Cellular Respiration & Fermentation
Cellular Respiration & Fermentation
Overview: Life Is Work
Living cells are open systems, constantly requiring a flow of energy to maintain their complex organization, carry out synthetic work (anabolism), transport substances, and perform mechanical work (e.g., muscle contraction).
Some animals, such as chimpanzees, obtain energy by consuming plants. These dietary patterns reflect the flow of chemical energy originating from primary producers (plants, algae) in an ecosystem.
Other organisms feed on other animals that themselves consume plants.
Energy flows into an ecosystem as sunlight and exits as heat. This adheres to the laws of thermodynamics: energy cannot be created or destroyed, but its transformations lead to an increase in entropy, often manifested as heat.
Photosynthesis plays a crucial role by generating O2 and organic molecules, which are utilized in cellular respiration. In chloroplasts, light energy is captured to convert carbon dioxide and water into glucose (a type of organic molecule) and oxygen. This process stores energy in the chemical bonds of glucose, acting as the foundation for most food chains.
Cellular respiration involves the use of chemical energy stored in organic molecules to regenerate ATP, which powers various cellular functions. Occurring primarily in mitochondria, cellular respiration systematically breaks down these organic molecules in the presence of oxygen, releasing the stored energy in a controlled manner to synthesize ATP, the cell's main energy currency. This ATP then drives endergonic cellular processes.
Adenosine Triphosphate (ATP)
ATP (adenosine triphosphate) is a nucleotide that contains three phosphate groups. ATP consists of an adenine base, a ribose sugar, and a chain of three phosphate groups. The energy currency of the cell is primarily stored in the high-energy phosphate bonds connecting the second and third phosphate groups.
Hydrolysis of these phosphate groups can yield ADP (adenosine diphosphate) or AMP (adenosine monophosphate). Specifically, the removal of the terminal phosphate group through hydrolysis yields ADP and inorganic phosphate (Pi), releasing a significant amount of free energy (\Delta G = -7.3 \text{ kcal/mol} under standard conditions, but often closer to -13 \text{ kcal/mol} in cellular conditions due to reactant concentrations).
The phosphate groups are negatively charged and interact favorably with water. The three phosphate groups are all negatively charged, making their repulsion a major contributor to the instability of the phosphate bond region. When these terminal bonds are hydrolyzed (broken by water), the new products (ADP and Pi) are more stable, and the relief of electrostatic repulsion, combined with increased entropy and stabilization by hydration, drives the exergonic nature of the reaction.
When phosphate groups are bonded together in ATP, water molecules cannot interact effectively, leading to an energetically unfavorable configuration.
As a result, energy is released when ATP is hydrolyzed to ADP or AMP. This released energy is then used to power various cellular activities, such as active transport, muscle contraction, and biosynthesis.
Phosphorylation Reactions
In phosphorylation reactions, the gamma phosphate of ATP is transferred and attached to a protein, facilitating cellular functions. Phosphorylation is the transfer of the terminal (gamma) phosphate group from ATP to another molecule, typically a protein or a reactant molecule. This transfer often renders the recipient molecule more reactive or changes its conformation. For example, in metabolic pathways like glycolysis, phosphorylation primes glucose for subsequent breakdown. In signal transduction, protein phosphorylation acts as a molecular switch, activating or deactivating enzymes and receptors, thereby regulating cell activities.
Energy Flow and Chemical Recycling in Ecosystems
Energy flow involves a continuous cycle where light energy from the sun is captured by chloroplasts during photosynthesis to synthesize organic molecules (e.g., glucose) from carbon dioxide and water. These organic molecules then serve as fuel for cellular respiration, which primarily takes place in the mitochondria. During cellular respiration, organic molecules are broken down to release energy, producing ATP, while also releasing carbon dioxide and water as byproducts. Crucially, at each energy transformation step, some energy is inevitably lost as heat, in accordance with the second law of thermodynamics, ensuring a continuous input of energy from the sun is required to sustain life.
Catabolic Pathways Yield Energy
Several processes are essential to the functionality of cellular respiration, including: Catabolic pathways are metabolic pathways that break down complex molecules into simpler ones, releasing energy in the process. Cellular respiration is a prime example, encompassing different types based on the presence or absence of oxygen:
Aerobic Respiration: This is the most efficient form of catabolism, carried out by most eukaryotic cells and many prokaryotes. It completely breaks down organic molecules (like glucose) using oxygen (O_2) as the final electron acceptor, generating a large amount of ATP. The complete oxidation of glucose is highly exergonic, meaning it releases energy spontaneously (\Delta G^\circ = -686 \text{ kcal/mol} or -2870 \text{ kJ/mol}). This energy is strategically harvested in a series of controlled steps.
