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Chapter 8
Photosynthesis:
Capturing Energy
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Photosynthesis is the process by which
plants, some bacteria, and some protists
use the energy from sunlight to produce
sugar.
Cellular respiration then converts sugar
into ATP, the "fuel" used by all living
things.
Photosynthesis
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Conversion of light energy to chemical bond
energy
Associated with the green pigment, chlorophyll
Equation:
Plants
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Plants are the only
photosynthetic
organisms that have
leaves.
A leaf is basically a
solar collector full of
photosynthetic cells.
Leaves
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Water enters the root and is transported
up to the leaves through specialized
plant cells known as xylem.
Land plants (which risk drying out) have
structures called stomata that allow for
gas exchange.
• However, while open, water still escapes.
Leaf Structure
Leaf Stomata
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http://upload.wikimedia.org/wikipedia/commons/thumb/0/09/Tomato_leaf_stomate_1-color.jpg/300px-Tomato_leaf_stomate_1-color.jpg
Light
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Light is composed of small particles that
travel as waves
Radiation varies in wavelength
• Red – longer wavelengths
• Violet – shorter wavelengths
Light
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Light can also behave as particles
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When a molecule absorbs a photon of
light energy, one electron is energized
• Photons – packets of energy
Chlorophyll
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Chlorophyll is a pigment
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Chlorophyll – green pigment common to
all photosynthetic cells
• Pigment is any substance that absorbs light.
• Color is from the wavelength of light reflected
• Different types of chlorophyll
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Accessory pigments – absorb different
wavelengths of light
Pigments
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All photosynthetic organisms have
chlorophyll a
Chlorophyll a absorbs its energy from
Violet-Blue and Red-Orange
wavelengths
Green/Yellow is reflected
Accessory pigments: chlorophyll b,
carotenoids – absorb other wavelengths
Chloroplasts
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Have a double membrane
• Inner membrane encloses the stroma (area
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between membranes)
Thylakoids are a third set of membranes
enclosing the thylakoid interior space
Stacks of thylakoids = grana
Chlorophyll and other pigments located in the
thylakoid membranes
Prokaryotes lack chloroplasts
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Thylakoids are still present, but without
chloroplast structures
Thylakoid membranes are formed from
infoldings of the plasma membrane
Photosynthesis
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Divided into 2 reactions:
• Light Dependent Reactions - thylakoid
• Carbon fixation/Calvin Cycle (Light
Independent) – stroma
Light-Dependent Reactions
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Light excites the electrons in chlorophyll a to a
higher energy state
In a series of reactions the energy is converted
(along an electron transport process) into ATP
and NADPH.
Water is split in the process, releasing oxygen as
a by-product of the reaction.
The ATP and NADPH are used to make C-C
bonds in the Light Independent Process (Dark
Reactions)
Calvin Cycle (Light Independent
Reactions)
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Not driven by light
Use products from Light Dependent
reactions
CO2 is captured and modified with the
addition of Hydrogen to form
carbohydrates