Describe how plants carry out gas exchange in the leaves.

Outline the causes and consequences of the enhanced greenhouse effect.

Explain the role of limiting factors in photosynthesis.

gases/O₂ and CO₂ enter/exit the leaf through the stomata;
by diffusion / down the concentration gradient;
photosynthesis maintains concentration gradients/high O₂ and low CO₂ in the leaf;
guard cells open the stomata during the day / close the stomata at night;
gases/O₂/CO₂ move through air spaces in the spongy (mesophyll);
CO₂ dissolves in moisture in (mesophyll) cell walls;

burning of (fossil) fuels/coal/oil/gas releases carbon dioxide;
deforestation/loss of ecosystems reduces carbon dioxide uptake;
methane emitted from cattle/livestock/melting permafrost/waste dumps;
heating of the atmosphere/global warming/climate change;
melting of ice caps/glaciers/permafrost / sea level rise / floods / droughts / changes in ocean currents / more powerful hurricanes / extreme weather events / other abiotic consequence;
changes in species distributions/migration patterns / increased decomposition rates / increases in pest/pathogen species / loss of ice habitats / other biotic consequence;

factor nearest its minimum/furthest from its optimum is limiting;
increasing a limiting factor with other factors constant increases the rate;
increasing a non-limiting factor with other factors constant has no effect on rate;
light intensity is limiting in dim/low intensity light / at night;
photosynthesis (directly) proportional to intensity up to plateau / graph to show this;
light intensity affects the light-dependent reactions/production of ATP/NADPH;
temperature limiting at low and high temperatures;
optimum temperature with lower rates above and below plateau / graph to show this;
low temperatures limit the rate of light-independent reactions/Calvin cycle;
RuBP carboxylase/rubisco does not fix carbon dioxide at high temperatures;
carbon dioxide concentration is limiting in bright light and warm temperatures;
photosynthesis is (directly) proportional to CO₂ concentration up to plateau / graph to show this;
low CO₂ concentration limits carbon fixation/reaction between CO₂ and RuBP;

This question was based on assessment statement 9.1.3, which includes the relationship between the structure of the leaf and its role in gas exchange. All that was needed was an outline of the structure of the spongy mesophyll, guard cells and stomata, in relation to the diffusion of carbon dioxide into the leaf and oxygen out. Scores were typically poor, with many candidates missing the basic points. More candidates for example for example seemed to state that CAM plants open their stomata for gas exchange at night than that most plants open their stomata in the day. 

Scores were mostly much better in this part of the question, with nearly all candidates at least mentioning warming due the enhanced greenhouse effect and an example of the abiotic and biotic consequences. The cause of the enhanced greenhouse effect was less well understood, with vagueness about what is causing carbon dioxide levels to increase and other greenhouse gases often not mentioned. There was considerable confusion, as so often, between ozone depletion and the greenhouse effect. It is easy to assume that candidates will be able to distinguish between these two phenomena easily and that little teaching is required, but all those who marked this exam will know that careful teaching is very much required.

This was another area of relatively poor understanding, perhaps because weaker candidates tended to choose question 7. A basic minimum was to know that light intensity, temperature and carbon dioxide concentration are the three main limiting factors of photosynthesis. Many failed at this first hurdle, omitting one or more of the main three and including instead pH, water availability or various other biotic and abiotic factors. Perhaps some candidates were confusing enzyme activity with photosynthesis. What was required for each of the three factors was a clear statement of the relationship between the level of the variable and the rate of photosynthesis, ideally by means of an annotated sketch graph, and then some details of the reasons for the rate of photosynthesis changing as the level of the variable changed. A common misconception was to say that the rate reduces at higher temperatures because of enzyme denaturation when in fact the rate reduction occurs at much lower temperatures than those at which this would happen. The problem at higher temperatures is due to RuBP carboxylase failing to fix carbon dioxide effectively.