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# 4.2: Photosynthesis: Sugar as Food

Difficulty Level: At Grade Created by: CK-12

## Lesson 4.2: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Photosynthesis provides almost all of the energy used by living things on Earth.

_____ 2. Earth’s oxygen comes from photosynthesis.

_____ 3. In photosynthesis, the Calvin cycle comes before the light reactions.

_____ 4. ATP and NADPH are the reactants of the light reactions.

_____ 5. Electron transport occurs in the thylakoid membranes.

_____ 6. All cells have chloroplasts.

_____ 7. During the Calvin cycle, NADPH and ATP are used to make glucose.

_____ 8. Photons of sunlight can excite and energize electrons.

_____ 9. A chemiosmotic gradient causes hydrogen ions to flow across the thylakoid membrane into the stroma.

_____ 10. Like photosynthesis, chemosynthesis also relies on sunlight.

_____ 11. Two turns of the Calvin cycle produce two molecules of glucose.

_____ 12. The Calvin cycle takes place in the stroma surrounding the thylakoid membranes of the chloroplast.

_____ 13. During the light reactions, water molecules are made.

_____ 14. Light is absorbed by photosystems in the thylakoid membranes of chloroplasts.

_____ 15. Both stages of photosynthesis need sunlight to proceed.

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

Photosynthesis Stage I: The Light Reactions

The first stage of photosynthesis is called the light reactions. During this stage, light is absorbed and transformed to chemical energy in the bonds of NADPH and ATP. You can read about this process below.

Steps of the Light Reactions

The light reactions occur in several steps, all of which take place in the thylakoid membrane.

• Step 1: Units of sunlight, called photons, strike a molecule of chlorophyll in photosystem II of the thylakoid membrane. The light energy is absorbed by two electrons (2 e-) in the chlorophyll molecule, giving them enough energy to leave the molecule.
• Step 2: At the same time, enzymes in the thylakoid membrane use light energy to split apart a water molecule. This produces:
• two electrons (2e-). These electrons replace the two electrons that were lost from the chlorophyll molecule in Step 1.
• an atom of oxygen (O). This atom combines with another oxygen atom to produce a molecule of oxygen gas (O2), which is released as a waste product.
• two hydrogen ions (2H+). The hydrogen ions, which are positively charged, are released inside the membrane in the thylakoid interior space.
• Step 3: The two excited electrons from Step 1 contain a great deal of energy, so, like hot potatoes, they need something to carry them. They are carried by a series of electron-transport molecules, which make up an electron transport chain. The two electrons are passed from molecule to molecule down the chain. As this happens, their energy is captured and used to pump more hydrogen ions into the thylakoid interior space.
• Step 4: When the two electrons reach photosystem I, they are no longer excited. Their energy has been captured and used, and they need more energy. They get energy from light, which is absorbed by chlorophyll in photosystem I. Then, the two re-energized electrons pass down another electron transport chain.
• Step 5: Enzymes in the thylakoid membrane transfer the newly re-energized electrons to a compound called NADP+. Along with a hydrogen ion, this produces the energy-carrying molecule NADPH. This molecule is needed to make glucose in the Calvin cycle.
• Step 6: By now, there is a greater concentration of hydrogen ions — and positive charge — in the thylakoid interior space. This difference in concentration and charge creates what is called a chemiosmotic gradient. It causes hydrogen ions to flow back across the thylakoid membrane to the stroma, where their concentration is lower. Like water flowing through a hole in a dam, the hydrogen ions have energy as they flow down the chemiosmotic gradient. The enzyme ATP synthase acts as a channel protein and helps the ions cross the membrane. ATP synthase also uses their energy to add a phosphate group (Pi) to a molecule of ADP, producing a molecule of ATP. The energy in ATP is needed for the Calvin cycle.

Questions

1. In one sentence, describe what happens during the light reactions.

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2. In which step(s) of the light reactions is sunlight absorbed?

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3. Why is water “split” during the light reactions?

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4. What is an electron transport chain? What is its role during these light reactions?

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5. How is ATP made during the light reactions?

