One Cell or Many

Put a drop of pond water under a microscope and you may see a Paramecium, a single cell shaped like a slipper, swimming fast. That one cell catches food, moves, pumps out extra water and splits in two to reproduce. Your body does the same kinds of jobs, but it shares them out among trillions of cells of many different kinds. How can one cell manage alone, and why are your cells so different from one another?

Explore: share out life’s jobs

In the model you get five life jobs: take in food, move, sense the surroundings, reproduce and remove extra water. You can give every job to one cell, or build a group of cells and give each cell one or two jobs. Each cell has a limited energy supply, and you can make the organism bigger and see which arrangement keeps up.

In this sketch: each job is 1.5 units of work for every step of size, one cell has an energy supply of 10 units, and all five jobs must be done. In this sketch, food is the only energy supply: if no cell takes in food, no cell has energy. Units are relative, not measured.

Organism size

1

One cell, all five jobs Five cells, one job each + Add a cell − Remove last cell Reset

Press a job to give it to a cell or take it away. The jobs are: take in food, move, sense the surroundings, reproduce, remove extra water.

Rule to keep in this sketch: a cell can only do as much work as its limited energy supply allows, so when the work grows, sharing the jobs among more cells lets the organism keep up. Real specialized cells still take in food and make ATP themselves; this is a sketch of the logic.

Try this.

1. Give all five jobs to one small cell. Predict whether it can keep up, then run it.
2. Make that single cell bigger, step by step. Predict what happens to how well it keeps up, then test.
3. Build a group of cells and give each one a single job. Predict whether the group keeps up as it grows, then run it.
4. Remove the cells doing one job, such as taking in food. Predict what happens to the rest of the group.

Explain: one cell that does it all

A unicellular organism is a living thing made of just one cell. A multicellular organism is made of many cells. Bacteria, many algae and many protists, such as Paramecium, are unicellular.

A Paramecium has no tissues or organs, yet its one cell does every job of a living thing:

• Moves. Its surface is covered in cilia, short hair-like structures that beat together like oars.
• Feeds. Food such as bacteria is swept into a groove and enclosed in food vacuoles, where it is digested.
• Removes extra water. Pond water has fewer dissolved particles than the cell, so water keeps moving in by osmosis. A contractile vacuole is a sac that fills with this extra water and squeezes it out. You met osmosis in that lesson.
• Reproduces. It can split into two cells by binary fission, a way of reproducing without a partner in which one cell divides into two.

Some organisms sit in between. Volvox, a green alga, lives as a hollow ball of many cells. In Volvox carteri, most cells swim and make food, while a few large cells are set aside for reproduction. Colonies like this show a first step toward division of labor.

Explain: specialized cells, from tissues to systems

In a multicellular organism, cells share the work. A specialized cell is a cell with a structure that suits it to a particular job. Similar cells working together form a tissue, such as muscle tissue. Several tissues working together form an organ, such as the heart. Related organs form an organ system, such as the circulatory system.

cell → tissue → organ → organ system → organism

A 70-kilogram reference adult man holds about 30 to 36 trillion cells, according to two careful estimates (2016 and 2023); the 2023 model gave about 28 trillion for a reference woman and about 17 trillion for a 10-year-old child. Textbooks often say there are about 200 kinds of cells. A 2022 atlas that sorted cells by the genes they use counted more than 400 types in the tissues it sampled, so the true number depends on how finely you sort.

Each specialized cell’s shape fits its job:

Cell	Structure	How it fits the job
Neuron (nerve cell)	Long, thin extension called an axon	Carries signals long distances, such as from the spine to the foot
Red blood cell	Small, flexible disc dented in the middle; no nucleus when mature	Large surface for oxygen to cross; bends to squeeze through narrow vessels
Skeletal muscle cell	Very long cylinder with many nuclei	Packed with protein fibres that shorten together to pull on bone
Root hair cell	Long tube growing out from the root surface	Adds surface touching the soil, where water and minerals enter
Guard cell	Two cells, often curved, around a pore in a leaf	Change shape to open and close the pore, letting gases in and out

Think of a restaurant kitchen, where one cook chops, one grills and one washes dishes. The analogy breaks in one place: a specialized cell still does its own basic work, such as taking in food molecules and making ATP for energy. A muscle cell cannot hand all of that to another cell.

