Biology is a very special application of the laws of chemistry and physics. Biology follows and never violates the laws of the physical sciences, but this fact can sometimes be obscured in the complexity and other special characteristics of biological chemistry and physics. Following is a review of some of the principles of chemistry and physics that have special application in anatomy and physiology. Some of these principles overlap — for example, probability is one factor that drives the process of diffusion. Although what follows are oversimplified explanations of very profound and complex matters, they might help you better understand anatomy and physiology. ENERGY CAN NEITHER BE CREATED NOR DESTROYED The first law of thermodynamics is that energy can be neither created nor destroyed — it can only change form. Throughout any process, the total energy in the system remains the same. This law is one of the fundamental concepts in physics, chemistry, and biology. Energy is the ability to bring about change or to do work. It exists in many forms, such as heat, light, chemical energy, and electrical energy. Light energy can be captured in chemical bonds, such as in the process of photosynthesis. In physiological processes, the energy in the bonds of ATP is transformed into work when the chemical bonds are broken — to move things, for example, and to generate heat. (And where did the energy in ATP come from in the first place? Ultimately, the sun via photosynthesis.) Although the total energy in a system always remains the same, the energy available for biological processes does not. Cells can use energy only in certain specific forms. A physiological process that uses ATP doesn’t use all the energy stored in those chemical bonds, but the leftover energy isn’t in a form that can be used in another physiological process. It is “lost” to physiology, mostly as heat flowing out into the surrounding environment. EVERYTHING FALLS APART Energy is required to create “order” — for example, to build the atomic and molecular aggregations —”matter” or “stuff.” Without continuous input of energy (maintenance), stuff falls apart. No news here for dwellers in the real world. As a physicist might put it, all systems tend toward increasing entropy (disorder). This is the second law of thermodynamics. Energy always moves from a point of higher concentration to a point of lower concentration, never the reverse. For example, where two adjacent objects are of different temperatures, heat flows only from the warmer object (higher energy) to the cooler object (lower energy). A state of order contains more energy than a state of disorder because of the energy that went into building the state of order. Energy flows outward into the relative chaos of disorder. Because living systems are highly ordered, the implications of the second law of thermodynamics are profound for physiology. The law means that physiological homeostasis (the maintenance of order) is an active process that requires energy. The energy that must be applied to drive any physiological process comes from releasing the chemical bonds in ATP. EVERYTHING’S IN MOTION Particles in a solution fly around constantly and collide with one another all the time. This kind of motion is called Brownian motion. The higher the temperature, the more frequent and harder the collisions. It’s the reason why any reaction that can happen will happen, because (most of) the particles required for the reaction will collide sooner or later. This is especially important when considering all the molecules (such as glucose and ions) that move through membranes by simple or facilitated diffusion. Brownian motion is also a mechanism of entropy. Each of the molecular collisions converts energy in the molecules to heat, in which form the energy is transferred to the surroundings. PROBABILITY RULES Everything that can happen will happen — some of the time. Other times, it won’t. The proportion of times it does happen depends on a lot of factors. If a solution contains large numbers of each of two molecules required for a reaction, the different types will collide frequently. So, concentration affects the chances that a reaction will actually occur. The higher the solution’s temperature, the more frequently molecules will collide and facilitate the reaction. But almost never will every possible reaction actually happen. Just by chance, some of these molecules won’t meet up with their counterpart molecule. That’s life. The chance, or randomness, can be quantified as probability. As with this hypothetical reaction, so with everything else related to biology and physiology: Probability, not certainty, rules. By the way, the existence of life itself is highly improbable. And the probability of the existence of the uniqueness that is you is more improbable still. POLARITY CHARGES LIFE A molecule is said to be polar when the positive and negative electrical charges are separated between one side of the molecule and the other because of unequal electron sharing. For example, a molecule of water is polar because the oxygen hogs the electrons concentrating the negative charge on the oxygen atom. So the water molecule has a