Ch.11  Photosynthesis in Higher Plants

• Photosynthesis is a process in which the plants use light energy to make carbohydrate from carbon dioxide and water. The overall reaction of photosynthesis can be given in a simple form as follows:

• In green plants, water is the hydrogen donor and it undergoes oxidation to produce oxygen.

• Photosynthesis occurs in two stages, viz. light reaction and dark reaction. The light reaction is light-dependent. Light energy is captured in this stage and is utilized to make the energy-storage molecules ATP and NADPH. Dark reaction is light-independent reaction. Dark reaction is utilized to capture and reduce carbon dioxide. Dark reaction doesn’t mean that it happens in the absence of light.

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CHLOROPLAST​

• Chloroplast is the cell organelle where photosynthesis takes place in plants and algae. A typical plant cell may contain about 10 to 100 chloroplasts.

• Chloroplast is enclosed by a membrane. This membrane is composed of an inner, outer and an intermediate membrane. An aqueous fluid; called stroma is present within the membrane.

• Stacks of thylakoids are present in the stroma. A stack is called a granum. Thylakoids are the sites of photosynthesis.

• A thylakoid is a flattened disc. It is bound by a membrane. The lumen or thylakoid space is present within the membrane. The thylakoid membrane is the site of photosynthesis. It contains integral and peripheral membrane protein complexes. Pigments which absorb light energy are also present on the membrane. The protein complexes and the pigments form the photosystems.

• Chlorophyll is the main pigment to absorb light. Additionally, carotenes and xanthophylls are also used by plants to absorb light energy. Algae also use chlorophyll for absorbing light.

• These pigments are embedded in plants and algae in special antenna-proteins. The pigments are ordered in these proteins so that they can work in perfect coordination. Such a protein is also called a light-harvesting complex.

• All the cells in the green parts of a plant have chloroplasts but most of the energy is captured in the leaves. The mesophyll of the leaf can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle which protects the leaf from excess evaporation of water. It also decreases the absorption of ultraviolet or blue light to reduce heating. The epidermal layer of the leaf is transparent. It allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

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Light Reaction​

• The light reaction is also called the Photochemical phase. It includes light absorption, water splitting, oxygen release and the formation of high-energy chemical intermediates (ATP and NADPH). Many complexes are involved in the process.

• The pigments are organized into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). The photosystems are named in the sequence of their discovery and it has nothing to do with their function during the light reaction.

• Each photosystem has all the pigments; except one molecule of chlorophyll a. The single chlorophyll a molecule forms the reaction centre. In PS, I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence it is called PS700. In PS II the absorption maxima are at 680 nm and hence it is called PS680.

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The Electron Transport​

• The reaction centre in PS II absorbs 680 nm wavelength of red light. This causes electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor and sent to an electrons transport system consisting of cytochromes.

• In terms of an oxidation-reduction potential scale, this movement of electrons is downhill. The electrons are not used up as they pass through the electron transport chain. They are passed on to the pigments of PS I.

• At the same time, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm. They are then transferred to another acceptor molecule which has a greater redox potential.

• Z Scheme: These electrons are then moved downhill again, to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH+H+. This whole transfer of electrons from PS II to the acceptor, to PS I, to another acceptor and finally to NADP+ is called the Z scheme, because of its characteristic shape.

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Splitting of Water​

• Water is split into H+, [O] and electrons. The splitting of water is associated with PS II. This creates oxygen. Photosystem II provides replacement for electrons removed from PS I.

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Cyclic and Non-Cyclic Photo-phosphorylation​

• Synthesis of ATP from ADP and inorganic phosphate in the presence of light is called photophosphorylation.

• When the two photosystems work in a series; first PS II and then the PS I; a process called non-cyclic photophosphorylation occurs.

• When only PS I is functional, the electron is circulated within the photosystem and the cyclic flow of electrons leads to phosphorylation. The stroma lamellae are the possible location of phosphorylation. The stroma lamellae lack PS II and NADP reductase enzyme. The excited electron does not pass on to NADP+ but is cycled back to the PS I complex. Hence, the cyclic flow results only in the synthesis of ATP but not of NADPH+H+. Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

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Chemiosmotic Hypothesis​

• Synthesis of ATP in chloroplast can be explained by chemiosmotic hypothesis. The way it happens in respiration, ATP synthesis during photosynthesis happens because of development of a proton gradient across a membrane, i.e. membrane of the thylakoid. The following steps are involved in development of proton gradient across the thylakoid membrane.

• When the electrons move through the photosystems, protons are transported across the membrane. The primary acceptor of electron is located towards the outer side of the membrane. It transfers its electrons no to an electron carrier but to an H carrier. Due to this, it removes a proton from the stroma while transporting an electron. When electron is passed to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

• The NADP reductase enzyme is located on the stroma side of the membrane. Protons are also necessary for the reduction of NADP+ to NADPH+H+. These protons are also removed from the stroma.

• Thus, protons in the stroma decrease in number and accumulate in the lumen. This results in development of a proton gradient across the thylakoid membrane. Additionally, there is a measurable decrease in pH in the lumen.

• The breakdown of this gradient leads to release of energy. The movement of protons across the membrane to the stroma results in breakdown of this gradient. The movement of protons takes place through the transmembrane channel of the F0 of the ATPase.

• The ATPase enzyme consists of two parts. One part is called the F0 and is embedded in the membrane. This forms a transmembrane channel which carries out facilitated diffusion of protons across the membrane. The other portion is called F1. It protrudes on the outer surface of thylakoid membrane on the lumen side.

• The breakdown of the gradient provides enough energy to cause a change in the F1 particle of the ATPase which results in synthesis of several molecules of energy-packed ATP.

