PHOTOSYNTHESIS:
Can it make
sense?
It looks so innocent…
Role of Photosynthesis
 Photosynthetic
organisms (like plants,
algae, and some bacteria) occupy the
lowest level of a food chain
 The most common chemical energy
produced during photosynthesis is glucose
 Glucose fuels Cell respiration
Behavior of Light
Photosynthetic pigments respond to visible light, a type of
electromagnetic energy ranging from 400nm to 740nm
 Sunlight is made up of many different colors of light that
can be seen after separation by a prism and is absorbed
in particles called photons
 ANY substance when struck by light either absorbs it for
use (color vanishes) or reflects it (color can be seen)
 Example: A red T-shirt absorbs wavelengths Blue and green
but reflects red
 Light energy absorbed in photosynthesis is used to form
covalent bonds between sugar molecules to make
glucose
 Fun fact: Because almost no green is absorbed, plants
can’t be grown in green light
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Chlorophylls and Carotenoids
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Chlorophyll , which is a green pigment, reflects
green and absorbs other wavelengths of light to
be used in photosynthesis
Plant leaves appear green because of the
chlorophyll found in the chloroplasts of their cells.
Plants also contain various other photosynthetic
pigments in their leaves like xanthophylls and
carotene
Chlorophylls and carotenoids are the major
photosynthetic pigment groups
These are found on the membranes of the thylakoids
These are organized into photosystems used
during the first stage of photosynthesis
Spectra Measurements
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A spectrophotometer measures absorption of light at
various wavelengths to measure a plant’s absorption
spectrum
An Absorption spectrum is the combination of the
absorption spectra of all the pigments in the
chloroplast
An Action spectrum is the rate of photosynthesis at a
particular wavelength of light—more specifically, it is
the percentage use of wavelengths of light in
photosynthesis
Action spectrums can be measured via oxygen
production; if its high, there is likely a high rate of
photosynthesis
Absorption and action spectra vary between plants
due to differences in pigment and their amounts
Two Stages of Photosynthesis
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The first stage of photosynthesis traps light energy
and converts it into chemical energy ATP
The second stage is a series of reactions that uses
ATP to bond carbon dioxide and water molecules
together to create a sugar, such as glucose
Overall, photosynthesis is an anabolic processcomplex molecules are built from simpler ones
Photosynthesis occurs in autotrophic organisms so
they can make their own food
First Stage of Photosynthesis
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This can also be called the Light-dependent reactions
Light energy is absorbed in chlorophyll and
used to 1.) Convert light energy into
chemical energy in the form of ATP and
2.) split a water molecule into hydrogen and
oxygen in a process called photolysis . Not
all the absorbed light energy is used.
In photolysis, oxygen is released as a waste
product and will be used in aerobic cell
respiration
Photosystems
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Light-dependent reactions occur in the thylakoids or grana of
the chloroplast
A stack of thylakoids is called a granum
Photosystems exist on the thylakoid membranes and are made
of 1) chlorophyll a molecules, 2) accessory pigments, and 3) a
protein matrix
The Reaction center within the photosystem contains 1) a pair
of chlorophyll molecules, 2) a matrix of protein, and 3) a primary
electron receptor
TERMINOLOGY: Photosystems are only types of complexes
decorating the thylakoid; there are more than two per thylakoid
Photosynthetic bacteria have only one type of photosystem,
but plants have a photosystem I and a photosystem II
Each photosystem is most efficient at absorbing a certain type
of light; photosystem I responds best to light at 700 nanometers
and photosystem II responds best to light at 680 nanometers
Steps of Light-Dependent Reactions
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1) A photon of light is absorbed by a photosystem II pigment and
transferred along other pigment molecules to reach a chlorophyll a
molecule in the reaction center. The photon excites one of the
chlorophyll a electrons to a higher energy state. The chlorophyll is
referred to as photoactivated
2) The electron is captured by the primary acceptor of this reaction
center
3)Water is split by an enzyme to produce electrons, hydrogen ions,
and an oxygen atom through a reaction controlled by light energy.
