LabManual_Photosynthesis&Respiration - Foundations...

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Foundations of Biological Sciences I Photosynthesis & Respiration INTRODUCTION All living organisms have a constant energy requirement, and they have mechanisms to gather, store, and use energy. These mechanisms are called metabolism. The metabolic pathway by which plants capture light energy and use it to make carbohydrates such as glucose is called photosynthesis . Carbohydrates are temporary energy stores. The process by which energy stored in carbohydrates is released is called respiration . The two processes are thus intimately intertwined (Fig. 1), and in today’s lab you will examine some of the important aspects of photosynthesis and respiration. Virtually all life on earth depends on photosynthesis for food and oxygen. Photosynthesis , the process that converts radiant energy (light) into chemical energy (sugar) is thus the most important series of chemical reactions on earth. Although water (H 2 0) is presented on both sides of the summary equation, these are not the same water molecules. The reactant water molecules (those entering the equation) are split to release electrons during the photochemical ( light-dependent ) reactions. The “product” water molecules (those leaving the equation) are assembled from hydrogen and oxygen released during the photochemical ad biochemical ( light-independent ) reactions. The light independent reactions are also referred to as the dark reactions of the Calvin cycle. Both the light-dependent and the light-independent reactions of photosynthesis occur in the chloroplasts (Fig. 1). The light-dependent reactions occur on the thylakoid membranes of the chloroplast and convert light-energy to chemical energy captured as ATP and NADPH. The light-independent reactions occur in the stroma (the thick fluid between the thyakoid stacks) of the chloroplasts, and uses the ATP and NADPH generated by the light reactions to reduce CO 2 to sugars. Summary equation for Photosynthesis 6 CO 2 + 12 H 2 O C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Carbon Dioxide Water Glucose Oxygen Water “Light” reaction “Dark” reaction photochemical biochemical Fast (practically instantaneous) Slower (but still very fast) Light-dependent Light-independent Splits H 2 O to release electron, protons and oxygen “Fixes” CO 2 to sugar Fig. 1. Photosynthesis and Respiration
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Foundations of Biological Sciences I Both autotrophs and heterotrophs undergo respiration . Photoautotrophs such as plants use the carbohydrates they have produced through photosynthesis to built new cells and to maintain cellular machinery. Heterotrophic organisms have to obtain material for respiration by digesting plant or animal material. In most cells, respiration begins with the oxidation of glucose to pyruvate via a set of chemical reactions called
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LabManual_Photosynthesis&Respiration - Foundations...

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