Specialised Cells and Their Functions

Hold a red blood cell and a nerve cell side by side in your imagination: one is a tiny flat disc the size of a full stop, the other is a thread that could stretch the length of your arm, and both are single cells doing completely different jobs. Here are five classic examples.

Notice the pattern: every odd feature has a clear reason. Structure fits function.

A red blood cell does one job: pick up oxygen in your lungs, deliver it to every part of your body, then return for more. To do that job well it has three weird features:

• No nucleus. When a red blood cell is finished growing, it ejects its nucleus. This frees up space, letting it pack in roughly 270 million haemoglobin molecules, each one carrying oxygen.
• Biconcave shape (like a flattened doughnut without the hole). This shape gives more surface area for oxygen to enter and leave, and lets the cell bend through narrow blood vessels.
• Full of haemoglobin the red protein that grabs onto oxygen. Without haemoglobin, blood couldn't carry oxygen at all.

Trade-off: because it has no nucleus, a red blood cell can't divide or repair itself. So they only live about 120 days, and your bone marrow has to keep making fresh ones.

Now look at the cell that makes them, because it is the opposite of everything above. A blood stem cell in your bone marrow is round rather than biconcave, has a large obvious nucleus, contains no haemoglobin, and would be hopeless at carrying oxygen. It is not a badly built red blood cell. It is unspecialised, and that is its job: because it keeps its nucleus it can keep dividing, and its daughter cells can become red blood cells, or white blood cells, or the cells that make platelets.

Put the two side by side and the trade-off comes into focus. Specialising is not free. Every feature that makes a red blood cell brilliant at one job is a feature it cannot undo, and the nucleus it threw away to make room for haemoglobin is the very thing it would need in order to repair itself or make a replacement. That is why a body cannot be made only of specialists, and why yours keeps a supply of unspecialised cells on hand for the whole of your life.

Three more cells where the shape gives the job away:

• Neuron is a nerve cell with a long extension called an axon, which can stretch from your spine to your toe. Its length lets one neuron carry a signal over that long distance without a handover. Length itself does not make the signal travel faster; signal speed also depends on features such as axon diameter and myelin insulation.
• Sperm cell a male reproductive cell shaped like a tiny tadpole. It has a long flagellum (tail) that whips side to side to push it forward, and its midpiece (just behind the head) is packed with mitochondria to power the long swim to the egg.
• Root hair cell a plant cell on the outside of a young root. It grows one super-long thin extension that pokes between soil particles. This dramatically increases surface area, so water and dissolved minerals can be sucked in faster.

In every case, the cell solves a job through its shape. Long for distance. Tailed for swimming. Hairy for absorbing.

Now a plant example. Just under the top surface of a leaf is a layer of tall, column-shaped cells called palisade mesophyll cells. They are the plant's main sunlight collectors.

Two clever design choices:

• Column shape. Standing tall and tightly packed lets more cells fit into the sunlit upper layer per square millimetre.
• Stuffed with chloroplasts. A single palisade cell can contain dozens (sometimes hundreds) of chloroplasts. More chloroplasts means more photosynthesis, which means more glucose for the plant.

This is why a leaf is green on top and paler underneath, the chloroplast-packed palisade cells live near the upper surface where the sunlight is brightest.

Every specialised cell you've met follows the same rule:

The shape and features of a cell match the job it has to do.

This is called structure-function fit, and it is one of the most important ideas in biology. What makes it useful is that it works at every scale at once. It is there inside a single cell, where a chloroplast is built from stacked sheets that catch more light and a mitochondrion is folded to fit more reacting surface into the same space. It is there in whole organs, where your lungs hold the area of a tennis court in a chest-sized box by dividing it into millions of tiny sacs. And it is there in whole animals, in a cheetah's long legs and in the ridged teeth a koala uses to grind gum leaves.

Notice that the same design problem keeps producing the same answer. A root hair cell, a lung, a mitochondrion and the folds of your small intestine are utterly different structures that all needed a lot of surface in not much space, and all four solved it the same way: go thin, go long, or go folded. When you meet a new structure and cannot see the point of its shape, that is often the question to try first.

So when you see a weird-looking cell, the first question to ask is: "What job does this shape make easier?" And the second, which is the one that catches errors, is: "What has this cell given up in order to do it?"