Thinking Process

Chloroplast is a type of plastid (a cell organelle) which is green in colour. It absorbs light energy and convert it to a chemical energy with the help of the chlorophyll pigments.

Answer

(c) Magnesium $(\mathrm{Mg})$ is present in the centre of porphyrin ring of the chlorophyll molecule. Other ions, i.e., iron, copper and zinc participate in other metabolic processes which are

$\mathrm{Fe}$ is an important part of cytochrome and ferridoxin.

Cu plays an important role in enzyme catalysing redox reactions.

$\mathrm{Zn}$ is associated with auxin (a phytohormone) synthesis.

Answer

(a) Chlorophyll-a acts directly to convert light energy to chemical energy.

The other pigments chlorophyll-b, xanthophyll and carotenoids are accessory pigments associated with the main pigment, i.e., chlorophyll-a to harvest the light energy.

Answer

(c) Photosynthetically Active Radiation (PAR) ranges from 400-700 nm. This is the visible range of light energy.

The range of other wavelength are associatic with

$$ \begin{array}{lcl} 100-300 \mathrm{~nm} & : &\text { Ultraviolet (UV) range } \\ 390-430 \mathrm{~nm} & : &\text { Infrared (NIR) range } \\ 760-100,00 \mathrm{~nm} & :& \text { Mid Infrared (MIR) range } \end{array} $$

Thinking Process

The visible spectrum wavelength ranges from 400 - $700 \mathrm{~nm}$ of which red light has $700 \mathrm{~nm}$.

Answer

(c) Red light is most effective in photosynthesis.

In other lights namely blue and violet the photosynthesis takes place but the rate is slow, whereas in green light the photosynthesis is least because plants do not absorb this range of light reflect back thus, they appear green.

Answer

(d) In Inorganic Chemicals Chemosynthetic bacteria were the first organism on earth synthesise their own food by obtaining energy from chemicals like $H_{2} \mathrm{~S}, NO_{2}$, etc. Photosynthetic bacteria have taken their origin from chemosynthetic bacteria.

Sun, infrared rays and organic substances are not used as the sources of energy for chemosynthetic bacteria.

Answer

(a) The synthesis of ATP is directly linked to the development of proton gradient across the thylakoid membranes of a chloroplast. It results when the water molecule splits inside the inner membrane and form $\mathrm{H}^{+}$and $\mathrm{OH}^{-}$ions.

The energy for ATP synthesis comes from proton gradient which develops along the inner membrane, e.g., in case of mitochondria in electron transport chain and in chloroplast in the PS II.

The other options (b), (c) and (d) are not involved in the synthesis of ATP molecule.

Answer

(c) Light dependent reaction uses solar power to generate ATP and $NADPH_{2}$ which provide chemical and reducing power respectively to sugar synthesising reaction of the Calvin cycle, while the $O_{2}$ in released as a by product of light dependent reaction. Other options are incorrect because

(a) ATP is produced in light reaction but sugar is produced in dark reaction.

(b) $H_{2}$ is produced by splitting of water $(\mathrm{H_{2} O \rightarrow 2 H^{+}+\frac{1}{2} O_{2} H^{+}})$are released. The $2 H^{+}$is accepted by NADP to form NADPH and oxygen $\left(\mathrm{O}_{2}\right)$ is of course produced but sugar is not.

(c) $\mathrm{O}_{2}$ donor and ATP is produced in light reaction but not the hydrogen.

Thinking Process

$C_{3}$ cycle is involved in all kinds of plants carrying out photosynthesis which produces glucose as the first product of photosynthesis. There are certain adaptations and modifications in dark reactions of photosynthesis in certain plants called $C_{4}$ plants.

Answer

(b) Dark reaction is called so because it does not depend on light. It is also called as light independent reaction.

