CBSE Biology Class 11 Photosynthesis In Higher Plants Notes. The article provides Photosynthesis in higher plants class 11 Notes Biology for quick preparation of CBSE exams and school based annual examinations.
‘Photosynthesis is defined as the photo-biochemical/physicochemical mechanism, incorporating anabolic, reductive and endergonic processes carried out by green plants, in which complex, energy-rich organic compounds (carbohydrates such as sugars, starches) are synthesized from simple inorganic raw materials composed of water (H2O) and carbon dioxide (CO2) in presence of electromagnetic radiations (light or solar energy) and light capturing pigments (chlorophyll, carotenoids) with release of oxygen (O2) as a byproduct.’
This mode of nutrition using photosynthesis process is termed as photoautotrophism.
The basic mode of photosynthesis occurs in the following steps:
Absorption and retention of electromagnetic radiations (composed of photons and waves) in the form of light energy from the sun. Conversion of the dynamic, solar light energy into stable, chemical potential (energy) which is stored. Hence, the overall reaction of photosynthesis is shown as
Certain bacteria are capable for photosynthesis e.g. – Chlorobium (Green sulphur), Rhodospirillum, Rhodopseudomonas (purple non-sulphur). Cyclic photophosphorylation is an important method in bacterial photosynthesis. PS II is absent. So non-cyclic photophosphorylation is absent. Pigment system of bacteria denoted by – B-890 or B-870. Evolution of O2 if any is not linked to bacterial photosynthesis, because water is not the H+ donor. The donor may be hydrogen sulfide.
Pigments Participating in Photosynthesis
There are three main types of photosynthetic pigments–Chlorophylls, Carotenoids and Phycobilins.
Chlorophyll – Chloros in Greek means green while phyllon means leaf. They are the photosynthetic pigments found in higher plants and many other photosynthetic organisms. They are the main pigments concerned with harvesting solar energy. They are specialized lipid molecules embedded in thylakoid membrane of the chloroplasts.
Arnoff and Allen (1966) recognized 9 types of chlorophylls. Some of them are (1) Chlorophyll-a; (2) Chlorophyll-b, (3) Chlorophyll-c etc.
Chlorophyll-a and chlorophyll-b are the two main types of chlorophylls found in plants. Generally light energy absorbed by other photosynthetic pigment is transferred to chlorophyll-a.
Empirical formula of chlorophyll-a is C55H72O5N4Mg. Chlorophyll a molecule has a porphyrin (a tetrapyrrole closed ring derivative) head and a phytol(C20H39OH) tail. A vinyl group is present at the second carbon position in the tetrapyrrole ring. A methyl group is present at the third carbon position of the tetrapyrrole ring. An Mg atom in nonionic form is held within the head with two covalent and two coordinate bonds. Chlorophyll-a absorbs violet blue and red lights with absorption maxima at 430 nm and 662 nm. Except for bacteria, it is found in all photosynthetic organisms.
The empirical formula of chlorophyll-b is C55H70O6N4Mg. It has a formyl (CHO) group at the third carbon position of the tetrapyrrole ring. Otherwise, it is similar to chlorophyll-a. It absorbs blue and orange wavelengths with the absorption maxima at 430 nm and 644 nm.
Chlorophyll-c lacks phytol esterification. It is found in brown algae, diatoms and dinoflagellates.
It absorbs in far red wavelength of light. It is found in brown algae and other organisms that thrive in moderately deep zones of water bodies.
Carotenoids are yellow to orange lipid compounds. They occur in almost all higher plants. Carotenoids are of two types- carotenes and xanthophylls.
Carotenes are reduced molecules. Their general form is C40H56. Carotenes are of several types like α and β. The most widespread and important carotene associated with chlorophyll inphotosynthetic organisms is β-carotene. It is orange-yellow in colour. A molecule of carotene is broken down into two molecules of vitamin A in vertebrates during digestion. A carotene called lycopene is responsible for the red colour of tomatoes.
Xanthophylls also contain oxygen along with carbon and hydrogen. They are yellow colored pigments. They are found in papayas peaches and prunes. They are present in the human eye and help to protect it against ionizing effect of some radiations. Examples include Lutein (C40H56O2), cryptoxanthin (C40H56O), etc.
They are also called biliproteins. They are accessory pigments found in red algae and cyanobacteria. They have open chain tetrapyrrole structure. They are soluble on hot water. Phycoerythrins and Phycocyanins are two types of Phycobilins. Generally it is seen that both types occur together. However the proportion may vary according to the species and environment.
