1.
Chapter 6
Photosynthesis

2. Section 1 Vocabulary Pretest
Cellular organelles where photosynthesis occurs
The first stage of photosynthesis
An organism that can make its own food
An organism that can not make its own food
The process of converting energy from the sun into chemical energy of food
A membrane system found inside chloroplasts
Solution surrounding the thylakoids inside chloroplasts
Autotroph
Photosynthesis
Heterotroph
Light Reactions
Chloroplasts
Thylakoid
Stroma

3. Primary Electron Acceptor Electron Transport Chain Chemiosmosis
Granum
Pigment
Chlorophyll
Carotenoid
Photosystem
Primary Electron Acceptor
Electron Transport Chain
Chemiosmosis
Compounds that absorb light
A stack of thylakoids
Yellow, orange and brown accessory pigments
A cluster of pigments that harvest light energy for photosynthesis
Green pigment in plants
Movement of protons down a gradient to make ATP
Movement of electrons from one molecule to another
Accepts electrons

4. Answer Key Autotroph C Photosynthesis E Heterotroph D
Light Reactions B
Chloroplasts A
Thylakoid F
Stroma G
Granum I
Pigment H
Chlorophyll L
Carotenoid J
Photosystem K
Primary Electron Acceptor O
Electron Transport Chain N
Chemiosmosis M

5. Obtaining Energy
The sun is the direct or indirect source of energy for most living things.
Autotrophs —organisms that can make their own food
Heterotrophs —organisms that can not make food. They obtain energy from eating food.

6. Photosynthesis
Photosynthesis is the process used by autotrophs to convert light energy from sunlight into chemical energy in the form of organic compounds.
Involves a complex series of chemical reactions known as a biochemical pathway.
Product of one reaction is consumed in the next reaction

7. Overview Photosynthesis is often summarized in the following equation:
6CO H2O C6H12O O2
The Reactants are carbon dioxide and water
The Products are glucose and oxygen
Light energy

8. The Stages of Photosynthesis
There are two stages to the process
Light Reactions —light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH
Dark Reactions (Calvin Cycle)—organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH

9. The Light Reactions Require light to happen
Take place in the chloroplasts
Chloroplasts contain pigments that absorb sunlight.
Pigment —a compound that absorbs light

10. The Structure of a Chloroplast
Surrounded by an outer and inner membrane
Thylakoids —membrane system arranged as flattened sacs. (from the Greek meaning “pocket”)
Grana (pl.) Granum (singular)—stacks of thylakoid membrane sacs
Stroma —solution that surrounds the grana

11. Thylakoids contain the pigments known as chlorophylls.
Chlorophylls —absorb colors other than green. Therefore, green is reflected and is visible. Two types:
Chlorophyll a and Chlorophyll b
Chlorophyll a —directly involved in the light reactions
Chlorophyll b —accessory pigment that assists in photosynthesis
Carotenoids —accessory pigments responsible for fall colors and also assist in photosynthesis

12. Converting Light Energy to Chemical Energy
Chlorophylls and carotenoids are grouped in clusters embedded in proteins in the thylakoid membrane.
These clusters are called photosystems
Two photosystems exist, each with its own job to do:
Photosystem I and Photosystem II
Plants have both photosystems. Prokaryotic autotrophs only have photosystem II. It is only numbered as II because it was the second one discovered. However, it probably evolved 1st.

13. How does it work?
Light is absorbed by accessory pigments in photosystem II.
When the absorbed energy from the light reaches the chlorophyll a molecules of photosystem II, it “excites” electrons to a higher energy level.
These excited electrons will leave the chlorophyll a molecule (this is an oxidation reaction)
The electrons are accepted by the primary electron acceptor (this is a reduction reaction) and begin to move from molecule to molecule down an “electron transport chain”
The energy they lose as they are transported is used to pump H+ ions from the stroma into the thylakoid space, creating a concentration gradient.

14. Stroma II
Thylakoid Space

15. At the same time that light is absorbed by photosystem II, it is also being absorbed by photosystem I, again, exciting electrons.
These electrons move down a different electron transport chain and are added to NADP+ to form NADPH.
The lost electrons from photosystem I are replaced by the electrons moving down the transport chain from photosystem II.

