Biology – Key Themes Unit 2
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 2-6
marks.
Here are some for you to learn:
1. Compare and contrast plant and animal cells (3 marks)
 Animal and plant cells both have a nucleus, cytoplasm, cell membrane,
mitochondria and ribosomes
 Only plant cells have a cell wall – for support
 Only plant cells contain chloroplasts – for photosynthesis
 Only plant cells contain a permanent vacuole – containing cell sap
2. Describe the structure of a bacterial cell (4 marks)
 Bacteria cells are much smaller than animal or plant cells
 They have a cell membrane and a cell wall (although the cell wall is made out
of a different substance to that of a plant cell)
 They don’t contain a nucleus, instead they have a chromosome or plasmids
which are ‘loose’ in the cytoplasm
 The bacteria can sometimes have a slime capsule or a flagellum (tail)
3. Describe the functions of the contents of an animal cell (4 marks)
 Nucleus – controls cells activities/contains DNA
 Cytoplasm – where many chemical reactions take place
 Cell membrane – controls the movement of materials in and out of the cell
 Mitochondria-where aerobic respiration takes place
 Ribosomes- where proteins are made
4. Describe 4 examples of an adaptation shown by a specialised animal cell (4 marks)
Examples include:
 Muscle cells contain many mitochondria as they require energy for
contraction
 Gland cells contain many ribosomes for producing proteins e.g. enzymes
 Sperm cells have a tail for movement
 Receptor cells are able to detect stimuli e.g. light-sensitive rod and cone cells
 Neurons are long and can carry electrical impulses around the body
5. Describe 2 examples of an adaptation shown by a specialised plant cell (2 marks)
 Plant cells in the leaf (e.g. palisade cells) contain many chloroplasts for
photosynthesis
 Root hair cells have a large surface area for absorbing water
 Root hair cells contain many mitochondria as they need energy for taking in
mineral ions by active transport
6. Describe the process of diffusion and explain 2 factors which can affect the rate of
diffusion (4 marks)
 Diffusion is the net movement of particles
 from a high to a low concentration
 The rate of diffusion increases when temperature increases
 The rate of diffusion increases when there is a high concentration gradient
 The rate of diffusion decreases, the greater the diffusion distance
7. Define a tissue and explain the role of 2 animal tissues and 2 plant tissues (5 marks)
 A tissue is a group of cells with a similar structure or job
 In animals - muscle tissue –contract to bring about movement
 In animals - glandular tissue – secrete substances such as enzymes or
hormones
 In animals - epithelial tissue – covers the external surface of the body e.g. skin
 In plants – epidermal tissue – which covers the plant
 In plants- mesophyll tissue – which can photosynthesise
 In plants – xylem and phloem – which can transport substances around the
plant
8. Explain, using graphs, 3 factors which can affect the rate of photosynthesis (6
marks)
 Rate of photosynthesis increases when carbon dioxide concentration
increases.
 Graph eventually levels off due to limiting factors (not enough light or wrong
temperature)
 Rate of photosynthesis increases when light increases
 Graph eventually levels off due to limiting factors (not enough carbon dioxide
or wrong temperature
 Rate of photosynthesis increases when temperature increases
 Rate of photosynthesis stops when temperature gets too high because
enzymes become denatured
 Graph descriptions could be substituted for a correctly labelled graph for each
point
9. Describe 5 ways in which glucose is used by a plant (5 marks)
 Converted to starch for storage
 Used for respiration
 Converted to fats and oils for storage
 Used to produce cellulose –strengthens cell wall
 Combined with nitrates to produce protein
10. Explain how a quadrat can be used to conduct a random sample of the distribution
of daisies in a field (4 marks)
 Divide field into squares the size of the quadrat
 Assign each square a number
 Randomly generate numbers e.g. using a calculator
 Put the quadrat in those squares and count the number of daisies in each
square
 Do plenty of repeats (10+)
 Calculate a mean number of daisies
 Multiply the mean number of daisies by the number of squares that could
have been on the whole field to estimate the number in the whole area
11. Explain how enzymes work (4 marks)
 Enzymes are biological catalyst – speed up reactions without being used
 They have an active site which binds to the substrates of the reaction
