Understanding the Cell covers the fundamental concepts of cell biology at the level taught in introductory college biology courses. For each section, we'll introduce key words or concepts. Each of these terms will be discussed, then reviewed, prior to moving on. Our first key words are: metabolism, the first and second laws of thermodynamics, energy, work, order, open system, free energy, heat energy, and entropy.
Metabolism is the set of chemical reactions that occur in living things. Metabolic reactions turn food into stored chemical energy while others draw upon this energy to perform the routine tasks of life. For instance, all the chemical components necessary for building a cat are found in cat food. Metabolism deals with how a kitten converts cat food into cat.
Living things are amazingly intricate chemical machines that capture energy from their environment and use that energy to pay for the work they perform. Work is any process that requires energy. For cells, work includes growth, repair, reproduction, movement, and the capture of food. Cells pay for their work with the chemical energy captured from digested food. Food is packed with chemical energy. The digestion of food causes the release of food's chemical energy, some of which the cell uses to produce small, specialized energy-carrying chemicals. These chemicals are then used by the cell to pay for the work that it performs. As a cell performs work, its energy-carrying chemicals are converted into other chemicals and structures that, themselves, may retain a measure of usable energy. But work is always associated with the conversion of some usable chemical energy into heat energy, which cells cannot use for work. Consequently, it's critical that cells be open systems. Open systems can acquire energy from their environments to pay for the work they perform. If cells could not acquire energy from their environments in the form of food they would be closed systems, doomed to convert all of their usable internal energy into heat as they perform work. As open systems, cells bring in food energy and release heat energy by convection, resulting in a two-way energy transfer between the cell and its environment. Open systems also exchange matter with their environment. Food represents matterjust as it represents chemical energy. When cells digest food, it's broken down and some of the breakdown products are returned to the environment as waste. So, as open systems, cells must exchange both energy and matter with their environment if they are to continue performing work. We've actually been discussing thermodynamics, a branch of physics and chemistry that deals with energy conversion and heat. The first law of thermodynamics states that a closed system performing work will exhaust its usable internal energy by producing unusable heat energy as it performs work. As we've discussed, cells too lose usable energy to heat production but, nonetheless, maintain usable internal energy. They do so by being open systems that acquire energy from their environment. The second law of thermodynamics states that the amount of order in a closed system is always declining, through decay of order into disorder. Again, cells can resist decay and preserve internal order because they're open systems. They acquire energy from their environment, in the form of food, and use this energy to pay for the work of repairing and replacing cellular components lost to decay. Cells acquire energy in the form of food. Food is matter possessing a high degree of order. When cells digest food, they destroy its order by breaking it down into small pieces. It is precisely the loss of order in food that causes the release of food energy that cells then capture. Food energy is captured in the production of energy-carrying chemicals that, themselves, represent a new form of order.
Consider, now, both the cell and its environment. As food is digested the net result is an increase in disorder because conversions of food energy and order into new forms of cellular chemical energy and order are never 100% efficient - some food energy is lost as heat energy. Consequently, a cell always destroys more order in its food than it can build from the capture of food energy. Actually, a cell's internal energy is classified as either free energy, which can be used for work, or as heat energy, which cannot. Digestion of food converts the chemical energy of food into both free energy and heat energy. Free energy is that part of food energy captured by the production of small, highly reactive, energy-carrying chemicals later used to pay for the work of building order. Let's develop the concept of order. A drop of water on a desk top in a dry room represents an enormous amount of order because, in general, water molecules are dispersed in the air. As the drop evaporates order is lost as individual water molecules escape from the drop's surface to whiz about the room chaotically and independently of other molecules. Order, then, is nonrandomness in the location or concentration of matter, including cellular chemicals. For example, when a cell consumes food it is concentrating particular chemicals, thereby, creating order. Also, order is a constraint or confinement on the ability of things to move about chaotically. This kind of order is created when cells use small chemical building blocks derived from food digestion to synthesize and repair the large chemicals of which they're largely composed. Starch is the major form of food in a potato. It consists of hundreds of sugar molecules linked together. Starch represents order because the linkage of sugar molecules prevents their independent movement and because each sugar represents a collection of reactive chemical groups. When starch is digested, it's first broken down into individual sugar molecules. Further digestion entails the breakdown of each sugar into tiny carbon dioxide molecules. Through the destruction of the order in starch, cells can capture energy for the work of rebuilding, repairing, growing, and reproducing - all representing new forms of order. The measure of disorder in a system is called entropy. Scientists symbolize entropy with the letter S. Since disorder always increases, the change in entropy, denoted as dS, is positive for all the reactions of metabolism. When a cell performs the work of repairing itself, growing, accumulating food, reproducing, or moving, free energy is consumed. Scientists symbolize free energy with the letter G. All metabolic reactions are associated with a net loss of free energy. In other words, dG, the change in free energy, is negative. Let's review the key terms: metabolism, work, energy, free energy, heat energy, the laws of thermodynamics, open systems, order, and entropy. Metabolism is all the chemical reactions performed by a cell. Some reactions break food down and capture food energy by producing highly reactive chemicals while other metabolic reactions use the energy of these reactive chemicals to perform work. Work is the process of building and maintaining order. Work must be paid for as it costs energy to perform work. Energy cannot be created but it can be converted from one form to another. A cell's internal energy consists of two basic types: free energy, which can be used for work, and heat energy which cannot be used for work. Every time a cell does work, some of its chemical energy is lost as heat energy. The first law of thermodynamics states that a system performing work must continually acquire energy from the outside because, as it performs work, its usable energy is converted into unusable heat energy. The second law of thermodynamics states that order is always declining. Therefore, if a system is to remain ordered it must acquire energy from an outside source to pay for the work of keeping that order. Open systems exchange both matter and energy with their environment. Cells are open systems. A cell brings in food matter from the environment, which represents chemical energy, and gives back heat energy. Order is the opposite of both chaotic motion and the randomness in the location of things. A cell accumulates building block chemicals then assembles them into larger, functional chemical products. Both of these actions establish order. Entropy is a measure of disorder. The next set of key words are: anabolism, catabolism, coupling, and ATP. Metabolism represents two very different kinds of chemical processes, anabolism and catabolism. They are nearly the opposites of each other. The adjective anabolic refers to building. Anabolic steroids are hormones used by some athletes to build muscle mass. Similarly, anabolism is the set of cellular chemical reactions that build. When a cell grows, reproduces, or replaces molecules lost to decay, it is building large to huge chemicals from small chemical building blocks. Anabolism builds order.