2.13 Photosynthesis
Epun Dissanayake
MBBS (UG) | Microsoft Specialist | GitHub Developer | Google Developer
Photosynthesis as an energy-fixing mechanism
Photosynthesis
Photosynthesis is a metabolic process by which light energy is trapped and converted to chemical energy. Chemical energy is stored in the chemical bonds of carbohydrates, fats, oils, and proteins. All life on Earth depends on photosynthesis either directly or indirectly. Photosynthesis also occurs in algae and certain prokaryotes.
Global importance of photosynthesis;
During photosynthesis CO2 is reduced by the H of H2O and simple sugars are made using light energy. In eukaryotic photosynthetic cells, chloroplasts are the sites of photosynthesis.
The process of photosynthesis consists of two main stages and they are integrated.
There are two types of photosynthetic mechanisms (pathways) based on the number of C atoms of the first stable product of CO2 fixation.
????????C3 Mechanism – No of?C atom of the first stable compound is three
????????C4 Mechanism – No of?C atom of the first stable compound is four
Light-dependent reactions of photosynthesis take place in the membrane system of thylakoids. They are flattened fluid-filled sacs, which form stacks called grana at intervals. Chlorophylls, carotenoids and electron acceptors are located on this membrane system of thylakoids.
Stroma is a gel-like structure containing soluble enzymes and other chemicals, which is the site of the Calvin cycle.
Photosynthetic pigments are substances which absorb visible light. In a leaf we see green colour because chlorophylls absorb violet, blue and red light and therefore, they transmit and reflect green colour. Different pigments absorb different wavelengths of light. In chloroplast, there are two types of chloroplast pigments such as chlorophylls and carotenoids. Chlorophyll a is the key light-capturing pigment and they participate directly in the light reaction of photosynthesis.
According to the action spectrum, chlorophyll a is more effective for the blue and red light. Chlorophyll b and carotenoids (carotenes and xanthophylls) are effective in the absorption of specific range wavelengths of corresponding to different colours.
Another important function of some carotenoids is photoprotection. Photoprotection is the absorption and dissipation of excess light energy, if not that excessive light may cause damage to the chlorophylls or interact with oxygen and form reactive oxidative molecules which are dangerous to the cell.
Absorption spectrum
An absorption spectrum is a graph of the relative amounts of light absorbed at different wavelengths by a pigment.
Action spectrum
An action spectrum is a graph showing the effectiveness of different wavelengths of light in stimulating photosynthesis.
Excitation of chlorophyll by light
When a molecule of chlorophyll or other photosynthetic pigment absorbs light it becomes excited. The energy from the light is used to boost electrons to a higher level and become positively charged. The excited state is unstable and returns to its original lower energy state. The excited electrons may pass through several electron carriers until they reach the final electron acceptor.
By Light energy; Chlorophyll ? Chlorophyll+??+ e- (electron)
Therefore chlorophyll is oxidized and the electron acceptor is reduced.
Photosystems
Chlorophyll molecules, other organic molecules and proteins are organized into complexes in the thylakoid membrane of chloroplasts. They are called photosystems.
A photosystem contains a reaction centre complex and light-harvesting complexes. The reaction centre complex also contains a primary electron acceptor.
There are two types of photosystems found in the thylakoid membrane. They are Photosystem I (PS I) and photosystem II (PS II).In the PS I the chlorophyll molecule is known as P700 since they absorb light at 700nm wavelength effectively. In the PS II, the reaction centre contains chlorophyll a molecule which is known as P680 which absorbs light having a wavelength of 680 nm.
Light-dependent reaction /Light reaction photosynthesis
Linear electron flow
Light is absorbed by the photosynthetic pigments and synthesizes ATP and NADPH due to the excitation of Photosystem I and Photosystem II which are embedded in the thylakoid membrane of the chloroplast. The key to this energy transformation is a flow of the electron in one direction through the photosystems and other molecular components built in the thylakoid. This process is called linear electron flow. The striking of photons of light on the pigments results in the excitation of electrons from photosystem II to the higher energy state.
