Photosynthesis
Turning Light into Sugar
By the end I will be able to…
o explain how energy is absorbed by pigments, transferred
through the reduction of NADP+ to NADPH, and then
transferred as chemical potential energy to ATP by
chemiosmosis
o explain how NADPH and ATP are used to reduce Carbon in
the light independent reactions for the production of glucose
o identify where in the chloroplast these steps occur
ATP
WHAT IS ATP?
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Universal Energy Molecule
Energy in a form the cell can use
Makes energy readily available
Continuously being remade
Stands for Adenosine Triphosphate
Adenosine
P
P
Adenosine
P
P
+
Tri=3
P
P
High Energy Bond
ATP
• What is ATP used for?
1.
2.
3.
4.
Motion
Transport of ions and molecules
Building molecules
Switching reactions on or off
How ATP works
Redox Reactions
• Redox reactions (reduction-oxidation reactions) happen
throughout photosynthesis as electrons get transferred
from one molecule to another.
• There are a couple of ways to remember what happens to
the electron
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OIL RIG (Oxidation is losing, Reduction is gaining)
LEO goes GER (lose electrons = oxidation; gain electrons = reduction)
• This is just a chain reaction of gaining electrons (e-) and
passing them to the next compound
Redox Reactions
Photosynthesis
• The process of storing light energy (photons) as chemical
energy (carbohydrates) in the cell.
The equation
If you haven’t memorized it in previous classes, now is the
time. The equation will provide you with an outline of the
process. Work your way to memorizing the bottom format.
Photosynthesis occurs in Chloroplasts
Chloroplast Parts
• Thylakoid
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1 disk
Granum = a stack of disks
Grana = plural granum (many stacks of disks)
Site of the light dependent reaction (holds the chlorophyll)
• Stroma
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Gel surrounding the thylakoids
Site of the light independent reaction
Parts of Chloroplast
Chlorophyll
• A green chemical which
traps sunlight energy
• Located in the thylakoid of
a chloroplast
• The pigments chlorophyll a
and chlorophyll b absorb
light wavelengths on the
red and blue ends of the
spectrum
• They reflect green light,
that’s why plants look
green.
Leaves in the fall
• Why do leaves change
colour in the fall if
chlorophyll a and b reflect
green light?
Pigments
• Leaves contain other light absorbing pigments (chemicals)
besides chlorophylls.
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Carotenoids (including carotene which reflects orange light,
xanthophyll which reflects yellow light, and lycopene which
reflects red light)
Flavonoids (more yellow reflecting pigments and anthocyanin
which reflects shades of red, blue, and purple)
• Different plants have different concentrations of these
chemicals.
• In the fall, chlorophylls begin to break down, and the other
reflected colours begin to show through without the green
to overpower them.
Light Absorption Spectrum
High peaks mean they absorb those colours
Low peaks mean they reflect those colours
Photosynthesis
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There are two main parts to photosynthesis
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Light dependent reaction
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Needs light, therefore it takes place in the thylakoid membrane
where the chlorophyll is
Takes in water and creates oxygen as a byproduct
Creates intermediary (not final) products of ATP and NADPH for
the Calvin Cycle
Light independent reaction
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Doesn’t need light so it happens outside the thylakoid in the
stroma
Also referred to as the dark reactions (misleading because it
doesn’t only happen in the dark)
Takes in CO2 and the ATP and NADPH from the thylakoid to make
glucose in a process called the Calvin Cycle.
Photosynthesis
Light Dependent Reaction
• All of these steps are happening at the same time, but for
simplicity’s sake we will learn them separately
1. Photosystem II (PSII)
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PSII is a group of chlorophyll pigments located on the thylakoid
membrane.
It absorbs the sunlight energy, transforming it into chemical
energy in the form of excited electrons (e-)
These e- are unstable and leave the chlorophyll, attaching to
molecules in the ETC (electron transport chain).
The chlorophyll loses its electrons, so it is oxidized (remember
LEO)
Photosynthesis
2.
Photolysis (Photo = light
lysis = split, therefore: using light to split)
‒ In order to replace the electrons lost from the chlorophyll, a water
molecule will be split apart
‒ Light energy is used to split H2O into its parts
‒ The Oxygen now leaves the thylakoid and the chloroplast and
goes into the air for us to breathe
‒ The H splits into its proton H+ and its electron e‒ The e- goes to the chlorophyll to replace the e- it lost
‒ The chlorophyll has gained more electrons, therefore it has
been reduced
PSII and Photolysis
Start of the ETC
Photosynthesis
3.
