PRIMARY EVENTS IN Photosynthesis

WHAT IS PHOTOSYNTHESIS?, IMPORTANCE OF PHOTOSYNTHESIS, STRUCTURAL FEATURE OF LEAF ADVANTAGE FOR PHOTOSYNTHESIS,LEAVES AND LEAF STRUCTURE,CHLOROPHYLL, TYPES OF REACTIONS, LIGHT REACTION AND DARK REACTION, CYCLIC AND NON-CYCLIC PHOTOPHOSPORYLATION, MECAHANISM OF ATP SYNTHESIS, SCHEMATIC PRESENTATION OF LIGHT REACTION, CRASSULACEAN ACID METABOLISM (CAM), C3 AND C4 PLANTS, FACTORS AFFECTING RATE OF PHOTOSYNTHESIS, INTERNAL FACTORS AND EXTERNAL FACTORS,

1. Prepared By: Vipin Kr Shukla Assistant Lecturer Photosynthesis

2. What is Photosynthesis? Photosynthesis is the process by which plants, some bacteria, use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll. Most of the time, the photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Oh yes, we need the food as well! We can write the overall reaction of this process as: Sun light 6CO2 + 12H2O -------------> C6H12O6+ 6O2+6H2O

3. six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen • Diagram of a typical plant, showing the inputs and outputs of the photosynthetic process.

4. Importance of Photosynthesis: Photosynthesis is the only process which produces enormous quantities of organic matter for sustaining the life on this globe. Photosynthetic products not only build up the bodies of organisms but also provide energy for carrying out metabolic activities and different types of movements. The chemical energy present in the organic food is the converted form of radiant or solar energy. Coal, petroleum and natural gas represent the photosynthetic capital of the past geological ages. Several materials derived from the organic world (and hence photosynthesis) are in our daily use. Examples: Natural fibers, drugs, vitamins, gum, tannins, turpentine, furniture, etc. By photosynthesis, green plants keep the concentration of the two gases almost constant by absorbing carbon dioxide and evolving oxygen during photosynthesis.

5. Structural feature of leaf advantage for photosynthesis: Upper epidermis and cuticle are transparent to light, thus allows most light to pass to photosynthetic mesophyll tissues. Palisade mesophyll cells are closely packed and contain many chloroplasts to carry out photosynthesis more efficiently. Extensive vein system allow sufficient water to reach the cells in the leaf and to carry food away from them to other parts of the plant . Spongy mesophyll cells are loosely packed with numerous large air spaces to allow rapid diffusion of gases throughout the leaf. Numerous stomata on lower epidermis allow rapid gaseous exchange with the atmosphere .

6. Contd…. Upper epidermis and cuticle are transparent to light, thus allows most light to pass to photosynthetic mesophyll tissues. Palisade mesophyll cells are closely packed and contain many chloroplasts to Carry out photosynthesis more efficiently. Extensive vein system allow sufficient water to reach the cells in the leaf and to carry food away from them to other parts of the plant . Spongy mesophyll cells are loosely packed with numerous large air spaces to allow rapid diffusion of gases throughout the leaf. Numerous stomata on lower epidermis allow rapid gaseous exchange with the atmosphere .

7. Leaves and Leaf Structure: Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen.

8. Leaf anatomy of C3 and C4 plants:

9. Contd…. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem. Stomata is the structure to allow gas to enter the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening of stoma. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost.

10. Contd…. In the electromagnetic spectrum the wavelengths between 400-700nm is called as PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR). Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometers to about 740 nm. while its speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second. Light, which is emitted and absorbed in tiny "packets“ called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality.

11. Contd…. The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet.

12. Contd…. Chlorophyll and Accessory Pigments. A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light reflected ( those not absorbed). Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment.

13. Chlorophyll: Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms have chlorophyll a Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae), xanthophylls, and carotenoids. Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange- Red wavelengths, and little from the intermediate (Green- Yellow-Orange) wavelengths.

14. The molecular structure of chlorophylls : Chlorophyll b (C55H70O6N4Mg) differs from chlorophyll a (C55H72O5N4Mg) by the substitution of a CHO group for the CH3 marked at position R.

