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PHOTOSYNTHESIS Virtually all energy on earth comes from sunlight. Plants use energy from the sun to make the bonds which hold organic molecules together.

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Presentation on theme: "PHOTOSYNTHESIS Virtually all energy on earth comes from sunlight. Plants use energy from the sun to make the bonds which hold organic molecules together."— Presentation transcript:

1 PHOTOSYNTHESIS Virtually all energy on earth comes from sunlight. Plants use energy from the sun to make the bonds which hold organic molecules together. When these bonds are broken the energy is ultimately transferred to ATP, which is then moved about cells and organisms to power their needs. Photosynthesis overview Photosynthesis overview 12H20 + 6CO2 ----- light -----> 6O2+ C6H12O6 + 6H20 Photosynthesis - The Basic Reaction CO2 +H2O +---Plants (Chloroplasts) Light Energy---Simple Sugars+ O2

2 ROLE OF SUNLIGHT Sunlight provides the energy Sunlight provides the energy Organisms depend upon photosynthesis Organisms depend upon photosynthesis 1. Photosynthesis uses sunlight as a source of energy to produce carbohydrates 2. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate source of food for most life 3. Most food chains start with photo synthesizers 4. The early atmosphere of the earth lacked oxygen and it took about 3 billion years of photosynthesis to produce the current 21% oxygen atmosphere we now enjoy. This oxygen: a. Made aerobic respiration possible b. Formed the ozone layer (O3), which protects us from harmful solar radiation

3 SOLAR RADIATION Solar Radiation 1. Solar radiation is described in terms of its energy content and its wavelength 2. Photons are discrete packets of radiant energy that travel in waves 3. The electromagnetic spectrum of solar radiation is based on wavelength a. Energy content of photons is inversely proportional to wavelength

4 Solar radiation contd. 4. Only 42% of solar radiation that hits earth's atmosphere reaches surface; most is visible light a. Higher energy wavelengths are screened out by ozone layer in upper atmosphere b. Lower energy wavelengths are screened out by water vapor and CO2 c. Consequently, both the organic molecules within organisms and processes, such as vision and photosynthesis, are adapted to radiation that is most prevalent in the environment

5 PHOTOSYNTHETIC PIGMENTS Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum 1. Pigment is a substance that absorbs visible light that behave as packets of energy called photons. 2. Two major photosynthetic pigments are chlorophyll a and chlorophyll b 3. Chlorophylls absorb violet, blue, and red wavelengths; they reflect green, this is why leaves appear green. Different colors/wavelengths of light (ROYGBIV). 3. Chlorophylls absorb violet, blue, and red wavelengths; they reflect green, this is why leaves appear green. Different colors/wavelengths of light (ROYGBIV).

6 CAROTENOIDS 1. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions 2. When pigments absorb light, electrons are boosted to a higher energy level and the energy is captured in a chemical bond

7 Photosynthesis occurs in chloroplasts Chloroplasts have two parts: Chloroplasts have two parts: 1. A double membrane encloses a fluid-filled space called the stroma or ground substance 1. A double membrane encloses a fluid-filled space called the stroma or ground substance 2. Thylakoids = flattened sacs organized into stacks called grana 3. Chlorophylls and other pigments involved in absorption of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy

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9 Chloroplast Structure 1. Outer membrane 2. Inner membrane systems 3. Thylakoid membranes 4. Thylakoid space (within the thylakoids) 5. Granum(a) (stack(s) of thylakoidsmembranes) 6. Stroma (the liquid area outside the thylakoid membranes)

10 REACTIONS IN PHOTOSYNTHESIS Photosynthesis has two sets of reactions 1. Light-dependent reactions = the energy from the sun is captured in energy carrying molecules 2. Light-independent reactions = energy carrying molecules from the light-dependent reactions are used to make carbohydrates

11 The Photochemical (Light) Reactions Capture of light energy Capture of light energy Thylakoid membranes Thylakoid membranes Photosystems II and I Photosystems II and I Chlorophyll and accessory pigments Chlorophyll and accessory pigments

12 Electron trasport chain - A specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates redoxelectronredoxelectron In Photosystem II, the electron which reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms like plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II, thus the P700+ of Photosystem I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photosystem In Photosystem II, the electron which reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms like plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II, thus the P700+ of Photosystem I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photosystem

13 LIGHT ABSORBANCE AND ENERGY TRANSFORMATIONS 1. Flow of Electrons 2. Splitting of water molecules 3. Release of oxygen molecules 4. Accumulation of H+ in thylakoid Spaces 5. Reduction of NADP to NADPH 6. Production of ATP 7. ATP synthase 8. Powered by flow of H+ 9. ADP + Phosphate --- ATP

14 Light-dependent reactions – light energy is converted to chemical energy (ATP&NADPH) A. Occur in the thylakoid membranes and require participation of one or two light-gathering units called photosystems B. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated C. Each photosystem has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid pigments) D. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a E. Electrons in reaction-center become excited; then move along an electron transport system, along the way ADP is converted to ATP and NADP is converted to NADPH F. The electrons are replaced by the splitting of water molecules G. The energy in the ATP and NADPH are used to power the light- independent reactions

