1. Photosynthesis in Detail
Light Reactions and Calvin Cycle
Honors Biology

2. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of two processes
1) The Light reactions
NEEDS LIGHT
Light Dependent Reactions
2) The Calvin cycle
A.k.a- Dark Reactions or Light Independent Reactions
DOES NOT NEED LIGHT

3. Convert solar energy to chemical energy
The Light Reactions
Occur in the grana (& thylakoids)
Convert solar energy to chemical energy
Chlorophyll absorbs solar energy
Split water
release oxygen gas (a by-product)
produce ATP (using chemiosmosis)
Forms NADPH from NADP+ (an e- acceptor)
Temporarily stores high energy e-’s
“Electron shuttle bus”

4. The Calvin Cycle Occurs in the stroma Forms SUGAR from carbon dioxide
Carbon fixation occurs (CO2 fixed carbon)
using ATP for energy and NADPH for reducing power (adding e-s to fixed carbon)
Fixed carbon  carbohydrate (glucose)

5. An overview of photosynthesis
CO2
Light
LIGHT REACTIONS
CALVIN
CYCLE
Chloroplast
GLUCOSE
(sugar)
NADPH
NADP 
ADP
+ P O2
ATP
G3P
Starch

6. Photosynthesis Photosynthesis Occurs in the chloroplast
Divided into 2 sets of reactions:
- Light Dependent Reaction
- Calvin Cycle (Dark Reaction)

7. Light Reaction Requires Light!!
Occurs in the thylakoid/grana (in chloroplast)
Electron Transport Chain (ETC) makes ATP and NADPH  sends it to the Calvin Cycle
WASTE PRODUCT: O2 Gas

8. You need to be able to draw this picture!
Part 1 – Light Reactions
You need to be able to draw this picture!
Photosystem II
ETC
Photosystem I
H2O Splits 

9. What are the main steps in the light reactions?
Uses energy from sunlight
Splits water into H+ and O2
Converts ADP into ATP and NADP+ into NADPH
Use sunlight and water
Produces Oxygen, NADPH,
and ATP
WRITE
Light Rxn Sequence of Events…
Photosystem II
ETC
Photosystem I

10. How is light absorbed in the thylakoid?
Photosystems absorb light
Clusters of chlorophyll and proteins trap energy from the sun
Energy is transferred to electrons  makes “excited” electrons
Photosystems are located on the thylakoid membrances

11. What are Electron Carriers?
Compound (NADP+) that can accept a pair of high-energy electrons and transfer them to another molecule
NADP+ grabs/carries 2 electrons and a H+  becomes NADPH

12. Photosystem II Photosystem II Steps →
1) Chlorophyll and other light-absorbing pigments absorb energy from the sun
Energy transferred to e-  high energy e- leave the chlorophyll and go to the ETC (electron transport chain)
2) H2O molecules are broken down
- Electrons come from H2O
- O-, H+ and e- are separated
For every TWO H2Os we get 4e-, 4H+, and 1 O2
Photosystem II comes first in the ETC but was discovered second. PS II needs water to function! Electrons come from water  2 H2Os make 4 e-

13. Photosystem II (cont’d)
3) The excited e- jump from protein to protein and enter the ETC (electron transport chain) on the thylakoid membrane
As e- jump through the ETC, they lose energy
This energy pumps/pulls H+ from the outside (stroma) to the inside (lumen - aka - thylakoid space)
H+ ions start to
build up inside the
thylakoid
Next, e- move to
Photosystem I
Photosystem II comes first in the ETC but was discovered second. PS II needs water to function! Electrons come from water!

14. Photosystem I Photosystem I → 4) More energy is absorbed from sunlight
Energy is added to e- from Photosystem II
Excited e- leave the molecules
5) E- are added to NADP+ which then becomes NADPH
NADPH functions like ATP….NADPH goes back into the light rxn and it begins again! H+ ions are diffusing from high (inside thylakoid) to low (outside). The difference in concentrations is called a GRADIENT

15. Photosystem I (cont’d)
Now, remember…those H+ ions are building up inside the lumen
6) H+ ions diffuse through a protein channel embedded in the thylakoid membrane
The protein channel is part of an enzyme called ATP Synthase
7) As the H+ ions flow through the protein channel, the enzyme (ATP Synthase) makes ATP by adding a P group to ADP
NADPH functions like ATP….NADPH goes back into the light rxn and it begins again! H+ ions are diffusing from high (inside thylakoid) to low (outside). The difference in concentrations is called a GRADIENT

