Photosynthesis In Higher Plants - Class 11 Medical Biology - Extra Questions
Write the schematic representation of Z-scheme of light reaction.
PQ: Plastoquinone
PC: Plastocyanin
Do reactions of photosynthesis called, as 'Dark Reaction' need light? Explain.
Some of these terms/chemicals are associated with the $$C_4$$ cycle. Explain.
Hatch slack pathway
How does a tree trunk exchange gases with the environment although it lacks stomata?
Some of these terms/chemicals are associated with the $$C_4$$ cycle. Explain.
Calvin cycle
Observe the diagram and answer the following.
What is the first product of $$C_4$$ cycle?
Some of these terms/chemicals are associated with the $$C_4$$ cycle. Explain.
PEP carboxylase
Value Based Questions :
What chemical substances would be exchanged between the cytoplasm and the mitochondria? Indicate whether they are entering or leaving the mitochondria.
Does photorespiration release energy ?
Short /Long answer type question.
Give a schematic representation of the Hatch and Slack cycle ($$C_4$$ cycle).
Define the calvin cycle?
Give an account of carboxylation ($$CO_2$$ fixation) stage of Calvin cycle.
Why is non-cyclic photo phosphorylation considered as a non-cyclic pathway?
In case of the non-cyclic photo phosphorylation, the electron is released from the oxidized photosystem after capturing the light energy. This electron is transferred from one accepter to other. The electron is finally accepted by the NADP molecule which gets reduced to form NADPH + H$$^+$$. The same electron never comes back to the same photosystem. The electron gap created in the photosystem is filled by the electron released due to splitting of the water molecule. Thus, the process is called as non-cyclic photo phosphorylation.
Explain the $$C_{4}$$ cycle of photosynthesis.
This pathway mostly takes place in monocot plants and in some dicot plants. In this pathway, the first product of CO22fixation is a 4 carbon compound, oxaloacetic acid, hence it is called C44 pathway.
The anatomy of leaves of C44 plants is different from leaves of C33 plants (Calvin cycle). Palisade cells in the mesophyll tissues are absent. There is bundle sheath around the vascular bundles. In the bundle sheath cells, large chloroplasts without grana or less developed grana are present, whereas, in mesophyll cells, there are chloroplasts with well-developed grana. This type of anatomy is called Kranz anatomy.
In C44 pathway, CO22 taken from the atmosphere is accepted by phosphoenol pyruvic acid present in the chloroplast of mesophyll cells, leading to the formation of a 4 carbon compound oxaloacetic acid. This is converted into malic acid (4 carbon compound) which enters into the chloroplast of bundle sheath cells. Here the malic acid dissociates into pyruvic acid and CO22. Pyruvic acid re-enters with RuBP gives rise to sugars and other carbohydrates in bundle sheath cells. There is no photorespiration in these plants. Example, Maize, Sorghum.
The anatomy of leaves of C44 plants is different from leaves of C33 plants (Calvin cycle). Palisade cells in the mesophyll tissues are absent. There is bundle sheath around the vascular bundles. In the bundle sheath cells, large chloroplasts without grana or less developed grana are present, whereas, in mesophyll cells, there are chloroplasts with well-developed grana. This type of anatomy is called Kranz anatomy.
In C44 pathway, CO22 taken from the atmosphere is accepted by phosphoenol pyruvic acid present in the chloroplast of mesophyll cells, leading to the formation of a 4 carbon compound oxaloacetic acid. This is converted into malic acid (4 carbon compound) which enters into the chloroplast of bundle sheath cells. Here the malic acid dissociates into pyruvic acid and CO22. Pyruvic acid re-enters with RuBP gives rise to sugars and other carbohydrates in bundle sheath cells. There is no photorespiration in these plants. Example, Maize, Sorghum.
What is photorespiration? Explain it with diagrammatic representation.
Photorespiration is the process in which the enzyme RuBisCo acts as oxygenase and combines with oxygen. This is the wasteful pathway which results in the loss of the carbon dioxide which was fixed. Photorespiration results in the light-dependent uptake of oxygen and release of carbon dioxide and is associated with the synthesis and metabolism of a small molecule called as glycolate.
Write three differences between $$C_3$$ and $$C_4$$ cycles.
What are the differences between $$C_{3}$$ and $$C_{4}$$ pathways?
Differentiate between cyclic and non-cyclic photophosphorylation.
Write any three differences between photo respiration and dark respiration.
Tabulate any eight differences between $$C_3$$ and $$C_4$$ plants/cycles.
State any three differences between photorespiration and dark respiration.
Explain chemiosmotic hypothesis for ATP synthesis.
The chemiosmotic hypothesis was proposed by Peter Mitchell. The ATP and NADH$$_2$$ generated during the light reaction is used for carbon dioxide fixation during the dark reaction.
