Study notes on Photosynthesis in Higher Plants
Study notes on Photosynthesis in Higher Plants
- Photosynthesis is a physiochemical process by which the plants use light energy to drive the synthesis of organic compounds.
- Photosynthesis is important because it is the primary source of all food on earth and it is also responsible for the release of oxygen into the atmosphere by the green plants.
- The key features of plant photosynthesis are that plants could use light energy to make carbohydrates from CO2 and water.
- The equation for this process is : CO2 + H2O —> CH20 + O2.
WHERE DOES PHOTOSYNTHESIS TAKE PLACE?
- Mesophyll cells in the leaves have a large number of chloroplasts.
- The chloroplasts align themselves along the walls of the mesophyll cells such that they get the optimum quantity of the incident light.
- The chloroplast has a membrane a system consisting of grana, the stroma lamellae and the fluid stroma.
- There is a clear division of labour within the chloroplast.
- The membranous system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.
- The light reaction takes place inside the stroma.
- In stroma, enzymatic reactions incorporate CO2 into the plant leading to synthesis of sugar which in turn forms starch.
- This reaction is directly driven by light so it is called light reaction.
- Other reactions are not directly light-driven but are dependent on the products of light reactions.
- Those reactions are called dark reactions.
PIGMENTS INVOLVED IN PHOTOSYNTHESIS
- There are four pigments responsible for the different colour of leaves.
- Chlorophyll a (bright or blue green), chlorophyll b(yellow and green) xanthophylls(yellow), carotenoid(yellow to yellow-orange).
- These pigments absorb lights of different wavelengths.
- Chlorophyll-a is the chief pigment associated with photosynthesis.
- Chlorophyll b, xanthophylls and carotenoids are called accessory pigments.
- These pigments absorb light and transfer the energy to chlorophyll a.
- They enable a wider range of wavelength of incoming light to be utilised for photosynthesis.
- They also protect chlorophyll from photo-oxidation.
- The light reaction is also called as a photochemical phase.
- This includes light absorption, water splitting, oxygen release and the formation of high energy chemical intermediates ATP and NADPH.
- In light reaction, the pigments are organised into two discrete photochemical light-harvesting complexes. (LHC).
- There are two systems photosystem I and photosystem II.
- These are named according to the sequence of their discovery.
- The pigment molecules in LHC are bound to proteins.
- Each of the photosystems has all the pigments except one molecule of chlorophyll a.
- These pigments help make photosynthesis more efficient by absorbing different wavelengths of light.
- A single chlorophyll a molecule forms the reaction centre.
- In photosystem 1 the reaction centre chlorophyll a has an absorption peak at 700 nm. Hence it is called p700.
- In photosystem II it has absorption Maxima at 680nm and is called p 680.
- The reaction centre in photosystem II absorbs 680 nm wavelength of red light.
- Due to this the electrons become excited and jump into an orbit farther from the atomic nucleus.
- The electrons are then picked up by an electron acceptor which passes them to an electron transport system consisting of cytochromes.
- According to the redox potential scale, this movement of electrons is downhill.
- The electrons are directly passed onto the pigments of photosystem I.
- On the other hand the pigments in photosystem I also get excited when they receive the red light of wavelength 700 nm.
- The electrons are transferred to another acceptor molecule that has a greater redox potential.
- The electrons then move downhill to a molecule of energy-rich NADP+.
- Addition of these electrons reduces NADP+ to NADPH + H+.
- This whole scheme of transfer of electrons is called as Z scheme.
- It is named due to its characteristics shape.
- This shape is formed when all the carriers are placed in a sequence on a redox potential scale.
SPLITTING OF WATER
- The electrons for photosystem II are available due to the splitting of water.
- Water is split into H+, O and electrons.
- The net product of photosynthesis which is oxygen is created here.
- To replace the removed electrons from photosystem I electrons are provided by photosystem II
2H2O –-> 4H+ + 4e-
- The water-splitting complex is associated with the PSII
- This complex is located on the inner side of the membrane of the thylakoid.
CYCLIC AND NONCYCLIC PHOTOPHOSPHORYLATION
- The process by which ATP is synthesized by cells is named as phosphorylation.