Answer

(a) $C_{4}$ plants have evolved PEP as primary acceptor of $CO_{2}$ to avoid the sensitivity of RuBP carboxylase-oxygenase to high concentration of oxygen, so that they can avoid photorespiratory loss of $CO_{2}$ occuring in them.

Other options (b), (c) and (d) are incorrect. PEP is not primary acceptor in them.

Answer

(d) The thylakoid is a photosynthetic unit in the chloroplast. It is a membrane bound structure. The membrane has photosystems I and II embedded in it, in the form of chemicals and molecules. Splitting of water is associated with PS-II, which occurs in presence of $\mathrm{Mn}^{2+}$ and $\mathrm{Cl}^{-}$ions on the inner surface of thylakoid membrane.

$2 \mathrm{H}_2 \mathrm{O} \xrightarrow[\mathrm{Cl}^{-}]{\mathrm{Mn}^{+}} 4 \mathrm{H}^{+}+\mathrm{O}_2+4 \mathrm{e}^{-}$

Photosystem I and II are associated with light reactions.

Thinking Process

The light reaction of photosynthesis is mainly involved in trapping of solar radiation and converting it to ATP and produce NADPH also. The excited electron move through $^{\text {The }}$ chains of pigments and their energy is used in ATP formation.

Answer

(a) Option (a) is the correct sequence of movement of electrons.

The options (b), (c) and (d) do not give this correct sequence of events.

Thinking Process

$C_{3}$ plants are those plants which have calvin cycle to produce glucose molecules in dark reaction. $C_{3}$ cycle is found in all photosynthesising plants. Some plants have problem in fixing atmospheric $CO_{2}$ because of deviation to property of RuBP carboxylase oxygenase enzyme. So they have evolved $C_{4}$ mechanism.

Answer

(b) PEP carboxylase enzyme is found in $C_{4}$ plants to carry out initial fixation of $CO_{2}$. The functions of other enzymes in options (a), (c) and (d) are as follows

(a) RuBP carboxylase operates in $C_{3}$ plants.

(c) NADP reductase is involved in electron transport chain.

(d) ATP synthase is used in ATP synthesis.

Answer

(c) In $C_{3}$ cycle RuBP carboxylase is used to fix atmospheric $CO_{2}$ whereas, in $C_{4}$ plants PEP carboxylase is involved in primary $CO_{2}$ fixation. So, both are used in $CO_{2}$ fixation but in different cycles.

Other options are incorrect because in (a) and (b) option is giving incomplete information and option (d) PGA synthase is not at all involved in $CO_{2}$ fixation.

Thinking Process

$C_{4}$ plants use PEP (phosphoenol pyruvate) to fix atmospheric $CO_{2}$. In a normal photosynthetic cycle RuBP carboxylase is used to trap $CO_{2}$ in $C_{3}$ plants.

Answer

(d) Oxaloacetate is a four carbon compound formed in $C_{4}$ cycle and is a first stable product, that is why such plants are called $C_{4}$ plants.

Rest of the options are incorrect as because

(a) Pyruvate is formed in glycolysis.

(b) Glyceraldehyde-3-phosphate is also an intermediate compound of glycolysis.

(c) Phosphoglycerate is also an intermediate compound of glycolysis.

Very Short Answer Type Questions

Answer

(a) The above figure show the chloroplast which is green in colour and performs photosynthesis in plants thus, The structure is present in plant cell.

(b) Yes, chloroplast has the power of self replication because of presence of extranuclear DNA.

(c) The metabolic that occurs in the marked places are as follows.

$A$-It is the stroma of chloroplast where dark reaction of photosynthesis takes place.

$B$-It is the structure of extra nuclear DNA and is responsible for replication of chloroplast, when it is required in the photosynthesising cells.

Thinking Process

Water is a universal solvent, every living cell contains $70 %$ water with protoplasm. It plays an important role in the process of photosynthesis.