15. Contd…. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria.

16. Structure of chloroplast: The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoid are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as Stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments.

17. Functions of Chloroplast: These cellular organs are the main sites of photosynthesis in which both light and dark reactions are found. Light reaction takes place in the grana region whereas dark reaction takes place in the Stroma region. Chloroplast is capable for some amount of protein synthesis which is used in the structural organizations and some photosynthetic enzymes also synthesize in chloroplast. Chloroplast has its own genetic system (DNA) and participates in Cytoplasmic inheritance.

18. Stages of Photosynthesis: Photosynthesis is a two stage process. The first process is the Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules. The Light Independent Process (or Dark Reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

19. Light Reactions: In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. Photosynthetic unit: A photosynthetic unit is the smallest group of pigment molecules which combined together to cause a photochemical act. i.e. absorption and migration of a light quantum to a trapping center where it brings about the release of an electron .

20. Contd…. Photons:- Light behaves as it is consists of tiny particles called photons. Quantum:- A photon has a certain energy value which is called quantum. About 2500 chlorophyll molecules ( under photosynthetic unit) combined together to evolve one molecule of O2 and that for 10 quanta of light are used. Red drop:- Emerson observed that photosynthesis decreased when a plant was shifted from short wave length (650 nm) to wave length longer than 680 nm. This decline in the rate of photosynthesis under long wave length of red is described as red drop. Emerson effect:- Emerson also observed that when both short and long wave lengths of red light acted on at the same time, the rate of photosynthesis was higher than the sum of photosynthetic rates obtained by using short and long wave lengths respectively. This effect described as Emerson effect.

21. PHOTOSYSTEM I & II: The reaction centre chlorophyll of photo system I absorbs maximally at 700 nm in its reduced state. Accordingly, it is named P 700 (the P stands for Pigments), photo system I, its associated antenna pigments, ATP synthase enzymes are found exclusively in the stroma lamella and at the edges of the grana lamellae. The reaction centre chlorophyll of photo system II absorbs maximum at 680 nm (P 680 ) so its reaction centre chlorophylls and associated electron transport proteins is located predominantly in the stacked regions of the grana lamellae. At a wavelength grater than 680 nm PSII cannot operate. Therefore, quantum yield decreased beyond 680 nm. This is what red drop is observed. Photo system I contains large amount of chlorophyll a, a small amount of chlorophyll b and some B carotene. Photo system II also contains chlorophyll a, B carotene but a large amount of chlorophyll b.

22. The light reaction: Light reactions or the „Photochemical phase include light ‟ absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680. Water is oxidized to oxygen by Photo system II: The chemical reaction by which water is oxidized is given by the following equation. 2H2O O2 + 4H+ + 4e- This equation indicates that four electrons are removed from two water molecules, generating an oxygen molecule and four hydrogen ions. Robert Hill demonstrated that isolated chloroplasts evolved Oxygen when they were illuminated in the presence of suitable electron acceptor, such as ferricyanide. The ferricyanide is reduced to ferrocyanide by photolysis of water .This reaction is now called as HILL REACTION and it explains that water is used as a source of electrons for CO2 fixation and Oxygen is evolved as a by-product.

23. Cyclic photophosphorylation & Non Cyclic Photophosporylation: It functions in a closed circle. Electron free from chlorophyll to acceptor for return to chlorophyll. NADPH2 is not formed and so assimilation of CO2 is retarded. O2 is not evolved. Bacteria have only this. It is insensitive to dichloro phenyl dimethyl urea (DCMU) Independent electron donor is essential . H2O is ultimate source of Electrons and NADP is final electron acceptor. NADPH2 is formed and it is used in CO2 assimilation. O2 is evolved. In green plants this system. DCMU stop it.

