The term photosynthesis means literally “synthesis using light.” Photosynthetic organisms use solar energy to synthesize carbon compounds that cannot be formed without the input of energy. More specifically, light energy drives the synthesis of carbohydrates from carbon dioxide and water with the generation of oxygen.
6 CO2 + 6 H2O C6H12O6 + 6 O2 equation 1
Carbon dioxide Water Carbohydrates Oxygen
We shall begin the discussion of photosynthesis by looking at:
- the role of light,
- the structure of the photosynthetic apparatus, and
- the processes that begin with the excitation of chlorophyll by light and culminate in the synthesis of ATP and NADPH.
Photosynthesis takes place in two phases: the light reaction phase otherwise called the thylakoid reaction phase and the so-called dark reaction or carbon-fixation reaction phase.
The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialized light-absorbing green pigments, the chlorophylls. In photosynthesis, the plant uses solar energy to oxidize water, thereby releasing oxygen, and to reduce carbon dioxide, thereby forming large carbon compounds, primarily sugars. The complex series of reactions that culminate in the reduction of CO2 include the thylakoid reactions and the carbon-fixation reactions.
The thylakoid reactions of photosynthesis take place in the specialized internal membranes of the chloroplast called thylakoids. The end products of these thylakoid reactions are the high-energy compounds ATP and NADPH, which are used for the synthesis of sugars in the carbon fixation reactions. These synthetic processes take place in the stroma of the chloroplasts, the aqueous region that surrounds the thylakoids.
In the chloroplast, light energy is converted into chemical energy by two different functional units called photosystems. The absorbed light energy is used to power the transfer of electrons through a series of compounds that act as electron donors and electron acceptors. The majority of electrons ultimately reduce NADP+ to NADPH and oxidize H2O to O2. Light energy is also used to generate a proton motive force across the thylakoid membrane, which is used to synthesize ATP.
ROLE OF LIGHT IN PHOTOSYNTHESIS
Light have the characteristics of both a wave and a particle. A wave is any disturbance in motion. A wave is characterized by a wavelength, denoted by the Greek letter lambda (λ), which is the distance between successive wave crests (Figure 6.1). The frequency, v is the number of wave crests that pass an observer in a given time. A simple equation relates the wavelength, the frequency,
and the speed of any wave: Fig 6.1 Wave nature of light. Electric vectors (E) and
magnetic vectors (H) oscillate at 90◦ to each other.
c = vλ equation 6.2
where c is the speed of the wave—in the present case, the speed of light (3.0 × 108 m s–1). The light wave is a transverse (side-to-side) electromagnetic wave, in which both electric and magnetic fields oscillate perpendicularly to the direction of propagation of the wave and at 90° with respect to each other.
Light is also a particle, which we call a photon. Each photon contains an amount of energy that is called a quantum (plural quanta). The energy content of light is not continuous but rather is delivered in these discrete packets, the quanta. The energy (E) of a photon depends on the frequency of the light according to a relation known as Planck’s law:
E = hv equation 6.3
where h is Planck’s constant (6.626 × 10–34 J s). Sunlight is like a rain of photons of different frequencies. Our eyes are sensitive to only a small range of frequencies—the visible-light region of the electromagnetic spectrum (Figure 6.2). Light of slightly higher frequencies (or shorter wavelengths) is in the ultraviolet region of the spectrum, and light of slightly lower frequencies (or longer wavelengths) is in the infrared region.
An absorption spectrum (plural spectra) displays the amount of light energy taken up or absorbed by a molecule or substance as a function of the wavelength of the light. More importantly, an absorption spectrum is like a fingerprint of the molecule. Every light-absorbing molecule has a unique absorption spectrum that is often a key to its identification The absorption spectrum for a particular substance in a non-absorbing solvent can be determined by a spectrophotometer.
Fig 6.2 The electromagnetic spectrum. Visible radiation, or light, represents only a very small portion of the total electromagnetic spectrum.
Measurement of light in Photobiology
There are three parameters of primary concern when describing light. The first is light quantity— how much light has the plant received? The second is the composition of light with respect to wavelength, known as light quality, spectral composition, or spectral energy distribution (SED). The third factor is timing. What are the duration and periodicity of the light treatment?
