The catabolism of lipids and proteins
Glucose is not the only fuel the cell uses. Picture a cross-country runner in Colorado at mile 10 of a long race: once the glucose runs low, her cells turn to stored fats and, as a last resort, to protein. All of these pathways, by different routes, end up converging on the same point: the citric acid cycle (Krebs cycle). At the end of the page we'll come back to the runner.
Energy per gram
Fats store about $9\text{ kcal/g}$, while carbohydrates and proteins store about $4\text{ kcal/g}$. The body uses carbohydrates first because they are quick to mobilize and the brain depends on glucose. Fat is the long-term reserve. Protein comes last: every protein has a structural or enzymatic job, so burning it damages the body.
Order the three types of energy biomolecules (carbohydrates, lipids, and proteins) according to the energy they provide per gram, and explain why the body preferentially…
Order the three types of energy biomolecules (carbohydrates, lipids, and proteins) according to the energy they provide per gram, and explain why the body preferentially uses carbohydrates and turns to proteins only in extreme situations.
The β-oxidation of fatty acids
One gram of fat stores $38\text{ kJ}$ ($9\text{ kcal}$), more than twice as much as one gram of glucose ($17\text{ kJ}$), which allows migratory birds to fly thousands of miles without eating: how does the cell break down those very long hydrocarbon chains to extract so much energy from them?
Explain what β-oxidation is, where in the cell it occurs, and what product it feeds into the Krebs cycle. State also which electron-carrier molecules are generated…
Explain what β-oxidation is, where in the cell it occurs, and what product it feeds into the Krebs cycle. State also which electron-carrier molecules are generated during the process.
In β-oxidation, fatty acids are shortened by two carbon atoms at a time. Explain why it receives that name (on which carbon of the chain the oxidation occurs) and what…
In β-oxidation, fatty acids are shortened by two carbon atoms at a time. Explain why it receives that name (on which carbon of the chain the oxidation occurs) and what fragment is released in each round of the process.
A saturated fatty acid with $18$ carbon atoms is completely degraded by β-oxidation. Calculate how many rounds of β-oxidation are needed and how many acetyl-CoA…
A saturated fatty acid with $18$ carbon atoms is completely degraded by β-oxidation. Calculate how many rounds of β-oxidation are needed and how many acetyl-CoA molecules are obtained, explaining your reasoning for the calculation.
From stored fat to acetyl-CoA
When you skip breakfast before a morning of classes, where does your body find fuel? In your fat cells.
Before being oxidized, fatty acids must be activated by binding to coenzyme A. Explain what coenzyme A is and what general role it plays as a carrier of acyl groups in…
Before being oxidized, fatty acids must be activated by binding to coenzyme A. Explain what coenzyme A is and what general role it plays as a carrier of acyl groups in metabolism.
Where are reserve fats stored in the body? Explain what happens in adipose tissue when the body needs energy (lipolysis) and which molecules are then released into the…
Where are reserve fats stored in the body? Explain what happens in adipose tissue when the body needs energy (lipolysis) and which molecules are then released into the blood.
Explain why 1 g of fat yields more ATP than 1 g of glucose, referring to the degree of reduction of the carbons in the hydrocarbon chain of fatty acids and the quantity…
Explain why 1 g of fat yields more ATP than 1 g of glucose, referring to the degree of reduction of the carbons in the hydrocarbon chain of fatty acids and the quantity of electrons they can donate to the respiratory chain.
The catabolism of proteins
During prolonged fasting, or on a strict high-protein diet, the body uses amino acids as fuel, and the urea in the urine increases dramatically: how does the cell use the carbon skeleton of a protein without poisoning itself with the nitrogen it contains?
Amino acids from the protein you eat have three possible fates: (1) building new proteins, (2) being converted into other nitrogen-containing molecules (such as nucleotides), or (3) being broken down for energy. On a balanced diet, building new proteins takes priority.
The amino acids of ingested proteins can follow three fates: building new proteins, being transformed into other nitrogenous molecules, or being broken down to obtain…
The amino acids of ingested proteins can follow three fates: building new proteins, being transformed into other nitrogenous molecules, or being broken down to obtain energy. Explain them briefly and state which one takes priority when the diet is balanced.
A meeting point: acetyl-CoA
Summary
A saturated fatty acid with $18$ carbon atoms is completely degraded by β-oxidation. Calculate how many rounds of β-oxidation are needed and how many acetyl-CoA…
A saturated fatty acid with $18$ carbon atoms is completely degraded by β-oxidation. Calculate how many rounds of β-oxidation are needed and how many acetyl-CoA molecules are obtained, explaining your reasoning for the calculation.