Cellular Respiration of Carbohydrates, Fats, and Proteins
Cellular Respiration: An Overview
Cellular Respiration is the step-by-step breakdown of organic molecules within cells to harvest usable energy in the form of ATP. While glucose is the most common fuel, cells can also derive energy from Fats and Proteins. All three nutrient classes ultimately feed their carbon skeletons into the same central metabolic pathways — primarily Glycolysis, the Krebs Cycle, and the Electron Transport Chain.
Four Stages of Aerobic Respiration: Glycolysis (cytosol), pyruvate oxidation (mitochondrial matrix), Krebs cycle (mitochondrial matrix), and the respiratory chain (inner mitochondrial membrane)
Aerobic vs Anaerobic: Aerobic respiration requires oxygen and completely oxidises glucose to $CO_2$ and $H_2O$, yielding approximately 36–38 ATP. Anaerobic respiration (fermentation) occurs without oxygen and yields only 2 ATP per glucose molecule
Role of Mitochondria: Mitochondria are the primary sites of aerobic respiration. The inner membrane folds (cristae) house the ETC proteins, while the matrix contains Krebs cycle enzymes. They act as the powerhouses of the cell
Aerobic Respiration of Glucose
Glycolysis is the first stage of glucose breakdown, occurring in the cytosol. A 6-carbon glucose molecule is split into two molecules of 3-carbon Pyruvate through ten enzyme-catalysed steps. Glycolysis does not require oxygen and produces a net gain of 2 ATP and 2 NADH molecules.
$$C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \longrightarrow 2C_3H_4O_3 + 2NADH + 2H^+ + 2ATP$$
One glucose molecule is oxidised to two pyruvate molecules with a net yield of 2 ATP and 2 NADH
$C_6H_{12}O_6$=Glucose (6-carbon sugar)(molecules)
$C_3H_4O_3$=Pyruvate (3-carbon compound)(molecules)
$NAD^+ / NADH$=Nicotinamide adenine dinucleotide (oxidised/reduced forms)(molecules)
$O_2$ is absent
→Pyruvate enters fermentation (lactic acid or alcoholic) instead of further oxidation
Preparatory Phase: Glucose is phosphorylated using 2 ATP to form fructose 1,6-bisphosphate, which is then split into two 3-carbon molecules — G3P and dihydroxyacetone phosphate (DHAP)
Oxidative (Payoff) Phase: Each G3P is oxidised, producing NADH and 1,3-bisphosphoglycerate (BPG). Substrate-level phosphorylation then generates 4 ATP (net 2) and 2 pyruvate molecules
Key Intermediate: Glyceraldehyde 3-phosphate (G3P) is the crucial 3-carbon intermediate. Glycerol from fat breakdown also enters glycolysis at this point as DHAP, which is isomerised to G3P
Glycolysis Energy Yield (per glucose)
1
2 ATP consumed (preparatory phase)
2
4 ATP produced (payoff phase)
3
Net: 2 ATP
4
2 NADH produced
5
2 Pyruvate produced
Pyruvate does not enter the Krebs Cycle directly. It is first transported into the mitochondrial matrix where it undergoes oxidative decarboxylation. Each pyruvate (3C) loses one carbon as $CO_2$ and the remaining 2-carbon fragment is attached to coenzyme A to form Acetyl-CoA. This step also generates one NADH per pyruvate.
$$C_3H_4O_3 + NAD^+ + CoA \longrightarrow CH_3CO \sim SCoA + CO_2 + NADH + H^+$$
Pyruvate is decarboxylated and oxidised to form acetyl-CoA, the substrate that enters the Krebs cycle
$CoA$=Coenzyme A (carrier molecule)(—)
$CH_3CO \sim SCoA$=Acetyl-CoA (active 2-carbon unit)(—)
$CO_2$=Carbon dioxide released by decarboxylation(—)
Pyruvate Dehydrogenase Complex: This multi-enzyme complex catalyses the conversion of pyruvate to acetyl-CoA. It requires five coenzymes: $NAD^+$, CoA, FAD, lipoic acid, and thiamine ($B_1$)
Link Reaction: This step links glycolysis to the Krebs cycle. For each glucose, two pyruvate molecules produce 2 acetyl-CoA, 2 $CO_2$, and 2 NADH
Irreversible Step: Pyruvate oxidation is a key irreversible step. Once pyruvate is converted to acetyl-CoA, it cannot return to glycolysis
The Krebs Cycle (citric acid cycle) is a series of eight enzyme-catalysed reactions in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of oxidations and decarboxylations, two carbons are released as $CO_2$ and oxaloacetate is regenerated, completing the cycle. Each turn produces 1 ATP (via GTP), 3 NADH, and 1 $FADH_2$.
