Cell Theory, Membrane Transport and Viral Biology Essentials

Principles of Cell Theory

Living organisms consist of one or more cells. The cell is the fundamental structural and functional unit of all living things and represents the most basic life form capable of performing all biological functions.

Endosymbiotic Theory

This theory suggests that certain organelles evolved from bacteria phagocytosed by an ancestral cell:

• Mitochondria: Derived from aerobic bacteria.
• Chloroplasts: Derived from photosynthetic bacteria.

These organisms established a symbiotic relationship, eventually forming eukaryotic cells.

The Plasma Membrane

The plasma membrane is a continuous sheath that surrounds the cell, providing individuality and separating it from its environment. It is composed of a lipid bilayer embedded with proteins and carbohydrates.

Membrane Protein Functions

Proteins perform essential roles within the membrane, including:

• Transporting molecules
• Acting as receptors
• Functioning as enzymes
• Serving as structural bridges

Fluid Mosaic Model

Biological membranes are fluid structures where lipids and proteins diffuse rapidly within the plane of the membrane. These membranes are characterized as asymmetric structures.

Cellular Transport Mechanisms

Passive Transport

A diffusion process that does not require energy, moving substances in favor of their concentration gradient:

• Simple Diffusion: For nonpolar molecules like O2, benzene, and N2.
• Facilitated Diffusion: For small polar molecules (e.g., glucose, amino acids) and ions via channel proteins.

Active Transport

This process occurs against the concentration gradient and requires metabolic energy. Transport proteins involved are known as pumps, such as the Na+/K+ pump.

Viral Biology

Viruses are biological entities containing only the necessary components to reproduce inside host cells.

Basic Structure

• Nucleic Acid: The genetic core.
• Capsid: A protein envelope.

Classification of Viruses

• By Capsid Shape: Polyhedral, cylindrical, and complex (e.g., bacteriophages with head, tail, and basal plate).
• By Nucleic Acid: Double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA viruses.
• By Host Cell: Plant viruses, animal viruses, and bacteriophages (including retroviruses).