all the answers to AQA AS biology unit one summary questions from the book.

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i have written out all the answers from the back of the text book with the questions and will paste them below so you can copy them onto a word document and print them out so they are easier to be used for studying. this is for unit one of the AQA AS biology.

1. What is a pathogen?
A microorganism that causes disease

2. Why is the digestive system and respiratory systems often the entry site for pathogens?
Diffusion takes place in these systems so they have a large surface area, are thins, moist and well supplied with blood vessels. This makes it easy for pathogens to attach to and penetrate them.

3. In which 2 ways do pathogens cause disease?
They damage host tissues and they produce toxins

4. Why are oral antibiotics not used to treat diarrhoea or vomiting?
Because it is unlikely that the antibiotic will remain in the body long enough to be absorbed.

5. Why are things sometimes labelled, for example, the ‘relative risk for lung cancer’
Because it is the risk of lung cancer when compared to a non-smoker.

6. How would 30 minutes of exercise reduce the chance of suffering from CHD?
It would lower blood pressure, lower blood cholesterol and lower the risk of obesity.

7. What is hydrolysis?
The breakdown of molecules by the addition of water to the bonds that hold these molecules together.

8. Why does the stomach not have villi?
Because villi increase surface area to speed up absorption of molecules but food in the stomach has not yet been broken down into soluble molecules so cannot be absorbed/
9. Why do most molecules contain carbon?
Carbon atoms readily link to one another to form a chain.

10. Why does Benedict’s reagent turn red when heated with a reducing sugar?
Because the sugar donates electrons which will reduce blue copper sulphate to orange copper oxide.

11. In lactose intolerant people, microorganisms in the large intestine convert the undigested lactose into gas by respiration. Why is the gas unlikely to be carbon dioxide?
Because carbon dioxide forms as a result of aerobic respiration and conditions in the small intestine are anaerobic.

12. What four different components make up an amino acid?
An amino group, a carboxyl group, an R group and a Hydrogen atom.

13. What is a catalyst?
A substance that alters the rate of a chemical reaction without undergoing permanent changes.

14. Why are enzymes effective in tiny quantities?
They are not used up in a reaction so can be reused repeatedly.

15. Explain why changing one of the amino acids that make up the active site could prevent the enzyme from functioning.
The changed amino acid may no longer bind to the substrate which will then not be positioned correctly, if at all, in the active site so no enzyme-substrate complexes can be formed.

16. Why might changing certain amino acids that are not part of the active site also prevent the enzyme from functioning?
The changed amino acid may be one that forms hydrogen bonds with other amino acids. If the new amino acid does not form hydrogen bonds then the tertiary structure of the enzyme will change, including the active site, so that the substrate may no longer fit so also cannot form enzyme substrate complexes.

17. A reaction is inhibited by an enzyme. How could you tell if this enzyme is acting as a competitive or non-competitive inhibitor?
Increase the substrate concentration. If the amount of inhibition is reduced it is a competitive inhibitor, if it stays the same it is a non-competitive inhibitor.

18. Chloroplasts have a greater mass than mitochondria (which is centrifuged at 3500 times gravity) but a smaller mass than nuclei (which is centrifuged at 1000 times gravity). Starting with a sample of plant cells, describe how you would obtain a sample rich in chloroplasts.
Keep the plants in a cold, isotonic, buffered solution. Break up the cells using a homogeniser and then filter the homogenate to remove cell debris. Centrifuge the homogenate at 1000 times gravity and remove the supernatant liquid leaving the nuclei behind in the sediment. Then centrifuge the supernatant liquid at 2000-3000 times gravity. The sediment will be rich in chloroplasts.

19. Why do specimens have to be kept in a vacuum in order to be viewed by an electron microscope?
Electros are absorbed by molecules in the air and so if there was air it would prevent electrons reaching the specimen.

20. Four cells have the following water potentials: -200kPa , -250kPa , -100kPa , -150kPa. In what order would the cells have to be placed in for the water to pass from one cell to the next if they are arranged in a line.
-100, -150, -200, -250.

