Pathogens, Disease and Transmission - Pathogens Leading to Disease (GCSE Biology)

Pathogens Leading to Disease

In this section, we will learn about communicable diseases. We previously learnt about non-communicable diseases such as Coronary Heart Disease. Here, we will learn how diseases can be caused by viruses, bacteria, protists and fungi.

Re-Cap of Communicable Disease

Communicable Disease

• Communicable diseases spread from one organism to another. Some types of diseases can be passed on between people. These are called communicable diseases or transmissible diseases. They are caused by many different types of pathogen (disease-causing organisms), and are also known as contagious or infectious diseases.
• Communicable diseases can be transmitted by vectors. Vectors are often pathogens which can transmit disease from one organism to another. For example, malaria is a communicable disease which can be transmitted by mosquitos (the vector).

Non-communicable Disease

• Non-communicable diseases cannot be passed on to another organism. Some diseases cannot be transferred from one organism to another. These are non-communicable diseases. They are often chronic (long-term), and examples are cancer and diabetes.

Types of Pathogen

Introduction to Pathogens

• Communicable diseases are spread by pathogens. Communicable diseases can be spread from person to person or animal to person. They are spread by pathogens, which are microorganisms that cause infectious disease.
• Pathogens are microorganisms. There are many different types of pathogens which can cause different types of disease. Bacteria, viruses, protists and fungi can all be pathogens – for example, the human immunodeficiency virus (HIV) causes HIV and AIDS.

Bacteria

Bacteria are prokaryotes. A prokaryotic cell does not contain a nucleus.

• Bacterial cells are prokaryotic cells. Prokaryotes cells are smaller and more simplistic than eukaryotic cell, and do not contain a nucleus. Bacterial cells therefore do not have a nucleus, and are around 1% the size of normal body cells. Instead they have genetic material in a loop, and also have smaller rings of DNA called plasmids.
• Bacterial cells have similarities to plant and animal cells. Animal, plant and bacterial cells all have a cytoplasm and a cell membrane. In addition to the cell membrane, bacterial cells have a cell wall which surrounds the cell membrane.
• Bacteria spread disease using toxins. Bacteria spread diseases by producing toxins. These toxins damage human tissues and make us feel ill. Bacteria can cause disease by reproducing within the human body very quickly.

Viruses

• Viruses are not living cells. Viruses are not cells, nor are they living organisms. Viruses are basically biological structures which are made up of genetic material surrounded by a coat of proteins and lipids. They are smaller than bacteria and come in a variety of shapes and sizes.

• Viruses invade host cells. Viruses are constantly invading host cells, which can be in any living organism, inside which they reproduce making them parasitic. Before the host can detect the virus as foreign and respond with an immune response, the virus may have reproduced and spread for example, in the influenza virus that causes the flu.
• Viruses have no cell organelles. Viruses have no nucleus, no ribosomes, and none of the membrane bound organelles. Viruses don’t have a normal nucleus, but they do carry genetic material (both DNA and RNA).
• The lifecycle of a virus begins with it infecting a host cell. They can then go down the lytic or lysogenic pathway:
  • Lytic pathway:
    1. Virus attaches to host cell and injects its genetic material
    2. Using the host’s proteins, the virus replicates its genetic material to build new viral components which are assembled to form new identical viruses
    3. The host cell bursts and dies whilst releasing the new viral cells which go on to infect other host cells

• Lysogenic pathway:
  1. Virus attaches to host cell and injects its genetic material.
  2. The genetic material combines with the host’s DNA.
  3. When the host cell divides, the viral genetic material is replicated alongside the host’s DNA.
  4. The virus is currently dormant as no new viral cells are made.
  5. When triggered e.g. by a chemical, the viral genetic material leaves the host’s DNA and enters the lytic pathway.

Fungi

• Fungal cells are eukaryotes. Fungi are neither plants nor animals and are an entire class of organisms on their own, but they are eukaryotic organisms. Examples include Mucor, which has the typical fungal hyphal structure, and yeast, which is single-celled.
• Fungi can be unicellular or multicellular. Fungi include organisms like mushrooms, which are multicellular, and yeast, which are unicellular. Fungi get their nutrition by breaking down dead or decaying matter.
• Fungi have a chitin cell wall. Fungi differ from plant cells in two ways – first they do not have chloroplasts because they do not photosynthesise, and second their cell wall is made out of chitin (not cellulose like in plants).
• Fungi feed saprotrophically. Fungi are not photosynthetic organisms. Instead, they feed via saprotrophic (saprophytic) nutrition. This involves secreting enzymes to digest food extracellularly, then absorbing the digested molecules through diffusion. They may store carbohydrate as glycogen.
• Fungi can cause infection. Many fungal cells can join together to form long threads called hyphae which contain many nuclei and are organised into mycelium. These hyphae can grow on organisms and cause infection. Fungi make and release spores, which can lead to spread of fungi across the organism. Fungi infect mouldy bread in a similar way.

Protists

• Protists are an ‘other’ group. Everything which doesn’t fit into animals, plants and fungi will be classified as protists. Protists are difficult to classify as different organisms resemble other microorganisms, for example, some are very similar to bacteria and others to fungi. Some are similar to animal cells like Amoeba that live in ponds and others are more like plants like Chlorella that have chloroplasts.
• Many parasites are protists. Many protists are parasites, which are organisms that live inside or on the surface of other organisms and gain benefit whilst sometimes causing damage. The parasites often use vectors to transport from organism to organism. For example, the protist Plasmodium Protozoa causes malaria, and uses mosquitos as a vector.