PHOTOSYNTHESIS What is photosynthesis The biochemical process in

PHOTOSYNTHESIS

What is photosynthesis? The biochemical process in which sun light fixes carbon dioxide into glucose in the presence of water is called photosynthesis.

Photosynthesis in Overview Process by which plants and other autotrophs store the energy of sunlight into sugars. Requires sunlight, water, and carbon dioxide. Overall equation: 6 CO 2 + 6 H 20 + sunlight C 6 H 12 O 6 + O 2 Occurs in the leaves of plants in organelles called chloroplasts. 6

Where does photosynthesis occur? Photosynthesis occurs in the leaves of a plant.

Leaf Structure Most photosynthesis occurs in the palisade layer. Gas exchange of CO 2 and O 2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface. Palisade Spongy

Chloroplast Structure Chloroplasts are organelles specialized subunits, in plant and algal cells. Their main role is to conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, and stores it in the energy storage molecules ATP and NADPH while freeing oxygen from water. They then use the ATP and NADPH to make organic molecules from carbon dioxide in the Calvin cycle.

STRUCTURE Two membranes contain and protect the inner parts of the 7 chloroplast. They are appropriately named the outer and inner membranes. The inner membrane surrounds the stroma and the grana (stacks of thylakoids). One thylakoid stack is called a granum. Chlorophyll molecules sit on the surface of each thylakoid and capture light energy from the Sun. As energy rich molecules are created by the light-dependent reactions, they move to the stroma where carbon (C) can be fixed and sugars are synthesized. The stacks of thylakoid sacs are connected by stroma lamellae. The lamellae act like the skeleton of the chloroplast, keeping all of the sacs a safe distance from each other and maximizing the efficiency of the organelle.

Photosynthesis-One of 8 Energy carriers- ATP is the phosphorylated version of ADP, the main functions of the which stores energy in a cell and powers most cellular activities. ATP chloroplast is its role in is the energized form, while ADP is photosynthesis, the process by the (partially) depleted form. NADP+ which light is transformed into is an electron carrier which ferries chemical energy, tosubsequently high energy electrons. In the light reactions, it gets reduced, meaning it produce food in the form of sugars. picks up electrons, becoming Water (H 2 O) and carbon dioxide NADPH. (CO 2) are used in photosynthesis, NADP+ reduction-Electrons are and sugar and oxygen (O 2) is made, often removed from the electron using light energy. transport chains to charge NADP+ with electrons, reducing it to Light reactions- The light NADPH. Like ATP synthase, reactions take place on the ferredoxin-NADP+ reductase, the enzyme that reduces NADP+, thylakoid membranes. They take releases the NADPH it makes into light energy and store it in the stroma, right where it is needed + NADPH, a form of NADP , and for the dark reactions. ATP to fuel the dark reactions.

Photosynthetic pigments There are three different categories of pigments present in plants: Chlorophylls. Carotenoids. Phycobilins. Out of these three, Chlorophylls and Carotenoids are water insoluble while the Phycobilins are water soluble. Chlorophyll A is the most important photosynthetic pigment.

Other pigments called antenna or accessory 10 pigments are also present in the leaf. Chlorophyll B Carotenoids (orange / red) Xanthophylls (yellow / brown) Phycobilins( red/ blue) These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

CHLOROPHYLLS. 11 A series of ten chlorophylls is known – chl a, b, c, d, e, bacteriochlorophylls a, b, c, d, e and bacterioviridin. but only two types- chl a and chl b are widely distributed in higher plants and green algae.

STRUCTURE OF CHLOROPHYLL. 12 The chlorophyll molecule is a porphyrin system in which four pyrrole rings are linked together by methane groups forming a ring system. The centre of tetrapyrrole is occupied by a divalent Mg++ which is complexed with nitrogen atoms of the four pyrrole rings.

Each chlorophyll has a fifth ring containing only carbon atoms. This porphyrin skeleton 13 bears an alcohol with 20 carbon atoms- the phytol chain, bound in ester linkage to the pyrrole ring IV. The phytol is responsible for lipoidal solubility of the chlorophylls. The empirical formula of chl a is C 55 H 72 O 5 N 4 Mg. The empirical formula of chl b is C 55 H 70 O 6 N 4 Mg. Chl b also occurs in two forms Chl b 640 and chl b 650 The chlorophylls vary in their structure of side chains attached to the pyrrole rings. The frequently occuring chl b differs from chl a in that chl a has a methyl group on ring II while chl b has an aldehyde group –CHO.

14

DIFFERENCES b/w chl a & chl b 15 Chl a Chl b It is C 55 H 72 O 5 N 4 Mg. It is C 55 H 70 O 6 N 4 Mg. In pure state, chl a is blue In pure state, chl b is olive green. It is a primary photosynthetic pigment. Carbon-3 contains methyl group. It absorbs more red wavelength than violet red wavelength. It is soluble in petroleum ether. green. It is accessory photosynthetic pigment. Carbon-3 contains aldehyde or formyl group. It absorbs more violet-blue wavelength than red wavelength. It is soluble in 92% methyl alcohol.

