Photosynthesis converts solar energy into chemical energy through two stages: the light reactions and the Calvin cycle. In the light reactions, photosystems use light to split water, producing oxygen and generating ATP and NADPH. The Calvin cycle then uses ATP and NADPH to incorporate carbon from CO2 into organic molecules like glucose. Chloroplasts are the organelles where photosynthesis occurs, containing chlorophyll and other pigments in thylakoid membranes that absorb light to drive the light-dependent reactions. The process ultimately produces sugars that plants use as energy and build other organic molecules, while releasing oxygen as a byproduct.

1. BIOL 101
Chapter 10
Photosynthesis
Rob Swatski
Associate Professor of Biology
HACC – York Campus

2. The Process That Feeds the Biosphere
Photosynthesis:
- converts solar energy into chemical energy
- directly or indirectly feeds entire living world
- in plants, algae & other protists, some prokaryotes

3. Autotrophs: survive without eating
anything derived from other organisms
- Producers: make organic molecules
from inorganic molecules (H2O and CO2)
- Photoautotrophs: use solar energy to
make organic molecules

4. (a) Plants
(c) Unicellular
protist
10 µm
1.5 µm
40 µm(d) Cyanobacteria
(e) Purple sulfur
bacteria
(b) Multicellular alga

5. Heterotrophs: obtain their organic material
from other organisms
- Consumers of the biosphere
- heterotrophs depend on photoautotrophs
for food and O2
Can an organism be both autotrophic and
heterotrophic?

6. Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
- their structure enables the chemical reactions of
photosynthesis to occur – how so?

7. Leaves: the major organs of photosynthesis
- chlorophyll: green pigment inside chloroplasts
- absorbs light energy that powers the synthesis of
organic molecules
CO2 enters and O2 exits the leaf through stomata

9. Chloroplasts: found in mesophyll cells in the leaf’s
interior
- 30-40 chloroplasts per mesophyll cell
Chlorophyll is in the thylakoid membranes of
chloroplasts
- thylakoids are stacked into grana (granum)
- stroma

10. 5 µm
Mesophyll cell
Stomata
CO2 O2
Chloroplast
Mesophyll
Vein
Leaf cross section

11. 1 µm
Thylakoid
space
Chloroplast
Granum
Intermembrane
space
Inner
membrane
Outer
membrane
Stroma
Thylakoid

12. Photosynthesis summary equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
Photosynthesis is a redox process:
- H2O is oxidized
- CO2 is reduced

13. Chloroplasts split H2O into hydrogen and oxygen,
incorporating the e- of H into glucose
Reactants: 6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6

14. The Two Stages of Photosynthesis: A Preview
Light reactions: the “photo” part
Calvin cycle: the “synthesis” part
Light Reactions (in thylakoids):
- split H2O
- release O2
- reduce NADP+ to NADPH
- generate ATP from ADP by photophosphorylation

15. Light
H2O
Chloroplast
Light
Reactions
NADP+
P
ADP
i
+

16. Light
H2O
Chloroplast
Light
Reactions
NADP+
P
ADP
i
+
ATP
NADPH
O2

17. Calvin cycle (in stroma)
- forms glucose from CO2
- uses ATP and NADPH
Begins with carbon fixation
- incorporate CO2 into organic molecules

18. Light
H2O
Chloroplast
Light
Reactions
NADP+
P
ADP
i
+
ATP
NADPH
O2
Calvin
Cycle
CO2

19. Light
H2O
Chloroplast
Light
Reactions
NADP+
P
ADP
i
+
ATP
NADPH
O2
Calvin
Cycle
CO2
[CH2O]
(sugar)

20. The Nature of Sunlight
Light is a form of electromagnetic energy (radiation)
- travels in rhythmic waves
Wavelength: distance between crests of waves
- determines the type of electromagnetic energy

21. Electromagnetic spectrum: the entire range of
electromagnetic radiation
Visible light: the wavelengths that produce colors
we can see
- includes wavelengths that drive PSN
- behaves as though it consists of discrete energy
“particles” (photons)

22. UV
Visible light
Infrared
Micro-
waves
Radio
wavesX-raysGamma
rays
103 m
1 m
(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energyHigher energy
Shorter wavelength

