![]() ![]() Because this process involves synthesizing a larger, energy-storing molecule, it requires an input of energy to proceed. During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO 2) into sugar molecules, like glucose (C 6H 12O 6). The breakdown of glucose, a simple sugar, is described by the equation:Ĭ 6H 12O 6 + 6O 2 → 6CO 2 + 6H 2O + energyĬarbohydrates that are consumed have their origins in photosynthesizing organisms like plants ( Figure 2). Living things consume sugar as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. The metabolism of sugar (a simple carbohydrate) is a classic example of the many cellular processes that use and produce energy. Carnivores eat the herbivores, and decomposers digest plant and animal matter. Plants use photosynthesis to capture sunlight, and herbivores eat those plants to obtain energy. Most life forms on earth get their energy from the sun. ![]() All of the chemical reactions that take place inside cells, including those that use energy and those that release energy, are the cell’s metabolism.įigure 1. Just as living things must continually consume food to replenish what has been used, cells must continually produce more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Scientists use the term bioenergetics to discuss the concept of energy flow ( Figure 1) through living systems, such as cells. Most experimental data are derived instead from two rodent model systems: primary adipocytes from rats, and mouse 3T3-L1 cells induced to differentiate into adipocytes.Theme 4: How Do Diet, Exercise and Weight Affect Health? Inputs (substrates) and outputs (products) of individual reactions are connected by black arrows blue lines connect output activated enzymes to the other reactions that they catalyze.ĭespite the undoubted importance of these reactions in normal human energy metabolism and in the pathology of diseases such as type II diabetes, they have been studied only to a limited extent in human cells and tissues. These events are outlined in the figure below. The last two reactions, dephosphorylation of perilipin and HSL, negatively regulate the pathway. The next four reactions are the hydrolysis reactions themselves: the hydrolysis of cholesterol esters, and the successive removal of three fatty acids from triacylglycerol. The first six of these involve activation: phosphorylation of HSL, dimerization of HSL, disruption of CGI-58:perilipin complexes at the surfaces of cytosolic lipid particles, phosphorylation of perilipin, association of phosphorylated HSL with FABP, and translocation of HSL from the cytosol to the surfaces of lipid particles. Here, HSL-mediated triacylglycerol hydrolysis is described as a pathway containing twelve reactions. Conversely, perilipin, a major constituent of the particle surface, appears to block access of enzymes to the lipids within the particle its phosphorylation allows greater access. This organization is dynamic: the inactive form of HSL is not associated with the particles, but is translocated there after being phosphorylated. The processes of triacylglycerol and cholesterol ester hydrolysis are also regulated by subcellular compartmentalization: these lipids are packaged in cytosolic particles and the enzymes responsible for their hydrolysis, and perhaps for additional steps in their metabolism, are organized at the surfaces of these particles (e.g., Brasaemle et al. The hormones to which it is sensitive include catecholamines (e.g., epinephrine), ACTH, and glucagon, all of which trigger signaling cascades that lead to its phosphorylation and activation, and insulin, which sets off events leading to its dephosphorylation and inactivation (Holm et al. A central part in this regulation is played by hormone-sensitive lipase (HSL), a neutral lipase abundant in adipocytes and skeletal and cardiac muscle, but also abundant in ovarian and adrenal tissue, where it mediates cholesterol ester hydrolysis, yielding cholesterol for steroid biosynthesis. Triacylglycerol is a major energy store in the body and its hydrolysis to yield fatty acids and glycerol is a tightly regulated part of energy metabolism. ![]()
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