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Life Science · Grades 6–8
Cells
Every living thing is built from them, and most are too small to see.
Your body is made of about 30 trillion human cells. That is the estimate from a 2016 study that added up cells organ by organ. A single bacterium is just one cell, and it is a complete living thing.
On this page you will meet the people who first saw cells, find out how small cells really are, and look inside plant and animal cells. Then you will sort clues in the Plant, Animal, or Either? game and take a six-question quiz.
What is a cell?
A cell is the smallest unit of life. It takes in materials, uses energy, gets rid of wastes, grows, and can make new cells. Some living things, such as bacteria and amoebas, are a single cell. Others, like you or an oak tree, are made of huge numbers of cells working together.
Scientists sum up what they know about cells in the cell theory. It has three parts:
- All living things are made of one or more cells.
- The cell is the basic unit of life.
- New cells come only from cells that already exist.
That third idea means no cell appears out of nothing. Every cell in your body came from a cell that divided, going all the way back to the single fertilized egg cell you started as.
Who discovered cells?
Nobody could see cells until people learned to grind glass lenses well enough to build microscopes. The cell theory took almost 200 years and many scientists to put together.
- 1665
Robert Hooke, an English scientist, publishes a book of microscope drawings called Micrographia. In a thin slice of cork he sees rows of tiny empty boxes and calls them "cells." The boxes probably reminded him of small rooms, like the cells monks lived in, though Hooke never explained the name. He was looking at the walls of dead plant cells.
- 1674
Antonie van Leeuwenhoek, a cloth merchant in Delft, in the Netherlands, builds his own single-lens microscopes that magnify more than 200 times. In a letter to the Royal Society in London, he describes tiny living things swimming in lake water. He calls them "animalcules," little animals.
- 1683
Leeuwenhoek scrapes the white plaque from between his teeth and finds it full of moving "animalcules." His drawings are among the first records of bacteria.
- 1838
Matthias Schleiden, a German botanist, concludes that all plants are made of cells.
- 1839
Theodor Schwann, a German scientist who studied animals, finds that animals are made of cells too. Together, their ideas become the first two parts of the cell theory.
- 1852
Robert Remak, a doctor and scientist working in Berlin, publishes evidence from frog embryos that new cells form when existing cells divide.
- 1855
Rudolf Virchow, a German doctor, argues that every cell comes from an earlier cell. He sums it up in Latin, often written omnis cellula e cellula: "every cell from a cell." That became the third part of the cell theory. Historians point out that Virchow used Remak's findings without giving him fair credit.
How small is a cell?
Cells are measured in micrometers (µm). One micrometer is one thousandth of a millimeter. With a few exceptions, single cells are too small to see without a microscope.
| Thing | Size across |
|---|---|
| The head of a pin | about 2 mm (2,000 µm) |
| A typical plant or animal cell | about 10 to 100 µm |
| A human red blood cell | about 8 µm |
| A bacterium | about 0.1 to 5 µm |
| The smallest detail a light microscope can show | about 0.2 µm |
Two kinds of microscopes
Light microscopes bend light through glass lenses. The microscopes in a school lab magnify up to about 400 times, and a special oil lens can reach about 1,000 times. Even so, two dots closer together than about 0.2 µm blur into one. Most cells are see-through, so scientists add colored stains to make the parts show up, though staining usually kills the cells.
Electron microscopes use a beam of electrons instead of light. They can magnify about 100,000 times, enough to see ribosomes and other tiny cell parts. The catch is that the way a sample is prepared kills it, so you cannot watch living cells with one. In 1931, Ernst Ruska and Max Knoll built the first electron microscope, and by 1933 Ruska's could see finer detail than any light microscope. Ruska shared the 1986 Nobel Prize in Physics for it.
Two kinds of cells
Scientists sort every cell on Earth into one of two kinds, based on one big question: does it have a nucleus, a compartment wrapped in its own membrane that holds the cell's DNA?
- Prokaryotic cells have no nucleus. Their DNA sits in an open area of the cell called the nucleoid. They have a cell membrane, cytoplasm, and ribosomes, and usually a cell wall. Bacteria and archaea are prokaryotes.
- Eukaryotic cells have a nucleus. The name means "true nucleus." They also have organelles, small parts wrapped in membranes that each do a job, such as mitochondria. Plants, animals, fungi, and protists are eukaryotes.
If you read Classifying Living Things, this split will look familiar. The domains Bacteria and Archaea are prokaryotes. The domain Eukarya is named for its eukaryotic cells.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No. DNA is in the nucleoid | Yes |
| Organelles with membranes | No | Yes |
| Cell membrane and ribosomes | Yes | Yes |
| Typical size | 0.1 to 5 µm | 10 to 100 µm |
| Examples | Bacteria, archaea | Plants, animals, fungi, protists |
Inside plant and animal cells
Plant and animal cells are both eukaryotic, so they share most of their parts. Think of a cell as a busy workshop: each part has a job. The drawings below are simplified and not to scale. Real cells hold many more of each part, and in three dimensions.