According to the chemiosmotic hypothesis, ATP is produced by due to the proton gradient created across the mitochondrial membrane. The essential components required for chemiosmosis are proton pump, proton gradient and ATP synthase. ATP synthase is an enzyme which helps in ATP synthesis. The enzyme ATP synthase has two part $$F_0$$ and $$F_1$$. $$F_0$$ is the part which transports protons across the membrane as a transmembrane channel while configuration changes in $$F_1$$ activates the enzyme. They phosphorylate ADP i.e., add one more phosphorus group to the ADP and converts ADP(adenosine diphosphate) to ATP (adenosine triphosphate). The proton gradient developed across the membrane is the driving force of ATP synthase.
According to the chemiosmotic hypothesis, ATP is produced by due to the proton gradient created across the mitochondrial membrane. The essential components required for chemiosmosis are proton pump, proton gradient and ATP synthase. ATP synthase is an enzyme which helps in ATP synthesis. The enzyme ATP synthase has two part $$F_0$$ and $$F_1$$. $$F_0$$ is the part which transports protons across the membrane as a transmembrane channel while configuration changes in $$F_1$$ activates the enzyme. They phosphorylate ADP i.e., add one more phosphorus group to the ADP and converts ADP(adenosine diphosphate) to ATP (adenosine triphosphate). The proton gradient developed across the membrane is the driving force of ATP synthase.
Name the scientists who have contributed to the following.
(i) Reverse transcription.
(ii) Photorespiration.
(iii) Principle of limiting factors.
(iv) Photolysis of water.
What is 'photophosphorylation'? Describe non-cyclic photo-phosphorylation with schematic representation. Give its significance.
Photophosphorylation is the process in which phosphate group is transferred to ADP for the synthesis of energy-rich ATP molecule in the presence of light.
Non-cyclic photophosphorylation takes place as follows-
-The electrons lost by P680 are taken up by P700. These electrons do not go back to P680 hence it is called non- cyclic phosphorylation.
- The electrons pass through the primary acceptor, plastoquinone, cytochrome complex, plastocyanin and lastly to P700.
- The electrons released by P700 combine with H+ (obtained by splitting up water) and reduce NADP to NADPH.
Significance of non-cyclic photophosphorylation-
Non-cyclic phosphorylation results in the formation of oxygen, NADPH and ATP molecules.
Tabulate any four differences between C$$_3$$ and C$$_4$$ pathways.
Write differences between $$C_3$$ and $$C_4$$ .
With the help of biochemical reactions, explain the various phases of the Calvin cycle.
Calvin cycle is the second phase of the photosynthetic reactions which occurs in the stroma of the chloroplast.
There are 3 phases in this pathway. The first phase is carbon fixation. The enzyme RuBisCo is responsible for the fixation of carbon dioxide which reacts with the primary acceptor RUBP (Ribulose-1,5- bisphosphate). The first stable product is 3- PGA (3-phosphoglyceric acid).
The second step is reduction. 3-PGA is converted to glyceraldehyde-3-phosphate (G3P). ATP and NADPH are used for this reaction.
The third step is formation of glucose molecule. Some of the molecules of G3P are used for the formation of glucose.
The fourth step is regeneration. This is the step when ceratin molecules of G3P are used to regenerate the RUBP for the next cycle.
What is the site of formation of glycoxylate from glycolate?
The site of formation of glycoxylate from glycolate is the peroxisome of mesophyll cells.
By looking at a plant externally can you tell whether a plant is $$C_3$$ or $$C_4$$. Why and how ?
By looking at which internal structure of a plant can you tell whether a plant is $$C_3$$ or $$C_4$$? Explain.
Give comparison between the following :
(a) $$C_3$$ and $$C_4$$ pathways,
(b) Cyclic and noncyclic photophosphorylation.
(c) Anatomy of leaf in $$C_3$$ and $$C_4$$ plants.
Distinguish between
(a)Respiration and photorespiration.
(b) Cyclic photophosphorylation and non-cyclic photophosphorylation.
Define :(a) Photon (b) Quantum (c)$$C_3$$ plant (d) $$C_4$$ plant (e) Light Reaction (f) Photorespiration (g)Limiting Factor.
Name any two $$C_4$$ plants, Give anatomical characters of these plants.
Why is photorespiration also called $$C_2$$ cycle?
A plant is believed to be releasing oxygen during night time. Do you believe that the statement is true? Justify your answer with reason.
What is photorespiration ? How are photorespiratory losses overcome by $$C_4$$ plants ?
Name the first stable product of $$C_4$$ cycle ?
Specify how $$C_4$$ photosynthetic pathway increases $$CO_2$$ concentration in bundle sheath cells of sugarcance.
What is the difference between chlorophyll a and chlorophyll b?
Give the importance of $$C_4$$ plants.
Where does non-cyclic photophosphorylation take place ? Describe the process. Why is process referred to as non-cyclic ?
Non-cyclic photophosphorylation takes place in the granal thylakoids of chloroplasts. Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both ATP and $$NADPH+H^{+}$$ are synthesised by this kind of electron flow. First, in PS II, the $$P_{680}$$ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit which is farther from the atomic nucleus.