 They have a complementary shape and fit together (lock and key)
 Every enzyme is specific for the reaction which it speeds up
12. Explain, using graphs, 3 factors which can affect the activity of an enzyme (6 marks)
 Rate of enzyme activity increases when temperature increases
 Because enzymes have increased kinetic energy – molecules move around
faster
 Rate of photosynthesis stops when temperature gets too high because
enzymes become denatured
 Draw correct graph (pg 50 in revision guide)
 Rate of enzyme activity is highest at the optimum pH for that membrane
(varies for different enzymes)
 The enzyme is denatured outside of its optimum pH
13. Describe the role of enzymes in the digestion of food (6 marks)
 Digestion requires food to be broken down into small, soluble molecules
 Amylase/carbohydrase is produced by the salivary glands and small intestine
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Amylase/carbohydrase breaks down starch into glucose
Protease is produced by the stomach, pancreas and small intestine
Protease breaks down proteins into amino acids
Lipase is produced by the pancreas and small intestine
Lipase breaks down fats (lipids) into fatty acids and glycerol
Glucose, amino acids, fatty acids and glycerol can all be readily absorbed in
the small intestine into the blood
14. Use examples to explain how enzymes can be used in industry (4 marks)
 Biological detergents – contain proteases and lipases that digest stains
 Baby food – proteases are used to pre-digest protein into amino acids
 Slimming products – use isomerase to convert glucose into fructose which is
sweeter so less sugar is needed in the food
 Sports drinks – carbohydrases are used to convert starch into sugar syrup
 Enzymes can be used in industry so that reactions can be done at a lower
temperature (the enzymes will speed up the reaction)
15. Describe what happens to your body during exercise (3 marks)
 Heart rate increases
 This allows more blood containing oxygen and glucose to reach the muscles
 Breathing rate increases
 This allows more oxygen to enter the blood and more carbon dioxide to leave
the blood
 Stores of glucose in the muscles – called glycogen- are broken down, releasing
glucose for use by the cells
16. Explain what happens during anaerobic respiration in humans (4 marks)
 Anaerobic respiration occurs when you are doing strenuous exercise and your
muscle cells are not getting enough oxygen
 Glucose is not completely broken down so a much smaller amount of energy is
released than during aerobic respiration
 Lactic acid is produced
 Lactic acid causes muscle fatigue – extra oxygen is required after anaerobic
respiration to remove this lactic acid
17. Compare and contrast mitosis and meiosis (4 marks)
 Mitosis and meiosis are both types of cell division
 Mitosis produces 2 daughter cells, meiosis produces 4 daughter cells
 Mitosis has 1 cell division, meiosis has 2 cell divisions
 Mitosis produces clones, meiosis produces cells with half the number of
chromosomes
 Meiosis produces sex cells (gametes) only
18. Use a genetic diagram to explain the probability of what could happen if two
carriers of the cystic fibrosis gene have a baby (4 marks)
 Carriers of cystic fibrosis are heterozygous e.g. Cc
 Show punnet square with 2 parents with Cc
 Probabilities – 25% chance of cc, 50% chance Cc, 25% chance CC
 CC – unaffected, Cc- carrier, cc –affected (cystic fibrosis)
19. Explain some of the reasons for and against the use of stem cells for medical
reasons (3 marks)
 For - stem cells are unspecialised and so have the potential to turn into any
kind of specialised cell – could be used to treat many illnesses e.g. Parkinson
disease, blindness, paralysis
 For - Embryonic stem cells are taken from spare embryos from IVF (which
would be otherwise wasted)
 Against - Research is expensive
 Against – Requires use of embryos which are a potential life, the embryo can’t
give permission
20. Describe 4 factors which could live to the extinction of a species (4 marks)
 A new disease killing all members of a species
 Change in the environment
 New predator introduced to the area
 New competitor introduced to the area
 Single catastrophic event
21. Explain how 2 new species can arise from one common ancestor (5 marks)
 Geographical isolation could occur
 The two isolated groups of the same species could be subject to different
selection pressures e.g. different food sources
 Natural selection occurs differently in each environment
 Different characteristics are beneficial for survival in each environment
 Speciation has occurred when the two populations can no longer successfully
interbreed.