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

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

3. Visible light is part of electromagnetic spectrumY O R

4. 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:

5. pigments Plants gather the sun’s energy with
light absorbing molecules called
_______________.
pigments
Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules
*Remember: Heterotrophs obtain their organic material from other organisms
By: VanderWal

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

7. Leaves are the major locations of photosynthesis
Their green color is from chlorophyll within chloroplasts
Chloroplasts are found mainly in cells of the interior tissue (mesophyll) of the leaf
Each mesophyll cell contains 30–40 chloroplasts
CO2 enters and O2 exits the leaf through microscopic pores called stomata

8. Pigments are found within the chloroplast
Chlorophyll & other pigments
embedded in thylakoid membrane
arranged in a “photosystem” (PS)
PS = a group of proteins and lipids that receive energy from the sun

9. 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

10. WHY DON’T WE SEE THE OTHER PIGMENTS?
Carotenoids are usually hidden by
the presence of chlorophyll
Other pigments:
Caroteniods– appear orange, red, and yellow
Xanthophyll- yellow
In the fall chlorophyll production shuts down and other pigments “show”
A spectrophotometer measures a pigment’s ability to absorb various wavelengths
This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

11. What is photosynthesis such a big deal?
Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
These organisms feed not only themselves but also most of the living world

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

13. Plant anatomy THYLAKOIDS GRANUM (pl. grana) = sac-like
photosynthetic = stack of thylakoids
membranes
inside chloroplast
GRANUM (pl. grana)
Image from BIOLOGY by Miller and Levine; Prentice Hall Publishing©2006

14. 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

15. Remember redox reactions?!
OIL RIG
Oxidation is loss
Reduction is gain
Loss and gain of electrons
Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

16. 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

17. 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

18. ELECTRON TRANSPORT CHAIN
ETC PROTEINS:
Plastoquinone
Cytochrome
Plastocyanin
Ferredoxin
Electron Transport Chain
membrane-bound proteins (don’t have to memorize names)
electron END NADP+ accepts)
Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
Diffusion of H+ (protons) across the membrane drives ATP synthesis

19. Light Dependent reactions
Electron Transport Chain
membrane-bound proteins in organelle
electron acceptors
END = NADPH
proton (H+) gradient across inner membrane
ATP synthase enzyme
Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

20. 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

22. 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!

23. 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!

24. 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

25. Photophosphorylation
noncyclic
photophosphorylation
cyclic
Cyclic electron flow is thought to have evolved before linear electron flow
Cyclic electron flow may protect cells from light-induced damage

27. 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

28. 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

29. 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

30. Calvin Cycle
Memorization of the steps in the Calvin cycle, the structure of the molecules and
the names of enzymes (with the exception of ATP synthase) are beyond the scope
of the course and the AP Exam.
Click to See Calvin cycle animation  remember that much of this is out of scope
for the class but sometimes it helps to see the big picture

31. CALVIN CYCLE
also called light independent reaction - does not require light but usually occurs during the day.
It happens in the stroma between thylakoids
NADPH donates hydrogen ions and electrons
ATP donates energy
CO2 donates carbon and oxygen
All to make G3P which later can combine into glucose or cellulose or any other sugar a plant may need

32. 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

33. Normal Carbon Fixation
C3 plants (Ex: rice, wheat, soybeans)
(1st product of carbon fixation has 3 Carbons)
On hot, dry days when plant shuts stomata
plant switches to photorespiration
Rubisco adds O2 to Calvin cycle instead of CO2
This creates a toxic byproduct that must be broken down by mitochondria/peroxisomes
Photorespiration is COUNTERPRODUCTIVE:
Makes NO ATP, Makes NO sugar, Uses ATP
Decreases photosynthesis by siphoning molecules from Calvin cycle

34. PROBLEMS ON HOT DRY DAYS
If stomata are open to receive CO2 results in water loss
On hot, dry days if plant shuts stomata to conserve water photosynthesis slows
Over 8000 species of angiosperms (plants that flower) have developed adaptations which minimize the losses to photorespiration. Many of these plants are considered C4 plants

35. ALTERNATIVE METHODS of CARBON FIXATION
SEE ANIMATION
Have evolved ways to avoid a decrease in photosynthesis due to photorespriation
C4 plants- Ex: corn & sugarcane
Such plants are well adapted to habitats with high daytime temperatures and intense sunlight.
Another kind of C4 plant called the CAM plants- Ex: succulents, cactus, pineapple
Well adapted to conditions of high daytime temperatures, intense sunlight and low soil moisture.

37. C4 plants
C4 plants form a 4-carbon molecule instead of the normal 3-carbon molecules of the Calvin cycle when they take in CO2.
1. CO2 diffuses into a mesophyll cell.
2. CO2 then joins other molecules to form a 4-carbon acid that is stored in the bundle sheath cells – these cells are deep in the leaf where it is hard for O2 to reach – thus preventing photorespiration
3. Then this acid can be broken down in the Calvin Cycle
CAM plants — separate their C3 and C4 cycles by time
CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them)
CO2 joins other molecules to form a 4-carbon acid which can accumulates during the night in the central vacuole of the cells.

38. CALVIN CYCLE found in BUNDLE SHEATH CELLS in C4 plants

39. photophosphorylation
Quick Vocab Check-
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

40. Hank Green on Photosynthesis
3 amazing photosynthetic ANIMALS is also a good video to watch