positive charge at one end and a negative charge at the other, similar to a magnet. It attracts and holds other polar molecules. Methane is nonpolar because the carbon shares the electrons with the four hydrogen atoms uniformly. Polarity underlies a number of physical properties of a substance, including surface tension, solubility, and melting and boiling points. In physiology, polarity strongly determines which molecules form bonds and which don’t — like how oil and water don’t mix. More specifically for the study of physiology, lipids and water don’t mix. Living cells use this principle to control the flow of substances into and out of the cell. Lipids are a large and varied group of organic compounds, including fats and oils. All lipids have hydrophobic portions to them — that is, they don’t mix with water. Why not? Because a lipid is nonpolar, so it can’t form bonds with water. Water molecules push nonpolar molecules aside to get closer to other polar molecules. WATER IS SPECIAL Water is arguably the most important molecule in physiology. It accounts for around 60 percent of an adult’s body weight. Water’s strong polarity gives it characteristics that make it uniquely suited to providing its numerous functions. Water has a high specific heat. A substance’s specific heat is the amount of heat required to raise the temperature of 1 gram of the substance 1 degree Celsius. Because water has a high specific heat, it can absorb heat from our active physiological process without increasing the body temperature. The polarity of water also separates molecules from each other; dissolving them. This makes it useful as a method of transport (like in blood). This also makes it an ideal environment for chemical reactions to occur. As such, nearly all our metabolic reactions take place in water. FLUIDS AND SOLIDS Physiological processes, generally speaking, take place in fluids, and the properties of fluids are very important in these processes. In everyday conversation, “fluid” means “liquid,” something that’s usually water-based, like juice, broth, or tea. In physics and chemistry, though, a watery solution is one kind of fluid, whether it’s one you’d care to drink or not. Air is another kind of fluid. Fats are fluids, even when they’re solid: Butter is exactly the same substance whether cold or warm, and so is every other form of fat. Technically speaking, glass and pure metals are fluids! Salt, in contrast, is a solid. Salt (NaCl) crystals flow out of their containers in every kitchen and dining room, yes, but that doesn’t make salt a fluid. It’s got to do with the molecular structure. In solids, atoms are tightly packed together in a geometrically precise formation called a crystalline lattice. Sodium chloride is the model for this: Equal numbers of sodium and chlorine ions, each linked to six other ions, all pull each other in as tightly as the forces of polarity (electrical charge) require and allow. Solids are rigid at the molecular level; once bound together in a crystalline lattice, every atom in the molecule remains in place relative to its surrounding molecules. In fluids, things move around more. Components come together in various ways — carbon dioxide and molecular oxygen (O2) dissolve from air into water and back into air (in the lungs). Fluids take the shape of their container. Air flows into and fills your alveoli. A watery mass in your stomach changes shape with every churning contraction. Gaseous fluids can be easily compressed because the molecules are already so far apart. However, the compressibility of liquids is very limited because the water molecules are already held together just about as tightly as they can be made to go. UNDER PRESSURE Boyle’s law describes the inverse relationship between the volume and pressure of a gas. If nothing else changes, such as temperature, an increase in volume brings about a decrease in pressure. When the pressure drops in a fixed space, it creates a vacuum. The mechanisms of breathing utilize Boyle’s law. When the diaphragm contracts, it increases the volume of the lungs, which decreases the pressure. The vacuum pulls air in through the upper respiratory tract. It’s also a driving force for the cardiac cycle — opening and closing valves to move blood through the chambers of the heart. REDOX REACTIONS TRANSFER ELECTRONS The concept of reduction-oxidation (or redox) reactions is basically this: An electron is transferred from one chemical entity (atom or molecule) to another. The entity that receives the electron is said to be REDuced. The entity that releases the electron is said to be OXidized. In a redox reaction, the reduction of one entity is always balanced by the oxidation of another. The entities are called a redox pair. The redox reaction changes the oxidation state of both entities. In some cases, the oxidized entity undergoes another reaction to acquire another electron. Note that this isn’t a simple reversal of a redox reaction but a new reaction that involves another electron “donor” and frequently requires an enzyme catalyst. Here’s a clever mnemonic to help with the terminology: OIL RIG — Oxidation Is Losing, Reduction Is Gaining (electrons, that is). Source
