5.6.5 factors affecting photosynthesis

• carbon dioxide • water (water stress) • light intensity (energy source) • temperature

light intensity is necessary: - provides energy for light-dependent stage, which produces ATP & NADPH for calvin cycle - also causes stomata to open for gaseous exchange & transpiration to allow uptake of water limiting factor: - if constant suitable carbon dioxide concentration & favourable temperature - light intensity low = rate of photosynthesis is low - light intensity increases = rate of photosynthesis increases - if light intensity increases but photosynthesis rate doesn’t, then another factor is limiting

• GP cannot be reduced to TP as not hydrogen available from NADPH • TP levels fall & GP accumulates • TP levels fall = RuBP cannot be regenerated

• RuBP cannot accept carbon dioxide, so it accumulates • GP cannot be made • thus, TP cannot be made

involves many enzyme-catalysed reactions

if plants have sufficient water, carbon dioxide & light intensity = the rate of photosynthesis increases as temperature increases

• (most plants) growth rates may reduce due to photorespiration = oxygen competes with carbon dioxide for RuBisCO’s active site • reduces amount of carbon dioxide being accepted by RuBP = reduces quantity of GP & TP being produced, whilst initially caused accumulation of RuBP • due to lack of TP, RuBP cannot be regenerated

• enzymes may denature • reduce concentrations of GP & TP • eventually reduce concentration of RuBP as cannot be regenerated due to lack of TP

• transpiration stream has cooling effect on plant • water passing from xylem to leaves keeps plant cells turgid • turgid guard cells keep stomata open for gaseous exchange

• roots unable to take up enough water to replace water lost through transpiration • cells lose water & become plasmolysed • plant roots produce abscisic acid & if translocated to leaves, causes stomata to close = reduces gaseous exchange • tissues become flaccid & leaves wilt • rate of photosynthesis greatly reduces