Photosynthesis may be broadly defined as the utilization of solar
energy by green plants and certain green and purple bacteria for the
synthesis of organic compounds. The dominant form of photosynthesis
on this planet is plant photosynthesis, carried on by unicellular and
multicellular green plants, in which CARBON DIOXIDE is the main
source of carbon and WATER is the source of the hydrogen atoms. The
main products of plant photosynthesis are carbohydrates and oxygen gas.
light
6CO2 + 12 H2O ---------------------> C6H12O6 + 6 H2O + 6 O2
chlorophylls a,b
Photosynthesis is the most important chemical process on our planet. The products of photosynthesis sustain all life by providing food for unicellular and multicellular organisms, including humans, and by continually replenishing the atmosphere with the oxygen that we breathe.
Plant photosynthesis may be divided into two phases:
(a) a PHOTOCHEMICAL PHASE in which OXYGEN is set free and light
energy is trapped and converted into chemical energy; and
(b) a CHEMICAL PHASE in which the chemical energy released by phase (a) is utilized to convert carbon dioxide into organic
compounds by a series of reactions that are independent of
light.
The ability of the living cell to utilize energy depends on the presence within the cell of a number of highly specialized molecules. These include chlorophyll and other light capturing pigments, cytochromes, and other electron carriers, and ATP, which is the energy transfer molecule involved in almost all the energy transactions of the cell.
The first step in the conversion of light energy to chemical energy is the ABSORPTION OF LIGHT by a PIGMENT system. In all photosynthetic cells (except the photosynthetic bacteria) the pigment system includes CHLOROPHYLL A, and it now agreed that this one pigment is essential for photosynthesis of the type carried out by plants. CHLOROPHYLL A occurs in all photosynthe-tic eukaryotes and in the prokaryotic cyanobacteria.
In the vascular plants, bryophytes, green algae, and euglenoid algae, CHLOROPHYLL B is also found. CHLOROPHYLL B is an accessory pigment, and shares with CHLOROPHYLL A the ability to absorb light energy and produce in the molecule some sort of excited state. When this occurs in a molecule of CHLOROPHYLL B, the excited molecule transfers its energy to a molecule of CHLORO-PHYLL A, which then proceeds to transform it into chemical energy during the course of photosynthesis.
Since CHLOROPHYLL B absorbs light of a different wavelength from
CHLOROPHYLL A, it extends the range of light that can be used for photosynthesis. In the leaves of green plants, CHLOROPHYLL B generally constitutes about 25% of the total chlorophyll content.
Two other classes of accessory pigments are involved in the capture of light energy in photosynthesis - the CAROTENOIDS. The carotenoids are red, orange, or yellow, fat-soluble pigments found in all chloroplasts. Carotenoids prevent the oxidation of chlorophyll caused by light; thus their most important role in the chloroplast seems to be a protective one. Carotenoids that do NOT contain oxygen are called CAROTENES and those that do contain oxygen are the XANTHOPHYLLS. Not being water soluble, carotenoids are not found free in the cytoplasm, but, like chlorophylls, are bound to proteins within the plastids (on thylakoid membranes).
The structure which houses these light absorbing pigments is the CHLOROPLAST. In higher plants the chloroplasts are lens or disc-
shaped, 2-6F in diameter, 0.5-1.0F thick. Chloroplasts may be as much as 20% of the total leaf volume. A single leaf cell may contain 40-50 chloroplasts. Chloroplasts seem capable of independent movement within the cytoplasm and orient their surfaces in relation to the light.
The chloroplast is surrounded by a double membrane and internally there are a series of membranes called the LAMELLAE. These are joined at the ends to form parallel membrane pairs called THYLA-KOIDS. In places the thylakoids are stacked together in structures called GRANA. All of the chlorophyll and the carotenoids are found in association with the lamella and grana.Light energy is trapped in the grana. The remainder of the chloroplast is a darkish, somewhat granular solution called the STROMA. The reactions in which sugar is formed from carbon dioxide and water, using energy from the grana, takes place in the stroma. The chloroplasts also contain their own DNA and nucleic acids as well as RIBOSOMES and are capable of at least semi-autonomous replication.
ABSORPTION VS. ACTION SPECTRA
When light falls on an object, certain colors are absorbed and others are reflected. A red rose absorbs green and blue light but reflects into our eyes the wavelength we interpret as "red." Similarly, chlorophyll is green because it absorbs blue and red strongly but not green. Green is reflected and this is what we see. Obviously, if chlorophyll is instrumental in capturing light energy, it must first absorb that energy.
