Structure and Function of the Plasma Membrane

Plasma Membrane: Structure and Function

Overview of the Plasma Membrane

The plasma membrane is a fundamental structure that separates the living cell from its external environment. It plays a critical role in maintaining the internal conditions necessary for life.

• Selective permeability: The plasma membrane allows some substances to cross more easily than others, regulating the movement of materials into and out of the cell.

• This selective barrier is essential for cellular homeostasis and communication with the environment.

Fluid Mosaic Model of Membrane Structure

The fluid mosaic model describes the structure of the plasma membrane as a dynamic arrangement of lipids and proteins.

• Phospholipids are the most abundant lipid in the plasma membrane.

• Phospholipids are amphipathic molecules, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.

• The membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to the phospholipid bilayer.

• Proteins in the membrane have hydrophilic regions exposed to water and hydrophobic regions embedded within the lipid bilayer.

Definition: Amphipathic molecules are molecules that contain both hydrophobic and hydrophilic regions, such as phospholipids.

Membrane Fluidity

Membrane fluidity is crucial for the function of the plasma membrane, affecting the movement of proteins and lipids within the layer.

• Phospholipids can move laterally within the bilayer, allowing the membrane to remain flexible and self-healing.

• Lateral movement of phospholipids occurs frequently (about 107 times per second), while "flip-flop" movement (transverse movement from one leaflet to the other) is rare (about once per month).

• Proteins also drift laterally, but some are anchored in place by the cytoskeleton or extracellular matrix.

Example: The fluidity of the membrane allows cells to change shape and enables the movement of membrane proteins involved in signaling and transport.

Key Terms and Concepts

• Selective permeability: The property of the plasma membrane that allows some substances to pass through more easily than others.

• Phospholipid bilayer: The fundamental structure of the membrane, consisting of two layers of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Hydrophobic region: The part of a molecule that repels water (e.g., fatty acid tails of phospholipids).

• Hydrophilic region: The part of a molecule that interacts with water (e.g., phosphate head of phospholipids).

Summary Table: Properties of the Plasma Membrane

Component	Structure	Function
Phospholipids	Amphipathic; form bilayer	Barrier to most water-soluble substances; provides fluidity
Proteins	Embedded or attached; hydrophilic and hydrophobic regions	Transport, signaling, structural support
Carbohydrates (Additional info: not shown in these slides but present in membranes)	Attached to proteins or lipids	Cell recognition, signaling

Additional info: The fluid mosaic model is supported by experimental evidence, such as the lateral movement of membrane proteins observed in cell fusion experiments.