Photosynthesis in Higher Plants
Chapter-13
Photosynthesis in Higher Plants
Points to Remember
Photosynthesis : Photosynthesis is an enzyme regulated anabolic process to manufacture organic compounds inside the chlorophyll containing cells from carbon dioxide and water with the help of sunglight as a source of energy.
Sun Light
6CHO2 + 12H2O ------------------> C6H12O6 + 6H2O + 6O2 ↑
Chlorophyll + enzymes
Historical Perspective
Josheph Priestly (1170) : Showed that plant have the ability to take up CO2 from atmosphere and release O2. (Candle with bell jar and mouse expt.)
Jan Ingenhousz (1779) : Release of O2 by plants was possible only in sun-light and only by the green parts of plans. (Expt. with aquatic plant in light & dark)
Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis to occur.
Julius Von Sahs (1854) : Green parts in plant produce glucose which is stored as strach.
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 reaction in which hydrogen form an oxidisable compound reduces CO2 to form sugar. He gave a simplified chemical equation of photosynthesis.
Sun Light
2H2A + CO2 ---------------------> 2A + CH2O + H2O
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 photosynthesis. 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. Biosyntheitic phase (dark reaction) is carried in stroma.
Importance of Photosynthesis -- (1) Synthesis of organic compounds (2) Change of radiant energy into chemical energy (3) Useful products are obtained form plants gums. oils timber fire wood, resins rubber fibers and drug, etc. (4) Balance the percantage of O2 and CO2 in atmosphere (5) Fossil fuels like coal, natural gas and petroleum have been formed inside the earth indirectly as a product of photosynthesis.
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. It is called universal photo-synthetic pigment.
Chlorophyll b : (Yellow green)
Xanthophylls : (Yellow)
Carotenoids : (Yellow to yellow-orange)
- In the blue and regions of spectrum shows higher rate of photosynthesis.
Light Harvesting Complexes (LHC) : The light Harvesting complexes are made up of hundreads of pigment molecules bound to protein within the photosystem I (PS-I) and photosystem II (PS-II). Each photosystem has all the pigments (except one molecules of chlorophyll 'a') forming a light harvesting system (antennae). The reaction centre (chlorophyll a) is different in both the photosystems.
Photosystem I (PS-I) : Chlorophyll 'a' has an absorption peak at 700 nm (P700).
Photosystem II (PS-II) : Chlorophyll 'a' has absorption peak at 680 nm (P680),
Process of photosynthesis : In includes two phases- Photochemical phase and biosyntheitic phase. (Formerly known as Light reaction and dark reaction)
(i) Photochemical phase (Light reaction) : This phase includes-light absorption, splitting of water, oxygen release and formation of ATP and NADPH. It occurs in grana region of cholorplast.
(ii) Biosyntheitic phase (Dark reaction) : It is light independent phase, synthesis of food material (sugars). (Calvin cycle). It occurs in stroma region of chloroplast.
Photophosphrylation : The process of formation of high-energy chemicals (ATP and NADPH) in presence of light.
Non-Cycle Photophosphrylation Two photosystems work in series-First PSII and the 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.
The electron transport (Z-Scheme) : In PS II, reaction centre (chlorophyll a) absorb 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 hil further to NADP+. NADP+ in then reduced to NADPH + H+.
The splitting of water : It is linked to PS II. Water splits into H+, [O] and electrons. 2H2O ---> 4H+ + 2 + 4e-.
Cyclic Photophosphrylation : Only PS-I works, the electron circulates within the photosystem. It happens in the stroma lamellae (possible location) beacuse is this region PSII and NADP reductase enzymes are absent. Hence only ATP molecules are synthesised. It occurs when only light of wavelength beyond 680 nm are available for excitaion.
Chemiosmotic Hypothesis : Chemiosmotic hypothesis expalin the mechanism of ATP synthesis in chloroplast. In photosynthesis, ATP synthesis is linked to development of a proton are accumulated inside of membrane of thylakoids (in lumen). ATPase enzyme has channel of that allow diffusion of protons back across the membrane. This release energy to activate ATPase enzyme that catalyes the formation of ATP.
Biosyntheitic phase in C3 plants :
ATP and NADPH, 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 molecules in RuBP (ribulose bisphosphate). The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discover 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 (present in stroma)
(2) Reduction : Carbohydrate is formed at the expense of ATP and NADPH.
In involves 2ATP for phsophorylation and 2NADPH for reduction per CO2 molecules fixed.
(3) Regeneration : The CO2 1,5-bisphosphate is formed again.
6 turns of Calvin cycles and 18 ATP molecules are required to synthesize one molecules of glucose.
6CO2 + 6RuBP + 18ATP + 12NADPH ----> C6H12O6 + 6RuBP + 18ADP + 18Pi + 12NADP
| Input |
Output |
| 6CO2 |
One Glucose |
| 18 ATP |
18 ADP |
| 18 NADPH |
12 NADP |
The C4 Pathway :
C4 plants such as maize, sorghum, sugarcane have a four cabon compound special type, of leaf anatomy, they tolerate higher temperatures. In this pathway, oxaloacitic 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 cell and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase. Oxaloacetic is reduced to maltate/asparate that reach into bunlde sheath cells.
The decarboxylation of maltate/asparate occurs with the release of CO2 and formation of pyruvate (3C). In high CO2 concentration RuBisCO behaves as carboxylase and not as oxygenase, hence the photosynthetic losses are preventd. RuBP operates now under Calvin cycle and pyruvate transported back to mesophyll cells and changed into phosphoenol pyruvate (PEP) to ekkp the cycle continue.
Photorespiration :
The light induced respiration in green plants in called photorspiration. In C3 plants some O2 binds with RuBisCO and hence CO2 fixatiin 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.
RuBP + O2 → PGA + Phosphoglycolate
There is neither synthesis of ATP nor NADPH2 or sugar. Rather it results in release of CO2 with utilisation of ATP. The biological function of Photorespiration is not known yet.
C4 Plants :
(1) Lack Photorspiration
(2) Show response to high light intensities
(3) Have greater productivity of biomass.
Adaptations in C4 Plants :
(i) Kraz Anatomy
(ii) Occurrence of two types of cells
(iii) Dimophic chloroplast
(iv) Presence of RuBisCO in Bundle Sheath cells and PEPcase in mesophyll cells.
(v) Mechanisms to increase CO2 concen-tration near RuBisCO in Bundle Sheath cells.
CAM (Crassulacean Acid Metabolism)
Plants-- Stomata open at night. e.g., Cacti, Bryophyllum, Pineapple.
Law of Limiting Factors : If a chemical process in affected by more than one factor, then its rate will be detemined 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 : Rate of photo-synthetic increase at low light intensities. At high intensities of light beyong a point the rate of CO2 fixatiin decreases. Longer hours of light duration favoour more photosynthesis rate.
2. Carbon dioxide : Increase in CO2 concentration causes increase in CO2 fixatiin. It is the major limiting factor for photosynthesis.
3. Temperatures : The rate of photosynthesis at optimum temperature is, high. It is 20oC-25oC for C3 plants and 30-45o for C4 plants.
4. Water : Water is one of the reactant in photosynthesis, but is effects the rate of CO2 fixatiin. Low water content causes the stomata to close and reduces the CO2 availability.
