1. AP BIOLOGY PHOTOSYNTHESIS Chapter 10 Light Reactions
AP BIOLOGY PHOTOSYNTHESIS Chapter 10 Light Reactions
tomatoes

2. The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture free energy present in light to yield ATP and NADPH, which power the production of organic molecules.
Evidence of student learning is a demonstrated understanding of each of the following:
1. During photosynthesis, chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I and II.
2. Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through an electron transport chain (ETC). [See also 4.A.2]
3. When electrons are transferred between molecules in a sequence of reactions as they pass through the ETC, an electrochemical gradient of hydrogen ions (protons) across the thykaloid membrane is established.
4. The formation of the proton gradient is a separate process, but it is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase.
 5. The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast.
Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere; prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.

3. Fun Fact of the Day
Photosynthesis first evolved in prokaryotic organisms; scientific evidence supports that prokaryotic (bacterial) photosynthesis was responsible for the production of an oxygenated atmosphere; prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.

4. different wavelengths as different ___________ wavelengths
Sunlight is made up
of many different
_______________
of light
Your eyes “see”
different wavelengths as
different ___________
wavelengths
colors

5. Visible light is part of electromagnetic spectrumY O R

6. pigments Plants gather the sun’s energy with
light absorbing molecules called
_______________.
pigments
By: VanderWal

7. The main energy absorbing molecule in green plants is
The main energy
absorbing molecule
in green plants is
__________________
CHLOROPHYLL a

8. CAROTENOID PIGMENTS appear ORANGE, RED, and YELLOW
CAROTENOID PIGMENTS appear ORANGE, RED, and YELLOW
Carotene
appears orange
Xanthophyll
appears yellow

9. Pigments of photosynthesis
Chlorophyll & other pigments
embedded in thylakoid membrane
arranged in a “photosystem”
structure-function relationship

10. Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light
chlorophyll a
absorbs best in red & blue wavelengths & least in green
other pigments with different structures absorb light of different wavelengths

11. WHY ARE PLANTS GREEN? We “see” reflected light
Light wavelengths that are reflected
bounce back to your eyes . . .
so leaves “LOOK” green.
Image modified from:

12. WHY DON’T WE SEE THE OTHER PIGMENTS?
Carotenoids are usually hidden by
the presence of chlorophyll

13. In the fall chlorophyll production shuts down and other pigments “show”

14. PHOTOSYNTHESIS HAPPENS IN CHLOROPLASTS
THYLAKOIDS
= sac-like
photosynthetic = stack of thylakoids
membranes
inside chloroplast
GRANUM (pl. grana)
Image from BIOLOGY by Miller and Levine; Prentice Hall Publishing©2006

15. SPACES THYLAKOID SPACE (lumen) STROMA cytoplasm
Gel-filled space inside chloroplast surrounding
thylakoid sac
Gel-filled space
Inside the
thylakoid
sac
cytoplasm
Gel-filled space OUTSIDE chloroplast but inside the cell membrane

16. PHOTOSYNTHESIS OVERVIEW
Pearson Education Inc; Publishing as Prentice Hall

17. ATP LIGHT DEPENDENT REACTIONS CHARGE UP ENERGY CARRIER = _____ Adenine
Phosphate groups
Ribose

18. Energy for cellular work (Energy- consuming)
ATP
Energy for cellular work
(Energy- consuming)
Energy from catabolism
(Energy- yielding)
ADP + P i

19. NADP+ + 2e- + H+ → NADPH High energy electron carrier = _____________
NICOTINAMIDE ADENINE DINUCLEOTIDE PHOSPHATE
High energy electron carrier = _____________
NADP+
NADP+ + 2e- + H+ → NADPH

20. Photosynthesis Light reactions Calvin cycle light-dependent reactions
energy production reactions
convert solar energy to chemical energy
Make ATP & NADPH
Calvin cycle
light-independent reactions
sugar production reactions
use chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6

21. Light Reactions
chlorophylls absorb free energy from light, boosting electrons to a higher energy level in Photosystems I and II.
Photosystems I and II are embedded in the internal membranes of chloroplasts (thylakoids) and are connected by the transfer of higher free energy electrons through an electron transport chain (ETC).

