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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 6
Where It Starts – Photosynthesis
(Sections 6.1 - 6.4)
Albia Dugger • Miami Dade College
6.1 Green Energy
• Autotrophs make their own food using energy from the
environment and inorganic carbon sources such as CO2
• In photosynthesis, plants and other autotrophs capture light
energy and use it to build sugars from water and CO2
• Heterotrophs get energy and carbon from molecules that
other organisms have already assembled
Key Terms
• photosynthesis
• Metabolic pathway by which most autotrophs capture light
energy and use it to make sugars from CO2 and water
• autotroph
• Organism that makes its own food using carbon from
inorganic molecules, and energy from the environment
• heterotroph
• Organism that obtains energy and carbon from organic
compounds assembled by other organisms
Biofuels
• There is an increasing demand for biofuels (oils, gases, or
alcohols made from organic matter that is not fossilized)
• Much of the material currently used for biofuel production
consists of food crops – typically expensive, damaging to the
environment, and competes with our food supply
• Making biofuels from other types of plants requires additional
steps to break down abundant cellulose
Biofuels
• Researchers work to find cost-effective ways to break down
the cellulose in fast-growing weeds such as switchgrass
BBC Video: Are Biofuels as Green as They
Claim?
6.2 Sunlight as an Energy Source
• Visible light drives photosynthesis, which begins when
photons are absorbed by photosynthetic pigment molecules
• Photosynthesis converts light energy to chemical energy
Properties of Light
• Light energy travels in waves, and is organized as photons
• Visible light is a very small part of the spectrum of
electromagnetic energy radiating from the sun
• We see light of particular wavelengths as different colors
• wavelength
• Distance between crests of two successive waves of light
Properties of Light
Properties of Light
shortest wavelengths
(highest energy)
gamma
rays
range of most
radiation reaching
Earth’s surface
longest wavelengths
(lowest energy)
visible light
near-infrared infrared microwaves
x-rays ultraviolet
radiation
radiation
radiation
400
500
A Wavelengths of visible light (in nanometers)
B Higher
energy
range of heat
escaping from
Earth’s surface
600
radio waves
700
Lower
energy
Fig. 6.2, p. 94
ANIMATION: Wavelengths of light
Pigments: The Rainbow Catchers
• Pigments are molecules that absorb light of particular
wavelengths; photons that are not captured by a pigment are
reflected as its characteristic color
• Chlorophyll a, the main photosynthetic pigment, absorbs
violet and red light, so it appears green
• Accessory pigments absorb additional wavelengths
Key Terms
• chlorophyll a
• pigment
• An organic molecule that selectively absorbs light of
certain wavelengths
• chlorophyll a
• Main photosynthetic pigment in plants
Some Accessory Pigments
• Accessory pigments color familiar roots, fruits, and flowers:
• Beta-carotene in carrots (orange)
• Zeaxanthin in corn (yellow)
• Lycopene in tomatoes (red)
• Anthocyanin in violets (blue)
Photosynthetic Pigments
Pigment Function
• Absorbing a photon excites electrons in the pigment and
boosts them to a higher energy level
• Photosynthetic cells can capture energy emitted from an
electrons as they return to a lower energy level
• When the energy is passed to a special pair of chlorophylls,
the reactions of photosynthesis begin
Pigment Structure
• The light-trapping part of a
pigment is an array of atoms
in which single bonds
alternate with double bonds
• Such arrays easily absorb
photons, so pigment
molecules function a bit like
antennas
6.3 Exploring the Rainbow
• In 1882, botanist Theodor Engelmann identified the colors of
light (red and violet) that drive photosynthesis in a
photosynthetic alga (Chladophora)
• His results constituted an absorption spectrum, which is a
graph that shows how efficiently the different wavelengths of
light are absorbed by a substance
Engelmann’s Experiment
• Engelmann directed light through a prism so that bands of
colors crossed a water droplet on a microscope slide
Engelmann’s Experiment
bacteria
alga
400
500
600
700
Wavelength (nanometers)
B Engelmann directed light through a prism so that bands
of colors crossed a water droplet on a microscope slide.
The water held a strand of Chladophora and oxygen
requiring bacteria. The bacteria clustered around the algal
cells that were releasing the most oxygen—the ones that
were most actively engaged in photosynthesis. Those cells
were under red and violet light.
Fig. 6.4b, p. 96
Absorption Spectra
phycoerythrobilin
100
chlorophyll b
phycocyanobilin
β-carotene
chlorophyll a
Light absorption (%)
80
60
40
20
0
Wavelength (nanometers)
C Absorption spectra of a few photosynthetic pigments.
Line color is the characteristic color of each pigment.
Fig. 6.4c, p. 96
Key Concepts
• The Rainbow Catchers
• The flow of energy through the biosphere starts when
chlorophylls and other photosynthetic pigments absorb the
energy of visible light
• In plants, some bacteria, and many protists, that energy
ultimately drives the synthesis of glucose and other
carbohydrates
6.4 Overview of Photosynthesis
• Photosynthesis occurs in chloroplasts
• Plant chloroplasts have two outer membranes, and are filled
with a semifluid matrix called stroma
• Stroma contains an inner, folded thylakoid membrane which
forms stacks of disks (thylakoids) connected by channels
Key Terms
• chloroplast
• Organelle specialized for photosynthesis in plants and
some protists
• stroma
• Semifluid matrix between the thylakoid membrane and the
two outer membranes of a chloroplast
• thylakoid membrane
• A chloroplast’s highly folded inner membrane system;
forms a continuous compartment in the stroma
The Chloroplast
The Chloroplast
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
Fig. 6.5b, p. 97
ANIMATION: Photosynthesis overview
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Photosynthesis
• An overall formula for photosynthesis:
6CO2 (carbon dioxide) + 6H2O (water) → light energy
→ C6H12O6 (glucose) + 6O2 (oxygen)
• Photosynthesis is a series of reactions that occur in two
stages: light-dependent reactions and light-independent
reactions
Key Terms
• light-dependent reactions
• First stage of photosynthesis (“photo-”)
• Occur at the thylakoid membrane
• Convert light energy to chemical energy (ATP, NADPH)
• light-independent reactions
• Second stage of photosynthesis (“-synthesis”)
• Occur in the stroma
• Use ATP and NADPH to assemble sugars from water and
CO2
Two Stages of Photosynthesis
Key Concepts
• What Is Photosynthesis?
• Photosynthesis has two stages in the chloroplasts of
plants and many types of protists
• In the first stage, sunlight energy is converted to chemical
energy
• Molecules that form in the first stage of photosynthesis
power the formation of sugars in the second stage
ANIMATION: Photosynthesis - an overview
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