Bioenergetics and Human Energy Systems

Comprehensive vocabulary flashcards covering human bioenergetics, including the ATP-PCr system, anaerobic glycolysis, aerobic carbohydrate metabolism, beta-oxidation of fats, protein catabolism, and energy system interactions during exercise.

Oxidative System

An aerobic metabolic pathway that uses oxygen to break down carbohydrates, fats, or proteins in the mitochondria to produce large quantities of ATP at a slow, sustainable rate during rest and exercise lasting longer than $2\text{ to }3\text{ minutes}$.

ATP-PCr System

The fastest metabolic energy system that resynthesizes ATP in the cytoplasm from stored phosphocreatine (PCr) and ADP via creatine kinase, providing maximal energy for explosive efforts lasting up to $10\text{ to }15\text{ seconds}$.

Phosphocreatine (PCr)

A high-energy intracellular phosphate molecule stored in limited quantities within muscle cells that transfers its phosphate group to ADP to resynthesize $1\text{ molecule of ATP}$ per molecule of PCr.

Creatine Kinase (CK)

The enzyme that catalyzes the transfer of an inorganic phosphate ($\text{P}_i$) from phosphocreatine (PCr) to ADP, producing creatine and resynthesizing ATP.

ATPase

The enzyme that catalyzes the hydrolysis of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and inorganic phosphate ($\text{P}_i$), releasing free energy to power cellular work.

Phosphorylation

The chemical process of adding an inorganic phosphate group ($\text{P}_i$) to an organic molecule, such as the bonding of $\text{P}_i$ to ADP to form ATP.

Substrate-Level Phosphorylation

The direct synthesis of ATP by transferring a high-energy phosphate group from a phosphorylated metabolic intermediate directly to ADP without requiring oxygen or the electron transport chain.

Oxidative Phosphorylation

The mitochondrial metabolic pathway that couples the energy released from the electron transport chain (via $\text{NADH}$ and $\text{FADH}_2$) to rephosphorylate ADP into ATP in the presence of oxygen.

Aerobic Glycolysis (AG)

The first phase of aerobic carbohydrate metabolism occurring in the cytoplasm, breaking down one glucose or glycogen molecule into $2\text{ molecules}$ of pyruvate and generating a net of $2\text{ ATP}$ (from glucose) or $3\text{ ATP}$ (from glycogen).

Anaerobic Glycolysis (AnG)

The metabolic pathway occurring in the cytoplasm that breaks down glucose or glycogen in the absence of oxygen to produce $2\text{ molecules}$ of lactate, yielding a gross of $4\text{ ATP}$ and a net of $2\text{ ATP}$ (glucose) or $3\text{ ATP}$ (glycogen).

Nicotinamide Adenine Dinucleotide (NAD+)

A coenzyme carrier molecule that removes hydrogen ions ($\text{H}^+$) and electrons from nutrient substrates during glycolysis, the transition step, and the Citric Acid Cycle.

Lactate Dehydrogenase (LDH)

The enzyme that catalyzes the reversible conversion of pyruvate to lactate by transferring hydrogen ions from $\text{NADH} + \text{H}^+$, thereby rapidly regenerating $\text{NAD}^+$ during anaerobic glycolysis.

Lactate

The end product of anaerobic glycolysis formed when pyruvate accepts two hydrogen atoms from $\text{NADH} + \text{H}^+$, functioning as a temporary hydrogen store to allow glycolysis to continue producing ATP at high rates.

Lactate Clearance Timeline

The physiological duration required for accumulated lactate to be converted back to pyruvate or cleared following exercise, which occurs within $1\text{ hour or less}$.

Metabolic Acidosis (H+ Accumulation)

The intracellular buildup of free hydrogen ions ($\text{H}^+$) that decreases muscle pH, which inhibits glycogen phosphorylase, impairs phosphofructokinase (PFK), and interferes with muscle contractile machinery.

Glycogenolysis

The breakdown of glycogen into glucose-6-phosphate, catalyzed by glycogen phosphorylase which is stimulated by epinephrine, norepinephrine, and glucagon.