Answer

(a) The reaction takes place in reaction centre PS II, located on the inner surface of thylakoid membrane. It is known as water splitting centre where electrons are extracted from water. The $\mathrm{Mn}^{+}$and $\mathrm{Cl}^{-}$ions catalyse this reaction.

(b) Splitting of water is an important event in photosynthesis as

(i) It releases molecular oxygen as byproduct of photosynthesis and is the significant source of oxygen in air, or is essential for all living beings on earth.

(ii) Hydrogen ions produced, are utilised in reducing NADP to NADPH, a strong reducing agent.

(iii) The electrons released are transferred from PS II to PS I through a series of electron carriers thus, creating a gradient for the synthesis of ATP.

Thinking Process

Cyanobacteria are simple, one cell prokaryotic organisms belonging to Monera. The process of photosynthesis have evolved in the organisms of this group.

Answer

The cyanobacteria and photosynthetic bacteria are prokaryotes. They do not have well defined membrane bound cell organelles, but these organisms have photosynthetic pigments in a membranous form, which are primitive in nature but can trap and use solar energy. So, they can carry out photosynthesis.

Answer

(a) NADP reductase enzyme is located on the outer side of thylakoid membrane. It is bounded to the thylakoid membrance in light and becomes free in stroma during dark.

(b) ATP Molecules The movement of $\mathrm{H}^{+}$ions across the membrane is coupled with the formation of ATP synthesis in presence of enzyme ATP synthase.

Thinking Process

Xylem and phloem are conducting tissues in angiosperms. Xylem transports water and minerals and phloem transport food. It has been proved by many experiments.

Answer

The girdling experiment cannot be done in monocots. The monocot stem has vascular bundles scattered all over the width of stem so, we cannot reach that specific band of the phloem tissues as we get in dicots.

Answer

(a) 2 molecules of ATP for phosphorylation and two molecules of NADPH for reduction are required per $\mathrm{CO}_{2}$ molecule fixed.

(b) The calvin cycle occurs in the stroma of the chloroplast.

Thinking Process

Solar radiation is the main source of energy for the survival of plant and animal life on the planet earth.

Answer

Plants cannot carry out photosynthesis in moonlight because it does not carry enough energy to excite chlorophyll molecule, i.e., reaction centres PS I and PS II, so light dependent reactions does not get initiate. Thus, no photosynthesis occurs in presence of moonlight.

Answer

The terms associated with $C_{4}$ cycle are

Hatch Slack Pathway The process of synthesis of glucose in $C_{4}$ plants is different from $C_{3}$ plants which was discovered by two scientists MD Hatch and CR Slack (1977). Hence, named Hatch Slack pathway.

PEP Carboxylase It is an enzyme present in mesophyll cells of $C_{4}$ plants. It fixes $CO_{2}$ to form 4-carbon compound, i.e., oxaloacetate (OAA).

Bundle Sheath Cells These are specialised sclerenchymatous cells present arround the vascular bundle, in the veins of monocot leaves. These have agranal chloroplast. $C_{3}$ cycle occurs in these cells to manufacture glucose in $C_{4}$ plants.

Calvin Cycle This cycle operates in bundle sheath cells and fixes $CO_{2}$ to form glucose molecules.

Answer

NADP reductase enzyme is located on the outer side of lamella or thylakoid of the chloroplast. This enzyme causes breakdown of proton gradient to release energy, i.e., $\mathrm{NADPH}$.

Thinking Process

Thylakoid is a photosynthetic unit present in the grana of chloroplast, the cell organelle. It is the site for many important photosynthetic reactions.

Answer

ATP synthase enzyme has two parts

(a) $\mathrm{F}_{1}$-head piece is a peripheral membrane protein complex and contain the site for synthesis of ATP from ADP + pi (inorganic phosphate).

(b) $\mathrm{F}_{0}$-integral membrane protein complex that forms the channel through which proton cross the inner membrane.