The measure of light quantity most widely accepted by plant photo-biologists is based on the concept of fluence. Fluence is defined as the quantity of radiant energy falling on a small sphere, divided by the cross-section of the sphere. Since light is a form of energy that can be emitted or absorbed as discrete packets or photons, fluence can be expressed in terms of either the number of photons or quanta (in moles, mol) or the amount of energy (in joules, J). Photon fluence (units = mol m-2) refers to the total number of photons incident on the sphere while energy fluence (units = J m-2) refers to the total amount of energy incident on the sphere. The corresponding rate terms are photon fluence rate (units = mol m-2 s-1) and energy fluence rate (units = J m-2 s-1 or W m-2). The term irradiance is frequently used interchangeably with energy fluence rate, although in principle the two are not equivalent. Irradiance refers to the flux of energy on a flat surface rather than a sphere.. The range of light that are limited to the portion of the spectrum between 400 nm and 700 nm is broadly defined as photosynthetically active radiation (PAR).
The term ‘‘light quality’’ refers to spectral composition and is usually defined by an emission or incidence spectrum. SED is measured with a spectroradiometer, an instrument capable of measuring fluence rate over narrow-wavelength bands.
How does chlorophyll react to absorbed light energy?
Absorption of light by a pigment molecule is a rapid, photo-physical, electronic event, occurring within a femtosecond the energy of the absorbed photon is transferred to an electron in the pigment molecule during that extremely short period of time. The energy of the electron is thus elevated from a low energy level, the ground state, to a higher energy level known as the excited, or singlet, state. In the higher excited state, chlorophyll is extremely unstable, very rapidly gives up some of its energy to the surroundings as heat, and enters the lowest excited state, where it can be stable for a maximum of several nanoseconds (10–9 s).
In the lowest excited state, the excited chlorophyll has four alternative pathways for disposing of its available energy.
- Excited chlorophyll can re-emit a photon and thereby return to its ground state—a process known as fluorescence.
- The excited chlorophyll can return to its ground state by directly converting its excitation energy into heat, with no emission of a photon.
- Chlorophyll may participate in energy transfer, during which excited chlorophyll transfers its energy to another molecule.
- A fourth process is photochemistry, in which the energy of the excited state causes chemical reactions to occur.
The photochemical reactions of photosynthesis are among the fastest known chemical reactions. This extreme speed is necessary for photochemistry to compete with the three other possible reactions of the excited state just described.
The chemical reactions of photosynthesis are complex. In fact, at least 50 intermediate reaction steps have now been identified, and undoubtedly additional steps will be discovered. An early clue to the chemical nature of the essential chemical process of photosynthesis came in the 1920s from investigations of photosynthetic bacteria that did not produce oxygen as an end product. From his studies on these bacteria, C. B. van Niel concluded that photosynthesis is a redox (reduction–oxidation) process. As a microbiologist, van Niel was interested in the photosynthetic sulfur bacteria that use hydrogen sulfide (H2S) as a reductant in place of water. Consequently, unlike algae and higher plants, the photosynthetic sulfur bacteria do not evolve oxygen. Instead, they deposit elemental sulfur according to the following equation:
CO2 + 2H2S (CH2O) + 2S + H2O equation 6.4
The reaction in equation 7.3 can also be written as two partial reactions:
2H2S 4e- + 4H+ + 2S equation 6.5
CO2 + 4e- + 4H+ (CH2O) + H2O equation 6.6
Equations 6.5 and 6.6 describe photosynthesis in the purple sulfur bacteria as a straightforward oxidation–reduction reaction. C. B. van Niel adopted a comparative biochemistry approach and argued that the mechanisms for oxygenic (i.e., oxygen-evolving) photosynthesis in green plants and anoxygenic (i.e., non-oxygen-evolving) photosynthesis in the sulfur bacteria both followed the general plan:
2H2A + CO2 2A + (CH2O) + H2O equation 6.7
In this equation, ‘A’ can represent either oxygen or sulfur, depending on the type of photosynthetic organism. According to equation 6.7, the O2 released in oxygenic photosynthesis would be derived from the reductant water.
