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8: Photosynthesis

Photosynthesis( photo =light, synthesis-= putting together) is the process of formation of simple sugars by green plants, some bacteria, and some protistans from water from soil and carbon dioxide from air in presence of sunlight and chlorophyll. These simple sugars are then converted into complex carbohydrates, such as starches. Starches are the way plants store energy -- plants produce glucose and chain the glucose molecules together to form starch. During photosynthesis solar energy is trapped by autotrophic organisms and stored in the form of chemical bonds of ATP.In green plants the process of photosynthesis takes place in the chloroplasts, specifically the green pigment chlorophyll,but in cyanobacteria and other bacteria which do not have chloroplasts, pigments are located on the thylakoids.

Light+6CO2 + 12H2O -------------------------------> C6H12O6 + 6O2 + 6H2O chlorophyll

or, in a more balanced form:

6H20 + 6CO2 -------------------> C6H12O6 + 6O2

Six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

Site of photosynthesis in plants

Chloroplasts Chloroplasts are the sites of photosynthesis. The chloroplast is bounded by a double membrane that encloses the stroma, a dense aqueous solution that contains DNA, RNA, metabolites, and the enzymes associated with the conversion of CO2 into organic matter. Membranes of the thylakoid system separate the stroma from the thylakoids. Thylakoids are concentrated in stacks called grana. Thylakoids contain the pigments chlorophyll a and b, carotenoid, and the enzymes associated with the oxidation (splitting) of water (H2O) and the production of oxygen.

Photosynthetic pigments There are two types of photosynthetic pigments present in higher plants :-

  1. Chlorophylls:- Chlorophyll molecules embedded in the thylakoid membrane are main pigment involved with harvesting or trapping of solar energy. Aronoff and Allen (1966) recognized nine types of chlorophyll. Out of these, the two important types of chlorophyll found in green plants are chlorophyll a and chlorophyll b. A chlorophyll molecules consists of two parts -
    • The porphyrin ring -The "head" of a chlorophyll molecule is a ring called a porphyrin. It is a flat, square structure containing four smaller pyrrole rings with a magnesium atom at the centre,is the part of a chlorophyll molecule that absorbs light energy.
    • A phytol tail The head is joined to a long hydrocarbon phytol tail.and hydrophobic "tail" that embeds the molecule into the thylakoid membrane.
    Chlorophyll a is most abundant photosynthetic pigment, which is the only pigment found in all photosynthetic plants. Chlorophyll absorbs light near both ends of the visible spectrum - the blue and red light and transmit or reflect green light. Therefore, these appear to be green in colour.
  2. Carotenoids Are yellow to orange pigments which absorb light strongly in the blue violet range.Carotenoids are perform two major recognized functions in photosynthesis. These serve as accessory light harvesting pigments, and transfer light to chlorophyll for use in photosynthesis, and the other shield pigments -protect the chlorophyll from photo oxidation (photo bleaching) reaction by light of high intensity and also from oxygen produced during photosynthesis Carotenoids are of two types Carotenes (light yellow to dark red colourings) and Xanthophylls (yellow)

Quantosome:- Park and Biggins (1964) coined the term quantosome for a group of pigment molecules needed to perform a photochemical reaction. Quantosomes are present as small units on the membranes of thylakoids. Each quantosome made up of nearly 250 to 300 chlorophyll molecules, carotenoids, quinone compounds, sulpholipids, phospholipids, protein, etc. including special type of chlorophyll molecules - P680 and P700, which constitute the photocentres or reaction centre. P680 and P700 - P = Pigment and 680 and 700 denote the wavelength of light these molecules absorb. Other accessory pigments and chlorophyll molecules are light-gatherers or antenna molecules. Antenna molecules capture solar energy and resonance transfer or inductive resonance transfer it on the reaction centres.

