Jugami Biology

High school biology · Field notebook no. 1

The Cell

Life's smallest working unit, and how we learned to see it.

Right now, roughly 30 trillion human cells are keeping you alive. Almost every one of them is too small to see without a microscope. Inside each one, proteins are being built and DNA is being copied while you read this.

Nobody had seen a cell until about 360 years ago. We start below with the parts of a cell. Later sections compare plant and animal cells and tell the story of how scientists worked it out.

Ink sketch of a brass microscope with its objective lens over a slide, beside a magnified animal cell drawn in a notebook circle fig. 1: a cell sketched from many microscope views.

What is a cell?

A cell is the smallest unit that can do everything we count as being alive, from taking in energy to making copies of itself. Some living things are a single cell. You are a crowd of trillions working together.

Biologists sum this up in three statements called cell theory.

Tenet 1

All living things are made of one or more cells.

Tenet 2

The cell is the basic unit of structure and function in living things.

Tenet 3

All cells come from pre-existing cells.

There are two big kinds of cells. Prokaryotic cells, like bacteria, have no nucleus; their DNA sits loose in the cytoplasm. Eukaryotic cells (plants, animals, fungi, protists) keep their DNA inside a nucleus and have organelles, small working parts, most wrapped in a membrane. Each organelle has its own job.

Most prokaryotic cells are about 0.1 to 5 micrometers across, and most eukaryotic cells about 10 to 100 micrometers. A micrometer (µm) is a thousandth of a millimeter. A period at the end of a sentence is a few hundred micrometers wide.

Explore a eukaryotic cell

Click or tap any part of the sketch, or press Tab to move between parts and Enter to select one. Switch between an animal cell and a plant cell to see what changes.

Interactive diagram of a eukaryotic cell. Each organelle is a button.

Pick a part

Select any organelle in the sketch to read its name and job here. The big brown circle, the nucleus, is a good place to start.

Specimen Hunt

Field notes on every part in the diagram

Cell membrane
A thin, flexible double layer of lipids with proteins in it. It surrounds every cell and controls what enters and leaves.
Cytoplasm
Everything inside the membrane except the nucleus. That means the jelly-like cytosol and the organelles floating in it. Many chemical reactions happen here.
Nucleus
The control center. It holds the cell's DNA inside a double membrane, the nuclear envelope, whose pores let messages (RNA) out.
Nucleolus
A dense spot inside the nucleus where ribosome parts are made.
Ribosomes
Tiny machines that build proteins by reading RNA instructions. Some float free; others stud the rough ER. Every cell, even a bacterium, has them.
Rough endoplasmic reticulum (rough ER)
A maze of folded membranes covered in ribosomes. It builds and folds proteins that will be shipped out or put in membranes.
Smooth endoplasmic reticulum (smooth ER)
ER without ribosomes. It makes lipids and stores calcium. In liver cells it also helps break down toxins.
Golgi apparatus
A stack of flattened sacs that finishes proteins and lipids and packs them into vesicles for delivery. Think of it as the cell's post office.
Mitochondrion (plural: mitochondria)
Where most of cellular respiration happens, releasing energy from food and storing it as ATP. Mitochondria have their own small loop of DNA. Plant cells have them too.
Lysosome
A sac of digestive enzymes that breaks down worn-out cell parts and anything the cell has swallowed. Typical of animal cells.
Centrioles
A pair of short tubes in the centrosome near the nucleus. In animal cells they help organize the fibers that pull chromosomes apart during cell division.
Vacuole
A storage sac, mostly for water and the things dissolved in it. Animal cells have small ones. Plant cells have one large central vacuole that pushes against the wall and keeps the plant firm.
Chloroplast
Plant (and algae) organelle where photosynthesis uses light energy to turn water and carbon dioxide into sugar. The green pigment chlorophyll sits in stacked membrane disks called thylakoids.
Cell wall
A stiff outer layer outside the membrane. In plants it is made mostly of cellulose, and it gives support and protection.

Plant cells vs. animal cells

Plants and animals are both eukaryotes, so their cells share most of the same equipment. Most of the differences make sense once you remember that a plant makes its own food and stays in one place.

StructureAnimal cellPlant cell
Cell membraneYesYes
Nucleus, ribosomes, ER, GolgiYesYes
MitochondriaYesYes (plants respire too)
Cell wallNoYes, made of cellulose
ChloroplastsNoYes, in green parts like leaves
VacuolesSmall and temporary, if anyOne large central vacuole
CentriolesYesNo (in most flowering plants and conifers)
LysosomesYesUsually none; the vacuole does this recycling
Typical shapeRounded or irregularBoxy, fixed by the wall

Common mix-up. "Plants have chloroplasts instead of mitochondria." Not so. Chloroplasts make sugar; mitochondria release the energy stored in it. Plant cells need both.

How do we know? Seeing the invisible

Cells are far too small for the naked eye, so nobody could study them until someone built a good enough lens. Almost every big step in cell biology followed a better microscope.

