Bio 112 Chapter 7 SIMPLIFIED

Energy in Living Systems

Cells primarily manage energy through the transfer of electrons rather than by storing free energy directly. This transfer occurs via oxidation-reduction (redox) reactions. In these reactions, a molecule that loses an electron is considered oxidized and consequently loses potential energy. Conversely, a molecule that gains an electron is reduced and gains potential energy. To facilitate this movement, cells utilize electron carriers, specifically $NAD^+/NADH$ and $FAD/FADH_2$. These carriers shuttle high-energy electrons between various metabolic reactions, allowing the cell to utilize energy incrementally rather than in a single, uncontrollable burst. The chemical forms $NADH$ and $FADH_2$ represent the “loaded” or reduced states that carry extra energy, while $NAD^+$ and $FAD$ represent the “empty” or oxidized states, which are prepared to accept new electrons.

Adenosine triphosphate ($ATP$) serves as the primary short-term energy currency for the cell. Because cells cannot store large amounts of free energy without risking internal damage, $ATP$ allows for the distribution of energy in small, manageable amounts. The molecule consists of adenosine bonded to three phosphate groups. Energy is released for cellular work—such as powering muscle contractions or operating the sodium-potassium pump—when the bond to the terminal phosphate group is broken, converting $ATP$ into adenosine diphosphate ($ADP$). The process of charging the molecule from $ADP$ to $ATP$ requires the addition of a phosphate, while discharging it from $ATP$ back to $ADP$ involves removing one. Most cellular $ATP$ is regenerated through the breakdown of glucose via two mechanisms: substrate-level phosphorylation, which is a direct transfer of a phosphate group to $ADP$, and oxidative phosphorylation through chemiosmosis, which accounts for approximately $90$ percent of total $ATP$ production.

Glycolysis

Glycolysis is a linear biochemical pathway that occurs in the cytoplasm of nearly all living cells and does not require oxygen to proceed. The overall result of this process is the breakdown of one $6$-carbon glucose molecule into two $3$-carbon molecules known as pyruvate. The pathway is divided into two distinct stages: the energy investment phase and the energy payoff phase. During the investment phase, the cell spends $ATP$ to chemically split the glucose molecule in half. In the subsequent payoff phase, energy is extracted in the form of $ATP$ and $NADH$.

In terms of molecular output, the total production across both phases of glycolysis includes $4$ molecules of $ATP$ and $2$ molecules of $NADH$. However, because $2$ molecules of $ATP$ are consumed during the initial investment phase, the net gain for the cell is $2$ molecules of $ATP$ and $2$ molecules of $NADH$ per glucose molecule. If oxygen is unavailable to the cell, the $NADH$ produced is recycled back into $NAD^+$ through the process of fermentation to allow glycolysis to continue, rather than being used for further $ATP$ production in the mitochondria.

Oxidation of Pyruvate and the Citric Acid Cycle

There is a fundamental structural difference between linear pathways like glycolysis and circular pathways like the citric acid cycle. Glycolysis proceeds in a straight line, beginning with glucose and ending with pyruvate without repeating. In contrast, the citric acid cycle is circular; it begins by attaching an acetyl group to a molecule called oxaloacetate. By the conclusion of the cycle, oxaloacetate is regenerated, allowing the cycle to repeat continuously. This circular nature provides metabolic flexibility, as intermediate molecules within the cycle can be diverted to synthesize other compounds, such as amino acids, without completely halting the energy-producing pathway.

Before entering the citric acid cycle, the pyruvate produced in the cytoplasm must be prepared within the mitochondria. During this preparation, one carbon atom is removed from pyruvate and released as carbon dioxide ($CO_2$). The remaining $2$-carbon fragment is oxidized, transferring electrons to $NAD^+$ to create $NADH$. This $2$-carbon piece then attaches to coenzyme A to form acetyl $CoA$. Acetyl $CoA$ then delivers the acetyl group into the citric acid cycle by combining it with oxaloacetate to form a $6$-carbon molecule called citrate.

Oxidative Phosphorylation

Oxidative phosphorylation involves the movement of electrons through the electron transport chain (ETC), a series of protein complexes and carrier molecules located in the inner mitochondrial membrane. Electron carriers $NADH$ and $FADH_2$ deposit their electrons at the beginning of this chain. $NADH$ enters the chain at Complex I, whereas $FADH_2$ enters later at Complex II. Because $FADH_2$ enters at a later stage, its electrons contribute to the production of less $ATP$ compared to those from $NADH$. As electrons move through the complexes, they lose energy incrementally. This released energy is used to pump hydrogen ions ($H^+$) across the membrane. At the end of the chain, the electrons combine with $H^+$ ions and oxygen to form water ($H_2O$), establishing oxygen as the final electron acceptor.