| Part | Job | Found in |
|---|---|---|
| Cell membrane | A thin, flexible layer around the cell that controls what goes in and out | Every cell |
| Cytoplasm | The jelly-like fluid and everything in it, between the membrane and the nucleus | Every cell |
| Nucleus | Holds the cell's DNA, the instructions for building and running the cell | Plant and animal cells |
| Ribosomes | Tiny builders that make proteins | Every cell, even bacteria |
| Endoplasmic reticulum (ER) | A folded network of membranes. Rough ER, dotted with ribosomes, works on proteins. Smooth ER makes fats and oils (lipids). | Plant and animal cells |
| Golgi apparatus | Sorts, labels, and packs proteins and lipids into tiny bubbles to ship them | Plant and animal cells |
| Mitochondria | Release energy from food, using oxygen | Plant and animal cells |
| Lysosomes | Sacs that break down food particles and worn-out cell parts | Mostly animal cells. In most plant cells, the vacuole does this job. |
| Vacuoles | Storage sacs. A plant's large central vacuole holds water and keeps the cell firm. | Plant cells have one large central vacuole. Animal cells may have small ones. |
| Chloroplasts | Use light energy to make sugar (photosynthesis) | Plant cells (and algae) |
| Cell wall | A stiff layer outside the membrane, made of cellulose in plants, that supports and protects the cell | Plant cells. Fungi and most bacteria have walls made of other materials. |
Plants need mitochondria too. Chloroplasts make sugar, but the plant still has to break that sugar down to use its energy. That is the mitochondria's job, in plants and animals alike.
Moving in and out of cells
A cell has to take in what it needs and get rid of wastes. The cell membrane is selectively permeable: it lets some substances pass through and blocks others.
Diffusion is the movement of particles from where they are crowded (high concentration) to where they are less crowded (low concentration). The cell doesn't have to spend energy on it. Think of the smell of popcorn spreading through a house. Oxygen and carbon dioxide cross cell membranes this way.
Osmosis is the diffusion of water across a selectively permeable membrane. Water moves toward the side where there is less water and more dissolved stuff, such as salt or sugar.
- A plant that hasn't been watered wilts. Water leaves the central vacuoles, they shrink, and the cells stop pushing against their walls. Water the plant, and the vacuoles fill up again.
- A red blood cell placed in pure water takes in water, swells, and can burst. In very salty water it loses water and shrivels.
From cells to organisms
In a living thing made of many cells, the cells don't work alone. Similar cells form tissues, tissues form organs, and organs work together in organ systems. Here is one path, from a single heart muscle cell to a whole person:
- Cell: a heart muscle cell, which can squeeze.
- Tissue: heart muscle tissue, many of those cells squeezing together.
- Organ: the heart, built from muscle tissue and other tissues.
- Organ system: the circulatory system, the heart plus blood vessels and blood.
- Organism: you, with all your organ systems working together.
Plants follow the same plan. A leaf cell that does photosynthesis is part of leaf tissue, the tissue forms a leaf (an organ), leaves and stems make up the plant's shoot system, and the shoot system and root system together make the whole plant. The same five levels build you, a mountain gorilla, and a sunflower.
How do we know?
Game: Plant, Animal, or Either?
Each clue describes a cell or something found in one. Decide whether it points to a plant cell, an animal cell, or could be either. A round has ten clues picked at random from sixteen.
How scoring works: each right answer is worth 1 point, so a perfect round is 10. Your best round is saved in this browser only.
This mini-game needs JavaScript turned on. You can still practice with the clues and answers below.
Use Tab to move between buttons and Enter or Space to choose. Read the clue, then pick the answer that fits.
Clue 1 of 10Score: 0 / 10Best: none yet
Clue
Quick quiz
Six questions. Choose one answer for each, then press Check my answers.
Talk about it
- Hooke saw cells in 1665, but nobody said that all living things are made of cells until the late 1830s. Why do you think that took so long?
- Virchow got most of the credit for "every cell from a cell," but Remak published the evidence first. How should scientists share credit for an idea?
- A friend says, "Plants don't need mitochondria, because they have chloroplasts." How would you explain why that is wrong?
- If you could shrink down and spend a day inside one organelle, which would you pick, and what would you expect to see?
- Is a virus a cell? What would you need to know about viruses to decide?
Sources
- OpenStax, Biology 2e, 4.1 Studying Cells (microscopes, cell theory, red blood cell and pinhead sizes).
- OpenStax, Biology 2e, 4.2 Prokaryotic Cells (nucleoid, cell sizes).
- OpenStax, Biology 2e, 4.3 Eukaryotic Cells (organelles, lysosomes and vacuoles, plant cell walls, chloroplasts, wilting, endosymbiosis).
- OpenStax, Biology 2e, 4.4 The Endomembrane System and Proteins (ER and Golgi).
- OpenStax, Biology 2e, 5.2 Passive Transport (diffusion, osmosis, red blood cells in water and salt water).
- OpenStax, Biology 2e, 1.2 Themes and Concepts of Biology (levels of organization) and 30.1 The Plant Body (shoot and root systems).
- Encyclopaedia Britannica, Cell theory.
- Hooke, R. (1665), Micrographia, Project Gutenberg edition.
- University of California Museum of Paleontology, Antony van Leeuwenhoek (1632–1723).
- Wright, N. A., and R. Poulsom (2012), Omnis cellula e cellula revisited: the presence of the stem cell, Journal of Pathology 226: 145–147 (Virchow and Remak).
- Sender, R., S. Fuchs, and R. Milo (2016), Revised estimates for the number of human and bacteria cells in the body, PLOS Biology 14(8): e1002533.
- The Nobel Prize, Ernst Ruska, facts (Nobel Prize in Physics 1986), and Ernst Ruska, biographical (the 1931 microscope with Max Knoll, and the 1933 microscope that beat the light microscope).
- Sagan, L. (1967), On the origin of mitosing cells, Journal of Theoretical Biology 14: 255–274.
Credits
Written by Dian, Jugami's life science teacher. All illustrations (the microscope and cork view, the prokaryotic and eukaryotic cells, the labeled plant and animal cells, and the levels-of-organization icons) are original hand-coded SVG made for Jugami. The share image was drawn in code. The cell drawings are simplified and not to scale.