1.4. Three Types of Cells

Learning check

Once you have watched the video, check your learning with this quiz.

There are three main types of cells (on which most biologists agree). They constitute the three domains of life:

1. Bacteria
2. Eukaryotes
3. Archaea

1. Bacteria

Colonies of bacterial E. coli cells on an agar plate. Each colony consists of millions of bacteria.

Gram-stained E. coli cells seen through a light microscope.

Scanning electron microscope image of a cluster of E. coli cells.

A typical bacterial cell.

A typical bacterial cell contains:

• Cell membrane with proteins that regulate what goes in to/out from the cell
• Cytoplasm (intracellular fluid
• DNA in a single circular chromosome
• Ribosomes that synthesize proteins

Many bacteria also have a cell wall, though not all of them. There are also many more things that a bacterium may contain…

2. Eukaryotes

There are three different kinds of eukaryotic organisms:

1. Animal cells, e.g. in humans
2. Plant cells, e.g. in trees
3. Fungal cells, e.g. in mushrooms (in many ways similar to animal cells, and not covered in detail in this course)

a. Animal cells

Epithelial cells stained with methylene blue.

A typical eukaryotic cell (with a bacterium for size comparison).

A eukaryotic animal cell contains:

• Cell membrane
• Cytoplasm
• Nucleus surrounded by a nuclear envelope, containing the chromatin (DNA in several linear chromosomes).
• Endoplasmic reticulum (ER)
• Mitochondria (the cell's "power plant", handles cellular respiration)
• … and much more!

b. Plant cells

Plant cells are eukaryotic cells with some "extras".

A typical plant cell.

A typical plant cell also contains:

• Cell wall made of cellulose.
• Chloroplast for photosynthesis.
• Vacuole to expand the cell.

The endosymbiotic theory

How eukaryotic cells first formed.

Eukaryotic cells formed when a "prekaryotic" cell engulfed, but did not degrade, a bacterium that could take advantage of oxygen.

• The bacterium (probably closest related to Rickettsia bacteria) eventually evolved into a mitochondrion.

How plant cells formed.

In a similar way, plant cells formed when a eukaryotic cell engulfed, but did not degrade a bacterium capable of photosynthesis.

• The bacterium (probably closest related to Cyanobacteria) eventually evolved into a chloroplast.

3. Archaea

Similar to bacteria:

• Same approximate size.
• No nucleus.

Found in extreme environments:

• Salty, hot and acidic.

”Prekaryote”?

• The cell which engulfed a bacterium and turned into a eukaryotic cell most likely was closest related to present-day Lokiarchaeota.

Some archaea (Halobacteria).

Some concepts to remember forget

Archebacteria

• Since the archaea are not bacteria, only the term "archaea" sholuld be used.

Eubacteria

• Since the archaea are not bacteria, there is no need to differ the "true" bacteria from archaea. Thus, only the term "bacteria" should be used.

Prokaryote (a group consisting of both bacteria + archaea)

• Since eukaryotes and archaea are more closely related than bacteria and archaea, the term "prokaryote" is misleading.
• Many biologists use the term "prokaryotes" when they actually mean "bacteria"!
• The term "prokaryote" should be avoided.