Fermentation: This is a partial degradation of glucose or other organic fuels that occurs in the absence of oxygen. It doesn't use an electron transport chain and produces a smaller amount of ATP compared to aerobic respiration, as it only partially oxidizes the fuel molecule.
Anaerobic Respiration: Also utilizes an electron transport chain, but unlike aerobic respiration, it uses electron acceptors other than oxygen at the end of the chain, such as sulfate (SO4^{2-}), nitrate (NO3^-), or ferric ion (Fe^{3+}). It is more efficient than fermentation but less efficient than aerobic respiration.
Redox Reactions and Their Importance
The transfer of electrons in chemical reactions releases energy stored in organic molecules. The rearrangement of electrons from a higher energy state (as in organic molecules like glucose) to a lower energy state (as in carbon dioxide and water) during chemical reactions releases energy. This energy is not released all at once but is captured in a stepwise manner to produce ATP. Electrons lose potential energy as they move from less electronegative atoms (e.g., carbon, hydrogen) to more electronegative atoms (e.g., oxygen).
This released energy is eventually harnessed to synthesize ATP.
Reactions that transfer electrons are termed oxidation-reduction (redox) reactions: Redox reactions are fundamental to energy metabolism.
Oxidation: Loss of electrons from a substance. Oxidation is the loss of electrons (or hydrogen atoms, which carry an electron), increasing the oxidation state.
Reduction: Gain of electrons by a substance. Reduction is the gain of electrons (or hydrogen atoms), decreasing the oxidation state. It's important to remember that oxidation and reduction always occur together; one substance cannot be oxidized without another being reduced.
Redox Reaction Examples
An example of a redox reaction includes sodium (Na) and chlorine (Cl) interaction: In the formation of table salt (NaCl), sodium (Na) readily donates an electron to chlorine (Cl). The sodium atom, having lost an electron, becomes the positively charged sodium ion (Na^+) and is oxidized. The chlorine atom, having gained an electron, becomes the negatively charged chloride ion (Cl^-) and is reduced. This is a clear example of electron transfer between elements.
Na becomes oxidized (loses electrons).
Cl becomes reduced (gains electrons).
Redox Terminology
The electron donor is referred to as the reducing agent, while the electron receptor is the oxidizing agent. The reducing agent is the substance that gets oxidized, and by losing electrons, it reduces another substance. Conversely, the oxidizing agent is the substance that gets reduced, and by gaining electrons, it oxidizes another substance. In biological systems, many redox reactions involve shifts in the sharing of electrons in covalent bonds rather than complete electron transfers. For instance, in the combustion of methane (CH_4), carbon is oxidized as its bonds with hydrogen (less electronegative) are replaced by bonds with oxygen (more electronegative), while oxygen is reduced.
Some redox reactions do not transfer electrons but alter the electron sharing in covalent bonds, as seen in methane and O2 reactions.
Oxidation of Organic Fuel Molecules
Through cellular respiration, glucose is oxidized and O2 is reduced, leading to energy production. During cellular respiration, organic fuel molecules like glucose (C6H{12}O6) are systematically oxidized to carbon dioxide (CO2), and oxygen (O2) is reduced to water (H2O). This often involves the transfer of hydrogen atoms (comprising a proton and an electron). The enzymes that mediate these transfers are usually dehydrogenases, which remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate and transfer them to an electron carrier. Oxidases are enzymes involved in oxidation, often reducing oxygen in the process.
The process involves:
Oxidation: Loss of electrons, gain of oxygen, loss of hydrogen (with reactions mediated by dehydrogenases).
Reduction: Gain of electrons, loss of oxygen, gain of hydrogen (with reactions mediated by oxidases).
Stepwise Energy Harvest From Fuel
Glucose and other organic molecules are degraded through a series of steps: Instead of releasing all the energy from glucose in one explosive burst, cellular respiration funnels electrons from glucose through a series of controlled steps. The electrons are first transferred to coenzymes such as NAD+ and FAD (flavin adenine dinucleotide).
