What is Photosynthesis?
organisms require energy for their chemical reactions. These reactions
may be involved with reproduction, growth, or other activities.
Photosynthetic organisms such as plants use light energy to produce
carbohydrate (glucose). Glucose can be used at a later time to supply
the energy needs of the cell. Photosynthesis is therefore a process in
which the energy in sunlight is stored in the bonds of glucose for
Equation for Photosynthesis
This experiment was published by Jan Baptisa van Helmont in 1648:
took an earthenware vessel, placed in it 200 pounds of soil dried in an
oven, soaked this with rainwater, and planted in it a willow branch
weighing 5 pounds. At the end of five years, the tree grown from it
weighed 169 pounds and about 3 ounces. Now, the earthenware vessel was
always moistened (when necessary) only with rainwater or distilled
water, and it was large enough and embedded in the ground, and, lest
dust flying be mixed with the soil, an iron plate coated with tin and
pierced by many holes covered the rim of the vessel. I did not compute
the weight of the fallen leaves of the four autumns. Finally, I dried
the soil in the vessel again, and the same 200 pounds were found, less
about 2 ounces. Therefore 169 pounds of wood, bark, and root had arisen
from water only."
Most of the weight of the tree described in the
above experiment came from carbon dioxide and water. The equation for
photosynthesis shows that these compounds are used to produce glucose.
6CO2 + 6H2O + Energy --> C6H12O6 + 6O2
Equation for Cellular Respiration
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy
Light behaves as if it were composed of "units" or "packets" of energy that travel in waves. These packets are photons.
The wavelength of light determines its color. For example, The wavelength of red is
about 700 nm and the wavelength of blue light is about 470 nm.
Visible light is a part of a larger spectrum of radiation called the electromagnetic spectrum.
Ultraviolet radiation (UV) is dangerous to cells because it breaks chemical bonds.
are molecules that absorb light. When a photon of light strikes a
photosynthetic pigment, an electron in an atom contained within the
molecule becomes excited. Energized electrons move further from the nucleus of the atom.
The excited (energized) molecule can pass the energy to another molecule or release it in the form of light or heat.
Chlorophyll A is the main photosynthetic pigment in all organisms except bacteria. Other pigments called accessory pigments absorb slightly different wavelengths of light. The combination of all
of the pigments increases the range of colors that plants can use in
Accessory pigments include chlorophyll b and a group of pigments called carotenoids. They do not participate directly in photosynthetic reactions but are able to pass their energy to chlorophyll a.
photograph below is an elodea leaf X 400. Individual cells are clearly
visible. The tiny green structures within the cells are chloroplasts.
Click the image (above) to enlarge it.
Thylakoids are membranous disk-like structures that are stacked together in larger
structures that resemble stacks of coins. Chlorophyll and carotenoid
pigments are located in the membranes of the thylakoids. The thylakoid
membranes also contain the electron transport system.
The diagram below is a drawing of a chloroplast showing the thylakoids.
The fluid-filled space surrounding the grana is the stroma. Many enzymes needed in photosynthesis are found in the stroma.
2 Sets of Reactions
The light-dependent reactions require light.
These reactions occur in the thylakoid membrane.
They produce ATP and NADPH, which are needed to produce glucose in the light-independent reactions (below).
Notice how the equation for photosynthesis relates to the reactions shown in the diagram below.
6CO2 + 6H2O + Energy --> C6H12O6 + 6O2
Light-independent reactions occur in stroma of the chloroplast in light or dark conditions.
They function to reduce CO2 to glucose.
The closely packed pigment molecules and the reaction center form a light harvesting complex.
of light that are picked up by any of the pigment molecules in the
antenna pass their energy to nearby pigment molecules until it is
eventually passed to a special molecule of chlorophyll a called the reaction center.
The reaction center molecule becomes ionized and it loses its
electron to an electron acceptor. This electron will need to be
The antenna, the reaction center, and the electron
transport molecules make up a photosystem. There are two kinds of
photosystems in eucaryotes. The reaction center chlorophyll molecule of
photosystem I absorbs 700 nm light best and is therefore called P700. The reaction center of photosystem II absorbs 680 nm light best and is called P680.
