Module 2F - Photosynthesis Photosynthesis As we saw in the previous module, all cells can break down organic molecules and use the energy that is released to make. In addition, some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis). 1 In this module, we will examine photosynthesis a process for manufacturing glucose from CO 2 and H 2 O using energy from light. During photosynthesis, light energy is captured and stored in the chemical bonds of glucose molecules. 2 Objective # 36 Objective 36 Write a summary chemical equation for photosynthesis and describe the origin i and fate of each substance involved. Photosynthesis: 6CO 2 + 6H 2 O + light energy C 6 H 12 O 6 + 6O 2 During i photosynthesis, h H is removed from H 2 O, leaving O 2 as a waste product. The electrons in the removed H are energized by light, and then used to reduce CO 2 to form glucose. 3 4 Objective # 37 Objective 37 Explain the role and importance of photosynthesis for life on earth, and identify if the main types of cells that carry out this process. 5 All living organisms require organic molecules for survival. Organic molecules are used as an energy source, and as structural materials to build cell components. Photosynthesis is the ultimate source of almost all organic molecules used by living organisms. It is also the main source of O 2 in the atmosphere. 6 1
Objective 37 Objective 37 Organisms that can manufacture their own organic molecules from inorganic substances are called autotrophs. Those that must consume organic molecules are called heterotrophs. Autotrophs that use photosynthesis to manufacture organic molecules include some monerans, some protists, and almost all plants. 7 Together, photosynthesis and aerobic respiration form a complex cycle that is responsible for most of the energy flow through the biosphere. Photosynthesis uses CO 2 + H 2 O to produce glucose + O 2. Aerobic respiration uses glucose + O 2 to regenerate CO 2 + H 2 O. 8 Photosynthesis Aerobic Respiration Objective # 38 II Sunlight Glucose I NADP+ Calvin NADPH O 2 H 2 O CO 2 Pyruvate Heat Electron Transport System NAD + Krebs NADH ADP+Pi Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 Identify the 2 main stages of photosynthesis and indicate where each stage takes place in both prokaryotic and eukaryotic cells. 10 Photosynthesis occurs in 2 main stages: 1) The light-dependent reactions, also called the energy capturing reactions 2) The carbon fixation reactions, also called the Calvin or C 3 cycle In eukaryotes, photosynthesis occurs inside chloroplasts. Each chloroplast is surrounded by a double membrane. Inside the inner membrane are flattened membranous sacs called thylakoids surrounded by a semi-fluid material called the stroma. The thylakoids are arranged in stacks called grana: 11 12 2
Structure of a Chloroplast Plant cell Inner membrane Outer membrane Granum Chloroplast Membrane Stroma Copyright Copyright The McGraw-Hill The McGraw-Hill Companies, Companies, Inc. Inc. Permission required for for reproduction or display. 13 Sunlight Stroma CO 2 H 2 O O 2 Light-Dependent Reactions Calvin NADP + NADPH Organic molecules Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. In eukaryotes, the light-dependent reactions of photosynthesis take place on the surface of the thylakoid membranes and the carbon fixation reactions (Calvin cycle) occur in the stroma. 14 In prokaryotes, which lack chloroplasts: the light-dependent reactions of photosynthesis take place on the inner surface of the plasma membrane or on invaginations of the plasma membrane called photosynthetic lamellae. the carbon fixation reactions occur in the cytoplasm. The photosynthetic lamellae (membranes) of this bacterial cell are colored green: 15 16 Objective # 39 Describe the light-dependent (energy capturing) reactions of photosynthesis with respect to: a) how pigment molecules in the antenna complexes are used to capture light energy b) how the captured energy is used for the production of and NADPH during non- cyclic photophosphorylation c) how the captured energy is used for the production of during cyclic photophosphorylation 17 Objective 39 During the light-dependent reactions (also called the energy capturing reactions) of photosynthesis, light energy is captured by pigment molecules l and transferred to electrons. The molecules used to carry out this process are organized into photosystems. 18 3
Each consists of 2 parts: Antenna complex Hundreds of accessory pigment molecules Gather photons and feed the captured light energy to the reaction center Reaction center 11 or more chlorophyll a molecules Passes excited electrons out of the photosystem 19 The Antenna Complex Also called light-harvesting harvesting complex Captures photons from sunlight and channels them to the reaction center chlorophylls In chloroplasts, light-harvesting harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins 20 A photosystem embedded in the thylakoid membrane of a chloroplast: 21 The Reaction Center Transmembrane protein pigment pigment complex When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level Light-energized electron can be transferred to the primary electron acceptor, reducing it Oxidized chlorophyll then fills its electron hole by oxidizing a donor molecule 22 Release of an excited electron by the reaction center chlorophyll (shown in green). Objective 39 High energy electrons released by the reaction center chlorophyll can follow one of 2 pathways: cyclic photophosphorylation non-cyclic photophosphorylation 23 24 4
Objective 39 In cyclic photophosphorylation, the high energy electrons are used to make by chemiosmosis. In non-cyclic photophosphorylation, first the high energy electrons are used to make by chemiosmosis. Next, they are reenergized by light and used to reduce NADP + to form NADPH. 25 26 Objective # 40 Describe the following events of the carbon fixation reactions (also called the Calvin or C 3 cycle) of photosynthesis: a) carbon fixation b) the reduction of PGA to form G3P (also called PGAL) c) the production of glucose from G3P d) the regeneration of RuBP from G3P 27 28 Objective 40 During the carbon fixation reactions (also called the Calvin or C 3 cycle) of photosynthesis, energy from and hd hydrogen from NADPH are used to reduce CO 2 in order to form glucose. 29 30 5