(5) BACTERIA (another example of prokaryotes, prokaryotic cells, prokaryota)

Bacteria (prokaryotic organisms) are microscopic single-celled organisms with a cell wall, cell membrane, cytoplasm and plasmids; but they lack a nucleus, instead have a circular chromosome of DNA. Certain bacteria can carry out photosynthesis but most feed off other living or dead organisms.

Examples of bacteria include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yoghurt from milk, and pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia in the lungs.

The diagram shows the principal subcellular structures of an bacterial cell.

Bacterial cells, single-celled microorganisms (unicellular), are much smaller than plant or animal cells with some quite distinct and different sub-cellular features.

A prokaryotic bacterial cell consists of cytoplasm within a membrane surrounded by a cell wall.

Bacteria do NOT have a real nucleus, chloroplasts or mitochondria.

 

Cell wall and inner membrane

The cell contents i.e. the cytoplasm, DNA etc. are all held together within the cell wall by the surface membrane which controls the passage of substances in and out of the cell.

The surrounding outer cell wall gives a bacterium extra structural support.

The cell wall can also be surrounded by a capsule.

Note, unlike plant cells, animal cells do not have a cell wall, but bacteria do have a cell wall.

On the cell surface membrane ATP is produced to help power cell chemistry.

 

Cytoplasm

The jelly like fluid in which most of the cells chemical reactions take place with the aid of enzyme catalysts.

Although they do not have mitochondria, bacterial cells can still respire aerobically in the cytoplasm.

 

Chromosomal DNA - the genetic material is not confined in a nucleus which doesn't exist in bacteria

The genes are not in a distinct true nucleus, the genetic material is a sort of jumbled cluster comprising of one long circular strand (loop) of DNA floating free in the cytoplasm sometimes accompanied by one or more small rings of DNA called plasmids.

As with any other cells the string of DNA controls the cell's activities and cell division for replication.

This single chromosome controls the cells functions and the cell division in replication.

The chromosomal DNA moves freely around in the cytoplasm and is not confined in a distinct nucleus as in plant and animal cells.

 

Plasmid DNA, not part of the chromosome

Plasmids are small hoops of extra DNA that are separate from the chromosomal DNA i.e. not part of the chromos.

Plasmids contain genes that help tolerance against drugs and this drug resistance can be passed from one bacteria to another - a problem in dealing with bacterial infectious diseases.

This is how the bacteria MSRA have evolved and become so dangerous because of their antibiotic resistance.

Not all prokaryote cells contain plasmids.

 

Shape and Flagella (flagella plural, flagellum singular)

Bacteria come in all sorts of shapes e.g. rods, spirals etc. and some have a tail!

The flagellum is a long thin tail, a hair-like structure that projects out of the body of the cell, and can rotate to move the bacteria along.

Some bacterial cells have more than one flagella (flagellum) protruding from the outer layers of the bacterium.

The 'tail' flagellum can be driven by a tiny biochemical electric motor with moving parts, mostly made of proteins!

A rotating flagellum is quite a remarkable piece of biochemical engineering - bioengineering!

The flagellum enables a bacterium away from harmful substances (e.g. toxins) and move towards beneficial materials like nutrients or oxygen.

 

Ribosomes

As with other cells, proteins are required, and ribosomes are the place of protein synthesis from decoding genetic material from chromosomes.

 

Other comments on prokaryotes like bacteria

Unlike eukaryotic cells, prokaryotic cells do not contain a defined nucleus nor do they contain mitochondria or chloroplasts.

 

See also cell division and bacteria

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Revision Notes: Structure of a Typical Bacterial Cell

Bacteria are unicellular organisms, meaning they consist of a single cell.

They belong to the domain Prokaryota and differ from eukaryotic cells in that they lack a true nucleus and membrane-bound organelles.

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Cell Structure and Function

Component	Function
Cell Wall	Provides structural support and protection. It is made of peptidoglycan in most bacteria, which helps maintain shape and prevent bursting due to osmotic pressure.
Inner Membrane (Plasma Membrane)	Regulates the movement of substances into and out of the cell through selective permeability. It is composed of a phospholipid bilayer and embedded proteins.
Cytoplasm	Jelly-like substance that fills the cell and contains enzymes, nutrients, and other essential molecules. It is where chemical reactions occur.
Chromosomal DNA	A single, circular DNA molecule that carries the genetic information required for cellular functions. It is found in the nucleoid region rather than a nucleus.
Plasmids	Small, circular DNA molecules separate from chromosomal DNA. They often carry genes for antibiotic resistance and can be transferred between bacteria through conjugation.
Flagella (Flagellum - singular)	Long, whip-like structures used for movement. Bacteria with flagella can swim toward nutrients or away from harmful substances (chemotaxis).
Ribosomes	Small structures responsible for protein synthesis. Unlike eukaryotic ribosomes, bacterial ribosomes are 70S ribosomes (made of 50S and 30S subunits).

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Key Differences Between Bacterial and Eukaryotic Cells

• No nucleus → Genetic material is found in the nucleoid region.

• No membrane-bound organelles → All cellular processes occur in the cytoplasm.

• Peptidoglycan cell wall → Eukaryotic plant and fungal cells have cellulose or chitin in their cell walls instead.

• May contain plasmids → These allow for genetic exchange and adaptation to changing environments.

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Summary of learning objectives and key words or phrases to do with bacteria and bacterial cells

Know the structure of a typical bacteria cell (bacterium), a unicellular organism, know the function of the organelles in bacterial cells, including the cell wall, inner membrane, ribosomes, cytoplasm, the chromosomal DNA, plasmids rings of DNA, and some may have a flagellum (flagella).

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