  1. 1665 Robert Hooke names the "cell." In Micrographia, published by the Royal Society of London, Hooke shaved a thin slice of cork and saw rows of tiny empty boxes. He was looking at the dead walls of plant cells, not living insides, but the name stuck.
  2. 1670s Antonie van Leeuwenhoek sees life swimming. A Dutch cloth merchant who ground his own tiny, powerful single lenses, he began writing to the Royal Society in 1673. In a letter of 7 September 1674 he described green spiral strands in lake water (the alga Spirogyra) and tiny moving creatures. In 1676 he reported "little animals" that we now recognize as bacteria and protists. Hooke later confirmed his observations.
  3. 1838 Matthias Schleiden says plants are made of cells. The German botanist argued that the cell is the basic unit of plant structure.
  4. 1839 Theodor Schwann adds animals. In Mikroskopische Untersuchungen, Schwann extended the idea to animal tissues. Together they get credit for cell theory, although both wrongly thought new cells could form out of formless material.
  5. 1855 Rudolf Virchow and omnis cellula e cellula. "Every cell comes from a cell." Virchow made the Latin phrase famous in 1855. Robert Remak had already published in 1852 that animal cells multiply by dividing, though Virchow didn't credit him at first. Together their work fixed Schleiden and Schwann's mistake and completed the three tenets.
  6. 1931–1933 The electron microscope. Ernst Ruska and Max Knoll built the first electron microscope in Berlin in 1931. By 1933 Ruska's instrument beat the best light microscopes. Because electrons have far shorter wavelengths than light, organelles like the ER and ribosomes would come into view over the next two decades. Ruska shared the 1986 Nobel Prize in Physics.
  7. Today Glowing proteins and frozen molecules. Green fluorescent protein (GFP), borrowed from a jellyfish, lets scientists tag a protein and watch it move in a living cell (Nobel Prize in Chemistry 2008). Super-resolved fluorescence microscopy beat the old light-microscope limit (Nobel 2014). Cryo-electron microscopy freezes molecules in place to map them in near-atomic detail (Nobel 2017).
"Next, in that these pores, or cells, were not very deep, but consisted of a great many little Boxes, separated out of one continued long pore, by certain Diaphragms…" Robert Hooke, Micrographia (London, 1665), Observation XVIII, "Of the Schematisme or Texture of Cork." Text from Project Gutenberg eBook #15491.

Why the lens matters. An ordinary light microscope cannot separate details closer than about 0.2 micrometers, because of the wavelength of light. A mitochondrion is visible as a smudge; its folded inner membrane is not. That is why so much of the cell's machinery stayed hidden until the 1940s and 1950s, when better ways to prepare cells let electron microscopes show it.

Quiz: Lab Practical

Talk about it

  1. Hooke saw only the empty walls of dead cork cells. Is it fair to say he "discovered" the cell? What would discovering it actually require?
  2. Schleiden and Schwann got the big idea right and the mechanism wrong. Can a theory be useful even when part of it is wrong? Think of another example.
  3. Viruses have genes and proteins but no cells, and they cannot reproduce on their own. Using the three tenets of cell theory, argue whether a virus is alive.
  4. Many breakthroughs here came from a better tool, not a better idea. Which matters more in science, and can you have one without the other?
  5. If you could design a new organelle for a human cell, what would it do, and what problem would it solve?

Sources

  1. Robert Hooke, Micrographia (1665), Observation XVIII. Project Gutenberg eBook #15491. https://www.gutenberg.org/ebooks/15491
  2. University of California Museum of Paleontology, "Robert Hooke (1635–1703)." https://ucmp.berkeley.edu/history/hooke.html
  3. University of California Museum of Paleontology, "Antony van Leeuwenhoek (1632–1723)." https://ucmp.berkeley.edu/history/leeuwenhoek.html
  4. Nick Lane, "The unseen world: reflections on Leeuwenhoek (1677) 'Concerning little animals'," Philosophical Transactions of the Royal Society B (2015). https://pmc.ncbi.nlm.nih.gov/articles/PMC4360124/
  5. Embryo Project Encyclopedia (Arizona State University), "Matthias Jacob Schleiden (1804–1881)." https://embryo.asu.edu/pages/matthias-jacob-schleiden-1804-1881
  6. Linda Hall Library, "Scientist of the Day: Matthias Schleiden." https://www.lindahall.org/about/news/scientist-of-the-day/matthias-schleiden/
  7. Theodor Schwann, Mikroskopische Untersuchungen (Berlin, 1839), Deutsches Textarchiv. https://www.deutschestextarchiv.de/book/show/schwann_mikroskopische_1839
  8. Paolo Mazzarello, "A unifying concept: the history of cell theory," Nature Cell Biology 1 (1999): E13–E15. https://doi.org/10.1038/8964
  9. Embryo Project Encyclopedia, "Rudolf Carl Virchow (1821–1902)." https://embryo.asu.edu/pages/rudolf-carl-virchow-1821-1902
  10. The Nobel Prize, "Ernst Ruska: Biographical" (Physics 1986). https://www.nobelprize.org/prizes/physics/1986/ruska/biographical/
  11. Keith R. Porter, Albert Claude and Ernest F. Fullam, "A study of tissue culture cells by electron microscopy," Journal of Experimental Medicine 81 (1945): 233–246 (first images of the endoplasmic reticulum). https://doi.org/10.1084/jem.81.3.233
  12. George E. Palade, "A small particulate component of the cytoplasm," Journal of Biophysical and Biochemical Cytology 1 (1955): 59–68 (ribosomes). https://doi.org/10.1083/jcb.1.1.59
  13. The Nobel Prize in Chemistry 2008 (green fluorescent protein). https://www.nobelprize.org/prizes/chemistry/2008/summary/
  14. The Nobel Prize in Chemistry 2014 (super-resolved fluorescence microscopy). https://www.nobelprize.org/prizes/chemistry/2014/summary/
  15. The Nobel Prize in Chemistry 2017 (cryo-electron microscopy). https://www.nobelprize.org/prizes/chemistry/2017/summary/
  16. OpenStax, Biology 2e, Chapter 4 "Cell Structure" (cell sizes, organelles, plant vs. animal cells). https://openstax.org/books/biology-2e/pages/4-introduction
  17. Ron Sender, Shai Fuchs and Ron Milo, "Revised Estimates for the Number of Human and Bacteria Cells in the Body," PLOS Biology (2016). https://doi.org/10.1371/journal.pbio.1002533

Every organelle is built from the elements in The Periodic Table, Explained.