Cells perform the functions of life through various chemical reactions. Metabolism is a term referring to all of the chemical reactions that take place within each cell. Catabolic reactions involve breaking down polymers into monomers, and anabolic reactions involve building up polymers from their monomers. Catabolic pathways of cellular respiration break glucose down and release energy. Anabolic pathways like photosynthesis build glucose require energy.

Anabolic pathways require energy to synthesize larger molecules. Catabolic pathways generate energy by breaking down larger molecules. Both pathway types are required for maintaining a cell’s energy balance.

Energy comes in many different forms. Objects in motion display kinetic energy. Objects that are not in motion with the potential to do work display potential energy. Kinetic energy is the energy of motion. Potential energy is stored energy. Chemical energy is a type of potential energy stored in chemical bonds. Chemical reactions either absorb or release energy. 

The term system refers to matter and its environment involved in energy transfers. Everything outside of the system is the surroundings. Single cells are biological systems. The laws of thermodynamics are a series of laws that describe the properties and processes of energy transfer. The first law states that the total amount of energy in the universe is constant. This means that energy cannot be created or destroyed, only transferred or transformed. The second law of thermodynamics states that entropy is increasing. Every energy transfer involves some loss of energy in an unusable form, such as heat energy, resulting in a more disordered system. In other words, no energy transfer is completely efficient, and all transfers trend toward disorder. The laws are stated below.

1. Energy cannot be created or destroyed.
2. Entropy is increasing. 

Entropy is a measure of randomness or disorder in a system. Gases have higher entropy than liquids, and liquids have higher entropy than solids.

ATP is the primary energy-supplying molecule for living cells. ATP is comprised of a nucleotide, a five-carbon sugar, and three phosphate groups. ATP is the cells’ primary energy currency like money is the currency people exchange for things they need. ATP powers the majority of energy-requiring cellular reactions. ATP is adenosine triphosphate, because it contains three phosphates. ADP is a diphosphate, containing two phosphates. Like most chemical reactions, ATP to ADP hydrolysis is reversible. The reverse reaction regenerates ATP from ADP + P_i. Cells rely on ATP regeneration just as people rely on regenerating spent money through some sort of income. Since removal of a phosphate from ATP releases energy, ATP regeneration must require an input of free energy. Most energy transformations in organisms occur in oxidation-reduction reactions involving the transfer of electrons.

ATP is the cell’s primary energy currency. It has an adenosine backbone with three phosphate groups attached.

Enzymes are chemical catalysts that accelerate chemical reactions by lowering their activation energy, which increases the rate of reaction. Enzymes are usually proteins consisting of one or more polypeptide chains. Enzymes have an active site providing a unique chemical environment perfectly suited to convert particular chemical reactants for that enzyme called substrates, into unstable intermediates called transition states. Enzymes and substrates bind with an induced fit, which means enzymes undergo slight conformational changes based on the location of their electrons.

Each enzyme has a specific shape allowing it to accept only one type of substrate. The figure below shows a substrate fitting the active site of the enzyme. Because enzymes are proteins, their structure is impacted by pH, temperature and other conditions. Optimal conditions result in greater enzyme activity. A protein that is not folded correctly is termed denatured and cannot function properly. Some enzymes need cofactors or coenzymes to function. Homeostasis requires regulation of enzymes. Inhibitors turn enzymes off either temporarily or permanently. Competitive inhibitors fit in the active site. Allosteric inhibitors alter the shape of the active site to prevent binding, sometimes called non-competitive inhibitors. To see moving images demonstrating enzyme reactions, look for enzyme gifs like this one (https://www.mrdubuque.com/home/biodub-my-gifs-to-you-enzyme-reactions).

According to the induced-fit model, both enzyme and substrate undergo dynamic conformational changes upon binding. The enzyme contorts the substrate into its transition state, thereby increasing the reaction’s rate.

When a chemical pathway is repeated, end products start to build up. Some enzymes get turned off (inhibited) by the end product of the reaction. When there is a buildup of this end product, the enzyme is allosterically inhibited until there is a need for the process to begin again because the supply of the end product has been used.

Enzymes are not randomly located in the cell, but they are localized where needed. Enzymes are required for glucose catabolism and ATP generation within mitochondria. Several enzymes needed for protein synthesis surround ribosomes. Alpha-amylase is one enzyme vital to starch metabolism, you can read more about it here.