External factors affecting photosynthesis

∆ Carbon dioxide
– needed as raw material in the dark reaction
– provides the carbon for organic molecular skeleton
– first fixed into a 3-carbon sugar PGA
– ultimately released as 6-carbon (hexose) molecule
– hexose may subsequently be incorporated into polymers (e.g starch) for storage
– carbon dioxide concentration above optimum do not result in increased photosynthesis
∆ Water
– needed as a source of electrons/hydrogen ions in the light reactions
– electrons are derived from photolysis of water
– used as a source of reducing power for electron transport chains
– electron transport chains generate ATP (energy)
– used in driving
– photolysis of water also releases oxygen as a by-product
∆ Light
– needed to provide energy for splitting of water
– Essential for the activation/excitation of the chlorophyll molecule
– initiates the sequence of the enzymatic reactions in the photosystems I and II (cyclic and noncyclic photophosphorylation)
– reaction generate energy (ATP) used in CO2 fixation
– increasing light intensity beyond the action spectrum will not increase rate of photosynthesis
∆ Temperature
– needed for optimum activity of enzymes
– extremely low temperatures slow down enzymes action
– very high temperature denature enzymes.