Photosynthesis

1. Photosynthesis

2. Photosynthesis & the Cycle of Life
•Captures the energy of light
–Chlorophyll traps the light of the sun
•Overall reaction:
6 CO
2
+ 12 H
2
O ® C
6
H
12
O
6
+ 6 O
2
•End product is glucose, a 6 carbon sugar
–the primary energy molecule for many living
organisms
•Uses CO
2
& produces O
2

3. In the Beginning
•Life on Earth originated 3.5 to 4 billion years ago.
•The atmosphere was composed of methane,
carbon dioxide, and water vapor.
•The cooling water collected in pools, assimilating
the nutrients from the rocks.
•As water evaporated, the nutrients concentrated,
forming a rich soup.
•The first organisms used these molecules for food,
breaking them down into water and carbon
dioxide through respiration.

4. Evolution of Photosynthesis
•Eventually, these food molecules grew scarce
•Some organisms were able (through random
mutation) to use the sun's energy to
synthesize large molecules from small
molecules.
–This is the process of photosynthesis
•Organisms who create complex molecules this
way are called autotrophs
–Autotrophs are found in the bacterial and in the
plant kingdom.
•Produced O
2
as a byproduct, changing the
atmosphere

5. Discovery of Photosynthesis
•Joseph Priestly - chemist and minister
–discovered that under an inverted jar, a candle would
burn out quickly
–found that a mouse could similarly "injure" air.
–showed that the "injured" air could be restored by a plant.
•Jan Ingenhousz – 1778 - Austrian court physician
–repeated Priestly's experiments
–discovered that it was the effect of sunlight on the plant
that caused it to rescue the mouse
•Jean Senebier – 1796- a French pastor
–showed that CO
2
was the "fixed" or "injured" air and that
it was taken up by plants in photosynthesis.
•Theodore de Saussure
–showed that the increased mass of a plant as it grows is
not due only to CO
2
uptake, but also to water.

6. Photosynthesis Reactions
6 CO
2
+ 12 H
2
O ® C
6
H
12
O
6
+ 6 O
2
•This is really a series of complex
reactions
•Involves 2 phases:
1. The Light Reactions
–use energy from sunlight
2. The Dark Reactions
–fix carbon

7. Overview
•Light energy entering the plant splits the water into
hydrogen and oxygen:
•H
2
O + light energy ® ½ O
2
+ 2H
+
+ 2 electrons
•Electrons travel through the membrane much like the
electrons in oxidative phosphorylation
•Their energy pumps protons through the membrane.
–This proton gradient can be used to synthesize ATP.
•The same electron reduces NADP
+
to NADPH.
–This molecule plays the same role in synthesis as does
NAD
+
in respiration
–a carrier of reductive power.
•This reductive power converts CO
2
to glucose

8. The Nature of Light
•Light behaves as both a wave and a particle
–particles of energy are called photons.
•As a wave. light has a wavelength (the
distance from one peak of the wave to the
next) and an amplitude (the distance the
wave oscillates from its centerline).
–Different wavelengths of light have different
characteristic energies and properties.
•Light travels at various speeds in different
media, producing a frequency at which the
wave travels.

9. Visible Light
•Visible light is only part of the electromagnetic
spectrum
–Only 1% of light that reaches planet
•Visible white light is actually made of different colors

10. Light Energy
•The energy in a light wave is related to
frequency and wavelength
•Different colors have different
wavelengths
–different wavelengths have different amounts
of energy
•Short wavelengths have high energies
and long wavelengths have lower
energies.
–Violet light has 2x energy of red

11. Pigments
•How is light captured by living things?
•Molecules, when struck by a wave or
photon of light, reflect some energy back
•Absorb some of the energy, and thus enter
into a higher energy or excited state.
•Each molecule absorbs or reflects its own
characteristic wavelengths of light.
•Pigments = molecules that absorb
wavelengths in the visible region of the
spectrum

12. Absorption of Light
•Light energy comes in "packets" called
photons
•Plants can utilize energy only from
absorbed wavelengths
•The color we see is the color reflected;
the rest is absorbed

13. Absorption & Action Spectra
•An absorption spectrum for a pigment describes
the wavelengths at which it can absorb light and
enter into an excited state.
•An action spectrum describes the efficiency of a
particular molecule at absorbing light
–Shows what wavelengths of light the molecule can
trap to conduct photosynthesis.
•The action spectrum closely follows the
absorption spectrum for a particular pigment
–the molecule has to be able to absorb light to enter
into its excited state and pass the energy on.