These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on redox potential scale. The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PS I ($$P_{700}$$) are excited when they receive light of wavelength 700 nm and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of $$NADP^{+}$$. The addition of these electrons reduces the $$NADP^{+}$$ to $$NADPH+H^{+}$$. It is also known as Z-scheme due to the zig-zag shape formed by the carriers.
This process is referred to as non-cyclic because the flow of electrons is non-cyclic. Electrons do not return back to PS I, they move to a molecule of $$NADP^{+}$$.
Describe the mechanism of Hatch and Slack pathway in $$C_4$$ plants.
Hatch and Slack pathway is a cyclic process. The primary $$CO_{2}$$ acceptor is a three-carbon molecule phosphoenolpyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyses this $$CO_{2}$$ fixation is PEP carboxylase or PEPcase. The mesophyll cells pf $$C_{4}$$ plants lack the enzyme RuBisCO. The 4-carbon oxaloacetic acid (OAA) is formed in the mesophyll cells. It is then converted to other four-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, these are then transported to the bundle sheath cells. In the bundle sheath cells, these $$C_{4}$$ acids are broken down to release $$CO_{2}$$ and three-carbon molecule. The $$CO_{2}$$ released in the bundle sheath cells enters the $$C_{3}$$ or the Calvin pathway. The bundle sheath cells are rich in an enzyme RuBisCO but lack PEPcase. The three-carbon molecule is transported back to the mesophyll cells where it is converted to PEP again, thus completing the cycle.
Describe $$C_3$$ pathway of photosynthesis. How does it differ from $$C_4$$ pathway?
Define carbon fixation in plants. What was the contribution of Melvin Calvin. Describe the three phases of Calvin cycle.
How are ATP, NADPH and oxygen produced in noncyclic photophosphorylation.
Name any two $$C_4$$ plants. Describe how carbon dioxide is fixed in such plants.
What is photorespiration ? Explain the mechanism and its significance.
What is meant by photophosphorylation ? Discuss the cyclic photophosphorylation.
Calvin cycle consists of three phases. What are they? Explain the significance of each of them.
Calvin cycle was discovered by Melvin Calvin while radioactive studies of alga. It occurs in all photosynthetic plants whether they have $$C_{3}$$ or $$C_{4}$$ pathway.
Calvin cycle can be described under three stages:
- Carboxylation: It is the fixation of $$CO_{2}$$ into a stable organic intermediate. In this, $$CO_{2}$$ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO and it results in the formation of two molecules of 3-PGA (3-Phosphoglyceric acid).
- Reduction: These reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction, per molecule of $$CO_{2}$$ fixed. The fixation of six molecules of $$CO_{2}$$ and six turns of the cycle are required for the removal of one molecule of glucose from the pathway.
- Regeneration: For the cycle to continue uninterrupted, regeneration of the $$CO_{2}$$ acceptor molecule is crucial. This step requires one ATP for phosphorylation to form RuBP. To make one molecule of glucose, six turns of the cycle are required. 18 ATP and 12 NADPH molecules are used to make a molecule of glucose. Hence, for every $$CO_{2}$$ molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.
What is the disadvantage of photorespiration? Explain how the photorespiratory losses are overcome in plants like sugarcane.
Where does cyclic photophosphorylation take place in the leaves ? Why is this process called cyclic? Describe cyclic photophosphorylation.
Cyclic photophosphorylation takes place in chloroplasts in the leaves. It takes place in the stroma lamellae membrane of chloroplasts. The process of cyclic photophosphorylation involves only photosystem I. This process is called cyclic because when only PS I is functional, the electron is circulated within the photosystem and photophosphorylation occurs due to the cyclic flow of electrons. The membrane or lamella of the grana have both PS I and PS II, the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $$NADP^{+}$$ and is cycled back to the PS I complex through the electron transport chain. Cyclic photophosphorylation is not connected with photolysis of water. Therefore, no oxygen is evolved. It synthesises ATP only. It does not take part in photosynthesis except in certain bacteria.
Compare $$C_4$$ and $$C_3$$ pathways of $$CO_2$$ fixation during photosynthesis. Give well labelled schematic sketch of $$C_4$$ pathway.
$$C_{4}$$ pathway is also known as Hatch and Slack pathway.
Comparison between $$C_{3}$$ and $$C_{4}$$ pathways
| $$C_{3}$$ pathway | $$C_{4}$$ pathway |
| The primary acceptor of $$CO_{2}$$ is RuBP - a six carbon compound. | The primary acceptor of $$CO_{2}$$ is PEP - a three carbon compound. |
| The first stable product is 3-phosphoglycerate. | The first stable product is oxaloacetic acid. |
| It occurs in the mesophyll cells of the leaves. | It occurs in the mesophyll and bundle-sheath cells of the leaves. |
| It is a slower process of carbon-fixation. | It is a faster process of carbon fixation. |
| 3 ATP are consumed to fix one $$CO_{2}$$. | 2 ATP are consumed to fix one $$CO_{2}$$. |
What is used, produced, and accomplished by the Calvin-Benson cycle?
