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Lec 20 Photosynthesis.docx

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BIOL 1010
Igor Kovalchuk

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Photosynthesis Lecture 20 Photosynthesis in Nature - Photosynthesis transforms solar light energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules - This process: o Synthsizes energy-rich organic molecules from the energy-poor molecules, CO2 and H2O o Directly or indirectly supplies energy to most living organisms Plants and other autotrophs are the producers of the biosphere - Organisms aquire organic molecules used for energy and carbon skeleton by one of two nutritional modes: o Autotrophic nutrition o Heterotrophic nutrition - Autotrophic nutrition: nutritional mode of synthesizing organic molecules from inorganic raw materials o Examples of autotrophic organisms are plants, which require only CO2, H2O and minerals as nutrients o Because autotrophic organisms produce organic molecules that enter an ecosystem’s food store, autotrophs are also known as producers - The energy source may be from light (photoautotrophic) or the oxidation or inorganic substances (chemoautotrophic) - Photoautotrophs – autotrophic organisms that use light as an energy source to synthesize organic molecules. Examples are photosynthetic organisms such as plants, algae and some prokaryotes. - Chemoautotroph- autotrophic organisms that use the oxidation of inorganic substances, such as sulfur or ammonia, as an energy source to synthesize organic molecules. Unique to some bacteria, this is a rarer form of autotrophic nutrition - Heterotrophic nutrition- nutritional mode of acquiring organic molecules from compounds produced by other organisms. Are unable to synthesize organic molecules from inorganic raw materials o Heteroptrophs are known as consumers o Examples are animal that eat plants or other animals o Examples also include decomposers Heterotrophs that decompose and feed on organic matter. Most fungi and many bacteria are decomposers. o Most heterotrophs depend on photoautotroph for food and oxygen (a by-product of photosynthesis) The site of photosynthesis in a plant - Leaves are the major organs of photosynthesis in plants - Gas exchange between the mesophyll and the atmosphere occurs through microscopic pores called stomata - Chloroplasts, found in mainly in the mesophyll, are bounded by two membranes that enclose the stroma, the dense fluid content of the chloroplast - Membranes of the thylakoid system separate the stroma from the thylakoid space - Thylakoids are concentrated in stacks called grana The structure of the chloroplast - The thylakoid is the structural unit of photosynthesis - Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane, thylakoid are stacked like pancakes in stacks known collectively as grana - The areas between grana are referred to as stroma the viscous fluid where photosynthesis occurs. - While the mitochondrion has two membranes systems, the chloroplast has three, forming three compartments. - Chloroplasts have lens-shaped organelles Leaf structure - Plants are the only photosynthetic organisms to have leaves - A leaf may be viewed as a solar collector full of photosynthetic cells - The raw materials of photosynthesis- water and carbon dioxide- enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Chloroplasts are the sites of photosynthesis in plants - Although all green plant parts have chloroplast, leaves are the major sites of photosynthesis in most plants o Chlorophyll is the green pigment in chloroplasts that gives a leaf its color and absorbs the light energy used to drive photosynthesis o Chloroplasts are primarily in cells of mesophyll, green tissue in the leaf’s interior. What is photosynthesis? - Photosynthesis is the process by which plants, some bacteria, some protozoan use the energy from sunlight to produce sugar. The conversion of unusable sunlight energy into usable chemical energy, is associated with eh actions of the green pigment chlorophyll  12 H 2 + 6 CO + 2ight energy - C H O 6 612O6+ 6O 2 2 The pathways of photosynthesis - Indicating the net consumption of water simplifies the equation:  6 H 2 + 6 CO +2light energy - C H O 6 612 6 2 - In this form, the summary equation for photosynthesis is the reverse of that for cellular respiration - Photosynthesis and cellular respiration both occur in plant cells, but plants to not simply reverse the steps of respiration to make food. -
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