Photosynthesis
Topic 3.8 and 8.2
Autotrophs
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Plants are autotrophs (meaning “self-feeders” in
Greek) in that they make their own food and
thus sustain themselves without eating other
organisms or even organic molecules.
Chloroplasts of plant cells capture light energy
that has traveled 150 million kilometers from the
sun and convert it to chemical energy that is
stored in sugar and other organic molecules.
Producers
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Plants, algae, some prokaryotes make their own
organic molecules and are the ultimate source of
organic molecules for almost all other
organisms.
Often referred to as the producers of the
biosphere because they produce its food supply
All organisms that produce organic molecules
from inorganic molecules using the energy of
light are called photoautotrophs.
Chloroplasts
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All green parts of a plant have chloroplasts in their cells
and can carry out photosynthesis.
In most plants, leaves have the most chloroplasts and
are the major sites of photosynthesis.
Chloroplasts are concentrated in the cells of the
mesophyll, the green tissue in the interior of the leaf.
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Each mesophyll has numerous chloroplasts
Carbon dioxide enters the leaf and oxygen exits via tiny
pores called stomata.
Water absorbed by the roots is delivered to the leaves in
veins.
Chloroplasts
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Membranes in the chloroplast form the framework where many of
the reactions of photosynthesis occur, just as mitochondrial
membranes do for cell respiration.
Similar to mitochondria, chloroplast has an outer membrane and
an inner membrane, with an intermembrane space between them.
Inner membrane is filled with a thick fluid called stroma
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Stroma is where sugars are made from carbon dioxide and water
Within stroma is a system of interconnected membranous sacs
called thylakoids
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Enclose a third compartment called the thylakoid space
Built into thylakoid membranes are the chlorophyll molecules that
capture light energy.
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Membranes also house much of the machinery that converts light energy
to chemical energy.
In some places, thylakoids are concentrated in stacks called grana.
Chloroplast
Photosynthesis is a redox
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6CO2 + 6H2O  C6H12O6 + 6O2
When water molecules are split apart, yielding O2, they are actually oxidized;
that is, they lose electrons along with hydrogen ions
Meanwhile, CO2 is reduced to sugar as electrons and hydrogen ions are
added to it.
Overall, cell respiration harvest energy stored in a glucose molecule by:
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oxidizing the sugar and reducing O2 to H2O, involving a number of energyreleasing redox reactions,
with electrons losing potential energy as they travel down an energy hill from
sugar to O2.
Along the way, the mitochondria uses some of the energy to synthesize ATP.
In contrast, photosynthesis redox reactions involve an uphill climb.
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As water is oxidized and CO2 is reduced, electrons gain energy by being boosted
up an energy hill.
Light energy captured by chlorophyll molecules in the chloroplast provides the
boost for the electrons.
Photosynthesis converts light energy to chemical energy and stores it in sugar
molecules.
Photosynthesis: Overview
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Photo, from the Greek word for light,
refers to the first stage.
Synthesis, meaning “putting together”
refers to the sugar construction in the
second stage
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1.
Photosynthesis
Overview
Occurs in two stages:
Light reactions
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Include the steps that convert light energy to
chemical energy stored in ATP and NADPH and
produce O2 gas as a waste product.
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Occur in thylakoid membranes
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Light energy absorbed by chlorophyll is used to
make ATP from ADP and phophate.
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Also used to drive a transfer of electrons from
water to NADP+, an electron carrier similar to
NAD+ that carries electrons in cellular respiration.
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NADP+ gets reduced to NADPH via enzymes by
adding a pair of light-excited electrons along with
an H+
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Reaction temporarily stores energized electrons which
originally came form water that is split and O2 is
released.
Photosynthesis Overview
2.
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Dark reactions, or Calvin Cycle
Occurs in the stroma
Does not require light directly
Cyclic series of reactions that assembles sugar
molecules using CO2 and the energy-containing
products (NADPH and ATP) of the light reactions.
Incorporation of carbon from CO2 into organic
compounds is called carbon fixation.
After carbon fixation, enzymes of the cycle make
sugars by further reducing the carbon compounds.
Light Reactions:
Converting Solar Energy to Chemical Energy
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Electromagnetic energy
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type of energy that is sunlight
Travels in space as rhythmic waves analagous to those made by
a pebble dropped in a puddle of water
Distance between the crests of two adjacent waves is called a
wavelength.
In the electromagnetic spectrum, shorter wavelengths have
more energy than longer ones.
Visible light- the radiation your eyes can see as different colors,
consists of wavelengths from about 380 nm to 750 nm
Light Reactions:
Converting Solar Energy to Chemical Energy
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Figure 7.6B shows what happens to visible
light in the chloroplast.
