Photosynthesis: Light and Dark Reaction

1. Photosynthesis

2. • Almost all plants are photosynthetic autotrophs, as
are some bacteria and protists
– Autotrophs generate their own organic matter through
photosynthesis
– Sunlight is transformed to energy stored in the form of
chemical bonds
(a) Mosses, ferns, and
flowering plants
(b) Kelp
(c) Euglena (d) Cyanobacteria
THE BASICS OF PHOTOSYNTHESIS

3. Why is Photosynthesis important?
Makes organic molecules (glucose)
out of inorganic materials (carbon
dioxide and water).
It begins all food chains/webs. Thus
all life is supported by this process.
It also makes oxygen gas!!

4. Photosynthesis-starts to ecological food webs!

5. WHY ARE PLANTS GREEN?
It's not that easy bein' green
Having to spend each day the color of the leaves
When I think it could be nicer being red or yellow or gold
Or something much more colorful like that…
Kermit the Frog

6. Electromagnetic Spectrum and Visible Light
Gamma
rays X-rays UV
Infrared &
Microwaves Radio waves
Visible light
Wavelength (nm)

7. Different wavelengths of visible light are seen by
the human eye as different colors.
WHY ARE PLANTS GREEN?
Gamma
rays
X-rays UV Infrared
Micro-
waves
Radio
waves
Visible light
Wavelength (nm)

8. Sunlight minus absorbed
wavelengths or colors
equals the apparent color
of an object.
The feathers of male cardinals
are loaded with carotenoid
pigments. These pigments
absorb some wavelengths of
light and reflect others.

9. Why are plants green?
Transmitted light

10. WHY ARE PLANTS GREEN?
Plant Cells
have Green
Chloroplasts
The thylakoid
membrane of the
chloroplast is
impregnated with
photosynthetic
pigments (i.e.,
chlorophylls,
carotenoids).

11. • Chloroplasts
absorb light
energy and
convert it to
chemical energy
Light
Reflected
light
Absorbed
light
Transmitted
light
Chloroplast
THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

12. Plants use sunlight to turn water
and carbon dioxide into glucose.
Glucose is a kind of sugar.
Plants use glucose as food for
energy and as a building block
for growing.
Autotrophs make glucose and
heterotrophs are consumers of
it.
Photo-synthesis means "putting together with light."

13. PHOTOSYNTHESIS
• Absorbing Light Energy to make chemical
energy: glucose!
– Pigments: Absorb different colors of white
light (ROY G BIV)
•Main pigment: Chlorophyll a
•Accessory pigments: Chlorophyll b and
Carotenoids
•These pigments absorb all wavelengths (light) BUT
not green!

14. Chloroplasts: Sites of Photosynthesis
• Photosynthesis
– Occurs in chloroplasts, organelles in certain
plants
– All green plant parts have chloroplasts and carry
out photosynthesis
• The leaves have the most chloroplasts
• The green color comes from chlorophyll in the
chloroplasts
• The pigments absorb light energy

15. • In most plants, photosynthesis occurs
primarily in the leaves, in the chloroplasts
• A chloroplast contains:
– stroma, a fluid
– grana, stacks of thylakoids
• The thylakoids contain chlorophyll
– Chlorophyll is the green pigment that captures
light for photosynthesis
Photosynthesis occurs in chloroplasts

16. • The location and structure of chloroplasts
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Chloroplast
Mesophyll
CHLOROPLAST Intermembrane space
Outer
membrane
Inner
membrane
Thylakoid
compartmentThylakoidStroma
Granum
StromaGrana

17. • Chloroplasts contain several pigments
Chloroplast Pigments
– Chlorophyll a
– Chlorophyll b
– Carotenoids
Figure 7.7

18. Chlorophyll a & b
•Chl a has a methyl
group
•Chl b has a carbonyl
group
Porphyrin ring
delocalized e-
Phytol tail

19. Different pigments absorb light differently

20. Excited
state
e
Heat
Light
Photon
Light
(fluorescence)
Chlorophyll
molecule
Ground
state
2
(a) Absorption of a photon
(b) fluorescence of isolated chlorophyll in solution
Excitation of chlorophyll
in a chloroplast
Loss of energy due to heat
causes the photons of light to be
less energetic.
Less energy translates into
longer wavelength.
Energy = (Planck’s constant) x
(velocity of light)/(wavelength of
light)
Transition toward the red end of
the visible spectrum.
e

21. Fall Colors
• During the fall, the green chlorophyll
pigments are greatly reduced revealing
the other pigments.
• Carotenoids are pigments that are either
red or yellow.