Cycle Entry: Acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C). CoA is released and reused
Oxidative Decarboxylation: Citrate → isocitrate → $\alpha$-ketoglutarate (5C), releasing $CO_2$ and producing NADH. $\alpha$-Ketoglutarate → succinate (4C) releases another $CO_2$, produces NADH, and generates 1 ATP (GTP)
Succinate to Oxaloacetate: Succinate → fumarate (produces $FADH_2$ via succinate dehydrogenase) → malate → oxaloacetate (produces NADH)
Per Glucose: Two turns of the cycle (one per acetyl-CoA) yield: 2 ATP, 6 NADH, and 2 $FADH_2$
Krebs Cycle Intermediates (in order)
1
Citrate (6C)
2
Isocitrate (6C)
3
$\alpha$-Ketoglutarate (5C)
4
Succinyl-CoA (5C)
5
Succinate (4C)
6
Fumarate (4C)
7
Malate (4C)
8
Oxaloacetate (4C)
The Electron Transport Chain (ETC) is located in the inner mitochondrial membrane (cristae). NADH and $FADH_2$ donate electrons to a series of carrier proteins — coenzyme Q, cytochromes (b, c, a, a₃) — which pass electrons to the final acceptor, molecular oxygen ($O_2$), forming water. The energy released pumps protons ($H^+$) across the inner membrane, creating a gradient that drives Oxidative Phosphorylation — the synthesis of ATP by ATP Synthase.
$$NADH + H^+ + \frac{1}{2}O_2 + 3ADP + 3P_i \longrightarrow NAD^+ + H_2O + 3ATP$$
Each NADH yields approximately 3 ATP through oxidative phosphorylation; each $FADH_2$ yields approximately 2 ATP
$NADH$=Electron carrier from glycolysis, pyruvate oxidation, and Krebs cycle(—)
$FADH_2$=Electron carrier from Krebs cycle (succinate oxidation only)(—)
$\frac{1}{2}O_2$=Final electron acceptor (half molecule of oxygen)(—)
$FADH_2$ donates electrons
→Electrons enter ETC at coenzyme Q (bypassing the first proton-pumping complex), so only about 2 ATP are produced per $FADH_2$
Cytochromes: These are haem-containing proteins that undergo reversible $Fe^{2+}/Fe^{3+}$ valency changes, transferring electrons along the chain. They include cytochrome b, c, a, and a₃
Chemiosmosis: Proton ($H^+$) pumping across the inner membrane creates an electrochemical gradient (proton motive force). $H^+$ flows back through ATP synthase, driving ATP synthesis. This mechanism occurs in the inner membrane folded into cristae
Total ATP from Glucose: Approximately 36–38 ATP: 2 (glycolysis) + 2 (Krebs substrate-level) + approximately 34 (ETC: 10 $\text{NADH} \times 3$ + 2 $\text{FADH}_2 \times 2$)
ETC Carriers in Order
1
NADH → Complex I
2
Coenzyme Q (ubiquinone)
3
Cytochrome b → Complex III
4
Cytochrome c
5
Cytochrome a/a₃ → Complex IV
6
$O_2$ (final electron acceptor) → $H_2O$
Anaerobic Respiration
When oxygen is unavailable, pyruvate from Glycolysis undergoes fermentation instead of entering the Krebs Cycle. This regenerates $NAD^+$ from NADH, allowing glycolysis to continue producing ATP. However, fermentation yields only 2 ATP per glucose — approximately 2% of the energy available in glucose — as the products (lactate or ethanol) still contain most of the original chemical energy.
Lactic Acid Fermentation: Each pyruvate is directly reduced to lactate by lactate dehydrogenase, converting NADH back to $NAD^+$. This occurs in muscle cells during intense exercise when oxygen supply cannot keep up with demand
Alcoholic Fermentation: Pyruvate is first decarboxylated to acetaldehyde (releasing $CO_2$), then reduced to ethanol by alcohol dehydrogenase (converting NADH to $NAD^+$). This occurs in yeast and some plant cells
Energy Limitation: Fermentation cannot sustain long-term energy needs because the organic end products still store most of glucose's energy — complete oxidation requires the Krebs cycle and ETC
Comparison of Fermentation Types
•
Lactic acid: Pyruvate → Lactate (occurs in muscle cells, some bacteria)
•
Alcoholic: Pyruvate → Acetaldehyde → Ethanol + $CO_2$ (occurs in yeast)
•
Both regenerate $NAD^+$, allow glycolysis to continue, and yield only 2 ATP per glucose
Respiration of Fats
Fats (triglycerides) are the body's primary long-term energy reserve, storing more than twice the energy per gram compared to carbohydrates. Before entering the central respiration pathways, fats must first be hydrolysed into glycerol and fatty acids, which then enter metabolism at different points along the glucose respiration pathway.
