Living things require energy in order to do cellular work. Fuels like sugar contain stored energy which can be utilized by living things in a process called respiration.
All of an organism's chemical reactions that occur are known as its metabolism. There are different types of chemical reactions. In catabolic reactions, energy is released by breaking bonds of more complex molecules into simpler products. In anabolic reactions, energy is used in chemical reactions where simple molecules are combined to build more complex molecules.
Since energy is needed for some pathways and is released for others, one needs to understand how energy is transformed. Cells can transform energy from one type to another.
Since organisms are open systems, energy and matter can be transferred between organisms and their surrounding environment. The First Law of Thermodynamics states that the energy of the universe is constant. This means it can be transformed from one form to another and it can be transferred, but never created or destroyed.
An example of the first law is when someone burns wood in a fireplace. The energy in the wood is chemical energy, and by the process of combustion, it will be transformed into heat and light energy along with smoke and ashes.
The Second Law of Thermodynamics states that every energy transformation adds to the entropy of the universe. Entropy is a measure of disorder or randomness. Therefore, in the example above, the energy in the log was stored energy in organized chemical bonds, but once it was burned, it produced both heat and light. Most of the increased entropy in the universe is heat (less organized) which is going to dissipate. Every time energy transfers occur, when heat is produced, it adds to the entropy of the universe. Heat energy is unavailable do work once it is released.
During catabolic chemical reactions like respiration, the chemical energy found in the organized bonds of the glucose molecule is broken down to the lower energy products of carbon dioxide and water with the release of heat which is a less organized form of energy.
In order to perform cellular work such as movement, growth, or synthesis, a cell requires energy. By carrying out respiration, a cell will add to the amount of heat being released to the environment and thus will add to the entropy of the universe. This illustrates the relationship between how a cell works and the Second Law of Thermodynamics.