Lake Tahoe Community College
Spring 2008
Written by: Sue Kloss
Botany
Instructor: Ralph Sinibaldi
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Ch. 10: Using Chapter 7 - Photosynthesis - From Campbell
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Intro: CO2 + H20 ----> CH2O + O2
too much sun is as bad as not enough
I. Overview of Photosynthesis (Ps)
A. Autotrophs are producers of biosphere
1. producers = primary productivity
2. light energy drives carbon fixation
3. plants, algae and some bacteria are photosynthetic
B. Ps occurs in chloroplasts
1. all green parts of plants have chloroplasts, most in leaves
2. Fig. 7.2 - site of Ps
a. chloroplasts concentrate in mesophyll cells, interior leaf tissue
b. CO2 and O2 diffuse thru stomates
c. chloroplast has inner and outer membrane, disks called thylakoids in stacks called grana in
stroma
d. sugar is made in stroma
e. inside grana, thylakoid space is continuous
f. thylakoid membranes contain many proteins necessary for Ps
3. Fig. 7.3 - O2 produced in Ps comes from splitting of water
a. CO comes from CO2 for sugar
b. H comes from water for sugar
C. Ps is a redox process
1. (Fig. 7.4) When water molecules are split, they are oxidized (lose electrons, along w/ H ions)
2. CO2 is reduced to sugar as H ions are added to store chem energy
3. cell respiration does the opposite to release chem energy
4. Ps involves a food producing redox reaction - electrons get energy by being boosted “uphill”
5. light energy boosts electrons, Ps converts light energy to chem energy and stores it in sugar
molecules
D. Ps occurs in 2 stages linked by ATP and NADH
1. 2 steps of Ps are light reactions and Calvin Cycle or light independent reactions
2. light reactions convert light energy to chem energy and produce O2 gas as waste
3. Calvin cycle assembles sugar molecule using CO2 and energy containing products of light
reactions
4. Photo = light, synthesis = putting together (sugar)
5. Fig. 7.5 - light reactions occur in grana on thylakoid membranes
6. light absorbed by chlorophyll in thylakoid membranes furnishes energy to power sugar production in Ps
7. Light energy makes ATP from ADP and P; also makes NADP+ into NADPH by adding light excited
electrons -NADP is electron carrier similar to NAD+
9. this reaction temporarily stores energy
10. As NADP+ is reduced, water is split (oxidized) giving off O2
11. summary - light reactions in grana capture light energy and store it in ATP and NADPH as chem
energy
12. sugar is made in the Calvin cycle in the stroma.
13. CO2 is reduced - change from gas state to solid - called carbon fixation
14. NADPH produced by light reactions provides hi energy electrons for reduction in Calvin Cycle
15. ATP from light reactions provides chem energy to power several steps of Calvin cycle
16. Calvin cycle does not require light directly, but most plants do it in daylight when light reactions
can power it using ATP and NADPH.
II. Light Reactions
C. In the light reactions, electron transport chains (etc) generate ATP, NADPH and O2
1. In the light reactions, the key events are:
a. absorption of light energy,
b. excitation of electrons,
c. formation of ATP and NADPH using energy made available by electrons cascading down etc
2. Etc similar to ones that function in cell respiration - carrier molecules arranged in a membrane
3. In Ps, carrier molecules are arranged in thylakoid membrane of the chloroplast
4. Fig. 7.8 - both photosystems absorb energy (yellow arrow)
a. excited electrons pass from reaction center chlorophylls (P680 or P700) to primary electron
acceptor
b. gold arrows represent electron flow
c. each primary acceptor is oxidized as it passes off electrons to the first carrier of etc
d. at each step, electron loses some energy - stored temporarily in ATP or NADPH
e. NADP+ is reduced with 2 high energy electrons and an H to NADPH
f. every NADPH molecule requires 2 electrons from photosystem
g. electrons to maintain the process come from etc in center of diagram in 7.8 for photosystem
P700; from p680, a water molecule is split to provide 2 electrons to P680
h. 2 H+ ions are left in the chloroplast and an O2 atom , which joins with another O2 and leaves
cell
I. products of light reactions = NADPH, ATP and O2
D. Chemiosmosis powers ATP synthesis in the light reactions
1. photosystems and etc’s located in thylakoid membranes of chloroplast
2. photosystems are arranged in such a way energy released in electron flow drives pumping of H+ ions
across thylakoid membrane, similar to cell respiration
3. Fig. 7.9 excited electrons (gold lines) pass along series of electron carriers (blue circles) within a
membrane as redox reactions occur; electrons give up energy and ATP is made by chemiosmosis
4. some electron carriers use energy released from electrons to actively transport H+ ions across
membrane, from thylakoid membrane in stroma, to inside of thylakoid membrane.
5. concentration gradient of H+ ions build up inside thylakoid
6. energy stored in this concentration is used to drive ATP synthesis
7. in Ps, chemiosmotic production of ATP is called photophosphorylation
8. final electron acceptor is NADP+, not O2
9.
O2 is produced when H2O is split to produce electrons for photosystem 2
III. The Calvin cycle converts CO2 to sugar
A. ATP and NADPH power sugar synthesis in the Calvin cycle
1. Calvin cycle is a sugar factory in a chloroplast in photosynthesis
2. CO2 from the air gets reduced, and ATP and NADPH from light reactions provide energy
3. plant constructs G3P to make glucose or other compounds the plant needs
4. G3P is an energy rich sugar molecule
5. Fig. 7.10
a. CO2 goes in one at a time, and gets bonded to a 5 carbon intermediate Ribulose
Bisphosphate RuBP
b. an enzyme called Rubisco attaches the CO2 to the 5C RuBP, which is then split into 3
molecules of 3PGA
c. ATP and NADPH are oxidized to provide energy to transform 3PGA into hi energy G3P
d. the hi energy G3P is combined to make glucose, used to make other organic molecules
or recycled to make RuBP.
IV. Photosynthesis reviewed and extended
A. Ps uses light energy to make food molecules
1. Fig. 7.11 light reactions occur on thylakoid membrane
2. Photosystem 2 harvests light in the form of excited electrons;
3. these are passed to photosystem 1 via a primary electron acceptor and then down the etc
4. Photosystem 1 gathers all the hi energy electrons and passes them off to another primary
electron acceptor;
5. electron transport chains use energy to reduce NADP+
6. Photosystem 1 replenishes its lost electrons by splitting water, using the electrons and H+, and
releasing the Oxygen as waste
7. H+ ions are used by ATP synthase to make ATP using the energy in the H+ gradient on the in
side of the thylakoid membrane
8. The sugar factory is the Calvin cycle -in the stroma (liquid of the chloroplast)
9. Plants use the sugars to do cellular respiration and to make cellulose, for example
10. plants stockpile the leftover sugar as starch in chloroplasts, tubers, roots and fruits
11. Ps by plants on earth produces billions of tons of organic matter each year, an output that
is not matched by any other chemical process
12. all food energy on earth ultimately comes from plants.
Lesson Objectives
1. Define terms autotroph and producer.
2. Describe structure and location of chloroplasts in a leaf.
3. Explain how plants produce oxygen.
4. Explain how Ps is based on redox reactions.
5. Compare products and reactants in Calvin Cycle and light reactions. Explain how the term photosynthesis
relates to these functions.
6. Explain how the electron transport chain and chemiosmosis generate ATP, NADPH and O2 in light
reactions.
7. Describe the reactants and products in Calvin cycle. Explain why this cycle is dependent on light reactions.
8. Describe overall process of light reactions and Calvin cycle, noting products, reactants and locations of
each major step.