 In this video I will define energy, define the first law of thermodynamics, define the second law of thermodynamics, define and contrast exergonic and endergonic chemical reactions, describe how humans obtain energy, and explain how ATP is used by the cell as an energy source. Energy is the potential to do work or heat an object. Energy flows into our planet from the sun. You can notice the energy of light being transformed into heat energy as it the sun warms your skin. Plants are producers that can transform the energy from sunlight into chemical potential energy in the process of photosynthesis. As plants produce carbohydrates like glucose, sucrose and fructose and the starches, the energy that drives these anabolic pathways comes from sunlight. As consumers, humans obtain their energy by eating other organisms like plants or animals or fungi. Fungi are an example of decomposers that digest the remains of dead plants and animals in order to obtain their energy. While energy can be transformed from light energy to chemical potential energy, ultimately the energy that flows into our planet will be transformed into heat, which is essentially the kinetic energy of molecules vibrating and moving around. A plant like an oak tree harnesses the energy from sunlight in order to drive anabolic pathways, then an animal like a human or this squirrel is a consumer that obtains energy by catabolic pathways that break down food. The energy released in these catabolic pathways can then be used to drive anabolic pathways in order to sustain life. The first law of thermodynamics states that energy may transfer from one place to another place or transform into a different form of energy, however, energy cannot be created or destroyed. An example of an energy transformation is the chemical energy stored in our food such as this ice cream cone. When we eat that ice cream cone and break it down, we digest that and then the resulting nutrients are broken down to release energy. The chemical energy is being transformed into kinetic energy in order to move our body around. For example, as we're riding a bike, that movement of our body is kinetic energy that comes from the chemical energy that is released as we break the covalent bonds between atoms and organic molecules like carbohydrates. So photosynthesis stores energy from the light in chemical bonds in the covalent bonds between atoms and then as we consume our food and break down our food, we're releasing the chemical potential energy as we break the covalent bonds between atoms and these organic molecules. The second law of thermodynamics states that energy transfer or transformation always increases the entropy of the universe. Entropy is a measure of randomness or disorder. The illustration here compares the entropy of a solid to the entropy of a liquid. You can see the atoms in the solid are highly packed together in a very organized structure. In contrast, the atoms in the liquid are less organized and more spread out. The energy is transferred into the solid as the solid melts to form the liquid and that energy is transferred into the kinetic energy, the movement of the molecules. We will see that some chemical reactions will produce highly organized, more complex structures and in order to create such a highly organized structure with such low entropy, another chemical reaction will be required to release energy and in the process increase the entropy of the overall system. So while one structure can become highly organized, there must be a greater increase in entropy and increase in disorder to pay for the highly organized structure being formed. Gibbs free energy is a measurement of the energy transfer during a chemical reaction and it's the amount of energy that's available after accounting for the increase in entropy. Therefore that Gibbs free energy is the amount of energy that would be available to do work following an exergonic chemical reaction that energy released could be harnessed to do work. So an exergonic chemical reaction is a chemical reaction in which energy is released. The reactants have a higher level of chemical potential energy than the products and as the reactants are converted to products, energy is released. Therefore an exergonic reaction is a chemical reaction that has a negative change in the Gibbs free energy. ATP hydrolysis is an example. ATP has a high level of chemical potential energy and that energy is released as ATP hydrolysis breaks down ATP and produces ADP and inorganic phosphate. So our exergonic reactions release energy and these reactions are spontaneous. They don't require the input of any energy. In contrast, an endergonic reaction is not spontaneous and does require energy to be added. The example of ATP synthesis is an endergonic reaction. The reactants ADP and inorganic phosphate have a lower chemical potential energy than the product ATP and therefore energy will need to be added in order to derive an endergonic chemical reaction. So an endergonic chemical reaction has a positive change in the Gibbs free energy as the products have a higher potential chemical energy than the reactants. While the sun is the ultimate source of energy that powers life and plants are harnessing that sunlight in the process of photosynthesis, humans and other animals obtain energy by eating food. The energy that is stored in covalent bonds between the atoms in organic molecules is released as we break down the molecules in our food. As we break down nutrients, we will send nutrients through catabolic pathways, releasing energy as the nutrients are broken down. So our food contains large nutrients like proteins, complex carbohydrates and triglycerides. As we digest our food, we break down those large molecules into smaller building blocks like amino acids, glucose, glycerol and fatty acids. While those building blocks like amino acids could be used to create larger molecules, amino acids could be used in protein synthesis in order to produce polypeptides, or glucose could be used to synthesize polysaccharides like glycogen, or the glycerol and fatty acids could be used to synthesize triglycerides to store in adipose tissue. We can also take those molecules and break them down through catabolic pathways, releasing energy that can then be used to fuel anabolic pathways. For example, glucose, catabolism releases energy that will then be used to fuel the endergonic chemical reaction of ATP synthesis. ATP stands for adenosine triphosphate. ATP contains three phosphate groups held together by high-energy covalent bonds. ATP hydrolysis releases one of the phosphate groups as an inorganic phosphate, producing adenosine diphosphate or ADP. ATP synthesis converts ABP and inorganic phosphate into ATP. Catabolic pathways provide energy that is used to drive the endergonic chemical reaction of ATP synthesis. Then the hydrolysis of ATP releases energy that can be used to drive anabolic pathways. mitochondria are the organelles that produce the majority of ATP in most of the cells of our body. Mitochondria are double membrane organelles where the inner membrane has a highly folded structure to form cristae. This folding of the cristae functions to increase the surface area of the inner mitochondrial membrane, which is embedded with proteins that carry out a metabolic pathway known as oxidative phosphorylation. Oxidative phosphorylation is the last metabolic pathway and a series of metabolic pathways referred to as aerobic cellular respiration. Aerobic cellular respiration converts glucose to carbon dioxide and water. Aerobic cellular respiration is a catabolic pathway that breaks down glucose and releases energy. The last step of aerobic cellular respiration is ATP synthesis, which is catalyzed by an enzyme known as ATP synthase that's found embedded in the inner mitochondrial membrane. Aerobic cellular respiration requires oxygen and will produce the waste product carbon dioxide. But as energy is released from aerobic cellular respiration of glucose and glucose is broken down, energy will be transferred into the chemical bonds of ATP molecules. ATP is used in order to drive endergonic reactions. The hydrolysis of ATP and exergonic reaction will provide the energy needed to fuel endergonic reactions. An example is the dehydration synthesis reaction that forms the peptide bond between amino acids during translation. The ribosome is the enzyme that catalyzes dehydration synthesis to form the peptide bond and the ribosome will perform hydrolysis of ATP in order to release energy that can then be used to fuel the synthesis of the polypeptides from amino acids. Primary active transport is another example of how a cell uses ATP as the sodium-potassium pump is forcing sodium out of the cell to create a high concentration of sodium in the extracellular space and forcing potassium into the cell to create a high concentration of potassium in the cytoplasm. ATP provides the energy in order to fuel this primary active transport mechanism. The sodium-potassium pump catalyzes hydrolysis of ATP to produce inorganic phosphate and ADP. The energy released by that exergonic reaction is the energy that fuels the non-spontaneous movement of sodium and potassium against their concentration gradients.