Class 11 Biology Chapter 13 Photosynthesis In Higher Plants Notes

Photosynthesis In Higher Plants

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 CO2 from atmosphere and release O2.

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

Theodore de Saussure (1804) : Water is an essential requirement for pho- tosynthesis 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 reac- tion 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 photo- synthesis. 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)

Xanthophyll : (Yellow)

Carotenoids : (Yellow to yellow-orange)

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

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 sys- tem (antennae). The reaction centre (chlorophyll a) is different in both the pho- tosystems.

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.

• 1. Photochemical phase (Light reaction) : This phase includes - light absorption, splitting of water, oxygen release and formation of ATP and NADPH.
  2. Biosynthetic phase (Dark reaction) : It is light independent phase, synthesis of food material (sugars).

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

Cyclic photophosphorylation : Two photosystems work in series - First PSII and then PSI. These two photosystems are connected through an electron transport chain (Z. Scheme). Both ATP and NADPH + H+ are synthesised by this process. PSI and PSII are found in lamellae of grana, hence this process is carried here.

Non-cyclic photophosphorylation : Only PSI works, the electron circulates within the photosystem. It happens in the stroma lamellae (possible location) because in this region PSII and NADP rectase enzyme are absent. Hence only ATP molecules are synthesised.

The electron transport (Z-Scheme) : In PS II, reaction centre (chlo. a) absorbs 680 nm wavelength of red light which make the electrons to become excited. These electrons are taken up by the electron acceptor that passes them to an electron transport system (ETS) consisting of cytochromes. The movement of electron is down hill. Then, the electron pass to PSI and move down hill further.

The splitting of water : It is linked to PS II. Water splits into H+, O and electrons.

2H2O → 4H++ O2 + 4e-

Chemiosmotic Hypothesis : Chemiosmotic hypothesis explain the mechanism of ATP synthesis in chloroplast. In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane. The electrons are accumulated inside of membrane of thylakoids (in lumen). ATPase has a channel that allows diffusion of protons back 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. 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 :

• • 1. Carboxylation : CO2 combines with ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme.
    2. Reduction : Carbohydrate is formed at the expense of ATP and NADPH.
    3. Regeneration : The CO2 acceptor ribulose 1, 5-bisphosphate is formed again .2 6 turns of Calvin cycles and 18 ATP molecules are required to synthesize one molecule of glucose.

The C4 pathway : C4 plants have special type of leaf anatomy, they tolerate higher temperatures. In this pathway, oxaloacetic acid (OAA) is the first stable product formed. It is 4 carbon atoms compound, hence called C4 pathway (Hatch and Slack Cycle).

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 maltate/asparate that reach into bundle sheath cells.

The oxidation of maltate/asparate occurs with the release of O2 and formation of pyruvate (3C). In high CO2 concentration RuBisCo functions as carboxylase and not as oxygenase, the photosynthetic losses are prevented. RuBP operates now under Calvin cycle and pyruvate transported back to mesophyll cells and changed into phosphoenol pyruvate to keep the cycle continue.

2 Photosrespiration : The light induced respiration in green plants is called photorespiration. 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.

2 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.

Factors affecting photosynthesis :

1. Light
2. Carbondioxide
3. Temperature
4. Water