The Fundamental Unit of Life

Every living thing you can name — a mango tree, an earthworm, a human being — is built from the same basic unit: the cell. Robert Hooke was the first person to see and name cells. In 1665, using a self-made microscope, he examined a thin slice of cork (dead bark tissue) and saw small, empty, box-like compartments. He called them 'cells' because they reminded him of the small rooms monks lived in.

What Hooke actually saw were the cell walls of dead cork cells; the living contents had dried up long before. It took better microscopes and more careful observation, over the next two centuries, for scientists to work out that cells are not empty boxes at all — they are the living, functioning units of every organism, and every organism, from a single bacterium to a whale, is built from one cell or from many cells working together.

An organism made of a single cell (like Amoeba, Paramecium or a bacterium) is called unicellular; that one cell carries out every life function — nutrition, respiration, excretion, growth and reproduction — on its own. An organism made of many cells (like a human being or a neem tree) is multicellular; here, groups of cells specialise: some form muscle, some form nerve, some form skin, and they cooperate to keep the whole organism alive. This is why the cell is called the structural and functional unit of life — every structure of the body is built from cells, and every function of the body is carried out by cells.

Cells are too small to see with the naked eye — most cells range from about 1 micrometre to a few hundred micrometres. Some cells are visible without an instrument: the egg of a hen is a single, extremely large cell packed with stored food (yolk), and the longest cell in the human body is the nerve cell, which can be over a metre long though extremely thin. As a rule, though, cells need magnification to be studied, and the story of the cell is really the story of better microscopes.

In 1674, Antony van Leeuwenhoek, using a simple microscope he ground and polished himself, became the first person to see and describe a living cell — free-swimming organisms in pond water. Robert Brown discovered a dense spherical structure inside the cell in 1831 and named it the nucleus. Building on decades of such observations, two German scientists put forward the first two parts of the cell theory: Matthias Schleiden (1838), a botanist, stated that all plants are composed of cells, and Theodor Schwann (1839), a zoologist, extended this to animals and proposed that the cell wall is a unique feature of plant cells. Together they proposed the cell theory: all living organisms are composed of cells and the cell is the basic unit of life. In 1855, Rudolf Virchow added the crucial third part: all cells arise from pre-existing cells (Omnis cellula-e cellula) — cells are not spontaneously created but divide to form new cells. Later, with the electron microscope, scientists discovered that the cell is bound by a membrane made of lipids and proteins, which refined the cell theory further: all organisms are composed of cells and cell products, and new cells are formed from pre-existing cells.

Every cell, however different it looks under a microscope, has three basic parts working together: the plasma membrane, which forms the cell's outer boundary and controls what enters and leaves; the nucleus (or, in some cells, a nucleoid region), which holds the genetic material and directs the cell's activities; and the cytoplasm, the jelly-like fluid inside the membrane in which the nucleus and all the organelles are suspended and where most of the cell's chemical reactions take place.

Not every cell has a well-formed nucleus enclosed by a membrane. On this basis, cells are divided into two kinds. In a prokaryotic cell, the genetic material (a single circular DNA molecule) lies directly in the cytoplasm in a region called the nucleoid, with no nuclear membrane around it; prokaryotic cells are also smaller (1–10 micrometres), lack most membrane-bound organelles, and their only membrane-bound structure is the cell membrane itself — bacteria and blue-green algae (cyanobacteria) are prokaryotes. In a eukaryotic cell, the genetic material is enclosed within a well-defined nucleus by a nuclear membrane, the cell is larger (5–100 micrometres), and it contains a full set of membrane-bound organelles — mitochondria, ER, Golgi apparatus and, in plants, plastids. All organisms other than bacteria and blue-green algae — protists, fungi, plants and animals — are eukaryotes.

The plasma membrane (also called the cell membrane) is the outermost boundary of the cell in an animal cell, and it lies just inside the cell wall in a plant cell. It is made of lipids and proteins, is flexible, and is porous enough to allow the automatic movement of some substances across it without the cell spending energy — this property is called selective/differential permeability: it allows some materials to pass through while blocking others.

Movement of substances across the membrane happens mainly by diffusion: molecules of a substance move on their own, from a region of their higher concentration to a region of their lower concentration, until the concentration becomes uniform, without the cell using any energy. Gases move by diffusion — oxygen enters a cell by diffusion (because the cell keeps using it up, so its concentration inside always stays lower than outside) and carbon dioxide leaves a cell by diffusion for the same reason.

Osmosis is a special case of diffusion — it is the movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a selectively permeable membrane, without the cell spending energy. Depending on the concentration of the solution surrounding a cell relative to the concentration inside the cell, three situations arise:

• If the medium outside the cell has a higher water concentration than the cell (a hypotonic solution), water moves into the cell by osmosis — this is called endosmosis. A cell placed in pure water swells up.
• If the medium outside has the same water concentration as inside (an isotonic solution), there is no net movement of water — the cell's size stays the same.
• If the medium outside has a lower water concentration than inside (a hypertonic solution), water moves out of the cell by osmosis — this is called exosmosis. A cell placed in a concentrated salt or sugar solution shrinks.

In plant cells, exosmosis causes the cytoplasm to shrink away from the cell wall — this is called plasmolysis, and it can be reversed if the cell is placed back in water. This is exactly what is seen when raisins are placed in plain water (they swell, endosmosis) versus in a concentrated sugar or salt solution (they shrink, exosmosis), and it is why a wilted vegetable becomes firm again when kept in water.

The cell wall is found only in plant cells (and in fungi and bacteria), not in animal cells. It lies outside the plasma membrane and is mainly made of cellulose, a complex sugar that gives strength to plants. Besides cellulose, plant cell walls may also contain pectin and other polysaccharides.