These electrons?will?be?accepted?by the primary electron acceptor of photosystem II.
Splitting of water takes place as a result of an enzyme-catalyzed reaction and yields O2 (g), H+ ions and electrons.
Electrons released as a result of hydrolysis may neutralize excited photosystem II (P680).
Striking of photons of light on the pigments results in the excitation of electrons from photosystem I (P700) to the higher energy state. Excited electrons will be accepted by a primary electron acceptor of PSI.
Excited electrons of PS II at the primary electron acceptor of PS II will pass through an electron transport?chain to PS I and neutralize the excited PS I. The energy released due to the passage of electrons from a higher energy state to a lower energy results in the synthesis of ATP. This is known as photophosphorylation. Excited electrons of PS I at the primary electron acceptor of PSI will pass through an electron transport chain and reduce NADP and yield NADPH. The reduction of NADP is catalyzed by an enzyme called NADP reductase.
Cyclic electron flow
This occurs in photosystem I but not in Photosystem II. Here some photoexcited electrons use an alternative cyclic pathway. This produces ATP but not NADPH and Oxygen?are released.
The Calvin cycle
The Calvin cycle takes place in the stroma of the chloroplast. Energy from ATP and NADPH produced by the light reaction is used to reduce CO2. The reactions are catalyzed by enzymes and their sequence was discovered by scientist Calvin. This is an anabolic reaction. The first stable product of the Calvin cycle is glyceraldehyde 3-phosphate (G3P). For the net synthesis of one molecule of G3P, the cycle must take place three times.
The Calvin cycle of photosynthesis can be described in three steps;
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Carbon fixation
The CO2 acceptor is a 5 C sugar, Ribulose bisphosphate (RuBP). The addition of CO2 to a RuBP is called carboxylation. The enzyme involves in this reaction is RuBP carboxylase oxygenase or Rubisco.
The first product of RuBP carboxylation is a 6C molecule which is unstable and breaks down immediately into two molecules of 3-phosphoglycerate (3-PGA). This is the first stable product of photosynthesis. The enzyme RuBP carboxylase oxygenase (Rubisco) is present in large amounts in the chloroplast stroma.
Reduction phase
1,3-Bisphosphoglycerate will be reduced to Glyceraldehyde 3- phosphate (G3P) through step by step. Enzyme-catalyzed reactions utilizing NADPH and ATP from the light reaction. G3P will act as a precursor for carbohydrate synthesis (glucose).
Regeneration of RuBP
RuBP is regenerated by undergoing a series of complex reactions. This process uses energy from ATP generated in light reactions.
Photorespiration
As its name suggests, Rubisco is capable of catalyzing two distinct reactions, acting as both a carboxylase and as an oxygenase.
The oxygenase reaction of Rubisco uses the same substrate, RUBP, but reacts with O2. The reaction is catalyzed on the same active site as the carboxylation reaction. Thus CO2 and O2 are competitive substrates. Therefore CO2 inhibits oxygenase and O2 Inhibits the carboxylase reaction.
The oxygenase reaction forms just one molecule of 3-PGA plus a two-carbon product, 2-phosphoglycolate which is of no immediate use in the Calvin cycle and higher concentrations it is toxic for the plant. It, therefore, has to be processed in a metabolic pathway called photorespiration. The photorespiratory pathway involves enzymes in the chloroplasts, peroxisome and mitochondria. (detail of this pathway is not expected).
Photorespiration is not only energy demanding, but furthermore leads to a net loss of CO2 Each time Rubisco reacts with O2 instead of CO2?the plants make 50% less 3-PGA than it would have done if CO2?had been used. This potentially eliminates the net gain in photosynthetic carbon and loss of productivity.
These two factors increase photorespiration relative to photosynthesis so an increasing proportion of carbon is lost as the temperature rises.
The CO2. required for photosynthesis enters a leaf via stomata. However, stomata are also the main avenues of transpiration. On a hot, dry day, most plants close their stomata to conserve water. At the same time O2. released from the light reactions begins to increase and this leads to further reduction of the (CO2.) to (O2) ratio in the cytosol. These conditions within the leaf favour wasteful process photorespiration under high temperature, dryness and high light intensities.
