The Tiny Rooms That Build You: Cell Structure

Here is something to think about: right now, as you read this, around 37 trillion tiny units are working inside your body — each one following its own set of instructions, taking in fuel, and making new copies of itself. Each unit is a single cell. A human hair is roughly 70 micrometres wide, and many of your cells are about that same size. You could line up around 40 average human cells across a single full stop on this screen. Scientists use microscopes to see these structures clearly. Robert Hooke first described cells in 1665 when he looked at a thin slice of cork under a simple microscope and saw tiny box-like compartments — he called them 'cells' because they reminded him of the small rooms (cellae) in a monastery.

All living organisms are made of cells. Cells are the basic structural and functional units of life. There are two main types of cell we study at this level: animal cells and plant cells. Both share several key structures, but plant cells have extra features that help them make their own food. **Structures found in BOTH animal and plant cells:** **Cell membrane** — A thin, flexible layer that surrounds every cell. Think of it as a security gate: it controls which substances enter and leave the cell, letting nutrients in and waste products out. **Cytoplasm** — A jelly-like fluid that fills most of the cell. Almost all of the cell's chemical reactions happen here. It holds the other structures in place and acts like a busy factory floor. **Nucleus** — Often called the 'control centre'. It contains DNA, which carries the genetic instructions that tell the cell what to do — what proteins to make, when to divide, and how to function. It is surrounded by its own membrane, called the nuclear envelope. **Mitochondria** (singular: mitochondrion) — Small, bean-shaped structures where aerobic respiration takes place. Aerobic respiration is the process of releasing energy from glucose using oxygen. Cells that need a lot of energy — such as muscle cells — contain hundreds of mitochondria. **Ribosomes** — Tiny structures (even too small to see clearly with a light microscope) where proteins are built from amino acid chains, following instructions sent from the nucleus. Every cell activity depends on proteins, so ribosomes are essential. **Structures found ONLY in plant cells:** **Cell wall** — A rigid outer layer made of a tough fibre called cellulose, found just outside the cell membrane. It gives plant cells a fixed, boxy shape and provides structural support — this is why plant stems can stand upright. **Chloroplasts** — Green, disc-shaped structures that contain a pigment called chlorophyll. Chloroplasts are where photosynthesis happens: the cell uses light energy, water, and carbon dioxide to produce glucose and oxygen. Animal cells cannot photosynthesise, so they have no chloroplasts. **Permanent vacuole** — A large, fluid-filled sac in the centre of a mature plant cell. It contains a liquid called cell sap, which helps keep the cell firm (a state called turgor). When a plant wilts, it is partly because the vacuoles have lost water. Animal cells may have small, temporary vacuoles, but nothing like the large, permanent one in plant cells.