Absorption and action spectra

Mechanism of photosynthesis Photosynthesis is a two stage process :-The energy-fixing reaction or Light dependent process( light reaction) and The carbon-fixing reaction or Light independent process (dark reaction).The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

I.  Photochemical phase (light reaction) The light reactions of photosynthesis occurs in the thylakoids of grana. In the light reactions, solar energy is trapped by chlorophyll,and excited electrons from chlorophyll flow through a cytochrome transport system along membranes of the thylakoid disks (thylakoid membranes). In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

  1. PhotosystemsThe light-dependent portion of photosynthesis is carried out by two consecutive photosystems (photosystem I and photosystem II) in the thylakoid membrane of the chloroplasts.Photosystem I (PS I):- The cluster of accessory pigment molecules along with P700 absorb light at or below 700 nm, constitute the Photosystem I, located both in stroma and grana lamellae.Photosystem II (PS II):- The cluster of pigment molecules which transfer their energy to P680 absorb light at or below the wavelength of 680nm. Along with P680 these molecules make up the Photosystem II is located in grana .It contains chlorophyll a, chlorophyll b and carotenoids.
  2. A photon absorbed anywhere in the trapping (or harvesting) zone of a P680 centre can pass its energy to the P680 molecules.
  3. PhotophosphorylationPhotophosphorylation is the synthesis of ATP from ADP and phosphate that occurs in a plant using radiant energy of sun absorbed during photosynthesis. This is accompanied by photolysis of water, as a result of which O2(oxygen) is released and H combines with NADP to form NADPH.
  4. Excitation of pigments by light:-Absorption of sunlight by pigments causes the excitation of certain electrons i.e. the electrons absorb energy.Such an excited chlorophyll molecule is unstable and will tend to return its original unexcited state (ground state) losing its energy of excitation.The excited pigments lose electrons and become oxidised :-

    Light energy

    Chlorophyll ------------------------------------------> Chlorophyll+(oxidised) + e- (electron)

    Each lost electron is accepted by another molecule, the electron acceptor.The chlorophyll is oxidised and electron acceptor is reduced, so this is an oxidation-reduction process.Therefore, instead of losing its energy, the electron is captured by an electron acceptor i.e. the conversion of light energy into chemical energy.
  5. Photophosphorylation in most higher plant can occur by two processes:-
    1. Non-cyclic photophosphorylation: - If chloroplasts are illuminated by light energy,the two photo systems, PS11 and PS1 transfer electrons from water (the donor molecule) via electron carries and ultimately to NADP (the accepter molecule) reducing it to NADPH.Due to electron transport the proton gradient is established which in turn facilitate ATP synthesis and this type of electron transport is called non cyclic or linear photophosphorylation(electrons are just transported from water to NADP and never come back.)

  • Hill and Bendall (1960) proposed Z Scheme for electron transfer in cynobacteria, algae and green plants.
  • When photosystem II(P680) in the thylakoid membranes of the chloroplast absorb a sufficient quantum of energy, it gets exited and emits an electron which passes to primary electron acceptor.
  • With high potential energy the electrons passes from the electron acceptor to plastoquinone (PQ).From plastoquinone (PQ) the electrons pass on to a complex of cytochromes.

  • The PQ reacts with two hydrogen ions from the stroma and the two electrons from photosystem II to form PQH2.
  • 2H+stroma + 2e- + PQ ------------->PQH2

  • PQ can then diffuse back across the membrane to repeat the process. The net result of the Q cycle is to move two hydrogen ions from the stroma to the lumen.

  • As the electrons move from PQ to the cytochrome complex they release enough energy to power the active transport of hydrogen ions from the stroma into the thylakoid space. This generates a large hydrogen ion gradient.

  • The electrons lost from P680(PS II) enter the P700(PS I). Sunlight now activates the electrons, and becomes excited to release electrons,which are transferred to primary electron acceptor from there electron is transferred to ferredoxin(an iron-containing protein).

  • Ferredoxin, in turn transfers them to NADP+(nicotinamide adenine dinucleotide phosphate) NADP+ picks up two hydrogen atoms from water molecules forming NADPH2, a powerful reducing agent along with ATP is formed in the energy-fixing reactions is used to convert carbon dioxide into glucose during the dark reactions of photosynthesis (also called the Calvin Cycle).

  • Electrons are shed by the excited PS II (oxidation). The oxidized P680 grabs its electrons by the photolysis of water into 2H+, 2e- and oxygen. PS II thus returns it to its unexcited state (reduction).

  • Each split water molecule releases two electrons that enter the chlorophyll molecules to replace those lost. The split water molecules also release two protons (H+) that enter into the thylakoid space, as a result proton gradient occurs . This Proton gradient can be used to generate ATP chemiosmotically (Chemiosmosis is the process of using proton movement to join ADP and Pi. This is accomplished by enzymes called ATP synthases or ATPases). The third product of the disrupted water molecules is oxygen. Two oxygen atoms combine with one another to form molecular oxygen, which is given off as the byproduct of photosynthesis; it fills the atmosphere and is used by all oxygen-breathing organisms, including plant and animal cells.