The cell wall gives the plant cell a fixed, rigid shape and provides structural strength, which is why plants can stand upright without a skeleton. It also protects the cell from mechanical injury and from infection, and it is freely permeable, letting most substances pass through easily to reach the plasma membrane.

The cell wall is also what stops a plant cell from bursting even when a very large amount of water enters it by endosmosis in a hypotonic medium: as water enters, the cell swells, but the rigid wall resists further expansion, so the cell becomes turgid (firm) instead of bursting — this turgor pressure is what keeps leaves and stems upright and crisp. An animal cell, having no wall, would simply burst under the same condition. When a living plant cell is placed in a concentrated (hypertonic) solution, water moves out and the cytoplasm shrinks and pulls away from the cell wall — this is plasmolysis, and it demonstrates that the cell wall and the plasma membrane are separate structures.

The nucleus is a large, spherical, membrane-bound organelle that is usually located in the centre of the cell. It is separated from the cytoplasm by a double-layered nuclear membrane, which has pores that allow the transfer of material between the nucleoplasm (fluid inside the nucleus) and the cytoplasm. In a non-dividing cell, the nucleus shows a dense, thread-like network called chromatin material; when the cell is about to divide, the chromatin condenses to form rod-shaped, thicker structures called chromosomes.

Chromosomes carry genetic information in the form of DNA (deoxyribonucleic acid) molecules. Each segment of DNA that codes for one protein, and thereby controls one particular characteristic (trait) in an organism, is called a gene. Genes are therefore the functional units of chromosomes; they control the transfer of hereditary characteristics from parents to offspring. The nucleus also contains one or more small, dense, spherical bodies called nucleoli, which are involved in ribosome formation, and it directs and controls all the cell's activities — including cell division, since it decides when and how a cell will divide to form new cells.

In prokaryotic cells there is no nuclear membrane, so the DNA-containing region is called a nucleoid rather than a nucleus. Some highly specialised cells, such as the mature red blood cells of mammals, lose their nucleus entirely once fully formed.

The cytoplasm is the jelly-like substance that fills the cell between the nuclear region and the plasma membrane. It is the site of most of the cell's chemical activities, and it is where the various organelles are suspended.

The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets running through the cytoplasm, connecting the nuclear membrane to the plasma membrane. There are two kinds: rough endoplasmic reticulum (RER) has ribosomes attached to its surface, which gives it a granular, rough appearance; because ribosomes are the site of protein synthesis, RER helps manufacture proteins for export from the cell. Smooth endoplasmic reticulum (SER) has no ribosomes on it; it helps synthesise fats/lipids, which, along with proteins made by RER, help build the cell membrane (a process called membrane biogenesis). In liver cells, SER also plays a role in detoxifying poisons and drugs.

The Golgi apparatus (discovered by Camillo Golgi) consists of a system of membrane-bound, fluid-filled flat sacs called cisternae, stacked parallel to one another. The material synthesised near the ER is packaged into vesicles by the Golgi apparatus, which then either stores it, modifies it further, or sends it to its target inside or outside the cell (secretion). It also forms lysosomes.

Lysosomes are small, spherical, membrane-bound sacs filled with digestive enzymes made by RER; these enzymes can break down all organic material — old, worn-out organelles, food particles and foreign invaders such as bacteria. Lysosomes act as the cell's waste-disposal system, keeping the cell clean. When a cell is damaged or dies, lysosomes may burst and their enzymes digest the cell's own components — for this reason lysosomes are nicknamed the 'suicide bags' of the cell.

Mitochondria (singular: mitochondrion) are rod-shaped or oval organelles bound by a double membrane, with the inner membrane folded into structures called cristae, which increase the surface area for chemical reactions. Mitochondria have their own DNA and ribosomes, so they can make some of their own proteins and can divide independently of the cell — a feature called autonomous organelles. Their key function is cellular respiration: they break down food (glucose) to release energy in the form of ATP (adenosine triphosphate), the energy currency of the cell — which is why mitochondria are called the 'powerhouse of the cell'.

Plastids are found only in plant cells. They are of three main kinds: chloroplasts contain the green pigment chlorophyll (and carotenoid pigments) and are essential for photosynthesis; chromoplasts contain pigments other than chlorophyll (yellow, orange or red) and give colour to flowers and fruits; and leucoplasts are colourless plastids that mainly store food such as starch, oils and protein. Like mitochondria, plastids have their own DNA and ribosomes.

Vacuoles are storage sacs for solid or liquid contents, bound by a single membrane. Animal cells have many small, temporary vacuoles. Plant cells, in contrast, have one large, permanent central vacuole that can occupy 50–90% of the cell's volume; it is filled with cell sap and maintains the turgor pressure of the cell, giving mechanical support. In Amoeba, a temporary structure called the food vacuole forms around ingested food and contains the digestive enzymes that break it down; in some fresh-water protozoans, a contractile vacuole pumps out excess water to maintain osmotic balance.

New cells are formed from pre-existing cells by cell division, and this happens by two processes, depending on need. Mitosis is the type of cell division that occurs during growth and repair of tissue in the body. A single parent cell divides once to produce two daughter cells, and each daughter cell is genetically identical to the parent cell (same chromosome number). Mitosis is the basis of growth in multicellular organisms, replacement of worn-out cells, and healing of wounds.

Meiosis is the type of cell division that occurs only in reproductive organs, to produce gametes (sperm and egg cells, or pollen and ovules in plants). One parent cell divides twice in succession to produce four daughter cells, and each daughter cell has exactly half the number of chromosomes as the parent cell (this halving is why the offspring formed after fertilisation, when a sperm and egg fuse, has the normal, full chromosome number again — half from each parent).