16. Stroma I II
Thylakoid Space

17. Photosystem II replaces its electrons by splitting water, using a water-splitting enzyme.
2H2O H e O2
For every two molecules of water that are split, four electrons become available to replace those lost by the chlorophyll molecules in photosystem II.
The hydrogen ions and the oxygen molecules are released into the thylakoid space. This is where the oxygen gas given off by photosynthesis comes from.

18. I II

19. The build up of H+ in the thylakoid space stores potential energy
The build up of H+ in the thylakoid space stores potential energy. This energy is harvested by an enzyme called ATP synthase.
As H+ ions diffuse through ATP synthase down their concentration gradient, the enzyme uses the energy of the moving ions to make ATP.
This is done by adding a phosphate group to ADP in a process called chemiosmosis.
ATP will then be used in the second stage of photosynthesis called the Calvin Cycle.

21. The Calvin Cycle Named for Melvin Calvin
Most common pathway for carbon fixation
Carbon fixation —changing CO2 into organic compounds (carbohydrates)
It is the second set of reactions in photosynthesis and does not require light.
It uses the energy that was stored in ATP and NADPH during the light reactions to produce organic compounds in the form of sugars.
The Calvin Cycle occurs in the stroma of the chloroplasts and requires CO2

22. The Calvin Cycle Step by Step
Step 1: Create 6 molecules of 3-PGA
Three molecules of CO2 diffuse into the stroma
An enzyme combines each CO2 molecule with a 5-carbon molecule called RuBP (ribulose bisphosphate) to make 3 very unstable 6-carbon molecules. Each immediately breaks down into two 3-carbon molecules called 3-PGA (3-phosphoglycerate). This results in 6 molecules of 3-PGA.
3 molecules of CO2
3 molecules
of RuBP
6 molecules of 3-PGA

23. Step 2: Convert 3-PGA to G3P
Each of the 6 molecules of 3-PGA is converted into a molecule of G3P (glyceraldehyde 3-phosphate)
This is a two-step process
First: 6 ATP molecules (from the light reactions) donate a phosphate group to the 3-PGA. (Changing ATP to ADP)
Second: 6 NADPH molecules (from the light reactions) donate a H+ (Changing NADPH to NADP+) and the phosphate group is released.
The result is 6 molecules of G3P. The ADP, NADP+ and phosphates that are released can be used again in the light reactions to make more ATP and NADPH
6- 3PGA
6 ATP
6ADP
6 molecules
of G3P
6NADPH
6NADP+
6 P

24. Step 3: Make organic compounds
One of the G3P molecules leaves the Calvin cycle and is used to make organic compounds (carbohydrates) in which energy is stored for later use.
glucose
1 molecule
of G3P
starch

25. Step 4: Convert G3P to RuBP
The remaining G3P molecules are converted back into RuBP by adding phosphate groups from ATP molecules. The RuBP is used again in the cycle.
3 ADP
3 CO2
3 ATP
3 RuBP
6- 3PGA
6 ATP
6 ADP
5 G3P
glucose
6 G3P
1 G3P
6 NADPH
starch
6NADP+
6 P

26. Plant species that fix carbon using the Calvin Cycle only are known as C3 plants because of the three-carbon compound that is initially formed in the process. They include most plants.

27. Alternative Pathways
Plants living in hot, dry climates have trouble using the Calvin Cycle to fix carbon.
This is because they must partially close their stomata to conserve water.
This allows less CO2 to enter and an excess of O2 to build up, both of which inhibit the Calvin Cycle
Two alternate pathways have evolved for these plants—both allow the plants to conserve water.
They are the C4 pathway and the CAM pathway

28. The C4 Pathway C4 plants include: corn, sugar cane and crab grass
Cells called mesophyll cells in C4 plants use an enzyme to fix CO2 into a four carbon compound
This compound travels to other cells where CO2 can be released and enter the Calvin Cycle
These plants lose about ½ as much water as C3 plants when producing the same amount of carbohydrates.

29. The CAM Pathway
CAM plants include: cactuses, pineapples, and jade plants.
These plants open their stomata at night and close them during the day (opposite of most plants).
CO2 absorbed at night can enter the Calvin Cycle during the day, allowing the stomata to stay closed and conserve water.
These plants lose less water than any other plants