14. Absorption & Action Spectra

15. Chlorophyll
•Chlorophylls are the green pigments in plants
–Chlorophyll a
–directly involved in the light reactions of
photosynthesis
–Chlorophyll b
–assists chlorophyll a; an accessory pigment
•Located in the membranes of the thylakoids
inside chloroplasts

16. Chlorophyll & Light
•When a photon strikes
chlorophyll, the photon's
energy is transferred to
an electron in the
chlorophyll molecule
–energized electrons can't
remain in this state
–as the electron returns to
its original energy level, it
releases absorbed energy.

17. Structure of Chlorophyll

18. Photosystems
•Clusters of several hundred pigment
molecules in the thylakoid membranes
•Two types:
–Photosystem I
–Photosystem II
•Both are involved in the light reactions

19. Accessory Pigments
•Accessory pigments absorb light in
other parts of spectrum & pass the
energy to chlorophyll:
–Xanthophylls - yellow pigments
–Carotenoids - orange pigments
–Anthocyanins – red pigments

20. The Chloroplast
•The chloroplast is the organelle of photosynthesis.
•Resembles the mitochondrion
–Both are surrounded by a double membrane with an
intermembrane space.
–Both have their own DNA.
–Both are involved in energy metabolism.
–Both have membrane reticulations filling their inner
space to increase the surface area on which reactions
with membrane-bound proteins can take place.
•Has three membranes: inner, outer, and thylakoid
•Has three compartments: stroma, thylakoid space,
and inter-membrane space.

21. Structures of Photosynthesis
•The compartments and membranes isolate different
aspects of photosynthesis.
•Chloroplasts have a highly organized array of
internal membranes called thylakoids
•These form stacks of flattened structures = grana
–Photosynthesis begins in the grana
–Pigments are embedded here
•Stroma are dark fluid-filled spaces between grana
and the outer membrane
–contain enzymes, DNA, RNA, ribosomes
•Light reactions take place on the thylakoid
membranes.
•Dark reactions take place in the stroma.

23. Materials for Photosynthesis
•CO
2
is the source of C & O used to make
glucose
•H
2
O is the source of H
•Oxygen from H
2
O is released into the air
and produces O
2
in the atmosphere
–O
2
drives cellular respiration in living organisms

24. The Light Reactions
•Use trapped energy to convert ADP to ATP,
which stores energy for later use
•Uses energy to split H
2
O to H and O
•The reactions leading to the production of
ATP and reduction of NADP+ are called the
light reactions because they are initiated by
splitting water by light energy.

25. Photosystems
•Non-cyclic photophosphorylation Involves two
sets of pigments:
–Photosystem 1 (PS1)
–Photosystem 2 (PS2)
•PS1 is better excited by light at about 700 nm
–sometimes called P-700
•PS2 can’t use wavelengths longer than 680 nm
–sometimes called P-680.

26. Non-cyclic Photophosphorylation
•Energy enters the system when PS2
becomes excited by light.
•Electrons are shed by the excited PS2
(oxidation), which grabs electrons from
water
–This produces a molecule of oxygen gas for
every two waters split.
•PS2 thus returns it to its unexcited state
(reduction) .
•The electrons are passed through a chain of
oxidation-reduction reactions.

