ATP: the cell’s energy source and how it works
This article aims to explain what adenosine triphosphate (ATP) is and how it works. The material has been compiled from various sources, including international literature. The author hopes you will not find the text below too confusing.
What is ATP?
Every cell contains adenosine triphosphate (ATP). In terms of its chemical structure, ATP is a nucleotide. Like every nucleotide, it consists of a nitrogenous base (adenine), a carbohydrate (ribose), and a phosphate group. However, an ATP molecule differs significantly from ordinary nucleotides: it contains three phosphate groups instead of one.
ATP hydrolysis and energy release
A cell’s internal environment usually has a near-neutral pH. ATP is present in the cell as a salt rather than an acid. Under these conditions, the OH groups of the phosphate groups are therefore replaced by negatively charged oxygen atoms (–O²). Like charges positioned close together repel each other. In this form, ATP’s molecular structure is unstable. Specific enzymes cause it to undergo hydrolysis, meaning it reacts with a water molecule and breaks down:
ATP + water = ADP + phosphoric acid
The terminal phosphate group forms phosphoric acid, and ATP becomes ADP, or adenosine diphosphate. This reaction releases energy (about 40 kJ per mole of phosphoric acid released).
ATP plays a central role in cellular metabolism. It is the immediate energy source for every cellular function. Movement, biosynthesis, the passage of substances through membranes, electricity generation, and light emission — every form of cellular activity is powered by the energy released through the ATP hydrolysis described above.
A cell’s ATP reserves are limited. In muscle, for example, there is only enough ATP for 20–30 contractions. Yet we know that muscles can work for hours, performing thousands of contractions. Cells must therefore regenerate ATP as they use it. Energy from the breakdown of carbohydrates, lipids, and other substances replenishes ATP reserves. During brief, intense exertion, muscles work almost exclusively on the ATP they contain. You breathe heavily after exertion — during this period, carbohydrates and other substances are broken down, and ATP reserves in your cells are restored. ATP is thus the common, universal energy source for cells.
Glycolysis — anaerobic breakdown
ATP synthesis takes place mainly in the mitochondria. In human cells, glucose provides the energy needed to synthesize ATP. Glucose is broken down in two successive stages. The first is called glycolysis, or anaerobic breakdown, and the second is aerobic breakdown.
Oxygen does not participate in glycolysis, which is why it is called the anaerobic stage. ADP and phosphoric acid, however, play essential roles. A small supply of these substances is always present in the cell because they are continually produced during cellular activity. Glycolysis results in the breakdown of glucose and the synthesis of two ATP molecules.
Glycolysis is a complex, multistep process involving many successive reactions. Each reaction is catalyzed by its own enzyme and produces a small change in the substance’s composition, but the overall difference is considerable: a six-carbon glucose molecule yields two three-carbon organic acid molecules. Each reaction releases a tiny amount of energy, but together they release an impressive total — 200 kJ per mole. Of this, 60% dissipates as heat and 40% is stored in ATP.
Aerobic breakdown in the mitochondria
Glycolysis is followed by the second stage — aerobic breakdown. This involves enzymes, water, oxidizing agents, electron carriers, and molecular oxygen. Intact mitochondrial membranes are the main requirement for aerobic breakdown to proceed normally.
The end product of glycolysis — an organic acid — enters the mitochondria, where enzymes cause it to react with water and break down. The resulting carbon dioxide passes through the mitochondrial membrane and diffuses into the surrounding environment. Enzymes in the membranes oxidize the hydrogen atoms (causing them to lose an electron).
Electrons and hydrogen cations (protons) bind to specialized carrier molecules and move in opposite directions. The electrons move to the inner side of the membrane, where they bind to oxygen.
The cations move to the outer side of the membrane. As a result, the concentration of anions inside the mitochondrion increases, while positively charged particles accumulate outside because the membrane is impermeable to them. As the concentrations of oppositely charged particles on either side of the membrane increase, so does the electrical potential difference.
Molecules of the enzyme that synthesizes ATP are known to be located in certain parts of the membrane. Each enzyme molecule contains a channel through which hydrogen cations can pass. This occurs when the potential difference exceeds a certain critical threshold. The electric field then drives hydrogen cations through the enzyme’s channel to the inner side of the membrane, where they react with oxygen to form water.
The movement of electrons from hydrogen atoms to oxygen and of cations through the channel of the enzyme responsible for ATP synthesis releases a substantial amount of energy. Of this, 45% dissipates as heat and 55% is stored as energy in ATP’s chemical bonds.
Summary
The breakdown of one glucose molecule into carbon dioxide and water enables the synthesis of 38 ATP molecules: 2 during the anaerobic stage and 36 during the aerobic stage. The aerobic process is therefore 18 times more efficient.
The breakdown of organic substances in cells is often compared to combustion because both processes involve the uptake of oxygen and the release of oxidation products. During combustion, all the energy released is converted into heat. During the oxidation of glucose within cells, however, about 45% of the energy released becomes heat and about 55% is stored in ATP.
Author: Mikk-Alvar Olle
Read also: ribose.
Source: WHO – physical activity.
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