IT ALL STARTS WITH SUNLIGHT Light from the sun is composed of wavelengths (colors 16 The shorter the wavelength the higher the frequency, thus the higher the energy The longer the wavelength the lower the energy, thus the lower the energy

Sunlight (a. k. a. white light) -sunlight is actually white light made of all wavelength colors 17 -sunlight is visible light -different colors=different wavelengths of light The Visible Spectrum violet-blue-green-yellow-orange-red Chlorophyll is a green photosynthetic pigment found in chloroplasts of plants 380 major nm 750 nm -Two photosynthetic pigments are chlorophyll a chlorophyll b.

Green is the least effective color for photosynthesis because it is reflected -Both chlorophylls absorb 18 violet, blue, and red wavelengths best. Very little green light is absorbed; most is reflected back; this is why leaves appear green.

Absorption Spectrums. Absorption spectrums 19 are graphs that plot a pigment’s light absorption vs. wavelength

Action spectrums. A. Action spectrums tell you how much photosynthesis is occurring at each wavelength. B. Made by illuminating chloroplast with different wavelengths of light and then plotting wavelength against some measure of photosynthetic rate. C. The photosynthetic rates could be measured by finding oxygen production, carbon dioxide absorption or light absorption 20

Comparison of absorption and action spectra 21 Absorption spectrum Action spectrum for chlorophyll *The photosynthetic rate is very low at green wavelengths

CAROTENOIDS. 22 Carotenoids are large group of pigments of yellow, brown, orange and red colours which occur in chloroplasts and chromoplasts. They are insoluble in water but soluble in fats and other organic solvents. They do not require light for their formation.

STRUCTURE & TYPES OF CAROTENOIDS. 23 Carotenoids are related to phytol. They have highly unsaturated molecules which are built up of isoprene units. Each molecule has long chain which contains conjugated double bonds between carbon atoms. The carotenoids are of two types Carotenes. Xanthophylls (Carotenols).

Carotenes. They are orange or red orange with general formula 24 C 40 H 56. Lycopene a fundamental type is red in colour having open chain is found in purple sulphur bacteria, tomatoes, fruits, roses. etc Carotene which is isomeric with lycopene has six carbon ring at one or both ends of the chain. Three main isomers of carotenes are α, β and γ. Carotene α is less abundant.

25 Carotene β is more abundant found in chloroplasts and is capable of absorbing light in violet and blue green parts of spectrum showing maximum absorption peak between 450 and 460 nm. Carotene γ is present in green sulphur bacteria. Carotenes are soluble in carbon disulphide, chloroform and ether. They are found in flowers like Calendula, fruits like tomato, several plants and red colour of carrot is also due to them.

Xanthophylls. They are also called as carotenols are yellow or 26 brown pigments which are oxygen containing derivatives of carotenes that may be present in hydroxyl, carboxyl, methoxyl or epoxide groups. Lutein or luteol having empirical formula C 40 H 56 O 2 is hydroxylated form of α-carotene. It is most common xanthophyll of leaves and some flowers like sunflower, dandelion. etc. Other common leaf Xanthophylls are Zeaxanthin-an isomer of lutein and hydroxylated form of β-carotene.

27 Cryptoxanthin (C 40 H 56 O) Flavoxanthin (C 40 H 56 O 3). Violaxanthin (C 40 H 56 O 4). Fucoxanthin (C 40 H 56 O 6). Functions of CAROTENOIDS. They help in converting elemental oxygen to molecular form. They prevent photo-oxidation of chlorophyll. Carotenoids make flowers and fruits conspicuous to animals for pollination and dispersal Carotene β produces vitamin A in animals.

PHYCOBILINS. 28 They are photosynthetically active pigments occuring in blue green algae and red algae. These are of twotypes Phycoerythrin. Phycocyanin or allophycocyanin. These are protein linked water soluble pigments having open chain tetrapyrroles. These are found to be linked together by special proteins forming phycobiliprotein located in granular structures called phycobillisomes that are twice as large as ribosomes and are found attached to the photosynthetic membranes of algal species.

29

PHOTOSYSTEMS. 30 Light phase consist of two or three photochemical processes which are carried out by two different photosystems. Each PS consists of chlorophylls , accessory light absorbing pigments and electron transfer particles that are associated with integral proteins present in thylakoid membranes. The two different photosystems are- PS I • Photosystem II. PS II

EMERSON EFFECT or PHOTOSYNTHETIC ENHANCEMENT. 31 Emerson found a sharp reduction or quantum yield when a monochromatic light beam more than 680 nm was used, because its wavelength belonged to red region of spectrum, thus was named as red or far red drop. Blinks observed that two different beams of light can be applied successively at shorter intervals with similar increased photosynthetic activity. This increase in photosynthetic activity on the successive application of beams of different wavelength is termed as EMERSON EFFECT or PHOTOSYNTHETIC ENHANCEMENT.

