Photsynthesis and respiration

Agree with above - for the first part, you should talk about light intensity being constant, this can be monitored using a light meter (that's literally what they're called, it's a bit weird!) and extinguishing all external light sources, temperature, which can be monitored using a heat screen, humidity (as water is used in photosynthesis), and maybe the time you're experimenting for, as the longer you leave the experiment the more photosynthesis will be done! For the second part, cellular respiration uses up Oxygen produced in Photosynthesis and provides CO2, which can then be used in photosynthesis. This means that in areas with low CO2 concentration, the plants can still photosynthesise a little provided they can respire and there's enough water around. Rate of photosynthesis increases with CO2 concentration if that is the limiting factor, which means that increased cellular respiration could equal increased photosynthesis.

The variables are probably the same as for most experiments with plants - so light intensity, length of exposure to light, temperature etc.
Not too sure on the second set of marks, but I think I would talk about how respiration will provide CO2 which can be used in photosynthesis etc.

Would you be able to name 6 controls if I gave you the experiment details? Details here: you cut a pond weed to ten centimetres, put it in a beaker of NaHCO3 and then in a beaker of tap water. positioned the lamp 15 cm away and then waited ten minutes before taking three 30 second measurements of bubbles from the stem.

You would only have one control, so I assume that you meant variables? I'm not just going to do it for you, but everything should be kept constant for valid results, especially anything that effects photosynthesis. You should think about things like the temperature of the water, whether or not the water is distilled, length of time in the NaHCO3 etc.