Membrane Structure and Function (Chapter 7) – Study Notes

Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all cells, serving as the boundary that separates the internal environment of the cell from its external surroundings. Its unique structure allows it to regulate the movement of substances into and out of the cell, maintaining homeostasis and enabling communication with the environment.

At the Edge: The Plasma Membrane

Structure and Selective Permeability

• Plasma Membrane: The outer boundary of the cell, composed primarily of a phospholipid bilayer with embedded proteins.

• Selective Permeability: The property of the membrane that allows some substances to cross more easily than others, enabling the cell to control its internal composition.

• Regulation of Traffic: The plasma membrane uses various mechanisms to regulate the movement of molecules, including passive transport, active transport, and bulk transport.

Key Functions:

• Maintains the integrity of the cell

• Controls exchange of materials (nutrients, waste, ions, etc.)

• Facilitates communication and signaling

Mechanisms of Membrane Transport

Types of Transport Across the Membrane

• Passive Transport: Movement of small molecules (such as O2 and CO2) across the membrane without energy input. May involve simple diffusion or facilitated diffusion via transport proteins.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP) and specific transport proteins.

• Bulk Transport: Movement of large molecules (such as proteins and polysaccharides) via vesicles. Includes processes like exocytosis (exporting materials) and endocytosis (importing materials).

Summary Table: Types of Membrane Transport

Type of Transport	Energy Required?	Direction (relative to gradient)	Example
Passive Transport	No	Down gradient	O2 diffusion
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes	Varies	Endocytosis, Exocytosis

Key Terms and Definitions

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

• Transport Proteins: Proteins embedded in the membrane that assist in the movement of substances across the membrane.

• Vesicle: A small, membrane-bound sac that transports substances within or between cells.

• Exocytosis: The process by which large molecules are secreted from the cell via vesicles that fuse with the plasma membrane.

• Endocytosis: The process by which large molecules are taken into the cell by the plasma membrane folding inward to form a vesicle.

Example: Selective Permeability in Action

• Oxygen and carbon dioxide can diffuse freely across the membrane due to their small size and nonpolar nature.

• Ions and large polar molecules require specific transport proteins to cross the membrane.

• Cells use endocytosis to engulf large particles, such as nutrients or pathogens, and exocytosis to release waste or signaling molecules.

Additional info:

• The plasma membrane's selective permeability is essential for maintaining the proper internal environment of the cell, supporting processes such as metabolism, signaling, and growth.

• Membrane proteins play diverse roles, including transport, signal transduction, cell recognition, and attachment to the cytoskeleton and extracellular matrix.