Unit 1: Overview of Cell and plasma membrane

1. Prokaryotic and Eukaryotic cells
2. Cell theory
3. Various models of plasma membrane structure
4. Transport across membranes
5. Cell junctions

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.

[Image of Prokaryotic cell vs Eukaryotic cell comparison]

Feature	Prokaryotic Cell (e.g., Bacteria)	Eukaryotic Cell (e.g., Animal)
Nucleus	Absent. Genetic material (DNA) is in a region called the nucleoid.	Present. A true nucleus enclosed by a nuclear envelope.
DNA Structure	Single, circular chromosome. May also have plasmids.	Multiple, linear chromosomes complexed with histone proteins.
Membrane-Bound Organelles	Absent (no mitochondria, ER, Golgi, etc.).	Present (mitochondria, ER, Golgi, lysosomes, etc.).
Ribosomes	70S (composed of 50S and 30S subunits). Free in cytoplasm.	80S (composed of 60S and 40S subunits). Free in cytoplasm and attached to the ER.
Cell Wall	Present in most (e.g., peptidoglycan in bacteria).	Absent in animal cells. Present in plants (cellulose) and fungi (chitin).
Cell Division	Binary fission (simple division).	Mitosis and Meiosis.
Size	Typically small (1-10 µm).	Typically larger (10-100 µm).

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:

All living organisms are composed of one or more cells.

The cell is the basic structural and functional unit of life.

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.

[Image of the Fluid Mosaic Model of plasma membrane]

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.

Exam Tip: The key difference is energy and direction.

Passive: No ATP, high-to-low gradient.
Active: Requires ATP, low-to-high gradient.
Facilitated Transport is PASSIVE; it just uses a protein helper.

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.

[Image of types of cell junctions: tight junctions, desmosomes, gap junctions]

Junction Type	Structure	Primary Function	Example Location
Tight Junctions	Belts of proteins (claudins and occludins) that fuse the plasma membranes of adjacent cells.	Forms a "watertight" seal; prevents leakage of fluids and molecules between cells.	Lining of the intestine, bladder.
Desmosomes	"Spot welds" or "rivets" that anchor cells together. Cadherin proteins link the cells, while intermediate filaments (keratin) provide internal support.	Provides strong mechanical adhesion; resists tearing and stretching.	Skin (epidermis), heart muscle.
Gap Junctions	Channels (made of connexin proteins) that directly connect the cytoplasm of two adjacent cells.	Allows for rapid chemical and electrical communication; ions and small molecules pass directly from cell to cell.	Heart muscle, neurons.