The Basics of Metabolism

Metabolic Processes, Energy, and Enzymes

Cells carry out chemical reactions that involve energy and enzymes.
Metabolic processes, or metabolic pathways, are all of the processes in a cell or living organism that are necessary for life. Photosynthesis and glycolysis are just two examples of the many metabolic processes that are critical to life. The set of metabolic processes that break down large molecules is known as catabolism. Catabolism includes breaking down and oxidizing food molecules. Anabolism is the synthesis of complex molecules from simpler ones. In addition to building molecules, anabolic reactions lead to the storage of energy. The purpose of catabolic reactions is to provide the energy and components needed for anabolic reactions. In living things, chemical reactions that occur spontaneously give off energy to their surroundings.

Most reactions that occur in organisms require the help of enzymes in order for a reaction to take place. An enzyme is the biological catalyst that increases the rate of a chemical reaction and causes specific biochemical reactions within a metabolic process.

Although a metabolic process generally refers to a specific process within a cell or living organism, a metabolic pathway is a series of intertwined chemical reactions in which the product of one enzyme acts as the substrate for the next.

Endergonic and Exergonic Reactions

Metabolic reactions require an input of energy or they produce energy.
The two forms of metabolic processes are anabolic reactions (those that build molecules) and catabolic reactions (those that break down molecules).

Anabolic reactions require energy to proceed. A chemical reaction that requires an input of energy to proceed is known as an endergonic reaction. Therefore, anabolic reactions are endergonic reactions. An example of an anabolic reaction is the synthesis of protein.

Catabolic reactions release energy as they proceed. A chemical reaction that releases energy is known as an exergonic reaction. Therefore, catabolic reactions are exergonic reactions. Cellular respiration is an example of a catabolic reaction.


The biological unit of energy is ATP.
In living things, the unit of energy is a molecule called adenosine triphosphate, or ATP. ATP 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. When a phosphate ion is released from ATP, energy is released along with it. The molecule that remains is adenosine diphosphate, or ADP. The formation and breaking of the bonds in these molecules provides the energy that drives the processes of life.
The conversion of adenosine triphosphate (ATP), a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups, to adenosine diphosphate (ADP) uses water and releases energy and a phosphate ion. The reverse reaction consumes energy and a phosphate ion.
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 the "charged" state and ADP is the "flat" state.


Enzymes increase the rate of reactions.
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, also called a biological catalyst, 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 made of long chains of amino acids. Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. Each enzyme is specific to the reaction it catalyzes; that is, each enzyme performs a single, specific task. 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 enzyme's active site is the place where the chemical reaction occurs, and the kind of molecule that reacts with an enzyme is known as its substrate. The active site consists of a specific sequence of amino acids that help the reaction by forming and breaking bonds with the molecule with which the enzyme interacts. The physical shape of the enzyme's active site aids in this specificity. The amino acids that are found in the active site and their location, order, and orientation give the active site a very specific size, shape, and chemical behavior. Because of these amino acids, an enzyme's active site is entirely unique and will only bind to a specific target—the enzyme's substrate or substrates.

The Calvin Cycle

The Calvin cycle creates the products plants need using ATP and NADPH.
Photosynthesis, a metabolic process, is the pathway through which plants capture light energy and generate chemical energy in the form of glucose. There are two major stages in photosynthesis: light-dependent reactions, which use light to make ATP and NADPH, and the Calvin cycle, which is a light-independent reaction. Plants synthesize carbohydrates from carbon dioxide (CO2) and organic molecules in a process called the Calvin cycle (also known as the Calvin-Benson cycle). The primary function of this final step of photosynthesis is to create what the plant needs (glucose, protein, and lipids), using ATP and nicotinamide adenine dinucleotide phosphate (NADPH), the products of the light reactions of photosynthesis. This cycle takes in energy, so it is an endergonic process. It also builds a carbohydrate molecule, so it is an 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 (ribulose-1,5-bisphosphate carboxylase/oxygenase).

  1. In the first stage of the cycle, RuBisCO combines three 5-carbon molecules (ribulose bisphosphate, or RuBP) with three carbon dioxide molecules and ATP to produce six molecules of a 3-carbon compound (3-phosphoglyceric acid, or 3-PGA). Because this process converts carbon dioxide into organic compounds used by living things, it is known as carbon fixation.
  2. In the second stage of the cycle, ATP and NADPH convert the six molecules of 3-PGA into a different 3-carbon compound, glyceraldehyde 3-phosphate (G3P). One of these G3P molecules exits the cycle to be made into a sugar.
  3. The remaining G3P molecules get recycled into RuBP, and the cycle repeats.
The Calvin cycle is the light-independent second stage of photosynthesis. In the first step, atmospheric carbon is combined with RuBP to make 3-phosphoglycerate. In the second step, 3-phosphoglycerate is reduced to G3P, a 3-carbon sugar. In the third step, RuBP is regenerated. It takes six turns of the cycle to make a single glucose molecule. Plants need glucose for energy.
The sugar produced is glucose, a 6-carbon sugar, so it takes six rounds of the Calvin cycle to produce one molecule of glucose (one for each carbon dioxide molecule fixed by the cycle). Glucose is a basic sugar used for energy by many living things.


Glycolysis is the first step in cellular respiration.
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 3-carbon compound, to produce energy. As such, it is a catabolic pathway that is exergonic. Glycolysis is the first step in the process of cellular respiration, which many organisms use to obtain the energy they need.

Steps of Glycolysis

Glycolysis, the first step of cellular respiration, breaks down glucose, producing ATP, a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups. The first half of the reaction consumes two ATP molecules, but the second half produces four ATP molecules, so the net change is two ATP molecules produced.
Unlike the Calvin cycle, glycolysis is a straight pathway. That is, the products do not return to form the reactants. Further, 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.