Key Notes: Photosynthesis in Higher Plants - NEET PDF Download

• Photosynthesis is a physio-chemical process by which green plants use light energy to drive the synthesis of organic compounds.
• Photosynthesis is important due to two reasons:
  - it is the primary source of all food on earth and
  - it is also responsible for the release of oxygen into the atmosphere
• Ultimately, all living forms on earth depend on sunlight for energy.

Photosynthesis

• Variegated Leaf Experiment: To prove that photosynthesis occurred only in the green parts of the leaves in the presence of sun light (by testing these leaves for the presence of starch)
• To Prove that CO₂ is essential for photosynthesis → By starch testing, the exposed part of the leaf tested positive for starch while the portion that was in the tube (having KOH), tested negative.
• Priestley discovered Oxygen in 1774. 
• Priestley Hypothesised: Plants restore to the air whatever breathing animals and burning candles remove.
• Ingenhousz (1730-1799) → Showed that sunlight is essential to the plant process that purifies the air. He showed that it is only the green part of the plants that could release oxygen.
• In 1854, Julius Von Sachs provided evidence for production of glucose when plants grow. Glucose is usually stored as starch. His later studies showed that the green substance in plants (chlorophyll as we know it now) is located in special bodies (later called chloroplasts) within plant cells.
• TW Engelmann(1843 – 1909) → By using green alga, Cladophora, placed in a suspension of aerobic bacteria. FIRST ACTION SPECTRUM OF PHOTOSYNTHESIS WAS DESCRIBED. It resembles the absorption spectra of chlorophyll a and b.
• Key features of plant photosynthesis that plants could use light energy to make carbohydrates from CO₂ and water.
• Van Niel (studies on Purple and Green Bacteria)→ Demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
• In Green plants H₂O is the hydrogen donor and is oxidised to O₂ the hydrogen reduces the carbon di oxide in glucose.

• Some organisms do not release O₂ during photosynthesis.  When H2S is the hydrogen donor (for purple and green sulphur bacteria), the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O₂.
• From this, he inferred that the O₂ evolved by the green plant comes from H₂O, not from carbon dioxide.
• This was later proved by using radio isotopic techniques (Rubed, Hasid & Kamen)
• Then finally correct equation of photosynthesis came in existence.

Site of Photosynthesis

• Photosynthesis takes place in the green leaves of plants but it also takes place in other green parts of the plants.
• The mesophyll cells in the leaves have a large number of chloroplasts.
• Usually the chloroplasts align themselves along with the wall of the mesophyll cells (perpendicular to the incident light) such that they get the optimum quantity of the incident light.

Types of Reaction in Photosynthesis

• Light Reaction:
  Takes place in membranous system of grana in chloroplast.
  The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.
• Dark Reaction:
  Takes place in stroma of chloroplast.
  It is a light independent reaction but dependent upon light reaction products called ATP and NADPH.H.
  In stroma, enzymatic reactions synthesise sugar, which in turn forms starch.

Light Reaction

• A chromatographic separation of the chlorophyll shows that the colour that different shades of green colour in leaves is not due to a single pigment but due to four pigments:
  (i) Chlorophyll a (bright or blue green),
  (ii) Chlorophyll b (yellow green),
  (iii) Xanthophylls (yellow) and
  (iv) Carotenoids (yellow to yellow-orange).

• Pigments are substances absorb light of specific wavelengths.
• Chlorophyll a is the most abundant plant pigment in the world. It is found in all photosynthetic organism except photosynthetic bacteria.
• Chl a and Chl b absorb light maximum in blue and red regions of visible light hence rate of photosynthesis is maximum at these regions. 
• Chlorophyll a (primary photosynthetic pigment) is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a.
• Because of accessory pigments, plant can perform photosynthesis in wider range of visible light.
• Accessory pigment also protects chlorophyll a from photo-oxidation.

• Light reactions or the ‘Photochemical’ phase include:
  - Light absorption,
  - Water splitting,
  - Oxygen release,
  - The formation of high-energy chemical intermediates, ATP and NADPH. 
• Several protein complexes are involved in the process. o The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II).
• The LHC are made up of hundreds of pigment molecules bound to proteins. o Antennae also called light harvesting system which has all the accessory pigments whereas light harvesting complex has accessory pigments as well as chlorophyll a also.
• The single chlorophyll a molecule forms the reaction centre.
• In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700.
• In PS II, the reaction centre chlorophyll’ a’ has an absorption maximum at 680 nm, and is called P680.

