Edexcel A-Level Biology: Biological Molecules
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Water makes up the majority of the cytoplasm of living cells, and many of its biological roles arise directly from the shape and bonding of the water molecule.
Describe and explain how the structure of a water molecule gives rise to its properties, and how those properties make water important to living organisms. In your answer you should refer to hydrogen bonding and to at least three distinct biological roles of water.
(6 marks)
A student investigated the enzyme invertase (sucrase), which catalyses the hydrolysis of sucrose into glucose and fructose. A fixed mass of invertase was added to a fixed volume of sucrose solution at 30 °C, and the cumulative mass of reducing sugar produced was measured over time. The results are shown below.
| Time / s | Cumulative mass of reducing sugar produced / mg |
|---|---|
| 0 | 0 |
| 20 | 18.0 |
| 40 | 31.0 |
| 60 | 40.0 |
| 80 | 45.0 |
| 100 | 47.0 |
(a) Calculate the mean rate of reaction, in mg s⁻¹, over the first 40 seconds. Show your working and give the units. (2 marks)
(b) The mean rate over the interval from 60 s to 100 s is much lower than the rate you calculated in part (a). Explain why the rate of reaction falls during the investigation, even though the temperature and the amount of enzyme are unchanged. (4 marks)
The enzyme lipase hydrolyses triglycerides to fatty acids and glycerol. A scientist measured the initial rate of reaction of a fixed concentration of lipase at a range of substrate (triglyceride) concentrations, keeping all other conditions constant. The results are shown below.
| Substrate concentration / mmol dm⁻³ | Initial rate of reaction / μmol min⁻¹ |
|---|---|
| 0.5 | 12 |
| 1.0 | 22 |
| 2.0 | 36 |
| 4.0 | 45 |
| 8.0 | 48 |
| 16.0 | 48 |
(a) Calculate the percentage increase in initial rate when the substrate concentration is raised from 2.0 mmol dm⁻³ to 4.0 mmol dm⁻³. Show your working. (2 marks)
(b) Using the data, explain why the initial rate stops increasing at high substrate concentrations. (3 marks)
A species of fish lives in deep, near-freezing polar water. Researchers found that the phospholipids in its cell-surface membranes contain a very high proportion of fatty acid tails with many C=C double bonds (highly unsaturated), whereas a related fish from warm tropical water has membrane phospholipids with mostly saturated fatty acid tails (no C=C double bonds).
A cell-surface membrane must stay fluid enough for the membrane to function at the temperature the organism experiences.
Suggest and explain why the cold-water fish benefits from having highly unsaturated fatty acid tails in its membrane phospholipids. (5 marks)
In cells, ATP acts as the immediate source of energy for processes such as active transport and muscle contraction. ATP is made from ADP and inorganic phosphate (Pi) and is broken back down to ADP and Pi when energy is released.
A newly developed antibiotic kills certain bacteria by permanently blocking the enzyme that joins ADP to inorganic phosphate, so that the bacteria can no longer make ATP, although their existing ATP is unaffected at first.
Explain why this antibiotic causes the affected bacterial cells to stop carrying out energy-requiring processes shortly after the drug is applied. (4 marks)
DNA is a polymer made from many nucleotide monomers joined together.
(a) Name the three components that make up a single DNA nucleotide. (2 marks)
(b) Name the type of bond that joins one nucleotide to the next in a single polynucleotide strand. (1 mark)