From all this reaction, it can be argued that water is the source of evolved oxygen in oxygenic photosynthetic organisms. This conclusion has been confirmed, and it has served as a fundamental concept on which all subsequent research on photosynthesis has been based
Based on these results, photosynthesis can be viewed as a photochemical reduction of CO2. The energy of light is used to generate strong reducing equivalents from H2O—strong enough to reduce CO2 to carbohydrate. These reducing equivalents are in the form of reduced NADP+ (or, NADPH + H+). Additional energy for carbon reduction is required in the form of ATP, which is also generated at the expense of light. The principal function of the light-dependent reactions of photosynthesis is therefore to generate the NADPH and ATP required for carbon reduction. This is accomplished through a series of reactions that constitute the photosynthetic electron transport chain
An action spectrum depicts the magnitude of a response of a biological system to light, as a function of wavelength. An action spectrum is a graph that shows the effectiveness of light in inducing a particular process plotted as a function of wavelength. The underlying assumption is that light most efficiently absorbed by the responsible pigment will also be most effective in driving the response.
In other words, the action spectrum for a light-dependent response should closely resemble the absorption spectrum of the pigment or pigments that absorb the effective light (Fig. 6.4). A comparison of an action spectrum with the absorption spectra of suspected pigments can therefore provide useful clues to the identity of the pigment responsible for a photosensitive process.
Some of the first action spectra were measured by T. W. Engelmann in the late 1800s (Figure 6.5). Engelmann used a prism to disperse sunlight into a rainbow that was allowed to fall on an aquatic algal filament. A population of O2-seeking bacteria was introduced into the system. The bacteria congregated in the regions of the filaments that evolved the most O2. These were the regions illuminated by blue light and red light, which are strongly absorbed by chlorophyll. Action spectra were very important for the discovery of two distinct photosystems operating in O2-evolving photosynthetic organisms.
Antenna complex and reaction centre complex
Antenna complex are majority of pigments which serve the role of collecting light and transferring the energy to the reaction centre. The reaction centres are where chemical oxidation and reduction reactions leading to long-term energy storage take place (Fig. 6.6). The reaction centers and most of the antenna complexes are integral components of the photosynthetic membrane. In eukaryotic photosynthetic organisms, these membranes are found within the chloroplast; in photosynthetic prokaryotes, the site of photosynthesis is the plasma membrane or membranes derived from it.
Fig. 6.6 Basic concept of energy transfer during photosynthesis. Many pigments together serve as an antenna, collecting light and transferring its energy to the reaction center, where chemical reactions store some of the energy by transferring electrons from a chlorophyll pigment to an electron acceptor molecule.
Fig 6.4 A typical action spectrum for leaf photosynthesis (A) compared with the absorption spectrum (B) of a pigment extract from a leaf containing primarily chlorophyll.
Fig. 6.5 Schematic diagram of the action spectrum easurements by T. W. Engelmann.
The graph shown in Fig. 6.7 permits us to calculate another important parameter of the light reactions of photosynthesis, the quantum yield. The quantum yield of photosynthesis (Ф ) is defined as follows:
Ф Number of photochemical productTotal number of quanta absorbed s
In the linear portion (low light intensity) of the curve, an increase in the number of photons stimulates a proportional increase in oxygen evolution. Thus the slope of the curve measures the quantum yield for oxygen production. The quantum yield for a particular process can range from 0 (if that process does not respond to light) to 1.0 (if every photon absorbed contributes to the process). In functional chloroplasts kept in dim light, the quantum yield of photochemistry is approximately 0.95, the quantum yield of fluorescence is 0.05 or lower, and the quantum
yields of other processes are negligible. The vast
|majority of excited chlorophyll therefore lead to photochemistry.