Lipolysis: Triglycerides are broken down by lipase enzymes into one glycerol molecule and three fatty acid molecules. This occurs in adipose tissue and the liver
Glycerol Entry: Glycerol is phosphorylated to glycerol-3-phosphate, then oxidised to Dihydroxyacetone Phosphate (DHAP). DHAP is isomerised to G3P, which enters glycolysis at the payoff phase — the same pathway glucose follows
Fatty Acid Entry: Fatty acids undergo Beta-Oxidation in the mitochondrial matrix, where they are sequentially cleaved into 2-carbon acetyl-CoA units that enter the Krebs cycle directly
Beta-Oxidation is the process by which fatty acids are broken down into Acetyl-CoA units in the mitochondrial matrix. Each cycle of beta-oxidation removes a 2-carbon fragment from the fatty acid chain, producing one acetyl-CoA, one NADH, and one $FADH_2$. A fatty acid with $n$ carbons undergoes $(n/2 - 1)$ cycles to produce $(n/2)$ acetyl-CoA molecules.
$$\text{Palmitic acid (C}_{16}) \longrightarrow 8\text{ Acetyl-CoA} + 7\text{NADH} + 7\text{FADH}_2$$
A 16-carbon fatty acid (palmitic acid) yields 8 acetyl-CoA, 7 NADH, and 7 $FADH_2$ through 7 cycles of beta-oxidation
$\text{Palmitic acid (C}_{16})$=16-carbon saturated fatty acid ($C_{16}H_{32}O_2$)(—)
$8\text{ Acetyl-CoA}$=Eight 2-carbon units that enter the Krebs cycle(—)
$7\text{NADH} + 7\text{FADH}_2$=Electron carriers from beta-oxidation, feeding the ETC(—)
Odd-chain fatty acids
→The final cleavage produces one propionyl-CoA (3C) instead of acetyl-CoA, which is converted to succinyl-CoA and enters the Krebs cycle
Each Beta-Oxidation Cycle: Two carbons are removed as acetyl-CoA, with simultaneous production of 1 NADH and 1 $FADH_2$. The process repeats until the entire fatty acid chain is cleaved
Total Energy from Palmitic Acid: 8 acetyl-CoA through Krebs + ETC = 96 ATP, plus 7 NADH yielding 21 ATP and 7 $FADH_2$ yielding 14 ATP, minus 2 ATP (activation cost) = approximately 129 ATP total
Comparison with Glucose: One palmitic acid molecule yields approximately 129 ATP, while one glucose molecule yields approximately 38 ATP. Fats are a far more energy-dense fuel source per molecule
Respiration of Proteins
Proteins are used as an energy source primarily during prolonged fasting or starvation when carbohydrate and fat reserves are depleted. Before amino acids can enter the respiratory pathways, they must first be deaminated — the amino group ($NH_2$) is removed and converted to urea for excretion. The remaining carbon skeleton then feeds into Glycolysis or the Krebs Cycle at various entry points, connecting protein respiration to the same central pathway used for glucose.
Proteolysis: Proteins are first digested into individual amino acids by protease enzymes (pepsin, trypsin, etc.) in the stomach and small intestine
Deamination: The amino group is removed by transaminases (transfer to $\alpha$-ketoglutarate) or by oxidative deamination in the liver. The nitrogen is converted to ammonia ($NH_3$), then detoxified to urea via the ornithine cycle for safe excretion
Carbon Skeleton Entry: Depending on the specific amino acid, the remaining carbon skeleton enters metabolism as pyruvate, acetyl-CoA, oxaloacetate, $\alpha$-ketoglutarate, succinyl-CoA, or fumarate
Different amino acids enter the central metabolic pathways at different points. Based on their degradation products, amino acids are classified as Glucogenic Amino Acids (those whose carbon skeletons can be converted to glucose), Ketogenic Amino Acids (those that produce acetyl-CoA or ketone bodies), or both. This classification determines exactly where each amino acid connects to the glucose respiration pathway.
