An In-depth Look at Bacteria Cells for GCSE Biology

The topic of bacteria cells, including their structure and function, is covered in a range of different parts of the GCSE biology course. To ace your exams you will need to make sure your knowledge of this topic is really strong. 

There are a broad range of different types of bacteria, from bacteria that live in hot conditions in hydrothermal vents under the ocean, to bacteria that live in our intestines and help our digestion, to pathogenic (disease-causing) bacteria that cause illness such as E. coli. 

In this article I will review the structure of bacteria cells, their method of reproduction, and how they can evolve to be resistant to antibiotics. 

What are Bacteria Cells?

All living organisms can be split into two categories: eukaryotes and prokaryotes. Eukaryotes include animals and plants, and all bacteria are prokaryotes. The main difference between the two types of cells is that prokaryotic cells do not possess membrane-bound organelles. These include:

• A nucleus

• Mitochondria

• Chloroplasts

• A vacuole 

Bacterial cells are much smaller than eukaryotic cells, with prokaryotic cells typically being around 0.1–5.0μm in size and eukaryotic cells being 10–100μm – although they vary depending on their cell specialism.

Bacteria Cell Components

A typical bacteria cell has the following components:

• Cell wall

• Cell membrane

• Cytoplasm

• DNA in the form of a large circular piece of chromosomal DNA, as well as multiple smaller loops of DNA called plasmids

• Ribosomes (although the ribosomes have a different structure from those found in eukaryotic cells)

Some bacteria cells can also have additional structures such as flagella, although this varies depending on the species of bacteria.

Cell wall and cell membrane

The cell wall of a bacterial cell plays an important structural role by supporting the bacterial cell and holding the shape of the cell. It also helps to stop the cell from bursting due to the osmotic pressure of water inside the cell. The cell wall of bacterial cells is made of a different material from plant cell walls, which are made of cellulose. Bacterial cell walls are made from a substance called peptidoglycan.

The cell membrane of the cell controls the movement of substances in and out of the cell. These substances include nutrition for the cell such as glucose, as well as water and oxygen. Waste products, such as carbon dioxide, also diffuse out of the cell through the cell membrane.

Cytoplasm and genetic material

The cytoplasm of the cell is the jelly-like substance that fills the majority of the cell. It acts as the location of the majority of the chemical reactions in the cell, including respiration. It contains a lot of dissolved solutes that move through the cytoplasm via diffusion.

The genetic material of the bacterial cell comes in two forms. Both forms are circular DNA. This is different from eukaryotic cells, which possess linear chromosomes. 

• The main chromosomal DNA is a large loop that contains most of the genetic material for the cell and holds most of the instructions for the proteins required for cell functioning

• Plasmids are much smaller loops of DNA. Plasmids can be transferred between bacteria. This is problematic to humans because it helps to spread Antibiotic Resistance within populations

Bacteria Cell Reproduction

Bacteria reproduce by a process called binary fission. Binary fission is a form of asexual reproduction where one cell divides into two genetically identical offspring cells. This process is not to be confused with mitosis, which is a similar type of cell division that takes place in eukaryotic cells. They are classified as different processes due to significant differences in the cell and chromosomal structures. 

In preparation for the process of binary fission, the DNA replicates so there are two copies for the two new offspring cells. The number of organelles, such as ribosomes, also increases and the cell grows in size. The cell splits into two and the new cells are genetically identical to the parent cell.

Bacteria reproduce very quickly. It can take around 4–20 minutes, although this varies depending on the species of bacteria and the conditions. In the correct conditions bacteria are capable of reproducing exponentially. This means that their population size doubles very quickly, again and again, resulting in a small bacterial population rapidly expanding to a very large one. 

This can be problematic for humans when these bacteria are pathogenic (disease-causing) and are found in hospitals, food, or inside the human body. 

However, this mechanism is also extremely useful to humans because small populations of bacteria can be cultured and grown in fermenters for practical use in industry and medicine. A single genetically modified bacterium can asexually reproduce to create a large population.