The arrangment of $F_{1}$ and $F_{0}$ in thylakoid membrane is as follaws

$\mathrm{F}_{0}$-portion is present within the thylakoid membrane.

$F_{1}$-portion of ATP synthase enzyme is present in the stroma if chloroplast.

The conformational change occurs in $F_{1}$ portion of ATP synthase thus, facilitaling the ATP synthesis.

Answer

ATP and NADPH formed during light reaction of photosynthesis are used in dark reaction for fixing $\mathrm{CO}_{2}$ and to form glucose molecule.

Answer

The basis for designating $C_{3}$ and $C_{4}$ pathways of photosynthesis is as follows

Short Answer Type Questions

Thinking Process

Succulents plants have water conserving tissue as they have adaptations to conserve water because they grow in xeric conditions. They have other mechanisms too, to conserve water/ to prevent loss of water.

Answer

Succulent plants grow in dry and xeric conditions so, they have to shut down the stomata during day time, to prevent water loss through transpiration. So and the gaseous exchange does not take place.

Thus plants have developed the mechanism to fix $CO_{2}$ during night in the form of malic acid, which is a 4 carbon compound and store $CO_{2}$, release it during day, inside the photosynthetic cells.

Answer

Accessory pigments are also photosynthetic pigments. These are chlorophyll-b, xanthophyll and carotenoids. These are not directly involved in emission of excited electrons, but they help in harvesting solar radiation and pass it on to chlorophyll-a.

This pigment itself absorbs maximum radiation at blue and red region. So, Chlorophylla is the chief pigment of photosynthesis and others (i.e., chlorophyll-b, xanthophyll and carotinoion) are accessory pegment.

Answer

Dark reactions are actually light independent reactions. $CO_{2}$ is reduced through various biochemical reactions to produce $C_{6} H_{12} O_{6}$ (glucose) which does not need light. But they depend on the products formed during light reactions, i.e., $NADPH_{2}$ and ATP.

Thinking Process

Photosynthesis and respiration both are important in plant metabolism. One is anabolic process (synthesising) other is catabolic (breakdown) reactions.

Answer

Photosynthesis and respiration are related, as in both mechanisms, the plants gain energy.

In photosynthesis, plants gain energy from solar radiations whereas, in respiration, they break down glucose molecule to get energy in the form of ATP molecules.

They are releted also because they are dependent on each other. The product of photosynthesis i.e., glucose (food) is utilised in respiration to yield energy (ATP). Which doing so, it releases many other simple molecules $\left(CO_{2}+H_{2} \mathrm{O}\right)$ whcih are utilised in photosynthesis to produce more sugers.

Answer

The plant in given conditions can not carry out photosynthesis. Light is must for any green plant to make its own food.

The plant should be watered properly for its survival.

Thinking Process

The major part of photosynthesis occuring on earth is performed under water by plants specially algae present in ocean.

Answer

Plant present at various depth in ocean are mostly algae. These show great variations in its photosynthetic pigments. These can absorb different wave lengths of light and performs photosynthesis.

Green algae-chlorophyll-a, (absorbs red) and b(absorbs blue violet).

Brown algae-chlorophyll-a, $c$ and fucoxanthin (absorbrs yollow).

Rhodophyceae-chlorophyll-a, $d$ and phycoerythrin.

Answer

The plants carry out photosynthesis in presence of light. The quality and intensity of light affect only the rate of photosynthesis. So, plants growing in different canopy will carry out photosynthesis, but rate could be different depending upon the intensity and quantity of light recevied.

Thinking Process

RuBisCo is the largest occurring enzme which is involved in photosynthesis.

Answer

RuBisCo is an enzyme which has dual nature. It acts as carboxylase, when $CO_{2}$ concentration is good enough in atmosphere. But if $O_{2}$ concentration increases, its nature changes and it binds with $O_{2}$ and acts as oxygenase enzyme, which forces $CO_{2}$ to enter $C_{2}$ cycle thus leading to photorespiration and loss of $CO_{2}$.