Glucogenic Amino Acids: Their carbon skeletons are converted to pyruvate or Krebs cycle intermediates (oxaloacetate, $\alpha$-ketoglutarate, succinyl-CoA, fumarate). These can feed into gluconeogenesis to produce glucose. Examples: alanine is directly transaminated to pyruvate; glutamate is converted to $\alpha$-ketoglutarate; aspartate is converted to oxaloacetate
Ketogenic Amino Acids: Their carbon skeletons are degraded to acetyl-CoA or acetoacetyl-CoA. Since animals cannot convert acetyl-CoA back to pyruvate, these amino acids cannot replenish blood glucose. Leucine and lysine are purely ketogenic
Both Glucogenic and Ketogenic: Some amino acids (isoleucine, phenylalanine, tyrosine, threonine, tryptophan) produce both glucogenic and ketogenic fragments upon degradation
Ketone Bodies: During prolonged fasting, excess acetyl-CoA from fatty acid and ketogenic amino acid breakdown is converted to ketone bodies (acetone, acetoacetate, $\beta$-hydroxybutyrate) in the liver, which serve as an alternative fuel for the brain and muscles
Amino Acid Entry Points into Metabolism
•
Pyruvate: Alanine, cysteine, glycine, serine, threonine
•
Acetyl-CoA: Leucine, lysine, isoleucine, tryptophan, phenylalanine, tyrosine
•
$\alpha$-Ketoglutarate: Glutamate, glutamine, proline, arginine, histidine
•
Succinyl-CoA: Methionine, isoleucine, valine, threonine
•
Oxaloacetate: Aspartate, asparagine
•
Fumarate: Phenylalanine, tyrosine
Correlating the Three Pathways
Carbohydrates, fats, and proteins all converge on a shared metabolic core. Glucose enters at the top through Glycolysis, glycerol from fats enters glycolysis midway (as G3P via DHAP), and fatty acids feed Acetyl-CoA directly into the Krebs Cycle. Amino acid carbon skeletons enter at multiple points throughout both pathways. Despite their different entry points, all three nutrient classes ultimately generate NADH and $FADH_2$ that feed the same Electron Transport Chain to produce ATP.
Common Final Pathway: Regardless of the original fuel source, all energy-yielding substrates converge on the Krebs cycle and ETC. This is why mitochondria are central to the respiration of all three nutrient classes
Fuel Preference: Cells preferentially use glucose first (easily accessible, quick ATP). When glucose is depleted, fatty acids become the primary fuel through beta-oxidation. Proteins are used as a last resort because they serve critical structural and enzymatic roles essential for survival
Interconversion Limits: Glycerol can be converted to glucose via gluconeogenesis. Fatty acids cannot be converted to glucose in animals. Certain amino acids can replenish both glucose and Krebs cycle intermediates depending on their classification
Entry Points Summary
•
Glucose → Glycolysis → Pyruvate → Acetyl-CoA → Krebs Cycle
•
Glycerol → DHAP/G3P → Glycolysis → Pyruvate → Acetyl-CoA → Krebs Cycle
•
Fatty Acids → Beta-oxidation → Acetyl-CoA → Krebs Cycle
•
Amino Acids → Pyruvate or Acetyl-CoA or Krebs cycle intermediates
•
All pathways → $\text{NADH}/\text{FADH}_2$ → ETC → ATP
Application Strategy
Worked Examples
Problem Statement
Calculate the total ATP yield from the complete aerobic respiration of one molecule of glucose.
GlycolysisKrebs CycleETCOxidative Phosphorylation
Strategic Toolkit
$$\text{ATP}_{\text{total}} = \text{ATP}_{\text{substrate-level}} + \text{ATP}_{\text{oxidative}}$$
Step 1
Glycolysis ATP
Glycolysis produces 4 ATP (substrate-level) but consumes 2 ATP in the preparatory phase, giving a net of 2 ATP. It also produces 2 NADH.
$$\text{Glycolysis: Net 2 ATP} + 2\text{NADH}$$
Step 2
Pyruvate Oxidation
Two pyruvate molecules are converted to two acetyl-CoA in the mitochondria, producing 2 NADH (one per pyruvate).
$$2\text{Pyruvate} \longrightarrow 2\text{Acetyl-CoA} + 2\text{NADH}$$
Step 3
Krebs Cycle ATP
Two turns of the Krebs cycle produce 2 ATP (GTP) via substrate-level phosphorylation, plus 6 NADH and 2 $FADH_2$.
$$2\text{ATP} + 6\text{NADH} + 2\text{FADH}_2$$
Step 4
ETC ATP from NADH
Total NADH from all stages: 2 (glycolysis) + 2 (pyruvate oxidation) + 6 (Krebs) = 10 NADH. At 3 ATP per NADH, this yields 30 ATP.