Where does Calvin cycle take place in chloroplast ? Explain the cycle.
It takes place in the stroma of chloroplast in photosynthetic organisms.
Calvin Cycle is a series of reaction, also known as the dark reactions, or light-independent reactions because they are not directly driven by light,
The Calvin cycle of photosynthesis can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.
1. Carbon fixation.
A CO₂ molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This results in a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.
2.Reduction.
In the second stage, ATP and NADPH are required to convert the 3-PGA molecules in the first stage into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).
Moreover, this stage is called 'reduction' stage because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P.
3. Regeneration.
Some G3P molecules go to make glucose, while others get recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions.
Write the short notes on $$C_4$$ and $$C_3$$ plant.
Name the following :
The part of chloroplast where the dark reaction of photosynthesis takes place.
A potted plant was taken in order to prove a factor necessary for photosynthesis. The potted plant was kept in the dark for 24 hours. One of the leaves was covered with black paper in the centre. The potted plant was then placed in sunlight for a few hours.
Draw a neat diagram of a chloroplast and label its parts.
Neat labelled diagram of chloroplast.
Fill in the blanks with the appropriate answer from the choices given in the brackets.
The dark reaction of photosynthesis is known as ............... (Hill reaction, cyclic phosphorylation, Calvin cycle).
Fill in the blanks with the appropriate answer from the choices given in the brackets.
Stroma is the ground substance in ............... (cytoplasm, chloroplast, ribosomes).
Short answer type questions
Describe the chemiosmotic mechanism of ATP synthesis
Explain why respiration is said to be the reverse of photosynthesis.
Short / Long answer type questions .
What.is the role of ATP in energy transformations within living organisms?
Write the full form of G-6-PD
Give a schematic representation of non-cyclic photophosphorylation showing both the photosystems.
The light-requiring part of photosynthesis in higher plants, in which an electron donor is required, and oxygen is produced as a waste product. It consists of two photoreactions, resulting in the synthesis of ATP and NADPH 2. When photosystem II absorbs light, an electron excited to a higher energy level in the reaction centre chlorophyll (P680) is captured by the primary electron acceptor. This reaction splits a water molecule into two hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O2. Each photoexcited electron (energized by light) passes from the primary electron acceptor in photosystem II to photosystem I via an electron transport chain. The production of ATP in the chloroplast is called photophosphorylation because the energy harnessed in the process originally came from light. The ATP generated in this process will provide the energy for the synthesis of glucose during the Calvin cycle (light independent reactions).
Discuss the major events in the Calvin cycle.
The Calvin cycle (also known as the Benson-Calvin cycle) is the set of chemical reactions that take place in chloroplasts during photosynthesis. The cycle is light-independent because it takes place after the energy has been captured from sunlight. The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.
A green plant starts evolving carbon dioxide instead of oxygen on a hot summer day. Explain.
Describe the flow of electrons in non-cyclic photophosphorylation and compare it with that of cyclic photophosphorylation.
When photosystem II absorbs light, an electron excited to a higher energy level in the reaction centre chlorophyll (P680) is captured by the primary electron acceptor. The oxidized chlorophyll is now a very strong oxidizing agent. This reaction splits a water molecule into two hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O2. Each photoexcited electron (energized by light) passes from the primary electron acceptor in photosystem II to photosystem I via an electron transport chain. As electron move down the chain, their lower energy level is harnessed by the thylakoid membrane to produce ATP (by chemiosmosis) called non-cyclic photophosphorylation. Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P700) but not photosystem II (P680). This is called cyclic photophosphorylation. The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises.
Very short answer type.
Explain $$CO_2$$ compensation point
Define photo-oxidation and photorespiration.
Explain Kranz Anatomy.
Give an example of a plant exhibiting Kranz Anatomy.
What is the use of ATP binding site?
What is tricarboxylic acid cycle? Describe its different steps.
TCA cycle
•TCA cycle is also known as citric acid cycle, carbon dioxide is released from organic intermediates
•ATP is synthesized from ADP and inorganic phosphate via substrate level phosphorylation and electrons are captured by coenzyme
•The citric acid cycle completes the energy yielding oxidation of organic molecules and these events take place within the mitochondria matrix
Steps of TCA cycle
•C-C Bond formation to make citrate
•Isomerisation via hydration/ rehydration
•Oxidative decarboxylation to give 2 NADH
•Substrate level phosphorylation to give GTP
•Dehydrogenation to give reduced $$FADH_2$$
•Hydration
•Hydrogenation to give NADH
Name the three organelles involved in photorespiration.
In $$C4$$ cycle to get 1 Molecule of glucose. How many $$ATP$$ and $$NADPH_2$$ are required.
What is Glucocorticoids?
- Glucocorticoids are a class of corticosteroids, which are a class of steroid hormones.
