The Cell: The Unit of Life - Part 01

Welcome to the fundamental journey into the very essence of life! The cell is the basic structural and functional unit of all living organisms. Understanding its intricate details is crucial for NEET aspirants.

Introduction to the Cell

Life on Earth exhibits an incredible diversity, yet all living organisms, from the smallest bacteria to the largest whales, share a common fundamental unit: the **cell**. The study of cells, their structure, function, and behavior, is known as **Cell Biology**.

Discovery of the Cell

• Robert Hooke (1665): First observed and described "cells" in thin slices of cork using his self-designed microscope. He used the term "cellula" (Latin for small room) to describe the honey-comb like structures, which were actually dead cell walls. His observations were published in his book **"Micrographia"**.
• Anton von Leeuwenhoek (1674): First observed and described **living cells**, including bacteria, protozoa, sperm cells, and red blood cells. He famously called them "animalcules."
• Robert Brown (1831): Discovered and named the **nucleus** in plant cells.

The Cell Theory

The Cell Theory is one of the foundational principles of biology, explaining the organization of life.

• Matthias Schleiden (1838): A German botanist, concluded that all **plants are composed of cells** and that cells are the basic unit of plant life.
• Theodor Schwann (1839): A German zoologist, independently concluded that all **animals are composed of cells**. He also proposed that cells have a **thin outer layer called the plasma membrane**. He then hypothesized that both plant and animal bodies are composed of cells and products of cells.
• Schleiden and Schwann's original Cell Theory stated that all living organisms are made of cells, and cells are the basic unit of life. However, it failed to explain how new cells were formed.
• Rudolf Virchow (1855): A German physician, modified the Cell Theory by stating that **"Omnis cellula e cellula,"** meaning **new cells arise from pre-existing cells**.

Overview of Cell Types: Prokaryotic vs. Eukaryotic Cells

Based on the complexity of their organization, cells are broadly classified into two main types:

Prokaryotic Cells

• Characterized by the **absence of a true nucleus** (genetic material is not enclosed by a nuclear membrane).
• Lack **membrane-bound organelles** (e.g., endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, plastids).
• Genetic material (DNA) is typically a single circular chromosome, found in a region called the **nucleoid**.
• Ribosomes are present and are of the **70S** type.
• Examples: Bacteria, Blue-green algae (Cyanobacteria), Mycoplasma, PPLO (Pleuro Pneumonia-Like Organisms).

Eukaryotic Cells

• Possess a **true nucleus** (genetic material enclosed within a nuclear envelope).
• Contain various **membrane-bound organelles**, each with specific functions.
• Genetic material consists of multiple linear chromosomes.
• Ribosomes are of the **80S** type (present in cytoplasm and associated with ER) and **70S** type (present in mitochondria and plastids).
• Examples: All organisms other than bacteria, blue-green algae, and Mycoplasma (e.g., Protists, Fungi, Plants, Animals).

Prokaryotic Cell Structure in Detail

Prokaryotic cells are generally smaller and multiply more rapidly than eukaryotic cells. They exhibit a wide variety of shapes and sizes.

Cell Envelope and its Modifications

Most prokaryotic cells, particularly bacterial cells, have a complex chemically composed **cell envelope** that provides structural protection. It consists of three tightly-bound layers:

1. Glycocalyx: The outermost layer. It can be a loose sheath called a **slime layer** or a thick and tough layer called a **capsule**. It provides protection against desiccation and phagocytosis.
2. Cell Wall: Located just beneath the glycocalyx. It gives shape to the cell, provides structural support, prevents the cell from bursting or collapsing, and is composed of **peptidoglycan** (murein).
3. Plasma Membrane (Cell Membrane): The innermost layer of the cell envelope. It is **selectively permeable** in nature and interacts with the outside world. It is structurally similar to that of eukaryotes.

Based on the cell wall composition and the way they respond to Gram stain developed by **Christian Gram**, bacteria are classified into:

• Gram-positive bacteria: Take up the Gram stain and appear purple/blue. They have a thick peptidoglycan layer.
• Gram-negative bacteria: Do not retain the Gram stain and appear pink/red (after counterstaining). They have a thin peptidoglycan layer sandwiched between an inner and outer membrane.

Mesosomes

These are unique to prokaryotes. They are **invaginations** (inward foldings) of the **plasma membrane** into the cytoplasm. They are formed as vesicles, tubules, and lamellae.

Chromatophores

In some prokaryotes like cyanobacteria (blue-green algae), there are other membranous extensions into the cytoplasm called **chromatophores**. These contain **photosynthetic pigments**.

Flagella, Pili, and Fimbriae (Surface Structures)

These are surface structures that extend from the cell wall of some bacteria but are not part of the cell envelope.

• Bacterial Flagella: Thin, filamentous extensions from the cell wall. They are primarily responsible for **motility** (movement). Composed of three parts: **filament**, **hook**, and **basal body**. The filament is the longest portion and extends from the cell surface to the outside. Made of protein **flagellin**.
• Pili: Elongated, tubular structures made of a special protein called **pilin**. Their main function is to help in **conjugation** (transfer of genetic material).
• Fimbriae: Small, bristle-like fibers sprouting out of the cell surface. They help bacteria to **attach** to rocks in streams and also to the host tissues (e.g., urinary tract in pathogenic bacteria).

Ribosomes

• In prokaryotic cells, ribosomes are about **15 nm by 20 nm** in size.
• They are made of **rRNA and proteins** and are not enclosed by any membrane.
• Prokaryotic ribosomes are **70S type** (S refers to Svedberg's unit, a measure of sedimentation coefficient).
• They consist of two subunits: a large **50S** subunit and a small **30S** subunit.
• Function: Site of **protein synthesis**. Several ribosomes may attach to a single mRNA to form a chain called a **polysome** or **polyribosome**. The ribosomes of a polysome translate the mRNA into proteins.

Inclusion Bodies (Storage Structures)

These are non-membrane bound storage structures found in the cytoplasm of prokaryotic cells. They are not enclosed by any membrane system and lie free in the cytoplasm.

• Examples: **Phosphate granules, cyanophycean granules, glycogen granules.**
• Gas vacuoles: Found in blue-green and purple and green photosynthetic bacteria. They help in buoyancy regulation.

Nucleoid and Plasmids

• Nucleoid: The genetic material (DNA) in prokaryotes is a single, circular chromosome, not enclosed by a nuclear membrane. This region where the genetic material is concentrated is called the nucleoid.
• Plasmids: Many bacteria also have small, circular DNA molecules outside the genomic DNA called plasmids. These plasmids confer certain unique phenotypic characters to such bacteria, for example, **antibiotic resistance**. Plasmids are also used as vectors in genetic engineering.

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