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## Lesson 4.2: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. Most autotrophs make “food” through the process of
1. cellular respiration.
2. chemosynthesis.
3. homeostasis.
4. photosynthesis.
2. The correct sequence of events in the light reactions is
1. absorption of sunlight, electrons flow down the electron transport chain, ATP is made, NADPH is made.
2. absorption of sunlight, splitting of water, electrons flow down the electron transport chain, ATP is made.
3. electrons flow down the electron transport chain, NADPH is made, ATP is made, water is split.
4. absorption of sunlight, electrons flow down the electron transport chain, NADPH is made, water is split.
3. The Calvin cycle occurs
1. in the granum of the thylakoid membranes of the chloroplast.
2. in the stroma surrounding the inner membrane of the chloroplast.
3. in the stroma surrounding the thylakoid membranes of the chloroplast.
4. in the granum inside the inner membrane of the chloroplast.
4. By the end of the light reactions, energy from sunlight
1. has been stored in chemical bonds of NADPH and ATP.
2. has been transferred to glucose.
3. has entered the Calvin cycle.
4. is ready for use in the cell.
5. ATP synthase is
1. both an enzyme that makes ATP and a channel protein, and helps hydrogen ions cross the thylakoid membrane.
2. both an enzyme that makes ATP and a channel protein, and helps hydrogen ions cross the chloroplast inner membrane.
3. both an enzyme that makes ATP and a carrier protein, and helps hydrogen ions cross the thylakoid membrane.
4. both an enzyme that makes ATP and a carrier protein, and helps hydrogen ions cross the chloroplast inner membrane.
6. Essentially, the oxygen we breathe is
1. necessary for the light reactions to proceed.
2. a waste product of photosynthesis.
3. a reactant of the Calvin cycle.
4. essential for the homeostasis of the plant cell.
7. The Calvin cycle
1. starts with the molecule RuBP.
2. uses the energy in ATP and NADPH from the light reactions.
3. turns twice to produce one molecule of glucose.
4. all of the above
8. How do bacteria that live deep below the ocean’s surface make food?
1. by photosynthesis
2. by chemosynthesis
3. by cellular respiration
4. They eat other organisms.

## Lesson 4.2: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. a green pigment

_____ 2. main product of photosynthesis

_____ 3. process in which chemical energy, instead of sunlight, is used to make “food”

_____ 4. process in which sunlight is used to make “food”

_____ 5. sac-like membranes that make up the grana within the chloroplast

_____ 6. organelle of photosynthesis

_____ 7. space outside the thylakoid membranes within the chloroplast

_____ 8. energy carrying molecule

_____ 9. series of electron-transport molecules, which pass electrons from molecule to molecule

_____ 10. groups of molecules where sunlight is absorbed during the light reactions

_____ 11. stage of photosynthesis in which the energy from sunlight is stored in ATP and NADPH

_____ 12. stage of photosynthesis in which glucose is made

Terms

a. Calvin cycle

b. chemosynthesis

c. chlorophyll

d. chloroplast

e. electron transport chain

f. glucose

g. light reactions

i. photosynthesis

j. photosystem

k. stroma

l. thylakoid membrane

## Lesson 4.2: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. ____________ are the organelles where photosynthesis takes place.

2. Stage I of photosynthesis is called the ____________.

3. Stage II of photosynthesis is called the ____________.

4. During the first stage of photosynthesis, a molecule of ____________ gas is released.

5. Making food with chemical energy instead of sunlight is called ____________.

6. Chloroplasts contain ____________, which are made out of sac-like membranes, known as ____________ membranes.

7. Most ____________ make food using photosynthesis.

8. The green pigment, ____________, absorbs light to start photosynthesis.

9. During the first stage of photosynthesis, two ____________ are passed from molecule to molecule down the electron-transport chain.

10. ____________ turns of the Calvin cycle produce one molecule of ____________.

11. During the light reactions, ____________ and ____________ are produced.

12. During the Calvin cycle, ____________ is produced.

## Lesson 4.2: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

What are the two stages of photosynthesis? Discuss these two stages and how they are related.

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