- Glucocorticoids are the corticoids that bind to the receptor, that is present in almost every vertebrate animal cell.
Name the following :
A photosynthetic stage which may take place in the presence and absence of light.
You would recollect from previous unit that the mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light. When do you think the chloroplasts will be aligned with their flat surfaces parallel to the walls ? when would they be perpendicular to the incident light ?
Why is photorespiration is considered as a destructive activity?
Explain $$CO_2$$ fixation by the Calvin-Benson cycle?
CO$$_2$$ fixation occurs by means of the Calvin cycle reactions, that occur in the stroma of the chloroplast. These reactions occur in three stages: fixation, reduction, and regeneration (shown in the image). During CO$$_2$$ fixation enzyme RuBisCO catalyzes a reaction between CO2 and RuBP, forming a six-carbon compound that is immediately converted into two three-carbon compounds (3-PGA). This process is called carbon fixation because CO2 is “fixed” from its inorganic form into organic molecules.
What is the importance of the following processes occurring during photosynthesis in plant.
Emission of electrons from chlorophyll.
Photolysis of water.
The G-actin polymerises to form the F-actin in the presence of ____________
With the help of a diagram explain Z scheme of light reaction
The “Z‐scheme” describes the oxidation/reduction changes during the light reactions of photosynthesis. The vertical axis in the figure represents the reduction potential of a particular species—the higher the position of a molecular species, the more negative its reduction potential, and the more easily it donates electrons. 
In the Z‐scheme, electrons are removed from water (to the left) and then donated to the lower (non‐excited) oxidized form of P680. Absorption of a photon excites P680 to P680*, which “jumps” to a more actively reducing species. P680* donates its electron to the quinone‐cytochrome bf chain, with proton pumping. The electron from cytochrome bf is donated to PSI, converting P700 to P700*. This electron, along with others, is transferred to NADP, forming NADPH. Alternatively, this electron can go back to cytochrome bf in cyclic electron flow.
Explain Calvin cycle.
What is the primary acceptor of $$CO_2$$ in $$C_4$$ plants? What is the first compound formed as a result of primary carboxylation in the $$C_4$$ pathway?
Write the differences between $$C_3$$ and $$C_4$$ pathway.
Find out how Melvin Calvin worked out the complete biosynthetic pathway for synthesis of sugar.
Write five differences between $$C_3$$ and $$C_4$$ plants.
Why photorespiration does not take place in C$$_4$$ plants?
$$CO_2$$ fixation rate in under high light condition in $$C_3$$ & $$C_4$$ is:
Six turns of Calvin cycle are required to generate one mole of glucose. Explain.
Some of these terms/chemicals are associated with the $$C_4$$ cycle. Explain.
Bundle sheath cells
A process is occurring throughout the day, in 'X' organism. Cells are participating in this process. During this process ATP. $$CO_2$$ and water evolved. It is not a light dependent process.
(a) Name the process.
(b) Is it a catabolic or an anabolic process?
(c) What could be the raw material of this process?
Give comparison between the following:
(a) C$$_3$$ and C$$_4$$ pathways
(b) Cyclic and non-cyclic photophosphorylation
(c) Anatomy of leaf in C$$_3$$ and C$$_4$$ plants.
Even though a very few cells in a C$$_4$$ plant carry out the biosynthetic Calvin pathway, yet they are highly productive. Can you discuss why?
What are phytoplankton?
Explain Calvin cycle with the help of diagram.
Calvin cycle is the second phase of the photosynthetic reactions which occurs in the stroma of the chloroplast.
There are 3 phases in this pathway. The first phase is carbon fixation. The enzyme RuBisCo is responsible for the fixation of carbon dioxide which reacts with the primary acceptor RUBP (Ribulose-1,5- bisphosphate). The first stable product is 3- PGA (3-phosphoglyceric acid).
The second step is reduction. 3-PGA is converted to glyceraldehyde-3-phosphate (G3P). ATP and NADPH are used for this reaction.
The third step is formation of the glucose molecule. Some of the molecules of G3P are used for the formation of glucose.
The fourth step is regeneration. This is the step when certain molecules of G3P are used to regenerate the RUBP for the next cycle.
Tabulate any eight differences between C$$_3$$ and C$$_4$$ cycles.
Describe the cyclic photophosphorylation. What is the purpose of proton gradient during the process in thylakoids?
RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $$C_4$$ plants ?
(i) How many turns of dark reaction yield one molecule of glucose ?
(ii) Explain main chemical steps that take place in the dark reaction or Calvin cycle of photosynthesis.
(i) Three turns.
(ii) The Calvin cycle of photosynthesis can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.
1. Carbon fixation
A CO₂ molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This results in a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.
2. Reduction
In the second stage, ATP and NADPH are required to convert the 3-PGA molecules in the first stage into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).
Moreover, this stage is called 'reduction' stage because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P.
3. Regeneration
Some G3P molecules go to make glucose, while others get recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions.