Cyclic photophosphorylation :-If electrons from PS1 are recycled back to lower energy level of the same photosystem that is PS1 facilitating more ATP synthesis, then the flow is called cyclic electron flow-cyclic photophosphorylation.

  • Some plants are also known to participate in cyclic energy-fixing reactions.
  • These reactions involve only photosystem I and the P700 reaction center.
  • The excited electrons resulting from the absorption of light in photosystem I are received by the primary electron acceptor and then transferred to ferredoxin, from ferredoxin to plastoquinone to the cytb6-f complex which acts as an electron transport chain.
  • The electrons recycled of Photosystem I through plastocyanin(PC), at a low energy ,where the cycle is ready to start all over.
  • Each electron powers the proton pump and , motivates the transport of a proton across the thylakoid membrane.
  • This process enhances the proton gradient and eventually leads to the generation of ATP. No reduction of NADP+ occurs in Cyclic Photophosphorylation.

II.  Carbon-fixing during dark reaction(Light independent process)The ATP and NADPH produced by the light reaction of photosynthesis are utilized during dark reaction to reduce carbon dioxide(CO2). to glucose (C6H12O2). and other compounds by a process called carbon fixation. In plants the cycle takes place in the stroma of the chloroplast can occur without the presence of sunlight.The process comprises a series of reaction controlled by enzymes. there are three known mechanisms for photosynthetic carbon fixation, one basic procedure and two modifications of it:-

  • Calvin cycle or C3 Pathway This method is used by most common temperate zone species. The sequence of reactions was determined in Chlorella and Scenedesmus by Calvin, Benson and Bassham using radioactive carbon 14C.The carbon-fixing reaction of photosynthesis involves a cyclic series of reactions , often called as the Calvin cycle. The dark reaction is also known asBlackman’s reaction.

  • The Calvin Cycle proceeds in three stages:-

    RuBP carboxylase

    RuBP +CO2 -------------------------> 2moles of PGA

    PGAkinase

    2 moles of PGA + 2ATP -----------------> 2 moles 1,3-bisphosphoglyceric acid

    G-3-P dehydrogenase

    2 moles 1,3-bisphosphoglyceric acid + 2NADPH + H+ -----------------> 2 moles G-3-P

    1. Carboxylation or Fixation - Three molecules of CO2 combine with three molecules of 5C compound called ribulose-1,5- bisphosphate (RuBP) Which is the acceptor molecule of CO2 give rise to an unstable 6-carbon compound. Carboxylation (addition of Co2 to any compound) of a molecule of CO2 to RuBP is catalyzed by the enzyme RuBP carboxylase or Rubisco. The resulting 6C compound is highly unstable. This molecule quickly breaks down to give two molecules of the three-carbon 3-phosphoglycerate (3PG), also called phosphoglyceric acid (PGA).
    2. Glycolytic reversal or Reduction - carbohydrate is formed at the expense of ATP and NADPH. The two molecules of PGA are reduced (reduction means that electrons are added to the molecule) to glyceraldehyde-3- phosphates , two PGA molecules are converted to 1,3 -bisphosphoglyceric acids by the enzyme PGA kinase. by adding a high-energy phosphate group from ATP.NADPH, produced during the light reactions, provides the high energy electrons for this process. Then two molecules of 1,3-bisphosphoglyceric acid are reduced to glyceraldehyde-3-phosphates by the enzyme glyceraldehyde- 3-phosphate dehydrogenase with NADPH, produced during the light reactions, provides the high energy electrons for this process. So, the Calvin cycle is energetically bond to the light reactions of photosynthesis.
    3. Regeneration - the CO2 acceptor RuBP reforms at the expense of ATP.The three molecules of RuBP that began the cycle are made up of 15 atoms of carbon. Three molecules of carbon from CO2 were then fixed for a surplus of three carbons in the cycle. Those three carbons are expelled from the cycle as one molecule of G3P. The remaining 15 carbons are still in the form of G3P. Therefore, they must be converted back to RuBP to start the process over. For every carbon fixation 3ATP and 2 NADPH + 2H+ are consumed.
    4. The reactions of regeneration of RuBP are :-
      • Some of the Glyceraldehyde 3-phosphate molecules are converted to dihydroxy acetone phosphates.
      • Glyceraldehyde 3-phosphate combines with dihydroxy acetone phosphate to form fructose1,6-bisphosphate.
      • Fructose 1,6-bisphosphate undergoes dephosphorylation to form fructose 6-phosphate.
      • Fructose 6-phosphate combines with glyceraldehyde 3-phosphate obtained from the fixation of second molecule of CO2 to form Ribose 5-phosphate (R5P) and Erythrose 4-phosphate (Er4P).
      • Erythrose 4-phosphate combines with DHAP obtained from the second CO2 fixation, to form sedoheptulose 1,7-bisphosphate.
      • Sedoheptulose 1,7-bisphosphate undergoes dephosphorylation to form sedoheptulose 7-phosphate.
      • Sedoheptulose 7-phosphate combines with glyceraldehyde 3-phosphate obtained by the third CO2 fixation, to form two molecules of 5C compounds – ribose 5-phosphate and xylulose 5-phosphate (Xy5P).
      • Ribose 5-phosphate and xylulose 5-phosphate molecules are transformed to ribulose 5-phosphate (Ru5P).
      • Ru5P molecules are then phosphorylated by ATP to form RuBP molecules, which again enter into the cycle of CO2 fixation.