15 Light-dependent reactions – light energy is converted to chemical energy (ATP&NADPH) A. Occur in the thylakoid membranes and require participation of one or two light-gathering units called photosystems B. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated C. Each photosystem has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid pigments) D. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a

16 LIGHT ENERGY CONVERTED TO CHEMICAL ENERGY E. Electrons in reaction-center become excited; then move along an electron transport system, along the way ADP is converted to ATP and NADP is converted to NADPH F. The electrons are replaced by the splitting of water molecules G. The energy in the ATP and NADPH are used to power the light-independent reactions

17 PIGMENTS IN LIGHT ABSORPTION Before a pigment absorbs light all its electrons in their ground state (lowest energy level) Before a pigment absorbs light all its electrons in their ground state (lowest energy level) The absorption of one photon of light excites one electron in a pigment molecule. The e- is in an excited state The absorption of one photon of light excites one electron in a pigment molecule. The e- is in an excited state For a photon to be absorbed, the energy of the photon must match the difference in energy between the ground state of some e- and a possible excited state. For a photon to be absorbed, the energy of the photon must match the difference in energy between the ground state of some e- and a possible excited state.

18 What happens to the energy of an excited state? It gives off heat It gives off heat Gives of light Gives of light Excites an e- in another pigment Excites an e- in another pigment Drives a photochemical reaction Drives a photochemical reaction The excited e- reduces an acceptor. The excited e- reduces an acceptor.

19 The Biochemical Reactions: The Calvin Cycle 1. "Fixing" CO2 2. Cyclic series of enzyme reactions 3. Rubisco (the enzyme that fixes CO2) 4. Stomates and CO2 availability 5. Addition of CO2 to a 5 carbon compound 6. Production of Carbohydrate 7. Energy input from ATP (ATP --- ADP + Phosphate) 8. Addition of H+ and energy 9. Production of carbohydrates for storage, transportation, and biosynthesis 10. Recycling of 5 carbon compound to fix more CO2

20 THE DARK REACTION (Light- independent reactions) Light-independent reactions (also called carbon fixation or Calvin - Benson cycle) A. Takes place in the stroma of the chloroplast B. Carbon dioxide enters through small pores in the leaves called stomates and enzymes extract and bond the carbon to a 5-carbon intermediate RuBP(ribulose-1,5-bisphosphate)

21 DARK REACTION C. Thorough a series of reactions energy from the light reactions (ATP, NADPH) is used to form several molecules of PGAL. PGAL is used to: 1. Regenerate RuBP so the light-independent reactions may continue; note this requires an input of ATP created during light reactions 2. PGAL that accumulates is also used to produce glucose, a 6-carbon sugar, and as a backbone for other organic molecules

22 PGAL or G3P or triosephosphate Glyceraldehyde 3-phosphate (G3P) or triose phosphate is a 3 carbon metabolic intermediate. Glyceraldehyde 3-phosphate (G3P) or triose phosphate is a 3 carbon metabolic intermediate. G3P is often referred to as 3- phosphoglyceraldehyde (PGAL) with respect to the product of photosynthetic carbon fixation during the Calvin cycle. G3P is often referred to as 3- phosphoglyceraldehyde (PGAL) with respect to the product of photosynthetic carbon fixation during the Calvin cycle.carbon fixationCalvin cyclecarbon fixationCalvin cycle glyceralaldehyde 3-P glyceralaldehyde 3-P

23 PGAL- end product of photosynthesis During plant photosynthesis, two molecules of glycerate 3-phosphate (GP, but also known as 3-phosphoglycerate (PGA)) are produced by the first step of the light- independent reactions when ribulose 1,5- bisphosphate (RuBP) and carbon dioxide are catalysed by the rubisco enzyme During plant photosynthesis, two molecules of glycerate 3-phosphate (GP, but also known as 3-phosphoglycerate (PGA)) are produced by the first step of the light- independent reactions when ribulose 1,5- bisphosphate (RuBP) and carbon dioxide are catalysed by the rubisco enzymephotosynthesisglycerate 3-phosphate3-phosphoglyceratelight- independent reactionsribulose 1,5- bisphosphatecarbon dioxiderubiscophotosynthesisglycerate 3-phosphate3-phosphoglyceratelight- independent reactionsribulose 1,5- bisphosphatecarbon dioxiderubisco The GP is converted to PGAL using the energy in ATP and the reducing power of NADPH as part of the Calvin cycle The GP is converted to PGAL using the energy in ATP and the reducing power of NADPH as part of the Calvin cycleATP NADPHCalvin cycleATP NADPHCalvin cycle

24 G3P This returns ADP, Pi, and NADP+ to the light-dependent reactions of photosynthesis for their continued functioning This returns ADP, Pi, and NADP+ to the light-dependent reactions of photosynthesis for their continued functioningADPNADP light-dependent reactionsADPNADP light-dependent reactions PGAL can then be converted to glucose. RuBP is regenerated for the Calvin cycle to continue PGAL can then be converted to glucose. RuBP is regenerated for the Calvin cycle to continueglucose PGAL is generally considered the prime end- product of photosynthesis. PGAL is generally considered the prime end-product of photosynthesis PGAL is generally considered the prime end- product of photosynthesis. PGAL is generally considered the prime end-product of photosynthesis

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