Metabolic processes or metabolic pathways are all of the processes in a cell or living organism that are necessary for life. We will be looking at the two types of metabolic reactions, anabolic and catabolic reactions, as well as the biological catalysts or enzymes needed for these reactions to occur. Anabolic reactions are reactions that are involved in the building of molecules, such as the synthesis of proteins from amino acids, and they require energy to proceed. A reaction that requires energy is known as an endergonic reaction.
Catabolic reactions are reactions that break down molecules, such as the breakdown of sugars during cellular respiration, and they release energy as they proceed. A chemical reaction that releases energy is known as an exergonic reaction. So anabolic reactions use energy and catabolic reactions release energy.
What exactly do I mean by energy? Well, I'm referring to ATP, the unit of energy used by cells. ATP stands for adenosine triphosphate.
It is a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups. ATP contains chemical energy in the bonds of its phosphate groups, so that when a bond is broken and a phosphate ion is released from ATP, energy is released along with it. The molecule that remains is adenosine diphosphate. or ADP.
ADP can then be rephosphorylated by cellular respiration to form ATP. The molecules of ATP and ADP cycle within cells, donating and receiving phosphate ions as they move between exergonic and endergonic reactions. It might be helpful to think of ATP as nature's rechargeable chemical battery.
ATP is a charged state and ADP is a flat state. Although many reactions in living cells would happen spontaneously because of the laws of thermodynamics, most reactions would occur too slowly to be useful to the organism. An enzyme speeds up a reaction by lowering the activation energy of the reaction without being used up in the process. Activation energy is the minimum energy required to start a chemical reaction. Typically, enzymes are proteins and each enzyme is specific to the reaction it catalyzes.
For example, the enzyme lactate dehydrogenase converts lactate to pyruvic acid, while the enzyme glycogen synthase synthesizes glycogen from glucose for storage in the liver. The reactant an enzyme acts on is referred to as the enzyme substrate and it results in the formation of an enzyme substrate complex. In most enzymatic reactions the substrate is held in the active site of the enzyme.
The active site is the specific region of the enzyme usually involving only a few of the enzymes amino acids that binds to the substrate. The active site is the place where the catalysis takes place. In other words, this is where bonds are formed or broken. The specificity of an enzyme is attributed to the specific shape of the active site, and thus changing its shape can result in an enzyme losing its function.
The Calvin cycle is the light independent stage of photosynthesis and is used to build carbohydrates. This stage takes in energy to both molecules, so it is an example of an endergonic and anabolic process. The Calvin cycle, which takes place after energy from the sun has been captured and converted to ATP, is catalyzed by an enzyme called RuBisCO. The main steps in this cycle are, first, RuBisCO combines a 5-carbon sugar, ribulose bisphosphate, or RuBP, with carbon dioxide.
A total of three carbon dioxide molecules are combined with three RuBP molecules. The result is a very unstable compound which splits in half, forming a total of six three-carbon molecules known as three-phosphoglyceric acid or 3PGA. Because this process converts carbon dioxide into organic compounds used by living things, it is known as carbon fixation.
In the second stage of the cycle, ATP and NADPH convert the six molecules of 3-PG-1-ATP into a different three carbon compound, glyceraldehyde 3-phosphate or G3P. One of these G3P molecules exits the cycle and will be used as the starting material to make other organic compounds such as glucose and other carbohydrates. The remaining G3P molecules get recycled into RuBP and the cycle repeats.
It takes six turns of the cycle to make one molecule of glucose. In order to use the chemical energy from carbohydrates, living organisms undergo cellular respiration, a metabolic process. The first step of cellular respiration is glycolysis, the process that breaks down glucose into two molecules of pyruvate, a three-carbon compound, to produce energy. As such, it is a catabolic pathway that is exergonic.
Unlike the Calvin cycle, glycolysis is a straight pathway. That is, the products do not return to form the reactants. Glycolysis consists of many steps, each step catalyzed by a different enzyme. Note that the first half of the glycolysis pathway is endergonic because it takes in two ATP molecules. However, the second half of the pathway produces four ATP molecules.
The net change in ATP is two ATP produced, making the entire glycolysis pathway energy producing and thus exergonic.