Chapter 10
Summary of Chapter 10, BIOLOGY, 10 TH ED Campbell, by J.B. Reece et al. 2014.
The sun is the energy source that keeps most organisms alive.
Photosynthesis converts radiant (solar) energy to chemical energy.
Almost all organisms depend on photosynthesis directly or indirectly.
Exception to this are the hot vent ecosystems found at the bottom of the oceans.
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 CO
2
and H
2
O.
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 CO
2
and H
2
O in the presence of chlorophyll.
1. CHLOROPLASTS
In eukaryotes, photosynthesis occurs in chloroplasts, which are located mostly in the mesophyll of the leaf.
The mesophyll is the tissue in the middle of the leaf in a cross section; it is found in between the upper and lower epidermis.

Meso = middle

Phyll = leaf
CO
2
enters the mesophyll through pores in the epidermis called stomata (sing. stoma ), and O
2 exists.
A typical cell has 30 to 40 chloroplasts.

Chloroplasts are organelles bound by a double membrane.

Dimensions 24 μm by 4-7 μm
The inner membrane encloses a fluid-filled 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 (singular 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
2. TRACKING ATOMS THROUGH PHOTOSYNTHESIS
Photosynthesis is the conversion of light energy into chemical bond energy.
6CO
2
+ 12 H
2
O + Light energy

C
6
H
12
O
6
+ 6O
2
+ 6 H
2
O
A simplified equation: 6CO
2
+ 6 H
2
O + Light energy

C
6
H
12
O
6
+ 6O
2
3. THE SPLITTING OF WATER
In a series of experiments scientists were able to figure out where the hydrogen, oxygen and carbon atoms of the reactants were incorporated into the products.
They found out that water was split and the hydrogen atoms were incorporated into the sugar, and oxygen, a waste, was released into the atmosphere.
The carbon atoms from CO
2 molecules were incorporated into the sugar, and the oxygen went to the sugar and water.
See fig. 10.5
4. PHOTOSYNTHESIS AS A REDOX REACTION
Water is split and electrons are transferred along with hydrogen ion from the water to carbon dioxide reducing it to sugar, glucose.
CO
2
becomes reduced to glucose, C
6
H
12
O
6

Because electrons need to increase their potential energy as they mover from water to sugar, the process requires energy. It is endergonic.
H
2
O becomes oxidized to O
2
Photosynthesis consists of two processes each with multiple steps. These processes are known as the light reactions and the Calvin cycle .
In the light reactions, solar energy is converted to chemical energy.

The chloroplasts split water into hydrogen and oxygen.

The O
2
from water is released to the atmosphere and restores the oxygen consumed during respiration.

Light provides the energy to transfer electrons and hydrogen from water to the acceptor
NADP + .

NADP + is reduced to NADPH.

ATP is also generated during the light reaction by a process called photophosphorylation .

ATP and NADPH are the products of the light-dependent reactions.

The oxygen in CO
2
is incorporated into carbohydrates and water.

The carbon is reduced into carbohydrates by the addition of hydrogen atoms (electrons).
This process is called carbon fixation .
Light is a form of energy called electromagnetic energy or electromagnetic radiation.
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.
The atmosphere filters out a large amount of radiation, but allows visible light to pass through.
PHOTOSYNTHETIC PIGMENTS: THE LIGHT RECEPTORS
The spectrophotometer is used to measure the amount of light a substance has absorbed.
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
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.

The porphyrin ring is very similar to the heme portion of the red pigment hemoglobin in red blood cells.

The porphyrin ring of chlorophyll contains an atom of magnesium in its center.

The heme ring of hemoglobin contains iron in its center.

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, CH
3
, 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.
Chloroplasts also have accessory pigments called carotenoids , which are yellow and orange.
Carotenoids absorb light at ranges different from chlorophyll and broaden the spectrum of light that provides energy for photosynthesis.
Some carotenoids act as protectors that dissipate excess light that might damage chlorophyll.
To view the structural formula of the chlorophyll molecules visit: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chlorophyll.html
When a molecule absorbs one photon of light, one of its electrons is energized and moves into a higher energy level (excited state).
1. The excited electron may return to its original lower energy level (ground state) emitting a less energetic photon. This is called fluorescence. They may also release energy in the form of heat.
2. An electron acceptor molecule may accept the excited electron.
The only photons absorbed are those whose energy is equal to the difference between the ground state and excited state.
A particular compound absorbs photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum.
The energy of the absorbed photon is converted to the energy of an electron raised from the ground level to an excited state.
Photosystems
Photosystems are light harvesting complexes made pigments and proteins. The various pigments enable the photosystem to harvest light over a larger surface and a larger portion of the spectrum.
Each photosystem consists of:
1. Light harvesting complexes made of chlorophyll, carotenoids, proteins and other molecules.
2. A reaction center made of two special chlorophyll a molecules, P
700
and P
680
and a primary electron acceptor.
Two types of photosynthetic units present in chloroplasts make up photosystems known as photosystem I and photosystem II, in order of their discovery.
Photosystem I is a trimer: made of three subunits.

