Photosynthesis In Higher Plants

Question 1

Explain Dark reaction

Answer 1

The second phase of biosynthesis reaction, which is independent of light.
It takes place in stroma of chlotoplqst.
CO2 is fixed to form glucose.

Question 2

Site of photosynthesis

Answer 2

o Green leaves, green stems and floral parts (sepal)

o Chloroplast - found in mesophyll cells of leaves

o In chloroplast – the membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH. Where stroma has enzymes for the reduction of CO2 in to carbohydrates (sugar

Question 3

Pigments Involved in Photosynthesis

Answer 3

4 types of pigments may be present in leaves:ChlorophyllaChlorophyllbXanthophyllsCarotenoidsAnabsorption spectrumis the graph plotted against the fraction of light absorbed by the pigment.Anaction spectrumis the rate of a physiological activity pl

Question 4

Light Reaction (Photochemical Phase)

Answer 4

This phase directly depends on light. The pigments absorb light energy and produce ATP.Includes:Light absorptionWater splittingOxygen releaseFormation of ATP and NADPH, which is then used in the biosynthetic phasePigment molecules bound to the proteins form LHC (light harvesting complexes). LHC are located within two photosystems − PSI and PSIIEach photosystem has two parts:Reaction centre − consisting of chlorophyllamoleculeAntennae − consisting of accessory pigments, which increase the efficiency of photosynthesis by absorbing different wavelengths of lightReaction centre is different in both photosystems:PSI − P700; since chlorophyllahas absorption peak at 700 nm herePSII − P680; since chlorophyllahas absorption peak at 680 nm here.

Question 5

Photo-Phosphorylation

Answer 5

The process of formation of ATP in chloroplast in the presence of sunlightPhoto-phosphorylation is of two types:Non-cyclic photo-phosphorylationCyclic photo-phosphorylation

Question 6

Non-Cyclic Photo-Phosphorylation

Answer 6

PSII absorbs 680 nm wavelength of red light, causing electrons to become excited and these electrons are then accepted by an electron acceptor, which sends them to an electron transport system.Electron transport system transfers the electrons to PSI.Electrons in PSI are simultaneously excited on receiving a wavelength of 700 nm.From the electron acceptor, electrons are transferred to the molecule of NADP+.Addition of these electrons reduces the NADP+ to NADPH+ H+.Since the electrons lost by PSII do not come back to it, this process of formation of ATP is called non-cyclic photo-phosphorylation.

Question 7

Cyclic Photo-Phosphorylation

Answer 7

In this scheme, only PSI is functional. Hence, the electrons are circulated within the photosystem.This results in a cyclic flow of electrons.This scheme could possibly be occurring instroma lamellaebecause it lacks both PSII and NADP reductase enzyme.This cyclic flow results only in the synthesis of ATP, and not of NADPH + H+.

Question 8

Splitting Of Water

Answer 8

Water splitting complex is associated with PSII.Manganese, chlorine, etc., play an important role.The light-dependent splitting of water is called photolysis
2H2O → 4H+ + O2 + 4e−Electrons formed are used for replacing the electrons lost from P680.P680 absorbs light and becomes as a strong oxidizing agent and splits a molecule of water to release oxygen. Oxygen is liberated as a by-product of photosynthesis.Protons are used for the formation of reducing power NADP to NADPH+.

Question 9

Chemiosmotic Hypothesis

Answer 9

· It is the mechanism of ATP synthesis in thylakoid of chloroplast.

· When electrons are transported through the electron transport system (ETS) and protons accumulate inside the thylakoid membrane due to photolysis of water.

· Now electrons are passed through PS and protons are transported across the membrane

Question 10

Chemiosmosis requires;

Answer 10

· A thylakoid membrane

· A protein pump

· A protein gradient

· ATP synthase enzyme

Question 11

Dark Reaction / Biosynthetic Phase:

Answer 11

Next stage is the biosynthetic phase. In this, ATP and NADPH are used for synthesising the food / Glucose.This stage is also called the dark phase as it is independent of light.It takes place in the stroma of chloroplasts.In some plants, the first product of CO2 fixation is a 3-carbon compound called 3-phosphoglyceric acid (PGA). These plants are said to adopt the C3 pathway.In other plants, the first CO2 fixation product is a 4-carbon compound called oxaloacetic acid. These plants are said to adopt the C4 pathway.

