1. Variation between individuals of the same species (within a species)

2. The spread of the data about the mean

3. There is variation due to environmental factors

4. Nitrogen-containing base, deoxyribose and phosphate group

5. T

6. Multiple codons may encode the same amino acid

7. Non-coding sections of genes

8. Circular chromosome and plasmids

9. Crossing over between homologous chromosomes during prophase I; independent segregation of chromosomes during anaphase I

10. A sudden large decrease in a population due to a chance event

11. Small number of individuals have a very limited gene pool; breeding occurs but successive generations have a small variety of alleles, so genetic diversity is low

12. Small mammals have a large SA:volume ratio so lose heat easily; right-shifted oxygen dissociation curve means reduced affinity of Hb for oxygen, so oxygen is more readily unloaded at low pO2s, i.e. at respiring tissues, for respiration to release energy to keep the body warm

13. Lowers pH of blood; Hb acts as a buffer, accepting protons which causes it to change shape and lowers its oxygen affinity (the Bohr effect)

14. Beta-glucose

15. Cellulose is a straight-chain, allowing it to be packed into microfibrils and macrofibrils with hydrogen bonds between adjacent chains, which gives it tensile strength, making it ideal for support and structure of plant cell walls. By contrast starch is branched, allowing it to be more easily hydrolysed as enzymes can operate on all ends of the branches, and is coiled and compact so that a lot of it can fit into a small space (i.e. starch grains), making it ideal for energy storage.

16. Chlorophyll, which traps light energy for photosynthesis

17. Separates the double helix by breaking hydrogen bonds, allowing new bases to be added onto the template strands

18. Prophase, metaphase, anaphase, telophase

19. The centromere holds chromatids together and attaches them to opposite ends of the spindle. At anaphase it allows them to split and move towards opposite ends of the spindle.

20. Growth of the cell; synthesis of new organelles; replication of DNA; regular checkpoints to prevent the cell from dividing uncontrollably

21. Different types of tissues coordinated to carry out a function or variety of functions

22. Blood and water flow in opposite directions, so a concentration gradient of oxygen is maintained at all points along the gill lamellae and thus the maximum amount of oxygen can diffuse from the water into the blood.

23. Stomata allow gases to diffuse in and out; air spaces between spongy mesophyll cells maximise surface area for diffusion

24. Spiracles, tracheae, tracheoles (any two)

25. Thin leaves with a small surface area; sunken stomata trap water vapour; hairs trap water vapour; waterproof waxy cuticle stops water passing through

26. Hepatic vein

27. Elastic tissue smooths out blood pressure; blood in arteries at higher pressure than in veins

28. Muscle contracts to constrict lumen and control blood flow; arterioles connect to capillaries which are much smaller in volume, so lumen must be constricted more

29. Soluble so lower the water potential of the blood, causing water to move from the surrounding tissues into the capillaries by osmosis

30. Reabsorption into capillaries; drainage into lymph vessels/lymphatic system

31. Apoplast is through cell walls whereas symplast is through cytoplasm/vacuoles; in apoplast pathway water moves due to cohesion between molecules but in the symplast pathway it moves by osmosis; apoplast pathway prevented by Casparian strips/stops at endodermis but symplast unaffected by Casparian strips (any two)

32. Waterproof so force water in apoplast pathway into the symplast pathway at the endodermis

33. As water enters the xylem by osmosis the root pressure created forces water slightly up the xylem. Water vapour diffusing out through the stomata causes water to move from the xylem into mesophyll cells by osmosis, creating a pull on the continuous column of water in the xylem due to intermolecular cohesion. Adhesion between water and the xylem walls also pulls water molecules upwards.

34. Humidity, wind speed, temperature, light (any three)

35. Organisms grouped according to evolutionary history/lineage

36. Class

37. Single strand of DNA from two organisms fused to form a hybrid and then heated to break hydrogen bonds until the strands separated. The greater the sequence identity (i.e. the closer related the organisms) the more hydrogen bonds form between complementary base pairs and thus the closer the evolutionary relationship.

38. Allows an individual to recognise a mate of the same species, synchronises fertility and stimulates production/release of gametes

39. Mutation occurs in one bacterium giving it an allele for antibiotic resistance. Antibiotics kill the non-resistant bacteria but surviving bacteria survive and reproduce, passing on the allele by vertical gene transmission so that the frequency of the allele in the population increases. Horizontal gene transmission by conjugation may occur between bacteria of different species.

40. The passing on of genes by asexual reproduction (binary fission)

41. Conjugation tube/pilus extends from donor bacterium and fuses with recipient bacterium; plasmid is replicated and moves through the pilus into the recipient.

42. Monoculture causes a large area of land to be dominated by a single species; pesticides kill insects and other animals (disrupting food webs); forest cleared, hedgerows chopped down and wetlands drained – less variety of habitats and food sources for animals; deforestation leads to fewer plant species and machinery may kill animals; eutrophication disrupts aquatic ecosystems; predators culled due to their impact on livestock (any two)

43. Simpson’s index of diversity takes into account the number of organisms of each species, which is important as an ecosystem may have many species but could be dominated by just one or two and thus have a low index of diversity.

	Created by Stephen Cole over 10 years ago

A-Level Biology Note on Unit 2 Revision questions and answers: the variety of living organisms, created by Stephen Cole on 23/05/2014.

Unit 2 Revision questions and answers: the variety of living organisms