PHOTOSYNTHESIS Section 6.2 – The Reactions of Photosynthesis. Pages 186-199 The Reactions of Photosynthesis • Chlorophyll and other pigments are in chloroplasts which capture packages of energy called photons from sunlight. • Photosynthesis – the captured energy is converted into chemical energy (glucose). – A process made up of a series of complex chemical reactions. Energy Containing Molecules Formed in Photosynthesis 1. ATP – principal energy supply molecule. It provides an immediate source of energy for cellular processes (such as growth and movement). 2. NADPH – electron donor involved in energy transfers. 3. Glucose – sugar; transport molecule; medium term energy storage in most cells (used to store energy for later use). ATP • Adenosine triphosphate (ATP): – Used by all living cells – Provides immediate energy for cellular functions (ex. Synthesis of needed chemicals, transport materials across membranes). – Formed by the addition of an inorganic phosphate group (Pi) to a molecule of lower energy adenosine diphosphate (ADP). – Same energy can be released to the cell by a reaction that splits ATP back to ADP and Pi. – Figure 1 on page 186 NADPH • Nicotinamide dinucleotide phosphate (NADP+) appears in several places during photosynthesis. – Accepts one hydrogen atoms and two electrons to form NADPH. – NADPH can donate electrons to other molecules and becomes NADP+ again. An Overview of Photosynthesis • Three distinct (but connected) stages: 1. Capturing solar energy and transferring it to electrons. 2. Using captured solar energy to make ATP and to transform high energy electrons to NADP+; make NADPH, which is then used as a high energy electron carrier molecule. 3. Using energy stored in ATP and high energy electrons carried by NADPH to form energy rich organic molecules, such as glucose, from CO2. An Overview of Photosynthesis • Light dependent reactions: – First 2 stages – Directly energized by light – Require chlorophyll and occur on the thylakoid membranes in chloroplasts – Light energy is transferred to carbohydrate molecules (last stage) • Carbon fixation: – 3rd stage – Incorporates CO2 into carbohydrate – Takes place in the stroma An Overview of Photosynthesis • Light-independent: – The second set of reactions in photosynthesis and does not require solar energy. • Calvin cycle: – A cyclic set of reactions occurring in the stroma of chloroplasts that fixes the carbon of CO2 into carbohydrate molecules and recycles coenzymes. Calvin Cycle Stage 1: Capturing Solar Energy • Photosystems: a cluster of photosynthetic pigments embedded in a thylakoid membrane of a chloroplast that absorbs light energy. – Photosystem I & II • These are the actual molecules responsible for capturing light energy. Stage 1: Capturing Solar Energy • Solar energy is captured when an electron in a chlorophyll molecule absorbs a photon. – Electrons are high energy particles present in atoms (excited). – Electrons have low energy before a photon hits them. – Photon has now been converted to chemical energy! Stage 1: Capturing Solar Energy • Electron transport chain: a series (steps) of progressively stronger electron acceptors; each time an electron is transferred, energy is released. • Photolysis: a chemical reaction in which a compound is broken down by light. – Electrons removed from the photosystem need to be replaced so this is where they come from. – Replacement electrons come from water. – In photolysis, the solar energy absorbed by the cholorphyll is used to split water into hydrogen ions (H+) and oxygen gas. – Occurs in the thylakoid lumen. Stage 1: Capturing Solar Energy • For every 2 water molecules consumed, 4 electrons and 1 oxygen are made. 2H2O (l) + energy 4H+ + 4e- + O2(g) Stage 2: Electron Transfer and ATP Synthesis • • Now that we have solar energy captured, we need to make ATP. Two tasks: 1. The electron transport train - Build up of charged particles 2. Oxidation-reduction reactions - Direct transfer of electrons The Electron Transport Chain • Solar energy is used to excite electrons which were removed from water. • This added energy lifts them up. • Potential energy is gradually released and electrons travel down the stairs to their original lower level. • Some energy is captured to make ATP • See figure 3 on page 189. Oxidation-Reduction Reactions • High energy electron donor passes an electron to a lower energy electron acceptor – this is an oxidation-reduction or a redox reaction. • Oxidation: a reaction in which an atom or molecule loses electrons. (an ox loses) • Reduction: a reaction in which an atom or molecule gains electrons. (red cat gains) Oxidation-Reduction Reactions • Electron donors such as NADPH tend to lose electrons. • Electron acceptors like NADP+ tend to gain electrons. Look at Figure 4 page 190 • animation Key steps in electron transfer during the light dependent reactions: 1. 2. 3. 4. Electrons from photosystem II are transferred along an electron transport chain and across the thylakoid membrane to the inner surface. Some of their energy is used to pull H+ ions across the membrane, making a build up of a positive charged lumen. The electrons, which have lost most of their energy, are then transferred to chlorophyll molecules in the photosystem I complex, where they absorb solar energy and reach an excited state again. High energy electrons from photosystem I are transferred to NADP+ to form NADPH. Chemiosmosis • H+ ions were pulled across the thylakoid membrane making a concentration gradient. • The H+ ions are now not able to escape from the except by special proteins embedded in the membrane – ATP synthase complexes. • ATP synthase complex: sepecialized protein complex in the thylakoid membrane that allows H+ ions to escape from the lumen and uses the resulting energy to generate ATP. Chemiosmosis • Chemiomosis: a process for synthesizing ATP using the energy of an electrochemical gradient and the ATP synthase enzyme. Stage 3: The Calvin Cycle and Carbon Fixation • Carbon fixation: formation of high energy organic molecules from CO2. – Referred to as the Calvin Cycle to honour the 1961 Nobel Prize winner Melvin Calvin. • Involves a large number of lightindependent reactions Stage 3: The Calvin Cycle and Carbon Fixation Figure 6 on page 193 Stage 3: The Calvin Cycle and Carbon Fixation • Utilizes both ATP and high energy electrons carried by NADPH to make G3P (sugar used to create glucose) • Carbon and oxygen atoms are supplied by CO2 and hydrogen atoms are supplied by the light dependent reactions. • 3 ATPs and 2 NADPHs are consumed for every CO2 that enters the cycle. Stage 3: The Calvin Cycle and Carbon Fixation • Building one simple sugar molecule (like glucose – C6H12O6) requires energy from 18 ATP molecules and 12 NADPH molecules. • Water produced is < consumed. There are 6 H2O molecules for every 1 glucose formed. Questions • Section 6.2 #1-7