$$10\text{NADH} \times 3\text{ ATP/NADH} = 30\text{ ATP}$$
Step 5
ETC ATP from FADH₂
Two $FADH_2$ from the Krebs cycle each yield 2 ATP through the ETC (entering at coenzyme Q, bypassing Complex I).
$$2\text{FADH}_2 \times 2\text{ ATP/FADH}_2 = 4\text{ ATP}$$
Step 6
Total ATP
Adding all contributions: 2 (glycolysis net) + 2 (Krebs substrate-level) + 30 (NADH via ETC) + 4 ($FADH_2$ via ETC) = 38 ATP. This may be reduced to 36 due to the cost of transporting glycolytic NADH into the mitochondrion.
$$2 + 2 + 30 + 4 = \boxed{38\text{ ATP}}$$
Problem Statement
A 16-carbon saturated fatty acid (palmitic acid, $C_{16}H_{32}O_2$) undergoes complete beta-oxidation and enters aerobic respiration. Calculate the total ATP yield.
Beta-OxidationKrebs CycleETC
Strategic Toolkit
$$\text{Cycles} = \frac{n}{2} - 1, \quad \text{Acetyl-CoA produced} = \frac{n}{2}$$
Step 1
Beta-Oxidation Products
For a 16C fatty acid: number of beta-oxidation cycles = 16/2 − 1 = 7. This produces 7 NADH, 7 $FADH_2$, and 8 acetyl-CoA.
$$7\text{ cycles} \longrightarrow 7\text{NADH} + 7\text{FADH}_2 + 8\text{Acetyl-CoA}$$
Step 2
ATP from Beta-Oxidation Carriers
The 7 NADH from beta-oxidation yield 21 ATP (7 × 3), and the 7 $FADH_2$ yield 14 ATP (7 × 2).
$$7\text{NADH} \times 3 = 21\text{ ATP}, \quad 7\text{FADH}_2 \times 2 = 14\text{ ATP}$$
Step 3
ATP from Acetyl-CoA via Krebs + ETC
Each acetyl-CoA entering one turn of the Krebs cycle yields 3 NADH (9 ATP), 1 $FADH_2$ (2 ATP), and 1 ATP (GTP) = 12 ATP per acetyl-CoA. For 8 acetyl-CoA: 8 × 12 = 96 ATP.
$$8\text{Acetyl-CoA} \times 12\text{ ATP/acetyl-CoA} = 96\text{ ATP}$$
Step 4
Subtract Activation Cost
Activating the fatty acid (attaching it to CoA) consumes 2 ATP equivalents before beta-oxidation can begin.
$$\text{Activation cost} = 2\text{ ATP}$$
Step 5
Total ATP
Adding all contributions and subtracting the activation cost: 21 + 14 + 96 − 2 = 129 ATP total from one molecule of palmitic acid.
$$21 + 14 + 96 - 2 = \boxed{129\text{ ATP}}$$
Problem Statement
Glycerol ($C_3H_8O_3$), released from a triglyceride by lipolysis, enters glycolysis as G3P via DHAP. Calculate the ATP yield from one glycerol molecule through complete aerobic respiration.
Glycerol MetabolismGlycolysisKrebs CycleETC
Strategic Toolkit
$$\text{Glycerol} \longrightarrow \text{DHAP} \longrightarrow \text{G3P} \longrightarrow \text{Pyruvate}$$
Step 1
Glycerol Activation and Oxidation
Glycerol is phosphorylated to glycerol-3-phosphate (consuming 1 ATP), then oxidised by NAD⁺ to DHAP, producing 1 NADH. DHAP is isomerised to G3P.
$$\text{Glycerol} \xrightarrow{-1\text{ATP}} \text{Glycerol-3-P} \xrightarrow{+1\text{NADH}} \text{DHAP} \rightarrow \text{G3P}$$
Step 2
G3P Through Payoff Phase
One G3P molecule through the payoff phase of glycolysis yields 2 ATP (substrate-level) and 1 NADH, producing 1 pyruvate.
$$\text{G3P} \longrightarrow \text{Pyruvate}: +2\text{ATP} + 1\text{NADH}$$
Step 3
Pyruvate Oxidation
One pyruvate is converted to acetyl-CoA, producing 1 NADH and 1 $CO_2$.
$$\text{Pyruvate} \longrightarrow \text{Acetyl-CoA} + 1\text{NADH}$$
Step 4
Krebs Cycle (1 turn) and ETC
One acetyl-CoA through the Krebs cycle yields 3 NADH, 1 $FADH_2$, and 1 ATP (GTP). Total NADH from all steps: 1 + 1 + 1 + 3 = 6 NADH. Total $FADH_2$: 1. Substrate-level ATP: 2 + 1 = 3.
$$6\text{NADH} \times 3 = 18\text{ ATP}, \quad 1\text{FADH}_2 \times 2 = 2\text{ ATP}, \quad 3\text{ substrate-level ATP}$$
Step 5
Total ATP
Subtracting the activation cost from all contributions: 18 + 2 + 3 − 1 = 22 ATP total from one glycerol molecule.
$$18 + 2 + 3 - 1 = \boxed{22\text{ ATP}}$$
Problem Statement
The amino acid alanine undergoes transamination to produce pyruvate and glutamate. Trace the fate of the pyruvate through complete aerobic respiration and calculate the total ATP yield.
