From Cells to Organisms

Imagine a drop of pond water under a microscope. A Paramecium, one cell shaped a bit like a slipper, swims fast across it. That one cell moves, eats, gets rid of extra water and wastes, and can split in two. Your body does the same kinds of jobs. But it shares them out among trillions of cells of many different kinds. Why does a body need so many cells, when one cell can do it all?

Explore: share the jobs

In the model, an organism has five life jobs: moving, getting food, gas exchange, response and reproduction. You choose how many cells it has: 1, 10, 100 or 1,000. Then you choose whether all the cells are alike or split into jobs, and you watch five bars show how well each job keeps up as the organism grows. A strip at the side zooms out from cell to tissue, organ, organ system and organism.

1. How many cells does the organism have?

2. Are the cells alike, or split into jobs?

Zoom strip: the levels this body reaches

Reset

Made up for this sketch, not measured: the bars are relative units. A bigger body of alike cells strains gas exchange and getting food the most, and reproduction the least. Five or more cells can be split into five job groups. Tissues, organs and organ systems appear at 10, 100 and 1,000 cells when cells are split into jobs. Real organisms do not follow cell-count cut-offs: the lesson lists Trichoplax (about 320 cells, a rough estimate) and Volvox carteri (about 2,000 cells) with no organs. No real organism is a ball of alike cells that fails: Volvox carteri has about 2,000 cells and only 2 kinds. Blocks are not to scale; at 1,000 cells one block stands for 10 cells.

Rule to keep: a bigger organism has more cells, and it tends to have more kinds of cells that share the work. This is a sketch of the logic, not a measurement of any real organism.

Try this.

1. Start with 1 cell. Predict whether all five bars keep up. Then test it.
2. Grow to 1,000 cells with all cells alike. Predict which jobs fall behind first in the model. Then test it.
3. Keep 1,000 cells and switch on jobs. Predict what happens to the bars. Then compare with step 2.

Explain: one cell does every job

A unicellular organism is a living thing made of just one cell. A multicellular organism is made of many cells. Bacteria and Paramecium are unicellular. You and an oak tree are multicellular. You met the idea that all living things are made of cells in that lesson.

A Paramecium has no tissues or organs. Its one cell still does every life job:

• It moves. Short, hair-like parts called cilia cover the cell and beat to push it along.
• It eats. Food, such as bacteria, is taken into small sacs called food vacuoles and digested.
• It removes extra water. Water keeps moving into the cell from the pond. A contractile vacuole is a sac that fills with this extra water and squeezes it out. That lesson shows why water moves in.
• It removes wastes. Bits of food it cannot digest leave through a small opening.
• It reproduces. One cell can divide to form two new cells.

So one cell is not simple. Its parts work as a system, as you saw in that lesson.

Volvox, a green alga, sits in between a Paramecium and you. It is a ball of many cells living together, but with only a few kinds of cells.

Explain: cells, tissues, organs, systems

In a multicellular organism, cells share the work. A specialized cell is a cell whose shape and parts suit it to one kind of job. Sharing the work this way is called division of labor. Cells are grouped into levels:

• A tissue is a group of similar cells doing one job, such as muscle tissue.
• An organ is made of different tissues working together, such as the heart.
• An organ system is a group of organs working together, such as the heart and blood vessels.

cell → tissue → organ → organ system → organism

Together, the levels help the whole organism grow, repair itself, reproduce, exchange gases, carry out metabolism and respond. The levels depend on each other. If heart tissue is damaged, the whole body can suffer.

A school play works a bit like this. One group builds sets, one runs lights, and one acts. The comparison breaks in one way: no cell chooses its job or plans the show.

Not every multicellular animal has all these levels. Trichoplax, a tiny ocean animal, has several kinds of cells but no organs.

Explain: a shape for every job

Specialized cells look very different. Their shapes and parts fit their jobs.

Cell	Shape and parts	How it fits the job
Muscle cell (in your arm)	Long and thin, with many mitochondria	Shortens to pull on bones and move the body
Nerve cell	Long branches	Carries signals over long distances, in some cases about a metre
Red blood cell	Small, flexible disc, dented on both sides; no nucleus when mature (in humans)	Carries oxygen and bends through very narrow vessels
Root hair cell (plant)	Long, thin tube growing into the soil	More surface touches the soil, where water comes in

Bigger organisms do not mostly get bigger cells. Most cells of one kind are about the same size in a mouse and an elephant. The elephant has far more of them.

Elaborate: more cells, more kinds of cells?

Scientists have counted or estimated the cells in many organisms. They have also sorted those cells into kinds.

Organism	Number of cells	Kinds of cells	How the numbers were found
A bacterium	1	not counted	One cell by definition
Paramecium	1	not counted	One cell by definition
Trichoplax adhaerens	about 320 (rough body-size estimate, not a count)	4	1997 table; later studies found more kinds
Volvox aureus	about 560 (estimate)	2	1997 table
C. elegans worm, adult hermaphrodite	959 body cell nuclei	24	Nuclei counted (1983); kinds from 1997 table
Volvox carteri	about 2,000 plus about 16 large cells	2	2018 study
Hydra	about 63,000 (estimate)	15	1997 table
Human, reference adult man	about 30 trillion	about 400	Cells: 2016 estimate; kinds: 2023 classification

Sources: Bell and Mooers, 1997, a table built from older studies, which estimated cell numbers from body size; Matt and Umen, 2018 (Volvox carteri, lab strain); Sulston and colleagues, 1983, and WormAtlas, 2009 (worm nuclei); Sender and colleagues, 2016, and Hatton and colleagues, 2023 (human). These are lab and reference values, not samples from Virginia, and the rows use different methods.

1. As you read down the table, the number of cells goes up. What happens to the number of kinds?
2. Compare the two Volvox rows. Does adding cells always add new kinds of cells?
3. How many times more kinds of cells does Hydra have than Volvox carteri?

Design problem

A Paramecium’s contractile vacuole pumps out extra water. Does adding a little salt to its water change how often the vacuole empties? Plan an investigation with a Paramecium culture from a science supply company.

One way to do it

• Question: Does the amount of salt in the water change how often the contractile vacuole empties?
• Independent variable: salt added, such as 0 g, 0.1 g and 0.2 g per 100 mL of spring water.
• Dependent variable: number of times the vacuole empties in 60 seconds.
• Constants: same culture, same temperature, same microscope, and the same wait (10 minutes) before counting.
• Control: spring water with no salt added.
• Repeated trials: count 5 paramecia in each kind of water, and find the mean.
• Safety: wash your hands, handle glass slides with care, never taste anything, and give the culture back to your teacher.
• What it means: if the counts differ, salt changed this one cell’s water job.

Evaluate: check your thinking

A Paramecium can live alone because its one cell has parts for every job: cilia to swim, food vacuoles to digest, and a contractile vacuole to pump out water. Your body shares the same kinds of jobs among trillions of cells. They are grouped into tissues, organs and organ systems, and each kind of cell has a shape that fits its job.

Science practices you used: using a model and naming its limits, reading a data table and comparing rows, and planning a fair test with a control and repeated trials.