Cell Biology Fundamentals

A Comprehensive Guide for CSIR-NET, GATE, and DBT-BET

1. What is a Cell?

The cell is the structural, functional, and biological unit of all known organisms. It is the smallest unit of life that can replicate independently. In a biochemical context, it is a membrane-bound compartment filled with a concentrated aqueous solution of chemicals and organelles.

2. Cell Theory

The classical cell theory was proposed by Schleiden and Schwann (1838-39) and later refined by Rudolf Virchow (1855).

• All living organisms are composed of one or more cells.
• The cell is the basic unit of structure and organization in organisms.
• Omnis cellula e cellula: All cells come from pre-existing cells.
• Modern addition: Energy flow (metabolism) occurs within cells, and they contain hereditary information (DNA) passed during division.

3. Three Domains of Life

Proposed by Carl Woese based on 16S rRNA sequences, life is categorized into three distinct domains:

1. Archaea: Prokaryotic, but possess biochemical pathways (like transcription/translation) more similar to eukaryotes. They often live in extreme environments.
2. Bacteria: "True" bacteria; prokaryotic cells with peptidoglycan in cell walls.
3. Eukarya: Organisms with membrane-bound nuclei and organelles (Protists, Fungi, Plants, Animals).

Phylogenetic Tree of the Three Domains

4. Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	Absent (Nucleoid region)	Present (Double membrane)
DNA	Circular, usually no histones	Linear, associated with histones
Organelles	Absent (No mitochondria/ER)	Present (Mitochondria, Golgi, etc.)
Ribosomes	70S (30S + 50S)	80S (40S + 60S)
Size	0.1 - 5.0 μm	10 - 100 μm

Structural Comparison

5. Animal Cell vs. Plant Cell

• Plant Cells: Possess a rigid cell wall (cellulose), chloroplasts for photosynthesis, and a large central vacuole. They lack centrioles (usually).
• Animal Cells: Lack a cell wall and chloroplasts. They have centrioles/centrosomes and small, temporary vacuoles.

Plant vs Animal Cell

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The Plasma Membrane: Advanced Review

Target Level: CSIR-NET / GATE / DBT-BET

The plasma membrane is a dynamic, selectively permeable phospholipid bilayer that maintains cellular homeostasis.

I. The Fluid Mosaic Model

Proposed by Singer and Nicolson (1972), this model describes the membrane as a "mosaic" of proteins floating in or on a fluid lipid bilayer.

II. Chemical Composition

1. Lipids:
   • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.
   • Sterols: Cholesterol (animals) and Phytosterols (plants) act as "temperature buffers," maintaining fluidity at low temps and stability at high temps.
   • Sphingolipids: Involved in cell signaling and forming "Lipid Rafts."
2. Proteins:
   • Integral (Transmembrane): Span the bilayer; usually α-helices.
   • Peripheral: Attached to the surface via electrostatic interactions.
3. Carbohydrates: Found only on the exoplasmic face (Glycocalyx), crucial for cell-cell recognition.

III. Membrane Fluidity Factors

• Fatty Acid Chain Length: Shorter chains increase fluidity.
• Saturation: Unsaturated "kinks" prevent packing, increasing fluidity.
• Temperature: Transition temperature (T_m) is the point where the membrane shifts from a gel-like to a fluid state.

IV. Membrane Transport

• Simple Diffusion: Non-polar molecules (O₂, CO₂, Steroids).
• Facilitated Diffusion: Via Channels or Carriers (e.g., GLUT transporters).
• Active Transport:
  • Primary: Direct ATP use (Na+/K+ ATPase - P-type pump).
  • Secondary: Uses electrochemical gradients (Symporters/Antiporters).

V. Interactive Visualization (SVG/CSS Animation)

Live Fluidity Simulation

The "kinks" in tails represent unsaturated fatty acids maintaining membrane flux.

CSIR Key Concept: Asymmetric distribution. Phosphatidylserine is normally on the inner leaflet; its appearance on the outer leaflet is a hallmark of Apoptosis.