Daily Exam Challenge: Biology (Higher)
Mark Scheme — 24 marks total
Higher TierMarking Instructions
- • Accept equivalent answers unless the mark scheme states otherwise.
- • M marks are for method — award even if the final answer is wrong.
- • A marks are for accuracy — only award if the method mark has been earned.
- • B marks are independent and can be awarded without other marks.
- • Where a range of answers is acceptable, this is indicated in the mark scheme.
*A student investigates the effect of surface area to volume ratio on the rate of diffusion using agar cubes of different sizes soaked in acid. (a) Describe how the experiment should be carried out. (b) Explain the expected results. (c) Relate this to why single-celled organisms do not need a transport system but multicellular organisms do.
Acceptable Answer
(a) Cut agar cubes containing indicator (e.g., phenolphthalein) to different sizes (1cm, 2cm, 3cm). Place in beaker of dilute acid. Time how long for the colour to fully change. Calculate SA:V ratio for each cube. (b) Smaller cubes (higher SA:V) will change colour faster because acid diffuses to the centre more quickly. Larger cubes take longer because the diffusion distance to the centre is greater. (c) Single-celled organisms have a large SA:V ratio — diffusion alone is sufficient for gas exchange and nutrient uptake. Multicellular organisms have a much smaller SA:V ratio. Diffusion alone would be too slow, so they need specialised transport systems (e.g., circulatory system) and exchange surfaces (e.g., lungs, villi) to move substances efficiently.
Evaluate the use of stem cells in treating human diseases. Discuss the scientific, ethical, and practical considerations.
Acceptable Answer
Scientific: embryonic stem cells can differentiate into any cell type, potentially treating conditions like diabetes (new insulin-producing cells), paralysis (new nerve cells), and heart disease. Adult stem cells are more limited but avoid some ethical issues. Therapeutic cloning produces patient-matched cells, reducing rejection. Ethical: embryonic stem cells require destruction of embryos, which some people believe have the right to life. Some religious groups oppose this. Adult stem cells and iPSCs (reprogrammed adult cells) avoid this concern. Practical: risk of tumour formation, immune rejection (unless patient's own cells used), currently very expensive, limited availability, regulatory hurdles. Long-term effects unknown. Conclusion: stem cells offer huge medical potential but ethical concerns and practical limitations must be balanced.
The graph shows how the population of a predator and prey species change over time in an ecosystem. A disease kills 50% of the predators at time X. Predict and explain the changes to both populations in the 12 months following time X.
Acceptable Answer
Immediately after X: predator numbers drop by 50%. With fewer predators, fewer prey are eaten, so prey numbers increase. As prey increases, there is more food for remaining predators, so predator numbers begin to recover. As predators increase again, prey numbers start to decrease. A new cycle establishes but it may take several cycles to return to the original equilibrium. The amplitude of oscillations may be larger initially. If predator numbers recover too slowly, prey may overshoot, potentially overgrazing and causing a population crash.