The pumping of $H^+$ ions from the mitochondrial matrix into the intermembrane space creates a concentration and charge difference known as an electrochemical gradient. Because $H^+$ ions cannot pass through the lipid core of the membrane independently, they must flow back into the matrix through a specialized channel called $ATP$ synthase. This protein functions like a turbine; the flow of $H^+$ ions causes $ATP$ synthase to spin, providing the mechanical energy necessary to attach a phosphate group to $ADP$, thereby synthesizing $ATP$. This process, known as chemiosmosis, is responsible for approximately $90$ percent of the $ATP$ generated during glucose catabolism.

Metabolism without Oxygen

When oxygen is absent, cells must use either anaerobic cellular respiration or fermentation to regenerate $NAD^+$ from $NADH$, ensuring that glycolysis can persist. The fundamental difference between the two lies in the final electron acceptor used. Fermentation utilizes an organic molecule—often pyruvate itself—as the final electron acceptor, converting it into substances like lactate or ethanol. Anaerobic respiration, however, uses an inorganic molecule other than oxygen as the final acceptor, essentially mimicking the aerobic process but with a different non-organic terminal point for electrons.

In animal cells, lactic acid fermentation is the primary anaerobic pathway. This process is also utilized by certain bacteria, such as those used in yogurt production. In animals, this pathway is initiated whenever oxygen levels cannot meet metabolic demands, such as during periods of high-intensity exercise. Additionally, red blood cells rely on lactic acid fermentation exclusively because they lack mitochondria. In this process, pyruvate accepts electrons from $NADH$, resulting in the formation of lactic acid (lactate) and the regeneration of $NAD^+$. The produced lactate is eventually transported to the liver, where it is converted back into pyruvate for further use. Notably, contemporary research has challenged the long-held belief that the accumulation of lactic acid is the direct cause of muscle soreness or fatigue.

Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways

Metabolic pathways for carbohydrates, proteins, and lipids are interconnected and feed into the central processes of glycolysis and the citric acid cycle. Carbohydrates other than glucose, such as fructose and galactose, as well as glucose derived from stored glycogen, are converted into various intermediates of glycolysis. Proteins are broken down into amino acids; after the removal of the amino group (which is converted into ammonia and subsequently urea for excretion), the remaining carbon skeletons enter either glycolysis or the citric acid cycle. Lipids are also integrated: glycerol from triglycerides enters glycolysis, while fatty acids undergo a process called beta oxidation. Beta oxidation breaks fatty acids into acetyl $CoA$ units, which then enter the citric acid cycle directly.

Metabolic pathways are not closed systems. They do not operate in isolation but rather involve a constant exchange of intermediates. Molecules generated in one pathway frequently serve as the starting substrates for another. Because of this continuous interplay, no single pathway can be considered a sealed loop that only interacts with itself.

Regulation of Cellular Respiration

Cellular respiration is heavily regulated through feedback inhibition to prevent the overproduction of intermediates or the wasting of cellular resources. Key enzymes, particularly those at the “first committed step” of a pathway, are regulated allosterically. This means that molecules such as $ATP$, $ADP$, $AMP$, $NAD^+$, and $NADH$ bind to the enzyme at a site other than the active site to alter its activity level. Generally, high-energy signals like high concentrations of $ATP$ or $NADH$ inhibit the pathway, while low-energy signals like high levels of $ADP$ or $AMP$ accelerate it.

Specific examples of this regulation include phosphofructokinase, which serves as the primary control point for glycolysis; it is stimulated by $ADP/AMP$ and inhibited by $ATP$ or citrate. Within the citric acid cycle, enzymes such as isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase are similarly regulated by the balance of $ATP$ and $ADP$. In contrast to these enzymatic pathways, the rate of the electron transport chain is not controlled by feedback inhibition of its enzymes. Instead, the pace of electron transport is determined by the ratio of $ADP$ to $ATP$. An increase in $ADP$ levels indicates energy consumption and speeds up electron transport, while an increase in $ATP$ indicates energy surplus and slows the process down.