|molecules||Fig. 6.7. Relationship of oxygen production to flash energy, the first evidence for the interaction between the antenna pigments and the reaction center. At saturating energies, the maximum amount of O2 produced is 1 molecule per 2500 chlorophyll molecules.|
Although the photochemical quantum yield under optimum conditions is nearly 100%, the efficiency of the conversion of light into chemical energy is much less. The energy needed to drive the photosynthetic reaction comes from light. Here’s a simpler form of Equation
CO2 + H2O (CH2O) + O2 equation 6.8
where (CH2O) is one-sixth of a glucose molecule. About nine or ten photons of light are required to drive the reaction. If red light of wavelength 680 nm is absorbed, the total energy input is 1760 kJ per mole of oxygen formed. This amount of energy is more than enough to drive the reaction in Equation 6.8, which has a standard state free-energy change of +467 kJ mol–1. The efficiency of conversion of light energy at the optimal wavelength into chemical energy is therefore about 27%, which is remarkably high for an energy conversion system. Most of this stored energy is used for cellular maintenance processes; the amount diverted to the formation of biomass is much less.
There is no conflict between the fact that the photochemical quantum efficiency (quantum yield) is nearly 1 (100%) and the energy conversion efficiency is only 27%. The quantum efficiency is a measure of the fraction of absorbed photons that engage in photochemistry; the energy efficiency is a measure of how much energy in the absorbed photons is stored as chemical products. The numbers indicate that almost all the absorbed photons engage in photochemistry, but only about a fourth of the energy in each photon is stored, the remainder being converted to heat
Oxygen-evolving organisms have two photosystems that operate in series
By the late 1950s, several experiments were puzzling the scientists who studied photosynthesis. One of these experiments carried out by Emerson, measured the quantum yield of photosynthesis as a function of wavelength and revealed an effect known as the red drop (Fig. 6.8). If the quantum yield is measured for the wavelengths at which chlorophyll absorbs light, the values found
Fig 6.8. Red drop effect. The quantum yield of photosynthesis Fig.6.9 Enhancement effect. The rate of photosynthesis (black curve) falls off drastically for far -red light of wavelengths when red and far-red light are given together is greater than greater than 680 nm, indicating that far-red light alone is the sum of the rates when they are given apart. inefficient in driving photosynthesis.
throughout most of the range are fairly constant, indicating that any photon absorbed by chlorophyll or other pigments is as effective as any other photon in driving photosynthesis. However, the yield drops dramatically in the far-red region of chlorophyll absorption (greater than 680 nm). This drop cannot be caused by a decrease in chlorophyll absorption because the quantum yield measures only light that has actually been absorbed. Thus, light with a wavelength greater than 680 nm is much less efficient than light of shorter wavelengths.
Another puzzling experimental result was the enhancement effect, also discovered by Emerson. He measured the rate of photosynthesis separately with light of two different wavelengths and then used the two beams simultaneously (Fig. 6.9). When red and far-red light were given together, the rate of photosynthesis was greater than the sum of the individual rates. This was a startling and surprising observation.
The photosynthetic electron transport chain
The key to the photosynthetic electron transport chain is the presence of two large, multimolecular, pigment-protein complexes known as photosystem I (PSI) and photosystem II (PSII) (Fig. 6.10). PSI consists of 18 distinct subunits whereas PSII consists of 31 individual subunits! These two photosystems operate in series linked by a third multiprotein aggregate called the cytochrome complex. Overall, the effect of the chain is to extract low-energy electrons from water and, using light energy trapped by chlorophyll, raise the energy level of those electrons to produce a strong reductant
Photosystem I preferentially absorbs far-red light of wavelengths greater than 680 nm; photosystem II preferentially absorbs red light of 680 nm and is driven very poorly by far-red light. This wavelength dependence explains the enhancement effect and the red drop effect. Another difference between the photosystems is that
- Photosystem I produces a strong reductant, capable of reducing NADP+, and a weak oxidant.
- Photosystem II produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by photosystem I.
The reductant produced by photosystem II re-reduces the oxidant produced by photosystem I. These properties of the two photosystems are shown schematically in Fig. 6.10
Fig. 6.10 The Z-scheme
Almost all the chemical processes that make up the light reactions of photosynthesis are carried out by four major protein complexes: photosystem II, the cytochrome b6 f complex, photosystem I, and the ATP synthase. These four integral membrane complexes are vectorially oriented in the thylakoid membrane to function as follows:
- Photosystem II oxidizes water to O2 in the thylakoid lumen and in the process releases protons into the lumen.