23. Photosynthetic Pigments: The Light Receptors
Pigments: chemicals that absorb visible light
- different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or
transmitted
- leaves appear green because chlorophyll reflects and
transmits green light

24. Reflected
light
Absorbed
light
Light
Transmitted
light
Granum

25. Spectrophotometer: measures a pigment’s ability to
absorb different wavelengths
- sends light through pigments
- measures the fraction of light transmitted at each
wavelength

26. Galvanometer
Slit moves to
pass light
of selected
wavelength
White
light
Green
light
Blue
light
The low transmittance
(high absorption) reading
-chlorophyll absorbs most
blue light
High transmittance
(low absorption) reading
-chlorophyll absorbs very little
green light
Refracting
prism Photoelectric
tube
Chlorophyll
solution
TECHNIQUE
1
2 3
4

27. Absorption spectrum: graph plotting a pigment’s light
absorption vs. wavelength
- the absorption spectrum of chlorophyll a suggests
that violet-blue and red light work best for PSN
Action spectrum: profiles the relative effectiveness of
different wavelengths of radiation in driving PSN

28. Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’s
experiment
Aerobic bacteria
RESULTS
Filament
of alga
Chloro-
phyll a Chlorophyll b
Carotenoids
500400 600 700
700600500400

29. Chlorophyll a: the main photosynthetic pigment
Accessory pigments:
- Chlorophyll b: broadens the PSN spectrum
- Carotenoids: absorb excess light that would
damage chlorophyll

30. Porphyrin ring:
light-absorbing “head” of molecule
-Mg atom at center
in chlorophyll aCH3
Hydrocarbon tail:
interacts with hydrophobic regions of
proteins inside thylakoid membranes of
chloroplasts
(H atoms not shown)
CHO in chlorophyll b

31. Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a stable
ground state to an unstable excited state
When excited e- fall back to the ground state,
photons are given off (fluorescence)
- if illuminated, a chlorophyll solution will fluoresce,
giving off light and heat

32. (a) Excitation of isolated chlorophyll molecule
Heat
Excited
state
(b) Fluorescence
Photon
Ground
state
Photon
(fluorescence)
Energyofelectron
e–
Chlorophyll
molecule
e–

33. Photosystem consists of:
- Reaction-center complex surrounded by
- Light-harvesting complexes (pigments bound to
proteins)
Funnels energy of photons to the reaction center

34. Primary electron acceptor (in reaction center
complex)
- accepts an excited e- from chlorophyll a
- the 1st step of the light reactions

35. THYLAKOID SPACE
(INSIDE THYLAKOID)
STROMA
e–
Pigment
molecules
Photon
Transfer
of energy
Special pair of
chlorophyll a
molecules
(P680 or P700)
Thylakoidmembrane
Photosystem
Primary
electron
acceptor
Reaction-center
complex
Light-harvesting
complexes

36. Two Types of Photosystems:
Photosystem II (PS II) functions first:
- best at absorbing wavelengths of 680 nm
P680: the reaction-center chlorophyll a of PS II

37. Photosystem I (PS I) functions second:
- best at absorbing wavelengths of 700 nm
P700: the reaction-center chlorophyll a of PS I

38. Linear Electron Flow
Two possible routes for e- flow during the light
reactions: cyclic and linear
Linear electron flow: the primary pathway
- involves both photosystems I and II
- produces ATP and NADPH using light energy

39. A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
- an excited e- from P680 is transferred to the
primary electron acceptor

40. Pigment
molecules
Light
P680
e–
2
1
Photosystem II
(PS II)
Primary
acceptor
e-

41. P680+ = P680 with one less e-
- is a very strong oxidizing agent
- H2O is split by enzymes
- its e- are transferred from H atoms to P680+
- reduces P680+ to P680
- O2 is released as a by-product of this reaction

42. Pigment
molecules
Light
P680
e–
Primary
acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
Photosystem II
(PS II)
e-
e-
e-

43. Each e- “falls” down an electron transport chain from
the primary electron acceptor of PS II to PS I
- energy released by the fall drives the creation of a
proton gradient across the thylakoid membrane
- the diffusion of H+ (protons) across the membrane
drives ATP synthesis

44. Pigment
molecules
Light
P680
e–
Primary
acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome
complex
5
ATP
Photosystem II
(PS II)
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-