22. Photosystems of photosynthesis
2 photosystems in thylakoid membrane
Both have a REACTION CENTER • CHLOROPHYLL a molecules • PRIMARY ELECTRON ACCEPTOR
Surrounded by light-gathering “ANTENNA COMPLEX” • Accessory pigments (chlorophyll b, carotenoids)
Collect light energy and pass it on to chlorophyll a
Photosystem II
P680 = absorbs 680nm wavelength red light
Photosystem I
P700 = absorbs 700nm wavelength red light

23. ETC of Photosynthesis Photosystem II Photosystem I
Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide

24. Light Dependent reactions
Electron Transport Chain
membrane-bound proteins in organelle
electron acceptors
NADPH
proton (H+) gradient across inner membrane
Where’s the double membrane?
ATP synthase enzyme
The formation of the proton gradient linked to the synthesis of ATP from ADP and inorganic phosphate
Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

25. Chloroplasts transform light energy into chemical energy of ATP
use electron carrier NADPH
ETC of Photosynthesis
Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide

26. LIGHT DEPENDENT REACTIONS
ETC produces from light energy
ATP & NADPH
go to Calvin cycle
PS II absorbs light
excited electron passes from chlorophyll to “primary electron acceptor”
need to replace electron in chlorophyll
enzyme extracts electrons from H2O & supplies them to chlorophyll
splits H2O
O combines with another O to form O2
O2 released to atmosphere
and we breathe easier!

27. ETC of Photosynthesis to the Calvin Cycle 3 1 H+ 4 H+
PS II absorbs light
Excited electron passes from chlorophyll to the primary electron acceptor
Need to replace electron in chlorophyll
An enzyme extracts electrons from H2O & supplies them to the chlorophyll
This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2
O2 released to atmosphere
Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state
Electron is handed off down chain from electron acceptor to electron acceptor
In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac.
Flow back through ATP synthase to generate ATP. 4 H+
ADP + Pi
ATP

28. ETC of Photosynthesis to the Calvin Cycle 3 1 2 H+ 4 H+ ATP ADP + Pi

29. $$ in the bank… reducing power
ETC of Photosynthesis
electron carrier 6
to the Calvin Cycle 5
Need a 2nd photon -- shot of light energy to excite electron back up to high energy state.
2nd ETC drives reduction of NADP to NADPH.
Light comes in at 2 points.
Produce ATP & NADPH
$$ in the bank… reducing power

30. ETC of Photosynthesis split H2O Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide
split H2O

31. MAKING ATP moves the electrons runs the pump pumps the protons
forms the gradient
drives the flow of protons through ATP synthase
attaches Pi to ADP
forms the ATP H+
ADP + Pi
ATP

32. Noncyclic Photophosphorylation
Light reactions elevate electrons in 2 steps (PS II & PS I)
PS II generates energy as ATP
PS I generates reducing power as NADPH
1 photosystem is not enough.
Have to lift electron in 2 stages to a higher energy level. Does work as it falls.
First, produce ATP -- but producing ATP is not enough.
Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle!

33. Cyclic photophosphorylation
PS I doesn’t pass electron to NADP… it cycles back to ETC & makes more ATP, but no NADPH
coordinates light reactions to Calvin cycle
Important in maintaining proportion of ATP & NADPH for Calvin
Calvin cycle uses more ATP than NADPH X

34. Photophosphorylation
cyclic
photophosphorylation
noncyclic
photophosphorylation

35. Experimental evidence
Where did the O2 come from?
radioactive tracer = O18
6CO2
6H2O
C6H12O6
6O2
light
energy  +
Experiment 1
6CO2
6H2O
C6H12O6
6O2
light
energy  +
6CO2
6H2O
C6H12O6
6O2
light
energy  +
Experiment 2
Proved O2 came from H2O not CO2 = plants split H2O

36. LIGHT DEPENDENT REACTION
Requires ______________
Molecules embedded in ________________________
Made up of __________________
connected by ______________________
& ___________________
Uses light energy to change
ADP + P → _______
NADP+ + 2e- + H + → _________
Breaks apart ______ molecules and releases _____________
LIGHT
THYLAKOID membranes
PHOTOSYSTEMS II & I
ELECTRON TRANSPORT CHAIN
ATP SYNTHASE
ATP
NADPH
H20
oxygen

37. LIGHT REACTIONS summary
Where did the energy come from?
Where did the electrons come from?
Where did the H2O come from?
Where did the O2 come from?
Where did the O2 go?
sunlight
From chlorophyll; replaced by H2O
In through roots
Made when water splits
Out through stomata

