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Photosynthesis Senior Biology • • • • Photosynthesis overview Equation Chloroplast & Pigments Photosynthesis & Cellular Respiration connection • The Light Reaction • The Light Independent Reaction/Calvin Cycle • Essential RuBisCO The Photosynthesis Song Photosynthesis • Process whereby plants convert CO2, water and the energy from the sun into sugar • Occurs in the leaves of plants in organelles called chloroplasts. carbon dioxide water sugar oxygen light energy Photosynthesis: An Overview • Why? – To make glucose • How? – Reduces CO2 into simple sugars like glucose • Who? – Any organism which contains chlorophyll (plants, cyanobacteria, algae, etc.) • Where? – Prokaryotes (e.g. bacteria) – infolding of cell membrane – Eukaryotes (e.g. plants) – chloroplasts Photosynthetic Equation • 6 CO2 + 6 H2O + light energy--> C6H12O6 + 6O2 • But not that simple • Involves 2 stages: 1) Making ATP and NADPH with energy from light (Light Reaction) 2) Making glucose from carbon dioxide using energy from ATP and NADPH (Light Independent Reaction) Photosynthesis • Occurs in the Chloroplast • Light interacts with the pigment chlorophyll which is located on the thylakoid membranes Chloroplast Structure • Inner membrane called the thylakoid membrane. • Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana) • Stroma is a liquid surrounding the thylakoid. Pigments • Chlorophyll is the most important photosynthetic pigment present in plants. • Other pigments are also present in the leaf. – Carotenoids (orange / red) – Xanthophylls (yellow / brown) Why are plants green? Light Review • Visible light comes in a spectrum from purple to red • Different substances absorb different colours depending on the structure. • Colours that the substance doesn’t absorb, it reflects. We see that reflection as the objects colour Chlorophyll a & b • Is a pigment that absorbs blue and red light but reflects green. The light is reflected into our eyes giving plants a green colour High absorption Low absorption/ High reflection How do plants capture light? • Chlorophyll a and b molecules absorb photons (light packets) with blueviolet and red regions of the spectrum. They reflect the green regions Carotenoids (other accessory pigments) • Other accessory pigments (called carotenoids) also absorb light energy. They reflect photons in the yellow-to-red range of the spectrum. • Gives them a yellow-red colour Why do some leaves turn colours in the fall? There are three main pigments in plants; Chlorophyll (reflect green), Carotenoids (reflect orange/red), and Xanthophylls (reflect yellow/brown). As the most abundant pigment, chlorophyll is what gives leaves their green hue in spring and summer. Plant Pigment Absorption Spectrums Why do some leaves turn colours in the fall? • During winter, there is not enough light or water for photosynthesis. Therefore, the trees cut off the circulation of water, nutrients and sugar to the leaves and live off the food they stored during the summer. • They begin to shut down their food-making factories. The green chlorophyll pigment disappears from the leaves leaving the other pigments behind. As the bright green fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along we just can't see them in the summer, because they are covered up by the green chlorophyll. Cellular Respiration Recall • The process by which sugar is converted into carbon dioxide, water and energy • The organism uses released energy for all cellular activities sugar oxygen carbon dioxide water energy Photosynthesis and Cellular Respiration Photosynthesis carbon dioxide water sugar oxygen light energy Notice Anything? sugar oxygen carbon dioxide water energy Cellular Respiration Photosynthesis and Cellular Respiration Photosynthesis carbon dioxide water sugar oxygen light energy sugar oxygen carbon dioxide water energy Cellular Respiration What is created in one reaction is used up in the other reaction! Photosynthesis and Cellular Respiration Photosynthesis carbon dioxide water sugar oxygen light energy sugar oxygen carbon dioxide water energy Cellular Respiration • What is created in one reaction is used up in the other reaction! • If one reaction were to stop, they both would come to a halt. Plants • Plants need to go through both processes. Why? • Photosynthesis only makes glucose, it still needs to be broken down for energy • Since plants don’t eat, they break down some of the glucose they made from photosynthesis and create ATP through cellular respiration! The Photosynthesis Song Photosynthesis and Cellular Respiration are Connected Photosynthesis vs. Respiration video Photosynthesis Respiration Song Metabolism – Photosynthesis Day 2 Photosynthesis The Light Reaction and The Calvin Cycle Photosynthesis Overview Video The Photosynthetic Equation • • • 6 CO2 + 6 H2O + light energy--> C6H12O6 + 6O2 But not that simple Involves 3 stages: 1) Capturing light energy 2) Making ATP and NADPH with energy from light 3) Making glucose from carbon dioxide using energy from ATP and NADPH Light Reaction 3 Stages 1. Photoexcitation -> absorption of a photon 2. ETC 3. Chemiosmosis Photosynthesis • Involves a series of reactions that are