chapter 12 p1

Question 1

Communicable diseases are caused by

Answer 1

infective organisms known as pathogens.
Each pathogen has particular characteristics that affect the way they are spread and the ways we can attempt to prevent or cure the diseases they cause.
A communicable disease can be passed from one organism to another.
In animals they are most commonly spread from one individual of a species to another, but they can also be spread between species.

Question 2

Communicable diseases in plants are spread

Answer 2

directly from plant to plant

Question 3

Vectors

Answer 3

which carry pathogens from one organism to another, are involved in the spread of a number of important plant and animal diseases.
Common vectors include water and insects.

Question 4

Types of pathogens:

Answer 4

bacteria, viruses, fungi, and protoctista

Question 5

Bacteria

Answer 5

There are probably more bacteria than any other type of organism.
A small proportion of these bacteria are pathogens, causing communicable diseases.
Bacteria are prokaryotes, so they have a cell structure that is very different from the eukaryotic organisms they infect. They do not have a membrane-bound nucleus or organelles.

Question 6

Bacteria can be classified in two main ways:

Answer 6

By their basic shapes

By their cell walls

Question 7

By their basic shapes

Answer 7

they may be rod shaped (bacilli), spherical (cocci), comma shaped (vibrios), spiralled (spirilla), and corkscrew (spirochaetes).

Question 8

By their cell walls

Answer 8

the two main types of bacterial cell walls have different structures and react differently with a process called Gram staining.
Following staining Gram positive bacteria look purple-blue under the light microscope, for example methicillin-resistant Staphylococcus aureus (MRSA).
Gram negative bacteria appear red, for example the gut bacteria Escherichia coli (E.coli).
This is useful because the type of cell wall affects how bacteria react to different antibiotics a compound that kills or inhibits the growth of bacteria).

Question 9

viruses p1

Answer 9

• Viruses are non-living infectious agents.
• At 0.02-0.3 pm in diameter, they are around 50 times smaller in length than the average bacterium.
• The basic structure of a virus is some genetic material (DNA or RNA) surrounded by protein.
• Viruses invade living cells, where the genetic material of the virus takes over the biochemistry of the host cell to make more viruses.
• Viruses reproduce rapidly and evolve by developing adaptations to their host, which makes them very successful pathogens.

Question 10

viruses p2

Answer 10

All naturally occurring viruses are pathogenic.
They cause disease in every other type of organism.
There are even viruses that attack bacteria, known as bacteriophages.
They take over the bacterial cells and use them to replicate, destroying the bacteria at the same time.
People now use bacteriophages both to identify and treat some diseases, and they are very important in scientific research.
Medical scientists consider viruses to be the ultimate parasites.

Question 11

Protoctista (protista)

Answer 11

The protoctista (now widely known as protista) are a group of eukaryotic organisms with a wide variety of feeding methods.
They include single-celled organisms and cells grouped into colonies.
A small percentage of protoctista act as pathogens, causing devastating communicable diseases in both animals and plants.
The protists which cause disease are parasitic - they use people or animals as their host organism.
Pathogenic protists may need a vector to transfer them to their hosts - malaria and sleeping sickness are examples - or they may enter the body directly through polluted water - amoebic dysentery and Giardia are examples of these.

Question 12

4. Fungi p1

Answer 12

• Fungal diseases are not a major problem in animals, but they can cause devastation in plants.
• Fungi are eukaryotic organisms that are often multicellular, although the yeasts which cause human diseases such as thrush are single-celled.
• Fungi cannot photosynthesise and they digest their food extracellularly before absorbing the nutrients.
• Many fungi are saprophytes which means they feed on dead and decaying matter.
• However some fungi are parasitic, feeding on living plants and animals.

Question 13

Fungi p2

Answer 13

These are the pathogenic fungi which cause communicable diseases. Because fungal infections often affect the leaves of plants, they stop them photosynthesising and so can quickly kill the plant.
When fungi reproduce they produce millions of tiny spores which can spread huge distances, this adaptation means they can spread rapidly and widely through crop plants.
Fungal diseases of plants cause hardship and even starvation in many countries around the world.

Question 14

Pathogens - modes of action

Answer 14

Damaging the host tissues directly:

Many types of pathogen damage the tissues of their host organism.
It is this damage, combined with the way in which the body of the host responds to the damage, that causes the symptoms of disease.
Different types of pathogens attack and damage the host tissues in different ways:

Question 15

Different types of pathogens attack and damage the host tissues in different ways:

Answer 15

Viruses take over the cell metabolism.

Some protoctista also take over cells and break them open as the new generation emerge,

Fungi digest living cells and destroy them.

bacteria produce toxins

Question 16

Viruses take over the cell metabolism.

Answer 16

The viral genetic material gets into the host cell and is inserted into the host DNA.
The virus then uses the host cell to make new viruses which then burst out of the cell, destroying it and then spread to infect other cells

Question 17

Some protoctista also take over cells and break them open as the new generation emerge,

Answer 17

but they do not take over the genetic material of the cell.
They simply digest and use the cell contents as they reproduce.
Proctists which cause malaria are an example of this.