Answer

Photosynthesis is an enzyme specific process. All enzymes act at an otptimum temperature (i.e., $25-35^{\circ} \mathrm{C}$ ). If temperature increases, enzyme get denature, thus leading to fall in the rate of photosynthesis.

Thinking Process

Chemiosmosis refers to the movement of protons $\left(\mathrm{H}^{+}\right)$from the region of higher concentration to lower concentration. Their movement is coupled with ATP synthesis in thylalkoid membrane.

Answer

In light reaction plants trap solar radiation by photosynthetic pigments which convert light energy into chemical energy. Main event of light reaction is photophosphorylation, i.e., formation of ATP from ADP + Pi by using energy of excited electron movement through electron transport chain, present in thylakoid membrane.

Chemiosmosis is the movement of ions across a selectively permeable membrane, down the electrochemical/ proton gradient.

Chemiosmosis hypothesis of ATP formation was first proposed by Mitchell (1961) according to which the enzyme ATP synthase generales ATP via a membrane, proton pump and proton gradient. ATP synthase allows ions $\mathrm{O}_{2}$ protons to pass through membrane and proton pump.

This creales a high cocentration of protons $\left(\mathrm{H}^{+}\right)$in the lumen and hence diffuses across the membrane to activale ATPase, releasing ATP molecules. One molecule of ATP is released for every two $\left(\mathrm{H}^{+}\right)$ions passing Through ATPase.

Thinking Process

Melvin Calvin was awarded Nobel Prize in 1961 for developing the technique to trace the path of carbon in glucose synthesis in dark reaction of photosynthesis.

Answer

Melvin Calvin used Chlorella as an experimental material and discovered the first stable compound of photosynthesis, i.e., 3 phosphoglyceric acid so as to trace the path of carbon by using a radioactive isotope of carbon $\left(\mathrm{C}^{14}\right)$ and autoradiography technique.

He then, compared the radioactive compounds on the chromatogram as a result of which he found and concluded that the PGA (phosphoglyceric acid), as the first stable product of photosynthesis and gradually the other sugars including hexoses, tetroses and pentoses etc.

Thus, he derived the pathway of $\mathrm{CO}_{2}$ fixation from these radioactive products (sugars) formed.

Answer

Ribulose 5 phosphate is a five carbon compound which accepts atmospheric $CO_{2}$ in presence of RuBisCo and form 2 molecules of 3PGA, a ${ }^{3 c}$ carbon compound. It uses 3 ATP and $2 \mathrm{NADPH}$ to fix one molecule of $CO_{2}$ per cycle of Calvin.

So, to fix $6 CO_{2}$ molecules to form 6 carbon compound glucose 6 cycles are required as mentioned below.

Answer

The following flow chart show cyclic photophosphorylation and the missing part of this flow chart are

Answer

Kranz anatomy refers to the dimorphism in the chloroplast structure. It is found in $C_{4}$ plants. The cells of leaves have two types of chloroplast in them.

Granal Chloroplast It is found in the mesophyll cells of leaves. Chloroplast have well developed grana in them. These chloroplast effectively fix $CO_{2}$ even if it is present in lower concentrations. PEP carboxylase is present which fix $CO_{2}$ and to form oxaloacetic acid (4 carbon compound).

Agranal Chloroplast Present in bundle sheath cells of the leaves. $C_{3}$ cycle occurs in these cells with the presence of RuBisCo enzyme.

The $C_{4}$ plants are well adapted to high $O_{2}$ concentrations and high temperature.

$C_{4}$ plants can absorb $CO_{2}$ even when $CO_{2}$ concentration in much low thus $C_{4}$ plants can perform high rate of photosynthesis even the stomata are closed or there is the shortage of water thus, they can conserve water.

