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Chapter 8
PHOTOSYNTHESIS
Producers are organisms that can make organic molecules from inorganic substance.
They are also known as autotrophs.
Most producers are photoautotrophs and use light as energy source for manufacturing organic
compounds from CO2 and H2O.
Organisms that cannot make their own organic compounds and must obtain them from other
organisms are called heterotrophs.
Consumers and decomposers are heterotrophs.
Photosynthesis is the biochemical process that utilizes radiant energy from sunlight to
synthesize carbohydrates from CO2 and H2O in the presence of chlorophyll.
LIGHT
Light is composed of particles of energy that travel as waves.
Light is part of the electromagnetic spectrum, the entire range of electromagnetic radiation.
Wavelength is the distance from one wave peak to the next.
Visible light consists of a mixture of wavelengths ranging from about 380 nm to 760 nm.

Violet has the shortest wavelength and red the longest.
Light also behaves as particles or packets of energy called photons.
The energy of the photon is inversely proportional to its wavelength: the shorter the wavelength
the greater the energy of the photon.
When a molecule absorbs on photon of light, one of its electrons is energized and moves into a
higher energy level.
1. The excited electron may return to its original lower energy level emitting a less
energetic photon. This is called fluorescence.
2. An electron acceptor molecule may accept the excited electron.
PHOTOSYNTHESIS
In eukaryotes, photosynthesis occurs in chloroplasts, which are located mostly in the mesophyll
of the leaf.
Chloroplasts are organelles bound by a double membrane.
The inner membrane encloses a space called the stroma, in which membranous flattened sacs,
the thylakoids are stacked.
The thylakoid membrane encloses the thylakoid interior space.
The stacks of thylakoids are called grana (sing. granum).
Usually there are 40 to 60 grana linked together by arms in each chloroplast.
Thylakoids are part of an overlapping and continuous system of membranes suspended in the
stroma.
Built in thylakoid membrane are the chlorophyll molecules and other pigments that capture light
energy.
The chlorophyll molecule is embedded in the thylakoid membrane by a long hydrophobic
hydrocarbon tail
Chloroplasts also have accessory pigments called carotenoids, which are yellow and orange.
Carotenoids absorb light at ranges different from chlorophyll and broadens the spectrum of light
that provides energy for photosynthesis.
Structure of the chlorophyll molecule.
There are several kinds of chlorophyll all of them containing a magnesium atom.
The molecule has two main parts: one captures the energy and the other holds the molecule in
place.
A complex ring called the porphyrin ring, made of joined smaller rings made of carbon and
nitrogen absorbs light, and a hydrocarbon or lipid tail.
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The porphyrin ring is very similar to the heme portion of the red pigment hemoglobin in
red blood cells.
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The porphyrin ring of chlorophyll contains an atom of magnesium in its center.
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The heme ring of hemoglobin contains iron in its center.
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The tail extends into the thylakoid membrane and is hydrophobic.
The most important is chlorophyll a.
Chlorophyll b is an accessory pigment that also participates in photosynthesis.

Chlorophyll a has a methyl group, CH3, on the porphyrin ring.

Chlorophyll b has carbonyl group, CHO, on the porphyrin ring.
Chlorophyll absorbs light mostly in the blue and red areas of the visible spectrum.
Green light is not appreciably absorbed but reflected by chlorophyll.
CHLOROPHYLL AND LIGHT.
The spectrophotometer is used to measure the relative abilities of different pigments to absorb
different wavelengths of light.
The absorption spectrum is a plot of the absorption of light of different wavelengths.
The action spectrum of photosynthesis shows how effective various wavelengths of light are in
carrying photosynthetic activity.
LIGHT-DEPENDENT REACTIONS OF PHOTOPSYNTHESIS
Photosynthesis is the conversion of light energy into chemical bond energy.
______oxidation______
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
6CO2 + 12 H2O  C6H12O6 + 6O2 + 6 H2O
_______reduction______
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Energy from sunlight splits water into H+ , O2 and electrons.
ATP and NADPH are the products of the light -dependent reactions.
Photosystems.
Two types of photosynthetic units present in chloroplasts make up photosystems known as
photosystem I and photosystem II.
Each photosynthetic unit of photosystem I consists of highly ordered groups of ...