The Electron Transport

• In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus.
• Then these electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes.
• This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale.
• The electrons pass through the electron transport chain and are passed on to the pigments of photosystem PS I.
• Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential.
• These electrons from PS I are moved downhill again and finally reduce the a molecule of energy-rich NADP+ to NADPH.
• In short, light fall on LHC II then electron moves from Chl a to electron acceptor then down hill movement through ETS then to pass on LHC I then again electron acceptor then reduce NADPH+ into NADPH.
• It is called the Z scheme, due to its characteristic shape.
• This shape is formed when all the electron carriers are placed in a sequence on a redox potential scale.

Splitting of Water

• The electrons that were moved from photosystem II is replaced by electrons available due to splitting of water.
• The splitting of water is associated with the PS II; water is split into 2H+, [O] and electrons.
• These electrons move to PS II and the electrons needed to replace those removed from photosystem I are provided by photosystem II and photolysis of water maintain the continuous supply of electrons to PS II.
• The water splitting complex is associated with the PS II, which is physically located on the inner side of the membrane of the thylakoids.

Cyclic and Non-Cyclic Photo-Phosphorylation

• Living organisms can extract energy from oxidizable substances and store this in the form of bond energy like in ATP.
• The process through which ATP is synthesised by cells (in mitochondria and chloroplasts) is named PHOSPHORYLATION from ADP and ip.
• Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
• In Z scheme /non-cyclic photophosphorylation,
  - both ATP and NADPH+.H+ are synthesised by electron flow in ETS.
  - It is operated in grana membrane as the membrane or lamellae of the grana have both PS I and PS II .
• In cyclic phosphorylation,
  - the excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain.
  - The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+.
• CYCLIC PHOSPHORYLATION possibly operated in stroma lamellae as it lacks PS II and NADP reductase enzyme.
• Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

Chemiosmotic Hypothesis of ATP Synthesis

• The chemiosmotic hypothesis has been put forward to explain the mechanism of ATP synthesis.
• ATP synthesis is linked to development of a proton gradient across a membrane inside the membranes of thylakoid (in lumen).
• Because splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids.
• As electrons move through the photosystems, protons are transported across the membrane.  
• This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. 
• Hence, this molecule removes a proton from the stroma while transporting an electron.  
• When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.
• The NADP reductase enzyme is located on the stroma side of the membrane.  
• Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH+ H+.  
• These protons are also removed from the stroma.
• Therefore, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. 
• This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
• The breakdown of this proton gradient that leads to the synthesis of ATP.
• The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF0 of the ATP synthase.

The ATP Synthase Enzyme

• Consists of two parts: one called the CF0 is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
• The other portion is called CF1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
• The breakdown of the gradient provides enough energy to cause a conformational change in the CF1 particle of the ATP synthase, which makes the enzyme synthesise several molecules of energy packed ATP.
• Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase.
• ATP synthase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATP synthase enzyme that catalyses the formation of ATP.
• The ATP will be used immediately in the biosynthetic  reaction taking place in the stroma, responsible for fixing CO₂, and synthesis of sugars.

Calvin Cycle/Dark Reaction

• It takes place in stroma. 
• It is light-independent but dependent on product of light reaction that is ATP and NADPH.
• This is the biosynthetic phase of photosynthesis leading to synthesis of sugar.
• The use of radioactive ¹⁴C by Calvin in algal photosynthesis studies led to the discovery that the first CO₂ fixation product was a 3-carbon organic acid.
• He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him.
• The first product 3-phosphoglyceric acid(PGA) in C-3 cycle but in C4 plant the first product is oxaloacetic acid which is a 4-carbon compound (c-4 cycle).
• The Primary Acceptor of CO₂ in Calvin cycle is 5-carbon sugar called Ribulose 1-5 bis phosphate.
• Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in a cyclic manner; the RuBP was regenerated.
• The Calvin cycle can be described under three stages: Carboxylation, Reduction and Regeneration.
• In carboxylation, CO₂ is utilised for the carboxylation of RuBP.
• This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.
• RuBisCO is the most abundant enzyme in the world.
• Reduction:
  (i) These are a series of reactions that lead to the formation of glucose.
  (ii) The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO2 molecule fixed.
  (iii) The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway.
• Regeneration: The regeneration steps require one ATP for phosphorylation to regenerate RuBP.
• To make one molecule of glucose 6 turns of the cycle are required. 18 ATP and 12 NADPH molecules will be required to make one molecule of glucose through the Calvin pathway.

