2. Photosynthesis: Life from Light , Air, and Water

3. ALL LIFE needs CONSTANT input of ENERGY
Heterotrophs (Animals)
Eat food (that is, organic molecules)
Convert food to energy via respiration
Autotrophs (Plants)
produce their own food
Convert energy of sunlight to food
That is…use the energy of sunlight to build organic molecules (CHO) from CO2
That’s called…photosynthesis
THEN convert food to energy via respiration
consumers
producers

4. making energy from ingesting organic molecules
How are they connected?
Heterotrophs
making energy from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6
6O2
6CO2
6H2O
ATP  +
oxidation = exergonic
Different
Forms of Energy
Though..
Autotrophs
making organic molecules from 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

5. Plants are NOT the only autotrophs…
Certain bacteria
cyanobacteria
Also Protists
Algae

6. What does it mean to be a plant
Need to…
collect light energy
transform it into chemical energy
store chemical energy
in a more STABLE form that can be moved around the plant or stored for later
need to get building block atoms from the environment
C,H,O,N,P,K,S,Mg
produce all organic molecules needed for growth
carbohydrates, proteins, lipids, nucleic acids
ATP
glucose
CO2
H2O N P K …

7. 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…

8. Leaf Structure Mesophyll – where most of the photosynthetic action is
Palasaide layer
photosynthesis
Spongy layer
Gas exchange
Lower epidermis
Stomate Lets gas in and out

9. stomate
transpiration
gas exchange

10. chloroplasts in plant cell chloroplasts contain chlorophyll
absorb sunlight & CO2
cross section of leaf
leaves
CO2
chloroplasts in plant cell
chloroplasts contain chlorophyll
chloroplast
make energy & sugar

11. Plant structure Chloroplasts Thylakoid membrane contains
compartment
Plant structure
Chloroplasts
double membrane
stroma
fluid-filled interior
Stacks of thylakoid sacs
Surrounded by stroma
granum = stack
Thylakoid membrane contains
chlorophyll molecules
Where the light trapping action happens
outer membrane
inner membrane
granum
thylakoid
stroma
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.

12. Chlorophyll Found embedded in the membranes of the thylakoids
Absorbs light energy which is then used by the plant to make sugar

13. Basics about Light and Chlorophyll
LIGHT behaves both as a wave and a particle
When light is behaving as a particle = a photon
This is a fixed quantity of energy
Chlorophyll is best at absorbing red and blue light in the visible light portion of the electromagnetic spectrum
Chlorophyll does NOT absorb green light – REFLECTS it. Hence, leaves appear green.

14. Types of Chlorophyll and other Pigments
Only pigment that DIRECTLY participates in the light reactions
Blue green in color
Accessory pigments
Absorb light and transfer its energy TO chlorophyll a
Chlorophyll b
Yellow green in color
carotenoids
Yellow/orange in color
Also serve in
photoprotection
Absorb and dissipate excessive light energy that might damage chloropyll

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

16. 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?

17. Molecules and absorption of light energy
When a molecule absorbs a photon, one of the molecule’s electrons is elevated to an orbital where it has MORE potential energy.
Normal orbital = ground state; Higher orbital = excited state
Particular compounds absorb photons at very particular wavelengths
Chlorophyll absorbs photons at wavelengths of 680 nm and 700 nm
Once a molecule’s electron is excited to a higher energy level, it will normally fall back down to its original level unless something “catches” the electron and holds it at the elevated state.

18. The Photosynthetic Reactions
It takes two sets of chemical reactions (two biochemical pathways) to make a sugar molecule
The two sets of reactions are called:
Light Reactions and Dark Reactions OR
Light Dependent Reactions and Light Independent Reactions OR
Light Reactions and Calvin Cycle

19. Light Reactions vs Dark (light independent / Calvin Cycle) Reactions
Fundamentally, Light Reactions MUST have light in order to proceed
Dark (light independent / Calving Cycle) reactions can occur with or without light being present, HOWEVER…
Dark reactions ARE dependent on the PRODUCTS of the LIGHT reactions in order to occur.

