1. Photosynthesis
Chapter 8

2. Photosynthesis
Energy for the majority of life on Earth ultimately comes from the sun through photosynthesis
Photosynthesis is the process that converts energy of sunlight the into chemical energy, especially carbohydrates
Carbon Fixation: Photosynthesis collects the energy of sunlight and captures it into a chemical form
Photosynthesis uses CO2 to build carbohydrates (C6H12O6)

3. Photosynthesis
Photoautotrophs: photosynthetic organisms that can produce their own ‘food’
these organisms fix the energy of sunlight into carbohydrate form
they then use the carbohydrates they produce for cellular respiration
Heterotrophs: organisms that require an alternative food source
these organisms consume photoautotrophs along with the carbohydrates they produce
they then use the carbohydrates they consume for cellular respiration 3

4. Photosynthesis Oxygenic photosynthesis: produces O2
carried out by cyanobacteria, 7 groups of algae, all land plants
Photosynthetic organisms serve as a food and energy source for the majority of life on on earth
Photosynthetic organisms directly or indirectly provide energy for the majority of the life on earth
most living things depend on the sun for energy
a few exceptions include chemoautotrophs: autotrophic organisms that utilize energy from other sources, live independently from the sun 4

5. Oxygenic Photosynthesis
Energy Fixation
Maintains O2 in the atmosphere
vital for aerobic respiration
Removes CO2 from the atmosphere
green house gases
H2O production 5

6. carbon dioxide water glucose water oxygen
Photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
carbon dioxide water glucose water oxygen
Reactants: 6CO2 + 12H2O
Products: C6H12O6 + 6H2O + 6O2
Photosynthesis vs. Cellular Respiration:
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
C6H12O6 + 6O CO2 + 12H2O 6

7. Photosynthesis vs. Cellular Respiration
Photoautotrophs
Chloroplasts
grana and thylakoids
ETC
NADP reduced to NADPH
ATP synthase, ATP production
Calvin Cycle
CO2 reduced to carbohydrates
Heterotrophs
Mitochondria
mitochondrial membrane, cisternae
Krebs Cycle
Carbohydrates oxidized to CO2
ETC
NAD+ reduced to NADH
ATP synthase, ATP production

8. Photosynthesis Occurs in stroma
Photosynthesis is divided into two reactions:
1. Light-dependent reactions
capture energy from sunlight
reduce NADP+ to NADPH
produce ATP
occur in chloroplasts
2. Carbon fixation reactions
use ATP and NADPH to synthesize organic molecules from CO2
Occurs in stroma

10. Photosynthesis Overview
Photosynthesis takes place in chloroplasts
Thylakoid membrane – internal membrane arranged in flattened sacs
contain chlorophyll and other pigments
Grana – stacks of thylakoid membranes
Stroma – semiliquid substance surrounding thylakoid membranes

11. Fig

12. Fig

13. Discovery of Photosynthesis
The work of many scientists led to the discovery of how photosynthesis works:
Jan Baptista van Helmont ( )
Joseph Priestly ( )
Jan Ingen-Housz ( )
F. F. Blackman ( )

14. Discovery of Photosynthesis
C. B. van Niel, 1930‘s
proposed a general formula:
CO2+H2A + light energy CH2O + H2O + 2A
H2A is the electron donor
van Niel identified water as the source of the O2 released from photosynthesis
Robin Hill confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation

15. Energy from Light Reactions Fuels Carbon Fixation
Photon: a particle of light
acts as a discrete bundle of energy
energy content of a photon is inversely proportional to the wavelength of the light
Photoelectric effect: removal of an electron from a molecule by light
occurs when photons transfer energy to electrons
Light energy is absorbed by pigments: molecules that absorb visible light

16. Energy from Light Reactions
Fuels Carbon Fixation

17. Pigments Pigments: molecules that absorb visible light
chlorophyll a
chlorophyll b
carotenoids
Each pigment has a characteristic absorption spectrum: the range and efficiency of photons it is capable of absorbing.

19. Pigments Chlorophyll a – primary pigment in plants and cyanobacteria
absorbs violet-blue and red light
Chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

20. Pigments Structure of pigments:
Porphyrin ring: complex ring structure with alternating double and single bonds
magnesium ion at the center of the ring
photons excite electrons in the ring
electrons are shuttled away from the ring

23. Pigments
Accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a
increase the range of light wavelengths that can be used in photosynthesis
include: chlorophyll b, carotenoids, phycobiloproteins
carotenoids also act as antioxidants

24. Photosynthesis: Photosystems
Photosystem: Enzyme complexes for photosynthesis
enzymes use light to oxidize H2O and reduce CO2 to carbohydrates
Located in the thylakoid membrane of plants, algae and cyanobacteria or the cytoplasmic membrane of photosynthetic bacteria 24

25. Photosystem Organization
A photosystem consists of:
1. an antenna complex of hundreds of accessory pigment molecules
2. a reaction center of one or more chlorophyll a molecules
Energy of electrons is transferred through the antenna complex to the reaction center