Hexokinase

The enzyme that phosphorylates circulating blood glucose into glucose-6-phosphate upon entering the cell, consuming $1\text{ molecule of ATP}$.

Phosphofructokinase (PFK)

The key rate-limiting enzyme in glycolysis that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-diphosphate, consuming $1\text{ molecule of ATP}$.

Cori Cycle

The metabolic cycle in which lactate released from active skeletal muscle into the bloodstream is transported to the liver, where it is converted back into glucose and stored as glycogen.

Transition Step (Pyruvate Dehydrogenase)

The mitochondrial reaction converting each $3\text{-carbon}$ pyruvate into a $2\text{-carbon}$ acetyl-CoA molecule, releasing $1\text{ CO}_2$ and producing $1\text{ NADH} + \text{H}^+$ per pyruvate ($2\text{ CO}_2$ and $2\text{ NADH}$ per glucose).

Citric Acid Cycle (Krebs Cycle)

A cyclical series of reactions within the mitochondrial matrix that completes the oxidation of macronutrients, producing $3\text{ NADH} + \text{H}^+$, $1\text{ FADH}_2$, $2\text{ CO}_2$, and $1\text{ ATP}$ via substrate-level phosphorylation per acetyl-CoA.

Electron Transport Chain (ETC)

A series of mitochondrial electron carriers that accepts electrons from $\text{NADH}$ and $\text{FADH}_2$ to drive ATP resynthesis, using oxygen as the final electron acceptor to produce water ($\text{H}_2\text{O}$).

NADH ATP Yield

The quantity of ATP generated when the hydrogen ions and electrons from one mitochondrial $\text{NADH}$ pass through the electron transport chain, equaling $2.5\text{ ATP}$.

FADH2 ATP Yield

The quantity of ATP generated when the hydrogen ions and electrons from one mitochondrial $\text{FADH}_2$ pass through the electron transport chain, equaling $1.5\text{ ATP}$.

Aerobic ATP Yield of Blood Glucose

The total energy harvested from one molecule of blood glucose through complete aerobic oxidation, yielding $32\text{ to }34\text{ ATP}$ gross and $30\text{ to }32\text{ ATP}$ net after the $2\text{ ATP}$ initial investment.

Aerobic ATP Yield of Muscle Glycogen

The total net energy harvested from one glucosyl unit derived from muscle glycogen through complete aerobic oxidation, equaling $31\text{ to }33\text{ ATP}$ due to an investment of only $1\text{ ATP}$.

Lipolysis

The enzymatic cleavage of a triacylglycerol molecule by lipase into one $3\text{-carbon}$ glycerol molecule and three free fatty acid (FFA) chains.

Beta-Oxidation (β-Oxidation)

The mitochondrial pathway that systematically cleaves $2\text{-carbon}$ fragments from fatty acyl-CoA molecules, generating $1\text{ Acetyl-CoA}$, $1\text{ NADH} + \text{H}^+$, and $1\text{ FADH}_2$ per cycle.

Glycerol ATP Yield

The net energy yield from the metabolism of the $3\text{-carbon}$ glycerol backbone, which requires an activation investment of $-1\text{ ATP}$ to form $3\text{-PG}$, yielding $16\text{ or }17\text{ ATP}$ gross and $15\text{ or }16\text{ ATP}$ net.

Triglyceride ATP Yield (3x 10-Carbon FFAs + Glycerol)

The total ATP produced from a triglyceride composed of three $10\text{-carbon}$ FFAs ($198\text{ ATP}$ gross, $-6\text{ ATP}$ activation) and glycerol ($16\text{ or }17\text{ ATP}$ gross, $-1\text{ ATP}$ activation), resulting in $214\text{ or }215\text{ ATP}$ gross and $207\text{ or }208\text{ ATP}$ net.

Deamination

The removal of the nitrogen-containing amino group from an amino acid molecule, enabling its remaining carbon skeleton to be converted into glucose, pyruvate, acetyl-CoA, or Citric Acid Cycle intermediates.