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Metabolism: the story on HearLore | HearLore
Metabolism
The word metabolism comes from the ancient Greek metabole, meaning change, yet it describes the most constant process in existence. Every second, trillions of chemical reactions occur within the human body, converting food into energy, building tissues, and eliminating waste without conscious thought. This invisible machinery allows organisms to grow, reproduce, and respond to their environments, functioning as the fundamental engine of life itself. The three main functions of metabolism are the conversion of energy in food into a usable form for cellular processes, the conversion of food to building blocks of macromolecules such as proteins, lipids, nucleic acids, and some carbohydrates, and the excretion of metabolic wastes. These enzyme-catalyzed reactions allow organisms to maintain their structures and respond to their environments, creating a dynamic equilibrium that defines life.
The Universal Language of Chemistry
A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species, from the unicellular bacterium Escherichia coli to huge multicellular organisms like elephants. The set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, suggesting these pathways appeared early in evolutionary history and were retained due to their efficacy. This universal language of chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions, creating a bridge between catabolism and anabolism. The most common set of catabolic reactions in animals can be separated into three main stages, where large organic molecules are digested into smaller components, taken up by cells, and converted to smaller molecules like acetyl coenzyme A, which releases some energy before being oxidized to water and carbon dioxide during the citric acid cycle and electron transport chain.
The Dance of Energy and Matter
Adenosine triphosphate, or ATP, serves as the energy currency of cells, acting as a bridge between catabolism and anabolism. There is only a small amount of ATP in cells, but as it is continuously regenerated, the human body can use about its own weight in ATP per day. Catabolic reactions generate ATP, while anabolic reactions consume it, creating a cycle of energy transfer that powers every cellular process. In oxidative phosphorylation, the electrons removed from organic molecules in areas such as the citric acid cycle are transferred to oxygen, and the energy released is used to make ATP. This process is done in eukaryotes by a series of proteins in the membranes of mitochondria called the electron transport chain, which uses the energy from reduced molecules like NADH to pump protons across a membrane, creating a proton concentration difference that drives protons back into the mitochondrion through the base of an enzyme called ATP synthase.
Common questions
What is the definition of metabolism in biological terms?
Metabolism describes the set of chemical reactions that occur within organisms to convert food into energy, build tissues, and eliminate waste. This process functions as the fundamental engine of life that allows organisms to grow, reproduce, and respond to their environments.
When were the first controlled experiments in human metabolism published?
The first controlled experiments in human metabolism were published by Santorio Santorio in 1614 in his book Ars de statica medicina. He described how he weighed himself before and after eating, sleeping, working, and excreting to measure food loss.
What is the role of adenosine triphosphate in cellular energy transfer?
Adenosine triphosphate serves as the energy currency of cells and acts as a bridge between catabolism and anabolism. The human body uses about its own weight in adenosine triphosphate per day as it is continuously regenerated to power every cellular process.
Which four basic classes of molecules make up most animal and plant structures?
Most structures that make up animals, plants, and microbes are made from four basic classes of molecules: amino acids, carbohydrates, nucleic acids, and lipids. Proteins are made of amino acids, carbohydrates store energy, nucleic acids store genetic information, and lipids form biological membranes.
Who discovered enzymes and when did this separate metabolism from cell biology?
Eduard Buchner discovered enzymes at the beginning of the 20th century and separated the study of chemical reactions of metabolism from the biological study of cells. This discovery marked the beginnings of biochemistry and allowed scientists to study metabolic reactions independently of whole cells.
Most of the structures that make up animals, plants, and microbes are made from four basic classes of molecules: amino acids, carbohydrates, nucleic acids, and lipids. Proteins are made of amino acids arranged in a linear chain joined by peptide bonds, and many proteins are enzymes that catalyze the chemical reactions in metabolism. Carbohydrates are the most abundant biological molecules, filling numerous roles such as the storage and transport of energy and structural components. Nucleic acids, including DNA and RNA, are critical for the storage and use of genetic information, and their interpretation through the processes of transcription and protein biosynthesis. Lipids are the most diverse group of biochemicals, with their main structural uses as part of internal and external biological membranes such as the cell membrane, and their chemical energy can also be used.
The History of a Changing World
The history of the scientific study of metabolism spans several centuries and has moved from examining whole animals in early studies to examining individual metabolic reactions in modern biochemistry. The first controlled experiments in human metabolism were published by Santorio Santorio in 1614 in his book Ars de statica medicina, where he described how he weighed himself before and after eating, sleep, working, sex, fasting, drinking, and excreting. He found that most of the food he took in was lost through what he called insensible perspiration. In the 19th century, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that fermentation was catalyzed by substances within the yeast cells he called ferments. It was the discovery of enzymes at the beginning of the 20th century by Eduard Buchner that separated the study of the chemical reactions of metabolism from the biological study of cells, and marked the beginnings of biochemistry.
The Regulation of Life's Rhythm
As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain a constant set of conditions within cells, a condition called homeostasis. Two closely linked concepts are important for understanding how metabolic pathways are controlled: the regulation of an enzyme in a pathway and the control exerted by this enzyme. A very well understood example of extrinsic control is the regulation of glucose metabolism by the hormone insulin. Insulin is produced in response to rises in blood glucose levels, and binding of the hormone to insulin receptors on cells then activates a cascade of protein kinases that cause the cells to take up glucose and convert it into storage molecules such as fatty acids and glycogen. The metabolism of glycogen is controlled by activity of phosphorylase, the enzyme that breaks down glycogen, and glycogen synthase, the enzyme that makes it, with these enzymes regulated in a reciprocal fashion.
The Evolution of Metabolic Pathways
The central pathways of metabolism described above, such as glycolysis and the citric acid cycle, are present in all three domains of living things and were present in the last universal common ancestor. This universal ancestral cell was prokaryotic and probably a methanogen that had extensive amino acid, nucleotide, carbohydrate, and lipid metabolism. The retention of these ancient pathways during later evolution may be the result of these reactions having been an optimal solution to their particular metabolic problems, with pathways such as glycolysis and the citric acid cycle producing their end products highly efficiently and in a minimal number of steps. Many models have been proposed to describe the mechanisms by which novel metabolic pathways evolve, including the sequential addition of novel enzymes to a short ancestral pathway, the duplication and then divergence of entire pathways, and the recruitment of pre-existing enzymes and their assembly into a novel reaction pathway.
The Future of Metabolic Science
Classically, metabolism is studied by a reductionist approach that focuses on a single metabolic pathway, but it is now possible to use genomic data to reconstruct complete networks of biochemical reactions and produce more holistic mathematical models that may explain and predict their behavior. A major technological application of this information is metabolic engineering, where organisms such as yeast, plants, or bacteria are genetically modified to make them more useful in red biotechnology and aid the production of drugs such as antibiotics or industrial chemicals such as 1,3-propanediol and shikimic acid. These genetic modifications usually aim to reduce the amount of energy used to produce the product, increase yields, and reduce the production of wastes. Using these techniques, a model of human metabolism has now been produced, which will guide future drug discovery and biochemical research, and these models are now used in network analysis to classify human diseases into groups that share common proteins or metabolites.