- Synthesis of ATP from ADP and inorganic phosphate in the presence of light is called photophosphorylation.
- When the two photosystems work in series first PSII and then PSI a process called non-cyclic photophosphorylation takes place.
- Both ATP and NADPH + H+ are synthesized by this kind of electron flow.
- When only PSI is functional the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.
- This takes place in the stroma lamellae.
- The membrane or lamellae of grana have both PSI and PSII
- The stroma lamellar membrane slack pH 2 as well as NADP reductase enzyme.
- In cyclic electron transport, the excited electron does not pass on to NADP+.
- It is cycled back to the PSI complex through the electron transport chain.
- The cyclic flow results only in the synthesis of ATP, but not of NADPH + H+.
- When only the light wavelength beyond 680nm are available cyclic phosphorylation takes place.
- It is to explain the mechanism of ATP synthesis in the chloroplast.
- In photosynthesis, ATP synthesis is linked to the development of proton gradient across the membrane.
- The membranes are the membranes of the thylakoid.
- The proton accumulates towards the inside of the membrane which is in the lumen.
- First, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of thylakoids.
- As the electrons move through the photosystems protons are transported across the membrane.
- This is because the primary acceptor of the electron which is located towards the outside of the membrane transfers its electron to an H carrier.
- Hence a proton is removed from stroma while transporting an electron.
- When the electron is passed to the electron carrier on the inner side of the membrane the proton is released into the inner side of the lumen.
- Protons are necessary for the reduction of NADP+ to NADPH + H+.
- Hence these protons are also removed from the stroma.
- Due to this within the chloroplast the protons in stroma decrease in number why in human there is the accumulation of protons.
- Hence proton gradient is created across the thylakoid membrane.
- Also the pH in the lumen decreases.
- The breakdown of the proton gradient leads to the release of energy.
- The gradient is broken due to the movement of protons across the membrane to the stroma.
- The protons move through the transmembrane channel of the Fo of the ATPase.
- Fo is the part of the ATPase enzyme.
- It is embedded in the membrane and forms the transmembrane channel that carries out facilitated diffusion of a proton across the membrane.
- The other part is the F1.
- It is towards the outer surface of the thylakoid membrane on the side that faces the stroma.
- The breakdown of gradient provides the energy that cause a conformational change in the F1 particle of the ATPase.
- This makes the enzyme synthesise several molecules of energy-packed ATP.
- So chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase.
- The channel in the enzyme allows diffusion of protons back across the membrane.
- The energy released due to this is enough to activate the ATPase enzyme that catalyses the formation of ATP.
- The ATP is also used in the biosynthetic reaction taking place in the stroma responsible for fixing CO2 and synthesis of sugars.
UTILISATION OF ATP AND NADPH.
- Product of light reaction is ATP, NADPH, O2.
- O2 diffuses out of the chloroplast.
- ATP and NADPH are used to drive the processes leading to the synthesis of food (sugars).
- This is called the biosynthetic phase of photosynthesis.
- This phase depends on the products of light reaction.
- Melvin Calvin uses radioactive 14C in algal photosynthesis.
- This phase is also called as Calvin cycle.
- Use of radioactive led to the discovery that the first CO2 fixation product was a three-carbon organic acid.
- The first product identified was 3 phosphoglyceric acid.
- In other plants the co2 fixation product is is a four-carbon organic acid which is oxaloacetic acid.
- Hence the co2 assimilation during photosynthesis is of two types.
- C3 pathway for three-carbon organic acid
- C4 pathway for four carbon organic acid.
PRIMARY ACCEPTOR OF CO2.
- Accepted molecule for CO2 was a 5 carbon ketose sugar.
THE CALVIN CYCLE
- The Calvin pathway is operated in a cyclic manner.
- The RuBP was regenerated.
- The Calvin cycle occurs in all photosynthetic plants.
- It has three stages carboxylation, reduction and regeneration.
- Carboxylation is the fixation of co2 into a stable organic intermediate.
- CO2 is utilised for the carboxylation of RuBP.
- This reaction is catalysed by the enzyme RuBp carboxylase.
- IT results in the formation of two molecules of 3- PGA.
- This enzyme has an oxygenation activity.