Since, PEP-carboxylase is insensitive to $O_{2}$ thus excess $O_{2}$ has us inhibitory effect in $C_{4}$ pathway and there is no photosynthesis in $C_{4}$ plant.

Thus, $C_{4}$ plants are better adapted to tropical and desert (hot acid habitats) areas than the plants, that lack this anatomy.

Answer

(a) The name of the process is cellular respiration.

(b) It is a catabolic process which involves break down of glucose molecule.

$$ \underset{\text { Carbohydrate +oxygen }}{\mathrm{C_{6} H_{12} O_{6}+6 O_{2}}} \rightarrow \underset{\text { carbon dioxide + water +(ATP) }}{6 CO_{2}+6 H_{2} \mathrm{O}+\text { Energy }} $$

(c) Raw material for this process is glucose molecule and oxygen, which are products of photosynthesis, occurring in plants.

Answer

The different colours other than green are present in the plant because of presence of chromoplasts. It is a kind of plastid and develops different coloured pigments which are not photosynthetic. Pigment lycopene present in tomato, carrot and chillies, imparts red colour to them.

Thinking Process

The plant cell is a eukaryotic cell. It has number of well developed cell organelles. Few are single membrane bound, few are double membrane bound (chloroplast and mitochondria) structures.

Answer

Both the cell organelles, chloroplast and mitochondria have extra chromosomal DNA and $70 S$ type of ribosomes. The presence of DNA enables then to duplicate, when cell need more mitochondria and the chloroplasts. So they are called as semi-autonomous cell organelles.

Answer

(a) Monocot plants belonging to Graminae/Poaceae family, e.g., sugarcane, maize etc., possess these two types of cells. i.e., bundle sheath and mesophyll cell (in kranz anatomy).

(b) A 4-carbon compound oxaloacetic acid is the first product of $C_{4}$ cycle.

(c) Mesophyll cells have PEP carboxylase to fix atmospheric $CO_{2}$ to form a4-carbon compound oxalo acetic acid, whereas bundle sheath cells have RuBP carboxylase which fix $CO_{2}$ by this enzyme to form 3-carbon compound 3 PGA (3 phosphoglyceric acid).

Answer

(a) Photorespiration is the process, which do not produce energy rather consum it.

(b) It is not needed for the survival of $C_{3}$ plant.

(c) The end product of this process is $H_{2} O_{2}$.

(d) This process involves three cell organelles of the plant cell. (i) Chloroplast
(ii) Peroxisome
(iii) Mitochondria

Answer

(a) Euphorbia is a CAM plant. It fixes $CO_{2}$ during night and uses it in day time. It will be able to survive in hot tropical climate.

(b) Maize being a $C_{4}$ plant is more efficient in terms of photosynthetic activity as it is able to use $CO_{2}$ at lower level as well as high $O_{2}$ and temperature.

(c) Maize plants show kranz anatomy in their leaves. They have granal chloroplast in mesophyll cells and agranal in bundle sheath cells. Euphorbia does not have $\mathrm{C}_{4}$ cycle so kranz anatomy is not found in them.

Thinking Process

Photosynthesis is a process of manufacturing food by plants. Many plant parts other than leaf also perform this function in some plants.

Answer

Photosynthesis is a process which mainly occurs in leaves of all green plants. The plants have designed their leaf in such a way that it is able to trap solar radiation and effectively convert solar/light energy to chemical energy.

But biology is science of exceptions. Some plants carry out photosynthesis in modified plant parts other then leaves.

Few examples are as follows

1. Root as Photosynthetic Organ

When roots develop chlorophyll and start photosynthesis, they are called assimilitory roots. Trapa and Tinospora are the examples of assimilatory roots.

2. Stem as Photosynthetic Organ

Phylloclade of Opuntia

In Opuntia, the stem gets modified to take up the function of leaves. It becomes flattened, thick and succulent and perform photosynthesis. Such structures are called phylloclade.

