EDEXCEL B A LEVEL BIOLOGY PAPER 3 GENERAL MARK SCHEME ANSWERS

EDEXCEL B A LEVEL BIOLOGY PAPER 3 GENERAL MARK SCHEME ANSWERS aseptic techniques to transfer bacterial cells growing on an agar gel plate to a tube containing a sterile broth - ANS--disinfect surfaces -work near bunsen flame -flame top of tube -heat wire loop -open lid of petri dish to small extent -allow wire loop to cool locust dissection - ANS-1. pin the locust 2. cut body open 3. submerge under water a student investigated the effect of different gases on the breathing rate of a locust. they blew exhaled air over a locust in a syringe and recorded its breathing rate. The student concluded that the carbon dioxide increased the breathing rate. What are the limits to the validity of the investigation - ANS--only one carbon dioxide concentration was tested -the experiment should be repeated to see if results are not anomalous -the locust should be allowed to recover because it is still affected by the exhaled air and needs to return to normal -exhaled air is warmer which could affect breathing rate -exhaled air contains other gases, not just carbon dioxide -needs a control with (normal) atmospheric air why do single cell organisms not have a gas exchange system - ANS-they have a large surface area to volume ratio so diffusion is sufficient due to a short diffusion pathway role of air sacs in gas exchange system of locusts - ANS--supply of oxygen -so respiration can occur in large/active locusts how to obtain more accurate results using quadrats - ANS--use larger quadrat -sample a larger area -use random number generator -achieve a consistent measure of the mean/use of running mean How to measure a named abiotic factor during sampling - ANS--eg light intensity -use of a light meter -measure in each quadrat/ measure in same place/ at same time of day how is urea produced - ANS--remove amino group from amino acids (deamination) -ammonia combines with carbon dioxide -ornithine cycle produces urea glomerulus - ANS-cluster of blood capillaries enclosed by the bowman's capsule role of bowman's capsule - ANS-involved in the excretion of urea in mammals name the process by which filtrate is produced from the bowman's capsule - ANSultrafiltration molecules in blood plasma of kidney vs filtrate produced in bowman's capsure - ANS-1. 0 protein in filtrate, but present in blood plasma, as protein is too big to pass through 2. same amount of molecules and ions in blood plasma and filtrate as they are small enough to pass through how to calculate how many more times urea becomes concentrated compared with chloride ions - ANS-1. increase in concentration of urea 2. increase in concentration of chloride 3. urea increase/chloride increase why is glucose concentration in the bladder higher - ANS--glucose has been selectively reabsorbed -by active transport -in the proximal tubule How is the loop of henle involved in the production of concentrated urine - ANS-1. sodium ions are moved out of the ascending limb by active transport 2. meanwhile, the ascending limb is impermeable to water 3. this results in a lower water potential in the medulla 4. The loop of henle acts as a counter-current multiplier 5. the collecting ducts are permeable to water 6. so water moves out of the collecting ducts by osmosis How to modify respirometer to find out the effect of fluoride ions on the anaerobic respiration of yeast - ANS--use yeast -glucose for yeast -coloured liquid put into tubing -scale/ruler -use apparatus to measure the carbon dioxide released -use a range of fluroide concentrations -control temp/volume of mixture/mass of yeast -allow yeast to acclimatise with the three-way tap open -wait for lag phase to finish before measure the rate of reaction -statistical analysis such as t test for seeing a difference in means -calculate standard deviation -calculate mean -distance/time why antibiotics aren't used to treat viruses - ANS--viruses are non living -anti viral drugs used because they inhibit replication of viruses mosquitoes are injected with dna, from gm mosquitoes. this dna contains a gene for fluorescence. how will this procedure help to produce large numbers of gm mosquitoes? - ANS-identify mosquitoes that fluoresce -as these mosquitoes contain the gene -allow these gm mosquitoes to interbreed -repeat for several generations -offspring with the gene for fluorescence are selected to produce a population of gm mosquitoes (selective breeding) Why should you use mosquito nets even if you've already contracted the zika virus - ANS--prevent (people with zika) being bitten -therefore preventing the spread of zika/increase in infected mosquitoes/biting uninfected people