Photosynthesis is a process where autotrophic organisms, primarily plants, convert light energy into chemical energy by transforming carbon dioxide and water into glucose and oxygen. This process occurs in chloroplasts and involves two main stages: the light-dependent reactions, which generate ATP and NADPH, and the light-independent reactions (Calvin cycle), which use these products to synthesize glucose. Factors that affect photosynthesis include light intensity, carbon dioxide concentration, and temperature, while photorespiration can hinder efficiency under certain conditions.

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

6. 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)

7. Sunlight minus absorbed
wavelengths or colors
equals the apparent color
of an object.

8. Why are plants green?

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

10. • 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

11. 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."

12. 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 (Carotene & Xanthophyll)
•These pigments absorb all wavelengths (light) BUT
not green!
•Maximum absorption of red & blue light.

13. 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

14. • 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

15. • 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

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

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

18. Different pigments absorb light differently

19. Excited
state
e
Heat
Light
Photon
Light
(fluorescence)
Chlorophyll
molecule
Ground
state
2
(a) Absorption of a photon
Excitation of chlorophyll
in a chloroplast
e

20. 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.

21. • 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

22. • 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

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

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

25. Oxidation Reaction
• The loss of electrons from a substance.
• Or the gain of oxygen.
glucose
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
Oxidation

26. Reduction Reaction
• The gain of electrons to a substance.
• Or the addition of hydrogen.
glucose
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
Reduction

27. 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 this ATP to make
simple sugars/ glucose

28. 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

29. 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

30. 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

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

32. 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

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

34. 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

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

36. Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product

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

38. 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.

39. 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)

40. • 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

41. 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

42. 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 1 glucose: it takes 2 turns and
uses 18 ATP and 12 NADPH.

43. Chloroplast
GranumThylakoid
Stroma
Outer Membrane
Inner Membrane

47. • 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

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

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

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

51. Concepts
• Photosynthesis: CO2 + Water --> Sugar + O2
– Photosynthesis is the production of sugar (stored
energy) and oxygen using energy from the sun to
combine carbon dioxide and water.
– CO2 is brought into plants and O2 is released from
plants through pores (stomata) in their leaves and
other tissues.
– RUBISCO is the enzyme plants use to undergo
photosynthesis.
+ Solar
Energy
Stomata

52. Concepts
• Respiration: Sugar + O2 --> CO2 + Water + E
– Respiration is the burning of sugar in the presence
of oxygen to release energy stored in the sugar and
produces carbon dioxide and water as by-products.
• Photorespiration: Occurs under high light/heat
when RUBISCO tends to react with O2 (undergoing
respiration) rather than CO2 (undergoing
photosynthesis). This slows rates of photosynthesis
under high light/heat (this is not what the plant wants
to happen).
Energy

53. Concepts
• Transpiration: Loss of water out of stomata
(pores) of plants during gas exchange (O2 and
CO2) while photosynthesizing and respiring.
• Water Use Efficiency (WUE): How good
a plant is at bringing in CO2 without losing too
much water. In other words it is the ratio of rate
of photosynthesis (energy generation) to rate of
transpiration (water lost).
Stoma

54. AP Biology
Leaf Structure
H2O
CO2
O2 H2O
phloem (sugar)
xylem (water)
stomate guard
cell
palisades
layer
spongy
layer
cuticle
epidermis
O2 CO2
Transpiration
vascular bundle
Gas exchange

55. AP Biology
Controlling water loss from leaves
 Hot or dry days
 stomates close to conserve water
 guard cells
 gain H2O = stomates open
 lose H2O = stomates close
 adaptation to
living on land,
but…
creates PROBLEMS!