directly energized by light. • Require chlorophyll and occurs on the thylakoid membrane in chloroplasts. • Light energy is absorbed and eventually transferred to carbohydrates in the last stage of the process – Light energy is in the form of a photon (packet of solar energy) 1) Photoexcitation • Light is absorbed by chlorophyll or accessory pigments that are associated with proteins in clusters. – These clusters = photosystems • Two Photosystems 1) Photosystem I - chlorophyll a- called P700 2) Photosystem II- chlorophyll a- called P680 2) ETC - Photosystems • Plants use Photosystem I and II to make NADPH and ATP Photosystem II • Is composed of proteins and P680 chlorophyll a molecules (680 because it absorbs 680nm λ) • Light strikes P680 and excites it (a) P680* (it’s excited) • The energized chlorophyll (P680*) then transfers a highenergy electron to acceptor A in the reaction centre (b). Photosystem II • The P680 Chlorophyll a molecule is now oxidized (missing an electron)- how does it get it back? • Water is split into 2 protons,1/2 O2 and 2 electrons - these electrons replace the missing one. + + + + - - - • After P680 oxidizes water (steals its electrons), it becomes neutral again. • The high-energy electron is transferred from the reaction centre (A-) to the carrier molecule plastoquinone (PQ) (c). Photosystem II • This process releases both oxygen gas and protons into the lumen Linear (non-cyclic) electron transport • Light hits photosystem II electron excited eventually transferred from Plastoquinone (PQ) Cytochrome Complex Photosystem I • Electrons pass through components of ETC similar to cellular respiration. As the electrons are transferred, protons are pumped from Stroma into thylakoid lumen creating a concentration gradient. Our Electron • By the time our poor electron reaches Photosystem I, it’s tired and needs a boost • It gets that from another photon strike at Photosystem I which re-excites it. Photosystem I • The pigment P700 within Photosystem I is also hit by light which re-excites our electron creating P700*. • Photosystem I’s excited electron eventually is passed to enzyme NADP+ reductase which uses H+ in the stroma to reduce NADP+ to NADPH – NADPH is used in the second phase of photosynthesis 3) Chemiosmosis • Protons that accumulate in the thylakoid lumen form an electrochemical gradient that drives ADP and Pi into ATP (like the ETC) – Recall that 3 H+ through an ATP synthase = 1 ATP – This is called Photophosphorylation – we have created ATP using energy gathered from sunlight. Energy Changes within the Electron The blue line represents the electron Required Numbers • 2 e- required to reduce 1 NADPH • For every 4 e- transferred in the light reaction, 12 H+ are added to the thylakoid lumen 4 ATP • 3H+ per 1 ATP – 12/3 = 4 Why is it called non-cyclic? • Has to do with the electrons…. • It’s a one-way street, electrons leave and never return home. The light reaction Photosynthesis (Light Reactions) The Light Independent Reaction – The Calvin Cycle • Does not require light, only the products from the Light Dependent (LD) reaction • In the stroma of the chloroplast, a series of 11 reactions uses NADPH (from the LD reaction) to reduce CO2 into sugar. • Reactions are cyclic and similar to the Krebs Cycle 3 stages in the Calvin Cycle 1. Carbon fixation 2. Reduction 3. Regeneration Phase 1: Carbon Fixation • CO2 is added to a 5 carbon molecule of ribulose 1, 5bisphosphate (RuBP) which splits into two 3 carbon molecules- 3phosphoglycerate (PGA) • Reaction occurs 3 times (3 CO2’s used) to make 6 PGAs • Reaction is catalyzed by the protein Ribulose 1,5bisphosphate carboxylase oxygenase (RuBisCO) Ribulose 1,5-bisphosphate carboxylase oxygenase (RuBisCO) • It is arguably the most important enzyme of the biosphere. • By catalyzing CO2 fixation in all photoautotrophs, it provides the source of organic carbon molecules for most of the world’s organisms. • It begins the conversion of about 100 billion tons of CO2 into carbohydrates annually. • There are so many RuBisCO molecules in chloroplasts that the enzyme makes up 50 % or more of the total protein of plant leaves. • As such, rubisco is also the world’s most abundant protein, estimated to total some 40 million tonnes worldwide Phase 2: Reduction • Each of the 6 PGA’s are phosphorylated by ATP – (6 ATP used from LD reaction) • 6 NADPH reduce the molecules to 6 glyceraldehyde 3phosphate (G3P) • 1 G3P molecule is sent away to be made into glucose The lone G3P • It takes three turns of the cycle (3 CO2 inputs) to create 6 G3P molecules • 1 G3P molecules leaves to be made into glucose • Essentially it takes three turns to create this one usable G3P molecule – Therefore, 3 CO2’s to make 1 G3P The lone G3P • The production of the 1 G3P is the goal of photosynthesis • To create it, the Calvin cycle uses 6 reduced NADPH and 9 ATP from the LD reaction and 3 CO2 molecules • We need 2 G3P to make one glucose therefore … Glucose • The production of a single glucose molecule requires the input of 6 CO2, 12 NADPH and 18 ATP Phase 3: RuBP regeneration • Remaining 5 molecules of G3P are rearranged to make 3 RuBP • The cycle can now occur again with the addition of more CO2 Video 1 Video 2