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5.E: Photosynthesis (Exercises)

5.1: Overview of Photosynthesis

All living organisms on earth consist of one or more cells. Each cell runs on the chemical energy found mainly in carbohydrate molecules (food), and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy used to hold these molecules together is released when an organism breaks down food.

Multiple Choice

What two products result from photosynthesis?

A. water and carbon dioxide
B. water and oxygen
C. glucose and oxygen
D. glucose and carbon dioxide


Which statement about thylakoids in eukaryotes is not correct?

A. Thylakoids are assembled into stacks.
B. Thylakoids exist as a maze of folded membranes.
C. The space surrounding thylakoids is called stroma.
D. Thylakoids contain chlorophyll.


From where does a heterotroph directly obtain its energy?

A. the sun
B. the sun and eating other organisms
C. eating other organisms
D. simple chemicals in the environment


Free Response

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

To convert solar energy into chemical energy that cells can use to do work.

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

Because lions eat animals that eat plants.

5.2: The Light-Dependent Reactions of Photosynthesis

How can light be used to make food? It is easy to think of light as something that exists and allows living organisms, such as humans, to see, but light is a form of energy. Like all energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is transformed into chemical energy, which autotrophs use to build carbohydrate molecules. However, autotrophs only use a specific component of sunlight .

Multiple Choice

What is the energy of a photon first used to do in photosynthesis?

A. split a water molecule
B. energize an electron
C. produce ATP
D. synthesize glucose


Which molecule absorbs the energy of a photon in photosynthesis?

B. glucose
C. chlorophyll
D. water


Plants produce oxygen when they photosynthesize. Where does the oxygen come from?

A. splitting water molecules
B. ATP synthesis
C. the electron transport chain
D. chlorophyll


Which color(s) of light does chlorophyll a reflect?

A. red and blue
B. green
C. red
D. blue


Free Response

Describe the pathway of energy in light-dependent reactions.

The energy is present initially as light. A photon of light hits chlorophyll, causing an electron to be energized. The free electron travels through the electron transport chain, and the energy of the electron is used to pump hydrogen ions into the thylakoid space, transferring the energy into the electrochemical gradient. The energy of the electrochemical gradient is used to power ATP synthase, and the energy is transferred into a bond in the ATP molecule. In addition, energy from another photon can be used to create a high-energy bond in the molecule NADPH.

5.3: The Calvin Cycle

Carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

Multiple Choice

Where in plant cells does the Calvin cycle take place?

A. thylakoid membrane
B. thylakoid space
C. stroma
D. granum


Which statement correctly describes carbon fixation?

A. the conversion of CO2 to an organic compound
B. the use of RuBisCO to form 3-PGA
C. the production of carbohydrate molecules from G3P
D. the formation of RuBP from G3P molecules
E. the use of ATP and NADPH to reduce CO2


What is the molecule that leaves the Calvin cycle to be converted into glucose?

B. G3P
D. 3-PGA


Free Response

Which part of the Calvin cycle 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.

Explain the reciprocal nature of the net chemical reactions for photosynthesis and respiration.

Photosynthesis takes the energy of sunlight and combines water and carbon dioxide to produce sugar and oxygen as a waste product. The reactions of respiration take sugar and consume oxygen to break it down into carbon dioxide and water, releasing energy. Thus, the reactants of photosynthesis are the products of respiration, and vice versa.