The electrons are primarily transferred to NAD+ (nicotinamide adenine dinucleotide), which acts as an oxidizing agent during cellular respiration. NAD+ accepts two electrons and one proton to become NADH (NAD^+ + 2e^- + H^+ \rightarrow NADH), and FAD accepts two electrons and two protons to become FADH2 (FAD + 2e^- + 2H^+ \rightarrow FADH_2).
Every NADH produced represents stored energy used for ATP synthesis. Both NADH and FADH2 are high-energy electron carriers; they function as 'electron shuttles,' carrying the harvested potential energy (in the form of electrons) to the electron transport chain, where the energy will be used for ATP synthesis.
Cellular Respiration Process Steps
Cellular respiration is a metabolic pathway that occurs in distinct stages, maximizing energy capture:
Glycolysis: Occurs in the cytoplasm. This initial stage "splits sugar," breaking down a six-carbon glucose molecule into two three-carbon pyruvate molecules. A small amount of ATP is produced via substrate-level phosphorylation, and NAD+ is reduced to NADH.
Pyruvate Oxidation and The Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix (eukaryotes). After glycolysis, pyruvate is converted to acetyl-CoA, which then enters the citric acid cycle. This cycle completes the oxidation of the organic fuel molecules, releasing carbon dioxide and generating more ATP by substrate-level phosphorylation, along with a significant amount of electron carriers (NADH and FADH2).
Oxidative Phosphorylation: Occurs at the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes). This stage comprises the electron transport chain and chemiosmosis. It utilizes the high-energy electrons harvested by NADH and FADH2 to drive the synthesis of the vast majority of ATP.
Substrate-level phosphorylation, a direct transfer of a phosphate group from a substrate molecule to ADP, accounts for a smaller but significant portion of ATP produced directly during glycolysis and the citric acid cycle.
Oxidative Phosphorylation
This process generates the majority (\sim90\%) of ATP through redox reactions, utilizing NADH and FADH2 as electron carriers. Oxidative phosphorylation is the primary mechanism of ATP synthesis, accounting for approximately 90\% of the ATP generated during aerobic respiration. It involves two main components: the electron transport chain (ETC) and chemiosmosis. NADH and FADH2, carrying high-energy electrons, donate these electrons to the ETC. The energy released as electrons move down the chain is used to pump protons (H^+) across the membrane, creating an electrochemical gradient. This gradient then drives ATP synthase to produce ATP.
Thus, for each glucose molecule catabolized, the cell can produce up to 32 ATP molecules. While the theoretical maximum yield is around 32 ATP per glucose molecule, actual yields can vary (e.g., 30-32 ATP) due to factors such as proton-motive force variations and the cost of moving ATP out of the mitochondria.
Glycolysis
Glycolysis, or the splitting of sugar, involves two primary phases: Glycolysis (from Greek 'glykys' for sweet and 'lysis' for split) is a universal metabolic pathway occurring in the cytoplasm of nearly all living cells. It is a ten-step process with a net breakdown of one six-carbon glucose molecule into two three-carbon pyruvate molecules.
Energy Investment Phase: This phase consumes 2 ATP molecules. Glucose is phosphorylated twice, preparing it for cleavage. For example, hexokinase phosphorylates glucose to glucose-6-phosphate, requiring 1 ATP. Phosphofructokinase, a key regulatory enzyme, phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate, requiring another ATP.
Energy Payoff Phase: This phase yields 4 ATP molecules (net gain of 2 ATP), 2 NADH molecules, and 2 pyruvate molecules per glucose. Substrate-level phosphorylation occurs in two steps, where phosphate groups are directly transferred from high-energy intermediates to ADP. For instance, the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate and phosphoenolpyruvate to pyruvate both generate ATP.
Glycolysis is characterized by its ability to function both in aerobic and anaerobic conditions. A key characteristic of glycolysis is its independence from oxygen, allowing it to function under both aerobic and anaerobic conditions, thus being a foundational pathway for virtually all life forms.
Pyruvate Oxidation
In O2 presence, pyruvate enters mitochondria where its oxidation occurs, transforming into acetyl CoA—an essential link to the citric acid cycle. Upon entering the mitochondrial matrix (in eukaryotes), each pyruvate molecule undergoes a crucial three-step oxidation process catalyzed by a multienzyme complex called the pyruvate dehydrogenase complex.
A carboxyl group is removed from pyruvate and released as carbon dioxide (CO_2).
The remaining two-carbon fragment is oxidized, and the electrons transferred to NAD+, reducing it to NADH.