27. Photosystem Organization
At the reaction center, the energy from the antenna complex is transferred to chlorophyll a
This energy causes an electron from chlorophyll to become excited
chlorophyll absorbs a photon, looses an electron
The excited electron is transferred from chlorophyll a to an electron acceptor, quinone
Water donates an electron to chlorophyll a to replace the excited electron

29. Light-Dependent Reactions
Light-dependent reactions occur in 4 stages:
1. Primary Photoevent – a photon of light is captured by a pigment molecule
2. Charge Separation – energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
3. Electron Transport – electrons move through carriers to reduce NADP+
4. Chemiosmosis – produces ATP (ATP synthase)

31. Light-Dependent Reactions
In sulfur bacteria, only one photosystem is used for cyclic photophosphorylation
1. Hydrogen sulfide oxidized
electrons joins a proton to produce hydrogen
elemental S and protons produced as products
2. An electron is recycled to chlorophyll
this process drives the chemiosmotic synthesis of ATP
This system only produces energy

32. Light-Dependent Reactions
In chloroplasts, two linked photosystems are used in noncyclic photophosphorylation
1. Photosystem I
reaction center pigment (P700) with a peak absorption at 700nm
2. Photosystem II
reaction center pigment (P680) has a peak absorption at 680nm
This system produces energy, O2, and NADPH for biosynthesis of carbohydrates

33. Light-Dependent Reactions
Photosystem II acts first:
accessory pigments shuttle energy to the P680 reaction center
excited electrons from P680 are transferred to the cytochrome/b6-f complex - connects photosystems II and I
electron lost from P680 is replaced by an electron released from hydrolysis: the splitting of water

34. Light-Dependent Reactions
The b6-f complex is a series of electron carriers
an electron transport chain
electron carrier molecules are embedded in the thylakoid membrane
protons are pumped into the thylakoid space to form a proton gradient

35. Light-Dependent Reactions
Photosystem I:
receives energy from an antenna complex
energy is shuttled to P700 reaction center
excited electron is transferred to a membrane- bound electron carrier
electrons are used to reduce NADP+ to NADPH
electrons lost from P700 are replaced from the b6-f complex

36. Z Diagram of Photosystems I and II
Fig. 8.13
Z Diagram of Photosystems I and II

37. Fig

38. Fig

39. Fig

40. Light-Dependent Reactions
ATP is produced via chemiosmosis:
ATP synthase is embedded in the thylakoid membrane
protons have accumulated in the thylakoid space
protons move into the stroma only through ATP synthase
ATP is produced from ADP + Pi

42. Carbon Fixation Reactions
Calvin cycle
biochemical pathway that allows for carbon fixation
incorporates CO2 into organic molecules
occurs in the stroma
uses ATP and NADPH as energy sources

43. Carbon Fixation Reactions
To build carbohydrates, cells need:
1. Energy
provided by ATP from light-dependent reactions
2. Reduction Potential: hydrogen atoms
provided by NADPH from photosystem I

44. Carbon Fixation Reactions
The Calvin cycle has 3 phases:
1. Carbon Fixation
RuBP + CO molecules PGA 3 phosphoglycerate
2. Reduction
PGA is reduced to G3P glyceraldehyde-3-phosphate
3. Regeneration of RuBP
G3P is used to regenerate RuBP

45. Carbon Fixation Reactions
Carbon fixation – the incorporation of CO2 into organic molecules
occurs in the first step of the Calvin cycle:
ribulose-bis-phosphate + CO (PGA)
5 carbon molecule carbon carbons
Reaction is catalyzed by rubisco

47. Fig

48. Fig

49. Fig

50. Carbon Fixation Reactions
Glucose is not a direct product of the Calvin cycle
2 molecules of G3P leave the cycle
each G3P contains 3 carbons
2 G3P are used to produce 1 glucose in reactions in the cytoplasm

51. Carbon Fixation Reactions
During the Calvin cycle, energy is needed. The energy is supplied from:
18 ATP molecules
12 NADPH molecules

52. Carbon Fixation Reactions
The energy cycle:
photosynthesis uses the products of respiration as starting substrates
respiration uses the products of photosynthesis as starting substrates

54. Photorespiration Rubisco has 2 enzymatic activities:
1. Carboxylation – the addition of CO2 to RuBP
favored under normal conditions
2. Photorespiration – the oxidation of RuBP by the addition of O2
favored in hot conditions
CO2 and O2 compete for the active site on RuBP.

55. Fig

56. Fig

57. Photorespiration
Some plants can avoid photorespiration by using an enzyme other than rubisco.
PEP carboxylase
adds CO2 to phosphoenolpyruvate (PEP)
a 4 carbon compound is produced
CO2 is later released from this 4-carbon compound and used by rubisco in the Calvin cycle

59. Photorespiration C4 plants use PEP carboxylase to capture CO2
CO2 is added to PEP in mesophyll cell
the resulting 4-carbon compound is moved into a bundle sheath cell where the CO2 is released and used in the Calvin cycle

62. Photorespiration CAM plants
CO2 is captured at night when stomata are open
PEP carboxylase adds CO2 to PEP to produce a 4 carbon compound
this compound releases CO2 during the day
CO2 is then used by rubisco in the Calvin cycle