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Chapter 7: Biochemical Pathways- Photosynthesis
Lecture Outline
Enger, E. D., Ross, F. C., & Bailey, D. B. (2012). Concepts in biology (14th ed.). New York: McGrawHill. 1
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Photosynthesis and Life
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During photosynthesis
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Organisms use the energy of light to build highenergy organic molecules.
Plants, algae, and some bacteria can do this.
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99.9% of all life on earth relies on
photosynthesis for energy.
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Can make their own food using light
Called photosynthetic autotrophs
Heterotrophs eat autotrophs.
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An Overview of Photosynthesis
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Photosynthesis occurs in
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Chloroplasts
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During photosynthesis
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Contain the pigment that captures the energy in light
– Chlorophyll
– Found in membranous sacs called thylakoids
– A stack of thylakoid is called a granum (pl. grana).
– Grana are suspended in a fluid-filled space called
the stroma.
The energy in light is used to make ATP.
The energy in ATP is used to make organic
molecules such as glucose.
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Photosynthesis, Chloroplasts,
and the Structure of a Leaf
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An Overview of Photosynthesis
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The chemical equation of photosynthesis:
Light energy + carbon dioxide + water  glucose and oxygen
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The three events of photosynthesis
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Light-capturing events
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Light-dependent reactions
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These reactions use the energy in the excited electrons to
make ATP and NADPH.
Light-independent reactions
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The pigment chlorophyll absorbs certain wavelengths of
light and some of its electrons become excited.
ATP and NADPH from the light reactions is used to
reduce carbon dioxide to make glucose.
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An Overview of Photosynthesis
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Light-capturing Events:
Fundamental Concepts
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Visible light
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Pigments are molecules that absorb light.
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Each pigment absorbs certain wavelengths of light.
The wavelengths that they do not absorb, they reflect.
 This is the color we see.
In photosynthesis, only the wavelengths that are absorbed can be
used to do work.
Chlorophyll: The main photosynthetic pigment
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Combinations of different wavelengths of light
Can be seen as different colors
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Two forms; a and b
Absorb light in the blue and red portions of the spectrum
Reflect green wavelengths
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The Electromagnetic Spectrum,
Visible Light and Chlorophyll
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Other Types of Pigments
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Accessory pigments
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Chlorophyll + accessory pigments
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Organized into photosystems that harvest the energy from
many wavelengths of light
Found in thylakoids of the chloroplasts
When pigments absorb light
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Include carotenoids
 Absorb blue and green wavelengths
 Reflect orange and yellow
 Found in leaves, masked by chlorophyll
– In the autumn, when chlorophyll disintegrates,
accessory pigments show through (fall colors).
Some of their electrons become “excited”
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Light-dependent Reactions:
Fundamental Concepts
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The excited electrons from chlorophyll
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Excited electrons passed to NADP+ to make NADPH
Water is split
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Electrons are donated to chlorophyll to replace the donated
electrons.
Oxygen is produced.
Occurs in the thylakoid membrane
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Are passed through an electron transport chain
The energy released is used to pump protons up their
concentration gradient.
When protons diffuse through ATP synthase, ATP is made.
ATP and NADPH move to the stroma to be used in the dark
reactions.
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Light-independent Reactions:
Fundamental Concepts
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The ATP and NADPH from the light-dependent
reactions
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Provide the energy and electrons needed to build sugar
from carbon dioxide
CO2 is captured by an enzyme called RuBisCO
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Combines CO2 with ribulose to form a 6-carbon molecule
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NADPH is used to reduce these molecules.
Glyceraldehyde-3-phosphate is formed.
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This is immediately split into two 3-C molecules.
Can be used to make sugars, proteins, or fats
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Photosynthesis: Fundamental
Description
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Light-capturing Events:
The Details
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Light
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Packaged as photons
Each photon has a distinct wavelength.
The energy in a photon is related to its
wavelength.
When a photon hits a pigment molecule, electrons
are excited.
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This means that the electrons jump up to a higher
energy level.
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Light-capturing Events:
The Details Continued
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Photosystems
– Structures in which the light-capturing events take
place
– Found in the thylakoid membranes
– Made up of antenna complexes and a reaction center
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Networks of chlorophyll and accessory pigments
Capture the energy in many different wavelengths of
light and pass it to the reaction center
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The reaction center is a special chlorophyll molecule that
passes its excited electrons to an electron acceptor.
There are two photosystems, I and II.
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Light Dependent Reactions:
The Details
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Photosystem II (PSII)
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Occurs first
Donates its excited electrons to the electron transport chain
Splits water to replace the electrons it donated
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Photosystem I (PSI)
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Occurs second
Donates its excited electrons to NADP+ to form NADPH
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O2 is released.
NADPH released into the stroma
Accepts electrons from the electron transport chain to
replace the electrons it donated
PS I and II are in the thlyakoid membrane.
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The Electron Transport Chain
and ATP Synthesis
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Between PSII and PSI
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Electrons are passed through an electron
transport chain (ETC).
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This releases energy.
This energy is used to pump protons from the stroma
into the thylakoid space.
This creates a proton concentration gradient.
Protons diffuse through ATP synthase.
ATP synthase makes ATP that is released into the
stroma.
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PS II and I: How They Interact
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Light Independent Reactions:
The Details
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Takes place in the stroma of the chloroplast
Uses
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CO2 (from the atmosphere)
ATP and NADPH (from the light-independent
reactions)
Ribulose (recycled)
Also called the dark reactions
Also called the Calvin cycle
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Carbon Fixation and Sugar
Formation
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Carbon dioxide gas is combined with a 5-carbon
sugar called ribulose.
Accomplished with an enzyme called ribulose
bisphospate carboxylase (RuBisCO)
Forms a 6-carbon molecule
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Immediately broken down into two 3-carbon molecules
These 3-carbon molecules are
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Energized by ATP
Reduced by NADPH
Glyceraldehyde-3-phosphate is formed.
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The Calvin Cycle
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Glyceraldehyde-3-phosphate:
The Product of Photosynthesis
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Glyceraldehyde-3phosphate can be used for
many things.
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Used to make glucose
Used to recycle ribulose for
the Calvin cycle
Used to make the sugars
needed to build ATP, DNA,
and RNA
Can be converted into lipids
Can be converted into
amino acids to make
proteins
Can be broken down in
glycolysis
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Other Aspects of Plant
Metabolism
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Plant cells can use the organic molecules
produced in photosynthesis to make
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Fats, proteins, and other carbohydrates
Toxins for their protection
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Vitamins
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Many of these are useful medicines.
Some can be used as natural insecticides.
Molecules that we cannot make, but that we need
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Interrelationships Between
Autotrophs and Heterotrophs
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Autotrophs use the energy in light to make food.
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Autuotrophs use the food they make in cellular respiration.
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Heterotrophs eat the autotrophs.
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Then use the food from the autotrophs to fuel cellular respiration.
They also use the excess oxygen given off by autotrophs.
All organisms respire
The circle of life:
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Plants use the sugar they make
Plants use the oxygen they make
Animals get sugar, oxygen, amino acids, fats and vitamins from
plants.
Plants get carbon dioxide, water and nitrogen from animals.
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The Relationship between Photosynthesis
and Cellular Respiration
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