The oxidized two-carbon acetate is attached to coenzyme A, forming acetyl CoA.
Thus, for each glucose molecule (which yields two pyruvates), 2 molecules of CO_2, 2 molecules of NADH, and 2 molecules of acetyl CoA are produced. Acetyl CoA is now ready to enter the citric acid cycle.
The Citric Acid Cycle
Krebs Cycle: Also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, this eight-step cycle completes the breakdown of the original glucose molecule. It takes place in the mitochondrial matrix. For each acetyl CoA that enters, the two carbons are completely oxidized and released as two molecules of carbon dioxide (CO_2).
Follows the oxidation of pyruvate and completes its breakdown to CO2.
Produces 1 ATP, 3 NADH, and 1 FADH2 per cycle. For each turn of the cycle, the following are produced: 3 NADH, 1 FADH2, and 1 ATP (or GTP, which is readily converted to ATP) via substrate-level phosphorylation. Since one glucose molecule yields two acetyl CoA molecules, the cycle runs twice per glucose, producing a total of 6 NADH, 2 FADH2, and 2 ATP (or GTP).
The cycle continuously occurs in the presence of adequate reactants.
Electron Transport Chain (ETC)
Located in the inner mitochondrial membrane (cristae), most chain components are proteins. The ETC is a series of multiprotein complexes (Complex I, II, III, and IV) and two mobile electron carriers (ubiquinone, Q, and cytochrome c). These components are embedded in the inner mitochondrial membrane, forming folds called cristae, which increase the surface area.
Carriers alternate between reduced and oxidized states as they transfer electrons, ultimately reducing O2 to form H2O. Electrons from NADH and FADH2 are passed down the chain from molecules of higher energy to molecules of lower energy, towards the highly electronegative oxygen. The energy released at each step drives the pumping of protons (H^+) from the mitochondrial matrix into the intermembrane space, building a proton gradient. At the end of the chain, oxygen (O2) acts as the final electron acceptor, picking up electrons and protons to form water (H2O). 2H^+ + 2e^- + 1/2 O2 \rightarrow H2O .
ATP Synthase
A complex molecular machine that utilizes the H+ gradient produced by the ETC to synthesize ATP from ADP and inorganic phosphate (Pi). ATP synthase is a remarkable enzyme complex found embedded in the inner mitochondrial membrane. It acts like a molecular rotary motor. The flow of protons (H^+) from the intermembrane space back into the mitochondrial matrix through a specific channel within ATP synthase causes part of the enzyme (the rotor) to spin. This mechanical rotation drives conformational changes in other parts of the enzyme (the catalytic knob), activating its catalytic sites to synthesize ATP from ADP and inorganic phosphate (P_i). This process is known as chemiosmosis.
Chemiosmosis
This mechanism couples the redox reactions of the ETC to ATP synthesis, utilizing a proton gradient (proton-motive force). Chemiosmosis is the process by which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work, such as ATP synthesis. The electron transport chain establishes a proton-motive force (PMF) across the inner mitochondrial membrane – an electrochemical gradient due to the difference in both hydrogen ion concentration (pH gradient) and electrical charge (membrane potential). Protons tend to diffuse back across the membrane, driven by this gradient. ATP synthase harnesses this flow of protons, allowing them to pass through its channel, leading to the synthesis of ATP.
Overview of ATP Production
The energy flow during cellular respiration follows: Summarizing the energy flow during aerobic cellular respiration: the chemical potential energy in glucose molecules is first transferred to electron carriers (NADH and FADH2) during glycolysis, pyruvate oxidation, and the citric acid cycle. These electron carriers then donate their electrons to the electron transport chain, where the energy is used to generate a proton-motive force across the inner mitochondrial membrane. This proton-motive force then powers ATP synthase to convert ADP and Pi into ATP through chemiosmosis.
Glucose \rightarrow NADH \rightarrow Electron Transport Chain \rightarrow Proton-Motive Force \rightarrow ATP
Approximately 34% of the energy from glucose is converted to ATP, resulting in about 32 ATP per glucose molecule. The overall efficiency of energy conversion from glucose to ATP is approximately 34\%, with the remaining energy released as heat. While a theoretical maximum of 36 or 38 ATP molecules was historically proposed, current estimates typically range from 30 to 32 ATP molecules per glucose molecule, accounting for variations in shuttle systems that transport electrons into the mitochondria and proton leakage.