3. Petiole as Photosynthetic Organ In Australian Acasia the petiole takes the shape and function of photosynthesis because leaf lamina soon falls off.

Answer

(a) Synthesis of ATP and NADPH takes place in outer side of thylakoid membrane.

(b) Photolysis of water occurs in inner side of thylakoid membrane.

(c) Fixation of $CO_{2}$ occurs in stroma of chloroplast

(d) Synthesis of sugar molecule occurs in chloroplast.

(e) Synthesis of starch occurs in cytoplasm.

Thinking Process

Plants are green because of the presence of the photosynthetic pigment called chlorophyll. They absorb all colours of light except green.

Answer

The chlorophyll pigments are present in the thylakoid membranes. They have the property of excitability and emits $\mathrm{e}^{-}$in the excited stage, though this $\mathrm{e}^{-}$is replaced and transferred by the $e^{-}$generated from splitting of water molecules.

Red and Blue Light have maximum energy which a chlorophyll pigment absorbs and get excited and initiate the process of photosynthesis. Also, its wavelength are (400-700 nm) i.e., between the Photosynthetic Active Radiation (PAR).

Thus, the rate of photosynthesis is higher in blue and red light.

Answer

Absorption Spectrum This depicts the absorption of light of different wavelength by chlorophyll-a, b, xanthophyll and carotenoids.

Action Spectrum This shows the rate of photosynthesis in the plant in the light of different wavelengths.

Super Imposed Absorption and Action Spectrum When we superimposed both action and absorption spectrum, it shows that in the region of red and blue light, the chlorophyll-a and $b$ harness the maximum light energy and are the main photosynthetic pigments.

So, the rate of photosynthesis is high in these two regions. It shows maximum activity peak at wavelength (red light) i.e., 660-670 nm, 430-470 nm (blue) and 390-430 nm (violet).

Thinking Process

The different plants show different adaptations in carrying out photosynthesis. This process is important for autotrophs as it synthesises food for survival of plants and animals as well.

Answer

$C_{4}$ plants are advantageous in following ways

(i) These plants can carry out photosynthesis even at low concentration of $CO_{2}$ in the atmosphere and in the shortage of water.

(ii) These plants can tolerate high $O_{2}$ concentration and temperature as enzyme PEP carboxylase in $C_{4}$ cycle in insensitive to $O_{2}$ and do not show- photorespiration in comparison to the $C_{3}$ plants, which start process of photorespiration and lose $\mathrm{CO}_{2}$ fixation in the form of glucose molecule.

Thus, $C_{4}$ plants are superior to $C_{3}$ plants.

Answer

(a) The effectiveness of different wavelengths of light on photosynthesis is measured and the rate of photosynthesis is plotted. This is called the action spectrum of photosynthesis.

(b) Absorption of different wavelengths of light by a particular pigment is plotted and the graph is called the absorption spectra of that pigment.

(c) Chlorophyll-a is responsible for light reaction of photosynthesis, but the action spectrum and absorption spectrum do not overlap because, though chlorophyll-a is the main pigment responsible for absorption of light, other thylakoid pigments like chlorophyll-b, xanthophylls, carotenoid, which are accessory pigments, also absorb and transfer the energy to chlorophyll-a.

Indeed they not only enable a wider range of wavelength of incoming light to be utilised for photosynthesis but also protect chlorophyll-a from photooxidation.

Answer

The important events of light reaction are

(i) Excitation of chlorophyll molecule to emit a pair of electrons and use of their energy in the formation of ATP from ADP $+\mathrm{Pi}$. This process is called photophosphorylation.

(ii) Splitting of water molecule

(a) $2H_{2}O \rightarrow 4H^{+}+4e^{-}+ O_{2} \uparrow$

(b) NADP $+2H^{+} \rightarrow NADPH_{2}$

End products of light reaction are NADPH and ATP. Reducing power is produced in the light reaction i.e., ATP and $NADPH_{2}$ molecules which are used up in dark reaction, $O_{2}$ is evolved as a by product by the splitting of water.