The  C₄ Pathway

• In C-4 pathway, plants have the C4 oxaloacetic acid as the first CO2 fixation product but they also use the C3 pathway or the Calvin cycle as the main biosynthetic pathway.
• C4 PLANTS ARE SPECIAL: They have a special type of leaf anatomy called KRANZ ANATOMY, they are capable to perform photosynthesis in low CO2 concentration, they tolerate higher temperatures, they show a response to high light intensities, they lack a process called photorespiration and have greater productivity of biomass.
• The large cells around the vascular bundles of the C4 plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’(wreath) anatomy.
• The bundle sheath cells may form several layers around the vascular bundles; they have large number of chloroplasts, thick walls impervious to
  gaseous exchange and no intercellular spaces. example – maize or sorghum and sugarcane.
• C-4 plants also have mesophyll cells but Calvin cycle do not operate here.
• In C-4 plants, Calvin cycle operates in bundle sheath cells whereas C4/Hatch & slack cycle completed in both mesophyll cells as well as bundle sheath cells. o In C-4 cycle, Rubisco is not exposed to oxygen so there is very less probability of photorespiration. Therefore, productivity of these plants is high.

Hatch and Slack Pathway

• The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells.
• The enzyme responsible for this fixation is PEP carboxylase or PEP case.
• Due to carboxylation of PEP, oxaloacetic acid (OAA) is formed which is C-4 compound then it is reduced by NADPH and convert into mailic acid.
• Malic acid is transported to bundle sheath cells where it release CO2 and pyruvic acid by decarboxylation.
• Pyruvic acid then moves to mesophyll cells where it regenerates PEP by phosphorylation. o Released CO2 in bundle sheath cells is accepted by ribulose 1-5 bis phosphate and Calvin cycle is operated.

Photorespiration

• Rubisco active site can bind to both CO2 and O2 ,when  it binds CO2 Calvin cycle operates but if it binds O2 then photorespiration occur.
• Rubisco has a much greater affinity for CO2 when the CO2: O2 is nearly equal than for O2. This binding is competitive.
• It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
• Photorespiration decreases the plant productivity.
• To prevent from photorespiration plants adapted for kranz anatomy.
• In C3 plants some O2 bind to Rubisco.
• When Rubisco bind with O2, The RuBP instead of being converted to 2 molecules of PGA it forms one molecule of phosphoglycerate and phosphoglycolate (2 Carbon) in a pathway called photorespiration.
• In the photo-respiratory pathway à neither synthesis of sugars, nor of ATP.
• Rather it results in the release of CO2 with the utilisation of ATP.
• In the photorespiratory pathway there is no synthesis of ATP or NADPH.
• Photorespiration is completed in chloroplast, peroxisome and mitochondria.

Factors Affecting Photosynthesis

• The rate of photosynthesis is very important in determining the yield of plants including crop plants.
• The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll.
• The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.
• The external factors would include the availability of sunlight, temperature, CO2 concentration and water.

Blackman’s (1905) 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”.

1. Light

• There is a linear relationship between incident light and CO2 fixation rates at low light intensities.
• At higher light intensities, gradually the rate does not show further increase as other factors become limiting.
• The light saturation occurs at 10 per cent of the full sunlight. Hence, except for plants in shade or in dense forests, LIGHT IS RARELY A LIMITING FACTOR IN NATURE.
• Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

2. Carbon Dioxide

• Major limiting factor for photosynthesis as the concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 percent).
• Increase in concentration up to 0.05 per cent can cause an increase in CO2 fixation rates;
• beyond this the levels can become damaging over longer periods.
• The C₃ and C₄ plants respond differently to CO₂ concentrations.
• At low light conditions neither group responds to high CO2 conditions.
• At high light intensities, both C₃ and C₄ plants show increase in the rates of photosynthesis with increase in CO₂ concentration.
• The C₄ plants show saturation at about 360 µlL-1 while C₃ responds to increased CO₂ concentration and saturation is seen only beyond 450 µlL-1. Thus, current availability of CO₂ levels is limiting to the C₃ plants.
• The C₃ plants respond to higher CO₂ concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper.
• They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.

3. Temperature

• The dark reactions being enzymatic are temperature controlled.
• The light reactions are also temperature sensitive they are affected to a much lesser extent.
• The C₄ plants respond to higher temperatures and show higher rate of photosynthesis while C₃ plants have a much lower temperature optimum.
• The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to.
• Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

4. Water

• Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis.
• Water stress causes the stomata to close hence reducing the CO₂ availability.
• water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.