This is called photolysis. The electrons are sent to the reaction
center’s chlorophyll a molecules
4)The excited electrons pass from the primary acceptor down an
electron transport chain, losing energy at each exchange. The first
of the three carriers is plastoquinine (PQ). The middle carrier is a
cytochrome complex
5)Energy lost from the electrons moving down the transport chain
drives chemiosmosis to phosphorylate ADP and produce ATP (a
proton is pumped into the thylakoid space via the electron’s
energy and travels back through ATP synthase). This is non-cyclic
photophosphorylation as opposed to cyclic phosphorylation.
Steps of Light-Dependent
Reactions (Continued)
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6) A photon of light is absorbed by pigment in photosystem I.
The energy is again transferred along accessory pigments to
a reaction center’s chlorophyll a molecule. One of the
chlorophyll a’s electrons is excited and transferred to the
reaction center’s primary acceptor. The void left behind is
filled with the de-energized electron from Photosystem II
7) Photosystem I’s electron is passed down a second
electron transport chain involving the carrier ferredoxin
8) Enzyme NADP reductase catalyzes the transfer of the
electron from ferredoxin to the energy carrier NADP+. Two
electrons are required to fully reduce NADP+ to NADPH, as
well as a H+ ion from the stroma.
Review: OIL RIG: OIL=Oxidation is loss (of electrons) and
RIG=Reduction is gain (of electrons)
The final products of the Light-Dependent Reactions are
some NADPH and ATPwhich supply chemical energy for the
light-independent reactions. Oxygen was also produced.
ATP: Cellular Respiration VS
Photosynthesis
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SIMILAR: Energy is released when electrons are exchanged from one
carrier to another in an electron transport chain in the involved
membranes
Electron transport chain is in the cristae membranes in cell
respiration and the thylakoids in photosynthesis
Released energy is used to actively pump hydrogen ion into the
intermembrane space in cell respiration; in photosynthesis it is
pumped into the thylakoid space
Hydrogen ions originate in the matrix in cell respiration, but in the
stroma in photosynthesis
ATP synthase allows hydrogen ions to diffuse back into the matrix in
cell respiration or the stroma in photosynthesis
ATP synthase catalyzes the oxidative phosphorylation of ADP to form
ATP in cell respiration. In photosynthesis, ATP synthase catalyzes the
photophosphorylation of ADP from ATP
In photosynthesis the b6-f cytochrome complex pumps hydrogen
ions into the thylakoid space to increase the concentration gradient
between the thylakoid space and the stroma
Second Stage of
Photosynthesis
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Also called the Light-independent reactions
ATP and hydrogen are used to convert CO2 and water
into useful organic molecules like glucose. Glucose is
made through the following reaction:
6CO2 + 6H2O  C6H12O6 + 6O2
Fixation is the conversion of an inorganic form of an
element (like CO2) into an organic form (like glucose). It
can also be described as the conversion of carbon
from a gas to a solid.
Photosynthesis can be described as a series of reactions
fixing CO2 and water to glucose, with water produced
as a byproduct (meaning this is a condensation
reaction)
Glucose is not the only organic molecule possible to
form during photosynthesis
Light-Independent Reaction
In-Depth
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Occurs in the stroma of the chloroplast
This involves the Calvin Cycle
1) Ribulose biphosphate (RuBP), a 5-carbon compound, binds to a
CO2 in carbon fixation to produce an unstable 6-carbon
compound. Rubisco catalyzes this reaction
2) The 6-carbon compound breaks into two 3-carbon compounds
called glycerate-3-phosphate
3) ATP and NADPH act on the glycerate-3-phosphate and form
new Triose phosphate (TP) compounds in a reduction reaction
4) TP can either leave the cycle and become sugar phosphates
that may become more complex carbohydrates. Most stay in the
cycle to reproduce the originating compound RuBP
5)ATP is used to recreate RuBP from TP
For every 12 TP molecules, 18 ATPs, and 12 NADPH: one 6-carbon
sugar and six RuBPs are created
TP can be used to produce simple sugars (glucose), disaccharides
(sucrose), or polysaccharides (cellulose or starch).