Photorespiration Otto Warburg showed in 1921 that increased amounts of oxygen inhibit photosynthesis in C3 plants. On hot days, stomata close, restricting the inflow of CO2 and raising the relative concentration of O2 inside the leaf. Under these conditions ( <50 ppm CO2), Rubisco starts fixing O2:-

> O2 + RuBP (in the presence of rubisco) ---------->6 phosphoglycolic acid + PGA

PGA stays in the Calvin cycle but phosphoglycolic acid leaves. No ATP is formed in photorespiration and rubisco operates at about 25% of its maximal rate.

C4 cycle (Hatch-Slack pathway):- An alternative, very efficient pathway used by plants living in areas with low levels of carbon dioxide, to convert carbon dioxide into a form usable by the plants during photosynthesis. In the 1966, two Australian scientists, M.D. Hatch and C.R. Slack show that C4 plants were much more efficient in CO2 utilisation than C3 plants. In 1967, Hatch and Slack describe the manner of CO2 fixation and reduction in such plants and this new carbon pathway in C4 plants is known as Hatch-Slack Pathway.

The leaf anatomy in C4 plants, the vascular bundles are surrounded by two rings of cells.In C4 plants, the mechanism of photosynthesis requires two types of photosynthetic cells:- the inner ring known as bundle sheaths and the outer ring- the mesophyll cells .(this peculiar anatomy is called Kranz anatomy) The C4 plants contain dimorphic chloroplasts i.e. chloroplasts in mesophyll cells are granal (with grana) whereas in bundle sheath chloroplasts are agranal (without grana). The The cell walls are relatively thicker to inhibit the diffusion of carbon dioxide and maintain a high concentration

Hatch-Slack pathway involves two carboxylation reactions. One takes place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells.

  • CO2 is picked up by phosphoenol pyretic (carboxylation) acid in the chloroplasts of mesophyll cells to form a 4C compound, oxaloacetic acidin the presence of enzyme phosphoenol pyruvate carboxylase
  • Oxaloacetic acid is converted into aspartic acid by the enzyme transaminase or reduced to malic acid by NADP+ specific malate dehydrogenase.
  • Malic acid or aspartic acid formed in chloroplast of mesophyll cells is transported to the chloroplasts of bundle sheath where it is decarboxylated to form CO2 and pyruvic acid in the presence of NADP+specific malic enzyme.4. Now, second carboxylation takes in chloroplasts of bundle sheath cells.
  • The CO2 released due to decarboxylation is incorporated into sugars in the chloroplasts of the bundle sheath cells following the Calvin-Benson cycle

C4 plants are photosynthetically more efficient than C3 plants, because the net requirement of ATP and NADPH2 for the fixation of one molecule of CO2 is considerably lower in C4 plants than in C3 plants.

CAM (crassulacean acid metabolism) Pathway:- Another Calvin cycle modification is made by succulents and other plants growing in areas of high temperatures, high light, and low moisture (deserts especially). In this modification, carbon fixation takes place at night in a pathway similar to C4 photosynthesis and, in addition, during the day carbon is fixed in the same cells using the C3 pathway. This pathway is named for the family of plants, Crassulaceae, in which it was first discovered.