- Cytochrome b6 f receives electrons from PSII and delivers them to PSI. It also transports additional protons into the lumen from the stroma.
- Photosystem I reduces NADP+ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase (FNR).
- ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma.
ORGANIZATION OF THE PHOTOSYNTHETIC APPARATUS
The site of photosynthesis in eukaryotes such as green plants and green algae is the chloroplast, a membrane enclosed organelle (Fig. 6.11). Like the mitochondrion, the chloroplast has inner and outer membranes and an intermembrane space. In addition, within the chloroplast are bodies called grana, which consist of stacks of flattened membranes called thylakoid disks. The grana are connected by membranes called intergranal lamellae. The thylakoid disks are formed by the folding of a third membrane within the chloroplast. The folding of the thylakoid membrane creates two spaces in the chloroplast in addition to the intermembrane space. The stroma lies within the inner membrane and outside the thylakoid membrane. In addition to the stroma, there is a thylakoid space within the thylakoid disks themselves. The trapping of light and the production of oxygen take place in the thylakoid disks. The dark reactions (also called light-independent reactions), in which CO2 is fixed to carbohydrates, take place in the stroma.
Fig. 6.11 Internal organization of chloroplast
CARBON FIXATION REACTION
The reactions catalyzing the reduction of CO2 to carbohydrate are coupled to the consumption of NADPH and ATP by enzymes found in the stroma, the soluble phase of chloroplasts. The stromalocalized reactions depend on the products of the photochemical processes, and are also directly regulated by light, they are more properly referred to as the carbon reactions of photosynthesis. There are three biochemical mechanisms for concentrating carbon dioxide that allow plants to mitigate the impact of photorespiration: CO2 pumps or Calvin cycle, C4 metabolism, and crassulacean acid metabolism (CAM).
The Calvin cycle
All photosynthetic eukaryotes, from the most primitive alga to the most advanced angiosperm, reduce CO2 to carbohydrate via the same basic mechanism: the photosynthetic carbon reduction cycle originally described for C3 species (the Calvin cycle, or reductive pentose phosphate [RPP] cycle). Other metabolic pathways associated with the photosynthetic fixation of CO2, such as the C4 photosynthetic carbon assimilation cycle and the photorespiratory carbon oxidation cycle, are either auxiliary to or dependent on the basic Calvin cycle.
The Calvin cycle was elucidated as a result of a series of elegant experiments by Melvin Calvin and his colleagues in the 1950s, for which a Nobel Prize was awarded in 1961. In the Calvin cycle, CO2 and water from the environment are enzymatically combined with a five-carbon acceptor molecule to generate two molecules of a three-carbon intermediate. This intermediate (3-phosphoglycerate) is reduced to carbohydrate by use of the ATP and NADPH generated photochemically. The cycle is completed by regeneration of the five-carbon acceptor (ribulose-1,5-bisphosphate, abbreviated RuBP) (Fig. 6.12)
The Calvin cycle proceeds in three stages
- Carboxylation of the CO2 acceptor ribulose-1,5-bisphosphate, forming two molecules of 3phosphoglycerate, the first stable intermediate of the Calvin cycle.
- Reduction of 3-phosphoglycerate, forming gyceraldehyde-3-phosphate, a carbohydrate
- Regeneration of the CO2 acceptor ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate
Fig. 6.12 The Calvin cycle showing the various stages of
An important property of rubisco is its ability to catalyse both the carboxylation and the oxygenation of RuBP. Oxygenation is the primary reaction in a process known as photorespiration. Because photosynthesis and photorespiration work in diametrically opposite directions, photorespiration results in loss of CO2 from cells that are simultaneously fixing CO2 by the Calvin cycle (Ogren 1984; Leegood et al. 1995). As alternative substrates for rubisco, CO2 and O2 compete for reaction with ribulose-1,5-bisphosphate because carboxylation and oxygenation occur within the same active site of the enzyme. Offered equal concentrations of CO2 and O2 in a test tube, angiosperm rubiscos fix CO2 about 80 times faster than they oxygenate. However, an aqueous solution in equilibrium with air at 25°C has a CO2:O2 ratio of 0.0416. At these concentrations, carboxylation in air outruns oxygenation by a scant three to one.
