The Cell: Structure and Functions

Introduction to the Cell

The cell is the basic structural and functional unit of all living organisms. Every living thing, from the simplest bacterium to the largest tree or animal, is made up of one or more cells. The scientific study of cells is called cytology. Cells were first observed by Robert Hooke in 1665, and the Cell Theory later established that all organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Types of Cells

There are two broad categories of cells based on their internal organisation:

• Prokaryotic cells — found in bacteria; they lack a true nucleus and membrane-bound organelles. Their genetic material floats freely in the cytoplasm.
• Eukaryotic cells — found in plants, animals, fungi and protists; they possess a true, membrane-bound nucleus and specialised organelles.

Structure of a Typical Eukaryotic Cell

A typical cell is enclosed by a cell membrane (plasma membrane), which controls the movement of substances in and out of the cell. Inside is the cytoplasm, a jelly-like fluid where most cell activities occur, and organelles such as:

• Nucleus — contains the chromosomes (DNA) and controls all cell activities.
• Mitochondria — the "powerhouse" of the cell; site of aerobic respiration and ATP (energy) production.
• Ribosomes — site of protein synthesis.
• Endoplasmic reticulum — transports materials within the cell; may be rough (with ribosomes) or smooth.
• Golgi apparatus — packages and modifies proteins for transport or secretion.
• Vacuoles — store water, food and waste; plant cells have one large permanent vacuole.

Plant Cells vs Animal Cells

While both are eukaryotic, plant cells have three features animal cells lack: a rigid cell wall made of cellulose, a large permanent vacuole, and chloroplasts (which contain chlorophyll for photosynthesis). Animal cells are usually irregular in shape, while plant cells are typically rectangular due to the cell wall.

Why the Cell Matters

Understanding cell structure helps explain how organisms grow, repair damage, reproduce, and carry out life processes such as respiration, nutrition, and excretion — all of which begin at the cellular level.