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Krishna: Atomic level? Hmmm.

This is a bit complicated. What will individual atoms do with energy in a complex living body? What has it got to do with the energy the cells require?

We study energy requirements at cellular and molecular levels. Okay some atoms might require energy to take part in reactions.

According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made.

An atom changes from a ground state to an excited state by taking on energy from its surroundings in a process called absorption. The electron absorbs the energy and jumps to a higher energy level. In the reverse process, emission, the electron returns to the ground state by releasing the extra energy it absorbed.

Cells require a constant supply of energy to generate and maintain the biological order that keeps them alive. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells. Sugars are particularly important fuel molecules, and they are oxidized in small steps to carbon dioxide (CO2) and water.

The breakdown, or catabolism of sugars produce ATP, NADH, and other activated carrier molecules in animal cells. Glucose breakdown is more important, since it dominates energy production in most animal cells. A very similar pathway also operates in plants, fungi, and many bacteria. Other molecules, such as fatty acids and proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways (1).

Schematic representation of the controlled stepwise oxidation of sugar in a cell

credit: Molecular Biology of the Cell. 4th edition.

Alberts B, Johnson A, Lewis J, et al.

Food is broken down in three steps to produce ATP:

The proteins, lipids, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either as a source of energy or as building blocks for other molecules. The breakdown processes must act on food taken in from outside, but not on the macromolecules inside our own cells. Stage 1 in the enzymatic breakdown of food molecules is therefore digestion, which occurs either in our intestine outside cells, or in a specialized organelle within cells, the lysosome. (A membrane that surrounds the lysosome keeps its digestive enzymes separated from the cytosol) In either case, the large polymeric molecules in food are broken down during digestion into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol—through the action of enzymes. After digestion, the small organic molecules derived from food enter the cytosol of the cell, where their gradual oxidation begins. As illustrated in Figure 2-70, oxidation occurs in two further stages of cellular catabolism: stage 2 starts in the cytosol and ends in the major energy-converting organelle, the mitochondrion; stage 3 is entirely confined to the mitochondrion. (1)

In stage 2 a chain of reactions called glycolysis converts each molecule of glucose into two smaller molecules of pyruvate. Sugars other than glucose are similarly converted to pyruvate after their conversion to one of the sugar intermediates in this glycolytic pathway. During pyruvate formation, two types of activated carrier molecules are produced—ATP and NADH. The pyruvate then passes from the cytosol into mitochondria. There, each pyruvate molecule is converted into CO2 plus a two-carbon acetyl group—which becomes attached to coenzyme A (CoA), forming acetyl CoA, another activated carrier molecule . Large amounts of acetyl CoA are also produced by the stepwise breakdown and oxidation of fatty acids derived from fats, which are carried in the bloodstream, imported into cells as fatty acids, and then moved into mitochondria for acetyl CoA production.

Stage 3 of the oxidative breakdown of food molecules takes place entirely in mitochondria. The acetyl group in acetyl CoA is linked to coenzyme A through a high-energy linkage, and it is therefore easily transferable to other molecules. After its transfer to the four-carbon molecule oxaloacetate, the acetyl group enters a series of reactions called the citric acid cycle. As we discuss shortly, the acetyl group is oxidized to CO2 in these reactions, and large amounts of the electron carrier NADH are generated. Finally, the high-energy electrons from NADH are passed along an electron-transport chain within the mitochondrial inner membrane, where the energy released by their transfer is used to drive a process that produces ATP and consumes molecular oxygen (O2). It is in these final steps that most of the energy released by oxidation is harnessed to produce most of the cell's ATP. (1)

Because the energy to drive ATP synthesis in mitochondria ultimately derives from the oxidative breakdown of food molecules, the phosphorylation of ADP to form ATP that is driven by electron transport in the mitochondrion is known as oxidative phosphorylation.

Through the production of ATP, the energy derived from the breakdown of sugars and fats is redistributed as packets of chemical energy in a form convenient for use elsewhere in the cell. Roughly 109 molecules of ATP are in solution in a typical cell at any instant, and in many cells, all this ATP is turned over (that is, used up and replaced) every 1–2 minutes.

It is in the last step in the degradation of a food molecule that the major portion of its chemical energy is released. In this final process the electron carriers NADH and FADH2 transfer the electrons that they have gained when oxidizing other molecules to the electron-transport chain, which is embedded in the inner membrane of the mitochondrion. As the electrons pass along this long chain of specialized electron acceptor and donor molecules, they fall to successively lower energy states. The energy that the electrons release in this process is used to pump H+ ions (protons) across the membrane—from the inner mitochondrial compartment to the outside . A gradient of H+ ions is thereby generated. This gradient serves as a source of energy, being tapped like a battery to drive a variety of energy-requiring reactions. The most prominent of these reactions is the generation of ATP by the phosphorylation of ADP.(1)

At the end of this series of electron transfers, the electrons are passed to molecules of oxygen gas (O2) that have diffused into the mitochondrion, which simultaneously combine with protons (H+) from the surrounding solution to produce molecules of water. The electrons have now reached their lowest energy level, and therefore all the available energy has been extracted from the food molecule being oxidized. This process is termed ‘oxidative phosphorylation’.

And finally the answer to your Q, if the energy is needed by the cell, molecules inside the cells, and even atoms inside the cells, they utilize this energy.

Instead of ‘nourish the atoms’, it would be appropriate to say ‘nourish the cell or cells’ in a living system. But yes, individual atoms might too ‘use this energy’ to take part in reactions in complex molecules and to even form molecules.

Footnotes:

  1. How Cells Obtain Energy from Food

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