Module 100. Cell Biology

Cell biology, also known as cytology, is the branch of biological science that studies the structure, function, and behaviour of cells—the basic unit of life. It explores how cells operate individually and collectively within organisms, focusing on their physical characteristics, chemical composition, molecular mechanisms, and interactions with their environment. This discipline forms the foundation for understanding physiology, genetics, biochemistry, and medicine, as nearly all biological processes can be traced to cellular activity.

Historical Background and Discovery

The study of cells began with the invention of the microscope in the 17th century. In 1665, Robert Hooke, an English scientist, first observed small compartments in a thin slice of cork and termed them “cells” after the Latin cellula, meaning small room. Later, Anton van Leeuwenhoek, using a more refined microscope, observed living cells such as protozoa and bacteria, marking the beginning of microbiology.
In the 19th century, German scientists Matthias Schleiden and Theodor Schwann proposed the Cell Theory, which stated that all living organisms are composed of one or more cells and that the cell is the basic unit of structure and function in all living things. Later, Rudolf Virchow expanded this theory by asserting that all cells arise from pre-existing cells (Omnis cellula e cellula).
The development of advanced microscopy, including electron microscopy in the 20th century, allowed scientists to observe subcellular structures such as mitochondria, ribosomes, and endoplasmic reticulum, giving rise to modern cell biology.

The Cell Theory and Its Principles

The modern Cell Theory comprises three core principles:

1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of life, carrying out all vital functions.
3. All cells originate from pre-existing cells through cell division.

Later refinements added that cells contain hereditary information (DNA) passed from one generation to the next, and that all cells share basic chemical composition and metabolic processes.

Structure and Components of Cells

Cells exist in two primary forms — prokaryotic and eukaryotic — which differ in structure and complexity.
Prokaryotic Cells:

• Found in bacteria and archaea.
• Lack a true nucleus and membrane-bound organelles.
• Genetic material (DNA) lies freely within the cytoplasm in a region called the nucleoid.
• Possess a simple structure with cell wall, plasma membrane, cytoplasm, and ribosomes.

Eukaryotic Cells:

• Found in plants, animals, fungi, and protists.
• Contain a well-defined nucleus enclosed by a nuclear membrane.
• Possess numerous membrane-bound organelles, each with specialised functions.

Key components of eukaryotic cells include:

• Cell Membrane (Plasma Membrane): A semi-permeable barrier controlling the entry and exit of substances. Composed of a phospholipid bilayer with embedded proteins, it maintains homeostasis.
• Cytoplasm: The jelly-like substance filling the cell, containing organelles and the cytoskeleton.
• Nucleus: Contains chromatin (DNA and proteins) and controls cellular activities such as growth and reproduction.
• Mitochondria: Known as the “powerhouse of the cell,” responsible for generating energy through cellular respiration.
• Endoplasmic Reticulum (ER): Exists in two forms — rough ER, studded with ribosomes for protein synthesis, and smooth ER, involved in lipid synthesis and detoxification.
• Golgi Apparatus: Modifies, packages, and transports proteins and lipids.
• Lysosomes: Contain digestive enzymes that break down waste materials and cellular debris.
• Ribosomes: Small structures responsible for protein synthesis, found either free in the cytoplasm or attached to the rough ER.
• Centrioles: Involved in cell division in animal cells, forming spindle fibres that separate chromosomes.
• Plastids: Found only in plant cells, such as chloroplasts, which perform photosynthesis.
• Cell Wall: Present in plants, fungi, and some bacteria, providing rigidity and structural support.
• Vacuoles: Fluid-filled sacs that store nutrients, waste, and water; large and prominent in plant cells.

Cellular Processes and Functions

Cells perform numerous biochemical processes essential for sustaining life.
1. Cellular Respiration: The process by which cells convert glucose and oxygen into energy (ATP), carbon dioxide, and water. Occurs primarily in mitochondria.
2. Photosynthesis: Occurs in chloroplasts of plant cells, where light energy is converted into chemical energy in the form of glucose.
3. Protein Synthesis: Takes place in two stages — transcription (DNA to RNA) in the nucleus and translation (RNA to protein) on ribosomes.
4. Cell Division: Cells reproduce by division, ensuring growth and tissue repair.

• Mitosis: Produces two genetically identical daughter cells for growth and maintenance.
• Meiosis: Produces four genetically distinct gametes for sexual reproduction.

5. Transport Mechanisms:

• Diffusion and Osmosis allow movement of substances across membranes.
• Active transport requires energy to move molecules against concentration gradients.

The Cell Cycle

The cell cycle describes the sequence of events that a cell undergoes in its life. It includes:

• Interphase (G₁, S, G₂ phases) – period of growth and DNA replication.
• Mitosis – division of the nucleus.
• Cytokinesis – division of the cytoplasm forming two daughter cells.

Control of the cell cycle is vital for preventing uncontrolled cell growth, which can lead to diseases such as cancer.

Specialised Cells and Cellular Differentiation

In multicellular organisms, cells differentiate to perform specialised functions. Examples include:

• Red blood cells: Transport oxygen through haemoglobin.
• Nerve cells (neurons): Transmit electrical impulses.
• Muscle cells: Contract to produce movement.
• Epithelial cells: Form protective layers and aid absorption.
• Guard cells: Regulate gas exchange in plant leaves.

Cell differentiation is regulated by genetic expression and environmental cues, enabling diverse tissue and organ formation.

Techniques in Cell Biology

Modern research in cell biology employs a range of advanced techniques:

• Microscopy: Light, fluorescence, and electron microscopy provide visualisation of cellular components.
• Cell Culture: Allows growth and study of cells in controlled laboratory environments.
• Flow Cytometry: Used for analysing and sorting cells based on size, shape, and protein markers.
• Centrifugation: Separates cellular components by density.
• Molecular Techniques: Include polymerase chain reaction (PCR) and Western blotting for studying gene and protein expression.

Applications and Significance

Cell biology has profound applications across multiple scientific and medical domains:

• Medicine: Understanding cell function aids in diagnosing diseases, developing vaccines, and creating targeted therapies such as cancer immunotherapy.
• Genetics: Cellular studies underpin genetic engineering, stem cell research, and cloning technologies.
• Biotechnology: Exploits cellular mechanisms to produce pharmaceuticals, enzymes, and biofuels.
• Agriculture: Enhances crop yields and disease resistance through plant cell culture and genetic modification.
• Environmental Science: Cellular processes are studied to assess pollutant impact and develop bioremediation strategies.

Modern Advances

Recent advances in cell biology include the discovery of stem cells capable of regenerating tissues, the use of CRISPR–Cas9 for genome editing, and the development of 3D cell culture systems mimicking natural tissues. Techniques such as confocal microscopy, single-cell sequencing, and live-cell imaging provide unprecedented insight into dynamic cellular processes.