1.Video Lesson

2.Objective

At the end of this lesson you will be able to:-

• Describe cyclic and non-cyclic photophosphorylation.
• Show an absorption spectra of chlorophyll a and chlorophyll b using graph.
• Differentiate between C3 , C4 plants, and CAM Plants.
• Justify the reason why the rate of photorespiration is less in C4 plants as compared to C3 plants.
• Summarize the process of photosynthesis using chemical equation.
• Conduct an experiment to show the presence of starch in green leaves.

Brainstorming questions

• What strategies can be developed to enhance the efficiency of light-dependent reactions in photosynthesis, particularly in terms of maximizing ATP and NADPH production for agricultural applications?
• How do the cyclic and non-cyclic photophosphorylation pathways complement each other in the overall process of photosynthesis, and what role do they play in balancing energy and electron flow in varying light conditions?

key words

• Light-Dependent Reactions: These are the steps of photosynthesis that convert solar energy into chemical energy. They occur in the thylakoid membranes of the chloroplasts.
• Photophosphorylation: The process of adding a phosphate group to ADP to form ATP, powered by light energy. This can be cyclic (where electrons return to the same photosystem) or non-cyclic (where electrons move from one photosystem to another).
• NADP+ (Nicotinamide Adenine Dinucleotide Phosphate): An electron acceptor that is reduced to NADPH during the light reactions. NADPH acts as a reducing agent in the Calvin Cycle.
• Chemiosmosis: The movement of protons (H+ ions) across the thylakoid membrane, which drives the synthesis of ATP.
• Photosystem: Complexes of chlorophyll and proteins where light-dependent reactions occur. Photosystems absorb light and drive the transfer of electrons.
• Plastoquinone (Pq): An electron carrier in the light-dependent reactions that transfers electrons from photosystem II to the cytochrome complex.
• Plastocyanin (Pc): A small protein that carries electrons from the cytochrome complex to photosystem I.
• Ferredoxin (Fd): An electron carrier that transfers electrons from photosystem I to NADP+ reductase.
• NADP+ Reductase: An enzyme that catalyzes the reduction of NADP+ to NADPH.
• Calvin Cycle: The set of light-independent reactions that occur in the stroma of the chloroplasts, using ATP and NADPH to convert CO2 into sugars.
• Carbon Fixation: The process where CO2 is incorporated into a 5-carbon molecule (RuBP) to form a 6-carbon compound that splits into two molecules of 3-phosphoglyceric acid (3-PGA). Catalyzed by the enzyme RuBisCO.
• RuBisCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase): The enzyme that catalyzes the fixation of CO2 in the Calvin Cycle.
• Reduction: The process where ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.
• Regeneration: The stage of the Calvin Cycle where some G3P molecules are used to regenerate RuBP, enabling the cycle to continue. This process requires ATP.
• Plant Types
• C3 Plants: Plants that fix CO2 into a 3-carbon compound (3-PGA). Examples include wheat, rye, and oats.
• C4 Plants: Plants that fix CO2 into a 4-carbon compound (oxaloacetate) which is later converted to CO2 for the Calvin Cycle. Examples include maize, sorghum, and sugarcane.
• CAM Plants (Crassulacean Acid Metabolism): Plants that capture CO2 at night and store it as malate in vacuoles. During the day, CO2 is released from malate and fixed in the Calvin Cycle. Examples include cacti and other succulent plants.
• Glycolate Pathway: The pathway involved in photorespiration where glycolate is processed and CO2 is released.
• CO2 Compensation Point: The concentration of CO2 at which the rate of CO2 uptake in photosynthesis equals the rate of CO2 release in photorespiration.

Photosynthesis

Photosynthesis occurs in two stages: –

• light-dependent reactions (cyclic and non-­ cyclic photophosphorylation)
• light-independent (Calvin Cycle) reactions

Light-dependent reactions Light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off O₂ as a by-product (Figure 22). Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. (The electron acceptor NADP⁺ is first cousin to NAD⁺, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP⁺ molecule.) The light reactions use solar energy to reduce NADP⁺ to NADPH by adding a pair of electrons along with an H⁺. The light reactions also generate do not produce sugar. ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells

photosystem I and photosystem II

1. Electrons (e-) in chlorophyll molecules in photosystem II are excited by the energy in
   photons of       light:    they     become            more energetic. Because of the extra energy, they escape from the chlorophyll and pass to an electron acceptor (the primary electron acceptor).
2. The conditions created in the chloroplast cause the following reaction to occur:

This light-dependent splitting of water 1s called photolysis. The electrons replace those lost from the chlorophyll molecule.

3.The primary electron acceptor passes the electrons to the next molecule in an electron transport chain (plastoquinone or ‘Pq’). The electrons then pass along a series of cytochromes (similar to those m the mitochondrial electron transport chain) and finally to plastocyanin (Pc) – the last carrier in the chain. The electrons lose energy as they are passed from one carrier to the next.

