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Chapter 7
Photosynthesis: Using Light to Make Food
Laura Coronado
Bio 10
Chap 7
Biology and Society:
Green Energy
– Wood has historically been the main fuel used to:
• Cook
• Warm homes
• Provide light at night
– Industrialized societies replaced wood with fossil
fuels.
– To limit the damaging effects of fossil fuels,
researchers are investigating the use of biomass
(living material) as efficient and renewable energy
sources.
Laura Coronado
Bio 10
Chap 7
Laura Coronado
Bio 10
Chap 7
Figure 7.0
Biology and Society:
Green Energy
– Fast-growing trees, such as willows:
• Can be cut every three years
• Do not need to be replanted
• Are a renewable energy source
• Produce fewer sulfur compounds
• Reduce erosion
• Provide habitat for wildlife
Laura Coronado
Bio 10
Chap 7
THE BASICS OF PHOTOSYNTHESIS
– Photosynthesis:
• Is used by plants, some protists, and some bacteria
• Transforms light energy into chemical energy
• Uses carbon dioxide and water as starting materials
– The chemical energy produced via photosynthesis
is stored in the bonds of sugar molecules.
– Organisms that use photosynthesis are:
• Photosynthetic autotrophs
• The producers for most ecosystems
Laura Coronado
Bio 10
Chap 7
PHOTOSYNTHETIC AUTOTROPHS
Photosynthetic Protists
(aquatic)
Photosynthetic Bacteria
(aquatic)
LM
Plants
(mostly on land)
Forest plants
Kelp, a large alga
Laura Coronado
Micrograph of cyanobacteria
Bio 10
Chap 7
Figure 7.1
Chloroplasts: Sites of Photosynthesis
– Chloroplasts are:
• The site of photosynthesis
• Found mostly in the interior cells of leaves
– Inside chloroplasts are membranous sacs called
thylakoids, which are suspended in a thick fluid,
called stroma.
– Thylakoids are concentrated in stacks called grana.
– The green color of chloroplasts is from chlorophyll, a
light-absorbing pigment.
– Stomata are tiny pores in leaves where carbon
dioxide enters and oxygen exits.
Laura Coronado
Bio 10
Chap 7
Chloroplast
Inner
membrane
Outer
membrane
Vein
Granum
O2
Leaf cross section
Interior cell
TEM
Stomata
Thylakoid
LM
CO2
Stroma
Laura Coronado
Bio 10
Chap 7
Figure 7.2-2
Light
energy
6 CO2
6 H2O
Carbon
dioxide
Water
C6H12O6
Photosynthesis
Laura Coronado
Bio 10
Glucose
Chap 7
6 O2
Oxygen gas
Figure 7.UN1
Photosynthesis
– Sunlight provides the energy
– Electrons are boosted “uphill” and added to carbon
dioxide
– Sugar is produced
– During photosynthesis, water is split into:
• Hydrogen
• Oxygen
– Hydrogen is transferred along with electrons and
added to carbon dioxide to produce sugar.
– Oxygen escapes through stomata into the
atmosphere.
Laura Coronado
Bio 10
Chap 7
A Photosynthesis Road Map
– Photosynthesis occurs in two stages:
• The light reactions convert solar energy to chemical
energy
• The Calvin cycle uses the products of the light reactions
to make sugar from carbon dioxide
Laura Coronado
Bio 10
Chap 7
CO2
H2O
Light
Chloroplast
NADP+
ADP
P
Calvin
cycle
Light
reactions
ATP
NADPH
Sugar
O2
(C6H12O6)
Laura Coronado
Bio 10
Chap 7
Figure 7.3-2
THE LIGHT REACTIONS: CONVERTING SOLAR
ENERGY TO CHEMICAL ENERGY
– Chloroplasts:
• Are chemical factories powered by the sun
• Convert solar energy into chemical energy
Laura Coronado
Bio 10
Chap 7
The Nature of Sunlight
– Sunlight is a type of energy called radiation, or
electromagnetic energy.
