Exercise Bioenergetics, Substrate Metabolism, and Energy Expenditure

Flashcards testing terminology, bioenergetic pathways, enzyme kinetics, and energy expenditure metrics from exercise physiology lecture notes.

Bioenergetics

The cellular-level process of converting food substrates into biologically usable energy.

Calorie

The amount of heat energy required to raise the temperature of $1\,\text{g}$ of water from $14.5\,^\circ\text{C}$ to $15.5\,^\circ\text{C}$, where $1{,}000\,\text{cal} = 1\,\text{kcal} = 1\,\text{Calorie}$.

Mass Action Effect

The principle that substrate availability influences metabolic rate, where higher abundance of a given substrate increases pathway activity and reliance on that substrate.

Activation Energy

The minimum energy required to initiate a chemical reaction, which is lowered by the presence of enzymes.

Holoenzyme

The complete, catalytically active enzyme system composed of the protein component (apoenzyme) combined with its nonprotein activator (cofactor or coenzyme).

Rate-Limiting Enzyme

An enzyme positioned early in a metabolic pathway that determines the overall reaction velocity through bottleneck control and negative feedback inhibition.

Adenosine Triphosphate (ATP)

A high-energy nucleotide composed of adenosine (adenine linked to ribose) and three inorganic phosphate groups, serving as the immediate fuel for biological work.

Substrate-Level Phosphorylation

The synthesis of ATP or GTP through direct transfer of a phosphate group from a phosphorylated metabolic intermediate to ADP or GDP, occurring independently of oxygen.

ATP-PCr System

An immediate, alactic anaerobic metabolic pathway in skeletal muscle that transfers an inorganic phosphate ($P_i$) from phosphocreatine to ADP via creatine kinase, supporting maximal exercise for $3$ to $15\,\text{s}$.

Creatine Kinase

The enzyme that catalyzes the cleavage of phosphocreatine into creatine and inorganic phosphate, activated by elevated concentrations of ADP and $P_i$ and inhibited by high ATP levels.

Brown Fat

A specialized adipose tissue rich in mitochondria that is activated by cold exposure to generate heat and burn regular body fat rather than storing energy.

Visceral Fat

Adipose tissue stored inside the abdominal cavity, wrapped around internal organs.

Subcutaneous Fat

Adipose tissue located directly beneath the epidermal and dermal layers of the skin.

GLUT-4

An insulin- and contraction-sensitive glucose transporter protein that translocates from intracellular vesicles to the sarcolemma in response to insulin signaling or increased intracellular $Ca^{2+}$.

Glycogenesis

The enzymatic conversion of glucose into glycogen for storage within skeletal muscle and liver cells.

Glycogenolysis

The biochemical breakdown of glycogen into glucose-1-phosphate to enter the glycolytic pathway.

Gluconeogenesis

The formation of glucose from non-carbohydrate precursors, including lactate, amino acids, and glycerol.

Hexokinase

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

Phosphofructokinase (PFK)

The rate-limiting enzyme of glycolysis that phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate using $1\,\text{ATP}$, stimulated by ADP and $P_i$ and inhibited by ATP, citrate, and $H^+$ ions.

Oxidative System

The most complex bioenergetic system, occurring within the mitochondria, that completely catabolizes carbohydrates and fats in the presence of oxygen ($O_2$) to generate large amounts of ATP.

Reactive Oxygen Species (ROS)

Toxic, chemically reactive oxygen-containing molecules formed when oxygen delivery excessively outpaces demand within the mitochondria.

Isocitrate Dehydrogenase

The rate-limiting enzyme of the Krebs cycle that is inhibited by ATP and activated by ADP, $P_i$, and $Ca^{2+}$.

Cytochromes

Iron-containing protein complexes located in the inner mitochondrial membrane that act as electron magnets, transferring electrons through successively higher-affinity transitions along the respiratory chain.

Cytochrome Oxidase

The rate-limiting enzyme of the electron transport chain (Complex IV) that catalyzes the reduction of oxygen by hydrogen ions and electrons to form water ($2H^+ + 2e^- + \frac{1}{2}O_2 \rightarrow H_2O$).

Lipolysis

The enzymatic breakdown of stored triglycerides into glycerol and three free fatty acids (FFAs), controlled by lipases.

Albumin

The blood plasma protein that binds free fatty acids released from adipose tissue to transport them through the circulation to working muscle.

β-Oxidation

A series of cyclical mitochondrial reactions that cleaves two-carbon acyl fragments from activated free fatty acids to produce acetyl-CoA, $1\,\text{NADH} + \text{H}^+$, and $1\,\text{FADH}_2$ per cycle.

Palmitic Acid

A 16-carbon saturated free fatty acid whose complete catabolism via $\beta$-oxidation, the Krebs cycle, and oxidative phosphorylation yields a net of $106\,\text{ATP}$.

