Energy in Living Systems

Transforming Chemical Energy

Cellular respiration is the process of transforming chemical energy into forms usable by the cell or organism.

Introduction: Cellular Respiration

An electrical energy plant converts energy from one form to another form that can be more easily used. For example, geothermal energy plants start with underground thermal energy (heat) and transform it into electrical energy that will be transported to homes and factories.

Like a generating plant, living organisms must take in energy from their environment and convert it into to a form their cells can use. Organisms ingest large molecules, like carbohydrates, proteins, and fats, and convert them into smaller molecules like carbon dioxide and water. This process is called cellular respiration, a form of catabolism, and makes energy available for the cell to use. The energy released by cellular respiration is temporarily captured by the formation of adenosine triphosphate (ATP) within the cell. ATP is the principle form of stored energy used for cellular functions and is frequently referred to as the energy currency of the cell.

The nutrients broken down through cellular respiration lose electrons throughout the process and are said to be oxidized. When oxygen is used to help drive the oxidation of nutrients the process is called aerobic respiration. Aerobic respiration is common among the eukaryotes, including humans, and takes place mostly within the mitochondria. Respiration occurs within the cytoplasm of prokaryotes. Several prokaryotes and a few eukaryotes use an inorganic molecule other than oxygen to drive the oxidation of their nutrients in a process called anaerobic respiration. Electron acceptors for anaerobic respiration include nitrate, sulfate, carbon dioxide, and several metal ions.

The energy released during cellular respiration is then used in other biological processes. These processes build larger molecules that are essential to an organism's survival, such as amino acids, DNA, and proteins. Because they synthesize new molecules, these processes are examples of anabolism.

Electrons and Energy

The transfer of electrons between molecules via oxidation and reduction allows the cell to transfer and use energy for cellular functions.

Electrons and Energy

The removal of an electron from a molecule via a process called oxidation results in a decrease in the potential energy stored in the oxidized compound. When oxidation occurs in the cell, the electron (sometimes as part of a hydrogen atom) does not remain un-bonded in the cytoplasm. Instead, the electron shifts to a second compound, reducing the second compound (oxidation of one species always occurs in tandem with reduction of another).

The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules via oxidation and reduction is important because most of the energy stored in atoms is in the form of high-energy electrons; it is this energy that is used to fuel cellular functions. The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion: in small packages rather than as a single, destructive burst.

Electron carriers

In living systems, a small class of molecules functions as electron shuttles: they bind and carry high-energy electrons between compounds in cellular pathways. The principal electron carriers we will consider are derived from the vitamin B group, which are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) is derived from vitamin B3, niacin. NAD⁺ is the oxidized form of niacin; NADH is the reduced form after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). It is noteworthy that NAD⁺must accept two electrons at once; it cannot serve as a one-electron carrier.

NAD⁺ can accept electrons from an organic molecule according to the general equation:

RH (Reducing agent) + NAD⁺ (Oxidizing agent) → NADH (Reduced) + R (Oxidized)

When electrons are added to a compound, the compound is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent and NAD⁺ is reduced to NADH. When electrons are removed from a compound, the compound is oxidized. In the above equation, NAD⁺ is an oxidizing agent and RH is oxidized to R. The molecule NADH is critical for cellular respiration and other metabolic pathways.

Similarly, flavin adenine dinucleotide (FAD⁺) is derived from vitamin B₂, also called riboflavin. Its reduced form is FADH₂. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD⁺ and FAD⁺ are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis.

ATP in Metabolism

ATP, produced by glucose catabolized during cellular respiration, serves as the universal energy currency for all living organisms.

ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would lead to excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the "energy currency" of the cell and can be used to fill any energy need of the cell.

ATP Structure and Function

The core of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group. Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP); the addition of a third phosphate group forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and, thus, repel one another when they are arranged in a series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation, releases energy.

Energy from ATP

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H⁺) and a hydroxyl group (OH^-) are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (P_i), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and, like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.

Obviously, energy must be infused into the system to regenerate ATP. In nearly every living thing on earth, the energy comes from the metabolism of glucose. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.

Phosphorylation

When ATP is broken down by the removal of its terminal phosphate group, energy is released and can be used to do work by the cell. Often the released phosphate is directly transferred to another molecule, such as a protein, activating it. For example, ATP supplies the energy to move the contractile muscle proteins during the mechanical work of muscle contraction. Recall the active transport work of the sodium-potassium pump in cell membranes. Phosphorylation by ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, using energy from ATP to pump ions against their electrochemical gradients.

Sometimes phosphorylation of an enzyme leads to its inhibition. For example, the pyruvate dehydrogenase (PDH) complex could be phosphorylated by pyruvate dehydrogenase kinase (PDHK). This reaction leads to inhibition of PDH and its inability to convert pyruvate into acetyl-CoA.

Energy from ATP hydrolysis

The energy from ATP can also be used to drive chemical reactions by coupling ATP hydrolysis with another reaction process in an enzyme. In many cellular chemical reactions, enzymes bind to several substrates or reactants to form a temporary intermediate complex that allow the substrates and reactants to more readily react with each other. In reactions where ATP is involved, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (~P). When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism. This is illustrated by the following generic reaction:

A + enzyme + ATP→[ A enzyme −P ] B + enzyme + ADP + phosphate ion