– The full range of radiation is called the
electromagnetic spectrum.
Laura Coronado
Bio 10
Chap 7
Increasing wavelength
10–5 nm 10–3 nm
Gamma
rays
X-rays
1 nm 103 nm
106 nm
Microwaves
Infrared
UV
103 m
1m
Radio
waves
Visible light
380 400
500
Wavelength (nm)
600
Wavelength =
Laura Coronado
Bio 10
580
nm
Chap 7
Figure 7.4
700
750
The Process of Science:
What Colors of Light Drive Photosynthesis?
– Observation: In 1883, German biologist Theodor
Engelmann saw that certain bacteria tend to cluster
in areas with higher oxygen concentrations.
– Question: Could this information determine which
wavelengths of light work best for photosynthesis?
– Hypothesis: Oxygen-seeking bacteria will congregate
near regions of algae performing the most
photosynthesis.
Laura Coronado
Bio 10
Chap 7
The Process of Science:
What Colors of Light Drive Photosynthesis?
– Experiment: Engelmann:
• Laid a string of freshwater algal cells in a drop of water on a
microscope slide
• Added oxygen-sensitive bacteria to the drop
• Used a prism to create a spectrum of light shining on the
slide
– Results: Bacteria:
• Mostly congregated around algae exposed to red-orange
and blue-violet light
• Rarely moved to areas of green light
– Conclusion: Chloroplasts absorb light mainly in the
blue-violet and red-orange part of the spectrum.
Laura Coronado
Bio 10
Chap 7
Light
Prism
Microscope slide
Number of bacteria
Bacterium
Algal cells
400
500
600
Wavelength
of light (nm)
Laura Coronado Bio 10 Chap 7
Figure 7.5
700
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light
Laura Coronado
Bio 10
Chap 7
Figure 7.6
Chloroplast Pigments
– Chloroplasts contain several pigments:
• Chlorophyll a:
– Absorbs mostly blue-violet and red light
– Participates directly in the light reactions
• Chlorophyll b:
– Absorbs mostly blue and orange light
– Participates indirectly in the light reactions
Laura Coronado
Bio 10
Chap 7
Carotenoids
• Absorb mainly blue-green light
• Participate indirectly in the light reactions
• Absorb and dissipate excessive light energy
that might damage chlorophyll
• The spectacular colors of fall foliage are due
partly to the yellow-orange light reflected
from carotenoids.
Laura Coronado
Bio 10
Chap 7
Laura Coronado
Bio 10
Chap 7
Figure 7.7
How Photosystems Harvest Light Energy
– Light behaves as photons, discrete packets of
energy.
– A photosystem is a group of chlorophyll and
other molecules that function as a lightgathering antenna.
– Chlorophyll molecules absorb photons.
• Electrons in the pigment gain energy.
• As the electrons fall back to their ground state,
energy is released as heat or light.
Laura Coronado
Bio 10
Chap 7
Excited state
(a) Absorption
of a photon
e–
Light
Heat
Light (fluorescence)
Photon
Chlorophyll
molecule
Ground state
(b) Fluorescence
of a glow stick
Laura Coronado
Bio 10
Chap 7
Figure 7.8
Chloroplast
Pigment molecules
Primary
electron
acceptor
Photon
Electron
transfer
Reactioncenter
chlorophyll a
Antenna pigment
molecules
Transfer of energy
Photosystem
Thylakoid
membrane
Laura Coronado
Bio 10
Chap 7
Figure 7.9-3
Reaction
center
How the Light Reactions Generate
ATP and NADPH
– Two types of photosystems cooperate in the
light reactions:
• The water-splitting photosystem
• The NADPH-producing photosystem
– The light reactions are located in the thylakoid
membrane.