Fermentation & Anaerobic Respiration
Anaerobic respiration utilizes an electron transport chain lacking O2. When oxygen is scarce or absent, cells can generate ATP through anaerobic pathways. Anaerobic respiration is distinguished from fermentation by its use of an electron transport chain. Instead of oxygen, it utilizes other inorganic molecules, such as sulfate (SO4^{2-}), nitrate (NO3^-), or ferric ion (Fe^{3+}), as terminal electron acceptors. This pathway is common in certain bacteria and archaea living in oxygen-depleted environments.
Fermentation, coupled with glycolysis, regenerates NAD+ enabling glycolysis to continue. Fermentation, on the other hand, is a metabolic process that occurs without an electron transport chain and without external electron acceptors. Its primary purpose, after glycolysis produces ATP and NADH, is to regenerate NAD+ from NADH. This regenerated NAD+ is crucial for glycolysis to continue, ensuring a continuous, albeit small, production of ATP via substrate-level phosphorylation when oxygen is unavailable.
Types include:
Alcohol Fermentation: This occurs in yeast and some bacteria. Pyruvate is first converted to acetaldehyde, releasing carbon dioxide (CO_2) as a byproduct. Acetaldehyde is then reduced by NADH to ethanol, regenerating NAD+. This process is used in brewing, winemaking, and baking.
Lactic Acid Fermentation: This occurs in human muscle cells during strenuous exercise when oxygen supply cannot meet demand, and also in certain bacteria (e.g., those used to make yogurt and cheese). Pyruvate is directly reduced by NADH to lactate, regenerating NAD+ without the release of carbon dioxide. The accumulation of lactate in muscle cells contributes to muscle fatigue and soreness.
Life Without Oxygen
Obligate anaerobes perform fermentation or anaerobic respiration and cannot tolerate O2. Organisms display varying tolerances to oxygen:
Obligate anaerobes are poisoned by oxygen because they lack the enzymes (e.g., catalase, superoxide dismutase) to detoxify reactive oxygen species. They rely exclusively on fermentation or anaerobic respiration for ATP production, thriving only in anaerobic environments.
Facultative anaerobes can switch between fermentation and respiration based on available O2. Facultative anaerobes are versatile; they can survive using either fermentation or aerobic respiration. If oxygen is present, they will preferentially perform aerobic respiration due to its much higher ATP yield. When oxygen is absent, they switch to fermentation to generate ATP, such as E. coli bacteria and yeast.
The Evolutionary Significance of Glycolysis
Glycolysis predates atmospheric oxygen, being a vital process for early anaerobic prokaryotes. Glycolysis is considered one of the most ancient metabolic pathways because it is ubiquitous across all domains of life (bacteria, archaea, and eukaryotes) and occurs in the cytoplasm, independent of membrane-bound organelles.
It signifies an ancient energy extraction route from organic molecules, requiring no advanced organelles. This suggests that it evolved in early prokaryotes before the accumulation of significant oxygen in Earth's atmosphere (the 'Great Oxidation Event'). Its ability to generate ATP in the absence of oxygen and its reliance on relatively simple enzymatic reactions underscore its foundational role in the evolution of energy metabolism.
Connecting Metabolism
The metabolic pathways include catabolism (energy producing) and anabolism (biosynthesis). Metabolic pathways are highly interconnected. Catabolism refers to the breakdown of complex molecules into simpler ones, releasing energy (e.g., cellular respiration). Anabolism refers to the synthesis of complex molecules from simpler ones, requiring energy input (e.g., protein synthesis, gluconeogenesis).
Glycolysis and the citric acid cycle serve as intersections for various pathways. Glycolysis and the citric acid cycle are central 'intersections' or 'funnels' in metabolism. Many catabolic pathways feed into them, and they also provide precursor molecules that can be diverted for anabolic pathways. This allows the cell to efficiently interconvert different types of organic molecules and adjust its metabolic activities according to energy needs and nutrient availability.
Alternative Energy Sources
Proteins: Digested to amino acids that feed into catabolic pathways. These amino acids can then enter cellular respiration at various points, either in glycolysis (e.g., converted to pyruvate) or in the citric acid cycle (e.g., converted to acetyl CoA, alpha-ketoglutarate, or oxaloacetate), after their amino groups are removed (deamination).
