Chapter 10 Guided Reading

3. Use both chemical symbols and words to write out the formula for photosynthesis (use the one that indicates only the net consumption of water). Notice that the formula is the opposite of cellular respiration.

6 CO2(carbon dioxide) + 6 H2O(water) + light energy → C6H12O6(glucose) + 6 O2(oxygen)

23. Explain what is meant by a C3 plant.

A C3 plant is a plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.

Photosystem

A photosystem is composed of a protein complex called a reaction center complex surrounded by several light-harvesting complexes.

D. Regeneration

Three turns of the Calvin cycle nets one G3P because the other five must be recycled to RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP. To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues.

25. Explain what is meant by a C4 plant.

A C4 plant is plant in which the Calvin cycle is preceded by reactions that incorporate CO2 into a four carbon compound, the end product of which supplies CO2 for the Calvin cycle.

d. As electrons fall from photosystem II to photosystem I, the cytochrome complex uses the energy to pump hydrogen ions. This builds a proton gradient that is used in chemiosmosis to produce what molecule?

ATP

1. As a review, define the terms autotroph and heterotroph. Keep in mind that plants have mitochondria and chloroplasts and do both cellular respiration and photosynthesis!

Autotroph: An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from oxidation of inorganic substances to make organic molecules from inorganic ones. Heterotroph: An organism that obtains organic food molecules by eating other organisms or substances derived from them.

29. What are CAM plants and what is their "advantage"?

Both C4 and CAM plants thrive in hot, dry conditions. Both have evolved methods to reduce water loss and to "fix" carbon dioxide in an intermediate compound before it enters the Calvin cycle. C4 plants have Kranz anatomy, with thick-walled bundle-sheath cells. CO2 is added to PEP to make a four-carbon intermediate so that CO2 will not be lost through photorespiration. CAM plants prevent water loss by closing their stomata during the day, but in order to have CO2 available, the CO2 is fixed in crassulacean acid when the stomata are open at night. In C4 plants, the initial steps of carbon fixation are separated structurally from the Calvin cycle. In CAM plants, the two steps occur at different times (temporal separation of steps).

24. Why is photorespiration such a "waste"?

During photorespiration, which is a metabolic process, the plant consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. Photorespiration generally occurs on hot, dry,bright days, when stomata close and the O2/CO2 ratio in the leaf increases, favoring the binding of O2 rather than CO2 by rubisco. Rice, wheat, and soybeans are C3 plants that are important in agriculture. When their stomata partially close on hot, dry days, C3 plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle, limiting growth.

28. Conceptually, it is important to know that the C4 pathway does not replace the Calvin cycle but works as a CO2 pump that prefaces the Calvin cycle. Explain how changes in leaf architecture (Figure 10.20) help isolate rubisco in regions of the leaf that are high in CO2 but low in O2.

In effect, the mesophyll cells of a C4 plant pump CO2 into the bundle sheath, keeping the CO2 concentration in the bundle-sheath cells high enough for rubisco to bind carbon dioxide rather than oxygen.

d. Primary electron acceptor

In the thylakoid membrane of a chloroplast or in the membrane of some prokaryotes, the primary electron acceptor is a specialized molecule that shares the reaction center complex with a pair of chlorophyll a molecules and that accepts an electron from them.

a. What is the source of energy that excites the electron in photosystem II?

Light

C. Reduction

NADPH donates electrons, and so is the source of the reducing power. In this reduction stage, the low-energy acid 1, 3-bisphosphoglycerate is reduced by electrons from NADPH to form the three-carbon sugar G3P. The carbohydrate produced directly from the Calvin cycle is not glucose, but the three-carbon compound glyceraldehyde 3-phosphate (G3P). Each turn of the Calvin cycle fixes one molecule of CO2; therefore, it will take 3 turns of the Calvin cycle to net one G3P.

15. What is the name of the chlorophyll a at the reaction center of PS I called?

P700

16. Linear electron flow is, fortunately, easier to understand than it looks. It is an electron transport chain, somewhat like the one we worked through in cellular respiration. While reading the section "Linear Electron Flow" and studying Figure 10.14 in your text, label this diagram number by number as you read.

See page 195 in your text for the labeled figure.

19. Use the diagram below to summarize the activities in the light reactions of photosynthesis.

See page 198 of your text for the labeled figure.

26. Use the diagram below to explain C4 plants.

See page 202 of your text for the labeled figure. 1. In mesophyll cells, the enzyme PEP carboxylase adds carbon dioxide to PEP. 2. A four-carbon compound conveys the atoms of the CO2 into a bundle-sheath cell via plasmodesmata. In bundle-sheath cells, CO2 is released and enters the Calvin cycle.

c. What is the source of O2 in the atmosphere?

Splitting of water

B. Carbon fixation

The Calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate. The enzyme that catalyzes this first step is RuBP carboxylase, or rubisco. The product of the reaction is a six-carbon intermediate so unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate. The enzyme responsible for carbon fixation in the Calvin cycle, and possibly the most abundant protein on Earth, is rubisco.

A. Calvin cycle

The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions.

13. Explain what occurs in the light reactions stage of photosynthesis. Be sure to use NADP+ and photophosphorylation in your discussion.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off O2 as a byproduct. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation.

14. Photosystem II (PS II) has at its reaction center a special pair of chlorophyll a molecules called P680. What is the explanation for this name?

The reaction center chlorophyll a molecules of PS II are so named because these molecules are best at absorbing light energy at 680 nm. Nearly identical chlorophyll a molecules found within the reaction center of PS I, however, absorb light energy best at 700 nm as a result of their association with different proteins in the thylakoid membrane that affect their electron distribution.

b. Light harvesting complex

This complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) captures light energy and transfers it to reaction center pigments in a photosystem.

c. Reaction center

This complex of proteins is associated with a special pair of chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, the pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.

27. Explain the role of PEP carboxylase in C4 plants, including key differences between it and rubisco.

This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has a much higher affinity for CO2 than does rubisco and no affinity for O2. Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot—that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to fall and O2 concentration to rise.

b. What compound is the source of electrons for linear electron flow?

Water