21. In the production of urine, glucose is initially lost from the blood but is then reabsorbed into the bloodstream by cells in the kidneys. Explain why it is important that this reabsorption occurs by active transport rather than diffusion.
Diffusion, at best, can only reabsorb 50% of the glucose lost from the blood. The other 50% will be lost from the body. Active transport can absorb all the glucose, leaving none to be lost from the body.

22.How does the cholera toxin cause diarrhoea?
The cholera toxin opens chloride channels in the cell-surface membrane of intestinal epithelial cells. Chloride ions therefore flood into the lumen of the intestine by diffusion. This raises the water potential of the epithelial cells while lowering the water potential of the lumen. Water then moves into the lumen by osmosis causing watery stools.

23. Give two reasons why Glucose is added to oral rehydration therapies?
Glucose stimulates the uptake of sodium ions from the intestine and provides energy as it is a respiratory substrate.

24. Give two reasons why sodium chloride is added to ORT?
The sodium ions replace those lost from the body and encourage the sodium-glucose transporter proteins to absorb more sodium

25. Sports drinks contain a lot of glucose. Explain in terms of water potential why sport drinks would not be suitable to rehydrate someone with diarrhoea.
Too much glucose would lower the water potential in the intestine so the water potential in the lumen would be lower than that of epithelial cells. Water will therefore pass out of the cells by osmosis, increasing dehydration.

26. From the graphs on figure 3 on page 77, determine the tidal volume.
0.48 dm3

27. From the second graph on page 77 of the text book, calculate the breathing rate in breaths per minute.
The time between two corresponding points is 3.5 second. This indicates one cycle and so is the time for one breath.
The number of breaths per minute is therefore 60÷3.5 = 17.14

28. Using the two graphs on page 77 of the text book. If the volume of air a person inhaled was 3000cm3 , what would the volume of air be in the lungs after the person had exhaled?
3000cm3 = 3 dm3.
The graph shows that the exhaled volume is 0.48dm3 less than the maximum inhaled volume (which is 3 dm3.)
So the exhaled volume is 3-0.48 = 2.52cm3 (you can also just use 3000cm3)

29. Explain how muscles create the change of pressure in the alveoli during the period from 0 seconds to half a second in the second graph above.
The muscles in the diaphragm contract causing it to flatten and mover downwards. The external intercostal muscles contract moving the rib cage up and outwards. Both actions increase the volume of the lungs and so the pressure in the alveoli is reduced.

30.Why is difficulty in breathing and shortness of breath a symptom of Asthma?
Because as a result of asthma, airways are constricted, lining of airways are inflamed and there is the additional mucus and fluid.

31. Why is difficulty in breathing and shortness of breath a symptom of Emphysema?
Due to emphysema, there is a loss of elasticity in the lungs which makes exhaling difficult. If the lungs cannot be emptied of much air then it is difficult to inhale fresh air and so the person feels breathless. It also leads to a reduced surface area of the alveoli leading to a shortness of breath.

32. Why is difficulty in breathing and shortness of breath a symptom of Fibrosis?
Because Fibrosis leads to a reduced lung volume as much of the air space within lungs is occupied by fibrous tissue, so less air and less oxygen is taken into the lungs at each breath. It also causes a reduction in elasticity of lungs making ventilation harder and a lengthened diffusion pathway due to the fibrous tissue.

33. Why does a person with emphysema have difficulty carrying out strenuous exercise?
The surface area of the alveoli surface area is reduced and there is a longer diffusion pathway therefore less oxygen diffuses into the blood. This means that there is not enough oxygen available for the increase in respiration. Doing strenuous exercise requires extra energy in the form of ATP that is usually realised during respiration. So as there is not enough oxygen for respiration to provide enough energy, the person finds exercise hard.

34. Out of the vena cava, the pulmonary artery and the left atrium, which structure carries oxygenated blood?
The left atrium. (The rest carry deoxygenated)

35. List the structures blood passes through on its journey from the lungs, to the heart, to the body and then back to the lungs.
Pulmonary vein, left atrium, left ventricle, aorta, vena cava, right atrium, right ventricle, pulmonary artery.

