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 and , 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.
One glucose molecule is oxidised to two pyruvate molecules with a net yield of 2 ATP and 2 NADH
=Glucose (6-carbon sugar)(molecules)
=Pyruvate (3-carbon compound)(molecules)
=Nicotinamide adenine dinucleotide (oxidised/reduced forms)(molecules)
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 and the remaining 2-carbon fragment is attached to coenzyme A to form Acetyl-CoA. This step also generates one NADH per pyruvate.
Pyruvate is decarboxylated and oxidised to form acetyl-CoA, the substrate that enters the Krebs cycle
=Coenzyme A (carrier molecule)(—)
=Acetyl-CoA (active 2-carbon unit)(—)
=Carbon dioxide released by decarboxylation(—)
Pyruvate Dehydrogenase Complex: This multi-enzyme complex catalyses the conversion of pyruvate to acetyl-CoA. It requires five coenzymes: , CoA, FAD, lipoic acid, and thiamine ()
Link Reaction: This step links glycolysis to the Krebs cycle. For each glucose, two pyruvate molecules produce 2 acetyl-CoA, 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 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 → -ketoglutarate (5C), releasing and producing NADH. -Ketoglutarate → succinate (4C) releases another , produces NADH, and generates 1 ATP (GTP)
Succinate to Oxaloacetate: Succinate → fumarate (produces 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

Krebs Cycle Intermediates (in order)

1
Citrate (6C)
2
Isocitrate (6C)
3
-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 (), forming water. The energy released pumps protons () across the inner membrane, creating a gradient that drives Oxidative Phosphorylation — the synthesis of ATP by ATP Synthase.
Each NADH yields approximately 3 ATP through oxidative phosphorylation; each yields approximately 2 ATP
=Electron carrier from glycolysis, pyruvate oxidation, and Krebs cycle(—)
=Electron carrier from Krebs cycle (succinate oxidation only)(—)
=Final electron acceptor (half molecule of oxygen)(—)
donates electrons
→
Electrons enter ETC at coenzyme Q (bypassing the first proton-pumping complex), so only about 2 ATP are produced per
Cytochromes: These are haem-containing proteins that undergo reversible valency changes, transferring electrons along the chain. They include cytochrome b, c, a, and a₃
Chemiosmosis: Proton () pumping across the inner membrane creates an electrochemical gradient (proton motive force). 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 + 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
(final electron acceptor) →

Anaerobic Respiration

When oxygen is unavailable, pyruvate from Glycolysis undergoes fermentation instead of entering the Krebs Cycle. This regenerates 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 . 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 ), then reduced to ethanol by alcohol dehydrogenase (converting NADH to ). 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 + (occurs in yeast)
•
Both regenerate , 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 carbons undergoes cycles to produce acetyl-CoA molecules.
A 16-carbon fatty acid (palmitic acid) yields 8 acetyl-CoA, 7 NADH, and 7 through 7 cycles of beta-oxidation
=16-carbon saturated fatty acid ($C_{16}H_{32}O_2$)(—)
=Eight 2-carbon units that enter the Krebs cycle(—)
=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 . 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 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 () 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 -ketoglutarate) or by oxidative deamination in the liver. The nitrogen is converted to ammonia (), 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, -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, -ketoglutarate, succinyl-CoA, fumarate). These can feed into gluconeogenesis to produce glucose. Examples: alanine is directly transaminated to pyruvate; glutamate is converted to -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, -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
•
-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 → → ETC → ATP