DeaminationTransaminationPyruvate OxidationKrebs CycleETC
Strategic Toolkit
$$\text{Alanine} + \alpha\text{-ketoglutarate} \xrightarrow{\text{transaminase}} \text{Pyruvate} + \text{Glutamate}$$
Step 1
Transamination of Alanine
Alanine transfers its amino group to $\alpha$-ketoglutarate via alanine transaminase, producing pyruvate and glutamate. The amino group is ultimately excreted as urea via the ornithine cycle.
$$\text{Alanine} \longrightarrow \text{Pyruvate} + NH_2\text{ (excreted as urea)}$$
Step 2
Pyruvate to Acetyl-CoA
The pyruvate carbon skeleton enters the mitochondrion and undergoes oxidative decarboxylation to acetyl-CoA, producing 1 NADH.
$$\text{Pyruvate} + NAD^+ + CoA \longrightarrow \text{Acetyl-CoA} + NADH + CO_2$$
Step 3
Krebs Cycle (1 turn)
One acetyl-CoA through the Krebs cycle produces 3 NADH, 1 $FADH_2$, 1 ATP (GTP), and 2 $CO_2$.
$$3\text{NADH} + 1\text{FADH}_2 + 1\text{ATP}$$
Step 4
ETC ATP Calculation
Total NADH: 1 (pyruvate oxidation) + 3 (Krebs) = 4 NADH, yielding 12 ATP. One $FADH_2$ yields 2 ATP. Substrate-level ATP from Krebs: 1.
$$4\text{NADH} \times 3 + 1\text{FADH}_2 \times 2 + 1 = 12 + 2 + 1$$
Step 5
Total ATP
One molecule of alanine yields approximately 15 ATP through complete aerobic respiration (after deamination removes the amino group).
$$12 + 2 + 1 = \boxed{15\text{ ATP}}$$
Memorize formulas
Glycolysis Net Equation
Net production per glucose molecule in the cytosol. Produces 2 pyruvate, 2 net ATP, and 2 NADH. Occurs with or without oxygen.
Formula
$$C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \longrightarrow 2C_3H_4O_3 + 2NADH + 2H^+ + 2ATP$$
Total ATP per Glucose
Complete aerobic respiration yield: 2 (glycolysis net) + 2 (Krebs substrate-level) + 30 (10 NADH × 3 ATP) + 4 (2 FADH₂ × 2 ATP) = 38 ATP. May be 36 due to glycolytic NADH shuttle cost.
Formula
$$2 + 2 + 30 + 4 = 38$$
Beta-Oxidation Cycle Count
For a fatty acid with n carbons: number of beta-oxidation cycles = n/2 − 1; number of acetyl-CoA produced = n/2. Each cycle yields 1 NADH and 1 FADH₂.
Formula
$$\text{Cycles} = \frac{n}{2} - 1, \quad \text{Acetyl-CoA} = \frac{n}{2}$$
ATP per Acetyl-CoA (Krebs + ETC)
Each acetyl-CoA through one Krebs cycle turn yields 3 NADH, 1 FADH₂, and 1 ATP. Through ETC: 3 × 3 + 1 × 2 + 1 = 12 ATP per acetyl-CoA.
Formula
$$3 \times 3 + 1 \times 2 + 1 = 12$$
Oxidative Phosphorylation (per NADH)
Each NADH donates electrons to Complex I of the ETC, producing 3 ATP. Requires ADP, inorganic phosphate, and oxygen as the final electron acceptor.
Formula
$$NADH + H^+ + 3ADP + 3P_i + \frac{1}{2}O_2 \longrightarrow NAD^+ + H_2O + 3ATP$$
Palmitic Acid Total ATP
C₁₆ fatty acid: 8 acetyl-CoA × 12 = 96 ATP, plus 7 NADH × 3 = 21 ATP, plus 7 FADH₂ × 2 = 14 ATP, minus 2 ATP (activation cost) = 129 ATP.
