Introduction to Cells

The cell is the fundamental unit of life. Every living organism — from a single bacterium to a human being — is made up of cells. The human body contains roughly 37 trillion cells, and there are over 200 different types, each with a specific job. Robert Hooke first described cells in 1665 when he observed a thin slice of cork under a microscope. He saw tiny box-like compartments and named them "cells" after the small rooms (cellulae) in a monastery. What he saw were actually the dead cell walls of plant tissue — but the name stuck. Later, in the 1830s, Matthias Schleiden (plants) and Theodor Schwann (animals) proposed the Cell Theory, now one of biology's foundational principles: 1. All living things are made of cells 2. The cell is the basic unit of life 3. All cells come from pre-existing cells This third point — cells only come from other cells — was confirmed by Louis Pasteur in 1859 and overturned the idea of spontaneous generation (the belief that life could arise from non-living matter).

Despite the enormous variety of cells in the body, all cells — from the tiniest bacterium to a large human nerve cell — share the same four basic components: 1. Plasma membrane — a thin, flexible boundary that separates the inside of the cell from the outside world. It controls what enters and exits. 2. Cytoplasm — the jelly-like fluid that fills the inside of the cell and suspends all the cell's contents. It is mostly water, with dissolved proteins, salts, and sugars. 3. DNA (genetic material) — the instructions for building and running the cell. Every cell's DNA contains the complete genetic blueprint — like having the entire instruction manual for the whole body in every room. 4. Ribosomes — tiny molecular machines that read the DNA's instructions (via messenger RNA) and build proteins. Proteins do almost everything in the cell — they are enzymes, structural supports, transporters, signals, and more. These four features are the minimum requirements for life. Anything with all four is, by definition, a living cell.

All cells fall into one of two fundamental categories — and the difference between them is one of the most important in all of biology. Prokaryotic cells (from Greek: "before nucleus") - No membrane-bound nucleus — their DNA floats freely in the cytoplasm in a region called the nucleoid - No membrane-bound organelles (no mitochondria, no ER, no Golgi) - Much smaller (1–10 micrometres) - Include all bacteria and archaea - Examples you will encounter clinically: Staphylococcus aureus (skin infections), Streptococcus pneumoniae (pneumonia), Mycobacterium tuberculosis (TB), Escherichia coli (urinary tract infections, food poisoning) Eukaryotic cells (from Greek: "true nucleus") - Have a true nucleus enclosed by a double membrane (nuclear envelope) - Contain many specialised membrane-bound organelles - Much larger (10–100 micrometres) - Include all animal, plant, and fungal cells - Your own cells are all eukaryotic Why this distinction matters clinically: Many antibiotics work by targeting features unique to prokaryotes — like their cell walls (penicillin), their ribosomes (which are structurally different from eukaryotic ribosomes — targeted by erythromycin, tetracycline), or their DNA replication machinery (targeted by quinolones). Because human cells lack these structures, the drugs can kill bacteria without harming the patient's own cells. This is the basis of selective toxicity — the cornerstone of antibiotic therapy.

Cells reproduce by dividing — one cell becomes two. This is how the body grows, heals wounds, and replaces old cells. The process is tightly controlled, and when that control fails, cancer can develop. The cell cycle has two main phases: Interphase (the "working and preparing" phase — ~90% of the cycle) - G1 phase — the cell grows and carries out its normal functions. It checks that conditions are right to divide. - S phase (synthesis) — the cell copies all of its DNA. Each of the 46 chromosomes is duplicated. - G2 phase — the cell continues to grow and checks that DNA has been copied correctly before dividing. M phase (mitosis — the actual division) - The duplicated chromosomes are pulled apart and one complete set goes to each new cell. - The cell physically splits in two (cytokinesis), producing two identical daughter cells. Cell cycle checkpoints are quality control points where the cell checks whether it is safe to proceed. Proteins called cyclins and cyclin-dependent kinases (CDKs) drive the cycle forward, while tumour suppressor proteins (like p53 and Rb) apply the brakes. Cancer happens when these controls break down — mutations in oncogenes (accelerators) or tumour suppressor genes (brakes) allow cells to divide uncontrollably, ignoring normal signals to stop. Most cancer treatments target rapidly dividing cells (chemotherapy) or specific mutated proteins that drive division (targeted therapies).

Every disease ultimately comes down to what is happening at the cell level. Understanding healthy cell biology is the foundation for understanding what goes wrong in disease. Cancer — uncontrolled cell division caused by mutations in cell cycle regulators. Different cancers are classified by which cell type they come from: carcinomas (epithelial cells), sarcomas (connective tissue), leukaemias (blood cells), lymphomas (lymphocytes). Infection — bacteria are prokaryotic cells that invade the body; viruses hijack the machinery of our own eukaryotic cells to reproduce. Understanding cellular machinery is why antivirals can target viral-specific enzymes (like HIV's reverse transcriptase or influenza's neuraminidase) without damaging human cells. Genetic diseases — mutations in cellular DNA disrupt the proteins a cell makes. Cystic fibrosis: a mutation in the CFTR chloride channel protein. Sickle cell anaemia: a single amino acid change in haemoglobin. Cell death — cells die in two ways: necrosis (uncontrolled, caused by injury — triggers inflammation) and apoptosis (programmed, controlled cell death — the cell tidily dismantles itself). Apoptosis is essential for development (sculpting the spaces between fingers in a foetus), immune function (killing infected cells), and cancer suppression. Many cancer cells block apoptosis to survive — and some cancer drugs work by reactivating it.