Biology – Key Themes Unit 1
There are several processes or ideas in biology which you need to be able to
explain clearly. These questions frequently arise in exams and are often worth
between 3-6 marks.
Here are some for you to learn:
How does someone become immune to a pathogen? /How do vaccines work? (3
marks)
 White blood cells produce specific antibodies to a pathogen/bacteria/virus
 If you encounter the same pathogen/bacteria/virus in the future the white
blood cells recognise it. They produce the same antibodies much quicker
 The antibodies attach to the pathogen/bacteria/virus and tell other white
blood cells to destroy it.
Explain how antibiotic resistance arises/why it is important to complete the whole
course of antibiotics (3 marks)
 Some bacteria can mutate randomly to make them more resistant to
destruction by antibiotics
 When antibiotics are taken the resistant bacteria are not destroyed straight
away
 When the person stops taking the antibiotics, the resistant bacteria survive
and reproduce to make more resistant bacteria
Describe the stages in a reflex (4 marks)
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A receptor detects a change in the environment (stimulus)
A nerve impulse is sent along a sensory neuron
The message goes to the CNS/Central Nervous System/Relay neuron
A nerve impulse is sent back along a motor neuron
An effector (muscle or gland causes a response to the stimulus)
Describe the role of hormones in the control of the menstrual cycle (4 marks)
 FSH is secreted by the pituitary gland/brain
 FSH stimulates an egg to mature and stimulates the ovaries to release
oestrogen
 Oestrogen tells the brain to release LH
 LH causes an egg to be released (ovulation)
 In addition, Oestrogen inhibits FSH production (negative feedback) This is why
Oestrogen is found in the contraceptive pill.
Describe the process of IVF (3 marks)
 Fertility drugs are given to help the woman make lots of eggs
 The eggs are combined in a petri dish with a sample of semen
 After fertilisation, several healthy embryos are implanted into the uterus
Explain how a plant uses hormones to grow towards light (4 marks)
 Plants produce auxins in the tip of the plant in response to light
 The auxins are distributed to the shaded side of the plant
 This causes cells on the shaded side to elongate so that the plant bends over
towards the light
 This is called phototropism
Explain how animals and plants can be adapted for extremely hot environments (3
marks)
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Adaptations increase an animal or plants chance of survival
Plants might reduce surface area by having fewer leaves e.g. cacti
Plants might store water by having a waxy cuticle e.g. cacti
Plants might have large root systems to allow them to collect lots of water e.g.
mesquite tree
Animals might increase surface area to allow heat to leave their bodies more
easily
Animals might have behaviours causing them to stay in the shade during the
hottest time of the day e.g. lions
Animals living in the desert have very fine or no fur e.g. elephant
Other valid examples
Explain how animals and plants can be adapted for extremely cold environments (3
marks)
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Adaptations increase an animal or plants chance of survival
Animals might have smaller ears e.g. polar bears
Animals might have a thick layer of insulation/blubber e.g. seal
Animals might have a thick fur coat e.g. Arctic Fox
Animals often build up a layer of fat in the Summer months when food is more
plentiful
 Other valid examples
Explain how animals and plants can be adapted to avoid being eaten (3 marks)
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Adaptations increase an animal or plants chance of survival
Camouflage to stay hidden e.g. peppered moth, frog, lizard
Mimicry e.g. looks poisonous
Defence mechanisms e.g. sting, spikes
Speed e.g. gazelle
Live in groups
Eyes on the side of head to give wider field of view e.g. rabbits
Other valid examples
Explain how living indicators can be used to measure environmental change (3
marks)
 Environmental change can affect the distribution of plants and animals in a
certain area
 Lichen is a good indicator of environmental change as it is sensitive to sulphur
dioxide levels
 Invertebrate species e.g. mayfly nymph are good indicators of environmental
change/water pollution
Describe how energy can be lost as it is transferred along a food chain (3 marks)
 Energy lost when not whole organism is eaten
 Energy lost in indigestible food/lost as faeces
 Energy lost when released by organisms in respiration e.g. movement,
maintaining body temperature
Describe what causes things to decay (3 marks)
 Detritus feeders, bacteria and fungi feed on dead/waste products
 They require warm and moist conditions
 They require plenty of oxygen for aerobic respiration
Explain how carbon is transferred within the carbon cycle (6 marks)
(You can start at any point since it is a cycle)
 Carbon is removed from the atmosphere by plants by the process of
photosynthesis. This turns carbon in carbon dioxide into the carbon stored in
carbohydrates, fats and proteins
 Carbon stored in plants is transferred into animals when they eat plants.
 Carbon is returned to the atmosphere as carbon dioxide from plants and
animals when they respire.
 When plants and animals die they are decomposed by bacteria which also
respire, returning carbon dioxide to the atmosphere
 Dead material can also be compressed over millions of years and turned into
fossil fuels
 When fossil fuels are burned, stored carbon is released back into the
atmosphere by the process of combustion.
Explain how plants can be cloned by tissue culture (3 marks)
 Plant cells are removed from a plant with desirable characteristics
 They are grown in sterile conditions with nutrients and hormones until they
develop into plantlets
 Plantlets then planted into compost and will develop into clones of the parent
plant.
Explain how animals can be cloned by adult cell cloning (3 marks)
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The nucleus is removed from the skin cell of an animal you want to clone
This is placed into an empty egg cell (one which has had its nucleus removed)
An electric shock is given to the cells to cause them to develop into an embryo
The embryo is inserted into a surrogate sheep who will then give birth to a
clone of the animal from which the skin cell was removed
Describe the process of genetic engineering (3 marks)
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A gene is cut out of the chromosome of an organism using enzymes
The gene is stuck into the chromosome of an organism of the same species
The organism will then have the desired characteristic
Alternatively, genes can be put into bacteria so they can be used as
instructions by the bacteria to make human proteins, such as insulin
Describe the process of natural selection (e.g. change in beak shape) (4 marks)
 There is variation in the population caused by mutations in DNA e.g. beaks are
different sizes (this is due to random chance)
 The organisms with certain characteristics will have advantages which allow
them to survive better than others.
 The organisms which survive are able to reproduce
 When they reproduce they pass on their genes, so their offspring also have
the favourable characteristic
 REMEMBER VSRG