The Evolution of Antibiotic Resistance in Bacteria

Bacteria are capable of evolving over time. One of the traits that humans are particularly interested in is the ability of pathogenic (disease-causing) bacteria to be resistant to (not killed by) antibiotics. 

Bacteria can become resistant to antibiotics through a process called natural selection. 

This process is as follows:

1. Random mutations in the DNA of bacterial cells occur in a population over time

2. Some of these mutations may, by chance, cause resistance to an antibiotic

3. The population is exposed to an antibiotic 

4. The non-resistant bacteria are more likely to die because they are killed by the antibiotic

5. The resistant bacteria survive and now have fewer other bacteria to compete with for space and resources like nutrition

6. The resistant bacteria reproduce and increase in number in the population

7. The next time the patient takes the same antibiotic it does not work 

It is a common misconception that bacteria mutate to produce resistance DNA because of the presence of antibiotics. This is incorrect. The mutations happen randomly and cause resistance by chance.

Producing Genetically Modified Bacteria for Use in Industry

(Please note that the content covered in this section is covered in the triple-science-only sections of some courses, and other exam boards cover this topic in different levels of detail so please check your course specifications or our Save My Exams revision notes for your course to make sure you only learn the relevant information you need to ace your exams.)

Bacteria are very useful organisms to genetically modify because they are able to take plasmids into their cells and express the DNA from the plasmid by creating new proteins. They then excrete the excess proteins for easy extraction. They also reproduce quickly, are easy to store in large numbers, and are easy to maintain over time. 

The process of genetic modification (also known as genetic engineering) involves adding DNA from one organism, such as a human, into another organism, such as a bacterium. The target gene, such as the gene for human insulin, is cut out of the double stranded human DNA using enzymes called restriction enzymes. This exposes unpaired bases called “sticky ends” that allow the DNA fragment to join to other pieces of DNA.

A bacterial plasmid is cut using the same restriction enzyme to expose the complementary sticky ends. This allows the human DNA bases to form base-pairs with the bacterial DNA bases. The DNA pieces are then stuck together using another type of enzyme called DNA ligase. The new plasmid is called a recombinant plasmid because it contains DNA from more than one species. 

The recombinant plasmid can then be taken into a bacterial cell. The plasmid acts as a vector by transporting the DNA into the cell. The genetically modified (GM) bacterium then reads the DNA and synthesises the new protein, for example human insulin.

The modified bacteria are then placed into machines called fermenters. These fermenters provide optimum conditions for the bacteria to asexually reproduce and to produce insulin. They have carefully regulated temperature and pH, there is oxygen and glucose provided for respiration, and other nutrients are added for growth. There is an outlet that allows the product to be extracted and purified for use in medicine.

Human insulin is one example of a product made by this process, but it can also be used for many other types of medicines that require proteins, or even for making chemicals for industry like cleaning products or foods.

Study Bacteria Cells for Biology GCSE Exam Success

Bacterial cells are prokaryotic cells with no membrane-bound organelles such as a nucleus. They instead possess a cytoplasm, cell membrane, cell wall, ribosomes and two types of circular genetic material. 

Bacteria reproduce through a type of asexual reproduction called binary fission where one parent cell splits into two identical offspring cells. 

Random mutations can occur over time that cause bacteria to become resistant to antibiotics. These mutations are likely to spread in populations exposed to antibiotics, resulting in natural selection of the bacterial population towards increased resistance to multiple types of antibiotic. 

Bacteria can be genetically modified to contain DNA from other species in order to produce proteins for use in medicine and industry. This is done by inserting new DNA into a plasmid by cutting it using restriction enzymes and sticking in the new DNA using DNA ligase, creating a recombinant plasmid that can be taken into a bacterial cell to become modified. 

To find out more information about bacteria to help you ace your GCSE biology exams, check out our awesome revision notes by following these links:

• Bacterial Cell Structure

• Binary Fission

• Evolution of Antibiotic Resistance

• Genetic Modification