CBSE Class 11 Biology Photosynthesis In Higher Plants Notes Set A

Photosynthesis in Higher Plants Class 11 Notes

POINTS TO REMEMBER :

• Photosynthesis: Photosynthesis is an enzyme regulated anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and water with the help of sunlight as a source of energy.

Historical Perspective :

• Joseph Priestley (1770) : Showed that plants have the ability to take up CO from atmosphere and release O₂.

• Jan Ingenhousz (1779) : Release of O₂ by plants was possible only in sunlight and only by the green parts of plants.

• Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis to occur.

• Julius Von Sachs (1854) : Green parts in plant produce glucose which is stored as starch.

• T. W. Engelmann (1888) : The effect of different wavelength of light on photosynthesis and plotted the first action spectrum of photosynthesis.

• C. B. Van Niel (1931) : Photosynthesis is essentially a light dependent reaction in which hydrogen from an oxidisable compound reduces CO₂ to form sugar. He gave a simplified chemical equation of photosynthesis.

• Hill (1937) : Evolution of oxygen occurs in light reaction.

• Calvin (1954-55) : Traced the pathway of carbon fixation.

• Hatch and Slack (1965) : Discovered C₄ pathway of CO₂ fixation.

Site for photosynthesis :

• Photosynthesis takes place only in green parts of the plant, mostly in leaves.

• Within a leaf, photosynthesis occurs in mesophyll cells which contain the chloroplasts.

• Chloroplasts are the actual sites for photosynthesis.

• The thylakoids in chloroplast contain most of pigments required for capturing solar energy to initiate photosynthesis.

• The membrane system (grana) is responsible for trapping the light energy and for the synthesis of ATP and NADPH. Biosynthetic phase (dark reaction) is carried in stroma.

Pigments involved in photosynthesis:

• Chlorophyll a : (Bright or blue green in chromatograph). Major pigment, act as reaction centre, involved in trapping and converting light into chemical energy.

• Chlorophyll b : (Yellow green)

• Xanthophylls : (Yellow)

• Carotenoid : (Yellow to yellow-orange)

• In the blue and red regions of spectrum shows higher rate of photosynthesis.

What is light reaction?

• Light reactions or the ‘Photochemical ‘phase includes light absorption, splitting of water, evolution of oxygen and formation of high energy compound like ATP and NADPH.

• Light Harvesting Complexes (LHC) : The light harvesting complexes are made up of hundreds of pigment molecules bound to protein within the photosystem I (PSI) and photosystem II (PSII).

• Each photosystem has all the pigments except one molecule of chlorophyll ‘a’ forming a light harvesting system (antennae).

• The reaction centre (chlorophyll a) is different in both the photosystems.

• Photosystem I (PSI) : Chlorophyll ‘a’ has an absorption peak at 700 nm (P700).

• Photosystem II (PSII) : Chlorophyll ‘a’ has absorption peak at 680 nm (P680).

Process of photosynthesis :

• It includes two phases - Photochemical phase and biosynthetic phase.

• Photochemical phase (Light reaction) : This phase includes - light absorption, splitting of water, oxygen release and formation of ATP and NADPH.

• Biosynthetic phase (Dark reaction) : It is light independent phase, synthesis of food material (sugars).

The electron transport :

• In photosystem centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and release two electrons from the atomic nucleus.

• These electrons are accepted by primary electron acceptor i.e. ferredoxin.

• The electron from the ferredoxin passed to electron transport system consisting cytochromes.

• The electron moved in down hill in terms of redox potential by oxidation-reduction reactions.

• Finally the electron reached photosystem-I.

• Simultaneously electron released from photosystem-I is accepted by electron acceptor.

• Electron hole created in PS-I is filled up by the electron from PS-II.

• Electron from PS-I passed down hill and reduce NADP into NADPH⁺ + H⁺.

Photolysis of water :

• PS-II loose electrons continuously, filled up by electrons released due to photolysis of water.

• Water is split into H⁺, (O) and electrons in presence of light and Mn²⁺ and Cl^-.

• This also creates O₂ the bi-product of photosynthesis.

• Photolysis takes place in the vicinity of the PS-II.

• 2H₂O → 4H⁺ + O₂ + 4e^-.

Photophosphorylation :

• The process of formation of high-energy chemicals (ATP and NADPH).

Non Cyclic photophosphorylation :

• Two photosystems work in series – First PSII and then PSI.

• These two photosystems are connected through an electron transport chain (Z. Scheme).

• ATP and NADPH + H⁺ are synthesized by this process. PSI and PSII are found in lamellae of grana, hence this process is carried here. 

 • In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane.

• The protons that are produced by the splitting of water are accumulated inside of membrane of thylakoids (in lumen).

• As the electron moves through the photosystem, protons are transported across the membrane.

• NADP reductase enzyme is located on the stroma side of the membrane, along with electrons from the acceptor it removes H+ from the stroma during reduction of NADPH + H+.

• This creates proton gradients across the thylakoid membrane as well as a measurable decrease in pH in the lumen.

• ATPase has a channel that allows diffusion of protons back to stroma across the membrane.

• This releases energy to activate ATPase enzyme that catalyses the formation of ATP.

Biosynthetic phase in C3 plants :

• ATP and NADH, the products of light reaction are used in synthesis of food. The first CO2 fixation product in C3 plant is 3-phosphoglyceric acid or PGA.

• In some other plants the first stable product is an organic acid called oxaloacetic acid a 4-C compound hence is called C4 plants.

The Calvin cycle :

• The CO2 acceptor molecule is RuBP (Ribulose bisphosphate).

• The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discoverer of this pathway. Calvin cycle proceeds in three stages:

• Carboxylation :

o Carboxylation is the fixation of CO2 into a stable organic intermediate.

o CO2 combines with Ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme.

