Assignment 11: Photosynthesis- Carbon Fixation

In the carbon fixation step of the Calvin cycle, ribulose-1,5-biphosphate (a five-carbon compound) is carboxylated to produce________.

2 three-carbon compounds

Part C - Redox reactions of photosynthesis In photosynthesis, a redox compound that is produced in the light reactions is required to drive other redox reactions in the Calvin cycle, as shown in this figure along with other components of photosynthesis. Drag the terms to the appropriate blanks to complete the following sentences summarizing the redox reactions of photosynthesis. Terms may be used once, more than once, or not at all.

1. In the light reactions, light energy is used to oxidize H2O to O2. 2. The electrons derived from this oxidation reaction in the light reactions are used to reduce NADP+ to NADPH. 3. The Calvin cycle oxidizes the light-reactions product NADPH to NADP+. 4. The electrons derived from this oxidation reaction in the Calvin cycle are used to reduce CO2 to G3P. In the light reactions, light energy is used to remove electrons from (oxidize) water, producing O2 gas. These electrons are ultimately used to reduce NADP+ to NADPH. In the Calvin cycle, NADPH is oxidized back to NADP+ (which returns to the light reactions). The electrons released by the oxidation of NADPH are used to reduce three molecules of CO2 to sugar (G3P), which then exits the Calvin cycle.

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

3

Melvin Calvin introduced radio labeled carbon dioxide into plants. Which of the following molecules should he have expected to see radio labeled first?

3-phosphoglycerate

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

6

Photorespiration is ___________.

A process involving the addition of oxygen to ribulose-1,5-biphosphate and the formation of the products 3-phosphoglycerate and 2-phosphoglycerate

Why is it not entirely accurate to adopt the common phrase "light-independent reactions" when referring to the Calvin cycle?

Although its reactions are not triggered directly by light, they depend on the products of light-capturing reactions.

Stomata, openings in the leaf, are important to photosynthesis for _________.

Entry of carbon dioxide that is used in the Calvin cycle.

Part A - Inputs and outputs of the light reactions From the following choices, identify those that are the inputs and outputs of the light reactions. (Recall that inputs to chemical reactions are modified over the course of the reaction as they are converted into products. In other words, if something is required for a reaction to occur, and it does not remain in its original form when the reaction is complete, it is an input.) Drag each item to the appropriate bin. If the item is not an input to or an output from the light reactions, drag it to the "not input or output" bin.

Input: light, water, NADP+, ADP Output: O2, ATP, NADPH not input or output: glucose, CO2,G3P In the light reactions, the energy of sunlight is used to oxidize water (the electron donor) to O2 and pass these electrons to NADP+, producing NADPH. Some light energy is used to convert ADP to ATP. The NADPH and ATP produced are subsequently used to power the sugar-producing Calvin cycle.

Part B - Inputs and outputs of the Calvin cycle From the following choices, identify those that are the inputs and outputs of the Calvin cycle. Drag each item to the appropriate bin. If the item is not an input to or an output from the Calvin cycle, drag it to the "not input or output" bin.

Input:ATP, NADPH, CO2 Output:ADP, NADP+, G3P not input or output:light, glucose, O2 In the Calvin cycle, the energy outputs from the light reactions (ATP and NADPH) are used to power the conversion of CO2 into the sugar G3P. As ATP and NADPH are used, they produce ADP and NADP+, respectively, which are returned to the light reactions so that more ATP and NADPH can be formed.

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

NADPH (supplies the electrons that reduce the phosphorylated 3-PGA

During the Calvin cycle, carbon dioxide is _________, in order to drive the formation of sugars.

Reduced

Why is measuring the rate of oxygen production in chloroplasts appropriate for estimating the rate of photosynthesis?

Oxygen is produced "splitting" water in the photosystem 2 complex.

Why is photosynthesis so important to life on Earth?

Photosynthesis captures light energy from the Sun and stores it in the bonds of glucose, which is then used during cellular respiration to drive the formation of ATP in all plants and animals.

Which of the following best explains this observation?

Photosystem 2 is not splitting water, and the ATP is being produced by cycling electrons via photosystem 1

Carbon fixation involves the addition of carbon dioxide to ________.

RuBP

The most abundant enzyme on Earth, which catalyzes the fixation of carbon dioxide in photosynthesis, is ________.

Rubisco

Why, with four active sites, might there be such a large concentration of rubisco in plant matter?

Rubisco is a very slow enzyme; what it lacks in speed, it makes up in numbers.

How does carbon fixation differ between C3 and C4 plants?

Rubisco is the primary enzyme that catalyzes carbon fixation in C3 plants, and phosphoenolpyruvate carboxylase can catalyze carbon fixation in C4 plants.

How are the light-dependent and light-independent reactions of photosynthesis related?

The products of light-dependent reactions are used in light-independent reactions.

Most of the enzymes of the Calvin cycle also function in other metabolic pathways. This should not be a surprising finding, because _________.

There is a close relationship between carbohydrate synthesis and carbohydrate metabolism

What is the main purpose of the light-dependent reactions of photosynthesis?

To produce NADPH and ATP

Part D - Chloroplast structure and function In eukaryotes, all the reactions of photosynthesis occur in various membranes and compartments of the chloroplast. Identify the membranes or compartments of the chloroplast by dragging the blue labels to the blue targets. Then, identify where the light reactions and Calvin cycle occur by dragging the pink labels to the pink targets. Note that only blue labels should be placed in blue targets, and only pink labels should be placed in pink targets.

a) stroma- the dense fluid surrounded between the inner envelope membrane and the thylakoid membranes b) thylakoid membrane- the third membrane system; surrounds the thylakoid; chrorophyll resides in the thylakoid membrane c)cytosol- the fluid portion of cytoplasm surrounding (outside) the chloroplast d) location of Calvin Cycle- the calvin cycle occurs in the stroma e) thylakoid space- the space within the thylakoid sacs f)location of light reactions- the light reactions occur in the thylakoids of the chloroplast g) envelope membranes- the inner and outer membranes of the chloroplast The chloroplast is enclosed by a pair of envelope membranes (inner and outer) that separate the interior of the chloroplast from the surrounding cytosol of the cell. Inside the chloroplast, the chlorophyll-containing thylakoid membranes are the site of the light reactions. Between the inner envelope membrane and the thylakoid membranes is the aqueous stroma, which is the location of the reactions of the Calvin cycle. Inside the thylakoid membranes is the thylakoid space, where protons accumulate during ATP synthesis in the light reactions.