Explain: same DNA, different genes

If your nerve cells and muscle cells look and work so differently, do they carry different instructions? Mostly no. Nearly every body cell with a nucleus carries essentially the same DNA, copied from the first cell you began as. You can see how that copying happens in that lesson.

The difference is gene expression, the use of a gene’s instructions to make a product such as a protein. Each kind of cell uses a different set of its genes. A muscle cell uses the genes for the proteins that shorten; a red blood cell in the making uses the genes for hemoglobin, the protein that carries oxygen. Many other genes stay unused in each cell.

There are a few exceptions. A maturing red blood cell pushes out its nucleus, so a mature one has no DNA left. Egg and sperm cells carry half a set of DNA. You will learn how genes are read in that lesson.

Elaborate: measuring specialized cells

The table collects real measurements of specialized cells from several published sources.

Cell or part measured	Size (with unit)	Structural feature	Where the value comes from
Human red blood cell	about 7 to 8 µm across	Dented disc; no nucleus when mature	Textbook description
Long human axon	up to about 1 m long	Long extension of a nerve cell	Textbook description, spine-to-toe example
Human oocyte (developing egg cell)	115 ± 12.1 µm across (mean ± standard deviation)	Large round cell	1,563 oocytes from 75 patients, measured after lab maturation
Human sperm head	4.96 µm long (mean)	Head measured on its own	10 men, live cells
Human sperm tail	38.22 µm long (mean)	Tail measured on its own	Same 10 men
Human skeletal muscle cell	up to about 30 cm long	Long cylinder, many nuclei	Textbook, a long thigh muscle
Arabidopsis root hair, young seedling	0.49 ± 0.11 mm long	Tube growing from a root surface cell	Seedlings 3 to 5 days old
Arabidopsis root hair, older plant	0.38 ± 0.19 mm long	Same kind of tube	Plants 10 to 14 days old
Arabidopsis root hair	9.9 ± 2.2 µm wide	Width of the tube	Average of earlier studies
Arabidopsis root surface cell body	146.2 ± 36.1 µm long	The cell the hair grows from	Average of earlier studies

Sources: OpenStax Anatomy and Physiology 2e (2022) for red blood and muscle cells; Alberts and colleagues, Molecular Biology of the Cell (2002) for the axon; Pors and colleagues (2022) for egg cells; Kamieniczna and colleagues (2022) for sperm; Choi and Cho, Scientific Reports (2019), Table 1, for Arabidopsis. The rows come from different studies with different methods, not one survey. One µm (micrometre) is a thousandth of a millimetre.

1. Convert the young root hair length to micrometres. How many times longer is the hair than it is wide?
2. About how many times wider is the egg cell than a red blood cell?
3. Can you add the sperm head and tail to get the whole cell’s length? Explain using what the table says was measured.

Design problem

Do radish seedlings grown on drier paper towels grow longer root hairs than seedlings grown on wetter towels? Plan an investigation using a hand lens or a microscope with a scale. Name your variables, constants, control, sample size and safety steps.

One way to do it

• Independent variable: how wet the paper towel is. Add a set volume of water per towel, such as 5 mL for drier and 15 mL for wetter.
• Dependent variable: length of root hairs, measured on the same part of each root with a scale in view.
• Constants: seed batch, number of seeds per dish, temperature, light, towel type and the day you measure.
• Control: the dishes given the usual amount of water used for sprouting seeds.
• Sample size and trials: several dishes per group, ten or more seedlings per dish, and at least ten hairs per seedling. Report the mean and the spread.
• Fairness: a partner codes the dishes so the person measuring does not know the group.
• Limit: this tests one plant species at one age.
• Safety: handle slides and cover slips with care, wash hands after handling seeds, and do not eat them.

Evaluate: check your thinking

A Paramecium survives alone because its one cell holds every structure it needs: cilia to swim, food vacuoles to digest, a contractile vacuole to pump out water and the ability to divide. Your body works differently. It shares those jobs among specialized cells whose shapes fit their work, nearly all carrying the same DNA but using different genes.

Science practices you used: using a model and naming its limits, reading a data table with means and spread, converting units and comparing sizes, and planning a fair test with enough repeated measurements.