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200 to 300 molecules of chlorophyll a,
small amounts of chlorophyll b,
carotenoids with proteins attached,
a special reaction-center molecule of chlorophyll a called P700.
These photosynthetic units are also called antenna complexes.
The remaining photosystem pigment molecules are called antenna pigments.
The photosynthetic units of photosystem II consists of ...
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
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200 or more chlorophyll a,
-carotene attached to a protein,
a little chlorophyll b,
a reaction-center of chlorophyll a called P680.
The number 700 and 680 refer to the peak absorption with wavelengths of 700 and 680 nm.
Ordinary chlorophyll a absorbs at 660 nm.
Each antenna complex traps light and transfers the energy to the reaction center.
The reaction-center molecule is the only one that can actually use the light energy.
Light energy is converted to chemical energy in the reaction center by a series of electron
transfer reactions.
Reactions.
A photon of light strikes P680 in photosystem II and boosts an electron to a higher energy level.
Excited electrons lose much of their energy as heat.
Many photons strike the photosystem molecules at the same time, with the energy of excited
electrons being transferred toward the P680 reaction center.
The energize electron is transferred to a an electron acceptor, pheophytin, and then to Pq,
plastoquinone within the thylakoid membrane.
Pq unloads the electrons to the iron-containing cytochromes found in the thylakoid membrane
facing the stroma of the chloroplast.
Cytochromes pass the electron to plastocyanin, a copper containing protein.
These electrons are eventually donated to P700 in photosystem I.
When electrons move along the transport system, protons move across the thylakoid
membrane by chemiosmosis and ATP is synthesized. This process is called
photophosphorylation.
A proton gradient is created between the stroma and the interior thylakoid space. The greater
proton concentration is in the interior thylakoid space.
The interior space has about 1000 fold higher concentration of H+ .
The electrons lost by the P680 are replaced by electrons extracted from water by a protein
complex called the oxygen-evolving complex.
This process of splitting water to obtain electrons is called photolysis.
The electrons extracted from water are passed to tyrosine also known as Z.
Z contains manganese required to split water.
Two water molecules produce oxygen and four protons.
Pigment molecules in the antenna complex of photosystem I absorb photons and pass their
energy to the reaction center, P700, and an electron is energized (excited).
The excited electrons are passed to an acceptor molecule, P430, which transfers it to
ferrodoxin, a membrane-bound iron-containing protein.
Ferrodoxin transfers the electron to FAD reducing it to FADH2.
FADH2 then reduces NADP+ to NADPH. Two electrons are required to reduce NADP+ to
NADPH.
NADPH is released into the stroma of the chloroplast.
Electrons from photosystem II replace the missing electrons of P700.
Follow the electrons:
e- removed from water (photolysis) are passed on to photosystem II, then to Pq, the
cytochrome chain, photsystem I, P430, ferrodoxin and finally NADPH.
In the process photophosphorylation occurred through chemiosmosis.
This entire process is also knows as non-cyclic photophosphorylation, a continuous linear
process.
An alternate path of electrons is called cyclic photophosphorylation, which produces ATP
but no NADPH
The electrons in P430 can be transferred to the electron transport chain between the two
photosystems and back into photosystem I.
LIGHT-INDEPENDENT REACTIONS OF PHOTOSYNTHESIS
During the carbon fixation reactions, ATP and NADPH are used to manufacture carbohydrate
molecules from CO2.
Most plants use the Calvin cycle, also known as the C3 cycle, to fix carbon.
Main steps of the Calvin cycle:
1. Six molecules of CO2 from air combine with 6 molecules of ribulose 1,5-biphosphate with
the aid of the enzyme Rubisco (RuBP carboxylase).

RuBP is a 5-C sugar constantly being formed in the cycle.
2. The resulting 6-C unstable complexes are immediately split into twelve 3phosphoglyceric acid (3PGA), the first stable compound formed in photosynthesis.
3. The NADPH and ATP supply the energy to convert the 3PGA to twelve molecule of
glyceraldehyde 3-phosphate (GA3P), a 3-C sugar phosphate.
4. The of the twelve glyceraldehyde 3-phosphate molecules are restructured and become
six 5-C molecules of RuBP, the sugar used in the first step of the Calving cycle.