38. LIGHT REACTIONS summary
Where did the H+ come from?
Where did the ATP come from?
What will the ATP be used for?
Where did the NADPH come from?
What will the NADPH be used for?
Split off of water
Produced by ATP synthase during light rxns
Make sugar in Calvin cycle
Receives e-’s at end of ETC
Make sugar in Calvin cycle
…stay tuned for the Calvin cycle

39. PHOTOSYNTHESIS Light-Dependent Reaction Light & Water Oxygen
ATP
NADPH
(CH2O)n
Carbon Dioxide
Light-Independent Reactions
CALVIN CYCLE

40.
CALVIN CYCLE

41. Calvin Cycle See Calvin cycle animation X 2
The energy captured in the light reactions as ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle, which occurs in the stroma of the chloroplast.
See Calvin cycle animation
X 2

42. The cycle builds sugar from smaller molecules
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules
Carbon enters the cycle as CO2, as well as the products ATP and NADPH from the light reactions
CO 2 enters and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

43. The Calvin cycle has three phases:
Carbon fixation (catalyzed by rubisco)
Reduction
Regeneration of the CO2 acceptor (RuBP)

44. LE 10-18_3 Input 3 (Entering one at a time) CO2
H2O
CO2
Input
Light 3
(Entering one
at a time)
NADP+
ADP
CO2
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
Phase 1: Carbon fixation
NADPH
Rubisco O2
[CH2O] (sugar) 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
CALVIN
CYCLE 3 6 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
G3P
(a sugar)
Glucose and
other organic
compounds
Output

45. CALVIN CYCLE ____________ require ____________
(also called _________________________)
____________ require ____________
Happens in _________ between thylakoids
NADPH donates _______________
ATP donates _________________
CO2 donates ______________
to make __________________________
LIGHT INDEPENDENT
DOES NOT
LIGHT
STROMA
Hydrogen ions + electrons
ENERGY
Carbon & oxygen
glyceraldehyde-3-phosphate (G3P)

46. To make one glucose molecule C6H12O6 the Calvin cycle uses
_____ molecules of CO2
_____ molecules of ATP
_____ molecules of NADPH 6 18 12
Campbell concept check 10.3

47. CALVIN CYCLE summary Where does the C in G3P come from?
Where does the H in G3P come from?
Where does the O in G3P come from?
Where does the ADP & NADP+ go?
Where does the G3P go?
CO2
From H2O via NADPH
CO2
Back to light reaction to recharge
Used to make glucose and other organic molecules

48. STOMA (pl. STOMATA)
GUARD CELLS

49. PROBLEMS ON HOT DRY DAYS
If stomata are open to receive CO results in water loss
On hot, dry days if plant shuts stomata to conserve water photosynthesis slows

50. PHOTORESPIRATION C3 ____ plants (Ex: rice, wheat, soybeans)
(1st product of carbon fixation has 3 C’s- 3PG)
On hot, dry days when plant shuts stomata
plant switches to ______________________
Rubisco adds O2 to Calvin cycle instead of CO2
Product broken down by mitochondria/peroxisomes to release CO2
COUNTERPRODUCTIVE:
Makes NO ATP
Makes NO sugar
Uses ATP
Decreases photosynthesis by siphoning molecules from Calvin cycle
PHOTORESPIRATION

51. ALTERNATIVE METHODS of CARBON FIXATION
______ plants (Ex: corn & sugarcane
______ Crassulacean acid metabolism (Ex: succulents, cactus, pineapple,)
WAYS TO AVOID DECREASE IN PHOTOSYNTHESIS DUE TO PHOTORESPIRATION
CAM
SEE ANIMATION

52. CALVIN CYCLE found in BUNDLE SHEATH CELLS in C4 plants*
PEP CARBOXYLASE
________________________ adds CO2 to make a 4 carbon molecule before entering Calvin Cycle

54. photophosphorylation
Process of using H+ gradient to generate ATP
= ________________________
(Can refer to ATP made in mitochondria too)
Process of creating ATP using a Proton gradient created by the energy gathered from sunlight.
Process that consumes oxygen, releases CO2, generates no ATP, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide
= ___________________________________
chemiosmosis
photophosphorylation
photorespiration

55. The Importance of Photosynthesis: A Review
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
In addition to food production, photosynthesis produces the oxygen in our atmosphere

56. Photosystem II Electron transport chain Photosystem I
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP + P i
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport
chain
Photosystem I
ATP
G3P
Starch
(storage)
NADPH
Amino acids
Fatty acids
Chloroplast O2
Sucrose (export)