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Light absorbing molecules called pigments,
built into the thylakoid membranes, absorb
some wavelengths of light and reflect or
transmit other wavelengths.
We do not see the absorbed wavelengths;
their energy has been absorbed by pigment
molecules
We see green wavelengths when we look at
plants that the pigments transmit and reflect.
Light Reactions:
Converting Solar Energy to Chemical Energy
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Pigments of chloroplast:
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Chlorophyll a
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Chlorophyll b
Absorbs mainly blue and orange light and reflects (appears) yellowgreen.
 Broadens the range of light that a plant can use by conveying
absorbed energy to chlorophyll a, which then puts the energy to
work in the light reactions
Carotenoids
 Absorb mainly blue-green light and reflects yellow-orange
 Some may pass energy to chlorophyll a, as chlorophyll b does
 Have a protective function: absorb and dissipate excessive light
energy that would other-wise damange chlorophyll
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Absorbs mainly blue-violet and red light
Participates directly in the light reactions
Looks grass-green because it reflects mainly green light
Light Reactions:
Converting Solar Energy to Chemical Energy
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The theory of light as waves explains most
of light’s properties.
However, light also behaves as discrete
packets of energy called photons:
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A fixed quantity of light energy, and the
shorter the wavelength, the greater the
energy.
Each type of pigment absorbs certain
wavelengths of light because it is able to
absorb the specific amounts of energy in those
photons.
Light Reactions:
Converting Solar Energy to Chemical Energy
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Photosystems
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Clusters of chlorophyll molecules along with other pigments and
proteins in the thylakoid membrane
Consists of a number of light-harvesting complexes surrounding
a reaction center.
Have chlorophyll a, chlorophyll b, and carotenoid pigments that
function collectively as a light-gathering antenna.
Pigments absorb photons and pass the energy from molecule to
molecule until it reaches the reaction center.
Light Reactions:
Converting Solar Energy to Chemical Energy
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Photosystems:
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Reaction center
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A protein complex that contains a chlorophyll a
molecule and a molecule called the primary
electron acceptor:
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Captures a light-excited electron from the reactioncenter chlorophyll molecule and passes it to an electron
transport chain
Light Reactions:
Converting Solar Energy to Chemical Energy
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Photosystems:
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Two types: Photosystem I and Photosytem II:
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Photosystem I:
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Occurs second in light reactions
Reaction center is called P700 because the wavelength of
light it absorbs best is 700 nm
Photosystem II:
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Occurs first in light reactions
Chlorophyll a molecule in reaction center is called P680
because the light it absorbs best is red light with a
wavelength of 680nm
Light Reactions:
Converting Solar Energy to Chemical Energy
Light Reactions:
Converting Solar Energy to Chemical Energy
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Light Reactions:
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Light energy is transformed into the chemical
energy of ATP and NADPH
In this process, electrons removed from water
molecules pass from photosystem II to
photosystem I to NADP+
Between the two photosystems, the electrons
move down an electron transport chain and
provide energy for ATP production.
Light Reactions:
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Converting Solar Energy to Chemical Energy
Flow of electrons in light reactions (Figure 7.8A):
1.
2.
3.
A pigment molecule in a light-harvesting complex
absorbs a photon of light. The energy is passed to
other pigment molecules and finally to the reaction
center of Photosystem II, where it excites an
electron of chlorophyll P680 to a higher energy level.
The electron is captured by the primary electron
acceptor.
Water is split, and its electrons are supplied one by
one to P680, replacign those lost to the primary
electron acceptor. The oxygen atom compbines with
an oxygen from another split water molecule to form
a molecule of O2.
Light Reactions:
Converting Solar Energy to Chemical Energy
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Flow of electrons in light reactions (Figure 7.8A):
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4. each photoexcited electron passes from photosystem II to
photosystem I via an electron transport chain. The exergonic
“fall” of electrons provides energy for the synthesis of ATP.
5. Meanwhile, light energy excites an electron of chlorophyll P700
in the reaction center of photosystem I. The primary electron
acceptor captures the excited electron and an electron from the
bottom of the electron transport chain replaces the lost electron
in P700.
6. The excited electrons of photosystem I is passed through a
short electron transport chain to NADP+, reducing it to NADPH
Light Reactions:
Converting Solar Energy to Chemical Energy
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Chemiosmosis
Drives ATP synthesis using the potential energy of
a concentration gradient of hydrogen ions across a
membrane
 Gradient is created when an electron transport
chain pumps hydrogen ions across a membrane as
it passes electrons down the chain.
