1. Chapter 10 Photosynthesis

2. Energy needs of life All life needs a constant input of energy
Heterotrophs (Animals)
get their energy from “eating others”
organic molecules
make energy through cellular respiration
Autotrophs (Plants)
get their energy sunlight
Build their own organic molecules (food) from CO2
synthesize sugars through photosynthesis
Then get energy from the sugars they made by performing cellular respiration.

3. How are they connected? Heterotrophs  Autotrophs 
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6
6O2
6CO2
6H2O
ATP  +
exergonic
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

4. Sites of Photosynthesis
Mesophyll cells: chloroplasts mainly found in these cells of leaf
stomata: pores in leaf (CO2 enter/O2 exits)
chlorophyll: green pigment in thylakoid membranes of chloroplasts

5. Plant structure Chloroplasts Thylakoid membrane contains
double membrane
stroma
fluid-filled interior
thylakoid sacs
grana stacks
Thylakoid membrane contains
chlorophyll molecules
electron transport chain
ATP synthase
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.

6. Tracking atoms through photosynthesis
Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis (C.B. van Niel)
Reactants:
Products:
6 CO2
12 H2O
C6H12O6
6 H2O
6 O2

7. Photosynthesis = Light Reactions + Calvin Cycle

8. Light Reactions  produces ATP produces NADPH
H2O
ATP O2
light
energy  +
NADPH
H2O
sunlight
produces ATP
produces NADPH
releases O2 as a waste product
Energy Building
Reactions
NADPH
ATP O2

9. Calvin Cycle  builds sugars uses ATP & NADPH
CO2
C6H12O6  +
NADP
ATP
NADPH
ADP
CO2
builds sugars
uses ATP & NADPH
recycles ADP & NADP back to make more ATP & NADPH
ADP
NADP
Sugar Building
Reactions
NADPH
ATP
sugars
C6H12O6

10. Putting it all together
CO2
H2O
C6H12O6 O2
light
energy  +
H2O
CO2
Plants make both:
Energy (ATP & NADPH)
sugars
sunlight
ADP
NADP
Energy Building
Reactions
Sugar Building
Reactions
NADPH
ATP
sugars
C6H12O6 O2

11. Why do Calvin Cycle? Because light reaction only produces 1 ATP
The glucose made from Calvin cycle goes to cells of plant for cellular respiration (yes, the plants do BOTH photosynthesis and cellular respiration) to produce lots of ATP!

12. Light Reactions

13. The light reactions convert solar energy to chemical energy of ATP and NADPH
Nature of sunlight
Light = energy = electromagnetic radiation
Shorter wavelength (λ): higher E
Visible light - detected by human eye
Light: reflected, transmitted or absorbed

14. 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

15. (absorption of chlorophylls a, b, & carotenoids combined)
Action Spectrum: plots rate of photosynthesis vs. wavelength
(absorption of chlorophylls a, b, & carotenoids combined)
Engelmann: used bacteria to measure rate of photosynthesis in algae; established action spectrum
Which wavelengths of light are most effective in driving photosynthesis?

16. Interaction of light with chloroplasts

17. Photosystems of photosynthesis
2 photosystems in thylakoid membrane
collections of chlorophyll molecules
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.
antenna pigments

18. Proton motive force generated by an Electron Transport Chain
H+ from water
H+ pumped across by cytochrome
Removal of H+ from stroma when NADP+ is reduced

19. Calvin Cycle

20. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
Uses ATP, NADPH, CO2
Produces 3-C sugar G3P (glyceraldehyde-3-phosphate)
Three phases:
Carbon fixation
Reduction
Regeneration of RuBP (CO2 acceptor)

22. To G-3-P and Beyond! Glyceraldehyde-3-P G-3-P = important intermediate
end product of Calvin cycle
energy rich 3 carbon sugar
“C3 photosynthesis”…..C3 Plants
G-3-P = important intermediate
G-3-P   glucose   carbohydrates
  lipids
  amino acids
  nucleic acids

23. Accounting The accounting is complicated
1 turn of Calvin cycle = 1 G3P
3 CO2 1 G3P (3C)
2 turns of Calvin cycle = 1 C6H12O6 (6C)
6 CO2 1 C6H12O6 (6C)
18 ATP + 12 NADPH  1 C6H12O6
any ATP left over from light reactions will be used elsewhere by the cell

24. Evolutionary Adaptations
On hot dry days when a plant is forced to close its stomata to prevent excess water loss, CO2 levels inside the leaf become low. If the plant continues to attempt to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations.
Less sugar is made….bad news for the plant.
This is called Photorespiration.
To prevent this process, two specialized biochemical additions have been evolved in the plant world: C4 and CAM metabolism

25. 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

26. 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

28. Importance of Photosynthesis
Plant:
Glucose for respiration
Cellulose
Global:
O2 Production
Food source

29. Review of Photosynthesis and Cellular Respiration

30. 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