Year 8 Inheritance & Disease Topic 7
Summary Sheets
Microbes
Microbes (short for micro-organisms) can only be seen using a microscope. There are three main
types: viruses, bacteria and fungi. The most common fungus microbes are yeasts.
Viruses
are smaller than
bacteria
which are smaller than yeasts.
Viruses are often not considered to be living because they do not carry out any of the seven life
processes for themselves.
Bacteria and yeast are important in making foods and drinks. Yeast is used to make bread dough
rise. It uses oxygen, from the air found in pockets in the dough, for aerobic respiration. This
process produces carbon dioxide which makes the bread rise.
glucose + oxygen  carbon dioxide + water (+ energy)
(a sugar)
Yeast are also used to make beer and wine. In this case there is no air and so they use anaerobic
respiration. When yeast use anaerobic respiration it is called fermentation. The ethanol is a waste
product of this reaction.
glucose  carbon dioxide + ethanol (+ energy)
(a sugar)
The numbers of an organism in an area are called a population. In good conditions (warm, moist,
plenty of sugar) a population of yeast will grow rapidly. The population stops growing if something
runs out (eg sugar). The thing that stops the population growing is called a limiting factor.
Diseases
Some microbes cause infectious diseases (diseases that can be spread from person to person). The
microbes are said to infect you. The effects the microbes have on your body are known as
symptoms. Microbes can be spread by the air, water, touch, food, animals and sex.
Disease
Microbe that causes it
Symptoms
How it is spread
Virus
Sore throat, running nose, fever
Air
Food poisoning
Bacteria
Vomiting, diarrhoea
Food
Cholera
Bacteria
Vomiting, diarrhoea
Water
Athlete’s foot
Fungus
Sore cracked skin between the toes
Touch
Thrush
Fungus
Soreness and irritation and discharge
Touch
measles
virus
Red spots
Air
chickenpox
virus
Colds and flu
Fever. Raised red spots with yellow
tops.
Air
Some ways that diseases can be stopped from spreading are:
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making sure sewage is treated and disposed of properly
adding chlorine to water to kill bacteria
pasteurising milk
using disinfectants, antiseptics and soaps
immunising people with vaccines.
Your body has natural defences to stop microbes getting in (eg skin, mucus in the windpipe and
nose, ciliated epithelial cells to sweep mucus along). Your body also has ways of destroying
microbes. These include:
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a chemical in tears that kills some bacteria
acid in the stomach that kills some bacteria
white blood cells that engulf microbes
other white blood cells that make antibodies to help destroy microbes.
Babies do not have fully developed immune systems. Antibodies can pass through the placenta and
are found in breast milk. These help the baby to fight infections.
For many diseases, once you have had the disease (or been immunised) you will not get it again
(e.g. chickenpox). This is because the antibodies against these microbes stay in the blood.
Some diseases can be cured using antibiotics which are medicines which kill off bacteria. Some
bacteria, however, are unaffected by antibiotics – they are resistant to them.
Inheritance and selection
Inherited variation
The features of organisms are called their characteristics (e.g. blue eyes). Offspring normally share
some characteristics with their parents and brothers and/or sisters. Offspring can inherit
characteristics from their parents. Characteristics can be different and this is known as variation
(e.g. brown eyes and blue eyes). Variation occurs in both plants and animals.
An organism’s characteristics are controlled by genetic information which is found inside the
nucleus of almost all of its cells. Genetic information is passed from parents to offspring during
reproduction. In sexual reproduction, a male sex cell or gamete (e.g. a sperm cell) and a female
gamete (e.g. an egg cell) fuse. This fusing (joining together) produces a fertilised egg cell which
grows into the new organism. Each gamete contains half the amount of genetic information that a
normal body cell has. So the fertilised egg cell gets half its genetic information from the male and
half from the female.
In many animals, when two egg cells are each fertilised by a sperm cell, non-identical twins are
born. Sometimes a fertilised egg cell splits into two and identical twins form.
Species, breeds and varieties
A species is a group of organisms that are able to produce
offspring that are also able to reproduce. Members of the
same species have very similar characteristics but there is
some variation in these characteristics.
A group of animals may have special differences in their
inherited characteristics from the rest of their species. A
group like this is called breed (e.g. different breeds of
dog). There are also breeds of plants and these are called
varieties.
All tigers have stripes but there is variation in
the stripes between each tiger.
Environmental variation
An organism’s surroundings are known as its environment. The conditions in an environment are
called environmental factors. Plants are affected by environmental factors like the amount of light,
the amount of water, the amount of warmth and the amount of mineral salts in the soil.
The cress seedlings on the left have not had enough light.
The plant on the left has not had enough water. It has
wilted.
Animals are also affected by environmental factors. Humans who get sunburnt or have scars are
examples.
Inherited variation
This is caused by features being passed from parents to their offspring.
In humans, natural eye colour and natural hair colour are both examples of inherited variation.
Variation
A species is a group of organisms that can reproduce with one another to produce offspring that will
also be able to reproduce. The differences between organisms are known as variation. There is
variation between different species and between members of the same species.
There is variation between different species. Lions
and tigers are different species. Tigers have stripes, lions
do not.
There is variation between members of the same species.
All tigers have different patterns of stripes.
Sometimes there is a relationship or correlation between two features. A relationship is normally
best shown on a line graph. The line will go steadily up or steadily down.
Variation can have environmental or inherited causes.
Relationship: people with longer arms have longer middle fingers.
Cloning
Cloning is when the nucleus of an egg cell is replaced with the nucleus of the organism to be
copied. It means an animal can be copied quickly.
8
1
Put these statements in order
By numbering them.