Absorption and Action Spectra
Absorption spectrum is represented as a graph obtained by plotting Absorption vs Wavelength for a particular pigment. Different photosynthetic pigments absorb only certain wave lengths. Action spectrum is a graph showing the effectiveness of different wavelengths of light in stimulating the process under investigation. It was first studied by Engelmann on Cladophora. Effectiveness is measured by analyzing quantum yield or amount of action which can be denoted through CO2 reduction, O2 release etc.
Photosystems are functional and structural units consisting of protein complexes involved in photosynthesis. They are located in the thylakoid membranes of plants and algae or in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: Photosystem I and Photosystem II. Both photosystems I and II are required for oxygenic photosynthesis. The photosystem I was named “I” since it was discovered before photosystem II, but this does not represent the order of the electron flow.
PSUs (Photosynthetic units) are present on the thylakoid membranes. PSUs are made up of 250 – 400 molecules of various pigments. The PS II is located in the appressed region of granal thylakoids. PS I is found in the non appressed region of grana and stoma thylakoids.
Mechanism of Photosynthesis
1. Light Reaction/Hill Reaction
Alternate names – Light reaction/Hill reaction/Photochemical reaction/Generation of assimilatory powers (NADPH2 + ATPs)/Photophase.
In cyclic photophosphorylation, only PS-I works. PS I consists of Chlorophyll –a 670, Chlorophyll-a 683, Chlorophyll-a 695, carotenoids, some molecules of Chlorophyll-b and reaction centre- Chlorophyll-a 700 or P-700. This form of photophosphorylation occurs on the thylakoid membrane. The electron begins PS I, passes from the primary acceptor to ferredoxin reducing substance (FRS), then to ferredoxin, then to cytochrome b6f and then to plastocyanin before returning to PS I. This process produces a proton-motive force, pumping H+ ions across the membrane thereby generating a concentration gradient that can be used to power ATP synthase during chemiosmosis. Cyclic photophosphorylation neither produces O2 nor NADPH.
Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons, they are instead sent back to cytochrome b6f complex. This process is mostly seen in bacteria and favored in anaerobic conditions.
Light- PS I –> FRS –> Ferredoxin –> Cytochrome b6f –> Plastocyanin –> PS I
It is also termed as Z-scheme. Both PS-I and PS-II involved in non-cyclic photophosphorylation. PS-II (P-680) consists of Chlorophyll-a 660, Chlorophyll-a 673, Chlorophyll-a 690, Chlorophyll-b, or Chlorophyll-c or Chlorophyll-d, carotenoids and phycobilins. Chlorophyll-a 680 is the reaction centre. It occurs in stroma lamellae. The electrons do not go back to the reaction centre but rather are accepted by NADP+.
Photolysis of water uses up the electrons and leads to the formation of ATP and NADPH2. The steps in the process begin with the PS II. Electrons are passed to Plastoquinone reducing substance (PQRS). PQRS passes them to plastoquinone which passes them to the cytochrome system. The cytochrome system passes them to plastocyanin which in in turn passes them to PS I. The steps afterwards include FRS, ferredoxin and NADP reductase.
It was put forth by Peter Mitchell to explain ATP formation. During the electron transport chain of photosynthesis the H+ concentration gradually increases in thylakoid lumen. There are three causes of difference in H+ ion concentration –
O Photolysis of H2O produces H+
O PQ shifting of H+ ion from stroma to lumen.
O NADP reductase mediated utilization of H+ form stroma.
A proton gradient and electrical potential is generated across the thylakoid membrane due to the differential H+ ion concentration. The gradient and the electrical potential are collectively called proton motive force (PMF). The passage of H+ ions leads to activation of ATP synthase which synthesizes ATP from ADP and Inorganic phosphate (Pi).
Dark Reaction/Calvin Cycle
Alternative names – Dark Reaction/Blackman Reaction/Calvin cycle/C3-Cycle/Biochemical phase/Carbon assimilation/photosynthetic carbon reduction cycle (PCR-cycle)/Reductive pentose phosphates pathway-
C3 cycle is comes under dark reactions, as no direct light is required for the process to be carried out. Calvin presented these reactions in a cyclic manner and it is thus called as Calvin cycle. A three carbon compound called PGA (Phosphoglyceric acid) is the first stable compound produced during Calvin cycle. Hence, the cycle is also called as C3– cycle.
Calvin carried out his experiment using an algae system, chromatography and radioisotopy with radioactive carbon- C14. Rubisco (Ribulose bis-phosphate carboxylase-oxygenase) is an important enzyme of the Calvin cycle. It is present in stroma. CO2-acceptor in Calvin cycle is RuBP. In order to form one glucose molecule, 6 turns of Calvin cycle are required. 12 ATP molecules are used up to form a molecule of glucose.
