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Photosynthesis
6.1 Light Cycle
6.2 Dark Cycle
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Sid Galloway
Autotrophs make their own food, and most use
photosynthesis to do it.
* Plants are the most common, but algae and some
bacteria use it.
* All life depends on autotrophs (directly or indirectly) for
food.
* Nearly all living things obtain energy either directly
(like plants) or indirectly (like heterotrophs that eat
plants) from the sun’s energy captured during
photosynthesis.
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Photosynthesis (Simply Summarized)
Leaves of plants have chloroplasts that are filled with
chlorophyll, which capture the energy from the
sunlight).
* Water enters through the roots.
* CO2 enters through stomata openings on the
underside of the leaves.
* Water and CO2 move to the chloroplasts.
* Chemical reactions there, produce (O2) and sugars
like glucose (C6H12O6).
* Cells then use the energy in the sugars
(carbohydrates) to function.
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Photosynthesis
• Plant chloroplast capture light energy and
converts it to chemical energy, which is stored
in the bonds of sugar and other organic
molecules synthesized from carbon dioxide
and water
• This captured light energy is converted and
stored as chemical energy know as
photosynthesis
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Water
Sun Light
Carbon Dioxide
Photosynthesis
Sugar
Oxygen
Cellular Respiration
Energy - ATP
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(Lab) Chloroplasts in Elodea
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Chloroplast
• Chloroplasts are the site of photosynthesis
• Are found in all green plant parts
• The leaves are the major site of photosynthesis
in most plants
• Chlorophyll is green pigment in chloroplast
that gives leaf its color
• Chloroplast are found primarily in cells of the
mesophyll ( of the leaf)
• Each mesophyll contains 30-40 chloroplast
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Parts of Chloroplast: 3
1. Stroma: fluid filled space outside thylakoid,
Calvin cycle occurs here
2. Thylakoid: flattened membranous sac inside
chloroplast, light reaction occurs here.
Grana: whole stack, while thylakoid is each
individual
3. Intermembrane space: a double membrane
that partitions its contents from cytosol
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Formula for Photosynthesis
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ATP Molecule
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• The overall photosynthetic equation has been
known since the early 1880s
• Glucose is the major product of photosynthesis
• Water appears on both sides of the equation
because 12 molecules are consumed and 6
newly formed during photosynthesis
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• The discovery in 1930s that oxygen given off
by plants is derived from water and not from
carbon dioxide was one of the earliest clues to
the mechanisms of photosynthesis and
indicates that chloroplasts split water into
hydrogen and oxygen
• Products of photosynthesis are: glucose,
oxygen, water and ATP
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2 Stages of Photosynthesis
• 1. Light Reaction: occurs in the thylakoid
• 2. Calvin Cycle: occurs in stoma, also called
dark reaction
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Light Reaction
• Occurs in Thylakoid
• Involves the conversion of light energy into
chemical energy (photolysis)
• Water is split during this process and oxygen is
released as a by product…reason plants make
oxygen
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• The coenzyme NADP+ picks up hydrogen
and electrons from the split water molecules
and stores them for use in the Calvin cycle
(dark reaction)
• ATP is also generated and stoed for energy for
the dark reaction
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The Light Reaction in the Thylakoid Membrane
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The Light Reaction in the Thylakoid Membrane
Primary electron acceptor Primary electron acceptor
Photosystem II
Electron Transport Chain
Photosystem I
Electron Transport Chain
Animation here: http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf
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Electron flow
• 2 possible routes for electrons to flow during
light reaction:
• 1. Cyclic flow: simplest pathway, generates
ATP only
• 2. Noncyclic flow: light reaction usually
happens this way NADP is formed
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Calvin Cycle ( dark reaction)
• During this cycle, carbon dioxide from the air
is fixed to a 5 carbon sugar (ribulose
bisphosphate ) by the addition of electrons
from NADP+ from the light reaction ( this is
called carbon fixation) resulting in a new
unstable 6 carbon sugar
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• This unstable 6 carbon sugar begins going
through various chemical reactions
• The 2 necessary components for carbon
fixation to occur here are: an electron source
(NADP+) and an energy source (ATP)…both
supplied by the light reaction
• Water is released as a by-product
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• Glucose is made as the plant energy/food
supply
• Ribulose bisphosphate is the 5 carbon sugar
made at the end of the cycle and is again used
to combine with carbon dioxide for carbon
fixation
• 9 molecules of ATP and 6 molecules of
NADPH are needed to synthesis one glucose
molecule
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• Also called dark reaction…does not require
light directly, but need the products of light
reaction to occur.