Summary of Photosynthesis
Light-dependent reactions occur in the Thylakoids;
light independent reactions occur in the stroma
 Light-dependent reactions use light energy to make
ATP and NADPH; light-independent reactions use ATP
and NADPH to make TP
 Light-dependent reactions split water in photolysis and
release oxygen into the atmosphere; lightindependent reactions return ADP, inorganic
phosphate, and NADPH to the light-dependent
reactions
 Light-dependent reactions include photosystems I and
II and two electron transport chains; light-independent
reactions involve the Calvin cycle
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Photosynthesis vs. Cell
Respiration
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Photosynthesis has CO2 as a reactant and
oxygen as a product; cell respiration has
oxygen as a reactant and CO2 as a product
Plants do both photosynthesis and cell
respiration but the two processes DO NOT
cancel each other out. Plants do not have
muscle or so many other ATP-demanding
tissues like animals and have a consistently
low rate of cell respiration because of this.
Photosynthetic rate is not constant and
responds to many environmental factors
Cyclic Phosphorylation
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Another way that a light-dependent reaction of photosynthesis
can produce ATP
Can only occur is light is not the limiting factor and when NADPH
has built up in the chloroplast
This is when light-energized electrons from photosystem I flow
back into the cytochrome complex of the electron transport
chain between photosystems I and II instead of the chain
involving ferredoxin
From the cytochrome complex, electrons move down the
remaining electron transport chain to allow chemiosmosis to
produce ATP
This means the electrons don’t flow to the second transport chain
to produce NADPH, of which there is already an abundance
(causing the photophosphorylation to occur in the first place)
The extra ATP goes to the Calvin cycle and causes it to take place
faster
Photosynthesis and Limiting
Factors
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A limiting factor is a factor that controls the rate of a process
like photosynthesis
At any time in the life of a plant, one of the following may be
the plant’s limiting factor—the one that alone alters
photosynthetic rate regardless of how the others behave
Only the limiting factor has effect because photosynthesis has
many steps with many end-products. When a limiting factor
slows one step, the rest are also impeded—that one step is
called the rate-limiting step
Stronger light intensity and higher CO2 concentration increase
the rate of photosynthesis—more CO2 is taken in and more
oxygen is produced up to maximum capacity. Higher
temperatures also increase photosynthetic rate until the plant’s
enzymes begin to denature
At night as temperatures cool and light intensity drops,
photosynthesis may halt entirely, although cell respiration
continues
Measuring Photosynthetic
Rate
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Photosynthetic rate can be measured by oxygen
production or CO2 intake, provided a correction is
made for cell respiration. It can also be measured
by keeping track of a change in biomass of
experimental plants (massing is less reliable as
many other factors affect biomass)
Example: oxygen production can be seen in the
volume of air bubbles released from aquatic
plants
Example: CO2 uptake can be measured in
aquatic plants by a rising water pH as CO2 is
absorbed
Chloroplast Structure
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All of photosynthesis occurs in the chloroplast
Chloroplasts and mitochondria have an extra outer
membrane, their own DNA, and are close to the size of
a prokaryotic cell
Chloroplasts are generally in the leaves of a plant, but
not exclusively
Extensive membrane surface area of the thylakoids
allows photosystems to absorb more light
Small lumen within the thylakoids allows faster proton
accumulation to create a concentration gradient
Stroma region similar to a cell’s cytosol allows an area
for the Calvin Cycle’s enzymes to work
Double membrane on the outside isolates working parts
and enzymes of the chloroplast from the surrounding
cytosol
Chloroplast Diagram