Question 18

Fungi digest living cells and destroy them.

Answer 18

This combined with the response of the body to the damage caused by the fungus gives the symptoms of disease.

Question 19

Producing toxins which damage host tissues:

Answer 19

Most bacteria produce toxins that poison or damage the host cells in some way, causing disease.
Some bacterial toxins damage the host cells by breaking down the cell membranes, some damage or inactivate enzymes and some interfere with the host cell genetic material so the cells cannot divide.
These toxins are a by-product of the normal functioning of the bacteria

Some fungi produce toxins which affect the host cells and cause disease.

Question 20

Plant diseases

Answer 20

Plant diseases threaten people, because when crop plants fail, people suffer.
They may starve, economies may struggle and jobs are lost.
Plant diseases threaten ecosystems too - entire species can be threatened.

Question 21

The threat to English oak trees:

Answer 21

Oak woodlands are a traditional part of the British countryside.
These mighty trees (Quercus robur) can live for centuries and are home to up to 284 species of insects.
As many as 324 different lichens have been identified on a single tree.
Future generations, however may not see these trees in the countryside - they are under threat from a new disease.
Acute oak decline first appeared in the UK the 1980s, having spread from continental Europe.
It causes dark fluid to ooze from the bark, with a rapid decline in the tree and often death.
Scientists still do not know exactly what the cause of this deadly tree disease is - which makes it very difficult to understand how to prevent it spreading.

Question 22

Some of the evidence so far includes:

Answer 22

The discovery of previously unknown bacteria in the tree which may play a role in the disease
Evidence of oak jewel beetle activity in infected trees - they may be important in disease development.
For example, they may act as a vector or their presence may just be a coincidence.
A massive research project is currently underway involving DNA analysis of the microorganisms on infected and healthy oak trees, along with a careful study of the behaviour of the oak jewel beetle.
For now, the advice is to try and avoid spreading the disease by careful hygiene procedures for both people who work in oak woodlands and their machinery.

Question 23

Plant diseases are caused by a range of pathogens. They include:

Answer 23

Ring rot
Tobacco mosaic virus (TV)
Potato blight (tomato blight, late blight)
Black sigatoka

Question 24

Ring rot

Answer 24

a bacterial disease of potatoes, tomatoes, and aubergines caused by the Gram positive bacterium Clavibacter michiganensis.
It damages leaves, tubers and fruit. It can destroy up to 80% of the crop and there is no cure.
Once bacterial ring rot infects a field it cannot be used to grow potatoes again for at least two years.

Question 25

Tobacco mosaic virus (TV)

Answer 25

a virus that infects tobacco plants and around 150 other species including tomatoes, peppers, cucumbers, petunias and delphiniums. It damages leaves, flowers and fruit, stunting growth and reducing yields, and can lead to an almost total crop loss. Resistant crop strains are available but there is no cure.

Question 26

Potato blight (tomato blight, late blight)

Answer 26

caused by the fungus-like protoctist oomycete Phytophthora infestans. The hyphae penetrate host cells, destroying leaves, tubers and fruit, causing millions of pounds worth of crop damage each year. There is no cure but resistant strains, careful management and chemical treatments can reduce infection risk.

Question 27

Black sigatoka

Answer 27

a banana disease caused by the fungus Mycosphaerella fijiensis, which attacks and destroys the leaves. The hyphae penetrate and digest the cells, turning the leaves black. If plants are infected it can cause a 50% reduction in yield. Resistant strains are being developed - good husbandry and fungicide (a chemical that kills fungi) treatment can control the spread of the disease but there is no cure.

Question 28

Banana diseases and food security:

Answer 28

Food security is one of the biggest issues globally. In an ideal world everyone would consistently have a balanced diet provided in a sustainable way. One of the main problems, however, for many people is getting enough to eat. If plant diseases threaten staple crops such as rice, maize, cassava, and bananas then they threaten food security and the survival of the population. * Bananas are grown in over 130 countries where they are important both as a food crop and economically as a cash crop. They are the 4th most important crop in the developing world after rice, wheat, and maize * In East Africa bananas (known as plantains] are the staple food for around 50% of the population. People eat around 400 kg of bananas per year 90% of the bananas cultivated are produced on small farms and eaten locally. In recent years, as a result of Black Sigatoka there has been a 40% fall in banana yields * Around 10% of bananas are produced on big plantations for Western supermarkets. These are all from the same clone of a variety called Cavendish so they are genetically very similar. Black Sigatoka is invading these plantations too.

Question 29

Animal diseases:

Answer 29

The diseases that affect animals - and in particular human beings - have a profound effect on human health and wellbeing - and on national economies. Communicable diseases range from mild to fatal. Examples include: . Tuberculosis (TB) Bacterial meningitis . HIV/AIDS (acquired immunodeficiency syndrome) Influenza (flu) Malaria . Ring worm Athlete's foot