C4 PLANTS Features to avoid photorespiration

WHAT IS A C4 PLANT? • Plants in which the Co2 combines with

RuBP to form the six carbon compound and then splits into 2 three carbon compound in the light independent stage, are C3 plants

• Maize plants are different

• The first compound that forms in the light dependent stage of photosynthesis is four carbon compound, these are called C4 plants.

WHY AVOID PHOTORESPIRATION • Enzyme rubisco catalyses the

reaction of CO2 with RuBP

• It also catalyses the reaction of oxygen with RuBP

• When this happens, less photosynthesis takes place and not anough Rubisco is left to combine with CO2

• This unwanted reaction is called photorespiration

• It happens most readily at high temperatures and light intensity

FEATURES TO AVOID PHOTORESPIRATION

• Tropical grasses- maize, sorghum, sugar cane have evolved with this method

• They keep the RuBP and enzyme rubisco away from high oxygen concentrations

• The cells with RuBP and rubisco are arranged around vascular bundles and are called bundle sheath cells BSC, which have no contact with outside air

FEATURES TO AVOID PHOTORESPIRATION

• CO2 is also absorbed by mesophyll cells which are in contact with the air

• These cells have enzyme, PEP carboxylase

• This catalyses combination of CO2 and the 3 carbon compound, PEP, Phosphoenolpyruvate

• The compound formed in this reaction is oxaloacetate

• Inside the cells, this is converted to malate which is passed to the BSC

• Now the CO2 is removed from the malate and given to RuBP by Rubisco, normally. The light independent stage then just continues.

• Enzymes in the C4 plants have higher optimum temperatures than in C3 plants., which is an adaptation to growing in hot climates• If the temperature gets lower the

enzyme loses around 70%, of its activity. • By contrast, the same enzyme in

peas, In C3 plants, was found to have an optimum temperature of 30 degrees.