Lactate Shuttle

The process whereby lactate produced in glycolytic Type II muscle fibers is transported via circulation or diffusion to adjacent Type I fibers, cardiac tissue, or the liver for oxidation.

Monocarboxylate Transport Proteins (MCT)

Transporter proteins in the sarcolemma that facilitate the passive diffusion or active exchange of lactate and hydrogen ions between muscle fibers and the bloodstream.

Cori Cycle

The inter-organ metabolic cycle in which muscle-derived lactate travels through the blood to the liver, where it is converted to pyruvate and then to glucose via gluconeogenesis before returning to working muscle.

Crossover Concept

The metabolic model describing the shift from lipid oxidation at rest and low intensities (below $60\%\,\dot{V}O_{2\max}$) to carbohydrate oxidation at high exercise intensities (above $75\%\,\dot{V}O_{2\max}$).

Crossover Point

The intersection where the percentage of energy derived from carbohydrate exceeds that derived from fat, which endurance training shifts rightward toward higher exercise intensities.

Succinate Dehydrogenase (SDH)

A representative Krebs cycle enzyme whose activity directly correlates with skeletal muscle oxidative capacity ($\dot{Q}O_2$) and is $2$ to $4$ times higher in endurance-trained individuals.

Direct Calorimetry

A method of measuring energy expenditure through the direct quantification of total body heat production inside a specialized insulated chamber.

Ice Calorimeter

An early direct calorimeter apparatus in which body heat from an organism placed inside a chamber surrounded by ice melts the ice, yielding a water volume directly proportional to the heat produced.

Indirect Calorimetry

A method of estimating total energy expenditure based on respiratory gas exchange measurements of oxygen consumption ($\dot{V}O_2$) and carbon dioxide production ($\dot{V}CO_2$) during steady-state aerobic metabolism.

Haldane Transformation

A mathematical formula that calculates unknown inspired air volume ($\dot{V}_I$) from known expired volume ($\dot{V}_E$) based on the physiological constancy of nitrogen ($N_2$) volumes across the lungs.

Respiratory Exchange Ratio (RER)

The ratio between the volume of carbon dioxide produced and the volume of oxygen consumed ($\text{RER} = \dot{V}CO_2 / \dot{V}O_2$), which indicates fuel substrate utilization ($0.70$ for pure fat, $1.00$ for pure carbohydrate).

Doubly Labeled Water Method

A gold-standard technique for measuring total energy expenditure in free-living humans, where ingested $^2\text{H}_2^{18}\text{O}$ allows carbon dioxide production to be calculated from the difference between $^{18}\text{O}$ elimination (water and $CO_2$) and $^2\text{H}_2$ elimination (water only).

Basal Metabolic Rate (BMR)

The minimal daily energy expenditure required to maintain essential cellular functions, measured in a supine position in a thermoneutral environment after $8\,\text{h}$ of sleep and $12\,\text{h}$ of fasting.

Resting Metabolic Rate (RMR)

A practical measurement of resting energy expenditure assessed under less stringent conditions than BMR, typically falling within $5\%$ to $10\%$ of BMR.

Slow Component of VO2 Kinetics

A progressive rise in oxygen uptake that continues beyond expected steady-state levels during constant high-power exercise, associated with the recruitment of less efficient Type II muscle fibers.

VO2 Drift

A slow, continuous increase in oxygen consumption observed over time during prolonged, low-power submaximal exercise, potentially linked to ventilatory work and hormonal shifts.

Maximal Oxygen Uptake (VO2max)

The highest rate of oxygen consumption achievable during maximal exercise, characterized by a plateau in $\dot{V}O_2$ despite increasing work rate, representing the gold standard for aerobic fitness.

VO2 Peak

The highest rate of oxygen consumption reached during a graded exercise test when a subject stops due to volitional exhaustion prior to demonstrating an official plateau.

Oxygen Deficit

The difference between the theoretical oxygen requirement of a given exercise intensity and the actual oxygen consumed during the lag period before steady-state aerobic metabolism is established.

Excess Postexercise Oxygen Consumption (EPOC)

The volume of oxygen consumed during recovery that exceeds resting baseline values, utilized to replenish ATP and phosphocreatine stores, remove lactate, reoxygenate hemoglobin/myoglobin, and clear accumulated $CO_2$.

Lactate Threshold

The exercise intensity or speed at which blood lactate concentration begins to accumulate exponentially, indicating that the rate of lactate production surpasses the rate of lactate clearance.

Economy of Effort

The efficiency of movement reflected by an athlete consuming less oxygen to maintain a given running or cycling velocity, independent of maximal aerobic capacity ($\dot{V}O_{2\max}$).