Even though a very few cells in a $$C_4$$ plant carry out biosynthetic Calvin pathway, yet they are highly productive. Can you discuss why ?
What is Kranz anatomy? Write a note on photosynthesis of those plants in which this anatomy is found.
$${ C }_{ 4 }$$ plants have Kranz anatomy which is the special structure of leaves where the tissue equivalent to the spongy mesophyll cells is clustered in a ring around the leaf veins, outside the bundle-sheath cells. The bundle-sheath cells contain large chloroplasts whereas the spongy mesophyll cells have a few chloroplasts.
$${ C }_{ 4 }$$ carbon fixation or the Hatch-Slack pathway is a photosynthetic process in $${ C }_{ 4 }$$ plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. $${ C }_{ 4 }$$ refers to the four-carbon molecule that is the first product of this type of carbon fixation.
$${ C }_{ 4 }$$ overcomes photorespiration where the enzyme RuBisCO wastefully fixes oxygen rather than carbon dioxide. This is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen. Carbon dioxide is shuttled from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells, carbon dioxide is released due to decarboxylation. $${ C }_{ 4 }$$ plants use PEP carboxylase to capture more carbon dioxide in the mesophyll cells. PEP carboxylase (three carbons) binds to carbon dioxide to make oxaloacetic acid (OAA). OAA then makes malate (four carbons). Malate enters bundle sheath cells and releases the carbon dioxide. This requires more energy in the form of ATP.
Using this extra energy, $${ C }_{ 4 }$$ plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or carbon dioxide.
(a) Schematically represent non-cyclic photophosphorylation in Angiosperms, giving all the components.
(b) Why is the process called non-cyclic ?
(a) Non-cyclic photophosphorylation takes place in the granal thylakoids of chloroplasts. Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both ATP and $$NADPH+H^{+}$$ are synthesised by this kind of electron flow. First, in PS II, the $$P_{680}$$ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit which is farther from the atomic nucleus.
These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on redox potential scale. The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PS I ($$P_{700}$$) are excited when they receive light of wavelength 700 nm and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of $$NADP^{+}$$. The addition of these electrons reduces the $$NADP^{+}$$ to $$NADPH+H^{+}$$. It is also known as Z-scheme due to the zig-zag shape formed by the carriers.
(b) This process is called non-cyclic because the flow of electrons is non-cyclic. Electrons do not return back to PS I, they move to a molecule of $$NADP^{+}$$ to form $$NADPH+H^{+}$$.
What is photophosphorylation ? Distinguish between cyclic and non-cyclic photophosphorylation. Give schematic sketches of both.
The production of ATP by the light-dependent reactions is called photophosphorylation, as it uses light as an energy source.
Photophosphorylation may be either a cyclic process or a non-cyclic process.
Photophosphorylation may be either a cyclic process or a non-cyclic process.
| Cyclic photophosphorylation | Non-cyclic photophosphorylation |
| 1. Predominant in bacteria. | Predominant in green plants. |
| 2. Only photosystem I (PS1) is involved. | Both photosystems 1 and 2 (PS1 and 2) are involved. |
| 3. Since PS I is involved, the active reaction centre is P₇₀₀. | The active reaction centre is P₆₈₀. |
| 4. The movement of electrons is cyclic. | The movement of electrons is non-cyclic. |
| 5. This process produces no NADPH and no O₂, but it does make ATP. | This process produces both NADPH and ATP, and O₂ is evolved as a by-product. |
| 6. Photolysis (water-splitting) is absent. | Photolysis (water-splitting) is present. |
| 7. The electrons travel back to PS1 and complete the cycle. | The electrons from PS1 are accepted by NADPH, making the cycle incomplete. |
(a) Give a schematic outline of photorespiration.
(b)Differentiate between photophosphorylation and photorespiration.
Very short answer type :
Mention the most significant function of bundle sheath.
Can ribulose biphosphate (RuBP) combine with both carbon dioxide and oxygen gas?
Short /Long answer type question.
Enlist the steps involved in $$C_4$$ pathhway of fixation. Explain how plants overcome the photo-respiratory losses by this mechanism.
$$\bullet $$ In this pathway, phospho-end pyruvate (PEP) combines with
atmosphere $$ CO_{2}$$ in the pressure of catalyst energy phosphoemol
pyruvate cortroylax (PEP case) to from oxaloectic acid
(OOA)
$$\bullet$$ $$Rx^{2}$$ occurs in the mesophyll cell og the plants OOA is then
consented into another 4 carbon compound malice acid
which is trans ported into bundle sheath cell from
mesophyll cells.
$$ \bullet $$ In the bundle sheath cells, the malice acid gets decarlroylased
and redoax $$ CO_{2}$$ and pyruric acid
$$ \bullet $$ $$CO_{2} $$ Liberated is used for calcine cycle in the bundle sheath
cell and pyruric acid transported back to moesphyll cell
which is used for regenerative of $$ PEP $$
$$\bullet $$ Photorespiratow is regarded as a wasted prows.
$$\bullet $$ In $$ C_{4} $$ plants phosphorylation does not occurs This is because they
have a mechanism that increase the concentration of
$$ CO_{2} $$ at the enzyme site
$$\bullet $$ This results in increasing the interstellar concentration
of $$ CO_{2} $$ which ensures that the Ru Bisco functions
cartorylase and minimize the oxygenase activity
Application based questions
Suggest some habitats in which (a) light intensity or (b) temperature might be limiting factors in photosynthesis.
