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Leaving Cert Biology Revision: Bacteria, Viruses & Fungi

Bacteria, Viruses & Fungi  

Bacteria: Bacteria are tiny single-celled organisms that can be found almost everywhere on Earth.  

While some bacteria can cause diseases, many are harmless and even helpful to humans.  

For example, bacteria living in our intestines help us digest food. Bacteria come in different shapes, including spheres (called cocci), rods (called bacilli), and spirals.  

They can reproduce quickly by dividing into two cells, making them very adaptable to changing environments. 

Viruses: Viruses are even smaller than bacteria and are not considered living organisms because they cannot survive on their own.  

Instead, they need to infect a host cell to replicate. Viruses can cause various diseases in humans, animals, and plants, such as the common cold, flu, and COVID-19. They consist of genetic material (either DNA or RNA) surrounded by a protein coat.  

Vaccines are used to prevent viral infections by training the immune system to recognize and fight specific viruses. 

Non-living features of Viruses

Unlike living things, viruses are not composed of cells. 

They lack components found in live cells, such as ribosomes, chloroplasts, and mitochondria. 

RNA or DNA can be found in viruses, but not both. 

They need to borrow the machinery from a living cell to replicate themselves. 

Viral Replication  

The process by which viruses duplicate themselves within live cells is known as viral replication. 

The virus affixes itself to a host cell's surface during the initial attachment stage. The virus releases its genetic material when it penetrates the host cell, which is known as penetration. 

*During the replication stage, the genetic material of the virus commands the host cell's machinery to generate new virus components. 

Stages of Viral replication 

The steps of viral replication are as follows, with a quick and easy explanation: 

  1.  The virus attaches itself to the host cell. 

  2. Penetration: The virus enters the cell that hosts it. 

  3. The virus releases its genetic material upon uncrating. 

  4. Replication and transcription: The virus commands the machinery of the cell to multiply itself. 

  5. Assembly: creating new viruses. 

  6. Release: New viruses spread to additional cells by escaping their host cell. 

The Value of Viruses in Medicine and the Economy: 

Though frequently for bad reasons, viruses are significant in both medical and economic situations. 

The Significance of Medicine  

Human Illness: A variety of illnesses, including the flu, the common cold, and COVID-19, can be brought on by viruses. These ailments, which can be minor to severe, influence people's health and well-being all over the world.


Relevance to the Economy 

Plant Disease: Viruses can also infect plants, causing illnesses that can damage crops.  

The tobacco mosaic virus, for instance, damages tobacco plants, lowering crop yields and quality.  

Similar effects on tomatoes are caused by the tomato spotted wilt virus, which causes large financial losses in the agricultural sector. 

Animal Disease: Viruses can also infect animals, leading to illnesses such as avian flu in birds, foot-and-mouth disease in cattle, and rabies in dogs and cats. 

These diseases can impair an animal's health and occasionally spread to humans, impacting livelihoods and food sources. 

Fungi: Fungi are a diverse group of organisms that include mushrooms, molds, and yeasts. 

Fungi play essential roles in ecosystems by decomposing organic matter and recycling nutrients. However, some fungi can cause infections in humans, such as athlete's foot and ringworm.  

Fungi reproduce by producing spores, which can be spread by wind, water, or animals. 


Kingdom Monera: Kingdom Monera is made up of tiny living things called bacteria. 

These bacteria are very small and don't have a nucleus or other parts inside them like we do. They're everywhere, from dirt and water to inside our bodies.  

Some bacteria are good for us, helping with things like digestion, while others can make us sick. They also help keep nature in balance by recycling nutrients in the environment. 


Generalized Structure of a Bacterial Cell

Amoeba: Amoeba is a tiny living thing that belongs to the group called Protists. 

These little guys have a nucleus and other parts inside them. Amoebas are special because they can change their shape.  

Feeding in Amoeba

Freshwater environments are home to single-celled organisms called amoeba. It feeds on food by a procedure known as phagocytosis.

They have little "feet" that they use to move and grab food. You can find amoebas in places with water, like ponds and lakes. They eat tiny things like bacteria and other small creatures.  

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