24. MECHANISM OF ATP SYNTHESIS: The phenomenon of synthesis of ATP in light reactions of photosynthesis is known as photophosphorylation. The mechanism of ATP synthesis is explained by the chemi-osmotic mechanism first proposed in 1960 by Peter Mitchell. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. The basic principle of chemi-osmosis is that in concentration differences and electrical potential differences across the membranes are a source of free energy that can be utilized by the cell for the synthesis of ATP. In the light reactions electron flow is coupled to proton translocation, creating transmembrane proton motive force. The energy in the proton motive force is then used for synthesis of ATP by the enzyme called ATP synthase.

25. Schematic presentation of light reaction :

26. Dark reaction: Dark reaction:-Carbon-Fixing Reactions are also known as the Dark Reactions (or Light Independent Reactions). Stomata to allow gas to enter and leave the leaf. The Calvin Cycle occurs in the stroma of chloroplasts. Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP-5C ). Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose.

27. Crassulacean: Xerophytes, such as cacti and most succulents, also use PEP carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). In contrast to C4 metabolism , which physically separates the CO2 fixation to PEP from the Calvin cycle, CAM only temporally separates these two processes. CAM plants have a different leaf anatomy from C4 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by rubisco.

28. CRASSULACEAN ACID METABOLISM (CAM ): CAM is an acronym for Crassulacean acid metabolism. The CAM mechanism is similar in many respects to the C4 cycle, but differs from it in two important features . 1) In C4 plants the formation of C4 acids is spatially, but not temporally, separated from the Decarboxylation of the C4 acids and re fixation of the resulting CO2 by the Calvin cycle . CAM plants lack the specialized leaf anatomy kranz leaf anatomy typical of C4 plants. 2) CAM plants open their stomata during the cool, desert nights and closed during the hot , dry days. This minimizes water loss. The CO2 is assimilated at night.

29. Light reaction & DARK REACTION: LIGHT REACTION DARK REACTION Also called as Hillman reaction. Occurs in presence of light. It takes place in grana. Photolysis of water takes place and energy rich compound generated ATP & NADPH2. Photo-chemical reaction. Also called as Blackman reaction. Occurs in absence of light. It takes place in stroma. CO2 is reduced to form hexose sugar with utilization of energy produced during light reaction. Bio-chemical reaction.

30. C3 Plants & C4 PLANTS: C3 PLANTS C4 PLANTS Plant which have Calvin cycle. Only one CO2 acceptor- RuBP. First stable product- 3 carbon compound , phosphoglyceric acid. Kranz type anatomy is absent. The optimum temp. range between 10-25 Oc. C3 plants are less efficient. Rate of photosynthesis- 15-35 mg/dm2 of leaf area per hour Plant which have Hatch-Slack cycle. Two CO2 acceptors, in mesophyll- PEP and in bundle sheath- RuBP. First stable product- 4 carbon compound , oxaloacetic acid. Kranz type anatomy is present. The optimum temp. range between 30-45 oC. C4 plants are more efficient. Rate of photosynthesis: 40-80 mg/dm2 of leaf area per hour

31. Factors affecting rate of photosynthesis: External factors:- (1) Light: (a) Reflection (b) Absorption (c) Intensity (d) Quality (e ) Duration (f) Destructive effect of light. Light is green, most of light reflected in green colour and absorbed in blue and red region. (2) CO2: 0.036 % in nature by volume. High conc. Of CO2 reduced rate of photosynthesis. (3) Temperature: Light reaction is independent of temp. Cold temp retard the rate by affecting enzyme activities. Ice formation affect the rate and injurious. 20-30 oC is optimum, higher than 45 oC adversely affect by denaturing the enzymes. (4) O2: It favors respiration rate, compete for certain common intermediate in the process of respiration and photosynthesis. (5) Water: In water stress photosynthesis rate reduced due to the close stomata .

32. Internal factors :: Chlorophyll content : Amount of chlorophyll has little influence on rate of photosynthesis. Hydration of protoplasm: It is absolutely essential for almost in all metabolic processes. Decrease hydration of the protoplasm reduced the rate of photosynthesis. Protoplasmic factors : It includes factors most probably enzymatic in nature and is called the “time factor”. Accumulation of carbohydrates: Accumulation of photosynthate in plant cells, if not translocated, slow down and finally stop the process.