1. Photosystems & Light Reactions AP Biology Ms. Day
Chapter 10 (Part 2)
Photosystems & Light Reactions
AP Biology
Ms. Day

2. Recall…Fill in the letters of the leaf anatomyB E C D

3. Recall…Fill in the letters of the leaf anatomy
A=Spongy mesophyll
B=Palisade m mesophyll
E= Air space
C= Stoma
D= guard cell

4. Recall… How is water formed in cellular respiration?
Label the parts of water as it is formed
+_____

5. Recall… How is water formed in cellular respiration?
e-’s H+’s  H20
+2e -
x 2

6. A B ?H K
I + J M L G E
D  F C

7. G3P(pGAL) An overview of photosynthesis
8.1a - Photosynthesis: The Flow of Energy Process
8.1b - Photosynthesis: Overview of Reactions
H2O
CO2
Light
LIGHT REACTIONS
CALVIN
CYCLE
Chloroplast
GLUCOSE
(sugar)
NADPH
NADP 
ADP
+ P O2
ATP
G3P(pGAL)
Starch

8. 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”

9. Light Reactions (in detail)
The light reactions convert solar energy to the chemical energy of ATP and NADPH

10. Spectrophotometer
Machine that sends light through pigments  measures fraction of light transmitted and absorbed at each wavelength
Produces an absorption spectrum

11. An absorption spectrum
graph plotting light absorption versus wavelength
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
Slit moves to
pass light
of selected
wavelength
Green
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
Blue 1 2 3 4
100

12. The absorption spectra of three types of pigments in chloroplasts

13. Chlorophyll a
The main photosynthetic pigment (primary pigment)
Accessory Pigments
Absorb different wavelengths of light
pass energy to chlorophyll a

14. Excitation of Chlorophyll by Light
When a pigment absorbs light
It goes from a ground state (stable) to an excited state (unstable)

15. Photosystems Reaction centers used in Light Reactions
Made of light harvesting proteins (complexes)
Funnel (move) energy of photons (light pieces) to the middle of reaction center
INSIDE thylakoid membrane
2 DIFFERENT CENTERS
Photosystem II (PSII)
Photosystem I (PSI)

16. (INTERIOR OF THYLAKOID)
Primary election
acceptor
Photon
Thylakoid
Light-harvesting
complexes
Reaction
center
Photosystem
STROMA
Thylakoid membrane
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
THYLAKOID SPACE
(INTERIOR OF THYLAKOID) e–
When a reaction-center “special” chlorophyll a molecule absorbs energy
An electron gets bumped up to a primary electron acceptor
edu/~cmallery/255/2 55phts/photosynthesi s.swf

17. 2 Different Photosystems
BOTH found in thylakoid phospholipid membrane
2 types
photosystems II (PII)
Uses chlorophyll a called P680
1ST photosystem in membrane
photosystems I (PI)
Uses chlorophyll a called P700
2ND photosystem in membrane

18. 2 types of e- flow Non-cyclic photophosphorylation

19. Recall… What are the 3 events in Chemiosmosis?
Create a proton gradient
A difference in amts ( [ ]’s) of H+ ions on either side of a membrane
Uses energy from e-’s falling down an ETC to PUSH H+ ions AGAINST [ ] gradient
Create a proton motive force
H+ ions use facilitated diffusion to move down ATP synthase (H  L [ ] )
ATP synthase makes ATP
ATP synthase spins from motive force so ADP + Pi  ATP

20. Non cyclic light reactions & Chemiosmosis

21. Noncyclic Electron Flow Steps
1. PII  excited e- to primary e- acceptor
2. Photolysis- water splits by enzyme e-s are replaced from lost chl a P H20  2 H e- + ½ O2
(2 O’s combine and O2 is released)
3. Electron Transport Chain
proteins in thylakoid membrane pass e-s (become reduced)
Flow of e-’s is exergonic  releases energy to make ATP
Proteins used = Cytochromes, PC, and PQ complexes

22. 4. Chemiosomosis – the process that forms ATP during light reactions
Protons (H+) are pumped ACTIVELY into thylakoid space (lumen) from stroma by electron transport system #1
AGAINST [ H+ ] gradient  LESS acidic
Protons (H+) from split water build up in thylakoid space (lumen) MORE acidic
H+’s then DIFFUSE down ATP synthase channels in stroma

23. 5. Photosystem I – P700 absorbs energy  e- becomes excited goes down electron transport chain #2
6. NADP+ becomes reduced using NADP+ reductase
NADP H e-  NADPH
from split from PSI
water

24. Non cyclic light reactions & Chemiosmosis
PHOTOLYSIS

25. Antennae pigments

26. Non Cyclic Electron Flow (Overview)
Light  P680  ATP made  P700  NADPH made O2 released In
Out
2 light events
Water
Oxygen gas
ADP+ Pi
ATP
NADP+
NADPH

27. Noncyclic Electron Flow Animations
/metabolism/photosynthesis.swf
hill.com/sites/ /student_view0/chapter 10/animations.html#
2nd animation
gy/Bio231/ltrxn.html
ons/content/harvestinglight.html

28. Tutorials (Light Reactions)m
htm
photosynthesis_1/photosynthesis_1.html

29. Cyclic Electron Flow Under certain conditions
Photoexcited electrons take an alternative path (shorter pathway)
Why use this pathway?
Sugar production (Calvin Cycle) uses a lot more ATP than NADPH
Sometimes, autotrophs run low on ATP needs to replenish ATP levels and uses cycle e- flow

30. Why is it “cyclic”?
This process is cyclic since electrons return to the reaction center (PS1)
Water is NOT required and oxygen is NOT produced
ONLY PS1 and P700 is used and PART of the ETC #1

31. Cyclic Electron Flow Steps
Photon hits PS1
e-s from PSI P700 travel BACK to cytochrome complex (ETC#1) in P680 electron transport chain
ATP is produced using ATP synthase and H+ diffusion…NO NADPH!!!

32. In cyclic electron flow
Only photosystem I is used
Only ATP is produced NO NADPH

33. NON CYCLIC

34. CYCLIC

35. NON CYCLIC
CYCLIC

36. CYCLIC PHOTOPHOSPHORALYTION OverviewIN
OUT
1 light event
ADP + P
ATP
NO OXYGEN and NO NADPH like non-cyclic photophosphorylation

37. Noncyclic Electron Flow Animations
YnMFkK78
hill.com/sites/ /student_view0/chapter 10/animations.html#
1st animation
EXCELLENT REVIEW OF CELLULAR RESPIRATION and PHOTOSYNTHESIS
(14 min)