Alternative names – CO2 concentrating mechanism/Co-operative photosynthesis/Dicarboxylic acid cycle (DCA cycle)/C4 cycle/Hatch and Slack pathway
Kortschak and Hatch first observed that 4C, OAA (Oxaloacetic acid) is formed in sugarcane leaves during dark reaction. A pathway for dark reactions in sugarcane and maize leaves was proposed by Hatch and Slack. C4-cycle occurs in 19 families of angiosperms, but mostlyin monocots, belonging to families Gramineae (True grasses) e.g. sugarcane, Maize, sorghum etc. and Cyperaceae (sedges) e.g. water chestnut.
Kranz Anatomy is seen in leaves of C4 plants. Green bundle sheath cells (BS cells) present around the vascular bundles. Two types of chloroplasts are present in the leaf cells. In mesophyll cells, chloroplasts are small and with grana while chloroplasts of B.S. cells are larger and without grana. PEPCase (Phosphoenol pyruvate carboxylase) enzyme is present in mesophyll cells while Rubisco is present in BS cells. In the C4-plants, C3-cycle occurs in bundle sheath cells, while C4-cycle occurs in mesophyll cells.
Photosynthetically C4 plants are more efficient as there is no photorespiration. BS cells do not release O2 and mesophyll cells pump more CO2 for C3 cycle. C4-plants are found in tropical habitats. They have adapted themselves to the environment with high temperature, low water availability and intense light.
Primary CO2 acceptor in C4 is PEP (phosphoenol pyruvate). It is a 3 carbon compound. First carboxylation in C4-cycle is catalyzed by PEPCase in thecytoplasm of mesophyll cells. The second carboxylation or final CO2 fixation occurs in BS cells by through the C3 cycle. For the production of 1 hexose (Glucose) molecule in C4-plants, 30 ATP molecules are used up. The enzyme pyruvate phosphate dikinase (PPDK) converts pyruvate to PEP by converting an ATP to AMP. This regeneration of PEP helps C4 plants increase the efficiency of CO2 fixation.
Photosynthetic carbon oxidation cycle/C2Cycle/ Photorespiration/Glycolate-Metabolism
Rubisco has some affinity for O2. Hence, sometimes oxygen is added to RuBP instead of CO2. Instead of PGA, phosphoglycolate (PA) is produced. PA is recycled to produce PGA via the photorespiratory pathway. Photorespiration is a wasteful process linked with C3 cycle. It consumes ATP. It occurs in chloroplast, peroxisomes and mitochondria.
Alternative names – CAM-Plants/Crassulacean acid metabolism/Dark CO2 fixation/Dark acidification
It is observed in succulent xerophyte plants e.g. Kalanchoe, Bryophyllum, Sedum, Kleinia etc. Primary acceptor of CO2 is PEP (Phosphoenol pyruvate). Oxaloacetic acid is the first product of the carboxylation reaction. In CAM plants stomata are of scotoactive type (they open at night). Organic acids are produced during night and they are broken down during the day. Final CO2 fixation (C3 cycle) occurs in day time. PEPCase induces carboxylation reaction in night. PEP carboxylase and Rubisco present in mesophyll cells. Kranz-anatomy is not seen. Synthesis of 1 molecule of glucose requires 30 ATPs. Thus, CAM plants leave the stomata closed during the day. This highly reduces the water loss.
Factors Affecting Photosynthesis
Law of limiting factors states then when the rapidity of a process is dependent on more than one factor, the rate at which the process occurs is controlled by the slowest factor. In case pf photosynthesis,water may be a limiting factor in dry regions, light may be limiting on cloudy days and in dense forests.
There is a linear relationship between light intensity and rate of photosynthesis at low light intensity. At extremely high light intensity photo-oxidation may occur and this may destroy the photosynthetic apparatus. Intensity of light, at which rate of photosynthesis, becomes equal to the rate of respiration in plants is known as light compensation point. Net photosynthesis or net primary productivity at this point is zero.
Optimum temperature for C3 plants for photosynthesis is 20o–25oC and 30o–40oC for C4 plants. The rate of photosynthesis decreases at higher temperature due to denaturation of enzymes. Dark reactions are more affected by temperature as compare to light reactions.
(iii) Concentration of CO2 (Between 0.03% and 0.04%)
An increase in CO2 concentration upto 0.05% boosts the rate of photosynthesis. Higher concentration of CO2 is toxic to plants and also leads to closure of stomata. The CO2 concentration at which CO2 fixation in photosynthesis is equal to volume of CO2 released in respiration when plant saturated with full light is called CO2 compensation point. CO2 compensation point for C4 plants is 8 – 10 ppm, while for C3 plants it is 40 – 100 ppm.
Reduction in availability of water reduces the rate of photosynthesis.
(v) Plant factors
Amount of chlorophyll present. Leaves- The leaf number, size, age and leaf orientation can affect the rate of photosynthesis.