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Three Steps of the Calvin Cycle:
a. CO2 combines with RuBP to form two molecules of PGA.
b. Each molecule of PGA is converted into a molecule of PGAL.
c. Most of the PGAL is converted back into RuBP, but some
PGAL can be used later to make different organic compounds.
* RuBP = five-carbon carbohydrate.
* PGA and PGAL are both three-carbon molecule
* C3 Plants (produce the 3-Carbon PGA),
and use only the Calvin Cycle for carbon fixation.
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The Two Processes Visualized
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Calvin
Cycle
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Calvin
Cycle
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Calvin
Cycle
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Photosynthetic pigments
• Pigments: substances that absorb visible light
• Different pigments absorb light of different
wavelengths
Types of pigments include: Chlorophyll A (
bright green)
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Types of pigments include:
•
•
•
•
•
•
Chlorophyll A ( bright green)
Chlorophyll B ( yellow green)
Chlorophyll contains the ion magnesium
Carotene: faint yellow
Xanthrophyll: yellow
Anthrocyanin: red
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*
- Accessory Pigments = indirectly assist other pigments.
( Chlorophyll “b” is an accessory pigment assisting “a”.)
Carotenoids are other accessory pigments
(yellow, orange, brown, etc.)
(They absorb blue and green light.)
(In the fall, plant leaves turn color because they lose
chlorophylls, which reflect green light.)
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Photosystems:
• Found in the thylakoid membrane of
chloroplast
• Only 1 chlorophyll a ( photosynthetic pigment)
is needed to start the light reaction
• All of the other chlorophyll a molecules,
chlorophyll b molecules are carotenes function
as light gathering antenaes
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• The entire complex of light gathering
molecules is called a photosystem.
• There are 2 types of photosystems:
• 1. photosystem I: called P700..absorbs light up
to 700nm wavelength
• 2. Photosystem II: called P600…absorbs light
up to 600 nm wavelength
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Electron Transport System of the light reactions.
* Photosystem = cluster of pigment molecules grouped
in the thylakoid membrane.
Two types:
Photosystem II – Actually begins the process
Photosystem I – Called “1” is believed to have
evolved first.
* Accessory pigment molecules start the light reactions
by absorbing light energy.
(It is passed to other pigment molecules until it
reaches chlorophyll a molecules.)
*
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Alternative Pathways
* First, remember that C3 Plants are those that only
use the Calvin Cycle to fix carbon.
- They are called C3 plants, since they fix CO2 into a
compound with 3 carbons (PGAL).
* Others in hot, dry climates supplement the Calvin
cycle with alternatives (C4 or CAM), because their
stomata openings in the leaves must close to preserve
moisture, so the amount of CO2 they absorb is
reduced.
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C4 Pathway – use an enzyme which fixes CO2 into
compounds with 4 carbons, which are then transported
to other cells where CO2 is available to then use the
Calvin Cycle.
(corn, sugar cane, are examples)
CAM Pathway – These plants open the stomata only at
night to reduce water loss.
They take in and fix it into compounds, which then
release it during the day for use in the Calvin Cycle.
(cactuses, pineapples, etc.)
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Rate of Photosynthesis
* Increases as either light intensity or CO2
increase, but eventually plateaus at a maximum.
* Increases as the temperature increases, up to
a certain temperature.
(Beyond a certain high temperature, the rate of
photosynthesis decreases.)
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