36. Why is it important to prevent the mixing of blood in the two sides of the heart?
The mixing of oxygen and deoxygenated blood would result in only partly oxygenated blood reaching the tissues and lungs meaning that the supply of oxygen to the tissues would not be adequate and therefore there would be a reduced diffusion gradient in the lungs, limiting the uptake of oxygen.

37. After training, the heart rate is often decreased when at rest although the cardiac output is unchanged, suggest an explanation.
Training builds up the muscles of the heart and to the stroke volume increases (more blood is pumped at each beat). This means that, is the cardiac output is the same, the heart rate (number of beats per minute) would decrease
38. If one cycle takes 0.8 seconds. Calculate the heart rate in beats per minute.
60÷0.8 = 75 beats per minute

39. State three ways in which high blood pressure increases the risk of heart disease.
The heart must work harder and so is more prone to failure. An aneurysm is more likely to develop due to overworking which would lead to a haemorrhage. The artery walls may thicken which would restrict blood flow.

40. Why would it be inaccurate to say the body takes days to “respond” to a pathogen?
Because the body responds immediately by ‘recognising’ the pathogen. The delay is only due to having to wait for the number of lymphocytes to build up numbers.

41. Lysozymes are used to break down bacterial cell walls during phagocytosis. Why are lysozymes also found in tears?
The tear ducts are potential entry sites for pathogens and so lysozymes will break down the cell walls of any bacterial pathogens and will destroy them before they can cause harm.

42. What is an antigen?
An organism or substance, usually a protein, that is recognised as foreign by the immune system and therefore stimulates and immune response.

43. State 3 similarities between B cells and T cells.
Both are types of white blood cells, they both have a role in immunity and are both produced from stem cells.

44. State 2 differences between B cells and T cells.
B cells mature in the bone marrow while T cells mature in the thymus gland. B cells are involved in humoral immunity while T cells are involved in cell mediated immunity.

45. Explain why the secondary immune response is much more faster than the primary immune response.
In the primary response the antigens of the pathogen have to be ingested, processed and presented by B cells. T helper cells then need to link with the B cells and stimulate them to clone with some developing into plasma cells that produce antibodies and others into memory cells. These processes occur consequently and so take time.
In the secondary response, memory cells are already present and the only processes are cloning and development into plasma cells. Therefore fewer processes means a quicker response.

46. What are the differences between cell mediated immunity and humoral immunity responses?
Cell mediated immunity involves T cells, is the first stage of the immune response, there are no antibodies produces and is effective through cells. Whilst Humoral immunity involves mostly B cells, is the second stage of the immune response after the cell mediated stage, it does produce antibodies and is effective through body fluids.

47. Name 3 organelles you would expect to find in large quantities in plasma cells that produce around 2000 protein antibodies each second.
Rough endoplasmic reticulum to make and transport the proteins of the antibodies. Golgi apparatus to sort, process and compile the proteins. Mitochondria to release the energy needed for such massive antibody production.

48. Why have proteins, rather than carbohydrates, evolved as the molecules that all antibodies are made of.
There must be a large variety of antibodies as each antibody responds to different antigens (and there are millions of antigens). Only proteins have the diversity of molecular structure to produce millions of different types.

49. What is the difference between an antigen and an antibody?
An antigen is a molecule that triggers and immune response by lymphocytes. An antibody is a molecule that has a complementary shape to the antigen and is produced in response to it.

50. What’s the difference between active immunity and passive immunity?
Active immunity is when individuals are stimulated to produce their own antibodies. This immunity is normally long lasting. Passive immunity is when antibodies are introduced from outside rather than being produced by the individual. Immunity is normally only short lived.

51.Why is it difficult to control tuberculosis by vaccine?
HIV infections means than more people have impaired immune systems. More people who are carriers of TB re moving frequently and are living in overcrowded conditions as there is a more mobile population. There are more elderly people who have less effective immune systems.
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Report 7 years ago
heyyyy which textbook do you use
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Report Thread starter 7 years ago
(Original post by maggie43)
heyyyy which textbook do you use
The one that is endorsed by AQA Image
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Report Thread starter 7 years ago
ive just finished the ones for unit 2. ill paste them below


1. State 3 ways in which genetic variation can be increased in sexually reproducing organisms.
Mutation, meiosis and fusion of gametes.

