Unit 4: Cell Metabolism

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Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. The three main purposes of metabolism are the conversion of food/fuel to energy to run cellular processes, the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates, and the elimination of nitrogenous wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.

4.1: Enzymes
Enzymes are catalysts. Most are proteins. (A few ribonucleoprotein enzymes have been discovered and, for some of these, the catalytic activity is in the RNA part rather than the protein part. Link to discussion of these ribozymes). Enzymes bind temporarily to one or more of the reactants — the substrate(s) — of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction.
4.2: ATP
ATP (Adenosine triphosphate) is a nucleotide that performs many essential roles in the cell. It is the major energy currency of the cell, providing the energy for most of the energy-consuming activities of the cell. It is one of the monomers used in the synthesis of RNA and, after conversion to deoxyATP (dATP), DNA. It regulates many biochemical pathways.
4.3: NAD and NADP
Nicotinamide adenine dinucleotide (NAD) and its relative nicotinamide adenine dinucleotide phosphate (NADP) are two of the most important coenzymes in the cell. NAD participates in many redox reactions in cells, including those in glycolysis and most of those in the citric acid cycle of cellular respiration. NADP is the reducing agent produced by the light reactions of photosynthesis and is consumed in the Calvin cycle of photosynthesis and used in many other anabolic reactions.
4.4: Glycolysis
Glycolysis is the anaerobic catabolism of glucose and occurs in virtually all cells. In eukaryotes, it occurs in the cytosol, where it converts a molecule of glucose into 2 molecules of pyruvic acid.
4.5: Cellular Respiration
Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water.
4.6: ATP Synthase
ATP synthase is a huge molecular complex (>500,000 daltons) embedded in the inner membrane of mitochondria. Its function is to convert the energy of protons (H+) moving down their concentration gradient into the synthesis of ATP. 3 to 4 protons moving through this machine is enough to convert a molecule of ADP and Pi (inorganic phosphate) into a molecule of ATP. One ATP synthase complex can generate >100 molecules of ATP each second.
4.7: Photosynthesis - Pathway of Carbon Fixation
Photosynthesis is the synthesis of organic molecules using the energy of light.
4.8: Photosynthesis - The Role of Light
4.9: Photosynthesis - Dicovering the Secrets
This chapter talks about various scientists and their path towards discovering photosynthesis.
4.10: Chemiosmosis
Several kinds of evidence support the chemiosmotic theory of ATP synthesis in chloroplasts. When isolated chloroplasts are illuminated, the medium in which they are suspended becomes alkaline — as we would predict if protons were being removed from the medium and pumped into the thylakoids (where they reduce the pH to about 4.0 or so). The interior of thylakoids can be deliberately made acid (low pH) by suspending isolated chloroplasts in an acid medium (pH 4.0) for a period of time.
4.11: Metabolism
All living things must have an unceasing supply of energy and matter. The transformation of this energy and matter within the body is called metabolism.
4.12: Intermediary Metabolism
The immediate source of energy for most cells is glucose. But glucose is not the only fuel on which cells depend. Other carbohydrates, fats and proteins may in certain cells or at certain times be used as a source of ATP. The complexity of the mechanism by which cells use glucose may make you fervently hope that a similarly-constructed system is not needed for each kind of fuel. And indeed it is not.
4.13: G Proteins
G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three different subunits) associated with the inner surface of the plasma membrane and transmembrane receptors of hormones, etc. These are called G protein-coupled receptors (GPCRs).
4.14: Secondary Messengers
Second messengers are molecules that relay signals received at receptors on the cell surface — such as the arrival of protein hormones, growth factors, etc. — to target molecules in the cytosol and/or nucleus. But in addition to their job as relay molecules, second messengers serve to greatly amplify the strength of the signal. Binding of a ligand to a single receptor at the cell surface may end up causing massive changes in the biochemical activities within the cell.
4.15: Bioluminescence
Bioluminescence is the ability of living things to emit light. It is found in many marine animals, both invertebrate (e.g., some cnidarians, crustaceans, squid) and vertebrate (some fishes); some terrestrial animals (e.g., fireflies, some centipedes); and some fungi and bacteria.