Does chlorophyll absorb light of those colors or wavelengths which are most effective in photosynthesis? This can be answered by determining exactly WHAT WAVELENGTHS are absorbed by CHLOROPHYLL (the ABSORPTION SPECTRUM) and comparing them with the wavelengths at which chloroplasts PHOTOSYNTHESIZE best (the
ACTION SPECTRUM). Since there is a close correspondence between the absorption spectrum of chlorophyll and the action spectrum for photosynthesis, it seems clear that chlorophylls are the major photosynthetic pigments of green plants.
Evidence for two stages of photosynthesis was presented in 1905 by the English plant physiologist, F.F. Blackman. He measured the individual and combined effects of changes in light intensity and temperature on the rate of photosynthesis. Based on his experiments, Blackman came to the following conclusions:
1. there is a set of LIGHT DEPENDENT reactions which are
TEMPERATURE INDEPENDENT; the rate of these reactions in
the dim to moderate light range could be accelerated by
increasing the light intensity, but they were not accel-
erated by increase in temperature
2. there is a second set of reactions that are TEMPERATURE-
DEPENDENT
Both sets of reactions are required for photosynthesis.
RATE CHANGES
When the rate of one set of reactions increases, the rate of the entire process increases until the second set of reactions begins to hold back the first (becomes RATE-LIMITING). Photosyn-thesis was shown to have a light-dependent stage, the "light reactions," as well as a light-independent stage, the "dark reactions." The so-called dark reactions occur in the light. They require the products of the "light" reaction. Light is NOT directly involved in the "dark" reactions.
At low light intensities, light is limiting, temperature and carbon dioxide concentration are not.
At high light intensities, the pigment molecules are saturated and the chemical reactions are limited due to changes in temperature and carbon dioxide concentration.
Light independent ("dark") reactions increase in rate as tempera-ture increases until about 30-40E C (dependent upon plant in question), after which the rate begins to decrease.Blackman concluded that these (dark) reactions are controlled by enzymes since this is the way enzymes respond to temperature.
In the light dependent phase of photosynthesis, light energy is used to form ATP from ADP and to reduce electron carrier molecules.
In the light independent phase of photosynthesis, the energy products of the first stage are used to reduce carbon from carbon dioxide to a simple hexose sugar.
The conversion of carbon dioxide into organic compounds is known as CARBON FIXATION.
THE PHOTOSYSTEMS
Chlorophyll and other pigment molecules are embedded in the thylakoids in organizational units called PHOTOSYSTEMS. Each photosystem is an assembly of 250-400 pigment molecules.
All pigments within the photosystem are capable of absorbing photons (particles of light) but only 1 chlorophyll molecule per photosystem can use the energy in photochemical reactions. This single chlorophyll molecule is known as the REACTION CENTER of the photosystem. The other chlorophyll molecules are called ANTENNA PIGMENTS.
Light energy absorbed by pigment molecules anywhere in the system is transferred to the reaction center which is a special form of CHLOROPHYLL A. When this CHLOROPHYLL A molecule absorbs the energy, one electron is boosted to a higher energy level and transferred to an acceptor molecule to initiate electron flow. The chlorophyll molecule becomes oxidized and positively charged at this point.
There are two different photosystems operating in most plants - Photosystem I (P700) and Photosystem II (P680). P700 and P680 represent optimal absorption peaks in nanometers. The photosystems probably evolved independently, with photosystem I evolving first.
MODEL OF THE LIGHT DEPENDENT REACTIONS
Light energy enters P680 of Photosystem II causing an electron pair of the reaction center molecule to become energized. This energized electron pair move to a higher energy level and are transferred to an acceptor molecule. P680 replaces its electrons by extracting them from a water molecule. Water is dissociated into H+ (protons) and oxygen gas. This light dependent splitting of water is called PHOTOLYSIS.
Electrons then pass downhill along an electron transport chain which includes two cytochromes, to Photosystem I.
Primary Electron Acceptor (Q or C550)
8 `
8 `
8 2e- Cytochrome b3
8 `
8 ADP + Pi ` 2e-
8 9 `
8 2e- 7 2H+ + O ATP `
P680 8 Cytochrome f
HOH `
PHOTOSYSTEM II PHOTOSYSTEM I
As electrons move along the chain ATP is formed. The process is known as PHOTOPHOSPHORYLATION.
In Photosystem I, light energy boosts electrons from P700 to the electron acceptor (X or P430) where they pass downhill to the coenzyme NADP. Hydrogen ions from the photolysis of H2O are used to reduce NADP 6 NADPH2. P700 becomes oxidized.
NADPH2 provides energy directly to biosynthetic processes in the cell. Electrons removed from Photosystem I are replaced by electron from Photosystem II.
Thus, in light, there is a continuous flow of electrons from water to Photosystem II to Photosystem I to NADP. The overall process involving two Photosystems is called NON CYCLIC PHOTO-PHOSPHORYLATION - it is a unidirectional electron flow.
