Unit 1: Overview of Cell and plasma membrane

1. Prokaryotic and Eukaryotic cells

All life is classified into two fundamental cell types: prokaryotic and eukaryotic. The primary difference is the presence of a true, membrane-bound nucleus.

2. Cell theory

The Cell Theory is the fundamental unifying concept of biology, proposed by Matthias Schleiden and Theodor Schwann and later expanded by Rudolf Virchow.

The Three Principles of the Cell Theory:

1. All 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 (a concept added by Rudolf Virchow).

3. Various models of plasma membrane structure

The plasma membrane is the selectively permeable barrier that surrounds the cell. Several models have been proposed to describe its structure.

Historical Models

• Davson and Danielli (1935): Proposed the "sandwich model" or "protein-lipid-protein" model. They suggested a phospholipid bilayer was sandwiched between two continuous layers of globular proteins.

The Fluid Mosaic Model (Singer and Nicolson, 1972)

This is the currently accepted model. It describes the plasma membrane as a dynamic and flexible structure.

Fluid Mosaic Model: The membrane is a "mosaic" of components (phospholipids, cholesterol, proteins, carbohydrates) that are "fluid" and can move and flow relative to each other.

• Phospholipid Bilayer: The foundation of the membrane. Phospholipids are amphipathic (hydrophilic head, hydrophobic tails), creating a barrier to water-soluble substances.
• Proteins:
  • Integral Proteins: Embedded within the bilayer, often spanning it (transmembrane proteins). Function as channels, pumps, or receptors.
  • Peripheral Proteins: Loosely bound to the surface, often attached to integral proteins.
• Cholesterol: (In animal cells) Tucked within the tails. It acts as a "fluidity buffer," maintaining membrane stability.

4. Transport across membranes

The plasma membrane controls what enters and leaves the cell using various transport mechanisms.

A. Passive Transport

• Definition: Movement of substances across the membrane without the expenditure of cellular energy (ATP).
• Direction: Always occurs down a concentration gradient (from high concentration to low concentration).
• Types:
  • Simple Diffusion: Small, nonpolar molecules (like O₂, CO₂) move directly through the lipid bilayer.
  • Facilitated Transport: Molecules (like glucose or ions) move down their gradient with the help of a specific transport protein (either a channel protein or a carrier protein).

B. Active Transport

• Definition: Movement of substances across the membrane that requires cellular energy (ATP).
• Direction: Moves substances against their concentration gradient (from low concentration to high concentration).
• Mechanism: Requires specific carrier proteins called "pumps".
• Example: The Sodium-Potassium (Na⁺/K⁺) pump, which actively pumps Na⁺ out of the cell and K⁺ into the cell.

5. Cell junctions

In multicellular tissues, cells are connected by specialized structures called cell junctions. These are crucial for tissue structure, communication, and barrier function.