5. This leaves a net gain of two glyceraldehyde 3-phosphate, which can contribute to either
an increase in the carbohydrate content of the plant (glucose, starch, cellulose, etc.) or
can be used in pathways that lead to the net gain of lipids and amino acids.
Light-dependent reaction
12 H2O + 12 NADP+ + 18 ADP + 18 Pi  6 O2 + 12 NADPH + 18 ATP
Light-independent reaction
12 NADPH + 18 ATP + 6 CO2 C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi + 6 H6O
by canceling the common items on the above reactions we obtain the overall reaction of
photosynthesis.
6 CO2 + 12 H2O  C6H12O6 + 6 O2 + 6 H2O
C4 PATHWAY
It is also called the Hatch-Slack pathway.
CO2 is not very abundant in the atmosphere: 0.03% of atmosphere.
Diffusion of gases occurs only across moist surfaces
Leaves and other structures are covered with waterproof substances to prevent dehydration.
Entry of CO2 is limited to the stomata, which lead to the interior of the leaf, the mesophyll.
Mesophyll cells contain chloroplasts and carry on photosynthesis.
When conditions are hot and dry, the stomata close to prevent excessive water loss, also
preventing CO2 from entering the leaf interior.
When the maximum amount of light is available for photosynthesis, the stomata are often closed
in hot and dry climates.
1. C4 reactions take place in the mesophyll of the cell.
2. The CO2 is fixed into the 4-C oxaloacetate, which is converted to malate and requires
NADPH.
3. Malate moves into the choroplasts of the bundle sheath cell,
4. CO2 is removed from malate and pyruvate is produced.
5. The generated CO2 then enters the Calvin cycle.
6. Pyruvate returns to the mesophyll where is converted to phosphoenopyruvate, PEP.
The key component of the C4 pathway is the enzyme PEP carboxylase.
PEP carboxylase has a very high affinity for CO2 even at very low concentrations.
PEP carboxylase catalyzes the reaction by which CO2 reacts with the 3-C compound
phosphoenopyruvate to form oxaloacetate, a 4-C molecule.
When light is abundant, the rate of photosynthesis is limited by the concentration of CO 2.
C4 do not have to have the stomata open as much.
Because PEP carboxylase has such a high affinity for CO2, C4 plants tolerate higher
temperatures and higher light intensities, lose less water by transpiration, and have higher rate
of photosynthesis and growth than plants that use only the Calvin cycle.
Examples of plants that use C4 pathway are sugar cane, corn and crabgrass.
At lower temperatures and light intensities, C3 plants are favored because they do not require as
much energy to fix CO2
C4 and C3 occur at different locations (cells) in the leaf.
CRASSULACEAN ACID METABOLISM OR CAM
CAM is similar to C4 pathway.
PEP carboxylase fixes CO2 at night in the mesophyll of the cell when the stomata are open.
Oxaloacetate is formed which is converted to malate and store in cell vacuoles.
During the day the stomata close to prevent excessive evaporation.
When light is available, CO2 is removed from malate by a decarboxylation reaction, and is made
available to the C3 cycle.
CAM and C3 occur at different times within the same cell.
Desert plants use CAM in photosynthesis.
PHOTORESPIRATION
Many plants do not yield as much carbohydrate as expected, especially during hot summer
spells.
1. On hot, dry days plants close their stomata to conserve water.
2. Once the stomata close, the plant uses rapidly the available CO2 and produces O2, which
accumulates in the chloroplasts.
3. Rubisco is the enzyme responsible for binding CO2 to RuBP to make PGA (phosphoglyceraldehyde).
4. O2 competes with CO2 for the active binding site of Rubisco.
5. When O2 binds to Rubisco some intermediates of the Calvin cycle are degraded to H20 and
CO2.
This process is called photorespiration because; it occurs in the presence of light, requires O 2
and produces CO2 and H2O.
No ATP is produced during photorespiration and the removal of Calvin cycle intermediates
reduces yield.
Photorespiration is negligible in C4 because the concentration of CO2 is always high in bundle
sheath cells where Rubisco is present.
Many important C3 (e.g. soybean, wheat, potatoes) carry out photorespiration.
Attempts are being made to transplant genes for the C4 pathway to C3 plants.