2. How is genetic variation increased in asexually reproducing organisms?

3. Which of the statements below apply to variation due to genetic factors and which due to environmental factors?
a) The ABO blood grouping in humans - GENETIC
b) It can be represented by a line graph - ENVIRONMENTAL
c) It is usually controlled by a single gene - GENETIC
d) It can be represented as a bar graph - GENETIC
e) A mean can be calculated – ENVIRONMENTAL
f) The length of body in rats – ENVIRONMENTAL

4. The mean blood pressure of a population is 125. The standard deviation is 10. If the population was 1000, how many of them have a blood pressure between 115 - 135?
680. Because 115 and 135 is one standard deviation from either side of the mean (as 115 is 10 below 125 and 135 is 10 above 125). We are told that 68% of the population would have to lie within one standard deviation of the mean in each direction. So 680 (68% of 1000) men should lie within 115-135.

5. Suggest a reason why the base pairings of adenine with cytosine and guanine with thymine DO NOT occur?
Because the bases are linked with hydrogen bonds and the molecular structures could be such that hydrogen bonds do not form with these pairs. The right pairings have different amounts of hydrogen bonds.

6. If 19.9% of the base pairings in human DNA are guanine, what percentage is thymine?
30.1%. Because if 19.9% is guanine hen, as guanine always pairs with cytosine, then cytosine will also make up 19.9%. So together they would make up 39.8%. So the remaining 60.2% must be adenine and thymine and, as these also pair, each must make up half of this (i.e. 30.1%)
7. What is a gene?
A section of DNA containing coded information for making polypeptides.

8. How many genes are required for making a chain of 6 consecutive amino acids?
18 (3 base pairs so 3x6)

9. Explain how the change in one base along a DNA might result in an enzyme become dysfunctional.
A different base might code for a different amino acid and so the sequence of amino acids in the polypeptide produced will be different. This change to the primary structure of the protein might result in a different shaped tertiary structure. Therefore the enzyme shape will be different and may not fit the substrate so no enzyme substrate complexes can be formed.

10. In which ways does the DNA from a prokaryotic cell differ from the DNA of a eukaryotic cell?
In prokaryotic cells the DNA is smaller, is linear and is no associated with proteins so does not have chromosomes.

11. Suppose the total length of all the DNA in one muscle cell is 2.3m.
a) If all the DNA were equally distributed between the chromosome, what would the length of DNA in each one be?
50mm as there is 46 chromosomes in every cell so you do 2.3÷46=0.05m which equals 55mm
b) What do you think the length of DNA is in a human brain cell?
2.3m as all cells have the same quantities of DNA

12. A mule is a cross between a horse (with 64 chromosomes) and a donkey (with 62 chromosomes). Mules therefore have 63 chromosomes. Suggest why mules cannot produce gametes and therefore are sterile.
Gametes are produced my meiosis and in meiosis homologous chromosomes pair up. With 63 chromosomes precise pairings are impossible. As meiosis is prevented so is gamete production.

13. Explain how a different in its DNA might lead to an organism having a different appearance and hence the species showing greater genetic diversity
Different DNA means different amino acids are coded for. Having different amino acids means a different protein shape which changes the feature determined by that protein and alters appearance leading to greater genetic diversity.

14. Explain how genetic bottlenecks reduce genetic diversity.
There is a drop is population numbers due to a chance event and so the few individuals that survive are likely to have fewer, less diverse alleles. As the population grows its alleles are equally less diverse and so there is a reduction in genetic diversity.

15. Describe the quaternary structure of haemoglobin
2 pairs of polypeptides link to form a spherical molecule. Each polypeptide has a haem group that contains a ferrous ion.

16. Explain how DNA leads to different haemoglobin having different affinities for oxygen
A different base sequence of DA means there is a different amino acid sequence and so a different tertiary and quaternary structure & shape so the haemoglobin has different affinities for oxygen

17. When at rest, only 1 of the 4 oxygen’s carried by the haemoglobin is released into the tissues. Why is this an advantage when the organism becomes more active?
Because if all oxygen molecules were released, there would be none in reserve to supply tissue when they were more active.

