If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Introduction to cells review

Key terms

CellThe smallest living unit of life
Cell theoryThe explanation of the relationship between cells and all living organisms
MicroscopeInstrument used to magnify objects too small to be seen with the naked eye
Simple light microscopeMicroscopy tool that uses visible light and one lens to magnify an object
Compound light microscopeMicroscopy tool that uses visible light and multiple lens to magnify an object
Electron microscopeMicroscopy tool that uses a beam of electrons to create a magnified image

Cell discovery and cell theory

Early 1600sRobert Hooke discovers dead cells using early microscope.
Late 1600sAnton von Leeuwenhoek develops a more powerful microscope that allows him to see living cells like bacteria.
Early 1800sMatthias Schleiden and Theodor Schwann conclude that all living organisms are made of cells, and that cells can be produced from other cells.
Mid 1800sRudolf Virchow confirms that all cells must come from pre-existing cells. (There is some evidence that this idea was stolen from Polish scientist Robert Remak.)
These events gave way to the modern cell theory, which states:
  1. All living things are composed of one or more cells.
  2. The cell is the basic unit of life.
  3. New cells arise from pre-existing cells.

Viewing cells

In order to view cells, scientists must use magnifying tools called microscopes.
Simple light microscopes, such as magnifying glasses, generally are not powerful enough to view cells. Therefore, scientists need to use compound light microscopes or electron microscopes to see detailed cell structures or very small objects, such as viruses.
Compound microscope
Compound microscope. Image from Public domain pictures, Public domain,

Comparing light microscopes and electron microscopes

Light microscopeElectron microscope
Uses visible lightUses beam of electrons
Lower resolution and magnificationHigher resolution and magnification
Cells can be alive or deadCells must be dead
Inexpensive, relatively smallExpensive, very large

Common mistakes and misconceptions

  • Not all cells are the same. Although cells are the basic units of life, there are many different kinds of cells that make up multicellular organisms. Some cells have specialized jobs that allow them to work with one another to perform an organism’s biological functions. Not all cells are the same shape or size either. For example, sperm cells are much smaller than, say, a muscle cell.
  • Cells are the smallest living thing, but they are not the smallest thing. Cells are small enough that we need microscopes to view them, but they are much larger than some other substances that we have learned about. In fact, cells are made out of many atoms, so they are larger than macromolecules and viruses!
  • Not all microscopes are just magnifying glasses! Magnifying glasses do qualify as simple microscopes since they have only one lens. However, there are also more complex microscopes. Microscopes with multiple lenses are known as compound microscopes, and they are able to bend light to produce a much more magnified image than a simple magnifying glass.

Want to join the conversation?

  • aqualine ultimate style avatar for user Maryam Nayeemuddin
    What happens when we use live cells in an electron microscope ?
    (23 votes)
    Default Khan Academy avatar avatar for user
  • starky tree style avatar for user chocomaniak!
    They say that cells are the smallest living unit. However, they also say that viruses are smaller. Aren't viruses considered living units as well?
    (17 votes)
    Default Khan Academy avatar avatar for user
    • spunky sam blue style avatar for user B.K.
      It depends who you ask. Generally the answer is that viruses are not living units, for two reasons: The first reason is just that we've decided a cell is the basic unit of life, and viruses don't intrinsically contain cells, therefore they are not life. The second reason is that viruses are more like parasites, and are dependent on cells to "reproduce", so they aren't really living on their own.
      (21 votes)
  • old spice man blue style avatar for user MAHIR
    why does the cell have to be dead in order for the electron microscope to work?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • winston default style avatar for user P®[]G®@/\/\/\/\|/\/G /\/\@$†∑®
    According to cell theory all cells must come from preexisting cells, but then how did the first cell come about, and more importantly, why can't that happen again?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user Tybalt
      We actually do not know this answer for sure. Some theories suggest that the primordial Earth was the perfect place for chemical reactions to occur, with molecules eventually coming together to form the first cellular mechanisms. One theory suggests that amino acid (protein building blocks) chains were able to survive in the hellish conditions on the early Earth, and that their composition and characteristics eventually gave way to cells. One other theory suggests that cells evolved from viruses. There are several guesses in the community, and without our very own primordial Earth or time machine, what lead to the first cells is unknown.

      On Earth, if a primordial cell were to develop again, it would likely die, due to the millions of other, fully developed cells that exist (competition and natural selection). However, should there be life on other planets, then it could be safely said that the process did happen again.

      Does this help?
      (16 votes)
  • blobby green style avatar for user minenhle.zuke
    how many cells does a human have
    (8 votes)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user BigCanada2020
    Is there something called a micrometer ruler?
    (10 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user txonsalvador
    ¿En que año hicieron el primer microscopio ?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user asenger2
      El primer microscopio se inventó en 1590, pero no está claro quién lo inventó realmente. La primera vez que se utilizó un microscopio para observar células muertas fue por Robert Hooke a principios del siglo XVII, si eso ayuda. ¡Espero que esto ayude!
      (7 votes)
  • boggle yellow style avatar for user Sunny
    can someone explain cells with detail and give more examples? thanks!
    (4 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user 💜PURPLE4VICTORY💜
      As mentioned in Cell Theory, all living things are composed of one or more cells, cells are the basic unit of life, and that new cells come from preexisting cells.

      There are many kinds of cells, but generally, they can be classified in two broad groups: prokaryotic and eukaryotic. Prokaryotic cells tend to be simpler, with no nucleus or membrane-bound organelles. Some examples of prokaryotic cells include bacteria and archaea.

      Eukaryotic cells, on the other hand, are more complex, with a nucleus and other membrane-bound organelles like mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, etc. Eukaryotic cells can even be classified down to specific cells like animal, plant, fungi, and protists.

      Even though I mentioned some of the main differences, keep in mind that there are organisms that are anomalies and may not strictly fit into one kind of classification.

      This is a brief summary of what cells are, and I provided some examples of prokaryotic and eukaryotic organelles and organisms. If you are interested in learning more, I encourage you to do some more research yourself, or you can watch some videos here on Khan Academy.

      I hope I answered your question as thoroughly as possible. Have an amazing day. :)
      (17 votes)
  • blobby green style avatar for user victoria omotolani
    In the introduction to cells review, three cell theories were given but in my biology textbook four cell theories were given which the last one is ( A cell contains information for its structural and functional development in its nucleic acid.This information is passed down from parent to offspring cells). so my question is how true is the fourth cell theory, should it be part of the cell theory or not?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user LandonB
    what would i use to examine the entercica cell
    (8 votes)
    Default Khan Academy avatar avatar for user