Two stages – one of which requires light and the other does not.
The photosynthetic process –light dependent phase
A molecule of chlorophyll is excited by a photon of light and an electron ‘escapes’. This energy is used to split molecules of water (H 2 O) into their component hydrogen and oxygen. The oxygen is released as a by-product into the atmosphere.
The energy released by breaking the chemical bonds in the water is incorporated, with the hydrogen, into energy carrying molecules called ATP and NADPH2
Light – only part of the wavelength of light from the sun is used (PAR – photosynthetically active radiation).
Carbon dioxide – more than enough of this in the open air, but under glass may become depleted.
Water –lack of water leads to the stomata closing, preventing carbon dioxide entering the leaf.
Temperature – the chemical reactions in photosynthesis are temperature sensitive. Ideal range 25 °C - 36°C.
Nutrients – nutrients from the soil are needed to make chlorophyll. E.g. magnesium deficient plants photosynthesise poorly.
At low light intensity, rate of photosynthesis is proportional to light intensity. plateau
plateau At low CO 2 concentration, rate is positively correlated with concentration CO 2 is a substrate in an enyme-catalysed light-dependent reaction.
Increased temp. gives increased energy and increased rate of photosynthesis Optimum temperature Above the optimum temp., enzymes are denatured and rate drops steeply. Which enzymes are used in respiration?
Xylem – transports water from the roots to the shoots; enters by osmosis and is drawn up by transpirational pull.
Xylem cells are long, wide and open ended; dead once mature. Linked end to end and also laterally through pores called pits
Effects of humidity and temperature on transpiration
High humidity =lower transpiration because the rate of diffusion will fall. This is why greenhouse growers damp down the floor of the greenhouse in hot weather.
The higher the temperature outside the leaf the more quickly water in the spaces in the spongy mesophyll will evaporate. The more water vapour within the leaf the faster the rate of transpiration through diffusion.