• To summarise, it can be said that chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is utilised to pump protons across a membrane, to create a gradient or protons within the thylakoid membrane. ATPase has a channel. This channel allows diffusion of protons back across the membrane. The diffusion of protons releases enough energy to activate ATPase enzyme. The ATPase enzyme catalyses the formation of ATP. NADPH and the ATP are used in the biosynthetic reaction which takes place in the stroma. This reaction is responsible for fixing CO2 and for synthesis of sugars.

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ATP and NADPH​

• The products of light reaction are ATP, NADPH and O2. The oxygen diffuses out of the chloroplast. ATP and NADPH are used to drive the processes which lead to the synthesis of food. These processes depend on the products of light reaction; apart from being dependent on CO2 and H2O.

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The Calvin Cycle​

• The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

• Carboxylation: Fixation of CO2 into a stable organic intermediate is called carboxylation. In this step, carbon dioxide is utilised for the carboxylation of RuBP. The enzyme RuBP carboxylase catalyses this reaction. The reaction results in the formation of two molecules of 3-PGA. This enzyme is also called RuBP carboxylase-oxygenase or RuBisCO; because it also has an oxygenation activity.

• Reduction: This step involves utilization of 2 molecules of ATP for phosphorylation and two of NADPH for reduction of each CO2 molecule fixed. For the removal of one molecule of glucose from the pathway, fixation of six molecules of CO2 and 6 turns of the cycle are required.

• Regeneration: This step involves regeneration of CO2 acceptor molecule RuBP. This is necessary for the cycle to continue without interruption. This step requires one ATP for phosphorylation to form RuBP.

• Thus, for each CO2 molecule entering the Calvin Cycle, 3 molecules of ATP and 2 of NADPH are required. Cyclic phosphorylation probably takes place to meet this difference in number of ATP and NADPH used in the dark reaction.

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The C4 Pathway​

• Plants which are adapted to dry tropical regions use the C4 pathway. While 3-PGA is the first carbon fixation product in C3 plants, it is oxaloacetic acid (4 carbon atoms) which is the first carbon fixation product in C4 plants. However, the main biosynthetic pathway remains the Calvin cycle; as in C3 plants.

• Kranz Anatomy: In C4 plants, large cells are found around the vascular bundles. These are called sheath cells. Leaves have special anatomy; called Kranz anatomy. The bundle sheath cells may form several layers around the vascular bundles. The cells are characterized by large number of chloroplasts, thick walls (impervious to gaseous exchange) and no intercellular spaces.

• This pathway is known as Hatch and Slack Pathway. It happens in following steps.

• Phoshpenol pyruvate (PEP) which is a 3-carbon molecule is the primary CO2 acceptor. It is present in the mesophyll cells. The enzyme PEP carboxylase or PEPcase is responsible for this fixation. RuBisCO enzyme is absent in mesophyll cells. Oxaloacetic acid (OAA) is formed in the mesophyll cells.

• Then other 4-carbon compounds; like malic acid or aspartic acid are formed in the mesophyll cells. These are then transported to the bundle sheath cells. These C4 acids are broken down in the bundle sheath cells; to release CO2 and a 3-carbon molecule.

• The 3-carbon molecule is transported back to the mesophyll. In the mesophyll, it is converted into PEP again. Thus, the cycle is completed.

• The CO2 released in the bundle sheath cells enters the Calvin cycle. The bundle sheath cells are rich in RuBisCO but lack PEPcase.

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PHOTORESPIRATION​

• RuBisCO is the most abundant enzyme in the world. Its active site can bind to both CO2 and O2. But RuBisCO has a much greater affinity for CO2 than O2. The relative concentration of O2 and CO2 determines which of them will bind to the enzyme.

• In C3 plants, some oxygen binds to RuBisCO and hence CO2 fixation is decreased. The RuBP; in this case; binds with oxygen to form one molecule of PGA and phosphogylcolate. This happens in a pathway called photorespiration. In the photorespiratory pathway, neither the sugar nor the ATP is synthesized. Rather utilization of ATP results in the release of CO2. Even NADPH is not synthesized in the photorespiratory pathway. Thus, it is a wasteful process.

• Photorespiration does not occur in C4 plants. They have a mechanism which increases CO2 concentration at the enzyme site. This happens when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2. Thus, intracellular concentration of CO2 is increased. This ensures that the RuBisCO functions as a carboxylase; with minimum oxygenase activity.

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FACTORS AFFECTING PHOTOSYNTHESIS​

Blackman’s (1905) Law of Limiting Factors:

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• “If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.”

• Light: At low intensities of light, there is a linear relationship between incident light and carbon fixation. At high intensities of light, the rate does not show further increase because other factors become limiting. Increase in incident light beyond a point results in the breakdown of chlorophyll and a decrease in photosynthesis.

• Carbon dioxide Concentration: Carbon dioxide is a major limiting factor for photosynthesis. It is important to remember that carbon dioxide is in very low concentration in the atmosphere (0.03 – 0.04%). Increase in concentration up to 0.05% can increase carbon fixation. An increase beyond this level can be damaging over longer periods.

• Temperature: The dark reactions are controlled by temperature because they are enzymatic. Light reactions are also sensitive to temperature but to a much lesser extent. The C4 plants have a much higher temperature optimum than C3 plants. The temperature optimum for a plant also depends on it natural habitat. Tropical plants have a higher temperature optimum than temperate plants.

• Water: The effect of water is more on the plant rather than directly on photosynthesis. Water stress results in closing of stomata and; thus, in reduced availability of CO2. Water stress also leads to wilting of leaves. Wilting of leaves means reduced surface area and reduced metabolic activity in leaves.