56. AP Biology
When stomates close…
xylem
(water)
phloem
(sugars)
H2O
O2 CO2
 Closed stomates lead to…
 O2 build up  from light reactions
 CO2 is depleted  in Calvin cycle
 causes problems in Calvin Cycle


57. AP Biology
Inefficiency of RuBisCo: CO2 vs O2
 RuBisCo in Calvin cycle
 carbon fixation enzyme
 normally bonds C to RuBP
 CO2 is the optimal substrate
 reduction of RuBP
 building sugars
 when O2 concentration is high
 RuBisCo bonds O to RuBP
 O2 is a competitive substrate
 oxidation of RuBP
 breakdown sugars
photosynthesis
photorespiration

58. AP Biology
Calvin cycle when O2 is high
5C
RuBP
3C
2C
to
mitochondria
–––––––
lost as CO2
without
making ATP
photorespiration
O2
RuBisCo

59. AP Biology
Impact of Photorespiration
 Oxidation of RuBP
 short circuit of Calvin cycle
 loss of carbons to CO2
 can lose 50% of carbons fixed by Calvin cycle
 reduces production of photosynthesis
 no ATP (energy) produced
 no C6H12O6 (food) produced
 if photorespiration could be reduced,
plant would become 50% more efficient
 strong selection pressure to evolve
alternative carbon fixation systems

60. AP Biology
Reducing photorespiration
 Separate carbon fixation from Calvin cycle
 C4 plants
 separate carbon fixation from Calvin cycle by ANATOMY
 different cells to fix carbon vs. where Calvin cycle occurs
 store carbon in 4C compounds
 different enzyme to capture CO2 (fix carbon)
 PEP carboxylase
 different leaf structure
 CAM plants
 separate carbon fixation from Calvin cycle by TIME OF DAY
 fix carbon during night
 store carbon in 4C compounds
 perform Calvin cycle during day

61. AP Biology
C4 plants
 A better way to capture CO2
 1st step before Calvin cycle,
fix carbon with enzyme
PEP carboxylase
 store as 4C compound
 adaptation to hot,
dry climates
 have to close stomates a lot
 different leaf anatomy
 sugar cane, corn,
other grasses…
sugar cane
corn

62. AP Biology
C4 leaf anatomy
PEP (3C) + CO2  oxaloacetate (4C)
CO2
CO2
O2
light reactions
C4 anatomy
C3 anatomy
 PEP carboxylase enzyme
 higher attraction for CO2 than O2
 better than RuBisCo
 fixes CO2 in 4C compounds
 regenerates CO2 in inner cells for RuBisCo
 keeping O2 away from RuBisCo
bundle
sheath
cell RuBisCo
PEP
carboxylase
stomate

63. AP Biology
CAM (Crassulacean Acid Metabolism) plants
 Adaptation to hot, dry climates
 separate carbon fixation from Calvin cycle by TIME
 close stomates during day
 open stomates during night
 at night: open stomates & fix carbon
in 4C “storage” compounds
 in day: release CO2 from 4C acids
to Calvin cycle
 increases concentration of CO2 in cells
 succulents, some cacti, pineapple

64. AP Biology
CAM plants
succulents
cacti
pineapple

65. AP Biology
C4 vs CAM Summary
C4 plants
separate 2 steps
of C fixation
anatomically in 2
different cells
CAM plants
separate 2 steps
of C fixation
temporally =
2 different times
night vs. day
solves CO2 / O2 gas exchange vs. H2O loss challenge

66. AP Biology
Why the C3 problem?
 Today it makes a difference
 21% O2 vs. 0.03% CO2
 photorespiration can drain away 50% of carbon
fixed by Calvin cycle on a hot, dry day
 strong selection pressure to evolve better way
to fix carbon & minimize photorespiration

67. AP Biology

68. AP Biology
FACTORS NECESSARY FOR
PHOTOSYNTHESIS
 A number of factors affect the process
of photosynthesis, as a result of which
productivity is affected. These are
 Carbon dioxide
 Water
 Chlorophyll
 Light

69. AP Biology
Principle of limiting factors
 The Principle of limiting factors also
states that when a biochemical process
is affected by several factors, its rate is
limited by that factor which is nearest
its minimum value. That factor (known
as limiting factor) directly affects the
biochemical process if its quantity is
changed.

70. AP Biology
CARBON DIOXIDE (CO2)
 Air contains 0.03% of CO2. It is released by
respiration, combustion of fossil fuels and
microbial decomposition.
 During early morning hours and evening
hours, CO2 released in respiration is
sufficient for photosynthesis. At this stage,
there is no exchange of gases between the
plant and the environment. This is called
compensation point.

71. AP Biology
WE HAVE TEST ON NEXT THRUSDAY