CHAPTER 6 - PHOTOSYNTHESIS Photosynthesis – Process by which some organisms capture light energy and store it in organic compounds (mainly carbohydrates, sugars) Autotrophs – make their own food through photosynthesis. They are at the start of every food chain. Exs.) plants, algae, some bacteria/protozoans. Photosynthesis = “Biochemical Pathway” = a complex series of chemical reactions in which the product of 1 reaction is consumed in the next reaction. It uses CO₂ and water to make organic compounds + oxygen. Energy is stored!!! Cellular Respiration – uses organic compounds and oxygen from photosynthesis and releases CO₂ and water. Energy is released!! See figure 6-1 on page 112. Light Reactions – initial reactions in photosynthesis (referred to as P.S. from this point forward). Begins with the absorption of light in chloroplasts. See figure 6-2 on pg. 112. Thylakoids – flattened sacs inside the chloroplast…the light reactions take place here. Granum (Grana – Plural) – stacks of thylakoids. Stroma – solution that surrounds the thylakoids…more later. Visible Spectrum of Light – from white light yields an array of colors….each color represents a different wavelength of light. See figures 6-3 and 6-4 on pg. 113. Pigments – compounds that absorb light. Light that is reflected is the color that the pigment appears to the eye. Chlorophyll a and b – green pigments located in the membrane of the thylakoid. Chlorophyll a - directly involved in the light reactions of P.S. Chlorophyll b – “accessory pigment” – assists chlorophyll a. Carotenoids – other accessory pigments – yellow, orange, red, brown. Red is the best light for P.S. See fig. 6-4, pg. 113. Photosystem – clusters of pigments in the thylakoid membrane. 2 types – photosystem I and photosystem II…..each has a different role in the light reactions. See pg. 114. Electron Transport Electron Transport – (light reactions of P.S.) – begin when accessory pigment molecules absorb light in both photosystems. Energy from the light is quickly passed to other molecules until it reaches a specific pair of chlorophyll a molecules. The rest of the events are divided into 5 steps and are outlined on the next slide. Refer to pages 114 and 115. Electron Transport Continued… 1. Light energy forces electrons (e-’s) to a higher energy level (“excited”) in the 2 special chlorophyll a molecules. 2. Excited electrons leave the chlorophyll a molecules. They are picked up by a molecule in the thylakoid membrane known as the Primary Electron Acceptor. 3. Electrons then enter the Electron Transport Chain = a series of molecules that move electrons through the thylakoid membrane. As the electrons flow, they lose their energy. This energy is used to pump protons (H+) into the thylakoid. Continued on next slide….. At the same time, light is also being absorbed by photosystem I…..e-’s become excited begin to flow through their own etransport chain. NOTE – e-’s lost by photosystem I are replaced by the ones flowing from photosystem II. 5. e-’s from photosystem I flow to outside of thylakoid membrane. e-’s combine with a proton (H+) and NADP+ (Nicotinamide Dinucleotide Phosphate) to yield NADPH. NOTE – Special chlorophyll a’s, from both photosystems, lose electrons, this is called OXIDATION. NADP+ accepts the e-’s, this is called REDUCTION. The whole process is known as a REDOX REACTION. NOTE #2 – as photosystem II loses e-’s, they are resupplied by the splitting of water molecules ( 2 H₂O → 4H+ + 4e- + Oxygen). This is the oxygen that we breathe! See fig. 6-6, pg. 115. Here ends the info about the Electron Transport System! 4. Chemiosmosis – (also part of the Light Reactions) – some protons are produced by the splitting of water; others are pumped from stroma to the inside of thylakoid. This sets up a “concentration gradient” (High concentration of H+ inside thylakoid; Low concentration of H+ outside thylakoid). This represents Potential Energy like that found in a car battery. In Chemiosmosis: ATP Synthetase harnesses the potential energy. H+’s flow through ATP Synthetase from high to low concentration (inside thylakoid to outside). As they flow they produce energy. Energy is used to tack a phosphate onto ADP forming ATP. See fig. 6-7, pg. 116. Summary of Light Reactions As light causes e-’s to flow from photosystems I and II, NADPH and ATP are produced. Both molecules have had energy stored in them. This energy will be used for the 2nd set of reactions (Calvin Cycle) in Photosynthesis. Light Energy → Light Reactions → NADPH + ATP →Calvin Cycle. Calvin Cycle Calvin Cycle – (Melvin Calvin 1911-1997) – occurs in the stroma. Carbon atoms from CO₂ are bonded or “fixed” into organic compounds. This is known as CARBON FIXATION. ** The Calvin Cycle has 3 major steps. Refer to fig. 6-8, pg. 117 as you study the steps. 3 Steps of the Calvin Cycle An enzyme combines CO₂ with a 5-carbon molecule called RuBP (Ribulose Bisphosphate) making a 6-carbon molecule that splits into two 3-carbon PGA molecules (phosphoglycerate). 2. Each PGA receives a phosphate group from ATP. Then NADPH donates a proton and releases a phosphate group. The resulting molecules are known as PGAL (phosphoglyceraldehyde). 3. It takes 3 turns of the Calvin Cycle. This yields 6 PGAL (5 are used to regenerate RuBP to keep the cycle going. The one PGAL that remains is built up into organic molecules that FEED THE WORLD!!! NOTE – 3 turns of Calvin Cycle uses 9 ATP and 6 NADPH from the light reactions and yields organic molecules (carbohydrates) THAT FEED THE WORLD!!! 1. Simple Equation for Photosynthesis CO₂ + H₂O + Light Energy → Glucose + Oxygen Factors that Effect the Rate of Photosynthesis 1. Light Intensity – P.S. goes faster (to a point) with more intense light. 2. CO₂ Level – P.S. goes faster (to a point) with more CO₂. 3. Temperature – P.S. goes faster (to a point) with higher temperatures.