Question 12

Calvin Cycle (C3 Cycle)

Answer 12

The path of carbon in the dark reaction was traced by Melvin Calvin using radioactive carbon (14C).The primary acceptor of CO2 was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP). RuBP is used in a cyclic manner (regenerated) and a sugar is synthesised.3 phases of Calvin cycle: Carboxylation, Reduction and Regeneration of RuBP

Question 13

Carboxylation

Answer 13

Ribulose 1, 5-bisphosphate combines with CO2, and fixes it to a stable organic intermediate 3C compound called 3-phosphoglycerate (2 molecules). 3 PGA is the first stable product of this cycle.Reaction catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase)

Question 14

Reduction

Answer 14

Here, two molecules each of ATP and NADPH are required for fixing one molecule of CO2.This stage contains a series of reactions.Glucose is formed as a result of this series of reactions.

Question 15

Regeneration

Answer 15

RuBP regenerates to enable the cycle to continue uninterrupted.1 ATP molecule is required.For the formation of one molecule of glucose, six molecules of CO2 need to be fixed; hence, six cycles are required.ATP required:
For fixing 1 molecule of CO2 − 3 (2 for reduction and 1 for regeneration)
For fixing 6 molecules of CO2 − 3 × 6 = 18 ATPNADPH required:
For fixing 1 molecule of CO2 − 2 (for reduction)
For fixing 6 molecules of CO2 − 2 × 6 = 12 NADPHThus, the synthesis of 1 molecule of glucose requires 18 ATP and 12 NADPH

Question 16

C4 Pathway (Hatch and Slack Pathway)

Answer 16

Occurs in plants like maize, sugarcane − plants adapted to dry tropical regions. The leaves of C4 plants haveKranz anatomy. These plants show 2 types of photosynthetic cells , mesophyll cells and bundle sheath cells. Chloroplasts are dimorphic i.e., those is the mesophyll cells are granal and in bundle sheath Cells are agranal.

· C4 plants can tolerate high temperature and high light intensity, show greater productivity of biomass, and lack photorespiration.

· Primary CO2 acceptor: Phosphoenol pyruvate (PEP) − a 3-carbon molecule.

· PEP Carboxylase fixes CO2 in the mesophyll cells. It forms the 4-carbon compound oxaloacetic acid (OAA), and then other 4-carbon compounds malic acid.

· These compounds are transported to the bundle sheath cells. There, C4 acid breaks down to form C3 acid and CO2, and carbon dioxide enters the C3 cycle).

· C3 acid, so formed, is again transported to the mesophyll cells and regenerated back into PEP.

· C3 cycle cannot directly occur in the mesophyll cells of C4 plants because of the lack of the enzyme RuBisCO in these cells.

· RuBisCO is found in abundance in the bundle sheath cells of C4 plants.

Question 17

Photorespiration

Answer 17

It is a process in which there is no formation of ATP or NADPH, but there is utilization of ATP with release of CO2. It is also considered a wasteful process.Photorespiration is responsible for the difference between C3 and C4 plants.· At high temperature and high oxygen concentration, In C3 plants, RuBP carboxylase function as oxygenase.

· RuBP oxidized into phosphoglycerate (3C) and phosphoglycolate (2C)

· 75% of carbon lost during oxygenation of RuBP

· There is loss of photosynthetically fixed carbon and no energy rich compounds are formed, so photorespiration is a wasteful process.

Question 18

Factors affecting rate of Photosynthesis

Answer 18

Blackmans law of limiting factors.

When a physiological process is controlled by a number of factors, the rate of reaction depends on the lowest factor, so the factor which is the least/ limiting will determine the rate of Photosynthesis.

Photosynthesis is influenced by internal (plant) factors and external factors.

Question 19

Light

Answer 19

Quality and intensity of light

· Wavelength of light between 400 nm 700 nm is called photosynthetically active radiation (PAR). High intensity of light destruct chlorophylls.

Question 20

Temperature

Answer 20

High temperature denatures enzymes of biosynthetic phase and low temperature inactivates.

Question 21

Carbon dioxide concentration.

Answer 21

In C3 plants upto 500 and in C4 plants upto 360

Question 22

Availability of water.

Answer 22

Less water leads to - water stress, stoma closes, less carbon dioxide, reduce leaf expansion and less photosynthetic area.