16. #7 #5 #3 #6 #1 #4 #2

17. Light Reaction Summary…
Chemiosmosis  The process that uses the proton gradient and ATP synthase to make ATP
e- travels from PS II  PS I  added to NADP+ makes  NADPH
Both ATP (via ATP Synthase) and NADPH are produced
During the light reaction  these
then enter the
Calvin Cycle
NADP+ is the final e- acceptor in photosynthesis

18. Calvin Cycle! Second part of PS Occurs in the stroma of the
chloroplast
Can occur with or
without light
Three turns shown above

19. Calvin Cycle Summary of Steps: ATP and NADPH used for energy
CO2 is taken into the plant
High-energy sugars are made
Also known as the sug ar factory

20. Carbon dioxide & Calvin Cycle
Enters cycle from the atmosphere
Carbon from the carbon dioxide molecule is used to make glucose (C6H12O6)
Plants then use the glucose for:
Food!
Plant energy!
Stored in bonds of the glucose molecule

21. Carbon “fixing” – process of turning CO2 into a solid compound
Calvin Cycle!
Main job:
Incorporates CO2 and rearranges the C’s to form sugar using the energy (ATP/NADPH) from the Light Reaction
The CC “fixes” CO2
- Carbon enters as CO2 and
leaves as sugar
Goal is to produce a sugar (G3P)
Each turn “fixes” one molecule of carbon, so one G3P takes 3 turns of the Calvin Cycle
INPUT:
Each turn needs:
- 3 ATP
- 2 NADPH
- 1 CO2
Carbon fixing - Carbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound.
Three turns shown above

22. The Calvin cycle has three phases 1. Carbon fixation 2. Reduction 3
The Calvin cycle has three phases 1. Carbon fixation 2. Reduction 3. Regeneration of the CO2 acceptor (RuBP)

23. Enzyme RUBISCO catalyzes reaction
The Calvin Cycle Steps
CARBON FIXATION
1. CO2 enters cycle and attached to a 5-carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable)
Enzyme RUBISCO catalyzes reaction
2. Unstable 6-C molecule immediately breaks down to 3 3-C molecules called 3-phosphoglycerate (3-PGA)

24. REDUCTION
3. Each 3-phosphoglycerate (3-PGA) gets an additional phosphate from ATP (from LIGHT RXN)  becomes 1,3 phosphoglycerate
4. NADPH reduces 1,3 phosphoglycerate to Glyceraldehyde-3-phosphate (G3P)
G3P = a sugar that stores potential energy
Every 3 CO2  yields 6 G3P’s BUT only 1 can be counted in net gain for carbohydrate (GLUCOSE) production

25. REGENERATION OF CO2 ACCEPTOR (RuBP)
5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules
ATP is used !!!!

26. MORE ATP is needed than NADPH!!
The Calvin cycle
NOTE:
MORE ATP is needed than NADPH!!
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction
Also known as PGAL

27. Calvin Cycle Overview For 1 G3P molecule made 9 ATP molecules are used
6 NADPH molecules are used
G3P (aka PGAL)= starting material to make other organic molecules (glucose, starch, etc.)

28. Calvin Cycle
Note: RuBP and Rubisco are NOT the same thing!
Steps:
Enzyme Rubisco attaches CO2 to RuBP  creates 6 x 3-PGA
6 x PGA are reduced to = 6 x G3P (sugar) via ATP/NADPH
1 x G3P exits as glucose
5 x G3P remain in the cycle. ATP rearranges the atoms  3 x RuBP

29. 6CO2 + 6H2O + sunlight  C6H12O6 + 6O2

30. Rate of Photosynthesis
The rate of PS is affected by 3 factors:
Light intensity
Amount of CO2
Temperature
Why wouldn’t roots need chloroplasts?
Root cells don't have chloroplasts, because chloroplasts catch sunlight! Since roots are underground, they are not exposed to the sun!

31. Alternate Pathways
If it is too hot out, the plant will close the stomata so that it doesn’t lose too much water and become dehydrated
However this eliminates the gas exchange!!
SO  the levels of CO2 drop and the levels of O2 increase
This results in…. PHOTORESPIRATION
Photorespiration adds oxygen to the Calvin Cycle instead of carbon dioxide
- This makes NO sugar or ATP
- This wastes all of the plants resources!
Two types of plants:
1) CAM
2) C4 plants

32. Alternate Pathways
Therefore, certain plants, (cacti, pineapple, etc.) only open their stomata at NIGHT!
This prevents them from drying out and still lets them get CO2!

33. What is wrong with this picture?
Nutrients vs. actual food (glucose, CO2, water)

34. For the test, you will need to know these pictures and labels:
Flower Reproductive
Leaf Structure
Chloroplast
Light Reactions
Calvin Cycle

36. Exit Slip-April 9, 2014