Fuels

What is beta-oxidation, and how does it lead to energy production from fatty acids?

When you eat foods with fat, your body stores those fats as fatty acids. These are long chains made of carbon and hydrogen.

To get energy from them, your cells break them down through a process called beta-oxidation, which happens in the mitochondria.

1.Activation of Fatty Acids

• In the cytoplasm, a fatty acid combines with Coenzyme A (CoA) to form fatty acyl-CoA.

• This step requires ATP.

2.Transport into the Mitochondria

• Fatty acyl-CoA can’t cross the mitochondrial inner membrane directly.

• It’s converted to acyl-carnitine via carnitine shuttle to enter the mitochondria, then re-converted back to acyl-CoA inside.

3.Beta-Oxidation Proper (in the mitochondrial matrix)

Each cycle of beta-oxidation shortens the fatty acyl-CoA by 2 carbon atoms, producing:

• 1 Acetyl-CoA (enters the Krebs cycle)

• 1 NADH (goes to electron transport chain for ATP)

• 1 FADH₂ (also goes to ETC)

The cycle repeats until the entire fatty acid is converted to acetyl-CoA.

Why can’t the body store excess amino acids?

Proteins in your diet are made of amino acids. Once your body takes in the amino acids, it uses what it needs to build muscles, enzymes, etc.

But excess amino acids cannot be stored like fat or carbs. So the body breaks them down.

a)Deamination:

• This is when the amino group (–NH₂) is removed from the amino acid.

• The amino group becomes ammonia (NH₃), which is toxic.

• Your liver quickly turns NH₃ into urea, which is then excreted in urine.

b)What happens to the rest?

• The rest of the molecule (called the carbon skeleton) can become:

  • Pyruvate

  • Acetyl-CoA

  • Krebs cycle intermediates

These can now be used to make ATP just like carbs or fats

Why is lactic acid fermentation important for regenerating NAD⁺ during glycolysis?

When oxygen is low, your body wants glycolysis to continue (because it gives 2 ATP).

But glycolysis needs NAD⁺ to keep going.

So your body uses lactic acid fermentation:

• It turns pyruvate (from glycolysis) into lactate.

• In the process, it converts NADH back into NAD⁺.

• This keeps glycolysis running even when there’s no oxygen.

Which energy substrates are preferred by skeletal muscle, cardiac muscle, brain, and red blood cells?

Tissue	Preferred Fuel	Why?
Brain	Glucose, switches to ketones during starvation	Needs a constant, steady energy source. Can’t burn fat directly.
Skeletal Muscle	Glucose (fed), fatty acids (rest), glucose again (intense exercise)	Highly flexible. Will use what’s available depending on effort level.
Heart Muscle	Mostly fatty acids, also lactate and ketones	Needs steady energy and loves fat.
Red Blood Cells	Only glucose →lactic acid	They have no mitochondria, so they rely fully on anaerobic glycolysis.

How do different tissues change their energy source preferences during fed, fasting, and starvation states?

Fed State (0–4 hours after eating):

• Your body uses glucose from your meal.

• Extra glucose is stored as glycogen (short-term) and fat (long-term).

Fasting State (4–30 hours):

• You run out of meal glucose.

• Your body uses stored glycogen and starts breaking down fat.

• The liver makes glucose from amino acids (called gluconeogenesis).

Starvation (after 30 hours):

• Brain switches from glucose to ketone bodies (made from fat).

• Liver keeps making ketones.

• Protein breakdown slows to preserve muscle.

• You become very energy-efficient—but it’s dangerous long-term