Photosystem I evolved very early; photosystem II evolved later.
diagrams that follow are less magnified views of the chloroplast and
thylakoid shown in the diagram above. The antenna shown above is
represented by a single green circle below. Notice that there are two
photosystems and therefore two antennas. The blue circles represent the
electron transport system (discussed later).
During the light reactions, pigment molecules within the P680 antenna absorb a photon of light energy. The energy from that molecule
is passed to neighboring molecules and eventually makes its way to the
reaction center molecule as previously described. When the reaction
center molecule becomes excited, it loses its electron to an electron
The electron transport system is found embedded within the thylakoid membrane and functions in the
production of ATP. The system contains membrane-bound electron carriers
that pass electrons from one carrier to another. As a result of gaining
an electron (reduction), the first carrier of the electron transport
system gains energy. It uses some of the energy to pump H+ into the thylakoid.
The carrier then passes the electron to the next carrier. Because it used some energy to pump H+, it has less energy (reducing capability) to pass to the next H+ pump.
This carrier uses some of the remainder of the energy to pump more H+ into the thylakoid.
The electron is passed to the next carrier which also pumps H+.
The electron transport system functions to create a concentration gradient of H+ inside the thylakoid. The concentration gradient of H+ is used to synthesize ATP.
ATP is produced from ADP and Pi when hydrogen ions pass out of the thylakoid through ATP synthase. This method of synthesizing ATP by using a H+ gradient in the thylakoid is called photophosphorylation.
At this point, the electron has little reducing capability (little energy is left). It is passed to the P700 antenna.
A pigment molecule in the P700 antenna absorbs a photon of solar energy.
The energy from that molecule is passed to neighboring molecules within the antenna. The energy is eventually passed to the reaction center of this antenna.
As a result of being energized, the P700 reaction center loses the electron to an electron acceptor.
The acceptor passes it to NADP+, which becomes reduced to NADPH. According to the following equation, NADP+ has the capacity to carry two electrons. NADP+ + 2e- + H+ --> NADPH
The electron transport system and photophosphorylation in the chloroplast is similar to the system found in the mitochondria to produce ATP during cellular respiration.
The diagram below is a summary of the light reactions. High-energy components of the system are shown nearer the top of the
diagram. The red arrows show the path of electrons from water to NADP+ to form NADPH.
The electron that was lost from the antenna complex of photosystem I is replaced by splitting water (see diagram above).
In the light reactions, electrons move one way from water to NADPH and the energy of sunlight is used to produce ATP.
Calvin Cycle (Light-independent Reactions)
The products of the light reactions (ATP and NADPH) are used to reduce CO2 to carbohydrate in the Calvin cycle.
The Calvin cycle produces a 3-carbon sugar called
glyceraldehyde 3-phosphate (G3P). G3P is be used to synthesize glucose
and other organic compounds needed by the cell.
The words "CO2 fixation" refer to the attachment of CO2 to an organic compound: each CO2 binds to a 5-carbon ribulose bisphosphate (RuBP) molecule.
Carbon dioxide fixation is catalyzed by RuBP carboxylase (rubisco).
diagram below shows that three carbon dioxide molecules are combined with three RuBP molecules to produce three molecules of a 6-carbon compound.
Each of the carbon dioxide molecules contains one carbon atom and each RuBP molecule contains 5 for a total of 6 carbons.
makes up 20 - 50% of the protein in chloroplasts. It acts very slowly,
catalyzing 3 molecules per second. This compares to 1000 per second for
typical enzymatic reactions. Large quantities are needed to compensate
for its slow speed. It may be the most abundant protein on earth.
of the resulting 6-carbon molecules formed by carbon dioxide fixation
(above) splits into two 3-carbon molecules (phosphoglycerate [PGA]) for
a total of six PGA molecules (below).
PGA is reduced to form G3P (glyceraldehyde 3-phosphate). The G3P molecules also have three carbon atoms each. This reaction requires energy from ATP and electrons from NADPH.
G3P is used to form glucose and other organic compounds. For each G3P used, five G3P
remaining five G3P (3 carbons each, total = 15 carbons) can therefore be reassembled into 3 RuBP (5 carbons each, total = 15 carbons). This
rearrangement requires phosphorylation from 3 ATP.
