Question 2a.  "Wht is the major problem that can affect the efficiency of photosynthesis in plants growing in the humid tropics? Can any plants overcome this difficulty?

    This question was less popular than 2b and the answers were generally not as good. What is special about the humid tropics? they are hot but wet, So there is temperature stress, possible light stress  but NO water stress.  The temperature stress is further exacerbated by the low level of evaporative cooling possible due to the high air humidity. Contrary to what many of you suggested low levels of carbon dioxide are NOT likely to be a special problem in this environment! Particularly inside the tropical rain forest the high levels of biomass with consequent high levels of respiration lead to levels of CO2 30 to 40% higher than the general atmospheric concentration. In any case all atmospheric levels of CO2 are sub-optimal for photosynthesis and minor fluctuations would be unlikely to be major problem specific to this habitat.  Gas exchange is not going to be a big problem either, because there is no water stress so the stomata are not going to close and this will not create a barrier to gas exchange.The only exception to this would be in epiphytes which would also use CAM which some of you mentioned, but unless these remarks were made with specific reference to epiphytic growth habit they were not really relevant. The major problem is clearly the high temperature which can give very high levels of photorespiration. Remember that the rate of photorespiration increases more rapidly than carbon fixation as temperature increases.
    A useful answer should then give a brief outline of the mechanics of C4 photosynthesis and explain how it can overcome the problem of photorespiration, detailing the carbon fixation step + PEP, transport of C4 acids to the gas-tight bundle sheath and the seperation of PS I and PSII to reduce oxygen generation inside the bundle sheath.  A description of C4 photosynthesis and how it relates to the tissues of the plant is greatly simplified by the use of diagrams but unfortunately very few of you took advantage of this option.   In comparing the efficiency of C3 to C4 it is of course necessary to point out that although C4 doesn't lose so much energy to photorespiration there is an energetic burden for the transport of CO2.
 

Question 2b.  "Translocation in plants requires energy but the transport of water does not". Is this statement true?

    This question was quite popular and most answers were able to give a reasonably accurate account of the differences between translocation and water transport. This satement is not a simple question of being right or wrong however. Most answers agreed with the statement on the basis that translocation requires energy for phloem loading while the xylem  is a dead tissue which can still function effectively for water transport. It is however perfectly possible to argue that in fact water transport does require energy, the law of conservation of energy tells us this! except that this energy comes from the sun and not from the plant. Equally it can be said that the actual movement of solutes in translocation doeas not require energy since it is only the creation of the water potential gradient that consumes energy. So this question can actually be answered in more ways than and I  was pleased to receive some answers that did give consideration to these points.

Most of the answers would have been helped by more structure in your essays, don't be afraid to use paragraphs and sub-headings. Not only does it make an answer easier to read but it will help you organise your thinking.

Common mis-spellings, Pholem for phloem,  Surcose for sucrose