Each photosynthetic unit of photosystem I consists of highly ordered groups of ...

200 to 300 molecules of chlorophyll a,

small amounts of chlorophyll b,

carotenoids with proteins attached,

a special reaction-center made of two molecules of chlorophyll a called P

primary electron acceptor molecules.
700
.
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...

200 or more chlorophyll a,
 
-carotene attached to a protein,

a little chlorophyll b,

2 molecules of chlorophyll of P

primary electron acceptor
680 in a reaction-center.
The number 700 and 680 refer to the peak absorption with wavelengths of 700 and 680 nm.
Both P
700
and P
680
are identical chlorophyll a molecules. Their slight difference in absorption is due to the associated proteins that affects the electron distribution in the molecules.
For a detail description of the structure of the two photosystems visit: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.html#antenna
Light harvesting
When any antenna complex molecule traps a photon, it transfers its energy from pigment molecule to pigment molecule until it reaches a particular molecule of chlorophyll a located in the region of the complex called the reaction center.
The reaction-center molecule is the only one that can actually use the light energy.
Absorption of a photon causes a transition of the chlorophyll molecule from its ground state to its excited state.
At the reaction center, a specialized molecule called the primary electron accepto r captures an excited electron from the reaction-center chlorophyll a molecule.

A redox reaction
The primary electron acceptor traps the energized electron of chlorophyll a before it returns to its ground state in the chlorophyll a molecule.

Light energy is converted to chemical energy in the reaction center by a series of electron transfer reactions.

An isolated chlorophyll molecule will lose its energy as heat and fluorescent light as the electron drops down to its original ground state.
During the light reactions of photosynthesis there are two possible routes for the electron flow: cyclic and non-cyclic.
Non-cyclic electron flow
1. Many photons strike the photosystem molecules at the same time, with the energy of excited electrons being transferred toward the P
680
reaction center.
The energy boosts an electron to a higher energy level.
The excited electron is taken by the primary electron acceptor and P
680
becomes oxidized.
The oxidized chlorophyll a now is a very strong reducing agent. It has room for one electron.
2. The electrons lost by the P
680
are replaced with electrons extracted from water by a protein complex (enzyme) 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, which is required to split water.
Two water molecules produce oxygen, O
2
, and four protons, H + .
3. The energized electron is transferred to 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, Pc, a copper containing protein.
These electrons are eventually donated to P
700
in photosystem I.
Photosynthesis electron transport chain: Pheophytin → Pq → two cytochrome complexes →
Pc
4. When electrons move along the transport system, protons move from the stroma across the thylakoid membrane to the interior thylakoid space by chemiosmosis and ATP is synthesized.

This process is called non-cyclic photophosphorylation.
A proton gradient is created between the stroma and the interior thylakoid space. The greater proton concentration is in the interior thylakoid space.
When chloroplasts are illuminated, the pH in the stroma increases to 8, and the pH in the thylakoid space decreases to 5, a difference of three pH units.
The interior thylakoid space has about 1000 fold higher concentration of H + .
5. The low energy electron now reaches the end of the chain and is donated to P
700
, the chlorophyll a molecule in the reaction center of photosystem I. The chlorophyll a molecule becomes reduced.
Pigment molecules in the antenna complex of photosystem I absorb photons and pass their energy to the reaction center, P
700
, and an electron is energized (excited).
6. The excited electrons are passed to an acceptor molecule, P
430
, which transfers it to three iron-sulfur-containing proteins and then to ferrodoxin, Fd, a membrane-bound ironcontaining protein.
Ferrodoxin transfers the electron to FAD reducing it to FADH
2
.
FADH
2
then reduces NADP + to NADPH with the help of the enzyme NADP reductase. 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.
Second electron transport chain:
Fd → FADH
2
→ NADP + reductase → NADPH
The energy stored in NADPH will provide reducing power for the synthesis of carbohydrates.
Cyclic electron flow
Under certain conditions, the excite electrons flow back from ferrodoxin to the cytochrome complex and from there continues on to P
700
.
This flow generates ATP again.
The Calvin cycle is the next part of photosynthesis and requires more ATP than NADPH. The cyclic flow makes up the difference.
Follow the electrons; a summary:

e - removed from water (photolysis) are passed on to photosystem II, then to Pq, the cytochrome chain, photosystem I, P
430, 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 can be transferred to the electron transport chain between the two photosystems and back into photosystem I.
Similarities:
Chloroplasts and mitochondria generate ATP by the same mechanism of chemiosmosis.

