Download Lecture Slides

Chapter 7
Photosynthesis: Using Light to
Make Food
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Green Energy
• Wood has historically been the main fuel used to:
– Cook
– Warm homes
– Provide light at night
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
• 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
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
• Organisms that use photosynthesis are:
– Photosynthetic autotrophs
– The producers for most ecosystems
© 2010 Pearson Education, Inc.
PHOTOSYNTHETIC AUTOTROPHS
Photosynthetic Protists
(aquatic)
Photosynthetic Bacteria
(aquatic)
LM
Plants
(mostly on land)
Forest plants
Kelp, a large alga
Micrograph of cyanobacteria
Figure 7.1
Chloroplasts: Sites of Photosynthesis
• Chloroplasts are:
– The site of photosynthesis
– Found mostly in the interior cells of leaves
© 2010 Pearson Education, Inc.
• 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 lightabsorbing pigment.
© 2010 Pearson Education, Inc.
• Stomata are tiny pores in leaves where carbon dioxide enters and
oxygen exits.
© 2010 Pearson Education, Inc.
Inner
membrane
Outer
Chloroplast
membrane
Vein
Granum Stroma Thylakoid
O2
Leaf cross section
Interior cell
TEM
Stomata
LM
CO2
Figure 7.2-2
The Overall Equation for Photosynthesis
• In the overall equation for photosynthesis, notice that:
– The reactants of photosynthesis are the waste products of cellular
respiration.
© 2010 Pearson Education, Inc.
Light
energy
6 CO2
6 H2O
Carbon
dioxide
Water
C6H12O6
Photosynthesis
Glucose
6 O2
Oxygen gas
Figure 7.UN1
• In photosynthesis:
– Sunlight provides the energy
– Electrons are boosted “uphill” and added to carbon dioxide
– Sugar is produced
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
CO2
H2O
Light
Chloroplast
NADP+
ADP
P
Calvin
cycle
Light
reactions
ATP
NADPH
O2
Sugar
(C6H12O6)
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
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
Increasing wavelength
10–5 nm 10–3 nm 1 nm 103 nm 106 nm
Gamma
rays
X-rays
UV
1m
MicroInfrared waves
103 m
Radio
waves
Visible light
380 400
500
Wavelength (nm)
600
700
750
Wavelength =
580
nm
Figure 7.4
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.
© 2010 Pearson Education, Inc.
• 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
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
Light
Prism
Number of bacteria
Microscope slide
Bacterium
Algal cells
400
500
600
Wavelength of light (nm)
700
Figure 7.5
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light
Figure 7.6
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light
Figure 7.6a
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
© 2010 Pearson Education, Inc.
– 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.
© 2010 Pearson Education, Inc.
Figure 7.7
How Photosystems Harvest Light Energy
• Light behaves as photons, discrete packets of energy.
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
Excited state
e–
Light
Heat
Light (fluorescence)
Photon
Chlorophyll
molecule
Ground state
(a) Absorption of a photon
Figure 7.8a
• A photosystem is a group of chlorophyll and other molecules that
function as a light-gathering antenna.
© 2010 Pearson Education, Inc.
Chloroplast
Pigment molecules
Primary
electron
acceptor
Photon
Electron Reactiontransfer
Reaction
center
center
chlorophyll a
Antenna pigment
molecules
Transfer of energy
Thylakoid
membrane
Photosystem
Figure 7.9-3
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
© 2010 Pearson Education, Inc.
Primary
electron
acceptor
Primary
electron
acceptor
Energy
ATP
to make
2e–
NADP
2e–
NADPH
2e–
Light
Light
Reactioncenter
chlorophyll
Reactioncenter
chlorophyll
H2O
NADPH-producing
photosystem
2e –
2 H+ +
1
2
O2
Water-splitting
photosystem
Figure 7.10-3
• The light reactions are located in the thylakoid membrane.
© 2010 Pearson Education, Inc.
• An electron transport chain:
– Connects the two photosystems
– Releases energy that the chloroplast uses to make ATP
© 2010 Pearson Education, Inc.
To Calvin cycle
Light
Light
H+
NADPH
ATP
NADP
H
ADP  P
Stroma
Thylakoid
membrane
Photosystem
Electron transport chain
ATP
synthase
Photosystem
Inside thylakoid
Electron flow
2e –
H2O
H
1 O
2
2
H
H
H
H+
Figure 7.11
To Calvin cycle
Light
Light
H
NADPH
ATP
NADP
H
ADP  P
Stroma
Thylakoid
membrane
Photosystem
Electron transport chain
ATP
synthase
Photosystem
Inside thylakoid
Electron flow
2e–
H2O
H
1 O
2
2
H
H
H
H
Figure 7.11a
e–
ATP
e–
e–
NADPH
e–
e–
e–
e–
Water-splitting
photosystem
NADPH-producing
photosystem
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
© 2010 Pearson Education, Inc.
CO2 (from air)
P
RuBP sugar
Three-carbon molecule
P
P
ADP  P
ATP
ADP  P
Calvin
cycle
NADPH
ATP
NADP
G3P sugar
G3P sugar
P
P
G3P sugar
P
Glucose (and
other organic
compounds)
Figure 7.13-4
Evolution Connection:
Solar-Driven Evolution
• C3 plants:
– Use CO2 directly from the air
– Are very common and widely distributed
© 2010 Pearson Education, Inc.
• 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
© 2010 Pearson Education, Inc.
ALTERNATIVE PHOTOSYNTHETIC PATHWAYS
C4 Pathway
(example: sugarcane)
Cell
type 1
CO2
Four-carbon
compound
Cell
type 2
CAM Pathway
(example: pineapple)
CO2
Night
Four-carbon
compound
CO2
CO2
Calvin
cycle
Calvin
cycle
Sugar
C4 plant
Day
Sugar
CAM plant
Figure 7.14