Formula
$$8(12) + 7(3) + 7(2) - 2 = 129$$
Transamination of Alanine
Alanine (a glucogenic amino acid) transfers its amino group to α-ketoglutarate, producing pyruvate and glutamate. The pyruvate carbon skeleton then enters normal aerobic respiration.
Formula
$$\text{Alanine} + \alpha\text{-ketoglutarate} \rightleftharpoons \text{Pyruvate} + \text{Glutamate}$$
Review
1
What are the net products of one molecule of glucose after glycolysis?
Ans
2 pyruvate, 2 NADH, 2 ATP
Review
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Glycolysis Net Equation
One glucose molecule is partially oxidized to two pyruvate molecules with a net gain of 2 ATP and 2 NADH.
$$C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \longrightarrow 2C_3H_4O_3 + 2NADH + 2H^+ + 2ATP$$
2
What is the total net ATP yield from the complete aerobic respiration of one glucose molecule?
Ans
38 ATP
Review
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Summing ATP from All Stages
Glycolysis net ATP (2) + Krebs cycle GTP (2) + NADH from glycolysis via ETC (4) + NADH from Krebs via ETC (18) + FADH₂ via ETC (4).
$$2 + 2 + 30 + 4 = 38\text{ ATP}$$
3
In the absence of oxygen, pyruvate in human muscle cells is converted into:
Ans
Lactic acid
Review
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Anaerobic Reduction of Pyruvate
Pyruvate accepts electrons from NADH, producing lactate and regenerating NAD⁺ for continued glycolysis.
$$C_3H_4O_3 + NADH + H^+ \longrightarrow C_3H_6O_3 + NAD^+$$
4
The Krebs cycle occurs in which cellular compartment?
Ans
Mitochondrial matrix
Review
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Compartmentalization of Respiration
Each stage of aerobic respiration occurs in a specific location: glycolysis in the cytoplasm, pyruvate oxidation and the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane.
Krebs Cycle → Mitochondrial Matrix
5
A saturated fatty acid with 18 carbon atoms undergoes beta-oxidation. How many acetyl-CoA molecules are produced?
Ans
9
Review
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Apply Beta-Oxidation Formula
Each cycle of beta-oxidation removes a 2-carbon unit as acetyl-CoA, leaving a shortened fatty acyl-CoA.
$$\text{Cycles} = \frac{18}{2} - 1 = 8, \quad \text{Acetyl-CoA} = \frac{18}{2} = 9$$
6
How many ATP are generated from one acetyl-CoA molecule passing through the Krebs cycle and the electron transport chain?
Ans
12 ATP
Review
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Krebs Cycle Products per Acetyl-CoA
Each acetyl-CoA yields 3 NADH, 1 FADH₂, and 1 GTP from one turn of the cycle.
$$3 \times 3 + 1 \times 2 + 1 = 12\text{ ATP}$$
7
Glycerol released from triglyceride breakdown enters cellular respiration at which stage?
Ans
Glycolysis, as dihydroxyacetone phosphate (G3P)
Review
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Glycerol Conversion Pathway
Glycerol is phosphorylated and oxidized to G3P (DHAP), an intermediate of glycolysis, then continues through glycolysis to pyruvate.
Glycerol → Glycerol-3-phosphate → DHAP (G3P) → Glycolysis → Pyruvate
8
Which of the following amino acids is classified as purely ketogenic?
Ans
Leucine
Review
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Classifying Amino Acids
Glucogenic amino acids yield pyruvate or Krebs cycle intermediates that can be converted to glucose. Ketogenic amino acids yield acetyl-CoA or ketone bodies that cannot.
Alanine → Pyruvate (glucogenic) | Leucine → Acetyl-CoA (ketogenic)
9
During transamination of alanine, the amino group is transferred to which molecule?
Ans
Alpha-ketoglutarate
Review
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Transamination of Alanine
The amino group from alanine is transferred to alpha-ketoglutarate, forming pyruvate and glutamate.
$$\text{Alanine} + \alpha\text{-ketoglutarate} \rightleftharpoons \text{Pyruvate} + \text{Glutamate}$$
10
Deamination of amino acids during catabolism results in the removal of:
Ans
An amino group as NH₃
Review
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Deamination Process
The amino group is cleaved from the amino acid, producing a keto acid and ammonia. Ammonia is toxic and is converted to urea in the liver.
Amino Acid → Keto Acid + NH₃ → (in liver) → Urea + H₂O
11
What is the approximate total ATP yield from the complete oxidation of palmitic acid (C₁₆)?
