1. Photosynthesis: Using Light to Make Food
Chapter 7
Photosynthesis: Using Light to Make Food

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

3. Autotroph: can use CO2 as a sole source of carbon to make other organic compounds
Heterotroph: cannot use CO2 as a sole source of carbon
Autotroph = producer
Heterotroph = consumer, e.g., animals
Autotroph
Photoautotroph : light as the energy source: e.g., plants, algae, photosynthetic bacteria
Chemoautotroph : chemical compounds as the energy source : e.g., H2-oxidizing bacteria

4. 7.1 Autotrophs are the producers of the biosphere
Plants, some protists, and some bacteria are photosynthetic autotrophs
They are the ultimate producers of food consumed by virtually all organisms

6. In aquatic environments, algae and photosynthetic bacteria are the main food producers
Figure 7.1C
Figure 7.1D

7. 7.2 Photosynthesis occurs in chloroplasts
In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
6CO2 + 6H2O  C6H12O6 + 6O2
All green parts of a plant have chloroplasts
Chloroplasts are the sites where photosynthesis occurs
Green pigment : chlorophyll (absorption of light energy)
Chloroplasts are most abundant in the mesophyll cells
Gas exchange (CO2/O2) occurs by way of tiny pores called stomata

8. The location and structure of chloroplasts
Figure 7.2_1
The location and structure of chloroplasts
Epidermis cell
Leaf Cross Section
Leaf
Mesophyll
Vein
Mesophyll Cell
CO2 O2
Stoma
Chloroplast

9. Inner and outer membranes
Figure 7.2_2
Chloroplast
Inner and outer membranes
Granum
Thylakoid
Thylakoid space
Figure 7.2_2 Zooming in on the location and structure of chloroplasts (part 2)
Stroma 9

10. 7.3 Plants produce O2 gas by splitting water
The O2 liberated by photosynthesis is made from the oxygen in water
Figure 7.3A

11. Experiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2
Reactants:
Products:

12. 7.4 Photosynthesis is a redox process, as is cellular respiration
Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas
These electrons and H+ ions are transferred to CO2, producing sugar
Energy input is required (light energy)
Reduction
Oxidation
Figure 7.4A
Oxidation
Reduction
Figure 7.4B

13. 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
The complete process of photosynthesis consists of two linked sets of reactions:
the light reactions and the Calvin cycle
The light reactions convert light energy to chemical energy and produce O2
The Calvin cycle assembles sugar molecules from CO2 using the energy-carrying products of the light reactions

14. LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma)
An overview of photosynthesis
H2O
CO2
Chloroplast
Light
NADP+
ADP + P
LIGHT REACTIONS (in grana)
CALVIN CYCLE (in stroma)
ATP
Electrons
NADPH O2
Sugar

15. 7.6 Visible radiation drives the light reactions
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
7.6 Visible radiation drives the light reactions
Light is a form of electromagnetic radiation
Light behaves like wave and particle

16. 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Increasing energy Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
380
400
500
600
700
750
Wavelength (nm)
650 nm

17. Visible light : 380 – 750 nm
Energy is inversely proportional to wavelength
E = hc/l
h: Planck’s constant
C: velocity of light (3 x 108 m/sec)

18. When a molecule absorbes a photon, one of its electrons is raised from a ground state to an excited state
The excited molecule can serve as a strong reductant to reduce another molecule
Pigment : a molecule that absorbs visible light

19. Pigments that are involved in photosynthesis
Chlorophyll : major photopigments in chloroplasts
Carotenoid : 광합성시 이용가능한 광범위를 넓혀줌
Protection chlorophylls from photodamage

20. Light Reflected light Chloroplast Absorbed light Thylakoid
Figure 7.6B
Light
Reflected light
Chloroplast
Absorbed light
Thylakoid
Transmitted light

21. Several pigments are built into the thylakoids of chloroplasts
Absorb some wavelengths of light; reflect or transmit others
Chlorophyll a
Absorbs blue-violet and red light, reflects green light
Participates directly in the light reactions

22. Chlorophyll b Carotenoids
Absorbs blue and orange light, reflects yellow-green
Conveys absorbed energy to chlorophyll a
Carotenoids
Yellow-orange pigments that absorb mainly blue-green light
May pass energy to chlorophyll a or protect it by dissipating excessive light energy

23. Chlorophyll b
CH3  CHO
Porphyrin ring with a Mg2+
Phytol

24. When an excited chlorophyll molecule returns to the ground state,
Emission of heat or fluorescence
Reduction of the primary electron acceptor in the reaction center

25. 7.7 Photosystems capture solar power
The site where light energy is absorbed and light reaction begins
Reaction center + light harvesting complex
Located in the thylakoid membrane
RC: Chl. a + primary electron acceptor
LHC: Chl. a and b + carotenoids RC
Antenna pigment molecules
Photopigment molecules are associated with membrane proteins

26. Pair of chlorophyll a molecules
Figure 7.7B
Photosystem
Light
Light-harvesting complexes
Reaction-center complex
Primary electron acceptor
Thylakoid membrane
Pigment molecules
Pair of chlorophyll a molecules
Transfer of energy