Electron Transport Chain (ETC)
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The e- from the PSII are transferred through the ETC like a
bucket brigade (one molecule accepts it, and then passes it onto
the next molecule)
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This is a series of redox reactions, as the molecule that accepts
the e- is being reduced, but then is oxidized when it passes the
e- onto the next molecule (** You don’t have to know the names of
the individual molecules**)
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When the e- are being transferred, some of their energy is used
to take H+ that are floating around in the stroma and pump
them into the thylakoid lumen (H+ are floating around
everywhere in cells)
Photosynthesis
ETC continued.
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The pumping of H+ into the thylakoid is an example of active
transport (watch a tutorial on membrane transport)
This means it is using energy (from the e-) and the movement is
against the concentration gradient.
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The H+ are being pumped from an area of low concentration to an area of high
concentration
Building up a H+ concentration gradient means that the inside of
the thylakoid is more positively charged than the stroma
This is a form of potential energy (electrochemical gradient)
The e- lose their excess energy as they are transferred in the ETC
until they reach Photosystem I (PSI)
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PSI comes after PSII, but they’re numbered backwards because that’s the order
they were discovered in
Photosynthesis
Active Transport
ETC
Photosynthesis
4.
Reduction of NADP+
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When the low energy ereaches PSI, it gets
reenergized by the sun (einto e-)
It leaves the thylakoid at
PSI and joins with H+ and
NADP+ (floating around
waiting to be reduced) in
the stroma to make
NADPH
NADPH is an electron
carrier. It moves the e- and
its H+ from the thylakoid to
the Calvin Cycle.
Photosynthesis
5.
Chemiosmosis
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This is the process of making ATP using the potential
electrochemical energy from the H+ concentration gradient.
The H+ concentration has built up in the thylakoid lumen thanks
to the active transport of H+ inside by the ETC.
The high concentration of H+ means that they want to diffuse out
in order to achieve equilibrium
The H+ cannot diffuse across the membrane because they are
charged, so they must use a protein channel (facilitated diffusion)
The protein channel they diffuse out through is called the ATP
Synthase Complex.
Photosynthesis
Photosynthesis
Chemiosmosis continued.
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As the H+ diffuse through the ATP Synthase channel, the structure
turns the potential energy (from H+ gradient) into kinetic energy
that turns the protein channel
This kinetic energy is then transferred to the chemical bond
between ADP and P to make the energy rich compound ATP.
• This website has a really good animation of the Light
Dependent Reaction.
• Following are some overview pictures as well.
Photosynthesis
• Light Independent Reaction
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This is takes place in the stroma of the chloroplast
It is also called the Calvin Cycle
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It is a complicated cycle involving many steps that you don’t need to know
In this step, the CO2 from the air is bonded to the e- and H+ from
NADPH to make glucose (C6H12O6) – Carbon fixation
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Oxidizing NADPH back into NADP+
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ATP donates its stored chemical energy to bond the C, H, and O
together by breaking apart into ADP and P
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Now you have the final product of glucose
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And the ADP, P, and NADP+ can go back to the thylakoid to be
used in the Light Dependent Reactions again
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They continue to cycle back and forth between the main stages
Photosynthesis
Photosynthesis
Summary
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Light energy is absorbed by chlorophyll and turned into chemical
energy by exciting electrons
Water is broken apart to replace lost e- in PSII
The electrons’ energy is used to create a H+ concentration gradient
(electrochemical potential energy now) in the ETC
This gradient in turn transfers the energy into stored chemical
energy in the ATP molecule when H+ diffuse through the ATP
synthase complex.
The electrons absorb more sunlight energy and are carried from
the thylakoid to the Calvin Cycle by NADPH
The energy from ATP bonds the e- and H+ from NADPH to the CO2
to make C6H12O6
Some helpful review videos:
Crash Course
Great Animation
Teacher Tutorial (up to 8:35)
Taylor Swift song 1
Taylor Swift song 2
Teacher Song
Recall
o
explain how energy is absorbed by pigments, transferred through the
reduction of NADP+ to NADPH, and then transferred as chemical potential
energy to ATP by chemiosmosis
o
o
explain how NADPH and ATP are used to reduce Carbon in the lightindependent reactions for the production of glucose
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Light Dependent Reaction (Photolysis, ETC, Chemiosmosis)
Light Independent Reaction (Calvin Cycle)
identify where in the chloroplast these steps occur
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Thylakoid and Stroma