22. Chlorophyll: A Light Absorbing Pigment
The Solar Panel Chemical!

24. Photosynthesis
Glucose provides the energy and
carbon needed to make other
plant materials like wax and
proteins.

25. • Photosynthesis is the process by which
autotrophic organisms use light energy to
make sugar and oxygen gas from carbon
dioxide and water
AN OVERVIEW OF PHOTOSYNTHESIS
Carbon
dioxide
Water Glucose Oxygen
gas
PHOTOSYNTHESIS

26. • The Calvin cycle makes
sugar from carbon
dioxide
– ATP generated by the light
reactions provides the energy
for sugar synthesis
– The NADPH produced by the
light reactions provides the
electrons for the reduction of
carbon dioxide to glucose
Light
Chloroplast
Light
reactions
Calvin
cycle
NADP
ADP
+ P
• The light reactions
convert solar
energy to chemical
energy
– Produce ATP & NADPH
AN OVERVIEW OF PHOTOSYNTHESIS

27. Oxygen and Sugar!

28. • In plants and simple animals, waste products are
removed by diffusion. Plants, for example, excrete
O2, a product of photosynthesis.

29. EQUATION FOR
PHOTOSYNTHESIS
6CO2 + 6H2O + ENERGY C6H12O6 + 6O2
CARBON
DIOXIDE
WATER
GLUCOSE
OXYGEN

30. Redox Reaction
• The transfer of one or more electrons from one
reactant to another.
• Two types:
1. Oxidation
2. Reduction

31. Oxidation Reaction
• The loss of electrons from a substance.
• Or the gain of oxygen.
glucose
6CO2 + 6H2O  C6H12O6 + 6O2
Oxidation

32. Reduction Reaction
• The gain of electrons to a substance.
• Or the loss of oxygen.
glucose
6CO2 + 6H2O  C6H12O6 + 6O2
Reduction

33. PHOTOSYNTHESIS
• 2 Phases
– Light-dependent reaction
– Light-independent reaction
• Light-dependent: converts light energy
into chemical energy; produces ATP and
NADPH molecules to be used to fuel light-
independent reaction
• Light-independent: uses ATP produced to
make simple sugars/ glucose

34. PHOTOSYNTHESIS
• Light-dependent reaction (LIGHT
Reaction)
– Requires light
– Occurs in chloroplast (in thylakoids)
– Chlorophyll (thylakoid) traps energy from
light
– Light excites electron (e-)
•Kicks e- out of chlorophyll to an electron transport
chain
•Electron transport chain: series of proteins in
thylakoid membrane
– Bucket brigade

35. PHOTOSYNTHESIS
• Light-dependent reaction (LIGHT
Reaction)
– Energy lost along electron transport chain
– Lost energy used to recharge ATP from ADP
– NADPH produced from e- transport chain
•Stores energy until transfer to stroma
•Plays important role in light-independent reaction
– Total byproducts: ATP, NADP, O2

36. 1. Light Reaction (Electron Flow)
• During the light reaction, there are two possible
routes for electron flow.
A. Cyclic Electron Flow
B. Noncyclic Electron Flow

37. Water-splitting
photosystem
NADPH-producing
photosystem
ATP
mill
• Two types of
photosystems
cooperate in
the light
reactions

38. A. Cyclic Electron Flow
• Occurs in the thylakoid membrane.
• Uses Photosystem I only
• P700 reaction center- chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates ATP only
ADP + ATPP

39. A. Cyclic Electron Flow
P700
Primary
Electron
Acceptor
e-
e-
e-
e-
ATP
produced
by ETC
Photosystem I
Accessory
Pigments
SUN
Photons

40. B. Noncyclic Electron Flow
• Occurs in the thylakoid membrane
• Uses PS II and PS I
• P680 rxn center (PSII) - chlorophyll a
• P700 rxn center (PS I) - chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates O2, ATP and NADPH

41. B. Noncyclic Electron Flow
P700
Photosystem I
P680
Photosystem II
Primary
Electron
Acceptor
Primary
Electron
Acceptor
ETC
Enzyme
Reaction
H2O
1/2O2 + 2H+
ATP
NADPH
Photon
2e-
2e-
2e-
2e-
2e-
SUN
Photon

42. B. Noncyclic Electron Flow
• ADP +  ATP
• NADP+ + H  NADPH
• Oxygen comes from the splitting of H2O,
not CO2
H2O  1/2 O2 + 2H+
(Reduced)
P
(Reduced)
(Oxidized)

43. Primary
electron acceptor
Primary
electron acceptor
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy for
synthesis of
by chemiosmosis
Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product

44. • The O2 liberated by photosynthesis is made
from the oxygen in water (H+ and e-)
Plants produce O2 gas by splitting H2O

45. 2 H + 1/2
Water-splitting
photosystem
Reaction-
center
chlorophyll
Light
Primary
electron
acceptor
Energy
to make
Primary
electron
acceptor
Primary
electron
acceptor
NADPH-producing
photosystem
Light
NADP
1
2
3
How the Light Reactions Generate ATP and NADPH

46. Chemiosmosis
• Powers ATP synthesis.
• Located in the thylakoid membranes.
• Uses ETC and ATP synthase (enzyme) to make
ATP.
• Photophosphorylation: addition of phosphate to
ADP to make ATP.