4. One of the molecules in the cytochromes complex is a proton (hydrogen ion) pump. As electrons are transferred to and then transferred from this molecule, the energy they lose powers the pump which moves protons from the stroma of the chloroplast to the space inside the thylakoid. This leads to an accumulation of protons inside the thylakoid, which drives the chemiosmotic synthesis of ATP.

5. Electrons in chlorophyll molecules in photosystem   I  are  excited  (as  this photosystem absorbs photons of light) and escape from the molecule. They are replaced by the electrons that have passed down the electron transport chain from photosystem IL

6. The electrons then pass along a second electron transport chain involving ferredoxin (Fd) and NADP reductase. At the end of this electron transport chain, they can react with protons (hydrogen ions) and NADP in the stroma of the chloroplast to form reduced NADP.

Products of Photosynthesis: In summary, the steps of the light reactions of photosynthesis produce three chemical products: O₂, NADPH, and ATP:

• O2 is produced in the thylakoid lumen by the oxidation of water by photosystem II. Two electrons are removed from water, which produces 2 H+ and 1/2 02. The two electrons are transferred to P680 molecules.
• NADPH is produced in the stroma using high-energy electrons that start Ill photosystem II and are boosted a second time in photosystem I. Two high-energy electrons  and  one  H+  are transferred to NADP+ to produce NADPH
• ATP is produced in the stroma via ATP synthase that uses an H+ electrochemical gradient

Light-Independent Reactions (Calvin cycle)

This is the second step in the mechanism of photosynthesis. The chemical processes of photosynthesis occurring independent of light are called dark reactions. It takes place in the stroma of the chloroplast. The dark reaction is purely enzymatic and it is slower than the light reaction. Dark reaction does not require light . In a dark reaction, the sugars are synthesized from CO2. The energy-poor CO2 is fixed to energy-rich carbohydrates using the energy-rich compound, ATP, and the assimilatory power, NADPH2 of light reaction. The process is called carbon fixation or carbon assimilation.

C3 Plants are plants capable of fixing CO2 into a 3-Carbon sugar called Phosphoglycerate (PGA). The energy from ATP and NADPH energy carriers generated by the photosystems is used to phosphorylate the PGA. In this process, carbon dioxide enters a plant through its stomata, and the enzyme Rubisco fixes carbon into sugar using the Calvin cycle. This fixation of carbon dioxide by Rubisco is the first step of the Calvin cycle. The plants that use this mechanism of carbon fixation are called C3 plants. Approximately 95% of plants on the earth are C3 plants. Some of the C3 plant examples are wheat, rye, oats, and orchard grass. The photosynthesis process can take place only when the stomata on leaves are open. C3 plants exhibit the C3 pathway. It is a  three-carbon compound (3-PGA). Here the first carbon compound produced has three carbon atoms hence the name “C3 pathway”(Figure 23). The light-independent reactions of the Calvin cycle can be organized into three basic stages:

• carbon fixation,
• reduction, and
• regeneration.

C4 plants: In some plants like maize, sorghum, and sugarcane , the first product of carbondioxide fixation is not the three carbon molecule phosphoglycerate but the four carbon compound oxaloacetic acid. Plants that utlize this pathway are commonly called the C4 or four carbon plants. The oxaloacetic acid is formed when carbondioxide is bound to a compound known as phophoenolpyruvate (PEP) in the mesophyll cell. The oxaloacetic acid is reduced to malic acid or converted to aspartic acid; and the malic acid(aspartic acid) is decarboxylated to yield CO2 and pyruvic acid in the bundle sheath cell (Figure 24). Then, CO2 enters to Calvin cycle.

CAM  (crassulacean  acid metabolism) Plants:- The CAM mechanism enables plants to improve water use efficiency. The CAM mechanism is similar in many respects to the C4 cycle. In C4 plants, formation of the C4 acids in the mesophyll 1s spatially separated from decarboxylation of the C4 acids and from refixation 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 1s captured by PEP carboxylase in the cytosol, and the malate that forms from the oxaloacetate product is stored in the vacuole. During the day time, 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.

Photorespiration

Home practical activity, Testing a leaf for starch

When iodine solution (yellow/brown) mixes with starch (white), they form a deep blue color. To test for starch, iodine solution is added to a leaf to see if it turns blue. However, a living leaf is impermeable to iodine, and chlorophyll masks any color change. Therefore, the leaf that have green and yellowish colors must be treated in the following way:

• Heat water to boiling in a beaker, then turn off the Bunsen burner. Use forceps to dip a leaf into the hot water for about 30 seconds. This process kills the cytoplasm, denatures the enzymes, and makes the leaf more permeable to iodine solution.
• Push the leaf to the bottom of a test-tube and cover it with ethanol (alcohol). Place the tube in the hot water.
• The alcohol will boil and dissolve out most of the chlorophyll. This makes colour changes with iodine easier to see.
• Pour the green alcohol into a spare beaker, remove the leaf and dip it once more into the hot water to soften it.
• Spread the decolorized leaf flat on a white tile and drop iodine solution on to it. The parts containing starch will tum blue; parts without starch will stain brown or yellow with iodine.

Contributions of photosynthesis for the continuity of life, for 02 and CO2 balance and global warming