Unit 1: Introduction to Cell Biology

Cell Theory

The Cell Theory is one of the most important foundational principles in biology. It was developed over centuries, with key contributions from Robert Hooke (who coined the term "cell"), Schleiden, Schwann, and Virchow.

The Tenets of Modern Cell Theory:

1. All known living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells through cell division (coined by Rudolf Virchow as Omnis cellula e cellula).
4. Hereditary information (DNA) is passed from cell to cell during division.
5. All cells are basically the same in chemical composition and metabolic activities.
6. Energy flow (metabolism) occurs within cells.

Ultrastructure of Prokaryotic and Eukaryotic Cells

The "ultrastructure" refers to the detailed structure of a cell as seen with an electron microscope. Life is primarily divided into two types of cells:

Prokaryotic Cells

• Definition: Cells that lack a true, membrane-bound nucleus. Their genetic material is found in a region called the nucleoid.
• Examples: Bacteria and Archaea.
• Key Features:
  • Nucleoid: A region in the cytoplasm containing a single, circular chromosome (DNA).
  • Ribosomes: Smaller 70S ribosomes, responsible for protein synthesis.
  • No Membrane-Bound Organelles: They lack mitochondria, ER, Golgi, lysosomes, etc.
  • Cell Wall: Most have a cell wall outside the plasma membrane, often containing peptidoglycan (in bacteria).
  • Plasma Membrane: The inner membrane that controls transport. Metabolic processes like respiration can occur here.
  • Other Structures: May have flagella (for movement), pili (for attachment), and a capsule (for protection).

Eukaryotic Cells

• Definition: Cells that have a true, membrane-bound nucleus containing their genetic material.
• Examples: Animals, Plants, Fungi, Protists.
• Key Features:
  • Nucleus: Contains multiple, linear chromosomes (DNA complexed with histone proteins).
  • Ribosomes: Larger 80S ribosomes in the cytoplasm and on the RER. (Also have 70S ribosomes in mitochondria and chloroplasts).
  • Membrane-Bound Organelles: Highly compartmentalized with specialized organelles like:
    • Mitochondria: Site of cellular respiration and ATP production.
    • Endoplasmic Reticulum (ER): (Rough and Smooth) for protein and lipid synthesis.
    • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
    • Lysosomes: (in animals) Contain digestive enzymes.
    • Chloroplasts: (in plants/algae) Site of photosynthesis.
  • Cytoskeleton: A complex network of protein filaments (microtubules, microfilaments, intermediate filaments) for shape, support, and movement.

Comparison of Prokaryotic and Eukaryotic Cells

Biological Membranes

Biological membranes (like the plasma membrane and organelle membranes) are essential for compartmentalization and controlling the passage of substances.

Components of Biological Membranes

1. Phospholipids: The fundamental component. They are amphipathic molecules (having both hydrophilic and hydrophobic parts) that spontaneously form a lipid bilayer in water.
   • Hydrophilic Head: Phosphate-containing group (polar, faces the watery environment).
   • Hydrophobic Tails: Two fatty acid chains (nonpolar, face inwards, away from water).
2. Proteins: Determine most of the membrane's functions (transport, signaling, etc.).
   • Integral (Transmembrane) Proteins: Span the entire membrane.
   • Peripheral Proteins: Loosely bound to one side of the membrane.
3. Cholesterol: (in animal cells) A steroid lipid that embeds in the bilayer. It acts as a fluidity buffer:
   • At high temperatures, it reduces fluidity.
   • At low temperatures, it increases fluidity (prevents packing).
4. Carbohydrates: Found only on the outer surface, attached to proteins (glycoproteins) or lipids (glycolipids). Form the glycocalyx.

Fluid Mosaic Model

Proposed by Singer and Nicolson in 1972, this is the accepted model for membrane structure.

Key Concepts of the Fluid Mosaic Model:

• Fluid: The membrane is not rigid. The phospholipids and many proteins can move laterally (sideways).
• Mosaic: The membrane is a "patchwork" or "mosaic" of different components (lipids, proteins, cholesterol) fitted together.

This fluidity is essential for functions like cell signaling, transport, and membrane fusion.

Cell Recognition and Membrane Transport

Cell Recognition

• Definition: The ability of a cell to distinguish one type of cell from another.
• Mechanism: Mediated by the glycocalyx (the carbohydrate "name tags" on the cell surface).
• Importance: Crucial for immune responses (distinguishing "self" from "non-self"), tissue formation, and blood type compatibility (e.g., A, B, O antigens are carbohydrates).

Membrane Transport

The plasma membrane is selectively permeable. Transport occurs via two main mechanisms:

1. Passive Transport (No Energy Required)

Movement of substances down their concentration gradient (from high to low concentration).

• Simple Diffusion: Small, nonpolar molecules (like O2, CO2) pass directly through the lipid bilayer.
• Facilitated Diffusion: Polar molecules or ions (like glucose, Na+) move down their gradient with the help of transport proteins (channels or carriers).

2. Active Transport (Requires Energy - ATP)

Movement of substances against their concentration gradient (from low to high concentration).

• Primary Active Transport: Uses ATP directly to "pump" a substance. The most famous example is the Sodium-Potassium (Na+/K+) pump, which pumps 3 Na+ out and 2 K+ in, maintaining the cell's membrane potential.
• Secondary Active Transport: Uses the gradient established by a primary pump as an indirect energy source (e.g., using a Na+ gradient to "drag" glucose into the cell).