1. Photosynthesis: Life from Light

2. How are they connected? Heterotrophs and Autotrophs  Autotrophs 
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6
6O2
6CO2
6H2O
ATP  +
exergonic
Where’s the ATP?
Autotrophs
So, in effect, photosynthesis is respiration run backwards powered by light.
Cellular Respiration
oxidize C6H12O6  CO2 & produce H2O
fall of electrons downhill to O2
exergonic
Photosynthesis
reduce CO2  C6H12O6 & produce O2
boost electrons uphill by splitting H2O
endergonic
making energy & organic molecules from light energy
+ water + energy  glucose + oxygen
carbon
dioxide
6CO2
6H2O
C6H12O6
6O2
light
energy  +
endergonic

3. Photoautotrophs

4. Plant structure Obtaining raw materials sunlight CO2 H2O Nutrients
leaves = solar collectors
CO2
stomates = gas exchange
Found under leaves
H2O
uptake from roots
Nutrients
N, P, K, S, Mg, Fe…

5. Plant structure Chloroplasts Chlorophyll & ETC in thylakoid membrane
double membrane
stroma
thylakoid sacs
grana stacks
Chlorophyll & ETC in thylakoid membrane
H+ gradient built up within thylakoid sac
A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid lumen or thylakoid space.
Thylakoids may be stacked into columns called grana. H+

6. Pigments of photosynthesis
Why does this structure make sense?
chlorophyll & accessory pigments
“photosystem”
embedded in thylakoid membrane
structure  function
Orientation of chlorophyll molecule is due to polarity of membrane.

7. Photosynthesis = Light Reactions + Calvin Cycle

8. Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light
chlorophyll a (dominant pigment)
absorbs best in red & blue wavelengths & least in green
other pigments with different structures absorb light of different wavelengths
Why are plants green?

9. Photosynthetic pigments
Pigments absorb different λ of light
chlorophyll – absorb violet-blue/red light, reflect green
chlorophyll a (blue-green): light reaction, converts solar to chemical E
chlorophyll b (yellow-green): conveys E to chlorophyll a
carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis
Types: xanthophyll (yellow) & carotenes (orange)
anthocyanin (red, purple, blue): photoprotection, antioxidants

10. It’s the Dark Reactions!
Photosynthesis
Light reactions
light-dependent reactions
energy production reactions
convert solar energy to chemical energy
ATP & NADPH
Calvin cycle
light-independent reactions
sugar production reactions
uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6
It’s the Dark Reactions!

11. Light Reactions Summary:
Light energy splits H2O to O2 releasing high energy electrons (e-)
Movement of e- used to generate ATP
Electrons end up on NADP+, reducing it to NADPH

12. Light reactions Electron Transport Chain (like cell respiration!)
membrane-bound proteins in organelle
electron acceptor
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.

13. Photosystem: reaction center & light-harvesting complexes (pigment + protein)

14. Photosystems 2 photosystems in thylakoid membrane reaction center
act as light-gathering “antenna complex”
Photosystem II
chlorophyll a
P680 = absorbs 680nm wavelength red light
Photosystem I
chlorophyll b
P700 = absorbs 700nm wavelength red light
reaction center
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths.

15. ETC of Photosynthesis ETC produces from light energy
ATP & NADPH
NADPH (stored energy) goes to Calvin cycle
PS II absorbs light
excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER.
splits H2O (Photolysis!!)
O2 released to atmosphere
ATP is produced for later use

16. 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

17. Electron Flow A. Linear (noncyclic) electron flow
Two routes for electron flow:
A. Linear (noncyclic) electron flow
B. Cyclic electron flow

18. Light Reaction (Linear electron flow)
Chlorophyll excited by light absorption
E passed to reaction center of Photosystem II (protein + chlorophyll a)
e- captured by primary electron acceptor
Redox reaction  e- transfer
e- prevented from losing E (drop to ground state)
H2O is split to replace e-  O2 formed

19. e- passed to Photosystem I via ETC
E transfer pumps H+ to thylakoid space
ATP produced by photophosphorylation
e- moves from PS I’s primary electron acceptor to 2nd ETC
NADP+ reduced to NADPH
MAIN IDEA: Use solar E to generate ATP & NADPH to provide E for Calvin cycle

20. 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!

21. Cyclic photophosphorylation
If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH
coordinates light reactions to Calvin cycle
Calvin cycle uses more ATP than NADPH X

22. From Light reactions to Calvin cycle
Chloroplast stroma
Need products of light reactions to drive synthesis reactions
ATP
NADPH
What is there
left to do?
Make sugar!