27. Primary electron acceptor
An excited Chl molecule in the
RC serves as a strong reductant
PHOTOSYSTEM
Photon
Reaction center
Pigment molecules of antenna
Figure 7.7C

28. 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2
12H2O  6O2 : synthesis of NADPH and ATP
Photosystems in higher plants
Photosystem II : water-splitting photosystem
Photosystem I : NADPH-producing photosystem

29. Reduction potential smaller
Primary electron acceptor
Electron transport
Primary electron acceptor
Electron transport chain
Photons
Energy for synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II
by chemiosmosis
Figure 7.8
Electron donor of the ETC : H2O
Final electron acceptor of the ETC : NADP+

30. Figure 7.8A
Electron transport chain Provides energy for synthesis of ATP by chemiosmosis
NADP  H
NADPH
Light
Light
Photosystem I 6
Photosystem II
Stroma 1
Primary acceptor
Primary acceptor 2
Thylakoid membrane 4 5
P680
P700
Thylakoid space 3
H2O 2 1 O2 H  2

31. Photosynthetic electron transport
Non-cyclic electron transport : synthesis of NADPH and ATP
Cyclic electron transport : synthesis of ATP

32. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
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

33. Electron transport chain
The production of ATP by chemiosmosis in photosynthesis
Chloroplast
To Calvin Cycle H+
ATP
Light
Light
ADP P
Stroma (low H+ concentration) H+
NADP+ H+
NADPH H+ H+
Thylakoid membrane H+ H+ H+ H+
H2O 1 2 H+ H+
Thylakoid space (high H+ concentration) O H+ H+ H+ H+ H+ H+
Electron transport chain H+ H+
Photosystem II
Photosystem I
ATP synthase

34. PQA and B: plastoquinone; cyt: cytochrome b6f complex
PQA and B: plastoquinone; cyt: cytochrome b6f complex
PC: plastocyanine; Fd: ferredoxin

35. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
The Calvin cycle occurs in the chloroplast’s stroma
This is where carbon fixation takes place and sugar is manufactured
INPUT
CALVIN CYCLE
Figure 7.10A
OUTPUT:

36. The Calvin cycle constructs G3P using
carbon from atmospheric CO2
electrons and H+ from NADPH
energy from ATP
Energy-rich sugar (G3P) is then converted into glucose
G3P: glyceraldehyde 3-phosphate

37. Details of the Calvin cycle
RuBP carboxylase/oxygenase
(Rubisco)
6CO C6H12O6
12NADPH + 18ATP

38. 3-phosphoglycerate is the first product of CO2 fixation in Calvin cycle (C3 plants)
Glyceraldehyde-3-phosphate is the sugar molecule made by Calvin cycle

39. 7.11 Review: Photosynthesis uses light energy to make food molecules
PHOTOSYNTHESIS REVIEWED AND EXTENDED
7.11 Review: Photosynthesis uses light energy to make food molecules
A summary of the chemical processes of photo-synthesis
Chloroplast
Light
Photosystem II Electron transport chains Photosystem I
CALVIN CYCLE
Stroma
Electrons
Cellular respiration
Cellulose
Starch
Other organic compounds
LIGHT REACTIONS
CALVIN CYCLE
Figure 7.11

40. Many plants make more sugar than they need
The excess is stored in roots, tuber, and fruits
These are a major source of food for animals

41. Photorespiration
Rubisco is a bifunctional enzyme
carboxylase: RuBP + CO2  2 3-PGA  Calvin cycle
oxygenase: RuBP + O2  phosphoglycolate + 3-PGA + H2O
In peroxisome
mitochondria
CO2
CO2와 O2분압의 비율이 Rubisco가 CO2/O2 고정 여부를 결정
Photorespiration이 일어나면 Calvin cycle을 통한 CO2고정이 감소

42. Photorespiration in a C3 plant
CALVIN CYCLE
(phosphoglycolate)
2-C compound
Figure 7.12A

43. 7.12 C4 and CAM plants have special adaptations that save water
Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle
In these types of plants, stomata on the leaf surface close when the weather is hot
This causes a drop in CO2 and an increase in O2 in the leaf
Photorespiration may then occur

44. Some plants have special adaptations that enable them to save water
CAM
PEP : phosphoenolpyruvate
PEP+CO2oxaloacetate
PEP carboxylase

45. Figure 7.11
Mesophyll cell
CO2
CO2
Night
4-C compound
4-C compound
Bundle- sheath cell
CO2
CO2
Calvin Cycle
Calvin Cycle
Figure 7.11 Comparison of C4 and CAM photosynthesis
3-C sugar
3-C sugar
Day
C4 plant
CAM plant
Sugarcane
Pineapple 45

46. Rubisco and other Calvin cycle enzymes are present only in bundle sheath cells of C4 plants
C4 plant : corn, sugarcane
Mesophyll cell
Bundle sheath cell

48. The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism
They open their stomata at night and make a four-carbon compound
It is used as a CO2 source by the same cell during the day
4-C compound
Night
Day
CALVIN CYCLE
CAM : Crassulacean acid metabolism
3-C sugar
Figure 7.12C