27. The Redox Chain
•Each element in the pathway is reduced by the
electrons
•Each element then reduces its neighbor in the
pathway by giving it the electrons
•Thus each element is reoxidized and ready for the
next electrons to pass through the photosystem.
•PS2 passes on the energy to move the electron
through the redox chain
–this pumps protons through the membrane to generate ATP.
•PS1 passes on the energy required to reduce NADPH.

29. Cyclic Photophosphorylation
•Sometimes an organism has all the reductive
power (NADPH) needed to synthesize new carbon
skeletons
–still needs ATP to power other activities in the
chloroplast.
•Many bacteria can shut off PS2, allowing the
production of ATP in the absence of glucose
•A proton gradient is generated across the
membrane using the mechanisms of
photosynthesis.
•This type of energy generation is called cyclic
photophosphorylation.

30. The Role of PS1
•The role of PS1 seems contradictory
–In noncyclic phototphosphorylation PS1 was responsible
for NADPH production
–In cyclic photophosphorylation it is needed for ATP
production.
•PS1 is a good candidate for noncyclic
photophosphorylation and for NADPH production.
–PS1 is good at transferring an electron, whether to NADP
or to ferredoxin (fd).
–It is a powerful reductant.

31. The Role of PS2
•PS2 is better at grabbing electrons from
water and transferring them to quinone (Q).
–It is a good oxidant.
•The electron transferred is not derived from
water, but from PS1 itself.
•It therefore must be recycled to PS1.

32. Steps of the Light Reactions - 1
Chlorophyll in the grana absorb photons of light
–energy from the photons boosts electrons from the
chlorophyll a molecules of Photosystem II to a higher
energy level

33. Steps of the Light Reactions
The excited electrons leave chlorophyll a
–They are transferred to a molecule
in the thylakoid membrane:
the primary electron acceptor

34. Steps of the Light Reactions - 2
•Electrons lost from
the chlorophyll are
replaced by
electrons from
water molecules.
–This splits H
2
O into
H ions & O
2
gas

35. Steps of the Light Reactions - 3
The primary electron
acceptor donates the
electrons to the first of a
series of molecules called
the electron transport
chain.
–energized electrons
move from one
molecule to another in
the electron transport
chain
–each time a transfer is
made, energy is
released

36. Steps of the Light Reactions - 4
•Chemiosmosis
–Energy from the proton
gradient created in the
thylakoid membrane fuel ATP
synthase, an enzyme
–Energy released from electrons
as they move down the
electron transport chain is used
to form ATP, combining ADP &
phosphates in the stroma of
chloroplasts
•Energy from the light reactions in
the form of NADPH and ATP will
fuel the dark reactions that follow

37. Steps of the Light Reactions - 5
•Light is also absorbed by Photosystem I
–Electrons move from chlorophyll a molecules to another
primary electron acceptor
–These electrons are replaced by the electrons that passed
through the electron transport chain in photosystem II

38. Steps of the Light Reactions - 6
•The primary electron acceptor of Photosystem I
donates electrons to a 2nd electron transport chain.
–These electrons reduce NADP
+
to NADPH

39. Summarizing the Light Reactions

40. The Dark Reactions
•The reduction of carbon dioxide to glucose, using
the NADPH produced by the light reactions, is
governed by the dark reactions
•Also known as the Calvin Cycle for Melvin Calvin,
described in 1950’s
–Requires several enzymes & produces several
byproducts
–Takes place in the stroma of the chloroplasts
•Fixes carbon from CO
2
to form glucose
•Begins and ends with a five carbon sugar, RDP
(ribulose diphosphate)

41. Products of The Calvin-Benson Cycle
•The cycle runs 6 times, each time incorporating a new
carbon.
•Ribulose is a five-carbon sugar and the
gylceraldehydes are three-carbon sugars
•Running through the cycle six times generates:
6(5-carbon sugars) + 6(incorporated carbon dioxides)
•Those six CO
2
are reduced to glucose by the
conversion of NADPH to NADP
+
.
•Glucose serves as a building block to make
polysaccharides, other monosaccharides, fats, amino
acids, nucleotides, and all the molecules living things
require.

