1. Photosynthesis converts sunlight into chemical energy through light-dependent and light-independent reactions. 2. The light-dependent reactions use sunlight to produce ATP and NADPH through electron transport. 3. The light-independent Calvin cycle uses ATP and NADPH to fix carbon from CO2 into organic three-carbon sugars like G3P.

1. PHOTOSYNTHESIS

2. Photosynthesis
-converts sunlight into chemical energy
-very complex
-general reaction:
6CO2 + 6H20 → C6H12O6 + 6O2

3. • Light from the sun is composed of wavelengths
(colors
• The shorter the wavelength the higher the
frequency, thus the higher the energy
• The longer the wavelength the lower the energy,
thus the lower the energy

4. Sunlight (a.k.a. white light)
-sunlight is actually white light made of all
wavelength colors
-sunlight is visible light
-different colors=different wavelengths of
light
The Visible Spectrum
violet-blue-green-yellow-orange-red
380 nm 750 nm

5. The Electromagnetic Spectrum
-This is what scientists call radiation waves
-Radiation=energy that travels and spreads as it
goes
Examples: X-rays, gamma rays, visible light,
microwaves, etc.
-The electromagnetic spectrum is organized
according to wavelengths

6. -Wavelengths are measured in nanometers
(nm)
-Gamma rays have the shortest wavelengths
= 10-5
nm
(highest frequency and energy)
-Radio waves have the longest wavelengths
=103
nm
(lowest frequency and energy)

7. PHOTONS
-Photon=quantum=discreet amounts of light
energy
-Photons are not objects, but each one has
a distinct amount of energy
Ex: violet photons contain almost twice as
much energy as red photons
*violet wavelengths=380 nm=high
frequency=high energy
*red wavelenghts=750 nm=low
frequency=low energy

8. Chlorophyll (the photosynthetic pigment)
-Chlorophyll is a green photosynthetic pigment
found in chloroplasts of plants
-There are two main types of chlorophyll
(chlorophyll a and chlorophyll b)
-Green is the least effective color for
photosynthesis because it is reflected
-What you see is reflected.
-Everything else is absorbed
**Thus, red and blue are most effective for
photosynthesis.

10. Absorption Spectrums
Absorption spectrums are graphs that plot a pigment’s light
absorption vs. wavelength
Absorption spectrum of chlorophyll
**Remember:
Green wavelengths
are between ~475
and 600 nm

11. Action spectrums
A. Action spectrums tell you how much photosynthesis is
occurring at each wavelength
B. Made by illuminating chloroplast with different
wavelengths of light and then plotting wavelength against
some measure of photosynthetic rate
C. The photosynthetic rates could be measured by finding
oxygen production, carbon dioxide absorption or light
absorption Action spectrum for chlorophyll

12. Comparison of absorption and
action spectra
Absorption spectrum
for chlorophyll
Action spectrum
for chlorophyll
*Almost no absorption at
green wavelengths
*The photosynthetic rate is
very low at green
wavelengths

13. Light energy and water
• In photosynthesis, light energy is used to split
water molecules
• This process is called photolysis = when a
chemical is broken down by photons
• Water is split into hydrogen ions, oxygen and
electrons by photons
• ATP is also produced
• ATP and H ions will be used to fix CO2 to make
organic molecules
• Photosynthesis relies on water and sunlight for
its initial reaction

14. General photosynthesis information
A. There are light dependent and light
independent reactions
B. Light dependent reactions require light
C. Light independent reactions do not
require light or darkness.
-they are independent of light or dark
-DO NOT REFER TO LIGHT
INDEPENDENT REACTIONS AS DARK
REACTIONS (darkness is not required)

15. ASSIGNMENT
• READ: The light reactions and the Calvin
cycle cooperate in converting light
energy to the chemical energy of food:
an overview (p. 172)
• Briefly outline the section

16. Light Dependent Reactions
A. Light absorption
• As chlorophyll absorbs light its electrons are
raised to a higher energy level by photons at
certain wavelengths
• The electrons at higher energy levels are said
to be excited electrons
• The excited electrons cause the chlorophyll to
become photoactivated
• Photoactivation is the activation of a particular
pigment’s electrons (It is caused by absorbing
energy from photons.)

17. 5. After photoactivation the electrons quickly
return to their ground state
6. When electrons return to their ground
state they give off a photon (discreet
amount of energy)
7. The photon (energy) is released in the
form of heat
8. This process explains the conversion of
light energy into heat energy

18. B. Chlorophyll organization and light
absorption
1. Chlorophyll is found in the thylakoids which are
found in chloroplasts
2. Within the thylakoids chlorophyll is arranged into
groups called photosystems
3. There are two photosystems:
-Photosystem I – best at 700nm (aka P700)
-Photosystem II – best at 680 nm (aka P680)

19. **Both photosystems are identical chlorophyll
a molecules, except that they interact with
different proteins of the thylakoids
6. Excited electrons that have absorbed
photons of light pass from molecule to
molecule until they reach the chlorophyll at
the center of the photosystem
7. The photosystem (the chlorophyll) will then
pass the excited electrons to a chain of
electron carriers