Ans
129 ATP
Review
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Beta-Oxidation of Palmitic Acid
A 16-carbon fatty acid requires 7 cycles, producing 8 acetyl-CoA, 7 NADH, and 7 FADH₂.
$$\text{Cycles} = \frac{16}{2} - 1 = 7, \quad \text{Acetyl-CoA} = \frac{16}{2} = 8$$
Total ATP Calculation
Acetyl-CoA through Krebs + ETC, plus NADH and FADH₂ from beta-oxidation cycles, minus 2 ATP activation cost.
$$8(12) + 7(3) + 7(2) - 2 = 96 + 21 + 14 - 2 = 129\text{ ATP}$$
12
A saturated fatty acid with 14 carbon atoms undergoes complete beta-oxidation. How many beta-oxidation cycles are required?
Ans
6
Review
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Apply Cycle Formula
Each cycle removes one acetyl-CoA (2 carbons) until the chain is fully cleaved.
$$\text{Cycles} = \frac{14}{2} - 1 = 6$$
13
The electron transport chain in eukaryotic cells is located on:
Ans
The inner mitochondrial membrane
Review
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ETC Location and Proton Gradient
The inner mitochondrial membrane houses the ETC complexes and ATP synthase, creating a proton gradient between the intermembrane space and the matrix.
Intermembrane Space (high H⁺) || Inner Mitochondrial Membrane (ETC + ATP Synthase) || Matrix (low H⁺)
14
What is the final electron acceptor in the electron transport chain during aerobic respiration?
Ans
Molecular oxygen (O₂)
Review
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Oxidative Phosphorylation Per NADH
NADH donates electrons through the chain to oxygen, the terminal acceptor, driving proton pumping and ATP synthesis.
$$NADH + H^+ + 3ADP + 3P_i + \frac{1}{2}O_2 \longrightarrow NAD^+ + H_2O + 3ATP$$
15
Which macromolecule yields the greatest amount of ATP per gram when completely oxidized?
Ans
Lipids (fats)
Review
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Energy Density Comparison
Fats are more reduced (more C–H bonds, less oxygen) than carbohydrates or proteins, so their complete oxidation releases more energy per unit mass.
Fats ≈ 9 kcal/g | Carbohydrates ≈ 4 kcal/g | Proteins ≈ 4 kcal/g
16
When pyruvate is oxidatively decarboxylated to acetyl-CoA, which coenzyme is reduced and what gas is released?
Ans
NAD⁺ is reduced to NADH and CO₂ is released
Review
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Pyruvate Decarboxylation
Pyruvate (3C) loses one carbon as CO₂ and is oxidized, producing acetyl-CoA (2C) and NADH.
$$\text{Pyruvate} + NAD^+ + CoA\text{-}SH \longrightarrow \text{Acetyl-CoA} + NADH + H^+ + CO_2$$
17
All of the following molecules can enter the Krebs cycle directly or as intermediates EXCEPT:
Ans
Pyruvate
Review
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Entry Points into the Krebs Cycle
Acetyl-CoA (from glucose, fatty acids, or ketogenic amino acids) condenses with oxaloacetate to form citrate, starting the cycle. Glucogenic amino acids may enter as various Krebs intermediates.
Pyruvate → (requires conversion) → Acetyl-CoA → Krebs Cycle
18
How many NADH molecules are produced from one glucose molecule during the Krebs cycle?
Ans
6 NADH
Review
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NADH from Krebs Cycle
Each turn of the Krebs cycle produces 3 NADH. Since one glucose generates 2 acetyl-CoA, there are 2 turns.
$$3\text{ NADH/turn} \times 2\text{ turns} = 6\text{ NADH}$$
19
If beta-oxidation of fatty acids is inhibited in a cell under fasting conditions, which process is most directly affected?
Ans
ATP generation from fat decreases
Review
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Consequence of Beta-Oxidation Block
Without beta-oxidation, fatty acids cannot be converted to acetyl-CoA, cutting off the major ATP yield from fats (up to 129 ATP for a C₁₆ fatty acid).
Fatty Acids → (blocked) ✗ → Acetyl-CoA → Krebs → ETC → ATP
20
Each FADH₂ yields fewer ATP than each NADH in the electron transport chain because FADH₂ donates electrons at:
Ans
Complex II (succinate dehydrogenase)
Review
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ETC Entry Points and ATP Yield
NADH enters at Complex I, pumping protons through Complexes I, III, and IV (3 ATP). FADH₂ enters at Complex II, pumping protons only through Complexes III and IV (2 ATP).
NADH → Complex I → III → IV → 3 ATP | FADH₂ → Complex II → III → IV → 2 ATP
Question 1 of 20
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1
What are the net products of one molecule of glucose after glycolysis?