Energy Systems & Metabolism

Vocabulary flashcards covering key terms and definitions from the lecture notes on energy systems, ATP metabolism, CP/PCr, glycolysis, oxidative phosphorylation, and carbohydrate metabolism.

ATP

Adenosine triphosphate; main cellular energy currency. Produced by hydrolysis of ATP to ADP + Pi via ATPase; stores energy in phosphate bonds.

ADP

Adenosine diphosphate; product of ATP hydrolysis; can be rephosphorylated to ATP.

AMP

Adenosine monophosphate; formed when ATP loses two phosphate groups (2 ADP → ATP + AMP via adenylate kinase).

ATPase

Enzyme that hydrolyzes ATP to ADP + Pi, releasing energy for cellular work.

CP/PCr

Creatine phosphate (phosphocreatine); high-energy phosphate reservoir that donates phosphate to ADP to rapidly regenerate ATP.

Creatine Kinase (CK)

Enzyme that transfers a phosphate from PCr to ADP to form ATP and creatine.

Adenylate Kinase (Myokinase)

Enzyme that catalyzes 2 ADP → ATP + AMP, contributing to rapid ATP resynthesis.

Phosphorylation

Transfer of a phosphate group to or from a molecule; energy stored in phosphate bonds; essential in energy transfer and metabolism.

Oxidative Phosphorylation

ATP production from ADP and Pi via the electron transport chain, powered by substrate oxidation; occurs in mitochondria.

Electron Transport Chain (ETC)

Series of protein complexes that transfer electrons from NADH/FADH2 to oxygen, pumping protons to drive ATP synthesis.

NADH

Reduced nicotinamide adenine dinucleotide; electron carrier delivering electrons to the ETC.

FADH2

Reduced flavin adenine dinucleotide; electron carrier entering the ETC at Complex II.

NAD+/NADH shuttle

Mechanisms to transfer reducing equivalents from cytosolic NADH into mitochondria (e.g., malate-aspartate shuttle).

FAD

Oxidized form of flavin adenine dinucleotide; accepts electrons in the ETC and becomes FADH2.

Coenzyme Q (CoQ)

Ubiquinone; lipid-soluble carrier in the ETC that shuttles electrons between Complexes I/II and III.

Cytochromes

Iron-containing proteins in the ETC that transfer electrons via redox reactions.

P/O ratio

ATP produced per oxygen atom reduced; NADH yields about 3 ATP per NADH, FADH2 yields about 2 ATP per FADH2.

Oxygen (O2)

Final electron acceptor in the ETC; combines with electrons and protons to form water.

CHO (carbohydrates)

Primary fuel source for energy; can generate ATP anaerobically and support fat oxidation as a fuel source.

Glycogen

Stored form of glucose in liver and skeletal muscle; muscle glycogen is abundant; liver glycogen stores support whole-body glucose availability.

Glucose Paradox (Liver glycogen from lactate)

Liver often forms glycogen from lactate rather than directly from glucose when glucose is available; lactate can be shuttled to liver for glycogen synthesis.

Glucose Transporters (GLUT1-GLUT4)

Membrane transporters that move glucose into cells; GLUT4 is insulin-responsive and prominent in skeletal muscle.

GLUT4

Insulin-responsive glucose transporter in skeletal muscle (also translocated to the membrane with exercise).

Glycolysis (Embden–Meyerhof pathway)

Cytosolic pathway that breaks glucose to pyruvate (or lactate); net yield of 2 ATP per glucose; occurs in the cytosol with investment, cleavage, and energy phases.

Hexokinase

Enzyme that phosphorylates glucose to glucose-6-phosphate; traps glucose in the cell; inhibited by its product (glucose-6-phosphate).

Phosphofructokinase (PFK)

Rate-limiting enzyme of glycolysis; activated by ADP/AMP; inhibited by ATP and citrate; controls glycolytic flux.

Pyruvate Kinase

Enzyme that converts phosphoenolpyruvate to pyruvate; hepatic isoform is regulated by hormonal signals; less active in skeletal muscle compared to liver.

Malate-Aspartate Shuttle

Pathway to shuttle cytosolic NADH into mitochondria by converting oxaloacetate to malate and back, enabling NADH-derived electrons to enter the ETC.

Glycerol-Phosphate Shuttle

Shuttle transferring cytosolic NADH electrons to mitochondrial FAD, forming FADH2 that enters the ETC at Complex II.

LDH (Lactate Dehydrogenase)

Enzyme catalyzing interconversion of pyruvate and lactate with NADH/NAD+; exists as multiple isoforms with tissue-specific distributions.

LDH Isoforms (LDH-1 to LDH-5)

Five LDH isoenzymes with tissue-specific distribution; heart muscle favors pyruvate-to-lactate/ lactate-to-pyruvate balance suitable for oxidative metabolism; muscle isoforms favor lactate production under anaerobic conditions.

Lactic acid system

Anaerobic glycolysis producing ATP with lactate as byproduct; predominant in high-intensity, short-duration efforts (roughly up to 2 minutes).

ATP-CP System (Phosphagen system)

Rapid ATP resynthesis from creatine phosphate; anaerobic; lasts about 10 seconds of maximal effort; replenishment can take ~2 minutes.

Aerobic System

Energy system relying on oxidative metabolism (glycolysis + Krebs cycle + ETC) to produce ATP; dominant in long-duration, lower-intensity activities.