Lecture 13: How Cells Obtain Energy from Food

Vocabulary practice flashcards covering cell biology lecture topics on cellular energy extraction, stages of catabolism, glycolysis, fermentation, fatty acid beta-oxidation, the citric acid cycle enzymes, and nutrient storage forms.

Catabolism

The set of metabolic pathways that captures chemical energy in $ATP$ and reduced electron carriers through three successive stages: digestion, acetyl-CoA formation, and complete oxidation.

Stage 1 of Catabolism (Digestion)

The breakdown of large extracellular food macromolecules into simple subunits: proteins into amino acids, polysaccharides into simple sugars, and fats into fatty acids and glycerol.

Stage 2 of Catabolism

The intracellular breakdown of simple subunits to acetyl-CoA in the cytosol and mitochondria, generating limited amounts of $ATP$ and $NADH$.

Stage 3 of Catabolism

The complete oxidation of the acetyl group of acetyl-CoA to $H_2O$ and $CO_2$ via the citric acid cycle and oxidative phosphorylation, producing large amounts of $ATP$ on the inner mitochondrial membrane.

Glycolysis

A 10-step catabolic pathway occurring in the cytosol that converts one molecule of glucose into two molecules of pyruvate, yielding a net result of $2\text{ pyruvate} + 2\text{ ATP} + 2\text{ NADH}$.

Kinase

An enzyme type that catalyzes the addition of a phosphate group to molecules; in glycolysis, kinases transfer a phosphate from $ATP$ to substrate in steps 1 and 3, and transfer a phosphate from substrate to $ADP$ to form $ATP$ in steps 7 and 10.

Isomerase

An enzyme type that catalyzes the rearrangement of bonds within a single molecule; in glycolysis, isomerases act in steps 2 and 5 to prepare molecules for subsequent chemical alterations.

Dehydrogenase

An enzyme type that catalyzes the oxidation of a molecule by removing a hydrogen atom plus an electron (a hydride ion, $H^-$); for example, glyceraldehyde 3-phosphate dehydrogenase produces $NADH$ in step 6 of glycolysis.

Mutase

An enzyme type that catalyzes the shifting of a chemical group from one position to another within a single molecule; for example, phosphoglycerate mutase shifts a phosphate in step 8 of glycolysis.

Coupled Reactions of Steps 6 and 7 of Glycolysis

The energetic coupling where $C\text{–}H$ bond oxidation in glyceraldehyde 3-phosphate drives $NADH$ production and high-energy phosphate bond formation in 1,3-bisphosphoglycerate, followed by $ATP$ generation from $ADP$, with an overall favorable free energy change of $\Delta G^\circ = -12.5\,kJ\,mol^{-1}$.

Fermentation in Muscle Cells

An anaerobic metabolic pathway operating in vigorously active muscle cells that reduces pyruvate to lactate to regenerate $NAD^+$ from $NADH$, allowing glycolysis to continue without producing extra $ATP$.

Pyruvate Oxidation

The enzymatic conversion of pyruvate into acetyl-CoA in the mitochondrial matrix by a multi-subunit complex, releasing $CO_2$ and producing $NADH + H^+$.

Fatty Acid Oxidation Cycle

A repetitive mitochondrial cycle that activates fatty acids released from triacylglycerol by coupling them to $HS\text{–}CoA$, shortening the fatty acyl chain by two carbons per turn and yielding acetyl-CoA, $FADH_2$, and $NADH$.

Citric Acid Cycle

An 8-step circular metabolic pathway in the mitochondrial matrix that oxidizes the two-carbon acetyl group of acetyl-CoA, producing $CO_2$, $NADH$, $FADH_2$, and $GTP$ while regenerating oxaloacetate.

Citrate Synthase

The enzyme catalyzing Step 1 of the citric acid cycle, joining acetyl-CoA ($2C$) and oxaloacetate ($4C$) through an S-citryl-CoA intermediate to form citrate ($6C$) and free $HS\text{–}CoA$.

Aconitase

The enzyme catalyzing Step 2 of the citric acid cycle, converting citrate into isocitrate via dehydration and rehydration through a cis-aconitate intermediate.

Isocitrate Dehydrogenase

The enzyme catalyzing Step 3 of the citric acid cycle, which oxidizes isocitrate to $\alpha$-ketoglutarate via an oxalosuccinate intermediate, releasing $CO_2$ and producing $NADH + H^+$.

$\alpha$-Ketoglutarate Dehydrogenase Complex

The enzyme complex catalyzing Step 4 of the citric acid cycle, oxidatively decarboxylating $\alpha$-ketoglutarate with $HS\text{–}CoA$ to form succinyl-CoA, releasing $CO_2$ and generating $NADH + H^+$.

Succinyl-CoA Synthetase

The enzyme catalyzing Step 5 of the citric acid cycle, converting succinyl-CoA into succinate and $HS\text{–}CoA$ while coupling high-energy bond breakage to synthesize $GTP$ from $GDP$ and inorganic phosphate.

Succinate Dehydrogenase

The enzyme catalyzing Step 6 of the citric acid cycle, oxidizing succinate to fumarate while transferring electrons to reduce $FAD$ to $FADH_2$.

Fumarase

The enzyme catalyzing Step 7 of the citric acid cycle, which adds a molecule of $H_2O$ across the double bond of fumarate to yield malate.

Malate Dehydrogenase

The enzyme catalyzing Step 8 of the citric acid cycle, which oxidizes malate back into oxaloacetate while reducing $NAD^+$ to $NADH + H^+$.

Glycogen Granules

Cytoplasmic aggregates of branched glucose polymers stored in liver and muscle cells that buffer fluctuating nutrient availability by mobilizing glucose on demand.

Fat Droplets

Specialized intracellular storage structures that store triacylglycerols, which are hydrolyzed and mobilized into fatty acids when cellular energy demands arise.