Based on 12 pairs of electrons from water to NADP, the energy harvest is 12 ATP and 12 NADPH2. To generate 1 molecule of NADPH2 four photons of light must be absorbed, 2 by Photosystem I and 2 by Photosystem II.
Primary Electron Acceptor
8 `
8 Ferredoxin Reducing System
8 2e- `
8 Ferredoxin
8 `
8 Flavoprotein
8 `
8 2NADP+
8 _ `
8 _ 2NADPH
P700 (Photolysis) 2H+ `
PHOTOSYSTEM I "DARK" REACTIONS
Photosystem I can work independently of Photosystem II. Electrons go from the reaction center to the electron acceptor downhill back to the reaction center. ATP is generated in this process. This is CYCLIC PHOTOPHOSPHORYLATION and is used by more primitive plants.
LIGHT INDEPENDENT "DARK" REACTIONS
Energy generated from the light reactions is used to reduce carbon. Carbon is available as carbon dioxide. A reaction pathway known as the Calvin or Calvin-Benson Cycle and is like the Krebs Cycle because by the end of each turn of the cycle, the starting compound has been regenerated.
Six revolutions of the cycle with the introduction of 6 CO2 are needed to produce a 6 carbon sugar.
Calvin-Benson Cycle (not balanced)
6 ATP
Ribulose 5-P ------------------------> Ribulose 1,5 Di or BiP
phosphopentokinase
Diphosphoribulose
Rb 1,5 DiP + 3 CO2 --------------->2 carboxyl 3ketoributol 6,5DiP
carboxylase
12 ATP
----> 2 Phosphoglyceric Acid (PGA) ------------------->
12 NADPH (from Photosystem I)
----> Phosphoglyceraldehyde (PGAL) -------> transformed as needed
10 PGAL molecules are used for the regeneration of Ribulose 5-P
2 PGAL are the net gain and are converted into hexose sugar
Overall Reaction on a Per Glucose Basis
6CO2 + 12 NADP+ + 12 H2O + 18 ATP --->
C6H12O6 + 12 NADP+ 18 ADP + 18 Pi + 6 H2O + 6 O2
Regeneration of the Carbon Dioxide Acceptor (Ribulose 5-P)
C3 --------> C2 ------------------> C5
_
_
C3 ----- C6
_ `
_ `
C3 `
`
C3 -------------C4 ----> C7 ------> C5
`
C3 ------------------------C2 ----> C5
The Fate of PGAL
The principal end product of photosynthesis is PGAL (phosphoglyceraldehyde or glyceraldehyde 3-P). It is a food and forms in the grana. It does not accumulate to any great extent because of 3 main fates:
1. used directly as a nutrient in the cell which produced it
2. it may be packaged for export to other cells
3. it may be packaged for storage
As a nutrient, PGAL is usable immediately in respiration. The reduction of PGA to PGAL is virtually the exact opposite of a principal oxidation step in glycolysis, the one in which PGAL is oxidized to PGA. In that oxidation step ATP and NAD-H is a byproduct, and in the present reduction ATP is used and NADP-H is a raw material.
PGAL is also usable directly as a building material and can form chlorophyll, NADP, or Ribulose 5-P or any of the enzymes taking part in carbon dioxide fixation.
More PGAL is usually manufactured than needed - the bulk is available for export to nonphotosynthesizing cells of the plant. PGAL is too reactive to be exported as such but is transformed into sugars such as sucrose, glucose, or fructose. Sugar is often the primary endproducts of photosynthesis.
Storage occurs primarily in roots and stems, some in leaves. Stored materials should take up as little space as possible, and should be relatively unavailable (unreactive) in constant activities. Storage fats are sometimes manufactured instead of or in addition to carbohydrate reserves (olive oil, castor oil, peanut oil, coconut oil). The larger the storage unit, starches for example, the smaller it is in terms of individual units because of the water loss during dehydration synthesis.
Plant that use ONLY the Calvin-Benson Cycle are known as C3 plants. The Calvin-Benson Cycle is accompanied by PHOTORESPIRATION - a process that consumes oxygen and releases carbon dioxide in the presence of light. It involves the oxidation of carbohydrates but no yield of ATP or NADH2. As much as 50 % of photosynthetically fixed carbon may be reoxidized to carbon dioxide during photorespiration. This reduces the efficiency of C3 carbon dioxide fixation.
The Calvin-Benson Cycle is not the only carbon-fixation pathway used in "dark" reactions. In some plants carbon dioxide is fixed to phosphoenolpyruvate (PEP) rather than Ribulose 1,5 BiP. These plants are known as C4 plants and the pathway is known as the Hatch-Slack Pathway. Some of the plants that use this pathway are corn, sugar cane, sugar beets, and crabgrass. These plants make better use of carbon dioxide, have photorespiration near zero, reduced water loss, and are adapted to high light and temperature and dryness. In short, they save more of the photosynthate (sugar) they make.
