• All living things are composed of cells that carry out the processes which make an organism alive.
• All cells share the same basic chemical composition.
• All life processes – nutrition, respiration, excretion, growth, and reproduction – take place inside cells.
• Cells contain genetic material (DNA) and can replicate.

Cells are classified into two main types based on whether they have a nucleus:

Prokaryotic Cells

These are simple cells that do not have a true nucleus. Their DNA floats freely in the cytoplasm in a region called the nucleoid.

Characteristics:

• No membrane-bound nucleus
• No membrane-bound organelles (like mitochondria or chloroplasts)
• Usually smaller in size
• Reproduce by binary fission
• Have various shapes: rod (bacilli), spherical (cocci), spiral (spirilla)

Examples: Bacteria and blue-green algae (cyanobacteria)

Eukaryotic Cells

These are complex cells that have a membrane-bound nucleus and other membrane-bound organelles.

Characteristics:

• Have a nucleus that contains genetic material (DNA)
• Contain organelles such as mitochondria, chloroplasts (in plants), endoplasmic reticulum, and Golgi apparatus
• Usually larger and more complex
• Found in both unicellular and multicellular organisms

Examples: Animal cells, plant cells, Amoeba, Euglena

Both animal and plant cells share some common structures, but plant cells have additional features.

Common Structures in Both Cell Types

Structures Found Only in Plant Cells

Structures Found Only in Animal Cells

• Centrioles – involved in cell division
• Lysosomes – contain digestive enzymes

Cell Membrane (Plasma Membrane)

• Acts as a thin barrier around the entire cell
• Controls movement of substances in and out of the cell (selectively permeable)
• Allows useful substances in and removes waste products

Cytoplasm

• A jelly-like substance made of water and dissolved chemicals
• Site where many chemical reactions take place
• Holds organelles in place

Nucleus

• Controls all cell activities
• Determines cell shape, size, and function
• Contains hereditary information (DNA) passed from parent to daughter cells

Mitochondria

• The "powerhouse" of the cell
• Site of cellular respiration where energy is released from glucose

Cell Wall (Plant cells only)

• Made of cellulose – a tough, rigid material
• Fully permeable (allows water and gases to pass through freely)
• Protects the cell and gives it a fixed shape

Chloroplast (Plant cells only)

• Contains green pigment called chlorophyll
• Absorbs light energy for photosynthesis

Vacuole (Plant cells only)

• Stores water, nutrients, and waste materials
• Contains cell sap (dissolved sugars and salts)
• Helps maintain cell pressure and turgidity

Cells become adapted to perform specific functions through a process called cell differentiation. Specialized cells work together in groups called tissues, which form organs, and organs work together in systems.

Examples of specialized cells:

• Red blood cells – transport oxygen (biconcave shape, no nucleus)
• White blood cells – engulf and destroy pathogens
• Root hair cells – absorb water and minerals (have extended surface area)
• Guard cells – control opening and closing of stomata
• Palisade cells – contain many chloroplasts for photosynthesis

The cell is the fundamental unit of life. Cells are of two main types – prokaryotic (without a nucleus) and eukaryotic (with a nucleus). Eukaryotic cells are found in plants and animals, and they contain various organelles that perform specific functions. Plant cells have additional structures like cell walls, chloroplasts, and large vacuoles that enable them to carry out photosynthesis and maintain structure.

In Tanzania, understanding cells helps explain how plants grow in your school garden or farm. For example, when a farmer applies fertilizer to tomato plants, the nutrients are absorbed by root hair cells through their cell membranes. These cells then transport the nutrients to other parts of the plant through the vascular tissues. Similarly, when you eat foods like beans or meat, your body breaks them down at the cellular level – your stomach cells produce enzymes, and intestinal cells absorb the nutrients your body needs for energy and growth.