Plant, Animal and Bacterial Cells

Biological cells are the basic units of life i.e. the smallest units that can be alive.

There are many types of biological cells.

Classification of Biological Cells:

Biological taxonomy is the identification and naming of species and the arrangement of them into biological classes (i.e. groups). A taxonomic rank is a level in a taxonomic hierarchy, of which 'kingdom' is one of the highest ranks - its exact position in the taxonomic hierarchy depends on the system used. The Five-Kingdom system of biological classification proposed by Robert Whittaker in 1969 is based mainly on differences in nutrition and groups life forms:

Life

Empire Prokaryota

Kingdom Monera (prokaryotes, i.e. bacteria & "blue-green algae")

includes Bacterial Cells

Empire Eukaryota

Kingdom Protista (single-celled eukaryotes)

 

Kingdom Plantae (plants)

includes Plant Cells

Kingdom Fungi (fungi)

 

Kingdom Animalia (animals incl. humans)

includes Animal Cells


Prokaryotic and Eukaryotic Cells


Some school biology courses e.g. UK A-Level (AS and A2) include the structures of:

The biggest difference between these is that unlike eukaryotic cells, prokaryotic cells do not have a cell nucleus or any membrane-bound organelles such as mitochondria. Instead, all of the contents of the cell is openly accessible within the same volume e.g. ribosomes are scattered throughout the cytoplasm (not attached to the surfaces of the endoplasmic reticulum as in animal cells).


This table compares key features in plant, animal and bacterial cells:

 

Cell Feature:

Plant Cells

Animal Cells

Bacteria Cells
(Prokaryotic cells)

1.

Cell Wall

Plant cell walls are made mainly of cellulose.

none

Prokaryotic cell walls are made of murein.

2.

Cell Nucleus

Controls the activity of the cell and contains the genetic material. Each cell nucleus is surrounded by a double nuclear membrane that is crossed by nuclear pores.

Controls cell activity and contains the genetic material. The largest cellular organelle in animal cells, surrounded by a double nuclear membrane crossed by nuclear pores. Most animal cells have one nucleus, but:

none

  • Some very large cells (e.g. some striated muscle fibres) have many nuclei. Such multinucleate cells are called coenocytes.
  • Some special cells (e.g. erythrocytes) have no nucleus. Cells that have no nucleus are called anucleated cells.

3.

Plasmids

none

none

A plasmid is a molecule of DNA that is separate from and can replicate independently of the chromosomal DNA. They are double-stranded, generally circular, and capable of replicating autonomously within a suitable host.

4.

Mitochondria (pl.)
the singular form is "mitochondrion"

Mitochondria provide the cells' energy requirements. In the same way as cell nuclei they have a double membrane. The outer membrane of a mitochondrion is smooth while the inner membrane forms folds called cristae to increase the inner membrane's surface area on which sugar combines with oxygen to produce ATP - the cell's primary energy source.

none

5.

Ribosomes

The ribosomes present in plant and animal cells are the larger (80 S) type of ribosomes but are the smallest and most numerous of the eukaryotic cell organelles. They are composed of protein and RNA and are the sites of protein synthesis.
Ribosomes can exist free in the cytoplasm but are usually found attached to rough endoplasmic reticulum (RER). The ribosomes in eukaryotic cells are made in the nucleolus - which is inside the cell's nucleus.

Ribosomes are present - but are very small. That is, the ribosomes in prokaryotic cells (i.e. bacterial cells) are the smaller (70 S) type.

6.

Chloroplasts

Chloroplasts are green (because they contain the pigment chlorophyll) and are members of a class of organelles called plastids. They capture light energy, store it in the molecules ATP and NADPH, then use it to produce organic molecules and free oxygen from carbon dioxide and water by the process of photosynthesis.

none

none

7.

Permanent vacuole

Contains water needed for turgor pressure (turgidity) - that pushes the plasma membrane, which is also called the cell membrane, against the cell wall. Plants need turgidity to maintain rigidity. Molecules and ions may also be stored in the permanent vacuole.

none

none

See also prokaryotic cell structure, animal cell structure, plant cell structure and an introduction to cell division.

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