An electron transport chain pumps electron across a membrane and a proton gradient is created.

Potential energy is stored in the form of H + gradient.
Both organelles have built in their membranes a similar ATP-synthase complex that couples the flow of electrons down the gradient with the phosphorylation of ADP.
Electron carriers, cytochromes and ATP synthase are very similar in both organelles.
Differences:
Mitochondria transfer chemical energy from food to ATP; chloroplasts convert radiant energy to chemical energy.

In mitochondria, the high-energy electrons are extracted from food molecules. Food molecules are oxidized.

Chloroplasts capture light energy, which is used to excite low-energy electrons extracted from water and drive them to the top of the chain.
The spatial organization of chemiosmosis is different in both organelles.

In the mitochondrion, the electrons are pumped out of the matrix into the intermembrane space. ATP is synthesized in the mitochondrial matrix.

In the chloroplast, the electrons are pumped into the thylakoid space from the stroma.
ATP is synthesized in the stroma.

THE CALVIN CYCLE (LIGHT INDEPENDENT REACTIONS OR DARK REACTIONS)
The Calvin cycle is a cyclic metabolic pathway.
Carbon enters the cycle in the form of carbon dioxide and leaves in the form of carbohydrate.
ATP is the source of energy and NADPH is the reducing power by adding high-energy electrons to the carbohydrate.
The carbohydrate produced directly from the Calvin cycle is G3P or glyceraldehyde-3phosphate .
Carbon fixation refers to the incorporation of CO
2
into an organic molecule.
During the carbon fixation reactions, ATP and NADPH are used to manufacture carbohydrate molecules from CO
2
.
Most plants use the Calvin cycle, also known as the C
3
cycle, to fix carbon.
Main steps of the Calvin cycle:
1.
: Three molecules of CO
2
from air combine with 3 molecules of ribulose 1,5-biphosphate, RuBP, 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 6 molecules of 3phosphoglycerate (3PGA) , the first stable compound formed in photosynthesis.
3.
: Each molecule of the 6 molecules of 3PGA receives a phosphate from ATP and become 1,3 biphosphoglycerate.
4. NADPH supplies electrons to reduce the carboxyl group of 1,3 biphosphoglycerate to the carbonyl group of glyceraldehyde 3 phosphate, G3P. G3P is the same sugar formed in glycolysis by the splitting of glucose.
5. The NADPH and ATP supply the energy to convert the 3PGA to 6 molecules of glyceraldehyde 3-phosphate (G3P), a 3-C sugar phosphate. An inorganic phosphate leaves the biphosphoglycerate molecules in the process.
6. One of the G3P exits the cycle to be used by the plant cell. The other five are must be recycle to regenerate the three molecules of RuBP
7.
2
: The other five 3-C molecules of glyceraldehyde 3-phosphate (G3P) are restructured in a series of complex reactions and become three 5-C molecules of RuBP, the sugar used in the first step of the Calvin cycle. Three ATP molecules are used in this process of regeneration of RuBP.

8. This leaves a net gain of one glyceraldehyde 3-phosphate, G3P, 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.
9. For the synthesis of one G3P, the Calvin cycle consumes a total of nine molecules of
ATP ad six molecules NADPH. The light reaction generates these molecules.
SUMMARY
Light-dependent reaction
12 H
2
O + 12 NADP + + 18 ADP + 18 P i

6 O
2
+ 12 NADPH + 18 ATP + 12 H +
Light-independent reaction
12 NADPH + 12 H + + 18 ATP + 6 CO
2

C
6
H
12
O
6
+ 12 NADP + + 18 ADP + 18 P i
+ 6 H
2
O
By canceling the common items on the above reactions we obtain the overall reaction of photosynthesis.
6 CO
2
+ 12 H
2
O

C
6
H
12
O
6
+ 6 O
2
+ 6 H
2
O
ALTERNATIVE MECHANISMS OF CARBON FIXATION
Dehydration is a danger for terrestrial plants.
Plants have to compromise between photosynthesis and transpiration.

Opening of the stomata allows CO
2
to enter the mesophyll of the leaf , the site of photosynthesis.