Emerson effect can be due to two reasons Photosynthetic pigments occuring in specific groups called Light harvesting group or Energy traps. There are two interconnected pigment systems, each having its own requirement of light energy. Emerson effect is due to presence of two photosystems carrying out different photo-reactions. 32

PHOTOSYSTEM I 34 It is located both in stroma and grana. The chl a to chl b ratio is 4: 1. The light harvesting complex(LHC 1 ) is smaller. The trap or reaction centre of PS I is chl a dimer which is known as P 700 because its red absorption band is at 700 nm. It is surrounded by long wave chl a molecules i. e. Chl a 705 -35 Chl a 690 -700 , Chl a 678 -687 , Chl a 670 -680 , Chl a 660 -670 , Chl b and carotenoids. PS I has reducing agent A or K, Fe. S complexes, ferrodoxin, PQ, cytochromes f and b 6. PS I can carry on cyclic photophosphorylation independently where it derives electron from PS II to NADP+ so that may combine with H+ ions.

PHOTOSYSTEM II 35 PS II is located in grana where it possesses both chl a, chl b and carotenoids. The Light harvesting complex is LHCII. It contains 50 -60% of total chlorophyll. The chl a to chl b ratio is 1: 2. The trap or reaction centre is a special dimer of chl a called P 680 surrounded by Chl a 685 -695 , Chl a 678 -687 , Chl a 670 -680, Chl a 660 -670 , chl b and carotenoids. PSII contains Mn, quinones, cyt b 6 , cyt f and plastocyanin which is connected with photolysis of water and oxygen evolution. Chlorophyll trapping centre gets reduced with the gain of an electron that later absorbs a photon of light leading to the expulsion of the electron.

36

The electron is picked up by the pheophytin which acts like a quencher of flouresence thus 37 oxidising the chlorophyll trap. The electron flows through a number of electron transport particles like Plastoquinone, cyt. b 6 , cyt. f and is passed over to P 700 of PS I through a copper enzyme plastocyanin. ATP is produced due to development of proton motive force by PQ. Photophosphorylation occuring during PS II is non cyclic.

Photophosphorylation. 38 It was disovered by Arnon et al (1954). The process of phosphate group transfer into ADP to synthesise ATP is called Photophosphorylation. Two types of photophosphorylation occurs in chloroplasts. Cyclic photophosphorylation. • Non cyclic photophosphorylation.

Cyclic photophosphorylation. 39 The electron expelled out by an excited chlorophyll trap is returned to the oxidised chlorophyll after passing through a series of electron carriers. The energy gained by electron 23 kcal per mole during photoact is dissipated in various forms from which part of it is used in pumping protons for creating proton motive force required for production of ATP. This process takes place in illuminated leaves of high plants under low light intensity and CO 2 supply. It is carried out by PS I only where P 700 on absorbing a photon of light extrudes an electron and becomes oxidised The expelled electron is picked up by electron acceptor A 0 a chl a molecule which is reduced to A 0 -.

A 0 - Is a strong reducing Across thylakoid membrane agent that transfers its that causes movement of electrons to phylloquinone protons through channels of 40 (A 1) three types of Fe-S F 0 complex from where it is ATPase that result in passed to ferredoxin and synthesis of ATP from ADP then plastoquinone(PQ) for and Pi. combining with H+ions and transferring to thylakoid lumen. Electron moves from PQ to cyt b 6 and supplies to cyt f which handsover electron back to P 700 through plastocyanin. Pumping of H+ions by PQ into thylakoid lumen creates proton motive force

Non-Cyclic photophosphorylation. 41 The electron expelled by the excited chlorophyll trap is never returned to chlorophyll. The oxidised chlorophyll receives an electron from the anionic decomposition product of water. It is carried out by PS II in conjunction with PS I. The electron extruded by the excited chlorophyll P 680 is picked by pheophytin and then. PQA and PQB where in the region of PQB the electrons help in picking up H+ions from stroma side and pass the same into thylakoid lumen where it creates proton motive force for synthesis of ATP from ADP and Pi in area of ATPase.

42

From PQB the electron passes to cytochrome b 6 cyt f complex and then to the blue coloured copper containing 43 soluble protein called plastocyanin or PC tht further hands over the electron to of PS I and cyclic phosphorylation continues. Here electron moves from Fe-S complex, ferredoxin and then to NADP+ through NADP- reductase taking cation from water. NADP+ gives rise to NADPH + H+.

THANK YOU. 44