Therefore plants developed different ways to cope with this situation during the evolution that resulted in a most successful solution to concentrate CO2 around Rubisco provided by the C4 ?photosynthetic pathway.
C4 pathway of photosynthesis
The establishment of the C4?photosynthetic pathway includes several biochemical and anatomical modifications that allow plants with this pathway to concentrate CO2 at the site of Rubisco. Thereby its oxygenase reaction and the following photorespiration are largely repressed in C4 plants.
In most C4 plants the CO2 concentration mechanism is achieved by a division of labour between two distinct specialized leaf cell types, the mesophyll and the bundle sheath cells. Compared to C3 plants the bundle sheath cells of C4 plants have expanded physiological functions. This is reflected by the enlargement and higher organelle content of these cells in the C4 species. For the efficient function of the C4 pathway, a close contact between mesophyll and bundle sheath cells is tightly interconnected to each other by high numbers of plasmodesmata. The bundle sheath cells enclose the vascular bundles and are themselves surrounded by the mesophyll cells and this type of leaf anatomy was termed Kranz anatomy.
Since Rubisco can operate under high CO2 concentrations in the bundle sheath cells, it works more efficiently than in C3 plants. Because of the CO2 concentration mechanism they can acquire enough CO2 even when keeping their stomata more closed and minimize the water loss by transpiration.
In the mesophyll cells of C4 plants, CO2 is converted to bicarbonate by carbonic anhydrase and initially fixed by phosphoenolpyruvate carboxylase using PEP as a CO2 acceptor. The resulting oxaloacetate (OAA) is composed of four carbon atoms, which is the basis for the name of this metabolic pathway. Oxaloacetate is rapidly converted to the more stable C4 acids malate or aspartate that diffuse to the bundle sheath cells. Here, CO2 is released by decarboxylating enzymes and the released CO2 is refixed by Rubisco, which exclusively operates in the bundle sheath cells in C4 plants. Chloroplasts found in mesophyll cells are different in anatomy in comparison to chloroplasts of bundle sheath cells.
Since chloroplasts of mesophyll cells carry out only light reactions, they are rich in grana. The grana of mesophyll chloroplasts are extensive and highly differentiated for the light response. Bundle sheath chloroplasts possess very few, and less differentiated grana or grana are absent. Moreover, the PSII in the bundle sheath cells is depleted to lower oxygen production in these cells.?
By carbonic anhydrase;??CO2 + H2O????????HCO3- ?+ H+
By PEP carboxylase;???????PEP + HCO3- ??????Oxaloacetate (4C)
This PEP carboxylase enzyme is much more efficient than the enzyme of RUBP carboxylase for two reasons.
1.????It reacts with bicarbonate (HCO-3) rather than with CO2’ The advantage of this is that there is a 50-fold higher concentration of HCO3- than CO2??in solution in the cytosol.
2.????It has no affinity for O2
Significance of the C4 pathway
Comparisons of C3 and C4 plants
Factors affecting photosynthesis
The rate of photosynthesis is an important factor in crop production. Rate is affected by various factors.
??????e.g. light intensity, CO2 concentration, temperature, water, pollutants and?inhibitors
Photosynthesis involves a series of reactions. Therefore various factors are involved in it.
Blackman who is a scientist first proposed the idea of the principle of limiting factors.
When a chemical process is affected by more than one factor, its rate is limited by the factor which is nearest its minimum value.??????????e.g. Light intensity
Light Intensity
The rate of photosynthesis increases linearly with increasing light intensity. Gradually the rate of increase falls off as the other factors become limiting. In very high light intensities chlorophyll may bleach and slow down photosynthesis. However, plants exposed to such conditions are usually protected by devices such as thick cuticles, and hairy leaves.
Under normal conditions, CO2 is the major limiting factor in photosynthesis. An increase in photosynthetic rate is achieved by increasing CO2 concentration. For example, some greenhouse crops such as tomatoes are grown in CO2-enriched atmospheres.
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