Integration of carbohydrate, protein and fat metabolism
Carbohydrate, protein and fat pathways meet at acetyl-CoA and pyruvate, then share the TCA cycle as the common final pathway for energy.
Junction points
| Junction | Comes from | Goes to |
|---|---|---|
| Glucose-6-phosphate | Blood glucose, glycogen | Glycolysis, glycogen, HMP shunt (pentose phosphate pathway) |
| Dihydroxyacetone phosphate (DHAP) | Glycolysis; glycerol from fat | Glucose (gluconeogenesis) or glycerol-3-phosphate for making fat |
| Pyruvate | Glucose (glycolysis), alanine and other glucogenic amino acids, lactate | Acetyl-CoA, oxaloacetate (for gluconeogenesis), alanine, lactate |
| Acetyl-CoA | Glucose (via pyruvate), fatty acids (β-oxidation), ketogenic amino acids | TCA cycle, fatty acids, cholesterol, ketone bodies |
| TCA intermediates (oxaloacetate, α-ketoglutarate) | Glucogenic amino acids | Glucose (via oxaloacetate), non-essential amino acids |
What can turn into what
| Conversion | Possible? | How or why |
|---|---|---|
| Carbohydrate → fat | Yes | Extra glucose → acetyl-CoA → fatty acids → triacylglycerol, stored in adipose tissue. |
| Protein → glucose | Yes | Glucogenic amino acids (e.g., alanine) give pyruvate or TCA intermediates, which make glucose by gluconeogenesis. |
| Protein → fat | Yes | Amino acids that give acetyl-CoA (e.g., leucine, lysine) can form fatty acids and ketone bodies. |
| Carbohydrate → protein | Partly | Glucose gives carbon skeletons only for non-essential amino acids. Essential amino acids must come from food. |
| Fat → glucose | No (only glycerol) | Pyruvate → acetyl-CoA is one-way. For every 2 carbons entering the TCA cycle as acetyl-CoA, 2 leave as CO₂, so no extra oxaloacetate is left to make glucose. Glycerol enters as DHAP. (Odd-chain fatty acids give a little propionyl-CoA, which is glucogenic.) |
Fed, fasting and starvation
| State | Main hormone | What happens |
|---|---|---|
| Fed (a few hours after a meal) | Insulin ↑ | Glucose is the main fuel. Extra glucose is stored as glycogen (liver, muscle) and fat (adipose tissue). Protein synthesis rises. |
| Fasting (overnight, early fast) | Glucagon ↑, insulin ↓ | Liver breaks down glycogen (glycogenolysis) and starts gluconeogenesis. Adipose tissue releases fatty acids for muscle and liver. |
| Starvation (several days) | Glucagon and cortisol ↑, insulin very low | Liver glycogen is used up. Glucose now comes from amino acids (muscle protein) and glycerol. Liver makes ketone bodies; the brain slowly switches to them, which spares body protein. |
Role of each organ
| Organ | Fed state | Fasting / starvation |
|---|---|---|
| Liver | Stores glycogen; makes fat and sends it out as VLDL (very-low-density lipoprotein) | Releases glucose (glycogenolysis, gluconeogenesis); makes ketone bodies but cannot use them |
| Muscle | Takes up glucose with the help of insulin; stores glycogen; builds protein | Uses fatty acids and ketone bodies; sends alanine and glutamine to the liver. Its glycogen serves only itself (no glucose-6-phosphatase) |
| Adipose tissue | Takes up glucose and fat; stores triacylglycerol | Lipolysis by hormone-sensitive lipase releases fatty acids and glycerol |
| Brain | Uses glucose only | Glucose; in prolonged starvation, ketone bodies too. Cannot use fatty acids as fuel |
| Red blood cells | Glucose only (no mitochondria) | Glucose only |
Because fat cannot become glucose, a crash diet very low in carbohydrate makes the body break down muscle protein for glucose and raises ketone bodies. Enough carbohydrate from rice, chapati or millets has a protein-sparing effect. The pyruvate → acetyl-CoA step needs thiamine (B1), riboflavin (B2), niacin (B3) and pantothenic acid (B5).
2 marks: “Why can fatty acids not be converted into glucose?” Pyruvate → acetyl-CoA is irreversible, and acetyl-CoA gives no net oxaloacetate in the TCA cycle. Only glycerol can form glucose.
5 marks: “Write a note on the metabolic changes in starvation.” Glycogen runs out, gluconeogenesis from amino acids and glycerol, lipolysis, ketone bodies rise, brain adapts to ketones and protein is spared.
10 marks: “Explain the integration of carbohydrate, protein and fat metabolism.” Draw the acetyl-CoA–TCA hub chart, explain junction points and interconversions, then fed–fasting states and organ roles.
"Sugar can make fat, but fat can't make sugar" (except glycerol). Pyruvate → acetyl-CoA is the one-way street.