There are differences in leaf anatomy between plants that have a C4 carbon cycle (called C4 plants) and those that photosynthesize solely via the Calvin photosynthetic cycle (C3 plants). A cross section of a typical C3 leaf reveals one major cell type that has chloroplasts, the mesophyll. In contrast, a typical C4 leaf has two distinct chloroplast-containing cell types: mesophyll and bundle sheath (or Kranz, German for “wreath”) cells (Fig. 6.13). There is considerable anatomic variation in the arrangement of the bundle sheath cells with respect to the mesophyll and vascular tissue. In all cases, however, operation of the C4 cycle requires the cooperative effort of both cell types. No mesophyll cell of a C4 plant is more than two or three cells away from the nearest bundle sheath cell (see Fig 6.13). In addition, an extensive network of plasmodesmata connects mesophyll and bundle sheath cells, thus providing a pathway for the flow of metabolites between the cell types.
Fig.6.13 The C4 metabolism pathway
The C4 Cycle Concentrates CO2 in Bundle Sheath Cells
The basic C4 cycle consists of four stages:
- Fixation of CO2 by the carboxylation of phosphoenolpyruvate in the mesophyll cells to form a C4 acid (malate and/or aspartate).
- Transport of the C4 acids to the bundle sheath cells.
- Decarboxylation of the C4 acids within the bundle sheath cells and generation of CO2, which is then reduced to carbohydrate via the Calvin cycle.
Comparison of the plants of C3 and C4 cycle
|C3 Plant||C4 Plant|
|1||Only C3 cycle is found||Both C4 and C3 cycles are found|
|2||The efficiency of CO2 absorption at low concentration is far less and hence, they are less efficient.||The efficiency of CO2 absorption at low concentration is quite high and hence, they are more efficient plants.|
|3||The CO2 acceptor is Ribulose-1, 5- diphosphate.||The CO2 acceptor is phosphoenol pyruvate.|
|4||The first stable product is phospho glyceric acid (PGA).||Oxaloacetate (OAA) is the first stable product.|
|5||Plants show one type of chloroplast (monomorphic type).||Plants show dimorphic type of chloroplast. The chloroplast of parenchymatous bundle sheath is different from that of mesophyll cells (dimorphic type). The chloroplasts in bundle sheath cell are centripetally arranged and lack grana. Leaves show
Kranz type of anatomy.
|6||In each chloroplast, two pigment systems (Photosystems I and II) are present.||In the chloroplasts of bundle sheath cells, the photosystem II is absent. Therefore, these are dependent on mesophyll chloroplasts for the supply of NADPH +H+.|
|7||The Calvin cycle enzymes are present in mesophyll chloroplast. Thus, the Calvin cycle occurs.||Calvin cycle enzymes are absent in mesophyll chloroplasts. The cycle occurs only in the chloroplasts of bundle sheath cells.|
|8||The CO2 compensation point is 50-150 ppm CO2.||The CO2 compensation point is 0-10 ppm CO2.|
|9||Photorespiration is present and easily detectable.||Photorespiration is present only to a slight degree or absent.
|10||The CO2 concentration inside leaf remains high (about 200 ppm).
|The CO2 concentration inside the leaf remains low (about 100 ppm).|
|11||The 13C/12C ratio in C-containing compounds remains relatively low (both 13CO2 and 12CO2 are present in air).
|The ratio is relatively high, i.e. C4 plants are more enriched with 13C than C3 plants.
|12||Net rate of photosynthesis in full sunlight (10,000 – 12,000 ft. c.) is 15-25 mg. of CO2 per dm2 of leaf area per hour.
|It is 40-80 mg. of CO2 per dm2 of leaf area per hour. That is, photosynthetic rate is quite high. The plants are efficient.
|13||The light saturation intensity reaches in the range of 1000-4000 ft. c.
|It is difficult to reach saturation even in full sunlight.|
|14||Bundle sheath cells are unspecialized.||The bundle sheath cells are highly developed with unusual construction of organelles.
|15||The optimum temperature for the process is 1025°C.
|In these plants, it is 30-45°C and hence, they are warm climate plants. At this temperature, the rate of photosynthesis is double than that is in C3 plants.|
|16||18 ATPs are required to synthesise one glucose molecule.||30 ATPs are required to synthesise one glucose molecule.