47. Chemiosmosis
H+ H+
ATP Synthase
H+ H+ H+ H+
H+ H+
high H+
concentration
H+
ADP + P ATP
PS II PS I
E
T
C
low H+
concentration
H+
Thylakoid
Space
Thylakoid
SUN (Proton Pumping)

48. • The electron transport chains are arranged
with the photosystems in the thylakoid
membranes and pump H+ through that
membrane
– The flow of H+ back through the membrane is
harnessed by ATP synthase to make ATP
– In the stroma, the H+ ions combine with NADP+
to form NADPH
Chemiosmosis powers ATP
synthesis in the light reactions

49. • The production of ATP by chemiosmosis in
photosynthesis
Thylakoid
compartment
(high H+)
Thylakoid
membrane
Stroma
(low H+)
Light
Antenna
molecules
Light
ELECTRON TRANSPORT
CHAIN
PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

50. PHOTOSYNTHESIS
• Light-independent reaction (Dark
Reaction)
– Does not require light
– Calvin Cycle
•Occurs in stroma of chloroplast
•Requires CO2
•Uses ATP and NADPH as fuel to run
•Makes glucose sugar from CO2 and Hydrogen

51. Calvin Cycle
• Carbon Fixation (light independent rxn).
• C3 plants (80% of plants on earth).
• Occurs in the stroma.
• Uses ATP and NADPH from light rxn.
• Uses CO2.
• To produce glucose: it takes 6 turns and
uses 18 ATP and 12 NADPH.

52. Chloroplast
GranumThylakoid
Stroma
Outer Membrane
Inner Membrane

56. • A Photosynthesis Road Map
Chloroplast
Light
Stack of
thylakoids ADP
+ P
NADP
Stroma
Light
reactions
Calvin
cycle
Sugar used for
 Cellular respiration
 Cellulose
 Starch
 Other organic compounds

57. PHOTOSYNTHESIS
• What affects photosynthesis?
– Light intensity: as light increases, rate of
photosynthesis increases

58. PHOTOSYNTHESIS
• What affects photosynthesis?
– Carbon Dioxide: As CO2 increases, rate of
photosynthesis increases

59. PHOTOSYNTHESIS
• What affects photosynthesis?
– Temperature:
•Temperature Low = Rate of photosynthesis low
•Temperature Increases = Rate of photosynthesis
increases
•If temperature too hot, rate drops

60. Photorespiration
• Occurs on hot, dry, bright days.
• Stomates close.
• Fixation of O2 instead of CO2.
• Produces 2-C molecules instead of 3-C sugar
molecules.
• Produces no sugar molecules or no ATP.

61. Photorespiration
• Because of photorespiration: Plants have
special adaptations to limit the effect of
photorespiration.
1. C4 plants
2. CAM plants

62. C4 Plants
• Hot, moist environments.
• 15% of plants (grasses, corn, sugarcane).
• Divides photosynthesis spatially.
• Light rxn - mesophyll cells.
• Calvin cycle - bundle sheath cells.

63. C4 Plants
Mesophyll Cell
CO2
C-C-C
PEP
C-C-C-C
Malate
ATP
Bundle Sheath Cell
C-C-C
Pyruvic Acid
C-C-C-C
CO2
C3
Malate
Transported
glucose
Vascular
Tissue

64. CAM Plants
• Hot, dry environments.
• 5% of plants (cactus and ice plants).
• Stomates closed during day.
• Stomates open during the night.
• Light rxn - occurs during the day.
• Calvin Cycle - occurs when CO2 is present.

65. CAM Plants
Night (Stomates Open) Day (Stomates Closed)
Vacuole
C-C-C-C
Malate
C-C-C-C
Malate Malate
C-C-C-C
CO2
CO2
C3
C-C-C
Pyruvic acid
ATP
C-C-C
PEP glucose

66. Check it!
1. The process that uses the sun’s energy to
make simple sugars is _____________.
A. Cellular respiration
B. Glycolysis
C. Photosynthesis
D. Photolysis

67. Check it!
2. The function accomplished by the light-
dependent reactions is ______________.
A. Energy storage
B. Sugar production
C. Carbon fixation
D. Conversion of sugar