The questions in this revision test were more challenging than last time. I hope that players found them interesting and useful.
The plant transport questions at the start were well answered overall. Osmosis is the only way water can ever cross a cell membrane and although active transport does occur in the root hair cells (pumping mineral ions such as nitrates into the cell against the concentration gradient), water cannot be directly pumped against its concentration gradient using energy from respiration.
The cloning questions were difficult but I think the low scores here were perhaps more to do with problems with my school wifi than with your abilities to answer them! Micropropagation is the way that you learned when a small part of a plant is cut out, sterilised, washed and then added to a culture medium that triggers cell differentiation. You probably did this experiment with explants from a cauliflower. The aim was to produce whole new plants from these small explants. This technique could not work with animals simply because animal’s bodies contain many more types of tissue and have a more complex internal architecture that requires a much more sophisticated genetic programme of development.
I want to talk about a few questions in the latter stages of the test that were not well answered. I am sure there is plenty you can all learn from these.
The first was the one that asked you what was meant by a “diploid cell”. More than half of you thought that diploid meant having 46 chromosomes. This is almost a trick question because of course in humans, diploid cells will have 46 chromosomes. But diploid can be applied to any cell that has chromosomes found in homologous pairs. The number 23 is only important to humans as for our species it is the number of homologous pairs of chromosomes found in our diploid cells. Different species have differing numbers of pairs of chromosomes, some less than the number in humans but in many species they have more.
The second big idea question was the true or false question on whether energy is recycled in the ecosystem like carbon atoms. It is vital you understand that there is absolutely no recycling of energy ever in an ecosystem. Energy enters in the form of light energy being trapped by plants in photosynthesis and all this energy ultimately ends up as heat energy in the atmosphere. To find out the details of how it gets there, please read the relevant sections on my blog. Try the tag energy from the Tag cloud on the right of the screen.
There was one question in the quiz which not a single player answered correctly and it is the one about which type of cells produce antibodies. Antibodies are made from a cell called a plasma cell. Plasma cells secrete antibodies in large numbers to combat an infection. Plasma cells are descended from B lymphocytes that have been activated by the presence of antigen. This clonal selection theory is one of the most complicated bits in iGCSE Biology so make sure you have looked carefully at it. The final question was about active v passive immunity. This is not specifically mentioned in the specification so perhaps is a bit mean to include but if you can understand it properly, you understand how immunity works. Passive immunity is the name for when antibodies are transferred, perhaps across the placenta for a foetus or in an injection as an adult. Antibodies are made of protein and so do not exist for long in the blood – after a month or two they will all have been broken down and cleared from the blood. So passive immunity cannot give long-lasting protection. Active immunity is when memory cells are produced via a clonal selection response. These memory cells can survive for an entire lifetime and so do provide long lasting protection.
By far the biggest thing you can learn from this quiz however was about virus structure. I asked you whether “viruses are made from a different kind of cell not found in animals or plants – true or false.” Almost everyone went for false but remember this can’t be correct: viruses are definitely not made of cells! They are much simpler than even the simplest cell and just consist of a protein coat with some genetic material (DNA or RNA) inside. No cell membrane, no cytoplasm, no metabolism – just two chemicals associated into one simple particle.
Anyway I hope you enjoyed the quiz – look out for the next one on my Twitter feed and please use the comment facility on this blog to get in touch if you have any questions or want more explanations.