Below: Summary of the Calvin Cycle
Two G3P are used to produce one glucose molecule. For each six CO2 molecules that enter the cycle one glucose molecule is produced.
About 30% of the energy available in ATP and NADPH is finally present in the glucose produced.
Stomata and CO2 Concentration
Stomata (singular stoma) are microscopic openings on the undersurface of leaves
that allow gas exchange and water evaporation from inside the leaf.
Because dehydration can be a serious problem, the stomata close when
the plant is under water stress. When closed, CO2 needed for the Calvin cycle cannot enter.
A stoma can be seen in the diagram of a leaf below.
Plants that undergo photosynthesis as described above are called C3 plants because the end result of CO2 fixation is two 3-carbon molecules (PGA). In C3 plants, CO2 is fixed to RuBP to form a 6 carbon compound by the enzyme rubisco.
When the concentration of CO2 is low, oxygen will bind to the active site of rubisco. The
resulting reactions do not produce sugar and they consume ATP. In
addition, organic compounds that are involved in photosynthesis are
|During photosynthesis, CO2 is fixed by the enzyme rubisco.||During hot, dry conditions, the level of CO2 drops and rubisco adds oxygen to organic molecules.|
The leaves of C4 plants are structured differently than those of C3 plants. Photosynthesis occurs within the mesophyll cells in C3 plants, which form a dense layer on the upper surface of the leaf and a
spongy layer on the lower surface. Bundle-sheath cells surrounding the
veins are not photosynthetic.
Below: A cross-section of a C3 leaf.
Dense layers of mesophyll cells surround the bundle-sheath cells of C4 plants. Both the bundle-sheath cells and the rings of mesophyll cells are photosynthetic.
Below: A cross-section of a C4 leaf.
C4 plants have a pathway that allows them to fix CO2 at lower concentration than C3 plants. This gives them an advantage during hot, dry conditions because they can keep the stomata closed for longer periods of time to prevent dehydration before photorespiration occurs.
In C4 plants, CO2 is fixed in the mesophyll cells by PEP carboxylase, an enzyme that is
more efficient than rubisco. The resulting organic compound is
transported to the mesophyll cells where the CO2 is released for the Calvin Cycle. The movement of CO2 into the bundle sheath cells maintains a higher concentration than would otherwise be possible.
CO2 fixation occurs when CO2 is bonded to a 3-carbon compound to produce a 4-carbon compound. This enzyme functions well even at extremely low CO2 concentrations because it is unaffected by oxygen. This CO2 fixation occurs within the mesophyll cells that surround the bundle
sheath cells. The 4-carbon compound then moves into the bundle sheath
and releases the CO2, thus raising the level of CO2 and preventing photorespiration.
The enzyme that fixes CO2 in the mesophyll cells is PEP carboxylase (pepco).
This process is called C4 photosynthesis because the product of carbon fixation is a 4 carbon compound.
The diagram below summarizes the path of CO2 in C4 plants.
Efficiency of C4 Photosynthesis
This mechanism requires extra ATP but under hot, dry conditions C4 plants are two to three times more efficient than C3 plants.
In moderate weather, C3 plants are at an advantage.
CAM (crassulacean-acid metabolism) photosynthesis is found in most desert
plants, particularly the succulents (plants that store water in thick,
CAM plants keep their stomata closed during the day to conserve water. At night, the stomata open to allow CO2 to enter. CO2 is incorporated into organic acids which are then stored within the mesophyll cells. During the day, CO2 is released from the organic acids to supply the Calvin Cycle.
CAM plants are 5 to 7 times more efficient than C4 plants.
The diagram below summarizes the path of CO2 in CAM plants.
Identify each component in the two diagrams below.
How many carbon atoms does each molecule have in the diagram below? The molecules are represented by letters.
Identify each component (except C and F) in the diagram below.
Fill in the table below.
|Light Reactions||Light-Independent Reactions|
|Inputs|| || |
|Produced|| || |
Light-Dependent Reactions - Draw details of the light-dependent reactions.
Light-Independent Reactions - Draw details of the light-independent reactions (Calvin Cycle).