Photosynthesis - Calvin cycle & C3/C4/CAM plants (Chapter 10)

The alternative pathways of photosynthesis using the C4 or CAM systems are said to be compromises. Why?

Both minimize photorespiration but expend more ATP during carbon fixation

CAM

During the night CO2 is stored in organic acids and released during the day. the guard cells are closed during the day and open at night when there is less water loss due to evaporation. the slowest and most inefficient in converting sun energy into glucose but the most efficient at preventing water loss. found often in desert plants.

In C3 plants the conservation of water promotes _____.

Photoresperation

Carbon fixation involves the addition of carbon dioxide to _____.

RuBP

Quantifying the inputs of ATP and NADPH and output of Pi

The Calvin cycle requires a total of 9 ATP and 6 NADPH molecules per G3P output from the cycle (per 3 CO2 fixed). In Phase 2, six of the ATP and all of the NADPH are used in Phase 2 to convert 6 molecules of PGA to 6 molecules of G3P. Six phosphate groups are also released in Phase 2 (derived from the 6 ATP used). In the first part of Phase 3, 5 molecules of G3P (1 phosphate group each) are converted to 3 molecules of R5P (also 1 phosphate group each). Thus there is a net release of 2 Pi. In the second part of Phase 3, 3 ATP molecules are used to convert the 3 R5P into 3 RuBP. Note that in the entire cycle, 9 ATP are hydrolyzed to ADP; 8 of the 9 phosphate groups are released as Pi, and the ninth phosphate appears in the G3P output from the cycle.

Which of the following reactions ensures that the Calvin cycle can make a continuous supply of glucose?

The regeneration of RuBP ensures that the Calvin cycle can proceed indefinitely, since RuBP fixes carbon dioxide into an organic molecule that is used to produce sugar.

Why are C4 plants able to photosynthesize with no apparent photorespiration?

They use PEP carboxylase to initially fix CO2.

Compared to C3 plants, C4 plants _____.

can continue to fix CO2 even at lower CO2 concentrations and higher oxygen concentrations

In C4 and CAM plants carbon dioxide is fixed in the _____ of mesophyll cells.

cytoplasm

CAM plants keep stomata closed in the daytime, thus reducing loss of water. They can do this because they _____.

fix CO2 into organic acids during the night

Malate

stable 4 carbon form in C4 and CAM pathways that is then turned back into CO2

C4

the C4 plant uses PEP to capture the CO2 molecules and move them into the chloroplast Active transport. It has anatomical adaptations which transfer CO2 into 4 carbon molecules that increases the flow of CO2 into the leaf. This allows for smaller Stomata (guard cell openings) therefore decreasing the loss of water. spends more ATP per molecule of CO2. found in tropical plants

C3

when hot the plant shifts to photoresperation to decrease water loss. most energy efficient but in warm dry conditions there are large losses to photoresperation and C4 has the advantage. most plants such as flowers and trees are C3

After 3-PGA is phosphorylated, it is reduced by _____.

NADPH

In the Calvin cycle, how many ATP molecules are required to regenerate RuBP from five G3P molecules?

3

How many carbon dioxide molecules must be added to RuBP to make a single molecule of glucose?

6

Phases of the Calvin Cycle

-Phase 1: Carbon fixation -Phase 2: reduction -Phase 3: regeneration of the CO2 acceptor RuBP

Counting Carbons and molecules at the steps of the Calvin cycle

-To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2 (1 carbon atom each). -The 3 CO2 molecules are added to 3 RuBP molecules (which contain 15 total carbon atoms), next producing 6 molecules of 3-PGA (18 total carbon atoms). -In reducing 3-PGA to G3P (Phase 2), there is no addition or removal of carbon atoms. -At the end of Phase 2, 1 of the 6 G3P molecules is output from the cycle, removing 3 of the 18 carbons. -The remaining 5 G3P molecules (15 total carbon atoms) enter Phase 3, where they are converted to 3 molecules of R5P. -Finally, the R5P is converted to RuBP without the addition or loss of carbon atoms.

C4 plants differ from C3 and CAM plants in that C4 plants _____.

In C3 and CAM plants carbon dioxide fixation and the Calvin cycle occur in the same cells.

Relationship between the light-dependent and light-independent reactions of photosynthesis?

Light energy drives the formation of ATP and NADPH during the light-dependent reactions; these energy molecules are then used during the light-independent reactions to form sugars.