ANSWERS
Name _____ANSWERS_______
Photosynthesis Lecture Review
You should answer these on a separate sheet of paper.
1. In a sentence, describe the significance of ATP to living things:
ATP is a compound in cells that stores a large amount of energy.
Energy released from ATP drives all of the chemical reactions that enable an organism to
function. (textbook, page 54)
2. What are the three main structural components of ATP? On what page of your textbook is there a
diagram of it?
ATP is composed of (1) a 5-carbon sugar, called RIBOSE; (2) a nitrogen-containing compounds,
called ADENINE; (3) 3 linked PHOSPHATE groups (textbook, page 54)
3. What is the difference between ADP and ATP, both structurally and energetically?
Structurally, ATP has three phosphates attached, whereas ADP only has two.
ATP contains more energy than ADP, due to the additional phosphate bond.
4. In the light-dependent stage of photosynthesis, ATP is made by harnessing the energy from the flow
of protons (H+) out of the thylakoid membrane. This process is known by what name?
CHEMIOSMOSIS
On what page of the textbook can you find a picture and explanation of this process?
Pg 118,
Fig. 6-8
5. In reference to question #4, what enzyme (protein) is needed to make this ATP?
ATP synthase
6. In reference to question #4, what is the source of the protons (hydrogen ions, H+) necessary to the
process? They come from the splitting of water during PSII.
7. What organelle in a plant cell traps light energy? The CHLOROPLAST
In what organ of the plant are these found (mostly)? The LEAF
8. Define “photosystem”. A cluster of pigment molecules and the proteins that the pigment
molecules are embedded in. (page 116 in textbook, 1st paragraph)
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ANSWERS
9. Define “electron transport chain”. A series of molecules located in the thylakoid membrane that
transfer electron from one molecule to the next (page 117, 2nd paragraph)
10. What happens to the excited electrons from the chlorophyll in PS2? The electron are passed to the
primary electron acceptor molecule in the electron transport chain of PS II, and then passed
down the chain.
How are they replaced? The electrons that result from the splitting of water are used to replace
the electrons lost from the chlorophyll in PSII. (see page 117, “Replacing Electrons in Light
Reactions”, 2nd paragraph from bottom)
11. What happens to the excited electrons from the chlorophyll in PS1? ? The electrons are passed to
the primary electron acceptor molecule in the electron transport chain of PS I, and then passed
down the chain.
How are they replaced? The electrons that are lost from these chlorophyll a molecules are
replaced by the electrons that have passed through the electron transport chain from PSII. (see
page 117, step 4, 3rd paragraph from top)
12. When water is split, (a) does it happen in PS2 or PS1? PS II
(b) What three things is it split into?
Hydrogen ions (H+)
Electrons
Oxygen
(page 117, bottom)
(c) What happens to the oxygen? It diffuses out of the chloroplast, then leave the plant (via
stomata). (page 117, bottom)
13. What molecule is made in PS2? ATP
What is it later used for? To provide energy for sugar production in the Calvin Cycle (in the
light-independent stage) of photosynthesis. (page 118, last sentence)
14. What molecule is made in PS1? NADPH
What is it later used for? To provide energy for sugar production in the Calvin Cycle (in the
light-independent stage) of photosynthesis. (page 118, last sentence)
15. Why are these reactions (involving the photosystems) called “light-dependent” reactions? They
require light to excite the chlorophyll (page 114)
16. Why are the light-independent reactions sometimes are called the “dark” reactions in older
textbooks? Because they do not require light (they can even take place at night). They instead
run on stored ATP and NADPH produced in the first stage.
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ANSWERS
17. Why is O2 given off by plants during daylight hours and not at night?
Because water is only split in PS II, which only runs when the sunlight excites the chlorophyll.
18. (a) What materials must be provided for a plant in order for photosynthesis to occur? CO2, H2O,
light energy
(b) What does the plant itself provide? pigments, enzymes (like ATP synthase), NADP+ , etc.
19. Are sugars actually manufactured in the light-dependent or the light-independent stage of
photosynthesis? light-independent stage
This process involves a complex series of biochemical reactions called the _CALVIN__ Cycle.
20. What are the final products of photosynthesis? Carbohydrates are made from the G3P produced
in the Calvin cycle. Also RuBP is regenerated so the cycle can run again. (page 121, see step 3 and
4)
21. Glucose is C6H12O6 (b) Carbon dioxide (CO2) is the source of the carbon atoms for glucose.
22. Write a complete equation for photosynthesis and indicate
(a) where such material (reactant) is obtained,
(b) in what reaction (light-dependent/light-independent) each is used, and
(c) in what reaction each product is produced.
(a) air
(b) LI
(c)
soil
LD
LD
LI
LD
23. What is the MAIN pigment used by most plants to capture sunlight? Chlorophyll a
What are some of the other pigments that may also be present in plants, but in lesser amounts?
Xanthophylls, carotenes, chlorophyll b
24. Describe the importance of accessory pigments to the survival of a plant. They enable the plant to
absorb other wavelengths of the visible light spectrum that chlorophyll might not absorb as
well. Greater absorption means more energy to power the photosystems.
25. Explain why leaves change color in the autumn. When the days get shorter and nights get
longer, average temperature drops. Chlorophylls are the most cold-sensitive pigments and
will begin to break down first. This exposes the other pigments (xanthophylls, carotenes, etc)
that were always there, but had been “masked” by the chlorophylls during the spring and
summer months.
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