PAPER 3 - June 19th 2024

• When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob. Describe how. [3 marks]

  1. Nerve impulse/depolarisation of membrane causes Calcium ion channel (proteins) to open
  2. Ca2+ enter by facilitated diffusion
  3. Causes synaptic vesicles to fuse with presynaptic membrane
• Use your knowledge of how myosin and actin interact to suggest how the myosin molecule moves the mitochondrion towards the presynaptic membrane [2 marks]

  1. Myosin head attaches to actin AND bends/performs Powerstroke OR myosin head changes shape / angle
  2. This pulls mitochondria past/along the actin
  3. Othernext myosin head attaches to actin and bends/performs powerstroke

  
• This movement of mitochondria happens when nerve impulses arrive at the synapse. Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse. [2 marks]

  1. (Mitochondria) supply (additional) ATP/energy
  2. To move vesicles/for active transport of ions/for myosin to move past actin OR Re-synthesis/reabsorption of neurotransmitter/named neurotransmitter;
• With some samples, the scientists decided they needed to carry out a series of dilutions of the sample before counting the bacteria. Use evidence from Figure 3 to explain why dilutions were necessary for some samples but not for others. [2 marks]

  • MP1 - USING THE LOGARITHMIC SCALE = Log scale on graph shows big RANGE in number of bacteria OR Use of suitable data from log scale to give the range in number of bacteria;
  • MP2. Some samples too many to count (so dilute) but some countable (so don’t dilute)
• Using only Figure 3, what can you conclude from these data about the effectiveness of the bacteriophage in treating this lung infection in mice? [3 marks]

  This question uses RANGES in graphs =

  • MP1 - State the obvious - bacteriophage does reduce the number of bacteria
  • MP2 - USING RANGE - range does not overlap, so reduces bacteria in ALL MICE
  • MP3 - The range is quite huge - BIG range of effect / some mice has a big reduction
  • OR Does not bring down bacteria to 0 in any mice / works for some mice but not for all
• Use the data in Figure 4 to evaluate the use of artificial fertilisers on grain-producing crops in India. [2 marks]

  1. Using more but getting less response over time
  2. The graph shows correlation but doesn’t prove changes in yield due to fertiliser/but there could be other factors
  3. Becomes less cost effective with time;

  • IDEA OF OVER TIME IS IMPORTANT (Q stem states between 1970 and 2005)

  
• Outline a method the ecologists could have used to determine the plant species richness at one site. [3 marks]

  • A method of selecting sampling sites at random
  • Grid with coordinates selected using random number table
  • Use of quadrat
  • Identify (plant) species (at site/in each quadrat) OR Count number of (different plant) species (at site/in each quadrat)
• When ATR phosphorylates other enzymes, these enzymes become able to bind to their substrates. Use your knowledge of enzyme structure to suggest why.

  1. Changes tertiary structure of the enzyme
  2. Enzyme ACTIVE SITE formed/able to be formed/active site becomes COMPLEMENTARY;
• The student wanted to determine the rate of water loss per mm2 of surface area of the leaves of the shoot in Figure 5. Outline a method she could have used to find this rate. [3 marks] = Q asks working out the area of a leaf

  1. Method for measuring area; eg draw round (each) leaf on graph paper and count squares
  2. Of both sides of (each) leaf
  3. Divide rate of water loss/uptake from potometer by (total) surface area (of leaves);
• The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant. Suggest one reason why. [1 mark]

  • Plant has roots OR xylem cells very narrow

  Markscheme patterns: when mentioning shoot, do not forget roots
• Aquaporins are channel proteins that allow the diffusion of water across membranes. One type of aquaporin, called PIP1, can also transport carbon dioxide molecules across membranes.

  1. Both small/similar size so fit channel
  2. Have a similar shape so bind to fit channel

  
• These trees all foreign DNA inserted. This DNA led to the production of single-stranded RNA that inhibited expression of the gene for PIP1. Using this information, what can you conclude about the effect of the foreign DNA in the transgenic poplar trees? = RNA interference - regulation of translation

  1. Single-stranded RNA (has base sequence) complementary to PIP1 mRNA
  2. Binds to mRNA (of PIP1)/leads to destruction of mRNA
  3. Prevents/reduces translation of PIP1
  4. Reduces photosynthesis/named process that uses water (because PIP1 transports water)
• When counting, cells that touch top or left lines are counted but cells that touch right or bottom lines are not counted.

  Suggest two reasons for this rule [2 marks]

  1. To avoid dealing with parts of cells;
  2. To avoid counting same cells twice / more than once
  3. To be consistent / get comparable results

  • Accept more accurate results
• M. luteus is not resistant to chloramphenicol.

  Suggest two reasons why the bacteria were able to grow in the culture containing chloramphenicol. (Bacteria were able to grow in the antibiotic, even though it is not resistant) [2 marks]

  1. The concentration (of chloramphenicol) was to low/dilute to kill all bacteria OR there were too many bacteria for antibiotic to kill
  2. Contamination with other resistant bacteria (think aseptic techniques)
  3. Mutation occurred (so allele for resistant bacteria formed)
  4. Antibiotic is not evenly mixed in the liquid culture / bottle (agar broth aseptic technique)
• Suggest how SCFR (AO3) is destroyed by a lysosome. [2 marks]

  • The vesicle FUSES/ binds WITH a LYSOSOME
  • Lysozyme hydrolyses / digests SCFR OR Hydrolytic enzymes breakdown / hydrolyses / digests SCFR

  
• Suggest how production of these proteins give the result seen in C-KIT+ group (INCREASING in ventricular blood pressure)

  1. (Connexin-43) allows impulses to pass to the bottom/apex of the heart/ventricles OR
  2. (Connexin-43) allows impulses to pass through Purkyne tissue/the bundle of His
  3. GATA-4 = More actinomyosin 'cross' bridges // More binding sites on actin // More myosin filaments / heads
  4. OR more binding of myosin to actin

  
• In 1997, the scientists recorded the highest species richness, but the lowest value of index of diversity over the 29 years.

  Describe and explain how these results for 1997 were possible.[2 marks]

  1. There were many/48 (different) species OR (1997 had) the highest number of (different) species;
  2. However one/a few species were present in very large numbers OR
  3. Most species were present in (very) small numbers;
• New cardiomyocytes increasing ventricular pressure:

  • Connexin-43) allows impulses to pass to the bottom/apex of the heart/ventricles
  • OR allows impulses to pass through Purkyne tissue / the bundle of His
  • So increases ventricular pressure
• • GATA-4 transcriptional factor that stimulates expression of genes for actin and myosin
  • This causes increase in ventricular pressure because more muscle contraction but in A-level terms:
  • More actinomyosin bridges
  • More binding sites on actin
  • More myosin filaments/heads;
  • OR more binding of myosin to actin
• Give two types of cell that can stimulate an immune response

  1. Pathogen OR Cells from an organism of a different species;
  2. Cells from other organisms of the same species;
  3. Abnormal body cells // cancer cells
  4. Antigen-presenting cells // cells infected with virus