42. Steps of the Dark Reactions - 1
•CO
2
from the
atmosphere combines
with RDP in series of
reactions which use
ATP as an energy
source.
•This forms PGA
(phosphoglyceric acid),
a molecule with 3
carbons

43. Steps of the Dark Reactions - 2
•PGA reacts with
hydrogen from the light
reactions to form PGAL
(phosphoglyceraldehyde)
–Most of the PGAL formed
is used to make more
RDP.
–RDP combines with more
CO
2
and the cycle
repeats.

44. Steps of the Dark Reactions - 3
•Some PGAL is
combined to form
glucose:
–2 PGALs form one
glucose C
6H
12O
6
•Excess glucose is
stored as starch
which can be used
as needed

45. Rubisco
•The key enzyme in the Calvin Cycle
catalyzes the transformation of the 5-C
sugar, ribulose-5-phosphate and the
single-C CO
2
to two 3-C 3-
phosphoglycerates.
•This reaction has a very high G of -12.4
kcal/mol.
•The enzyme is called ribulose-1-5-
biphosphote carboxylase, or Rubisco .

46. Abundance of Rubisco
•Rubisco accounts for 16% of the protein
content of the chloroplast
•The most abundant protein on Earth.
•Why is this protein so abundant?
–It is crucial to all life to have a source of carbon
fixation
–The enzyme is very inefficient

47. Light Regulation of the Calvin Cycle
•The energy required for the Calvin
Cycle, in the form of ATP and NADPH,
comes from the light reactions.
•The plant or photosynthesizing
bacterium needs to tightly regulate the
Calvin Cycle with photosynthesis.
–It would be wasteful to run the process
using ATP generated for other plant
metabolism.

48. Linkage of Late & Dark Reactions
•The pH of the stroma increases as
protons are pumped out of it through
the membrane
–The enzymes of the Calvin Cycle function
better at this higher pH.
•As the reduction potential of ferredoxin
(fd) increases, it reduces a protein
called thioredoxin.

49. Linkage of Late & Dark Reactions
(continued)
•This reduction breaks a disulphide bridge in
thioredoxin.
–The enzyme now has free cysteines that can
compete for the the disulphide bonds in other
enzymes.
–Several enzymes of the Calvin Cycle are
activated by the breaking of disulphide bridges.
–So the activity of the light reactions is
communicated to the dark reactions by an
enzyme intermediate.

50. Linkage of Late & Dark Reactions
(continued)
•The reactions of the Calvin cycle stop when
they run out of substrate
–As photosynthesis stops, there is no more ATP
or NADPH in the stroma for the dark reactions
to take place.
•The light reactions increase the permeability
of the stromal membrane to cofactors such
as Mg++ which are required for the Calvin

51. Photosynthesis Summary

52. Alternative Pathways
•Many diverse environments
•Led to different approaches to photosynthesis
•Some organisms forgo the use of light for
energy production
•Others modify photosynthetic pathways to
make then more compatible with
environmental conditions
•All of these adaptations are variations on the
same basic pathways of photosynthesis and
respiration

53. Adaptations to Hot Climates
•In hot, dry climates plants need special
adaptations
–Water loss through stomata which must open to
exchange CO
2
for O
2
would be damaging
•Plants that fix carbon through the Calvin Cycle
are C
3
plants
•2 alternative pathways:
–C
4
pathway
–CAM pathway

54. Rubisco
•Rubisco is the most abundant enzyme
on Earth.
•It is a very important
•It is believed that Rubisco is so
abundant because of its inefficiencies.
•Rubisco will sometimes recognize
oxygen as a substrate instead of carbon
dioxide.

