Unit 1: Overview of the Cells

1. History of Discovery of Cell

The discovery of the cell is a cornerstone of biology, built upon by several key scientists over centuries.

• Robert Hooke (1665): An English scientist who observed thin slices of cork under a primitive microscope. He noticed tiny, box-like compartments, which he named "cells" (from the Latin *cellula*, meaning "small room"). He was observing dead plant cell walls.
• Antonie van Leeuwenhoek (1674): A Dutch merchant and amateur scientist, he was the first to observe and describe living cells. Using his simple but powerful microscopes, he discovered single-celled organisms in pond water, which he called "animalcules" (like protozoa and bacteria). He also observed blood cells and sperm.
• Robert Brown (1831): A Scottish botanist who discovered the nucleus within the cells of orchids.
• Matthias Schleiden (1838) & Theodor Schwann (1839): German scientists (a botanist and a zoologist, respectively) who formalized the first two principles of the Cell Theory.

2. Diversity of Cell Size and Shape

Cells are incredibly diverse, with their size and shape directly related to their specific function.

Cell Size

• Smallest Cells: Mycoplasma (a type of bacteria) are the smallest known cells, measuring about 0.1 to 0.3 micrometers (µm) in diameter.
• Largest Cell: The single cell of an unfertilized ostrich egg is the largest, visible to the naked eye.
• Longest Cells: Nerve cells (neurons) can be extremely long. For example, the sciatic nerve in a human can have axons that extend over a meter, from the base of the spine to the foot.

Cell Shape

A cell's shape is dictated by its function and environment. Common shapes include:

• Spherical/Ovoid: e.g., Egg cells, some bacteria (cocci).
• Discoidal (Disc-shaped): e.g., Red blood cells (biconcave shape maximizes surface area for oxygen transport).
• Spindle-shaped (Fusiform): e.g., Smooth muscle cells, allowing them to contract and relax.
• Polygonal: e.g., Plant cells in tissues, tightly packed.
• Branched: e.g., Nerve cells (neurons) with dendrites and axons for transmitting signals.
• Irregular: e.g., Amoeba (can change shape) or white blood cells (can move through tissues).

3. Cell Theory

The Cell Theory is the fundamental unifying concept of biology. It was 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. (This tenet, "Omnis cellula e cellula", was added by Rudolf Virchow in 1855).

Modern Additions to the Cell Theory:

Today, the theory also includes these concepts:

• Cells contain hereditary information (DNA) which is passed from cell to cell during division.
• All cells are basically the same in chemical composition and metabolic activities.
• Energy flow (metabolism) occurs within cells.

4. General Characters of 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.

5. Structure of Virus

Viruses are acellular (not cells). They are obligate intracellular parasites, meaning they can only replicate inside a living host cell by hijacking its machinery.

Key Components:

• Genetic Material: A core of nucleic acid, which can be DNA or RNA (but not both). It can be single-stranded (ss) or double-stranded (ds).
• Capsid: A protein coat that encloses and protects the genetic material. It is made of repeating protein subunits called capsomeres. The capsid gives the virus its shape (e.g., helical, icosahedral, complex).
• Envelope (in some viruses): A lipid membrane derived from the host cell's membrane. It surrounds the capsid and contains viral proteins (spikes) that help in attaching to new host cells. (e.g., Influenza virus, HIV). Viruses without this are called "naked" viruses.

6. Structure of Viroids

Viroids are even simpler than viruses. They are the smallest known infectious agents.

• They are infectious agents of plants (no known human viroids).
• They consist only of a short, circular, single-stranded RNA molecule.
• They have no capsid (protein coat) and no envelope.
• The RNA molecule does not code for any proteins. It is thought to cause disease by interfering with the host plant's gene expression.

7. Structure of Mycoplasma

Mycoplasma is a genus of bacteria. Unlike viruses or prions, they are living cells, but they are unique.

• They are the smallest known cells capable of independent growth and reproduction.
• Their defining feature is the complete lack of a rigid cell wall. (This makes them resistant to antibiotics like penicillin that target cell wall synthesis).
• Because they lack a cell wall, their shape is variable (pleomorphic).
• They are prokaryotic cells, containing a cell membrane, cytoplasm, 70S ribosomes, and a circular DNA nucleoid.

8. Structure of Prions

Prions are unique infectious agents composed only of protein.

• A prion is an abnormal, misfolded version of a normal cellular protein known as prion protein (PrP).
• They contain no genetic material (no DNA or RNA).
• The misfolded prion protein (PrP^Sc) can induce normal PrP proteins (PrP^C) to misfold into the prion form, setting off a chain reaction.
• This accumulation of misfolded proteins forms plaques in the brain, leading to neurodegenerative diseases.
• Examples of prion diseases: Creutzfeldt-Jakob Disease (CJD) in humans, Bovine Spongiform Encephalopathy (BSE or "Mad Cow Disease") in cattle, and Scrapie in sheep.