20. 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 (build) C6H12O6
ATP
It’s not the Dark Reactions!

21. The Light Reactions Begin With Photosystems
Complexes containing chlorophyll, proteins and other pigments and organic molecules
Embedded in thylakoid membrane
Photosystems contain a special region called the antenna complex
Light gathering structure
Cluster of chlorophyll a and other pigment molecules
Acts like a “catcher’s mitt” to catch photons from the sun
Energy from these photons is bounced around in the antenna complex until it “lands on” a particular chlorphyll a molecule

22. Photosystem: Antenna Complex Diagram

23. 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

24. Antenna Complex – The Reaction Center
Contains the particular chlorophyll a molecule upon which light’s energy ultimately lands.
Also contains a compound called the primary electron acceptor

25. Excited Electrons and the Primary Electron Acceptor
Energy landing on the chlorophyll a in the reaction center elevates the electrons of this chlorophyll a to a higher energy level.
Before these electrons can lose this energy and return to their original energy level, the electrons are “caught” by the primary electron acceptor which keeps them at this elevated state
POTENTIAL ENERGY provided by LIGHT is now STORED in CHEMICAL FORM.

26. 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

27. The LIGHT REACTIONS – Step by Step
There are TWO possible paths electrons can take in the light reactions
Noncyclic electron flow
The one we will focus on NOW
Cyclic electron flow
We’ll focus on this one LATER

28. The LIGHT REACTIONS – Step by Step
Also…Remember that electrons are going to be MOVING in these reactions.
Just like in cell respiration, we depend on REDOX (oxidation-reduction) reactions to pass electrons from one compound to another
As electrons move from one compound to the next they are moving from a compound that is less electronegative to one that is more electronegative

29. Light Reactions: Non Cyclic Electron Flow – Step 1
Photon strikes antenna complex of Photosystem II (P680)
Energy is bounced on molecules in antenna complex until they land on chlorophyll a in the reaction center
Electrons of reaction center chlorophyll a are excited to a higher energy level and CAUGHT by the primary electron acceptor

30. Light Reactions: Non Cyclic Electron Flow – Step 2
An electron “hole” is left behind in the chlorophyll a in the reaction center of Photosystem II
An enzyme extracts electrons from water found in the chloroplast (stroma) and supplies these electrons to the chlorophyll a molecule so that it can work again.
This enzyme splits a water molecule into TWO H+ ions and an oxygen atom that will combine with another oxygen atom to form O2.
THIS IS WHY WATER IS NEED FOR PHOTOSYNTHESIS
THIS IS WHERE THE OXYGEN GAS COMES FROM IN PHOTOSYNTHESIS

31. Light Reactions: Non Cyclic Electron Flow – Step 3
Electrons pass from the primary electron acceptor of Photosystem II to Photosystem I by way of an electron transport chain
This is VERY much like the ETC found in the mitochondrion
As electrons fall down the chain their energy is used to PUMP PROTONS from the stroma INTO the thylakoid space
This creates a PROTON GRADIENT
HIGH inside the thylakoid; LOW outside in the stroma
ATP is made as protons flow WITH their concentration gradient THROUGH a molecule of ATP synthase
ADP and Pi are combined at ATP Synthase
ATP

32. ATP

33. Light Reactions: Non Cyclic Electron Flow – Step 3, cont.
Remember: ATP synthesis using a proton gradient and ATP synthase is called CHEMIOSMOSIS
Using Light Energy to drive this type of ATP synthesis is called PHOTOPHOSPHORYLATION
Remember substrate level phosphorylation and oxidative phosphorlyation? This is the same type of thing.
The ATP generated by this method will be sent on to provide energy to the “DARK” REACTIONS to provide energy for making glucose.

34. Light Reactions: Non Cyclic Electron Flow – Step 4
When electrons reach the bottom of the ETC, they are passed to chlorophyll a of the reaction center of photosystem I (P700)
Here, these electrons fill an electron hole left in this reaction center when electrons of this reaction center’s chlorophyll a are excited by their own photon of light.
Just as in Photosystem II, the electrons here are also caught and passed on by the primary electron acceptor of photosystem I. This, again, leaves an electron hole to fill.