45. In PS I (like PS II), transferred light energy excites
P700, which loses an e- to an electron acceptor
P700+: P700 that is missing an e-
- accepts an e- passed down from PS II via the electron
transport chain

46. Pigment
molecules
Light
P680
e–
Primary
acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome
complex
5
ATP
Photosystem II
(PS II)
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
Photosystem I
(PS I)
Light
Primary
acceptor
e–
P700
6
e-
e-

47. Each e- “falls” down an electron transport chain from
the primary electron acceptor of PS I to the protein
ferredoxin (Fd)
- the e- are then transferred to NADP+ and reduce it to
NADPH
- the e- of NADPH are available for the reactions of the
Calvin cycle

48. Pigment
molecules
Light
P680
e–
Primary
acceptor
2
1
e–
e–
2 H+
O2
+
3
H2O
1/2
4
Pq
Pc
Cytochrome
complex
5
ATP
Photosystem I
(PS I)
Light
Primary
acceptor
e–
P700
6
Fd
NADP+
reductase
NADP+
+ H+
NADPH
8
7
e–
e–
6
Photosystem II
(PS II)
e-

49. Mill
makes
ATP
e–
NADPH
e–
e–
e–
e–
e–
ATP
Photosystem II Photosystem I
e–

50. A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
Both generate ATP by chemiosmosis, but use
different sources of energy:
Mitochondria transfer chemical energy from food
to ATP
Chloroplasts transform light energy into the
chemical energy of ATP

51. Spatial Organization of Chemiosmosis:
Mitochondria: protons are pumped into the
intermembrane space
- diffusion of protons back into the matrix drives
ATP synthesis
Chloroplasts: protons are pumped into the
thylakoid space
- diffusion of protons back into the stroma drives
ATP synthesis

52. Key
Mitochondrion Chloroplast
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
Electron
transport
chain
H+ Diffusion
Matrix
Higher [H+]
Lower [H+]
Stroma
ATP
synthase
ADP + P i
H+
ATP
Thylakoid
space
Thylakoid
membrane

53. ATP and NADPH are produced on the side facing the
stroma, where the Calvin cycle takes place
In summary:
- light reactions generate ATP
- they increase the potential energy of e- by moving
them from H2O to NADPH

54. Light
Fd
Cytochrome
complex
ADP
+
i
H+
ATP
P
ATP
synthase
To
Calvin
Cycle
STROMA
(low H+ concentration)
Thylakoid
membrane
THYLAKOID SPACE
(high H+ concentration)
STROMA
(low H+ concentration)
Photosystem II Photosystem I
4 H+
4 H+
Pq
Pc
Light
NADP+
reductase
NADP+ + H+
NADPH
+2 H+
H2O
O2
e–
e–
1/21
2
3

55. The Calvin cycle regenerates its starting material as
molecules enter and leave the cycle
- analagous to the citric acid cycle
- builds sugar from smaller molecules using ATP and
the reducing power of e- carried by NADPH

56. Carbon enters the Calvin cycle as CO2
Carbon leaves as the sugar, glyceraldehyde-3-
phospate (G3P)
For net synthesis of 1 G3P:
- the Calvin cycle must turn 3X, fixing 3 molecules of
CO2

57. Three Phases of the Calvin Cycle
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)

58. Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Short-lived
intermediate
Phase 1: Carbon fixation
(Entering one
at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP

59. Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Short-lived
intermediate
Phase 1: Carbon fixation
(Entering one
at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
6
6 NADP+
NADPH
i
Phase 2:
Reduction
Glyceraldehyde-3-phosphate
(G3P)
1 P
Output G3P
(a sugar)
Glucose and
other organic
compounds
Calvin
Cycle

60. Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Short-lived
intermediate
Phase 1: Carbon fixation
(Entering one
at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
6
6 NADP+
NADPH
i
Phase 2:
Reduction
Glyceraldehyde-3-phosphate
(G3P)
1 P
Output G3P
(a sugar)
Glucose and
other organic
compounds
Calvin
Cycle
3
3 ADP
ATP
5 P
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
G3P

61. Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
CO2
NADP+
ADP
P i
+
RuBP 3-Phosphoglycerate
Calvin
Cycle
G3PATP
NADPH
Starch
(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2