Short /Long answer type question.
Trace the non-cyclic electron pathway (Z-scheme), naming and explaining all the events that occur as the electrons move from water to $$NADP^+$$
A route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATP, NADPH, and oxygen. The neet electron flow is from water to NADP+.
The following are the steps in which it occurs:
• Firstly the light from the sun strikes the PS II system in which it results in splitting of the water molecule by the use of 2 photons.
• Then due to the above reaction 2 electron is generated which is absorbed by the reaction center of PS II.
• Then these electron is passed to the primary acceptor which then pass it to the cytochrome complex where one ADP is used and ATP is released.
• Then it is passed to the PS I reaction center and it’s is then passed to ferredoxin.
• In this, the enzyme NADP reductase is acted on the NADP$$^+$$ and proton which result in the release of NADPH.
The following are the steps in which it occurs:
• Firstly the light from the sun strikes the PS II system in which it results in splitting of the water molecule by the use of 2 photons.
• Then due to the above reaction 2 electron is generated which is absorbed by the reaction center of PS II.
• Then these electron is passed to the primary acceptor which then pass it to the cytochrome complex where one ADP is used and ATP is released.
• Then it is passed to the PS I reaction center and it’s is then passed to ferredoxin.
• In this, the enzyme NADP reductase is acted on the NADP$$^+$$ and proton which result in the release of NADPH.
Short /Long answer type question.
Trace the cyclic electron pathway, naming and explaining the main events that occur as electrons cycle.
Short /Long answer type question.
What is the Z-scheme of photosynthesis ? What are the products of photoreaction and how they are used in the process of $$CO_2$$ fixation ?
Short /Long answer type question.
How the tracer technique is useful in determining the reaction sequence of the metabolic process? Explain with the help of Calvin cycle.
In the radioactive tracer technique, radioactive nucleotides are used to follow the behavior of elements or chemical species in chemical and other processes. In 1913, Hevesy and Paneth succeeded in determining the extremely low solubility of lead salts by using naturally occurring 210Pb as a radioactive tracer. Now many artificially radioactive tracer elements are available.
In 1946 Calvin began his work on photosynthesis.
After adding carbon dioxide with trace amounts of radioactive carbon-14 to an illuminated suspension of the single-cell green alga Chlorella pyrenoidosa, he stopped the alga’s growth at different stages and used paper chromatography to isolate and identify the minute quantities of radioactive compounds. This enabled him to identify most of the chemical reactions in the intermediate steps of photosynthesis—the process in which carbon dioxide is converted into carbohydrates. This was the first use of a carbon-14 tracer to explain a chemical pathway.
The cycle is light-independent because it takes place after the energy has been captured from sunlight.
The steps in the cycle are as follows:
1. Grab: A five-carbon carbon catcher called RuBP (Ribulose bisphosphate) catches one molecule of carbon dioxide and forms a six-carbon molecule.
2. Split: the enzyme RuBisCO (with the energy of ATP and NADPH molecules) breaks the six-carbon molecule into two equal parts.
3. Leave: A trio of carbons leave and become sugar. The other trio moves on to the next step.
4. Switch: Using ATP and NADPH, the three carbon molecule is changed into a five-carbon molecule.
5. The cycle starts over again.
Very short answer type.
Give the exact location and function of grana
Write notes on
(a) Emerson enhancement effect(b) Warburg effect(c) Red drop(d) Photorespiration.
Explain the Hatch-Slack pathway.
HSK pathway (Hatch-Slack pathway) is also called as C$$_4$$ pathway. In C$$_4$$ plants , the CO$$_2$$ is not directly absorbed by RuBP in presence of enzyme helps to absorb CO$$_2$$ having concentration of 550 ppm or more. However, C$$_4$$ plants, due to high temperature and high light intensity, keep their stomata partially closed due to which the CO$$_2$$ concentration goes down to 2 ppm and calvin cycle cannot be started. Therefore C$$_4$$ plants follow C$$_4$$ pathway in which CO$$_2$$ is absorbed by PEP in presence of enzyme PEP-carboxylase.
The main steps of C$$_4$$ pathway are given below:
- CO$$_2$$ of low concentrations enters the mesophyll cells through stomata and is absorbed by phosphoenol pyruvic acid (3C) with the help of PEP-carboxylase to form oxaloacetic acid (4C).
- OAA (4C), being unstable, gets converted into stable 4C acids like malic acid or aspartic acid.
- These acids diffuse into bundle sheath where they release CO$$_2$$ to form pyruvic acid (3C).