Crassulacean acid metabolism
A third mechanism for concentrating CO2 at the site of rubisco is found in crassulacean acid metabolism (CAM). Despite its name, CAM is not restricted to the family Crassulaceae (Crassula, Kalanchoe, Sedum); it is found in numerous angiosperm families. Cacti and euphorbias are CAM plants, as well as pineapple, vanilla, and agave. The CAM mechanism enables plants to improve water use efficiency. Typically, a CAM plant loses 50 to 100 g of water for every gram of CO2 gained, compared with values of 250 to 300 g and 400 to 500 g for C4 and C3 plants. respectively. Thus, CAM plants have a competitive advantage in dry environments. The CAM mechanism is similar in many respects to the C4 cycle. In C4 plants, formation of the C4 acids in the mesophyll is spatially separated from decarboxylation of the C4 acids and from re-fixation of the resulting CO2 by the Calvin cycle in the bundle sheath. In CAM plants, formation of the C4 acids is both temporally and spatially separated. At night, CO2 is captured by PEP carboxylase in the cytosol, and the malate that forms from the oxaloacetate product is stored in the vacuole (Fig. 6.14). During the day, the stored malate is transported to the chloroplast and decarboxylated by NADP-malic enzyme, the released CO2 is fixed by the Calvin cycle, and the NADPH is used for converting the decarboxylated triose phosphate product to starch.
Fig.6.14 Crassulacean acid metabolism (CAM) pathway
The stomata of CAM plants open at night and close during the day
CAM plants such as cacti achieve their high water use efficiency by opening their stomata during the cool, desert nights and closing them during the hot, dry days. Closing the stomata during the day minimizes water loss, but because H2O and CO2 share the same diffusion pathway, CO2 must then be taken up at night. CO2 is incorporated via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. The malate accumulates and is stored in the large vacuoles that are a typical, but not obligatory, anatomic feature of the leaf cells of CAM plants (see Fig. 6.14). The accumulation of substantial amounts of malic acid, equivalent to the amount of CO2 assimilated at night, has long been recognized as a nocturnal acidification of the leaf (Bonner and Bonner 1948).
With the onset of day, the stomata close, preventing loss of water and further uptake of CO2. The leaf cells deacidify as the reserves of vacuolar malic acid are consumed. Decarboxylation is usually achieved by the action of NADP-malic enzyme on malate (Drincovich et al. 2001). Because the stomata are closed, the internally released CO2 cannot escape from the leaf and instead is fixed and converted to carbohydrate by the Calvin cycle.
Factors Influencing Photosynthesis
The factors influencing rate of photosynthesis can be classified into two categories,
- Internal factor and
- External factor (environmental)
A. Internal Factors
The amount of chlorophyll present has a direct relationship with the rate of photosynthesis, since, it is the pigment, which is photoreceptive and is directly involved in trapping the light energy.
2. Photosynthetic Enzyme Systems
The amount and nature of enzymes play a direct role on the rate of photosynthesis. Greater enzyme activity at higher light intensity increases the capacity of the leaf to absorb more light and thus increases the photosynthetic rate.
3. Leaf Resistance
Photosynthesis shows close dependence upon leaf resistance. For C4 plants, leaf resistance (primarily controlled by stomatal aperture) appears to regulate photosynthesis, but in C3 plants, the internal resistances including carboxylation efficiency offer greater limitation to CO2 fixation than stomatal resistance. Environmental factors such as light intensity, photoperiod, CO2 concentration, humidity and soil moisture also affect photosynthesis via stomatal resistance.
4. Demand for Photosynthate
Because of the greater demand, the rapidly growing plants show increased rate of photosynthesis in comparison to the mature plants. However, if the demand for photosynthesis is lowered by removal of meristem, then the photosynthetic rate declines.
