membrane, serving various functions such as transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins or lipids on the extracellular surface, contributing to cell recognition and signaling.

Example: Figure 5.2 and Figure 5.3 show models of the plasma membrane, highlighting the arrangement of lipids and proteins.

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure where proteins float in or on the fluid lipid bilayer. This model emphasizes the flexibility and diversity of membrane components.

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

• Peripheral proteins: Attached to the membrane surface, often involved in structural support or signaling.

• Fluidity: Lipids and proteins can move laterally within the layer, allowing the membrane to remain flexible and self-healing.

The Fluidity of Membranes

Factors Affecting Membrane Fluidity

Membrane fluidity is crucial for proper function, affecting processes such as transport, signaling, and cell movement. Several factors influence fluidity:

• Type of fatty acids: Unsaturated hydrocarbon tails (with double bonds) prevent tight packing, increasing fluidity. Saturated tails pack closely, making the membrane more viscous.

• Cholesterol: Acts as a fluidity buffer, stabilizing the membrane at different temperatures by restraining phospholipid movement.

• Temperature: Higher temperatures increase fluidity, while lower temperatures decrease it.

Example: Figure 5.5 compares membranes with unsaturated vs. saturated hydrocarbon tails and the effect of cholesterol.

Factor	Effect on Fluidity
Unsaturated hydrocarbon tails	Increase fluidity (looser packing)
Saturated hydrocarbon tails	Decrease fluidity (tighter packing)
Cholesterol	Stabilizes fluidity across temperature changes

Experimental Evidence: Do Membrane Proteins Move?

Experiments have shown that membrane proteins are not static; they can move within the lipid bilayer. This lateral movement is essential for membrane function and flexibility.

• Example experiment: Human and mouse cell membranes were fused, and proteins were observed to mix over time, demonstrating protein mobility.

Summary Equations and Key Terms

• Selective permeability: The property of membranes that allows some substances to cross more easily than others.

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

• Fluid mosaic model: Describes the dynamic arrangement of lipids and proteins in the membrane.

Relevant equation:

$\text{Fluidity} \propto \frac{\text{Number of unsaturated tails}}{\text{Number of saturated tails}}$

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.