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Photosynthesis Photosynthesis • Photosynthesis is the process by which organisms use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO2 and water) • Photosynthesis transforms the energy of the sun into chemical energy (glucose) • Provides the oxygen we breath, removes CO2, and provides food, energy Photosynthesis • Many organisms participate in photosynthesis: – Nearly all plants – Some Protists (Euglena, kelp) – Some bacteria = cyanobacteria Photosynthesis 6 CO2 + 6 H2O Carbon dioxide Water Light energy Photosynthesis C6H12O6 Glucose + 6 O2 Oxygen • Carbon dioxide and water are waste products of cellular respiration • Photosynthesis takes these products and converts them to the glucose and O2 Photosynthesis • Photosynthesis occurs on a cellular level • Chloroplasts are organelles which carry out photosynthesis • Chloroplasts contain chlorophyll, a lightabsorbing pigment which give autotrophs their distinctive color The Chloroplast • A chloroplast contains two membranes (as do mitochondria) • A thick fluid called the stroma fills the inner compartment of the chloroplast • Suspended in the stroma are the thylakoids, a system of interconnected membranous sacs, which enclose another compartment known as the thylakoid space – The thylakoids are stacked onto one another to form a granum Chloroplast Outer and inner membranes Thylakoid Stroma Granum Thylakoid space Intermembrane space Pigments • Pigments are light-absorbing molecules built into the thylakoid membranes of the cholorplast • Pigments absorb some wavelengths of light, but reflect others • We do not see the absorbed wavelengths (because their energy has been absorbed by the pigment molecules); we see the wavelengths that the pigment reflects • The pigment built into the thylakoid membranes that allow for photosynthesis to occur is called chlorophyll – capture light energy Chlorophyll • Chlorophyll absorbs light mainly in the blue-violet (high energy) and red (low energy) wavelengths • Chlorophyll reflects and transmits light mainly in the green (medium energy) wavelengths Light Reflected light Chloroplast Thylakoid Absorbed light Transmitted light Increasing energy 10–5 nm 10–3 nm Gamma rays X-rays 1 nm 103 nm UV 1m 106 nm Infrared Microwaves 103 m Radio waves Visible light 380 400 600 500 Wavelength (nm) 700 650 nm 750 Photosynthesis • Photosynthesis occurs in 2 stages 1. Light reactions convert light energy into chemical energy (ATP), and split molecules of water to produce H and O2. 2. Light-independent reactions (the Calvin Cycle) assembles glucose molecules using CO2 (carbon fixation) and the energy-rich products of the light reactions CO2 H2O Chloroplast Light NADP+ ADP P LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) ATP NADPH O2 Sugar 1. Light (dependent) Reactions • Light reactions occur in the thylakoid membranes of the chloroplast • Light energy absorbed by chlorophyll molecules is used to split a molecule of water – gives off protons, electrons and O2 as byproducts H2O Æ 2H + 1/2 O2 (2 H2O Æ 4H + O2) • The H are transferred to the coenzyme NADP to create NADPH to be used in the Calvin Cycle • Light energy absorbed by chlorophyll molecules is also used to generate ATP to power the Calvin Cycle Photosystems • In the thylakoid membrane, chlorophyll molecules are organized into clusters (with other pigments and proteins) called photosystems • A photosystem consists of a number of capturing pigments bound to proteins surrounded by a reaction center complex Photosystem Photon of Light Light-harvesting complexes Reaction center complex Thylakoid membrane Primary electron acceptor e– Transfer of energy Pair of Chlorophyll molecules Pigment molecules How do Photosystems capture light energy? • The pigments absorb light energy and pass the electrons (energy) from molecule to molecule until it reaches the reaction center • The reaction center complex contains a pair of chlorophyll molecules and an electron acceptor • When light is absorbed by the pigments, energy passes from pigment to pigment molecules until it reaches the reaction center of the photosystem where it excites an electron of chlorophyll to a higher energy state that is transferred to the primary electron acceptor 2 Separate Photosystems are Required • There are 2 photosystems: Photosystem II and Photosystem I, each capable of capturing light energy, but each photosystem uses that energy differently • Photosystems I and II were named in order of their discovery, but Photosystem II functions first in the sequence of steps that make up the light reactions Photosystem II • Photosystem II uses light energy to split H2O into H+ and O2. • The transfer of an electron from chlorophyll to the electron acceptor causes a molecule of water to be split. When water is split, it supplies electrons to the chlorophyll molecule which lost its electrons to the primary electron acceptor, releasing O2 and H+ as byproducts Photosystem II • The generation of H+ creates a gradient within the chloroplast that is used to synthesize ATP (just like in cellular respiration in the mitochondria). • The electron from Photosystem II is transferred to Photosystem I to supply an electron to its chlorophyll following the absorption of light by Photosystem I Photosystem I • Photosystem I uses light energy to convert the coenzyme NADP into NADPH. • Following the absorption of light, an electron is transferred from chlorophyll to the electron acceptor. From there the electron is transferred a molecule of NADP causing it to become negatively charged. – the electron lost from chlorophyll in Photosystem I is replaced by the electron created in Photosystem II A mechanical analogy of the light reactions e– ATP e– e– e– Mill makes ATP NADPH e– n Photo e– Photon e– Photosystem II Photosystem I 2. Light-independent Reactions • Light-independent reactions occur in the stroma of chloroplasts • Consist of the Calvin Cycle, which assembles sugar molecules using CO2 and H from NADPH generated during the Light Reactions • The incorporation of Carbon from CO2 into organic compounds is called carbon fixation 2. Light-independent Reactions • The Calvin Cycle does not require light, but occurs during daylight hours when the light reactions power the cycle by supplying NADPH and ATP • Often called “dark reactions” • NADPH provides the hydrogens to combine with CO2 to form glucose – ATP powers the cycle Photosynthesis: a review • Light reactions occur in the thylakoid membrane – 2 photosystems capture solar energy which energizes electrons – Photosystems transfer these excited electrons through electron transport chains which produces ATP and NADPH – Water is split and O2 is released Photosynthesis: a review • In the stroma of the chloroplast, sugars are produced via the Calvin Cycle (light-independent reactions) • CO2 is combined with H to form glucose Photosynthesis: a review • The sugar produced by plants during photosynthesis provides the starting materials to make structural components such as cellulose • 50% of this sugar goes toward cellular respiration (plants respire)