Ch 10 Calvin Cycle, Photorespiration, C3/C4/CAM plants

Occurs in the stroma CO 2 enters the cycle and leaves as sugar Spends the energy of ATP and NADPH Glucose not produced - yield is: glyceraldehyde-3-phosphate (G3P) Each turn of the Calvin cycle fixes 1C For net synthesis of one G3P molecule, cycle must occur 3X, fixing 3CO 2 To make one glucose molecule: 6 cycles and the fixation of 6CO 2

The Calvin Cycle

Evolution What is the biggest problem that organisms that live on land must conquer? Water loss. How do plants combat water loss?

How do the leaf structures help the leaf to function?

What happens to the guard cells when the plant is losing excessive water?

Transpiration Water evaporates out of the stomata. When the transpiration rate is too extreme, then the plant will close the stomata. The guard cell become flaccid, and stomata close. Besides removing water from the plant, what other purpose is served by the stomata? Gas exchange.

What environmental conditions will induce stomata closing? Hot, dry, sunny days. As the stomata close what gas accumulates in the spongy mesophyll? Oxygen, since the light reactions are still occurring. Rubisco, (RuBP carboxylase), is the enzyme that fixes CO 2 into RuBP. Rubisco can accept O 2 as well as CO 2 at its active site.

As concentrations of O 2 accumulate in the leaf, rubisco accepts oxygen in place of carbon dioxide. This process is called photorespiration. Problems? –Photorespiration drains as much as 50% of C from Calvin Cycle

How can you reconcile the fact that rubisco can accept either CO 2 or O 2 ? Rubisco evolved before there was an accumulation of O 2 in the atmosphere (no specificity) Due to the abundance of O 2 in the atmosphere a certain amount of photorespiration is inevitable. What is a C3 plant? –RuBisCo binds CO 2,splits product – 2C cmpd leaves chloroplast, molecule rearranged and CO 2 released –Most plants (rice, wheat, soy)

Photorespiration Adaptations Recall: –Photorespiration drains as much as 50% of C from Calvin Cycle Certain plants have developed adaptations to minimize photoresp. and maximize Calvin Cycle –C4 (spatial) –CAM (temporal)

C4 plants Very common pathway – sugarcane, corn Mesophyll cells incorporate CO 2 into organic molec (4C – oxaloacetate) PEP Carboxylase has a higher affinity for CO 2 than RuBisCo (and NO affinity for O 2 ) PEP pumps CO 2 into bundle sheath, so RuBisCo won’t bind to O 2

CAM plants The CAM plants are desert plants such as cacti. How do these plants solve the photorespiration problem? Temporal separation of CO 2 fixation and Calvin Cycle

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