18. Using the graph below…
a) [IMG]file:///C:/Users/User/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg[/IMG]At what partial pressure of oxygen is the haemoglobin 50% saturated with oxygen?
5 kPa
b) What is the percentage saturation of haemoglobin with oxygen when the partial pressure of oxygen is 9 kPa?
c) In an exercising muscle the partial pressure of oxygen is 4 kPa. What percentage of the oxyhaemoglobin from the lungs will have released its oxygen to an exercising muscle?
70% (95%-25%)

19. What is the effect of an increased carbon dioxide concentration on oxygen disassociation? And how does this change the saturation of haemoglobin with oxygen?
The curve is shifted to the RIGHT. It causes the haemoglobin to become less saturated

20. A rise in temperature shifts the curve to the right. How could this enable an exercising muscle to work more efficiently?
Exercising muscles release heat, shifting the curve to the right this causes the haemoglobin to release more oxygen to fuel the muscular activity.

21. What 3 structures would you find in a plant cell but never in an animal cell?
Cellulose cell wall, chloroplasts, starch grains.

22. During demi conservative replication, if an inhibitor of DNA polymerase were introduced into a cell, what would the effect be on cell replication?
The linking together of new nucleotides could not take place. The nucleotides would link up but would not join together to form a new strand.

23. What is a tissue?
A collection of similar cells aggregated together to perform a particular function.

24. Why is an artery described as an organ but a blood capillary is not?
An artery is made up of more than one tissue whereas a blood capillary is made up of only one tissue.

25. Name 4 things that need to be exchanged between organisms and their environment.
Respiratory gases, nutrients, excretory products, heat.

26. Calculate the surface area to volume ratio of a cube that has sides that are 10mm long.
0.6. 10 x 10 = 100 x 6= 600. 10x10x10 = 1000. 600 ÷1000 = 0.6

27. How do insects prevent excessive water loss from their tracheal system?
They have valves that can close spiracles with the insect is inactive.

28. Why does the tracheal system limit the size of insects?
Because the tracheal system relies on diffusion to bring oxygen to the respiring tissues. If insects were large it would take too long for oxygen to reach the tissues rapidly enough to supply their needs.

29. What differences in the gills would be seen in an active fish opposed to fish that aren’t as active.
More gill lamellae, more gill filaments, large surface areas.

30. State the similarities between gas exchange in a plant leaf and gas exchange in a terrestrial insect.
No living cell is far from the external air, diffusion takes place in the gas phase, they need to avoid excessive water loss, and diffusion takes place through pores in their covering.

31. Give one advantage of having thick elastic tissue in arteries.
It allows recoil and hence maintains blood pressure. It also allows for a smooth and consistent blood flow.

32. Explain an advantage of having thick muscle walls in arterioles.
Muscles can contract which would constrict the lumen and therefore will control the flow of blood through the capillaries.

33. What forces tissue fluid out of the blood plasma in capillaries and into the surrounding tissues?
Hydrostatic pressure (due to the pumping of the heart)

34. When seedlings are transported they sometimes wilt and die. Suggest two possible reasons for this.
The root hair cells are delicate and may get broken. There may be air pockets rather than soil solution around the roots when transplanting.

35. Why is transpiration often described as a 'necessary evil'?
Transpiration is necessary because plants need to evaporate water in order to create a pull that pulls water up the leaves. Transpiration is the unavoidable result of plants having leaves adapted for photosynthesis (having a large surface area and many stomata). However these features lead to a considerable loss of water by transpiration.

36. When a potted plant is places inside a black polythene bag it's transpiration rate falls. Why?
There is Hugh levels of humidity in the bag as water vapour cannot escape, there is still air and it is in darkness so the stomata is closed.

37. What two modifications to reduce water loss are shared by both plants and insects?
Waterproof coverings and the ability to close the openings of the gas exchange system through stomata or spiracles.