• Reduction :

o These are a series of reactions that lead to the formation of glucose.

o 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO2 molecule fixed.

o The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose.

• Regeneration :

o Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.

o Regeneration steps required one ATP for phosphorylation to form RuBP.

• Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.

The C4 pathway :

• Plants that are adapted to dry tropical regions have the C4 pathway.

• C4 oxaloacetic acid is the first CO2 fixation product.

• These plants have special type of leaf anatomy, they tolerate higher temperatures.

• The leaf has two types of cells: mesophyll cells and Bundle sheath cells (Kranz anatomy).

• Initially CO2 is taken up by phosphoenol pyruvate (PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase.

• Oxaloacetate is reduced to malate/asparate that reaches into bundle sheath cells.

• In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule i.e. pyruvic acid.

• The CO2 released in the bundle sheath cell enters the C3 cycle because these cells are rich in enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO).

• The pyruvate formed in the bundle sheath cell transported back to the mesophyll cell, get phosphorylated to form phosphoenol pyruvate.

Photorespiration:

• The light induced respiration (evolution of CO2) in green plants is called photorespiration.

• Active site of RuBisCO has active site for both O2 and CO2.

• In C3 plants some O2 binds with RuBisCo and hence CO2 fixation is decreased.

• In this process RuBP instead of being converted to 2 molecules of PGA, binds with O2 to form one molecule of PGA and phosphoglycolate.

• In the photorespiratory pathway there is neither synthesis of sugar, nor of ATP. Rather it results in the release of CO2with utilization of ATP.

• In the photorespiratory pathway there is no synthesis of ATP or NADPH.

• Therefore photorespiration is a wasteful process.

• In C4 plant photorespiration does not occur:

o RuBisCO enzyme is present in the bundle sheath cells.

o Primary carboxylation is takes place in the mesophyll cell by PEP carboxylase.

o CO2 supplied to bundle sheath cell by C4 acid intermediate.

o Hence C4 plants are photosynthetically more efficient than C3 plant.

Law of Limiting Factors :

• 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.

Important Questions for NCERT Class 11 Biology Photosynthesis in Higher Plants

Ques. CAM helps the plants in
(a) conserving water (b) secondary growth
(c) disease resistance (d) reproduction.

Answer: A

Ques. In kranz anatomy, the bundle sheath cells have
(a) thin walls, many intercellular spaces and no chloroplasts
(b) thick walls, no intercellular spaces and large number of chloroplasts
(c) thin walls, no intercellular spaces and several chloroplasts
(d) thick walls, many intercellular spaces and few chloroplasts.

Answer: B

Ques. Kranz anatomy is one of the characteristics of the leaves of
(a) potato (b) wheat
(c) sugarcane (d) mustard. 

Answer: C

Ques. The C4 plants are photosynthetically more efficient than C3 plants because
(a) the CO2 efflux is not prevented
(b) they have more chloroplasts
(c) the CO2 compensation point is more
(d) CO2 generated during photorespiration is trapped and recycled through PEP carboxylase.

Answer: B

Ques. In leaves of C4 plants, malic acid synthesis during CO2 fixation occurs in
(a) bundle sheath (b) guard cells
(c) epidermal cells (d) mesophyll cells. 

Answer: D

Ques. In the leaves of C4 plants, malic acid formation during CO2 fixation occurs in the cells of
(a) bundle sheath (b) phloem
(c) epidermis (d) mesophyll. 

Answer: D

Ques. As compared to a C3-plant, how many additional molecules of ATP are needed for net production of one molecule of hexose sugar by C4-plants?
(a) Two (b) Six
(c) Twelve (d) Zero 

Answer: C

Ques. Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because
(a) effective pumping of CO2 into bundle sheath cells
(b) RuBisCO in C4 plants has higher affinity for CO2
(c) four carbon acids are the primary initial CO2 fixation products
(d) the primary fixation of CO2 is mediated via PEP carboxylase. 

Answer: D

Ques. In sugarcane plant 14CO2 is fixed to malic acid, in which the enzyme that fixes CO2 is
(a) ribulose biphosphate carboxylase
(b) phosphoenol pyruvic acid carboxylase
(c) ribulose phosphate kinase
(d) fructose phosphatase. 

Answer: B

Ques. Stomata of CAM plants
(a) are always open
(b) open during the day and close at night
(c) open during the night and close during the day
(d) never open. 

Answer: C

Ques. Which pair is wrong?
(a) C3-maize (b) C4-kranz anatomy
(c) Calvin cycle-PGA
(d) Hatch and Slack cycle - OAA 

Answer: A

Ques. Which is the first CO2 acceptor enzyme in C4 plants?
(a) RuDP carboxylase (b) Phosphoric acid
(c) RuBisCO (d) PEP- carboxylase

Answer: D

Ques. In C4 plants, CO2 combines with
(a) phosphoenol pyruvate
(b) phosphoglyceraldehyde
(c) phosphoglyceric acid
(d) ribulose diphosphate. 

Answer: A

Ques. In C4 plants, CO2 fixation is done by
(a) sclerenchyma
(b) chlorenchyma and hypodermis
(c) mesophyll cells
(d) guard cells. 

Answer: C

Ques. The CO2 fixation during C4 pathway occurs in the chloroplast of
(a) guard cells
(b) bundle sheath cells
(c) mesophyll cells
(d) spongy parenchyma. 

Answer: C

Ques. Which one is a C4-plant?
(a) Papaya (b) Pea
(c) Potato (d) Maize/corn 

Answer: D

Ques. The enzyme that catalyses carbon dioxide fixation in C4 plants is
(a) RuBP carboxylase (b) PEP carboxylase
(c) carbonic anhydrase (d) carboxydismutase.

Answer: B

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