The cell theory is a fundamental concept in biology that describes the nature of cells. It was developed through the contributions of several scientists:

• Matthias Schleiden (1838), a Belgian botanist, observed plant cells
• Theodor Schwann (1839), a German zoologist, studied animal cells
• Rudolf Virchow (1855) established that all cells arise from pre-existing cells

Main Ideas of Cell Theory

The classical cell theory states:

1. All living organisms are composed of one or more cells — from simple unicellular organisms like bacteria to complex multicellular organisms like humans
2. Cells are the basic or fundamental units of life — all life processes (nutrition, respiration, excretion, growth) are controlled by cells
3. All new cells arise from pre-existing cells — cells divide through processes like mitosis and meiosis

Modern Additions to Cell Theory

Modern biology has extended the cell theory to include:

• Cells contain genetic material (DNA) that carries hereditary information passed to daughter cells
• Energy flow occurs within cells during metabolic processes
• All cells have similar basic chemical composition
• Structure and functions of cells complement each other

Shortcomings of Cell Theory

Despite its importance, the cell theory has limitations:

• Viruses: They are non-cellular but exhibit living characteristics when inside host cells
• Mature red blood cells and sieve tube cells lack nuclei but are still considered living
• The theory does not explain the origin of the first cell

Cells are broadly classified into two types: prokaryotic and eukaryotic.

Prokaryotic Cells

These are primitive cells that lack a true nucleus. Examples include bacteria and blue-green algae (cyanobacteria).

Characteristics:

• Diameter: 0.5–10 μm
• No membrane-bound nucleus; genetic material (circular DNA) floats freely in cytoplasm
• Small ribosomes (70S)
• No membrane-bound organelles (mitochondria, chloroplasts absent)
• Cell wall contains peptidoglycan (murein)
• Flagella, if present, lack "9+2" microtubule arrangement

Key structures: Cell wall, plasma membrane, cytoplasm, nucleoid, ribosomes, plasmids, flagella, pili

Eukaryotic Cells

These are complex cells with a true nucleus and membrane-bound organelles. Examples include plant and animal cells.

Characteristics:

• Larger size (10–100 μm)
• True nucleus with nuclear membrane
• Large ribosomes (80S)
• Membrane-bound organelles present
• DNA associated with histone proteins to form chromosomes
• Cell wall (in plants) contains cellulose; absent in animals

Nucleus

• Control centre of the cell
• Contains genetic material (DNA) in chromosomes
• Directs protein synthesis through RNA synthesis
• Controls cell division

Mitochondria

• "Powerhouse of the cell"
• Site of aerobic respiration producing ATP
• Double membrane organelle with cristae increasing surface area
• Contains its own DNA (similar to prokaryotes)

Chloroplasts (Plant Cells)

• Site of photosynthesis
• Contains chlorophyll for capturing light energy
• Has thylakoids arranged in grana
• Also contains DNA similar to prokaryotes

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; site of protein synthesis and transport
• Smooth ER: Synthesizes lipids and carbohydrates

Ribosomes

• Sites of protein synthesis
• 70S in prokaryotes; 80S in eukaryotes
• Found free in cytoplasm or attached to rough ER

Golgi Apparatus

• Modifies, packages, and transports secretions
• Forms lysosomes
• Involved in cell wall formation in plants

Lysosomes

• Contain digestive enzymes
• Break down waste materials, old organelles (autophagy)
• Can cause self-destruction (autolysis) — called "suicide bags"

Vacuoles (Plant Cells)

• Large central vacuole stores water, nutrients, and waste
• Maintains turgor pressure for structural support

Cells contain four major classes of organic molecules:

Carbohydrates

• Monosaccharides: Glucose, fructose (simple sugars; C₆H₁₂O₆)
• Disaccharides: Sucrose, lactose, maltose (two sugar units)
• Polysaccharides: Starch, cellulose, glycogen (many sugar units)

Functions: Energy source, structural components (cellulose in cell walls), energy storage (starch in plants, glycogen in animals)

Lipids

• Simple lipids: Fats and oils (triglycerides)
• Compound lipids: Phospholipids (membrane structure)
• Derived lipids: Steroids, terpenes

Functions: Energy storage, insulation, structural components of membranes, hormones

Proteins

• Composed of amino acids linked by peptide bonds
• Have primary, secondary, tertiary, and quaternary structures

Functions: Structural components, enzymes (catalysts), transport, defense, regulation

Enzymes

• Biological catalysts that lower activation energy
• Specific to their substrates (lock-and-key model)
• Affected by temperature, pH, substrate concentration, and enzyme concentration

A student examining an onion epidermal cell under a light microscope would observe:

1. Cell wall: Outer rectangular boundary
2. Cell membrane: Thin layer beneath cell wall
3. Nucleus: Dark-staining spherical structure, usually near the periphery
4. Cytoplasm: Jelly-like substance filling the cell
5. Large vacuole: Central clear space (may displace nucleus)

Using iodine solution stains the nucleus darker, making it more visible.

Understanding cell biology has practical applications in Tanzania's healthcare and agriculture. For example, knowledge of mitochondria helps explain why some tropical diseases affect specific tissues — the liver cells with abundant mitochondria are major sites for metabolizing antimalarial drugs. Additionally, plant cell biology informs Tanzanian farmers why certain crops (like cassava and sweet potatoes) store starch in leucoplasts, enabling better post-harvest storage decisions and value-addition processing for markets.