- It can be called as RuBP carboxylase oxygenase.
- In reduction reactions the glucose is formed.
- In this two molecules of ATP is utilised for phosphorylation.
- For each CO2 molecule to be fixed two molecules of NADPH are utilised.
- For the removal of one molecule of glucose from the pathway 6 molecules of O2 is required to fix and 6 terms of cycle is required.
- Regeneration steps require 1 ATP for phosphorylation to form RuBP.
- For every CO2 molecule entering the Calvin cycle 3 molecules of ATP and 2 of NAPH are required.
- Six molecule of co2 18 molecules of ATP and 12 molecules of NADPH goes in the calvin cycle.
- One molecule of glucose 18 molecules of ADP and 12 molecules of NADPH goes out of Calvin cycle.
THE C4 PATHWAY
- C4 pathway takes place in the plants that are adapted to dry tropical regions
- But this plant use the c3 pathway on the Calvin cycle as the main biosynthetic pathway.
- The leaves of C4 plants have special type of leaf anatomy.
- The C4 plants have bundle sheath cells and so the anatomy is called as krans anatomy.
- The bundle sheath cells have several layers around the vascular bundles.
- They are characterized by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
- The primary CO2 acceptor is a three carbon molecule phosphoenolpyruvate.
- It is present in the mesophyll cells.
- The enzyme responsible for this fixation is PEP carboxylase.
- The mesophyll cells lack RuBp carboxylase oxygenase.
- The C4 acid OAA is formed in the mesophyll cells.
- Then the four carbon compounds like malic acid or aspartic acid is formed in the mesophyll cells.
- Then they are transported to the bundle sheath cells.
- In the bundle sheath they are broken down to release CO2 and a three carbon molecule.
- The three carbon molecule is transported back to the mesophyll.
- There it is converted to PEP again.
- Hence the cycle completes.
- The co2 released in the bundle sheath cells and the c3 of the Calvin pathway.
- The bundle sheath cells are rich in an enzyme Rai bular biphosphate carboxylase oxygenase.
- RuBisCo is characterized by the fact that its active site can bind to both CO2 and O2.
- In some plants O2 bind to RuBisco.
- Due to this RuBp instead of being converted to two molecules of PGA.
- It forms one molecule and phosphoglycolate in a pathway.
- This pathway is called photorespiration.
- In this pathway there is neither synthesis of sugars nor of ATP.
- This results in the release of co2 with utilisation of ATP.
- As there is no synthesis of ATP this pathway is a wasteful process.
- C4 plants have a mechanism that increases the concentration of co2 at the enzyme site.
- Hence in C4 plants photorespiration does not occur.
- The intracellular concentration of CO2 increases because the C4 acid from the mesophyll is broken down into the bundle cells to release CO2.
- As the C4 plants lack photorespiration probably they have higher productivity and yields.
FACTORS AFFECTING PHOTOSYNTHESIS
- The plant factors affecting photosynthesis are :
- Number of leaves
- Size of leaves
- Orientation of leaves
- Mesophyll cells
- Internal CO2 concentration
- Amount of chlorophyll.
- The external factors are:
- CO2 concentration
- Law of limiting factors states 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 affect the process if its quantity is changed.
- It is a factor that affects photosynthesis .
- There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
- At higher in intensities the rate does not show any increase.
- Because the other factors become limiting.
- Increasein incident light beyond the. Causes the breakdown of chlorophyll and decrease in photosynthesis.
CARBON DIOXIDE CONCENTRATION
- It is the major limiting factor of photosynthesis
- Increase in concentration of 0.05% can cause an increase in CO2 fixation rates.
- C3 and C4 plants respond differently to CO2 concentrations.
- Both of them do not respond to high CO2 conditions at low light condition.
- At high light intensities both the plants show increase in the rates of photosynthesis.
- The dark reactions are enzymatic so they are temperature controlled.
- C4 plants respond to higher temperatures and show higher rate of photosynthesis.
- C3 plants have a much lower temperature optimum.
- Water stress causes the stomata to close hence reducing the CO2 availability.
- Water stress makes the leaves wilt.
- This reduces the surface area of the leaves and their metabolic activity as well.