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Light Reactions:
Converting Solar Energy to Chemical Energy
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Chemiosmosis (ctd)
Relationship between chloroplast structure and
function in light reactions:
 The two photosystems and e.t.c. are all located in
the thylakoid membrane of a chloroplast.
 As photoexcited electrons are passed down the
e.t.c. connecting the two photosystems, H+ are
pumped across the membrane from the stroma
into the thylakoid space. This generates a
concentration gradient across the membrane.
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Light Reactions:
Converting Solar Energy to Chemical Energy
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Chemiosmosis (ctd)
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Similar ATP synthase complex in mitochondria
Energy of concentration gradient drives H+
back across the membrane through ATP
synthase
ATP synthase couples the flow of H+ to the
phosphorylation of ADP: called
photophosphorylation
Light Reactions:
Converting Solar Energy to Chemical Energy
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Chemiosmosis (ctd)
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In photosynthesis, light energy is used to drive electrons to the
top of the transport chain (whereas, cell respiration, high-energy
electrons pass down the e.t.c. coming from oxidation of food
molecules)
Chloroplasts transform light energy into the chemical energy of
ATP (whereas, mitochondria transfer chemical energy from food
to ATP)
In photosynthesis, the final electron acceptor is NADP+
(whereas, in cell respiration, O2 is)
In photosynthesis, electrons are stored in at a high state of
potential energy in NADPH (whereas, in cell respiration, they are
at a low energy level in H20)
Light Reactions:
Converting Solar Energy to Chemical Energy
Dark Reaction (Calvin Cycle)
Converting CO2 to sugars
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Figure 7.10A: Overview of Calvin Cycle
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CO2 (from air), energy from ATP and high
energy electrons from NADPH (both
generated by light reactions) , the Calvin
Cycle constructs an energy-rich, three-carbon
sugar, glyceraldehyde-3-phosphate (G3P).
A plant cell uses G3P to make glucose and
other organic molecules as needed.
Dark Reaction (Calvin Cycle)
Converting CO2 to sugars
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Figure 7.10B: Details of the Calvin Cycle
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Carbon fixation: the enzyme rubisco attaches CO2
to RuBP (5-C). The unstable 6-C product splits into
two molecules called 3-PGA.
1.
For three CO2, six 3-PGA result
Reduction: NADPH reduces the organic acid six 3PGA to six molecules G3P with the assistance of ATP
Dark Reaction (Calvin Cycle)
Converting CO2 to sugars
3. Release of one molecule of G3P:
1.
Five G3Ps remain in the cycle, and one G3P will
leave. Plant cells use two G3P molecules to make
one molecule of glucose.
4. Regeneration of RuBP
energy from ATP drives a series of chemical
reactions to rearrange the atoms in the five G3P
molecules to form three RuBP molecules. These can
start another turn of the cycle.
Dark Reaction (Calvin Cycle)
Converting CO2 to sugars
Absorption spectrum
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As light meets matter, it may be reflected,
transmitted, or absorbed.
Pigments
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Substances that absorb visible light
Different pigments absorb light of different
wavelengths
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Chlorophylls absorb red and blue-violet light and appear
green
Carotenoids absorb blue-violet and appear orange, yellow, or
red
Measured with a spectrophotometer.
Absorption spectrum
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Absorption spectrum
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Light absorption vs. the wavelength
Absorption spectrum of different photosynthetic
pigments provides clues to their role in
photosynthesis, since light can only perform work if it
is absorbed.
Accessory pigments (chlorophyll b and carotenoids)
absorb wavelengths of light that chlorophyll a cannot,
pass the energy to chlorophyll a, broadening the
spectrum that can effectively drive photosynthesis.
Absorption Spectrum
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Action spectrum
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Profiles the effectiveness of different
wavelength light in fueling photosynthesis.
It is obtained by plotting wavelength against
some measure of photosynthetic rate.
Action Spectrum
Abiotic factors impact on
photosynthetic rate
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Photosynthetic rate is depended on
environmental factors:
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Amount of light available
Level of carbon dioxide
temperature
Light intensity
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Up to a certain intensity, photosynthesis
increases as more light is available to the
chlorophyll.
When all the chlorophyll molecules are
activated (saturated) by the light, more
light has no further effect.
Light Intensity
Temperature
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Increased temperature increases
photosynthetic rate until an optimal
temperature is reached.
Above the optimal temperature, enzymes
cannot function properly and
photosynthesis will decrease.
Temperature
Carbon Dioxide levels
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Increased carbon dioxide levels increases
photosynthesis, unless limited by another
factor, then levels off.
Carbon Dioxide
Measuring photosynthesis
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Production of oxygen or uptake of carbon
dioxide
Increase in biomass