Fats: Break down into glycerol (for glycolysis) and fatty acids (for acetyl CoA generation). Fats (Triglycerides) represent a highly efficient form of energy storage. They are hydrolyzed into glycerol and fatty acids. Glycerol can be converted into glyceraldehyde-3-phosphate, an intermediate of glycolysis. Fatty acids undergo beta-oxidation, a process that breaks them down into two-carbon units that are converted to acetyl CoA, which then enters the citric acid cycle.
Fat oxidation yields significantly more ATP than carbohydrates due to their dense energy content. The extensive hydrocarbon chains of fatty acids are highly reduced, meaning they contain many high-energy electrons. Consequently, the complete oxidation of fats yields more than twice the amount of ATP per gram compared to carbohydrates or proteins, making them a dense energy source.
Regulation of Cellular Respiration
Feedback inhibition is a common regulatory mechanism. Cellular respiration is precisely regulated to match the cell's energy demands. The most common regulatory mechanism is feedback inhibition, where a product downstream in a pathway inhibits an enzyme operating earlier in the pathway.
ATP levels directly influence the respiration rate and efficiency of pathways, focusing on enzyme activation or inhibition. The primary control points are key enzymes that catalyze irreversible steps early in glycolysis and the citric acid cycle.
Phosphofructokinase: An allosteric enzyme responsive to ATP and AMP levels. An enzyme in glycolysis, is a crucial control point. It is an allosteric enzyme regulated by changes in ATP and AMP concentrations. High levels of ATP (an end product of catabolism) act as an allosteric inhibitor, decreasing the enzyme's activity and slowing down glycolysis when energy reserves are high. Conversely, high levels of AMP (derived from ADP when ATP is utilized) act as an allosteric activator, stimulating the enzyme and speeding up glycolysis when the cell needs more ATP.
Other control points include the pyruvate dehydrogenase complex and enzymes in the citric acid cycle, which are also regulated by ATP/ADP ratios and the concentrations of other intermediates like citrate and NADH, ensuring metabolic balance.
Homeostatic Imbalance
Cyanide poisoning disrupts oxidative phosphorylation, inhibiting ATP production via ATP synthase. Toxins and poisons can severely impair cellular respiration. For example, cyanide poisoning is lethal because cyanide binds irreversibly to the cytochrome c oxidase complex (Complex IV) in the electron transport chain. This binding blocks the transfer of electrons to oxygen, effectively shutting down the entire electron transport chain. Without electron flow, no proton gradient can be established, and thus, ATP synthase cannot produce ATP via oxidative phosphorylation. Cells quickly deplete their ATP reserves, leading to cellular death and ultimately organismal death (suffocation at cellular level).
Other Glucose Processes
Glycogenesis: Conversion of glucose to glycogen for energy storage, especially in the liver and muscles. This is the anabolic pathway for synthesizing glycogen from glucose. It occurs primarily in the liver and skeletal muscles when blood glucose levels are high (e.g., after a meal) and there is sufficient ATP. Glycogen serves as a readily accessible store of glucose.
Glycogenolysis: Breakdown of glycogen into glucose when needed, particularly in response to low blood glucose. This is the catabolic breakdown of stored glycogen into glucose. In the liver, glycogenolysis helps maintain normal blood glucose levels during fasting or between meals. In muscles, stored glycogen is broken down to provide glucose for muscle contraction. This process is stimulated by hormones like glucagon (primarily liver) and epinephrine (liver and muscle) and inhibited by insulin.
Gluconeogenesis: Formation of glucose from non-carbohydrate sources, serving as an essential process during fasting or metabolic imbalances. 'New glucose formation' is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors, such as lactate, certain amino acids (e.g., alanine), and glycerol. This vital anabolic process occurs mainly in the liver (and to a lesser extent in the kidney) during prolonged fasting, starvation, or intense exercise to maintain blood glucose homeostasis and supply glucose to glucose-dependent organs like the brain and red blood cells.
Carbo-loading
A strategy employed by endurance athletes involving complex carb intake days before an event for increased glycogen storage to fuel prolonged activity. Carbo-loading, or carbohydrate loading, is a dietary strategy employed by endurance athletes (e.g., marathon runners, long-distance cyclists) before a competition. It involves increasing the intake of complex carbohydrates (like pasta, rice, bread) for several days leading up to an event, often combined with a reduction in training intensity. The goal is to maximize the storage of glycogen in muscles and the liver. Glycogen is the most accessible form of stored glucose, and increasing these reserves provides a larger fuel supply, delaying fatigue and improving endurance performance during prolonged, high-intensity aerobic activity.