Answer

A-Electron acceptor

B-Electron transport system

C-Chlorophyll (photosystem I) $\mathrm{P}_{700}$

The cyclic photophosphorylation is shown in the above figure.

Thinking Process

RuBisCo is the largest occurring enzyme on earth.

Answer

RuBP carboxylase and oxygenase has dual nature. It has affinity for both $CO_{2}$ and $O_{2}$ but has more affinity for $CO_{2}$ than $O_{2}$. Thus, the concentrations of two determines which of the two will bind to the enzyme.

Consider the following two situations

(i) In a normal condition when $CO_{2}$ and $O_{2}$ concentrations are normal, it acts as carboxylase and fix $CO_{2}$ by combining with ribulose bisphosphate and $C_{3}$ cycle operates normally, producing glucose molecule as an first product of photosynthesis.

(ii) If $O_{2}$ concentration goes up and $CO_{2}$ goes down, it starts acting as an oxygenase enzyme and $C_{2}$ cycle (photorespiration) starts where RuBP binds with $O_{2}$ to frorm phosphoglycolate.

(iii) $C_{4}$ plants have mechanisms to increase the concentration of $CO_{2}$ at enzyme site, and increasing the intracellular concentration of $CO_{2}$. Thus, here RuB is $Co$ acts as carboxylase, minimising the affect of oxygenase.

Answer

Kranz Anatomy, is the feature exhibited by $C_{4}$ plant. These possess two types of chloroplast in their leaves. Agranal chloroplast found in bundle sheath cells whereas granal chloroplast is found in the mesophyll cells.

Bundle sheath cell perform $C_{3}$ cycle (dark reaction) where as mesophyll cell perform $C_{4}$ cycle.

$C_{4}$ plants are more efficient even in high $O_{2}$ concentration and temperature as compared to

$C_{3}$ plants. Many important crop plants (monocots) show $C_{4}$ cycle like maize, sorghum, sugarcane and millet.

Answer

The important enzyme of $C_{3}$ cycle is RuBP carboxylase oxygenase which catalyses reaction of carboxylation of ribulose bis-phosphate, which is 5-carbon compound. to form PGA, the first stable product in $C_{3}$ cycle.

In $C_{4}$ cycle, the important enzyme is phosphoenol pyruvate carboxylase (PEP carboxylase) which help in fixing $CO_{2}$ to form oxaloecetate (4-carbon compound), the first stable product of dark reaction is $C_{4}$ cycle.

Answer

RuBisCo, now termed as RuBP carboxylase-oxygenase, is widely occurring enzyme because RuBisCo is used in the Calvin cycle to catalyse the first major step of carbon fixation.

RuBisCO is thought to be the most abundant protein in the world since, it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin cycle.

It makes about $20-25 %$ of the soluble protein in leaves and is made on the earth at the rate of about $1000 \mathrm{~kg} / \mathrm{s}$. It is estimated that every person on earth is supported by about $44 \mathrm{~kg}$ of RuBisCo.

Answer

Photorespiration is associated with $C_{3}$ cycle, where plant lose $CO_{2}$ fixation because of the increase in concentrate ion of $O_{2}$ and change the nature of activity of RuBP carboxylase-oxygenase.

$C_{4}$ plants have evolved a mechanism to avoid loss of $CO_{2}$. There is not a direct contact of RuBP carboxylase-oxygenase as $C_{3}$ cycle operates in bundle sheath cells (where both temperature and oxygen level low).

$\mathrm{CO}_{2}$ fixation is done by another enzyme PEP carboxylase in mesophyll cells and oxaloacetate is formed which is converted to malic acid and transported to bundle sheath cells.

There, it gives off $CO_{2}$ which is used in Calvin cycle, operating in bundle sheath cells of $C_{4}$ plants.