# 8.E: Photosynthesis (Exercises)

## 8.1: Overview of Photosynthesis

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules.

### Review Questions

Which of the following components is not used by both plants and cyanobacteria to carry out photosynthesis?

1. chloroplasts
2. chlorophyll
3. carbon dioxide
4. water

A

What two main products result from photosynthesis?

1. oxygen and carbon dioxide
2. chlorophyll and oxygen
3. sugars/carbohydrates and oxygen
4. sugars/carbohydrates and carbon dioxide

C

In which compartment of the plant cell do the light-independent reactions of photosynthesis take place?

1. thylakoid
2. stroma
3. outer membrane
4. mesophyll

B

Which statement about thylakoids in eukaryotes is not correct?

1. Thylakoids are assembled into stacks.
2. Thylakoids exist as a maze of folded membranes.
3. The space surrounding thylakoids is called stroma.
4. Thylakoids contain chlorophyll.

B

### Free Response

What is the overall outcome of the light reactions in photosynthesis?

The outcome of light reactions in photosynthesis is the conversion of solar energy into chemical energy that the chloroplasts can use to do work (mostly anabolic production of carbohydrates from carbon dioxide).

Why are carnivores, such as lions, dependent on photosynthesis to survive?

Because lions eat animals that eat plants.

Why are energy carriers thought of as either “full” or “empty”?

The energy carriers that move from the light-dependent reaction to the light-independent one are “full” because they bring energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. There is not much actual movement involved. Both ATP and NADPH are produced in the stroma where they are also used and reconverted into ADP, Pi, and NADP+.

## 8.2: The Light-Dependent Reactions of Photosynthesis

Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules. However, autotrophs only use a few specific components of sunlight.

### Review Questions

Which of the following structures is not a component of a photosystem?

1. ATP synthase
2. antenna molecule
3. reaction center
4. primary electron acceptor

A

How many photons does it take to fully reduce one molecule of NADP+ to NADPH?

1. 1
2. 2
3. 4
4. 8

B

Which complex is not involved in the establishment of conditions for ATP synthesis?

1. photosystem I
2. ATP synthase
3. photosystem II
4. cytochrome complex

C

From which component of the light-dependent reactions does NADPH form most directly?

1. photosystem II
2. photosystem I
3. cytochrome complex
4. ATP synthase

B

### Free Response

Describe the pathway of electron transfer from photosystem II to photosystem I in light-dependent reactions.

A photon of light hits an antenna molecule in photosystem II, and the energy released by it travels through other antenna molecules to the reaction center. The energy causes an electron to leave a molecule of chlorophyll a to a primary electron acceptor protein. The electron travels through the electron transport chain and is accepted by a pigment molecule in photosystem I.

What are the roles of ATP and NADPH in photosynthesis?

Both of these molecules carry energy; in the case of NADPH, it has reducing power that is used to fuel the process of making carbohydrate molecules in light-independent reactions.

## 8.3: Using Light Energy to Make Organic Molecules

The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

### Review Questions

Which molecule must enter the Calvin cycle continually for the light-independent reactions to take place?

1. RuBisCO
2. RuBP
3. 3-PGA
4. CO2

D

Which order of molecular conversions is correct for the Calvin cycle?

1. $$\text{RuBP} + \text{G3P} \rightarrow \text{3-PGA} \rightarrow \text{sugar}$$
2. $$\text{RuBisCo} \rightarrow \text{CO}_2 \rightarrow \text{RuBP} \rightarrow \text{G3P}$$
3. $$\text{RuBP} + \text{CO}_2 \rightarrow \text{[RuBisCo]} \enspace \text{3-PGA} \rightarrow \text{G3P}$$
4. $$\text{CO}_2 \rightarrow \text{3-PGA} \rightarrow \text{RuBP} \rightarrow \text{G3P}$$

C

Where in eukaryotic cells does the Calvin cycle take place?

1. thylakoid membrane
2. thylakoid lumen
3. chloroplast stroma
4. granum

C

Which statement correctly describes carbon fixation?

1. the conversion of CO2 into an organic compound
2. the use of RuBisCO to form 3-PGA
3. the production of carbohydrate molecules from G3P
4. the formation of RuBP from G3P molecules
5. the use of ATP and NADPH to reduce CO2

A

### Free Response

Why is the third stage of the Calvin cycle called the regeneration stage?

Because RuBP, the molecule needed at the start of the cycle, is regenerated from G3P.

Which part of the light-independent reactions would be affected if a cell could not produce the enzyme RuBisCO?

None of the cycle could take place, because RuBisCO is essential in fixing carbon dioxide. Specifically, RuBisCO catalyzes the reaction between carbon dioxide and RuBP at the start of the cycle.

Why does it take three turns of the Calvin cycle to produce G3P, the initial product of photosynthesis?

Because G3P has three carbon atoms, and each turn of the cycle takes in one carbon atom in the form of carbon dioxide.