Free practice · VCE Units 3 & 4
These 10 VCE Biology practice questions cover Units 3 and 4: nucleic acids and proteins, enzymes and biochemical pathways, immunity, genetic change and evolution, and molecular tools. They are original, VCAA-style questions written by ATARMAxxing, not official VCAA material, and we are not affiliated with or endorsed by the VCAA. Every question is answerable without a diagram. Work through each before checking the mark-by-mark scheme.
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Describe the role of mRNA and tRNA in the process of translation. (3 marks)
1 mark: mRNA carries the genetic code (sequence of codons) from the nucleus to the ribosome / acts as the template read during translation. 1 mark: tRNA carries a specific amino acid and has an anticodon that is complementary to a codon on the mRNA. 1 mark: tRNA anticodons base-pair with mRNA codons at the ribosome so amino acids are added in the correct order to form the polypeptide. Total: 3 marks.
A signal peptide is found at the start of many newly synthesised polypeptides destined for secretion from the cell. Explain the function of this signal peptide and outline the path the protein follows from synthesis to secretion. (4 marks)
1 mark: the signal peptide directs the ribosome/polypeptide to the rough endoplasmic reticulum (RER), so the protein enters the endomembrane system. 1 mark: the protein is synthesised into / processed within the RER. 1 mark: vesicles transport the protein from the RER to the Golgi apparatus, where it is modified/packaged. 1 mark: the protein is packaged into a secretory vesicle that fuses with the plasma membrane, releasing the protein by exocytosis. Total: 4 marks.
Competitive and non-competitive inhibitors both reduce the rate of an enzyme-catalysed reaction. Distinguish between the way each type of inhibitor acts on the enzyme. (4 marks)
Competitive inhibitor (2 marks): 1 mark for binding to the active site of the enzyme; 1 mark for competing with the substrate / its effect can be reduced by increasing substrate concentration. Non-competitive inhibitor (2 marks): 1 mark for binding to a site other than the active site (an allosteric site); 1 mark for changing the shape of the active site so the substrate can no longer bind effectively / its effect is not overcome by increasing substrate concentration. Total: 4 marks.
Explain why a 10 degree Celsius rise in temperature above an enzyme's optimum can cause a sharp decrease in the rate of reaction, while the same 10 degree rise below the optimum increases the rate. (3 marks)
1 mark: below the optimum, raising temperature increases the kinetic energy of substrate and enzyme molecules, so there are more frequent successful collisions and the reaction rate increases. 1 mark: above the optimum, the increased energy disrupts the bonds maintaining the enzyme's tertiary structure, causing denaturation / a change in the shape of the active site. 1 mark: the substrate can no longer bind to the altered active site, so the rate of reaction falls sharply. Total: 3 marks.
Photosynthesis and aerobic cellular respiration are described as complementary biochemical pathways. Using the inputs and outputs of each, justify this description. (4 marks)
1 mark: stating photosynthesis inputs are carbon dioxide and water (with light energy) and outputs are glucose and oxygen. 1 mark: stating aerobic respiration inputs are glucose and oxygen and outputs are carbon dioxide and water (with ATP/energy released). 1 mark: identifying that the outputs of one pathway are the inputs of the other (the products of photosynthesis are the reactants of respiration, and vice versa). 1 mark: linking this to a cycling of matter (carbon, oxygen) between the two pathways, justifying the term complementary. Total: 4 marks.
Distinguish between the role of a phagocyte and the role of a plasma cell in defending the body against a bacterial infection. (4 marks)
Phagocyte (2 marks): 1 mark for being part of the innate/non-specific immune response; 1 mark for engulfing and destroying pathogens by phagocytosis (and/or presenting antigens). Plasma cell (2 marks): 1 mark for being part of the adaptive/specific immune response, formed from activated B cells; 1 mark for secreting large numbers of antibodies specific to the pathogen's antigen. Total: 4 marks.
Explain why a person who has recovered from a particular viral infection usually responds more quickly and strongly if exposed to the same virus a second time. (3 marks)
1 mark: during the first (primary) exposure, memory B and T cells specific to the virus's antigen are produced and persist after the infection clears. 1 mark: on a second exposure, these memory cells recognise the same antigen and are rapidly activated / proliferate, so the secondary response has a shorter lag period. 1 mark: the secondary response produces a higher concentration of antibodies (or eliminates the pathogen faster), often before symptoms develop. Total: 3 marks.
A new pesticide is applied to a large population of insects. Within a few generations the pesticide no longer controls the population. Using the principles of natural selection, explain how this resistance arose and spread. (4 marks)
1 mark: there was pre-existing genetic variation in the population, with a few insects carrying an allele(s) conferring resistance to the pesticide (variation due to mutation, present before the pesticide was applied). 1 mark: the pesticide acts as a selection pressure, killing non-resistant insects while resistant individuals survive. 1 mark: surviving resistant insects reproduce and pass the resistance allele(s) to their offspring (differential reproductive success / heritability). 1 mark: over successive generations the frequency of the resistance allele increases in the population, so the population becomes resistant. Total: 4 marks.
Explain how the formation of a geographical barrier can lead to the development of two separate species from one original population. (3 marks)
1 mark: the barrier separates the original population into two (or more) populations, preventing gene flow / interbreeding between them. 1 mark: different selection pressures, mutations and/or genetic drift cause the allele frequencies of the two populations to change in different ways over time. 1 mark: the populations become so genetically different that, even if reunited, they can no longer interbreed to produce fertile offspring, so they are separate species (allopatric speciation). Total: 3 marks.
A forensic scientist has a very small DNA sample from a crime scene and needs to compare it with samples from several suspects. Name a molecular tool used to increase the amount of DNA, and explain how gel electrophoresis is then used to compare the samples. (4 marks)
1 mark: naming the polymerase chain reaction (PCR) as the tool used to amplify / make many copies of the DNA. 1 mark: stating that gel electrophoresis separates DNA fragments according to size (or length). 1 mark: explaining that an electric current moves the negatively charged DNA fragments through the gel, with smaller fragments travelling further than larger ones. 1 mark: explaining that the banding pattern (positions of fragments) produced for each sample is compared, and a match between a suspect's pattern and the crime-scene pattern indicates the DNA may be from that person. Total: 4 marks.
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