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

2. Plant Power
Photosynthesis makes sugar and oxygen gas from carbon dioxide and water
Carried out by plants, algae, and some bacteria
The ultimate source of all food eaten by animals
A major source of heat, light, and fuel

4. Using photosynthesis to meet growing energy demands
Energy plantations of trees
Renewable energy source
Productive
Environmentally safer than fossil fuels

6. AN OVERVIEW OF PHOTOSYNTHESIS
7.1 Autotrophs are the producers of the biosphere
Autotrophs produce their own food
Autotrophs produce the food supply for the global ecosystem
Photoautotrophs produce organic molecules using light energy
Plants, algae, and photosynthetic bacteria
Video: Cyanobacteria (Oscillatoria)

11. 7.2 Photosynthesis occurs in chloroplasts
In plants, photosynthesis occurs primarily in leaves
CO2 enters and O2 exits through stomata
Chloroplasts are the site of photosynthesis
Concentrated in mesophyll cells of leaves

12. Chloroplast structure Stroma
Fluid enclosed by inner membrane
Location of sugar synthesis
Thylakoids
Suspended in stroma
Interconnected sacs
Stacked in grana
Contain chlorophyll in membranes

13. LE 7-2 Leaf Cross Section Mesophyll Cell Leaf Mesophyll LM 2,600 Vein
Chloroplast
Stoma
CO2 O2
Outer
membrane
Chloroplast
Inner
membrane
TEM 9,750
Stroma
Intermembrane
space
Grana
Stroma
Granum
Thylakoid
Thylakoid space

14. 7.3 Plants produce O2 gas by splitting water
Experiments have made it possible to follow all the atoms in photosynthesis
O2 is both a product and a reactant in photosynthesis
The O2 liberated by photosynthesis is made from the oxygen in water

17. LE 7-3c
Reactants:
6 CO2
12 H2O
C6H12O6
6 H2O
6 O2
Products:

18. 7.4 Photosynthesis is a redox process, as is cellular respiration
H2O is oxidized
CO2 is reduced
Electrons gain energy
Cellular respiration
Glucose is oxidized
O2 is reduced
Electrons lose potential energy

21. 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
Light reactions
Occur in thylakoid membranes
Convert light energy to chemical energy as ATP and NADPH
Produce O2 as a waste product
Calvin cycle
Occurs in stroma
Assembles sugar molecules from CO2 using ATP and NADPH from light reactions

22. H2O CO2 O2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN
LE 7-5
H2O
CO2
Chloroplast
Light
NADP+
ADP  P
LIGHT
REACTIONS
CALVIN
CYCLE
(in stroma)
(in thylakoids)
ATP
Electrons
NADPH O2
Sugar

23. THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
7.6 Visible radiation drives the light reactions
Sunlight is radiation, or electromagnetic energy
Radiation travels as waves
The distance between waves is a wavelength
Visible light is only a small part of the electromagnetic spectrum
Light also behaves as discrete photons
Animation: Light and Pigments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

24. 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m LE 7-6a Increasing energy
Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
380
400
500
600
700
750
Wavelength (nm)
650 nm

25. 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

26. Chlorophyll b
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

27. Light Reflected light Chloroplast Absorbed light Transmitted light
LE 7-6b
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light

28. Light also behaves as photons
A fixed quantity of energy
Specific amounts of energy in photons absorbed by different pigments

29. 7.7 Photosystems capture solar power
When a pigment molecule absorbs a photon
An electron is raised in energy from a ground state to an unstable excited state
The electron rapidly drops back to the ground state
The electron releases excess energy as heat
Some pigments also emit light (fluorescence)

31. LE 7-7b Excited state Heat Energy of electron Photon Photon
(fluorescence)
Ground state
Chlorophyll
molecule

32. A reaction center that contains
The thylakoid membrane contains two types of photosystems, which consist of
Light-harvesting complexes of chlorophyll and other pigments and proteins
A reaction center that contains
A chlorophyll a molecule
A primary electron acceptor molecule
Receives excited electrons from the chlorophyll and transfers them to an electron transport chain

33. Photosystem Photon Thylakoid membrane Chlorophyll a molecule
Light-harvesting
complexes
Reaction
center
Primary electron
acceptor
Photon
To electron
transport
chain
Thylakoid membrane
Transfer
of energy
Pigment
molecules
Chlorophyll a
molecule

34. 7.8 In the light reactions, electron transport chains generate ATP and NADPH
Photosystem II
A pigment molecule absorbs a photon
The energy is ultimately transferred to chlorophyll P680
The excited electrons from P680 are passed to the primary electron acceptor, then to an electron transport chain

35. Electrons shuttle down the chain from photosystem II to photosystem I, providing energy to make ATP
Water is split, replacing electrons lost by P680 and releasing O2
Photosystem I
A photon is absorbed and excites an electron of chlorophyll P700
The excited electron passes through a short electron transport chain, reducing NADP+ to NADPH

36. LE 7-8a Animation: Light Reactions Photon Photon Photosystem I
Photosystem II
NADP+ + H +
NADPH
Stroma e– e–
Thylakoid membrane
P700
P680
Thylakoid
space
Electron transport chain
Provides energy for synthesis of
by chemiosmosis
ATP
H2O 1 O2
+ 2 H +
Animation: Light Reactions

