How cells make organisms grow

Growth is a lasting increase in an organism's size and usually its mass. At the scale of cells, three processes can contribute, but they do different jobs:

• cell division produces more cells;
• cell elongation makes an individual cell longer;
• cell differentiation changes a cell so that it becomes specialised for a particular function.

In an animal, repeated cell division increases the number of cells. Some of the new cells then differentiate. This means that growth can produce organised tissues and organs rather than just a larger collection of identical, unspecialised cells.

In a plant, cell division also increases cell number. In a growing root or shoot, many newly produced cells then elongate: each cell becomes longer, so a row of elongating cells increases the length of the organ. Cells also differentiate to form specialised plant cells and tissues. The processes can overlap in a real growing region, but their contributions remain distinct.

Cell elongation is not cells moving apart, and differentiation is not another round of division. One changes cell length; the other changes cell specialisation.

Worked comparison: more cells or longer cells?

Imagine one straight row of root cells, with no gaps. Initially there are five cells, each $20\,\mu\mathrm{m}$ long. The row is $5\times20=100\,\mu\mathrm{m}$ long. After cells have divided and grown back to the same length, ten cells give a row of $10\times20=200\,\mu\mathrm{m}$. If those ten cells then elongate to $40\,\mu\mathrm{m}$ each, the row becomes $10\times40=400\,\mu\mathrm{m}$ long.

The first comparison holds cell length constant to reveal the effect of cell number; the second holds cell number constant to reveal elongation. Real root growth combines these processes. Division itself partitions a cell, so a growing tissue must also make new cell material; splitting a fixed amount of material alone does not increase total size.

Why differentiation matters

A multicellular organism needs different jobs done at the same time. Differentiation creates this division of labour. Specialised cells can work together in tissues, and different tissues can work together in organs.

For example, an animal muscle cell develops structures that can shorten. This produces force, allowing a muscle tissue and then an organ to cause movement. A plant root hair cell develops a long extension; this gives a larger surface area for absorbing water and mineral ions. In each case the reasoning is:

specialised structure → physical consequence → useful function

Cell division and differentiation therefore make different contributions to development:

1. Cell division increases the number of cells available.
2. Differentiation makes some cells structurally and functionally suited to different roles.
3. Groups of specialised cells can form working tissues and organs.

An organism could become a larger mass of cells without enough differentiation, but it could not develop the same range of functioning tissues. This is why differentiation is important, rather than merely a change in appearance.

Quick recap

Growth needs new cell material as well as division. Differentiation gives cells the structures needed for their roles; it is not another name for making more cells.