1. Photosynthesis: Life from Light and Air

2. Energy needs of life All life needs a constant input of energy
Heterotrophs
get their energy from “eating others”
eat food = other organisms = organic molecules
make energy through respiration
Autotrophs
produce their own energy (from “self”)
convert energy of sunlight
build organic molecules (CHO) from CO2
make energy & synthesize sugars through photosynthesis
consumers
producers

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

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

5. stomate
transpiration
gas exchange

6. *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 builds up within thylakoid sac
outer membrane
inner membrane
granum
stroma
thylakoid
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.

7. It’s not the Dark Reactions!
Photosynthesis
*Light reactions
light-dependent reactions
energy conversion reactions
convert solar energy to chemical energy
ATP & NADPH
*Calvin cycle
light-independent reactions
sugar building reactions
uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6
It’s not the Dark Reactions!

8. Light reactions Electron Transport Chain
thylakoid
chloroplast H+
Light reactions H+
ATP
Electron Transport Chain
like in cellular respiration
proteins in organelle membrane
electron acceptors
NADPH
proton (H+) gradient across inner membrane
find the double membrane!
ATP synthase
Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

9. *ETC of Respiration
Mitochondria transfer chemical energy from food molecules into chemical energy of ATP
use electron carrier NADH
NADH is an input
ATP is an output
O2 combines with H+’s to form water
Electron is being handed off from one electron carrier to another -- proteins, cytochrome proteins & quinone lipids
So what if it runs in reverse??
generates H2O

10. *ETC of Photosynthesis
Chloroplasts transform light energy into chemical energy of ATP
use electron carrier NADPH
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
generates O2

11. ATP Synthesis *sunlight breakdown of C6H12O6 moves the electrons
photosynthesis
respiration
*sunlight
breakdown of C6H12O6 H+
ADP + Pi
moves the electrons
runs the pump
pumps the protons
builds the gradient
drives the flow of protons through ATP synthase
bonds Pi to ADP
generates the ATP
ATP

12. Pigments of photosynthesis
How does this molecular structure fit its function?
*Chlorophylls & other pigments
embedded in thylakoid membrane
arranged in a “photosystem”
collection of molecules
structure-function relationship

13. A Look at Light
The spectrum of color V I B G Y O R

14. *Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light
chlorophyll a
absorbs best in red & blue wavelengths & least in green
accessory pigments with different structures absorb light of different wavelengths
chlorophyll b, carotenoids, xanthophylls
Why are plants green?

15. Photosystems of photosynthesis
2 photosystems in thylakoid membrane
collections of chlorophyll molecules
act as light-gathering molecules
*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

16. ETC of Photosynthesis *Photosystem II *Photosystem I chlorophyll a
chlorophyll b
*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. ETC of Photosynthesis e e O split H2O H+ sun sun to Calvin Cycle ATP
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 O
to Calvin Cycle
split H2O
ATP

18. ETC of Photosynthesis ETC uses light energy to produce
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!

19. 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.

20. 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!

21. 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!
ATP

22. 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 
ATP
18 ATP + 12 NADPH 
1 C6H12O6

23. Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP

24. Photophosphorylation
Cyclic – used to generate ATP in “ancient” organisms, like bacteria
NADPH and sugars are not made
Noncyclic – used to generate NADPH for the Calvin Cycle

25. Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems
Figure Noncyclic Electron Transport Uses Two Photosystems As chlorophyll molecules in the reaction centers of photosystems I and II absorb light energy, they pass electrons into a series of redox reactions, ultimately producing NADPH and ATP. The term “Z scheme” describes the path (blue arrows) of electrons as they travel through the two photosystems. In this scheme the vertical positions represent the energy levels of the molecules in the electron transport system.

26. Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP
Figure Cyclic Electron Transport Traps Light Energy as ATP Cyclic electron transport produces ATP but no NADPH.

27. Occurs in the stroma of the chloroplast.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates
Calvin cycle: the energy in ATP and NADPH is used to “fix” CO2 in reduced form in carbohydrates
Occurs in the stroma of the chloroplast.
Each reaction is catalyzed by a specific enzyme.
The cycle is composed of three distinct processes.

28. Figure 6.22 The Calvin Cycle
1. Carbon Fixation
2. Reduction
3. Regeneration
Figure The Calvin Cycle The Calvin cycle uses the ATP and NADPH generated in the light reactions to produce G3P from CO2. The G3P is used as a starting material for the production of glucose and other carbohydrates. Six turns of the cycle are needed to produce one molecule of the hexose glucose.

29. Figure 6.23 RuBP Is the Carbon Dioxide Acceptor
1. Fixation of CO2
3PG
Figure RuBP Is the Carbon Dioxide Acceptor The enzyme rubisco adds CO2 to the five-carbon compound RuBP. The resulting six-carbon compound immediately splits into two molecules of 3PG.
Rubisco: ribulose bisphosphate carboxylase
the most abundant enzyme in the world!

30. 2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates
2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).

31. 3. The CO2 acceptor RuBP is regenerated from G3P.
Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates
3. The CO2 acceptor RuBP is regenerated from G3P.
Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose).
When glucose accumulates, it is linked to form starch, a storage carbohydrate.

32. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor an apparently wasteful process called photorespiration

33. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
C3 plants: rice, wheat, soybeans

34. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

35. C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
Utilize enzyme PEP carboxylase in an alternate pathway to fix carbon
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

36. Photosynthetic cells of C4 plant leaf PEP carboxylase
Figure 10.20
C4 leaf anatomy
The C4 pathway
Mesophyll cell
Mesophyll cell
CO2
Photosynthetic cells of C4 plant leaf
PEP carboxylase
Bundle- sheath cell
Oxaloacetate (4C)
PEP (3C)
Vein (vascular tissue)
ADP
Malate (4C)
ATP
Pyruvate (3C)
Bundle- sheath cell
Stoma
CO2
Calvin Cycle
Figure C4 leaf anatomy and the C4 pathway.
Sugarcane, corn, tropical grasses
Sugar
Vascular tissue 36

37. CAM Plants
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids (storage)
Stomata close during the day, when light reactions generate ATP and NADPH
Then, CO2 is released from organic acids and used in the Calvin cycle
Pineapple, agave