Opening of the stomata allows the escape of water from the leaf: transpiration .
The problem
In hot, dry days, most plants close their stomata in order to conserve water.
The concentration of CO
2
in the spaces in the mesophyll of the leaf begins to decrease as soon as the stomata close, and the concentration of O
2
begins to increase.
1) Photorespiration
The first organic product of carbon fixation is a 3-C compound called 3-phosphoglycerate.
Plants that produce this 3-C compound are called C
3
plants .
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 CO
2
and produces O
2
, which accumulates in the chloroplasts.
3. Rubisco is the enzyme responsible for binding CO
2
to RuBP to make PGA
(phosphoglyceraldehyde).
4. O
2
competes with CO
2 for the active binding site of Rubisco.
5. When O
2
binds to Rubisco some intermediates of the Calvin cycle are degraded to CO
2
.
Phosphoglycolate, a 2-C is produced with the consumption of a lot of energy.
6. Phosphoglycolate + O
2
+ ATP → PGA + CO
2
+ ADP + 2P i
7. Photorespiration occurs in chloroplasts, peroxisomes, and mitochondria of plant cells.
This process is called photorespiration because; it occurs in the presence of light, requires O
2 and produces CO
2
and H
2
O.
No ATP is produced during photorespiration and the removal of Calvin cycle intermediates reduces yield. No carbohydrates, food, are produced.

Ribulose-1,5bis -phosphate carboxylase/oxygenase
Photorespiration may be an evolutionary relic from a time when the concentration of oxygen in the atmosphere was very low and rubisco, in its evolution, did not develop a way of excluding O
2 from its active site.
Rubisco may be conserving this original affinity for O
2
, even though the concentration of O
2
in the atmosphere is now very high, 18%.
Rice, wheat and soybean may lose up to 50% of its yield in photorespiration.
For a good explanation of the photorespiration chemical reaction see: http://www.steve.gb.com/science/photorespiration.html
C
4
plants
It is also called the Hatch-Slack pathway .
CO
2
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 CO
2
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 CO
2
from entering the leaf interior.
 C
4
plants fix carbon in the form of a four-carbon compound before entering the Calvin cycle.
 C
4
plants have a unique leaf anatomy: bundle sheath cells and mesophyll cells .
 The bundle sheath cells form a tight ring around the veins and veinlets of the leaf.
The stomata are often closed in hot and dry climates when the maximum amount of light is available for photosynthesis.
1. C
4
reactions take place in the mesophyll of the cell.
2. The CO
2
is fixed into the 4-C oxaloacetate by the enzyme PEP carboxylase, which is converted to malate and requires NADPH.
3. Malate moves into the chloroplasts of the bundle sheath cell ,
4. CO
2
is removed from malate and pyruvate is produced.
5. The generated CO
2
then enters the Calvin cycle.
6. Pyruvate returns to the mesophyll where is converted to phosphoenolpyruvate, PEP .
The key component of the C
4
pathway is the enzyme PEP carboxylase , an enzyme found in the mesophyll cells of leaves.
Compared to rubisco, PEP carboxylase has a very high affinity for CO
2
even at very low concentrations.
C
4 must have the stomata partially open in order to conserve water. This condition causes the concentration of CO
2
decrease and that of O
2
increase.
PEP carboxylase catalyzes the reaction by which CO
2
reacts with the 3-C compound phosphoenolpyruvate to form oxaloacetate, a 4-C molecule.
When light is abundant, the rate of photosynthesis is limited by the concentration of CO
2
.
PEP carboxylase can fix carbon when rubisco cannot.
Because PEP carboxylase has such a high affinity for CO
2
, C
4
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.
Several thousand species of plants exhibit the C
4
metabolic pathway e. g. sugar cane, corn and crabgrass.

At lower temperatures and light intensities, C
3
plants are favored because they do not require as much energy to fix CO
2
C
4
and C
3
occur at different locations (cells) in the leaf.
THE CRASSULACEAN ACID METABOLISM OR CAM.
CAM is similar to C
4
pathway.
Desert plants use CAM in photosynthesis, e.g. succulents, cacti, euphorbs, orchids and members of the pineapple family.
The stomata close during the day to prevent excessive evaporation and open at night.
PEP carboxylase fixes CO
2
at night in the mesophyll of the cell when the stomata are open.
Oxaloacetate is formed which is converted to malate and stored in cell vacuoles.
When light is available, CO
2
is removed from malate by a decarboxylation reaction, and is made available to the Calvin cycle.
CAM and C
3
occur at different times within the same cell.

C
3
and C
4
plants show a spatial separation of metabolic steps.

C
3
and CAM plants show a temporal separation of metabolic steps.

C
4
and CAM plants show a temporal separation of metabolic steps.
See: http://www.steve.gb.com/science/photorespiration.html
Only the green cells of the plant are autotrophic. All other cells consume but do not produce food, e. g. roots, trunk wood and most parts of branches.
Most plants manage to produce more organic material than they need for respiration and growth.
The extra carbohydrates are stored in roots, stems, fruits, seeds, and leaves.
On the global scale, the estimate food produced by plants is 160 billion metric tons of carbohydrate per year.

1 metric ton = 1.1 tons or 1,000 kg or 2,200 pounds.
Photosynthesis provides the energy and building material for ecosystems.