2.2: Thermodynamics
Thermodynamics is the branch of science that studies how energy is transformed from one form to another. We study thermodynamics in biology because organisms are involved in many energy transactions. In other words, organisms are thermodynamic systems. Biochemical reactions must follow the laws of thermodynamics to predict whether reactions will occur spontaneously, or without any energy required. For example, living things need the ability to move. Energy gives this power, but it must be harnessed and transformed from one form of energy to another. Living things need usable forms of energy.
Two of the four laws of thermodynamics are especially important in biology:
- The first law of thermodynamics states that energy cannot be created or destroyed, although it can be transferred and transformed. This is also known as the law of conservation of energy.
- The second law of thermodynamics states that every energy transaction increases the entropy (disorder) of the universe. This second law implies that every energy transaction involved some loss of usable energy as heat, so no energetic process (including those occurring in organisms) can ever be perfectly efficient.
We will review thermodynamics again in Unit 5 when we study metabolism and metabolic pathways.
Energy is a basic process common among all living organisms. We define energy as the capacity to do work, which refers to some sort of change. For example, moving an object from one place to another requires work, and energy is required for that work. Heat is energy in the form of the movement of particles (atoms, ions, or molecules) within a substance.
Heat is energy that is unavailable for performing work. Temperature is a measure of the average speed of the particles in an object. Temperature and heat are not the same thing. Temperature does not depend on how much matter is present, whereas heat does.
For example, a swimming pool has the same temperature as a cup of water from that swimming pool, but the swimming pool contains much more heat than the cup of water because it contains much more matter.
Read this section to learn how energy flows through a living system and how enzymes catalyze chemical reactions.
Energy can be readily converted between forms. For example, a book that falls from a shelf converts potential energy into kinetic energy. When a person moves the book back to the shelf, they convert kinetic energy into potential energy. The metabolism of life involves countless interactions between matter and energy and countless conversions between energy forms, so it is important to understand the distinction between matter and energy.
Watch this video, which offers an overview of the concept of energy and discusses how it relates to atoms and molecules. All atoms and molecules have kinetic energy or movement. We measure kinetic energy as temperature.
Watch this lecture, which explains the importance of the first law of thermodynamics to living organisms. Many scientists call the first law of thermodynamics the law of conservation of energy since It states that energy can be neither created nor destroyed, but it may change form. For example, imagine a campfire: the energy is stored in chemical bonds in the wood and is released as light and heat.
After watching the video, make sure you can identify the different forms of energy and how they are transferred and transformed.
Watch this video for a discussion of the Gibbs Law of Free Energy. After you have read this section, you should be able to write and define each component (G, H, S, T) of the equation for free-energy change, distinguish between exergonic and endergonic reactions in terms of free energy change, predict which reactions are spontaneous, and explain why organisms do not violate the second law of thermodynamics.