20. C. Oxygen production
1. Photosystem II absorbs light
2. Its electrons become excited
3. Photosystem II donates its electrons to an electron
transport chain and the flow of electrons will
generate an ATP molecule
4. Photosystem II has been oxidized (LEO)
5. To get the electrons back (that were donated) an
enzyme in the center of photosystem II breaks a
water molecule (photolysis)

21. 6. The water is split into hydrogen ions,
oxygen and electrons
7. Electrons are donated to PS II (GER)
8. Oxygen and hydrogen ions are byproducts
9. Oxygen is released to the atmosphere
10. The production of oxygen in
photosynthesis is done by photolysis and
requires sunlight

22. GET A BOOK FOR
YOUR TABLE

23. D. ATP Production
1. an excited electron from the center of PS
II is donated and passed along a chain of
electron carriers
2. The energy for ATP is generated via a
proton gradient that is created as electrons
move through an ETC (chemiosmosis)

24. 3. ATP is eventually formed when the H+
ions move through ATP synthase
**their energy is harnessed to bring a
phosphate group and ADP together
4. The electrons from PS II are eventually
donated to PS I (after they go through the
ETC)
5. When ATP is produced in this manner it is
called non-cyclic photophosphorylation
(the book calls it non-cyclic electron flow)

25. E. NADPH Production
1. NADPH = nicotinamide adenine
dinucleotide phosphate-oxidase
2. After PS I accepts the electrons that were
donated by PS II (the ones that went through
the ETC), PS I becomes photoactivated
3. Next PS I donates its excited electrons to
NADP+ reductase via another ETC
4. NADP+ reductase is an enzyme that
assists in the reduction of NADPH

26. 5. The reduction happens when NADP+
accepts two excited electrons from PS I
and a H+ ion from the stroma
6. NADPH is then formed
NADP+
+ H+
+ 2E-
NADP+
reductase
NADPH
***Non-cyclic electron flow is responsible for
generation of NADPH and ATP***
The purpose of NADPH and ATP production is to
provide reducing power and chemical energy to
drive the Calvin cycle (to make sugar)

27. F. Cyclic photophosphorylation
(in the book it is cyclic electron flow)
1. PS II is not involved
2. Produces ATP but not NADPH
3. ATP is made via chemiosmosis (the same way
as non-cyclic photophosphorylation)
4. How it works:
a. PS I absorbs light
b. the excited electrons are given to an electron
acceptor
c. the electrons pass through an ETC to
produce ATP via chemiosmosis
d. At the end of the ETC the electrons go back
to PS I and the process starts again

28. **Remember in the chloroplast. . .**
2. Chemiosmosis involves the pumping of H+
ions through the membrane.
3. The protons go from the stroma to the
thylakoid space.
3. This creates a proton gradient.
4. The protons later flow through ATP synthase
(back to the stroma) and their energy is
captured in order to join a phosphate with ADP
5. This produces ATP.

29. Go to web animation of light-dep

30. Assignment
#1. Draw and annotate a chloroplast.
-Include: grana
thylakoids
thylakoid membrane
stroma
ribosomes
double membrane
circular DNA
fat/oil droplets

31. HOMEWORK
Complete the coloring worksheet on
light reactions
TURN IT IN TODAY (2-19)
GET YOUR BOOK
GO TO P. 180-181

32. Light-independent reactions
(light not required)
A. Calvin cycle-
1. takes place in the stroma
2. begins with a 5 carbon sugar called
ribulose biphosphate
3. Ribulose biphosphate = RuBP
4. ATP and NADPH from the light
dependent reactions drive the Calvin cycle
5. ATP provides the energy
6. NADPH provides reducing power

33. 7. RuBP is a carbon dioxide acceptor
8. The reaction is catalyzed by the enzyme ribulose
biphosphate carboxylase
9. RuBP carboxylase=rubisco
10. 3RuBP and 3CO2 form:
6 3-Phosphoglycerate
11. ATP is broken down to convert
6 3-Phosphoglycerate to
6 1,3-Biphosphoglycerate
12. NADPH reduces
6 1,3-Biphosphoglycerate to
6 Glyceraldehyde 3-phosphate

34. 13. Only one of the G3P molecules will be
converted to glucose, sucrose, starch,
fatty acids or amino acids
14. Five G3P molecules will be converted
back to RuBP to keep the Calvin cycle
continuing

36. Calvin cycle (more info.)
A. Carbon is:
-absorbed as carbon dioxide
-released as sugar
B. ATP=energy for reactions
NADPH=reducing agent
C. Net sugar production per turn (3 carbon
dioxide and 3 RuBP) is 1 G3P.