- When enough of CO$$_2$$ is accumulated, itis absorbed by RuBP-carboxylase of calvin cycle.
- Pyruvic acid diffuses back to mesophyll cells.
- Pyruvic acid (3C) is phosphorylated by ATP to form phosphoenol pyruvic acid to continue HSK pathway.
How does the length of a neuron affect the speed with which the nerve impulse Travels along it?
$$C4$$ plants ultimately use RUBISCO, yet have lower rate of photorespiration than do $$C_3$$ plants. Explain.
Calculate how many ATP are synthesised and deduct the number of ATP utilised during glycolysis.
During the glycolytic pathway, 1,3 bisphosphoglycerate is converted into three phosphoglycerate and released phosphate yields one molecule of ATP. In another step, phosphoenolpyruvate is converted into pyruvate and forms the second molecule of ATP. These two steps take place two times in glycolysis. Hence, four molecules of ATP are synthesized. The steps such as conversion of glucose to glucose-6 phosphate and fructose-6 phosphate to fructose-1, 6 bisphosphate consumed two molecules of ATP. Thus net gain of ATP molecules in glycolysis is 2.
Whether Photorespiration is present at low light intensities.
Granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.
What do you understand by photophosphorylation? Explain the process in detail.
Formation of ATP molecule from ADP and inorganic phosphate (Pi) in the presence of sunlight is called photophosphorylation. This process takes place in the grana region of chloroplast and solar energy is stored in the form of chemical energy in the ATP molecule.
- The formation of ATP from ADP in chloroplast by the use of light energy is called photophosphorylation.
- This was discovered by Arnon et. al. (1954).
- Cyclic photophosphorylation
- non-cyclic photophosphorylation
- In cyclic photophosphorylation electrons ejected out from $$P_{700}$$ return back to $$P_{700}$$ and during their transport through various electron acceptors bring about ATP synthesis. Hence it is called cyclic photophosphorylation.
- In this process, only PS-l is involved.
- Electrons ejected out from reaction centre $$P_{700}$$ travel through different electron acceptors such as $$A(FeS) \rightarrow Fd \rightarrow Cyt. b_{6} \rightarrow Cyt f\rightarrow Plastocyanin and are finally recycled to $$P_{700}.$$
- During this transport sufficient energy is released when electrons jump from Cyt. $$b_{6}$$ to Cyt f. and the energy is used in the synthesis of ATP.
- In this process electrons ejected out from $$P_{680}$$ do not return back to $$P_{680}$$ and travel to different electron acceptors and during this flow brings about ATP synthesis. Hence it is called as non-cyclic photophosphorylation.
- In this process, both PS-l and PS-ll are involved.
- Electrons ejected out from reaction centre $$P_{680}$$ flow through a series electron acceptors such as Pheophytin
- From $$P_{700}, $$ on electrons flow to ferredoxin where they bring about a reduction of NADP to NADPH + $$H^{+}$$.
- During this flow sufficient energy is released when electrons jump from plastoquinone to cytochrome $$b_[6}$$, and the energy is used in the synthesis of ATP.
| Cyclic photophosphorylation | Non-cyclic photophosphorylation | |
| 1. | Only PS-l participates | Both PS-l and PS-ll participate |
| 2. | Photolysis of water does not take place | Photolysis of water takes place |
| 3. | $$O_{2}$$ is not released | $$O_{2}$$ is released |
| 4. | $$NADPH_{2}$$ is not generated | $$NADPH_{2}$$ is generated |
- The net result of the reactions of PS-l and PS-ll is the liberation of $$O_{2}$$ by photolysis of water, synthesis of ATP molecules, and generation of NADPH + $$H^{+}$$.
- The ATP and NADPH + $$H^{+}$$ are used during the dark reaction in the reduction of $$CO_{2}$$ and formation of carbohydrate.
Give the diagrammatic representation of $$C_4$$ cycle.
With the help of a suitable diagrammatic representation explain HSK pathway.
HSK pathway or Hatch-Slack pathway is a cyclic process. It was discovered by two Australian scientists Hatch and Slack. It is also known as a $$C_4$$ pathway because it is found in $$C_4$$ plants. Carbon dioxide fixation is done in this pathway.
1) In Mesophyll cells, The primary $$CO_2$$ acceptor is three-carbon molecule phosphoenolpyruvate (PEP). PEP carboxylase enzyme helps in the fixation of the $$CO_2$$. Here, $$CO_2$$ gets converted into 4- Carbon oxaloacetate and then to malate.
2) In Bundle sheath cell, the malate gets converted into $$CO_2$$ and Pyruvate. This pyruvate transferred to the mesophyll cell to form phosphoenolpyruvate and $$CO_2$$ gets into the Calvin cycle and with the help of RuBisCO enzyme, it gets converted into sugar molecule.
In $$C_4$$ pathway carboxylation occurs twice. Discuss.
During photorespiration oxidation, deamination and decarboxylation all takes place in :-
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