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Eukaryotes and prokaryotes

It is unfortunate to start this introduction to Biology with words that are derived from Greek.

There are two main categories of living organisms, based on the structure of their cells .

It all depends on the genetic material

Genetic material controls activities that go on in the cell, and it must be passed on every time cells divide.

Eukaryotes (plants, animals and fungi) can usually be seen to have a (single) nucleus inside every cell. This contains their genetic material, which is mostly DNA, packaged with proteins into chromosomes.

Prokaryotes (bacteria and a few similar groups of organisms) have a slightly simpler loop (or loops) of genetic material, not enclosed in a nucleus. Their genetic material is also mostly DNA.

Along with this distinction go a few more differences inside the cells, mostly based on membranes.

This distinction is reflected in the classification of living organisms:
Plants, Animals and Fungi (and perhaps the single-celled Protista) are seen as separate Kingdoms, but included in the Domain Eukaryota.

Bacteria (including blue-green 'algae' - now called Cyanobacteria) and simpler organisms called Archaea are two groups (Kingdoms or Domains) which are Prokaryotic.

Not Eukaryotic, Not Prokaryotic, Not Cells

Viruses - which do not consist of cells - contain (smaller amounts of) DNA or RNA as their genetic material.

Eukaryotic cells - big on nuclei as well as organelles and membranes

These cells have a membrane surrounding their nucleus, as well as a cell or plasma membrane on the outside of the cell, enclosing the cytoplasm, which also includes other smaller structures (organelles) made of membranes. A eukaryotic cell is larger than a prokaryotic cell.

Other membranes within cells form cavities with different functions. These are generally only seen using the electron microscope.

Mitochondria are organelles with two layers of membranes (the inner one folded to give more area), and they may be seen within most animal and plant cells. Aerobic respiration takes place here.

Chloroplasts - organelles only found in some cells in plants - also have a differently folded inner membrane and they are found in some plant cells. Photosynthesis takes place here, so they contain the green pigment chlorophyll.

Plant cells typically also have a cell wall made of cellulose on the outside of the cell membrane, and some fungi have a slightly different cell wall.

Most eukaryotes are composed of large numbers of cells but there are also a few single-celled examples. Amoeba is often studied in Biology laboratories, and Plasmodium - the organism that causes malaria - is another example.

Human Liver Cells
under the light microscope
section (48K)
Each cell can be seen to contain a single nucleus, and is surrounded by a thin membrane.
Mitochondria cannot be seen at this magnification.

The cells seen above are about 25 µm in diameter.

Human Liver Cells
diagram showing features seen with the electron microscope
liverdrawing_labels (159K)
Mitochondria and internal membranes are visible.

Cells in a lilac leaf
under the light microscope
Lilacmesophyll_labels (112K) Nuclei have been stained red, but chloroplasts keep (some of) their green coloration.

Palisade cells which form columns within the top of the leaf are about 24 µm in length.

Prokaryotic cells - keeping it simple

Prokaryotes are single-celled organisms, noticeably smaller than eukaryotic cells. They cannot be seen without quite a good microscope, and this just shows their shape as 'dots' or 'dashes'.

Mixed bacterial cells seen with the light microscope cocci_and _bacilli (242K)
These bacteria have been stained to identify and classify them.

Details of the internal structure of prokaryotes can only be seen with an electron microscope.

Prokaryotic cells also have a membrane on the outside of their cytoplasm, but no membrane-bound structures inside the cell. Their genetic material simply forms a diffuse blob-like mass inside the cytoplasm, and there may also be one or more smaller rings of DNA called plasmids.

There is a cell wall (usually not made of cellulose) on the outside of the cell.

Bacteria-Cell-Structure-570x342 (157K)

This cell - a bacillus - is about 2 µm long.

In fact one of the plant cells in the lilac leaf shown on the left is about 12× as large as this bacterial cell in linear dimensions
- but its volume is 1728 (12 3) × as great.

Notes on words:

Eukaryote means "true nucleus"
The adjective eukaryotic comes from this

Prokaryote means "substitute nucleus"
The adjective prokaryotic comes from this

This topic has connections with other units on this site:
Cell biology
Animal cells
Plant cells
Yeast cells
Bacterial cells

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