Photosynthesis: The Process of Converting Light Energy into Chemical Energy, Summaries of Cellular and Molecular Biology

Download Photosynthesis: The Process of Converting Light Energy into Chemical Energy and more Summaries Cellular and Molecular Biology in PDF only on Docsity! lOMoARcPSD|39591929 M. Jones, G. Jones, Cellular biology Reading Notes Chapter 10- Photosynthesis Photosynthesis- the process of capturing light energy from the sun and convert it to chemical energy that is stored in sugar and organic molecules Autotrophic Nutrition • <self-feeders= • Sustain themselves without eating anything derived from other living begins • Produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment • Almost all plants are autotrophs (specifically photoautotrophs) Heterotrophic Nutrition • <other-feeding= • Unable to make their own food, they live on compounds produced by other organisms • Decomposers (fungi and most prokaryotes) are heterotrophs Chloroplasts: The Sites of Photosynthesis in Plants • Leaves are the major sites of photosynthesis in most plants • Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf • A typical mesophyll cell has about 30-40 chloroplasts • CO2 enters the leaf and oxygen exits through microscopic pores called stomata • A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma • Suspended within the stroma is a third membrane system made up of sacs called thylakoids, which segregates the stroma from the thylakoid space • Chlorophyll is the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast • Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast • When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex • The reaction-center complex also contains a molecule capable of accepting electrons and becoming reduced, this is called the primary electron acceptor • The solar powered transfer of an electron from the reaction-center chlorophyll a to the primary electron acceptor is the first step of the light reactions • As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it • The thylakoid membrane contains 2 types of photosystems that cooperate in the light reactions of photosynthesis 1. Photosystem II (PS II)- chlorophyll a in this photosystem is best at absorbing light having a wavelength of 680nm 2. Photosystem I (PS I)- chlorophyll a in this photosystem is best at absorbing light having a wavelength of 700nm Linear Electron Flow • Light drives the synthesis of ATP and NADPH by energizing the two photosystems embedded in the thylakoid membranes • Linear electron flow occurs during the light reactions of photosynthesis 1. A photo of light strikes one of the pigment molecules in a light-harvesting complex of PS II. One of its electrons is energized, when it falls back to ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. The process continues until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state 2. This electron is transferred from P680 to the primary electron acceptor. The resulting form of P680 is missing an electron, and becomes P680+ 3. An enzyme catalyzes the splitting of a water molecule into two electrons, two H+ ions, and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. The H+ are released into the thylakoid space. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O2 4. Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain. This chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc) 5. The exergonic <fall= of electrons to a lower energy level provides energy for the synthesis of ATP. As electrons pass though the cytochrome complex, H+ are pumped into the thylakoid space, contributing to the proton gradient 6. Meanwhile, light energy has been transferred via light-harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a. The photoexcited electron is then transferred to PS I9s primary electron acceptor, creating P700+, which can now act as an electron acceptor, accepting an electron that reaches the bottom of the transport chain from PS II 7. Photoexcited electrons are passed in a series of redox reactions from the primary electron accept of PS I down a second electron transport chain through the protein ferredoxin (Fd). This chain does not create a proton gradient and thus does not produce ATP 8. The enzyme NADP+ reductase catalyzes the transfer of electrons from ferredoxin to NADP+. Two electrons are required for its reduction to NADPH. This molecule is at a higher energy level than water, so its electrons are more readily available for the reactions of the Calvin cycle. This process removes an H+ from the stroma. • The big picture: the light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power to the carbohydrate-synthesizing reactions of the Calvin cycle Cyclic Electron Flow • In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses PS I but not PS II • The electrons cycle back from ferredoxin to the cytochrome complex and from there continue on to a P700 chlorophyll in PS I reaction-center complex • There is no producing of NADPH and no release of oxygen that results from this process. On the other hand, it DOES generate ATP • Photosynthetic bacteria are known to have a single photosystem. For these species, cyclic electron flow is the one and only means of generating ATP during photosynthesis • Cyclic electron flow can occur in photosynthetic species that possess both photosystems Comparing Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis • An electron transport chain pumps H+ protons across a membrane as electrons are passed through a series of carriers that are progressively more electronegative. Thus, electron transport chains transform redox energy to a proton-motive force • An ATP synthase complex couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP • In chloroplasts, the high-energy electrons dropped down the chain come from water. Chloroplasts do not need molecules from food to make ATP, their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain • The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (interior) which functions as the H+ reservoir. ATP is synthesized as the H+ ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes. Thus, ATP is formed in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle Summary of Light Reactions: Electron flow pushes electron from water, where they are at a low state of potential energy, ultimately to NADPH, where they are stored at a high state of potential energy. The light-driven electron flow also generates ATP. Thus, the equipment of the thylakoid membrane convers light energy to chemical energy stored in ATP and NADPH (oxygen is a byproduct) Light reactions capture solar energy and use it to make ATP and transfer electrons from water to NADP+, forming NADPH The Calvin Cycle • The cycle is anabolic, building carbohydrates from smaller molecules and consuming energy. Spends ATP as an energy source and consumes NADPH as reducing power for adding high-energy electrons to make sugar