36. Light Reactions: Non Cyclic Electron Flow – Step 5
Electrons from Photosystem I are passed from its primary electron acceptor to a compound called NADP+ Reductase
This enzyme passes electron to a coenzyme called NADP+
NADP+ will become NADPH when it picks up the electrons. (reduced form of NADP+)
NADPH takes the electrons (and their ENERGY!) to the Calvin Cycle (Dark Reactions) where this energy will be used to make sugar.

38. Light Reactions: BOTTOM LINE
The light reactions use LIGHT power to generate ATP and NADPH
ATP and NADPH provide CHEMICAL ENERGYand REDUCING POWER to the GLUCOSE-MAKING reactions of the CALVIN CYCLE.
WITHOUT THE CHEMICAL ENERGY (ATP/NADPH) PROVIDED BY the LIGHT REACTIONS, the “DARK” REACTIONS CAN’T OCCUR.

39. 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

40. Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP

41. Photosynthesis 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?
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?
Where did the energy come from? The Sun
Where did the electrons come from? Chlorophyll
Where did the H2O come from?
The ground through the roots/xylem
Where did the O2 come from? The splitting of water
Where did the O2 go? Out stomates to air
Where did the H+ come from? The slitting of water
Where did the ATP come from?
Photosystem 2: Chemiosmosis (H+ gradient)
What will the ATP be used for?
The work of plant life! Building sugars
Where did the NADPH come from?
Reduction of NADP (Photosystem 1)
What will the NADPH be used for? Calvin cycle / Carbon fixation
…stay tuned for the Calvin cycle

42. You can grow if you Ask Questions!

43. The ATP that “Jack” built
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
… that evolution built
ATP

44. 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

45. Photosystem II P680 chlorophyll a
ETC of Photosynthesis
sun 1 e
PS II absorbs light
Excited electron passes from chlorophyll to the primary electron acceptor
Need to replace electron in chlorophyll
An enzyme extracts electrons from H2O & supplies them to the chlorophyll
This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2
O2 released to atmosphere
Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state
Electron is handed off down chain from electron acceptor to electron acceptor
In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac.
Flow back through ATP synthase to generate ATP.
Photosystem II P680 chlorophyll a

46. Photosystem II P680 chlorophyll a
Inhale, baby!
ETC of Photosynthesis
thylakoid
chloroplast H+ H+
ATP
Plants SPLIT water! 1 O H 2 e O O H e- H+ +H e e
fill the e– vacancy
Photosystem II P680 chlorophyll a

47. energy to build carbohydrates Photosystem II P680 chlorophyll a
ETC of Photosynthesis
thylakoid
chloroplast H+ H+
ATP e H+ 3 1 2 e H+
ATP 4
to Calvin Cycle H+
ADP + Pi
energy to build carbohydrates
Photosystem II P680 chlorophyll a
ATP

48. Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a
ETC of Photosynthesis e e
sun
fill the e– vacancy e 5
Need a 2nd photon -- shot of light energy to excite electron back up to high energy state.
2nd ETC drives reduction of NADP to NADPH.
Light comes in at 2 points.
Produce ATP & NADPH e e
Photosystem I P700 chlorophyll b
Photosystem II P680 chlorophyll a

49. Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a
ETC of Photosynthesis
electron carrier e 6 e 5
sun
NADPH to Calvin Cycle
Need a 2nd photon -- shot of light energy to excite electron back up to high energy state.
2nd ETC drives reduction of NADP to NADPH.
Light comes in at 2 points.
Produce ATP & NADPH
Photosystem I P700 chlorophyll b
Photosystem II P680 chlorophyll a
We’re goin’ for
another joy ride!

50. 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

51. 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!

52. 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!

53. 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

54. 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 enzyme
Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

55. 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

56. 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

57. Ghosts of Lectures Past (storage)

58. Stomates