Unit 3: Defence Against Disease

Question 1

What is disease?

Answer 1

This term broadly refers to any abnormal condition that impairs normal function. Commonly, this term refers specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, Protozoa, multicellular parasites, and aberrant proteins known as prions. An infection that does not produce clinically evident impairment of normal functioning is not considered a disease.

Question 2

What is a pathogen?

Answer 2

Any disease-causing entity is called a pathogen. Pathogens tend to be host specific and can be cellular or non cellular.

Question 3

What are the non-cellular pathogens?

Answer 3

Non-cellular pathogens are prions, viruses and viroids.

Question 4

What are cellular pathogens?

Answer 4

Cellular pathogens are organisms that cause disease and include bacteria, protozoans and worms.

Question 5

What are prions?

Answer 5

Prions are infectious proteins that cause disease. The body makes its own prion proteins which occurs in nerve cells however the is function is unknown. If a defective prion enters the body it changes normal proteins into prion proteins. The polypeptide then folds in a different and irregular way, thus enzymes can’t function.

Question 6

What are viruses?

Answer 6

Viruses obligate intracellular parasites; that is they can only replicate in a host cell. They lack ribosomes to make proteins for replication. Mostly host specific; only affect one organism. Bacteriophages are viruses which infect bacterium.

Question 7

Explain the reproduction of a bacteriophage.

Answer 7

1. Protein coat attaches to bacterium wall and DNA is injected into the cell.
2. The DNA takes over the functioning of the cell, the cell structures are used to produce protein coats and DNA.
3. Thus material is assembled; the cell lyses and the new bacteriophages infect other bacterium.

Question 8

Explain the reproduction if the human viruses.

Answer 8

In most cases the DNA/RNA is injected into the cell as with bacteriophages and the whole virus can be enclosed by the cell membrane. The process is then essentially the same as with bacteriophages. The genetic material can either be incorporated into the host DNA or remain separate. Acute infections are when the cell dies quickly after infection.

Question 9

What are viroids?

Answer 9

Viroids are the smallest infectious agent and are small, naked pieces of RNA. Viroids only cause disease in plants and as they have no genes for protein synthesis, are completely reliant on the host cell for reproduction.

Question 10

What is bacteria?

Answer 10

Bacteria that cause disease are called pathogenic bacteria and can cause disease in humans, animals and plants. Depending on the host specificity of the bacteria, they can only make one particular host ill or cause trouble in a number of hosts. Bacteria can replicate very quickly and can interrupt normal cell functioning by reproducing in ver large numbers, or by releasing their own toxic waste products. Examples are botulism and tetanus.

Question 11

What are fungi?

Answer 11

Fungi are unicellular or multicellular organisms and consist of eukaryotic cells with cell walls composed of chitin. Yeasts are unicellular fungi and moulds are multicellular fungi that can invade tissue by hyphae. Many plant diseases such as Dutch elm disease and rust diseases are caused by moulds. In animals ringworm and athletes foot are common mould diseases. Fungi cells have nuclei and reproduce by budding.

Question 12

What are protozoan’s?

Answer 12

Protozoan’s are unicellular, colonial or simple multicellular organisms which consist of eukaryotic cells. Examples of protozoan’s include Amoebas that cause amoebic dysentery (a severe intestinal infection. Plasmodium is a protozoan that causes malaria.

Question 13

What are worms?

Answer 13

Worms are multicellular, eukaryotic, specialised for the parasitic way of life. They usually have well adapted mouth parts that form hooks, and body adaptions that allow them to survive in the digestive system of their host. Nematode worms include round worm and hook worm. Intestinal worms such as fluke and tapeworms can cause pain and increased appetite and interfere with absorption of nutrients.

Question 14

How are pathogens controlled?

Answer 14

Quarantine involves isolating the host from other hosts preventing the spread of the pathogen.
Another method is to disrupt the life cycle. Destroy the habitat of the pathogen, this may include destroying the vector.

Question 15

Describe physical barriers to infection in plants.

Answer 15

Physical barriers prevent the entry of the pathogen into the plant. Waxy cuticle or hair on the life surface is an example.

Question 16

Describe chemical barriers to infection in plants.

Answer 16

Some plants are able to produce antibiotics, toxins and even enzymes in response to specific pathogens. The production of the chemicals can either prevent the pathogen from spreading to the rest of the plant, or act as deterrent to further infection or damage.

Question 17

Explain the lymphatic system.

Answer 17

The lymphatic system is a network of lymph vessels and lymphoid organs found throughout the body. This system of lymphatic veins and nodes acts to drain excess fluids (lymph) from the tissues and move it back into the circulatory system. It also collects lipids in lacteals found in the villi of the small intestine and transport them to the bloodstream. The immune system relies heavily on the lymphatic system to help protect the body from pathogens. The lymphatic systems helps fight infection by helping to make special white blood cells (lymphocytes) that produce antibodies. Macrophages in side the lymph nodes which swallow up and kill any foreign particles like germs also assists.