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Light Dependant Reactions ` Photosynthesis converts energy of the sun into chemical energy of glucose ` Small portion of Sun’s energy reaches earth and even smaller portion gets converted into energy ` Still, photosynthesizing organisms synthesize about 1.4 X 1015 kg of energy-storing glucose and other sugars in one year ` 1,400,000,000,000,000 ` Most of the glucose is converted to cellulose and other structural tissues. ` Glucose also converted to other sugars as well as storage forms of carbohydrates (starch) ` Also, sugars produced by photosynthesis are involved in the synthesis of other cellular substances ` Eg. Amino acids – proteins ` Products of photosynthesis account for 95% of dry weight of plants ` → 6CO2 (g) + 6H2O(l) + energy C6H12O6 (s) + O2(g) ` Arrow is misleading, there are over 100 reactions that occur that lead to the end products ` Photo means capturing light energy ` Synthesis means to produce (or in this case, production of carbohydrates) ` Photosynthesis is made up of two types of reactions ` Light-dependant reactions – solar energy is trapped to generate two high energy compounds: ATP and NADPH (reduce nicotinamide adenine dinucleotide phosphate), has large amounts of reducing power ` Light independent reactions – the energy of ATP and the reducing power of NADPH are used to reduce carbon dioxide to make glucose which can than be converted into starch for storage ` Pigments in thylakoid membranes absorb light energy ` Chlorophyll solution reflects yellow and green light ` Previous figure shows chlorophyll has two chlorophylls a and b ` Also contains another pigment called beta- carotene ` Beta-carotene is part of large class of pigments called carotenoids ` They absorb blue and green light so they are red, yellow and orange in colour. ` Chlorophyll is bound to the membranes of the thylakoids inside the chloroplast ` Chlorophyll and other pigments are arrange in the thylakoid membranes in clusters called photosystems ` Two photosystems ◦ Photosystem I (PSI) (700 nm) ◦ Photosystem II (PSII) (680 nm) These were named in the order that they were discovered, NOT in the sequence they occur ` Made up of pigment molecules that include a dozen or more chlorophyll molecules as well as a few carotenoid molecules ` Also present is a molecule that accepts electrons ` All pigments molecules can absorb energy of various wavelengths ` However, they always pass the energy to one specialized, electron accepting chlorophyll a molecule called the reaction centre. (antenna analogy) ` Each pigment absorbs light of different colours ` Having a variety of pigments enables a plant to use a greater percentage of the Sun’s light ` Chlorophyll is bound to membranes of thylakoids in the chloroplasts ` Chlorophyll is arranged in clusters called photosystems ` Plant and algae chloroplasts have two photosystems ` Photosystem I (PSI) ` Photosystem II (PSII) ` Named after the order scientists discovered them ` When reaction centre receives the energy, the electron in the reaction centre becomes “excited” ` This means the electron is raised to a higher energy level ` The electron is then passed on to an electron-accepting molecule ` A series of progressively stronger electron acceptors; each time an electron is transferred, energy is released. ` ETS important in energy production in cells. ` The e- leave the reaction center from PS II and goes to the electron-acceptor ` This leaves the reaction center short an e- ` This electron must be replaced before PSII can accept light energy to excite ANOTHER e- ` The new e- comes from the splitting of a H2O molecule. O2 that is released by plants comes from this type of reaction ` Next from the electron-acceptor the energized e- is transferred along a series of electron carrying molecules. ` Together these are called the Electron transport system (ETS) ` With each transfer, the e- releases a small amount of energy ` This energy released helps to push H+ ions from the stroma across the thylakoid membrane and into the thylakoid space (area inside the thylakoid) ` When hydrogen ions are forced from the stroma to the thylakoid, they can’t diffuse back across the membrane ` Membrane is impermeable to these ions ` Embedded in membrane is a special structure called ATP synthase ` This is the only pathway for the hydrogen ions to move down their concentration gradient ` This pathway is linked to a mechanism that