23. Calvin Cycle: Uses ATP and NADPH to convert CO2 to sugar
Occurs in the stroma
Uses ATP, NADPH, CO2
Produces 3-C sugar G3P (glyceraldehyde-3-phosphate)
Three phases:
Carbon fixation
Reduction
Regeneration of RuBP (CO2 acceptor)

24. From CO2  C6H12O6 CO2 has very little chemical energy
fully oxidized
C6H12O6 contains a lot of chemical energy
reduced
endergonic
Reduction of CO2  C6H12O6 proceeds in many small uphill steps
each catalyzed by specific enzyme
using energy stored in ATP & NADPH

25. Calvin cycle 1C 5C 6C 3C 3C 3C 2x x2 2x CO2 1. Carbon fixation
ribulose
bisphosphate
1. Carbon fixation
3. Regeneration
of RuBP 5C
RuBP
Rubisco 6C
3 ADP
3 ATP
-enzyme that
Binds CO2
to RuBP
PGAL
to make
glucose 3C 2x
PGA 3C x2
PGAL
sucrose
cellulose
etc.
RuBP = ribulose bisphosphate
Rubisco = ribulose bisphosphate carboxylase
PGA = phosphoglycerate
PGAL = phosphoglyceraldehyde
2. Reduction
6 NADP
6 NADPH
6 ADP
6 ATP 3C 2x

26. Calvin cycle PGAL   important intermediate
Six turns of Calvin Cycle = 1 glucose
PGAL   glucose   carbohydrates
  lipids
  amino acids
  nucleic acids

27. Summary Light reactions Calvin cycle produced ATP produced NADPH
consumed H2O
produced O2 as by product
Calvin cycle
consumed CO2
produced PGAL
regenerated ADP
regenerated NADP
ADP
NADP

28. Factors that affect Photosynthesis
Enzymes are responsible for several photosynthetic processes, therefore, temperature and pH can affect the rate of photosynthesis.
The amount and type of light can affect the rate.
A shortage of any of the reactants,CO2 and/or H2O, can affect the rate.

29. Supporting a biosphere
On global scale, photosynthesis is the most important process for the continuation of life on Earth
each year photosynthesis synthesizes 160 billion tons of carbohydrate
heterotrophs are dependent on plants as food source for fuel & raw materials

30. The Great Circle of Life!
Energy cycle
sun
Photosynthesis
glucose O2
H2O
CO2
Cellular Respiration
The Great Circle of Life!
Where’s Mufasa?
ATP

31. Photosynthesis Calvin Cycle Light ENERGY Light Reaction involves both
in which
stored in
CO2 fixed to RuBP
organic molecules
energized electrons
H2O split
pass down
Reduce NADP+ to
C3 phosphorylated and reduced
by mechanism of
ETC
NADPH
O2 evolved
using
to form
regenerate RuBP
ATP
G3P
chemiosmosis
using
in process called
glucose & other carbs
photophosphorylation

32. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Photorespiration
Metabolic pathway which:
Uses O2 & produces CO2
Uses ATP
No sugar production (rubisco binds O2  breakdown of RuBP)
Occurs on hot, dry bright days when stomata close (conserve H2O)
Why? Early atmosphere: low O2, high CO2?

33. Evolutionary Adaptations
Problem with C3 Plants:
CO2 fixed to 3-C compound in Calvin cycle
Ex. Rice, wheat, soybeans
Hot, dry days:
partially close stomata, ↓CO2
Photorespiration
↓ photosynthetic output (no sugars made)

34. C4 Plants: CO2 fixed to 4-C compound Ex. corn, sugarcane, grass
Hot, dry days  stomata close
2 cell types = mesophyll & bundle sheath cells
mesophyll : PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath
bundle sheath: CO2 used in Calvin cycle
↓photorespiration, ↑sugar production
WHY? Advantage in hot, sunny areas

35. C4 Leaf Anatomy

36. CAM Plants: Crassulacean acid metabolism (CAM)
NIGHT: stomata open  CO2 enters  converts to organic acid, stored in mesophyll cells
DAY: stomata closed  light reactions supply ATP, NADPH; CO2 released from organic acids for Calvin cycle
Ex. cacti, pineapples, succulent (H2O-storing) plants
WHY? Advantage in arid conditions

37. LIGHT REACTIONS
Calvin cycle

38. Mitochondria
Chloroplast

39. Comparison RESPIRATION PHOTOSYNTHESIS Plants + Animals
Needs O2 and food
Produces CO2, H2O and ATP, NADH
Occurs in mitochondria membrane & matrix
Oxidative phosphorylation
Proton gradient across membrane
Plants
Needs CO2, H2O, sunlight
Produces glucose, O2 and ATP, NADPH
Occurs in chloroplast thylakoid membrane & stroma
Photorespiration
Proton gradient across membrane

40. Summary of photosynthesis
6CO2
6H2O
C6H12O6
6O2
light
energy  +
Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the H2O go?
Where did the energy come from?
What’s the energy used for?
What will the C6H12O6 be used for?
Where did the O2 come from?
Where will the O2 go?
What else is involved that is not listed in this equation?