55. The Inefficiency of Rubisco
•When Rubisco uses oxygen as a substrate
instead of carbon dioxide, it does not fix CO2
into sugar
•Instead, it creates phosphoglycolate, a nearly
useless compound.
•This wastes energy
•This reaction, directly competes with the
regular reaction at the same site on the
enzyme.
•The result is very detrimental to
photosynthesis

56. Alternate Fate of Rubisco

57. The Effect of Temperature
•At 25°C, the rate of the carboxylase
reaction is 4x that of the oxygenase reaction
–the plant is only about 20% inefficient.
•As temperature rises, the balance between
O
2
and CO
2
in the air changes (due to
changing solubility in the ocean)
•The carboxylase reaction is less and less
dominant.
•Plants living in warm climates have to
overcome this handicap

58. Balancing CO
2
Input & Water Loss
•Plants in arid climates close the stomata
(pores) in their leaves when it is very
dry
•This creates a closed environment
•As CO
2
is used up in photosynthesis, the
relative concentration of O
2
increases
•The oxygenase reaction begins to
dominate.

59. The C4 Solution
•Plants living in these dry conditions
have evolved a mechanism to make the
CO
2
concentration very high in the
immediate environment of Rubisco
•This prevents the oxygenase reaction
•This is called the C4 pathway because it
involves a 4 carbon intermediate in the
outer cells.

60. The C4 Pathway
•The conventional pathway is called the C3 pathway
–it involves only the 3-carbon sugars.
•In the C4 pathway, a 4-carbon intermediate brings
a molecule of CO
2
into the bundle sheath cells
–it is dropped right next to the location of the Calvin
Cycle.
•This ensures that the concentration of CO
2
at the
site of Rubisco is very high,
•Only the carboxylase reaction can take place.
•The C4 pathway still uses the Calvin Cycle with its
3-carbon sugar intermediates
–it makes use of 4-carbon sugars to bring the carbon
dioxide closer to the site of fixation.

61. Picturing the C4 Pathway

62. Chemistry of the C4 Pathway

63. Why Aren’t All Plants C4?
•Why don't the C4 plants out-compete the
C3 plants, which are inefficient?
•It takes ATP to bring the CO
2 to the
Rubisco.
•In moderate temperatures, this energy
burden outweighs the advantage of
eliminating the 1 in 5 times that Rubisco
binds O
2 instead of CO
2.
•In warmer climates the C4 plants win

64. CAM Plants
•Another strategy used in hot or dry
climates
–Prevents water loss
•Plants open their stomata at night and
close them during the day
•Take in CO
2
at night and fix it in organic
compounds
•Later, release carbon from these
compounds to enter the Calvin cycle
•Steps of the photosynthetic pathway are
separated in time

65. Comparing C4 & CAM Strategies

66. Lithotrophs
•Some autotrophs don’t use energy from
sunlight
•These bacteria derive reductive power by
oxidizing compounds such as hydrogen gas,
carbon monoxide, ammonia, nitrite,
hydrosulphuric acid, sulphur, sulphate, or iron.
•These organisms are called lithotrophs or
“rock-eaters”
•This process = Chemosynthesis
–oxidizing an inorganic substance and transporting
electrons through the membrane

67. Chemosynthesis
•Chemosynthesis oxidizes inorganic substances &
transports electrons through the membrane
–like in oxidative phosphorylation and photosynthesis
–(remember - oxidation is a loss of electrons, so this
inorganic substance is the electron donor).
•This electron transport pumps protons through the
membrane generating a proton gradient which can
be used to form ATP.
•These organisms make so much ATP that they can
drive the electron transport chain backwards to
generate NADH.
•This NADH provides the reducing power needed to
synthesize carbon structures from carbon dioxide.

68. Methanogens
•The methanogens are a class of anaerobic bacteria.
•They derive energy by reducing CO
2
to methane
•They use CO
2
as an energy source rather than treating
it as an energy-depleted waste product.
•The methanogens can oxidize hydrogen gas to directly
reduce NAD+ to NADH,
•Don’t have to waste energy making ATP through
chemosynthesis and then driving it backwards through
the electron transport chain.
•These organisms still incorporate their carbon into the
Krebs Cycle for processing into amino acids, nucleic
acids, and sugars.