37. ATP NADPH Mill makes ATP Photosystem II Photosystem I
LE 7-8b e–
ATP e– e–
NADPH e– e– e–
Mill
makes
ATP
Photon e–
Photon
Photosystem II
Photosystem I

38. 7.9 Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chain
Pumps H+ from the stroma into the thylakoid space
Generates a concentration gradient across the thylakoid membrane

39. Photophosphorylation
Energy of the concentration gradient drives H+ back across the membrane through ATP synthase
ATP synthase couples flow of H+ to the phosphorylation of ADP to produce ATP
Light provides the initial energy for the phosphorylation

40. LE 7-9 + + + + + + + + + + + + + + + + Chloroplast Stroma (low H+
concentration)
Light
Light H + H +
+ 
ATP H +
NADP+ + H +
NADPH H +
Thylakoid
membrane
H2O H + H + 1 2 O2
+ 2 H H + + H + H + H + H + H +
Photosystem II
Photosystem I H +
Electron
transport
chain
ATP synthase H +
Thylakoid space
(high H+ concentration)

41. THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
The Calvin cycle makes sugar in the chloroplast
Inputs
Carbon from CO2
Energy from ATP
High-energy electrons from NADPH
Output
Energy-rich G3P
Can be used to make organic molecules

42. LE 7-10a
CO2
Input
ATP
NADPH
CALVIN
CYCLE
Output:
G3P

43. Animation: Calvin Cycle
Steps of the Calvin cycle
Carbon fixation
Reduction
Release of one molecule of G3P
Regeneration of RuBP
Animation: Calvin Cycle

44. LE 7-10b Carbon fixation Input: 3 In a reaction catalyzed by rubisco,
CO2 is added to RuBP.
CO2 3 P P 6 P
Reduction
RuBP
3-PGA 6
ATP
3 ADP
6 ADP + P 3
ATP
CALVIN
CYCLE 6
NADPH
Release of one
molecule of G3P 6
NADP+ 5 P 6 P
G3P
G3P
Regeneration of
RuBP
Glucose
and other
compounds
Output: 1 P
G3P

45. PHOTOSYNTHESIS REVIEWED AND EXTENDED
7.11 Review: Photosynthesis uses light energy to make food molecules
The light reactions
Capture light energy
Generate NADPH and ATP
Release O2 and water
The Calvin cycle
Manufactures sugar
Cells use many of the same mechanisms in photosynthesis and cellular respiration

46. H2O CO2 + LE 7-11 Chloroplast NADP+ ADP P RUBP CALVIN CYCLE
Light
NADP+
ADP + P
RUBP
Photosystem II
CALVIN
CYCLE
(in stroma)
Electron
transport
chains
3-PGA
Thylakoid
membranes
Photosystem I
ATP
Stroma
NADPH
G3P
Cellular
respiration
Cellulose
Starch O2
Sugars
Other organic
compounds
LIGHT REACTIONS
CALVIN CYCLE

47. 7.12 C4 and CAM plants have special adaptations that save water
C3 plants
Corn, soybeans, wheat, rice
Use CO2 directly
Rate of photosynthesis decreases in dry weather
Stomata close, CO2 supply reduced
Calvin cycle diverted to photorespiration

48. Include corn and sugarcane Stomata closed in hot, dry weather
C4 plants
Include corn and sugarcane
Stomata closed in hot, dry weather
CO2 fixed into a 4-carbon compound that acts as a carbon shuttle
Enables plant to continue making sugar
Prevents photorespiration and water loss

49. Pineapple, cactus, succulents Adapted to very dry climates
CAM plants
Pineapple, cactus, succulents
Adapted to very dry climates
Open stomata and admit CO2 only at night
Fix CO2 into a 4-carbon compound that banks CO2 for release to Calvin cycle during the day

50. LE 7-12 Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound CO2 CO2
CALVIN
CYCLE
CALVIN
CYCLE
Sugarcane
Pineapple
Bundle-sheath
cell
3-C sugar
3-C sugar
Day
C4 plant
CAM plant

51. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH'S ATMOSPHERE: CONNECTION
7.13 Photosynthesis moderates global warming
The greenhouse effect
Like a greenhouse, the atmosphere traps CO2 and other gases that absorb heat radiated from Earth's surface
Excess greenhouse gases in the atmosphere contribute to global warming
Photosynthesis, which removes CO2 from the atmosphere, moderates warming
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

53. LE 7-13b
Some heat
energy escapes
into space
Sunlight
Atmosphere
Radiant heat
trapped by CO2
and other gases

54. TALKING ABOUT SCIENCE
7.14 Mario Molina talks about Earth's protective ozone layer
Nobel Prize winner Mario Molina has studied how pollutants are affecting Earth's ozone layer
Solar radiation converts O2 high in the atmosphere to ozone (O3)
The ozone layer shields organisms on Earth's surface from damaging UV radiation
CFCs have caused dangerous thinning of the ozone layer
International restrictions on CFC use are allowing a slow recovery

56. LE 7-14b
Southern tip of
South America
Antarctica