Laura Coronado
Bio 10
Chap 7
Primary
electron
acceptor
Primary
electron
acceptor
Energy
to make
ATP
2e –
2e –
NADPH
2e –
Light
Light
Reactioncenter
chlorophyll
Reactioncenter
chlorophyll
H2O
NADPH-producing
photosystem
2e –
1
2 H+ +
O2
2
NADP
Water-splitting
photosystem
Laura Coronado
Bio 10
Chap 7
Figure 7.10-3
– An electron transport chain:
• Connects the two photosystems
• Releases energy that the chloroplast uses to make ATP
Laura Coronado
Bio 10
Chap 7
To Calvin cycle
Light
Light
H
NADPH
NADP
H
ATP
ADP  P
Stroma
Thylakoid
membrane
Photosystem
Electron transport chain
ATP
synthase
Photosystem
Inside thylakoid
Electron flow
2e –
H2 O
H
H
H
H
1 O
2
2
Laura Coronado
Bio 10
Chap 7
H
Figure 7.11a
e–
ATP
e–
e–
NADPH
e–
e–
e–
e–
NADPH-producing
photosystem
Water-splitting
photosystem
Laura Coronado
Bio 10
Chap 7
Figure 7.12
THE CALVIN CYCLE: MAKING SUGAR FROM
CARBON DIOXIDE
– The Calvin cycle:
• Functions like a sugar factory within the stroma of a
chloroplast
• Regenerates the starting material with each turn
Laura Coronado
Bio 10
Chap 7
CO2 (from air)
P
Three-carbon molecule
RuBP sugar
P
ADP  P
ATP
P
ADP  P
Calvin
cycle
NADPH
ATP
NADP
G3P sugar
G3P sugar
P
P
G3P sugar
Glucose (and
other organic
compounds)
P
Laura Coronado
Bio 10
Chap 7
Figure 7.13-4
Evolution Connection:
Solar-Driven Evolution
– C3 plants:
• Use CO2 directly from the air
• Are very common and widely distributed
– C4 plants:
• Close their stomata to save water during hot and dry
weather
• Can still carry out photosynthesis
– CAM plants:
• Are adapted to very dry climates
• Open their stomata only at night to conserve water
Laura Coronado
Bio 10
Chap 7
ALTERNATIVE PHOTOSYNTHETIC PATHWAYS
C4 Pathway
(example: sugarcane)
Cell
type 1
CAM Pathway
(example: pineapple)
CO2
Night
CO2
Four-carbon
compound
Four-carbon
compound
Cell
type 2
CO2
CO2
Calvin
cycle
Calvin
cycle
Sugar
C4 plant
Laura Coronado
Day
Sugar
CAM plant
Bio 10
Chap 7
Figure 7.14
Light
energy
6 CO2
6 H2O
Carbon
dioxide
Water
C6H12O6
Photosynthesis
Laura Coronado
Bio 10
Glucose
Chap 7
6 O2
Oxygen gas
Figure 7.UN1
Light
CO2
H2O
Light
reactions
NADP
ADP
P
Calvin
cycle
ATP
NADPH
Sugar
(C6H12O6)
O2
Laura Coronado
Bio 10
Chap 7
Figure 7.UN2
Light
CO2
H2O
NADP
ADP
P
Light
reactions
Calvin
cycle
ATP
NADPH
Sugar
(C6H12O6)
O2
Laura Coronado
Bio 10
Chap 7
Figure 7.UN3
Chloroplast
Light
CO2
H2O
Stack of
thylakoids
NADP+
Stroma
ADP
P
Light
reactions
Calvin
cycle
ATP
NADPH
Sugar
(C6H12O6)
O2
Laura Coronado
Bio 10
Chap 7
Figure 7.UN5
ADP
e–
acceptor
ATP
e–
acceptor
2e –
2e –
NADPH
2e –
Photon
Photon
Chlorophyll
H2O
Chlorophyll
NADPH-producing
photosystem
2e –
2 H + + 1 O2
2
NADP+
Water-splitting
photosystem
Laura Coronado
Bio 10
Chap 7
Figure 7.UN6
CO2
ADP
ATP
Calvin
cycle
NADPH
P
NADP
G3P
P
Laura Coronado
Bio 10
Chap 7
Glucose and
other compounds
Figure 7.UN7