Cell Membrane (Plasma Membrane)

Overview and Definition

The cell membrane, also called the plasma membrane, is a fundamental structure in all living cells. It serves as a boundary between the cell's internal environment and the external surroundings, regulating the movement of substances in and out of the cell.

• Selective Permeability: The cell membrane is selectively permeable, meaning it allows certain substances to pass through while blocking others.

• Function: Helps maintain homeostasis by controlling the internal environment of the cell.

Fluid Mosaic Model

The fluid mosaic model describes the structure of the cell membrane. According to this model, the membrane is composed of a variety of molecules that are free to move laterally within the layer, giving the membrane flexibility and dynamic properties.

• Components: The membrane contains many different types of molecules, including phospholipids, proteins, and cholesterol.

• Fluidity: Molecules in the membrane move around and change positions, contributing to the membrane's fluid nature.

Phospholipids and Membrane Structure

Phospholipids are the primary building blocks of the plasma membrane. They form a bilayer, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Bilayer Formation: The phospholipid bilayer creates a semi-permeable barrier that separates the cell from its environment.

• Source: The cell synthesizes phospholipids for the membrane, and the endoplasmic reticulum is involved in their production.

Proteins in the Cell Membrane

Membrane proteins perform various functions, including transport, communication, and structural support.

• Transport Proteins: Help substances pass through the membrane.

• Enzymatic Proteins: Perform catalysis and other chemical reactions.

• Recognition Proteins: Help the cell identify and interact with other cells.

Summary Table: Major Components of the Cell Membrane

Key Terms and Definitions

• Selective Permeability: Ability of the membrane to allow some substances to pass while restricting others.

• Homeostasis: Maintenance of a stable internal environment.

• Phospholipid Bilayer: Double layer of phospholipids that forms the core of the cell membrane.

• Fluid Mosaic Model: Concept describing the dynamic and varied composition of the cell membrane.

Example: Transport Across the Membrane

Small nonpolar molecules like oxygen and carbon dioxide can diffuse directly through the phospholipid bilayer, while ions and large polar molecules require transport proteins.

Equation for Diffusion Rate:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Additional info: The notes mention the endoplasmic reticulum synthesizing lipids for the cell membrane, which is accurate; the smooth ER is responsible for lipid and phospholipid synthesis.