5. Leaf Age
The Photosynthetic rate is higher in the newly expanding leaves and reaches a maximum as the leaves achieve full size. The rate declines as the leaf ages due to reduced chloroplast functions and other anabolic reactions.
B. External Factors
1. Carbon dioxide
CO2 is one of the raw materials for photosynthesis; therefore, its concentration affects the rate of photosynthesis markedly. Rate of photosynthesis increases with the increase in the atmospheric CO2 concentration up to a certain extent. Because of its very low concentration in atmosphere (current level of 350-360ppm), it acts as a limiting factor in natural photosynthesis. Rate of photosynthesis increases with increase in the atm. CO2 level of up to 1000 ppm beyond which, there is a general decline in photosynthesis. At this enhanced level of CO2, the increase in the photosynthetic rate may be 10 to 30 times more than the normal CO2 level.
Light affects the rate of photosynthesis in several ways. In general, photosynthesis can occur under artificial lights of sufficient intensity. Role of light on photosynthesis can be discussed under the following sub-heads:
a. Intensity of light:
a. Light intensity
Wolkoff (1966) found that the arte of photosynthesis is directly proportional to light intensity. But the extremely high light intensities do not favour for higher photosynthetic rates. The high light intensity which fails to accelerate photosynthesis is called light saturation intensity. Of the light falling on a leaf, about 80 per cent is absorbed, 10 per cent is reflected and 10 % is transmitted. The rate of photosynthesis is greater in intense light than in diffused light. The plants are grouped into two types on the basis of light requirement.
i. Heliophytes (Sun plants) ii. Sciophytes (Shade plants)
At a specific light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 released in respiration are volumetrically equal. This specific light intensity is known as light compensation point.
At very high light intensity, beyond a certain point, the photosynthetic cells exhibit photo oxidation. This phenomenon is called solarisation and a result of this, inactivation of chlorophyll molecules, bleaching of chlorophyll molecules and even inactivation of some enzymes take place resulting in the destruction of whole photosynthetic apparatus. In general, low light intensity favours stomatal closure and in turn reduced rate of photosynthesis.
With the increase in light intensity, the rate of photosynthesis increases, i.e., the rate of photosynthesis is directly proportional to light intensity. However, at stronger light intensity, increase in rate of photosynthesis is not proportional to light intensity. Except on cloudy days, light is never a limiting factor in nature. At certain light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 produced in respiration are volumetrically equal. This point of light intensity is known as Light Compensation Point. Light compensation point is frequently in the order of 100 to 200 f.c. for sun loving leaves; while, the value is 100f.c. for shade-loving leaves. Thus, in shade-loving plants the compensation point lasts for a much shorter period than in sun loving plants.
- Wavelength of light: For photosynthesis, the visible range of spectrum of light (PAR: 400 to 700 nm) is essential. Maximum photosynthesis is known to occur in the red part of the spectrum with the next peak in blue part and minimum in the green region (red ,blue and green). he region between 575 and 750nm (yellow to red) is quite congenial for photosynthesis. Ultra violet light has a lethal effect on plants if exposure is for a prolonged period.
- Duration of light: Photosynthesis may be sustained for relatively long periods of time without any noticeable damaging effect on plants.
- Photo-oxidation: When the light intensity for photosynthesizing tissue is increased beyond ascertain limit, the cells become vulnerable to chlorophyll photo-oxidation; due to this, many more chlorophyll molecules become excited than can possibly be utilized. This causes damaging effect to the chloroplast membrane system. In presence of O2, the damaging effect of photo-oxidation is severe. It results in bleaching of chlorophyll and inactivation of some important enzyme involved in photosynthesis. Effect to temperature on photosynthesis is little than on other process. Very high and very low temperatures affect the photosynthetic rate adversely. The rate of photosynthesis increases with rise in temperature from 5 to 350C; beyond which, there is a rapid fall in photosynthesis. In the optimum range of temperature, the Temperature Quotient (Q10) is found to be 2.0 for the rate of photosynthesis (Q10=2.0)
THE END OF CRP 201 LECTURE NOTE 5.