The Fundamental Unit of Life

Just as all matter is built from atoms, all living things are built from cells. A cell is the smallest unit that can carry out the processes of life. Some organisms are a single cell, while others, like us, are made of trillions working together. Understanding the cell (its discovery, its types and its parts) is the foundation of all biology.

The cell is the basic structural and functional unit of life. It was first observed in 1665 by Robert Hooke. He looked at a thin slice of cork through a microscope and saw tiny box-like compartments. He named them "cells".

Later study led to the cell theory. It has three points: all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. Organisms made of a single cell are unicellular (like the amoeba). Those made of many cells are multicellular.

Cells are of two fundamental types, divided by whether they have a true nucleus:

• Prokaryotic cells have no true nucleus: their genetic material lies free in the cell and they lack membrane-bound organelles. They are small and simple. Bacteria are prokaryotes.
• Eukaryotic cells have a membrane-bound nucleus that holds the genetic material, and many specialised organelles. They are larger and more complex. Plants, animals and fungi are eukaryotes.

A eukaryotic cell contains tiny structures called organelles, each with its own job, like the rooms of a busy factory:

• Nucleus: the control centre, holding the DNA that directs the cell.
• Mitochondria: the "powerhouse", which release energy through respiration.
• Ribosomes: where proteins are made.
• Endoplasmic reticulum: a network that transports materials.
• Golgi apparatus: packages and dispatches substances.
• Lysosomes: the "suicide bags", which digest waste and worn-out parts.
• Vacuoles: storage sacs for water and food.

Surrounding everything is the cell membrane, which controls what enters and leaves the cell.

Both plant and animal cells are eukaryotic and share most organelles, but plant cells have three features that animal cells lack:

• A rigid cell wall of cellulose outside the membrane, giving the plant cell shape and support.
• Chloroplasts, which contain the green pigment chlorophyll and carry out photosynthesis.
• A large central vacuole that stores water and keeps the cell firm.

Animal cells, by contrast, have only a flexible membrane, no chloroplasts, and small or no vacuoles. Plants make their own food, while animals must eat. These structural differences reflect that.

Every living thing grows, repairs itself and reproduces. All of this comes down to cells making new cells. The cell cycle is the orderly sequence of events by which a cell grows and divides into two. It has two broad phases:

• Interphase: the long growing phase. The cell grows, carries out its work, and copies its DNA in preparation for division.
• The dividing (M) phase: the cell actually splits.

The cycle is carefully controlled. Faults in this control can lead to uncontrolled division, which is the basis of cancer.

Mitosis is the division that produces two identical daughter cells, each with the same number of chromosomes as the parent.

It is used for:

• growth: building a body from a single fertilised cell, and
• repair and replacement: healing wounds and replacing worn-out cells, like skin and blood cells.

Because the daughter cells are genetically identical to the parent, mitosis keeps the body's cells consistent.

Meiosis is a special division that halves the chromosome number, producing four cells, each with half the chromosomes of the parent. It occurs only in the formation of gametes, the sex cells: sperm and eggs.

Meiosis matters for two reasons:

• it ensures that when two gametes fuse at fertilisation, the normal chromosome number is restored, not doubled each generation, and
• it shuffles the genes, creating variation among offspring, the raw material of evolution.

So mitosis makes identical body cells, while meiosis makes varied sex cells with half the chromosomes.

A single cell can do only so much. In larger organisms, cells of the same kind group together to share the work. A tissue is a group of similar cells, working together to perform a particular function. Tissues in turn build organs, and organs build organ systems.

Tissues exist because of the division of labour. Instead of every cell doing everything, different tissues specialise. Some support, some transport, some contract, some sense. This makes a large, multicellular body far more efficient than a mass of identical cells.

Plant tissues are of two broad kinds:

• Meristematic tissue: made of actively dividing cells, found at growing points such as root and shoot tips. It is responsible for the growth of the plant.
• Permanent tissue: cells that have stopped dividing and taken on a fixed role. These include the protective surface tissue, the supporting and packing tissues, and the conducting tissues: xylem (carrying water and minerals up) and phloem (carrying food).

This is why a tree can grow taller at its tips while its trunk supports and transports.

Animals have four main types of tissue, each with a clear job:

• Epithelial tissue: covers and lines the body and its organs, such as skin and the gut lining, giving protection.
• Connective tissue: joins, supports and packs, including bone, cartilage, blood and fat.
• Muscular tissue: can contract, producing movement.
• Nervous tissue: carries electrical messages, enabling sensation and control.

Together these four tissue types build every organ in the animal body, from the heart to the brain.