38. Insects limit water loss by having a small surface area to volume ratio. Why is this not a feasible way of limiting water loss in plants?
Plants photosynthesise and so need a large surface area to capture light.

39. How does the rolling up of leaves help to reduce transpiration?
Water evaporating from the leaf (water vapour) is trapped and so the region within the rolled up leaf becomes saturated with water vapour so there is no water potential gradient between the inside and outside of the lead and therefore transpiration is reduced.

40. What are the two main things that all members of a species share?
They are similar to one another but different from members of other species. They are capable of breeding to produce fertile offspring.

41. What are the three features of a natural system of classification?
It is based on evolutionary relationships between organisms and their ancestors. It classifies species into groups using shared characteristics derived from their ancestors. It is arranged in a hierarchy in which groups are contained within larger groups with no overlap.

42. During DNA hybridisation, why is DNA heated?
To separate the two complimentary strands of DNA

43. Why is species recognition important in courtship?
It ensures mating only takes place between members of the same species as only they can produce fertile offspring.

44. Other than species recognition, what other functions does courtship perform?
Identification of a mate that is capable of breeding. Formation of a pair bond to raise offspring. Synchronisation of mating.

45. In which two ways does genetic diversity occur in living organisms?
Mutation or the recombination of existing DNA from two individuals.

46. What is a mutation?
A sudden change in the amount or arrangement of DNA that results in different characteristics.

47. Suggest a sequence of events, starting with a mutation and ending with cells being changed and unable to produce certain enzymes.
Mutation results in a change in base sequence of DNA and so a different amino acid sequence being coded for. This results in different polypeptides and so either a different or no protein is made. The protein could be an enzyme.

48. Distinguish between horizontal and vertical gene transmission.
Horizontal gene transmission is the transfer of DNA/genes from one species to another. Vertical gene transmission is the transfer of DNA/genes within a species from one generation to the next.

49. What is an antibiotic?
A substance produced by a living organism that kills or prevents the growth of other living organisms.

50. How do antibiotics kill bacteria?
They prevent the synthesis of cross lick she's in bacterial cell walls and as the cross linkages hold the cell wall together, without them the wall is weakened. Water will then enter the bacterium by osmosis and it will swell. Normally the cell wall will prevent swelling and hence prevent osmosis. It can no longer do this and so water continues to enter and the cell will burst - osmotic lysis.

51. How is antibiotic resistance transferred from species to species?
The transfer of DNA/genes for resistance during conjugation - horizontal gene transmission

52. Explain why antibiotic resistance is likely to increase the more antibiotics are used.
Antibiotics will only kill the non-resistant bacteria. This reduces competition and makes it easier for the resistant bacteria to survive and pass on the resistance to subsequent generations and to other species.

53. Explain how multiple-drug-resistive strains of bacteria may develop.
Drug-resistant strains of bacteria exchange DNA by conjugation (horizontal gene transmission) and so the bacteria exchanges genes for resistance. Some bacterial strains can then accumulate genes for resistance to many drugs.

54. Why are multiple-drug-resistant strains more likely to arise in hospitals?
Because many different types of antibiotics are used, in relatively large amounts. This creates a selection of pressure favouring multiple resistant strains.

55. What is meant by species diversity?
The number of different species and the proportion of each within a given area or community.

56. Explain why it is more useful to calculate a species diversity index rather than just to record the number of species present?
Species diversity measures both the number of species and the number of individuals and it therefore takes account of species that are only present in small numbers.

57. How has agriculture reduced species diversity?
The few species possessing desirable qualities are selected for and bred, other species are excluded as far as possible by culling or by the use of pesticides. Having many individuals of a few species means there is a low species diversity.

58. Why is there a reduction in species diversity when a forest is replaced by grassland for grazing sheep or cattle?
Because forests have many layers with many habitats and many different species. They therefore have a high species diversity. Grasslands have a single layer with feeder habitats, fewer selfies and therefore a lower species diversity.

59. Why does the loss of tropical rainforest have a greater effect on global biodiversity than the loss of any other ecosystem?
Because tropical rain forests have a greater number and variety of species than any other ecosystems and so they have the highest species diversity.

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