bonds a free phosphate group to ADP to form ATP. ` As hydrogen moves down the concentration gradient through the ATP synthase, the energy of the gradient is used to generate ATP molecules. ` The linking of the movement of hydrogen ions to the production of ATP is called chemiosmosis ` We will see this again in the mitochondria during C/R ` Plants use chlorophyll to trap solar energy and convert it to chemical energy. ` We’ve figured out how to do this as well, in a large scale, our space stations have large solar panels ` On small scale, solar calculators ` On Earth though, solar cells cannot provide enough energy that society needs ` We use fossil fuels for energy but produce to much CO2 ` If we could use hydrogen cells it would be great for a fuel source as its byproduct is water ` Unfortunately there is Not enough H2(g) in environment ` It takes more energy to split water than hydrogen combustion releases ` Scientists are trying to mimic PSII ` When there is enough NADPH and ATP in the stroma, the energy from these molecules can be used to synthesize glucose ` Series of reactions that synthesize glucose is called the Calvin-Benson cycle ` Named after Melvin Calvin and Andrew Benson ` Used a radioactive carbon tracer to discover the reaction ` 3 steps in this reaction Consumption: Water Formation: ATP, NADPH+, and oxygen ` Step 1 – Fixing Carbon Dioxide: carbon atom in carbon dioxide is chemically bonded to a pre-existing molecule in the stroma ` The molecule is a five-carbon compound called ribulose bisphosphate, aka: RuBP ` The resulting six carbon compound is unstable and immediately breaks down into two identical three carbon compounds ` These three carbon compounds are the first stable products of the products ` These three carbon compounds are the first stable products of the process ` Plants that demonstrate this process are called C3 plants ` Fixing Carbon Dioxide ` → → CO2 + RuBP unstable C6 2 C3 ` Step 2 – Reduction: The newly three carbon compounds are in a low energy stage ` To convert them into a higher energy state they are first activated by ATP and then reduced by NADPH ` The result of this reaction is two molecules of PGAL (short for glyceraldehyde -3 –phosphate) ` In their reduced state some of the PGAL molecules leave the cycle and may be used to make glucose ` The remaining PGAL molecules move on to the third stage ` Step 3 – Replacing RuBP: most of the reduced PGAL molecules are used to make more RuBP molecules ` Energy is required to break and reform the chemical bonds to make the five carbon RuBP from PGAL ` This cycle must be completed 6 times to synthesize one molecule of glucose. ` Of the 12 PGAL molecules made, 10 are used to regenerate RuBP, 2 are used to make glucose ` The process of moving from PSII to PSI, is not considered to be “cyclic” because the electrons that are first excited in PSII are not recycled through the system. ` Water replenishes the supply at PSII ` NADPH carries e- from PSI to the Calvin- Benson cycle. ` In the Calvin cycle there is a larger demand for ATP than for NADPH. ` Thus, when NADPH concentrations are sufficient, the cell will switch to cyclic photophosphorylation. This means the PSII will not be used in order to avoiding the splitting of water and addition NADPH production. ` Two types of reactions: ◦ Light-dependant and light-independent ` Two photosystems (PSI and PSII) ` PSII releases electrons when it is excited by photons of light – it transfers the e- to and then it is passed in the ETS ` Energy released in the ETS is used to force H ions across the thylakoid membrane ` Energy from this gradient is used to help generate ATP from ADP and phosphate by means of chemiosmosis ` As H ions move through the gradient, they drive the reaction that generates ATP to be used in the ‘Calvin-Benson cycle ` An e- from water that is split replaces the e- released in the PSII the oxygen molecule is converted to O2 and released ` When an e- from PSI is excited it is eventually used to reduce NADP+ to NADPH ` Calvin Benson cycle occurs in the stroma and synthesizes carbohydrates ` CO2 combines with RuBP to form a 6 C compound the splits into 2 Three-carbon compounds ` ATP and NADPH from the light dependent reactions provide energy and reducing power to for PGAL from the newly formed 3 carbon compounds ` 6 cycles produces 12 PGAL molecules 10 which regenerate PGAL and 2 which form glucose.
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