37. Phases of the Calvin Cycle
A. Carbon fixation:
1. Every RuBP is attached to a CO2
3RuBP
Rubisco
3CO2
(a 5 carbon
sugar)
converts to a very unstable
6 carbon molecule that is
immediately converted to
6 3-carbon molecules
6 glycerate-3-phosphate
**For every RuBP and CO2 pair, two three carbon molecules are formed
3RuBP + 3CO2 → 6 glycerate-3-phosphate

38. B. Reduction
1. molecules of glycerate-3-phosphate are
phosphorylated to glycerate-1,3-biphosphate
*when ATP is hydrolyzed to ADP
2. glycerate-1,3-biphosphate is reduced when
NADPH donates its electrons
*NADPH→NADP+
3. 6 molecules of triose-phosphate are produced
*one is removed from the Calvin cycle and
used by the plant to produce sugar
*the other 5 are recycled back into the Calvin
cycle and converted back to 3RuBP

39. C. Regeneration (of RuBP)
1. 5 triose-phosphate (G3P) molecules go
through a complex series of reactions to
form 3 RuBP
2. The Calvin cycle starts over
3. CO2 will be received by RuBP again

40. More on the Calvin Cycle
A. Start with 15 total carbons in 3 RuBP
*Remember RuBP is a 5 C molecule
• 3 CO2 is added for a total of 18 carbons
• 1 triose-phosphate (G3P) is released (a 3 C
molecule is released)
• The other 5 triose-phosphate molecules are
recycled back into 3RuBP(15 C are
recycled)

41. E. Net gain of carbons=3 (the single triose
phosphate that was released)
B. Energy consumed during the Calvin
cycle =9 ATP and 6 NADPH
C. ATP and NADPH will regenerate in the
light-dependent reaction
D. There must always be light dependent
reactions for light independent reactions
to occur
E. The products of the light reactions are
used as fuel for the Calvin cycle

42. 1. Outline photosynthesis (light-dependent and
light-independent)
2. Explain how the light-independent reaction
depends on the light –dependent reaction

43. Measuring Photosynthesis
*can be done three ways
A. Production of oxygen
1. Aquatic plants release oxygen in
bubbles during photosynthesis
2. If the bubbles are collected, their
volume can be measured

44. B. Carbon dioxide absorption
EX: 1. Leaves take in CO2 from the air
2. You could pot a plant in an enclosed
environment and measure the CO2
before and after
EX: 1. Aquatic plants absorb CO2 from the
water
2. If plants take up CO2, the pH of the
water will rise
3. You could use pH indicators to measure
pH before and after

45. C. Increase in Biomass
(measure the increase in sugar molecules)
1. Measure how much mass a plant gains
over time
2. to do this the plant must be completely
dehydrated (dead)
3. It is best to measure batches of plants
-select a few from each bunch to be
dehydrated at different times)
4. Biomass is an indirect measurement of
the photosynthetic rate

46. Limiting Factors in Photosynthesis
A. For photosynthesis to occur the following
criteria must be met:
-suitable temperature
-presence of: chlorophyll
light
carbon dioxide
water

47. B. Changes to one limiting factor will change
the rate of photosynthesis
C. Limiting factors are those that are near
their minimum or maximum level
D. Limiting factors determine the rate-limiting
step
For example: If light intensity is the limiting
factor, the light dependent reaction will
limit the rate of photosynthesis.
The limiting-step will be the reduction
reaction in the Calvin cycle (when the
products of the light dependent stage are
needed)

48. E. Light as a limiting factor
1. At low light NADPH and ATP are not
produced (b/c they are light-dependent products)
2. If NADPH and ATP are not produced
the Calvin cycle will stop at the reduction
and phosphorylation reactions
Rate
of
photosynthesis
Light intensity
The effect of light intensity on photosynthesis

49. *At high intensity photosynthesis plateaus
*Light intensity is directly proportional to
the rate of photosynthesis
*Light is not usually the limiting factor

50. F. Carbon dioxide as a limiting factor
1. If there is little or no carbon dioxide the
Calvin cycle is limited at carbon fixation
2. RuBP and NADPH will acculmulate
**Carbon dioxide is often a limiting factor
because it is never at a high concentration
in the atmosphere

51. Rate
of
photosynthesis
Carbon dioxide concentration
The effect of carbon dioxide on photosynthesis
*There is no photosynthesis when carbon dioxide
is low
*Carbon dioxide and photosynthesis are directly
proportional
*At high carbon dioxide concentrations
photosynthesis plateaus

52. G. Temperature as a limiting factor
1. At low temperature the enzymes that
catalyze the reactions work slowly
2. At high temperature rubisco is
ineffective (it is denatured)
3. Carbon fixation becomes the rate-
limiting step

53. Rate
of
photosynthesis
Temperature
The effect of temperature on photosynthesis
*As temperature increases so does the rate of photosynthesis
*After the optimum temperature is surpassed the rate quickly falls

54. Review Questions
1. Compare action spectra and absorption spectra.
2. What are the functions of ATP and NADPH
produced in non-cyclic photophosphorylation
3. What is the purpose of cyclic
photophosphorylation?
4. What is the advantage of non-cyclic
photophosphorylation over cyclic
photophosphorylation?
5. What is the purpose of the Calvin cycle?
6. Explain the relationship between the structure of
the chloroplast and its function.