Breakdown of Fats with Special Reference to β-Oxidation and its Energy Balance (In Plants)

1. Importance of Fats in Plants

Fats, mainly in the form of triacylglycerols (TAGs), are important reserve food materials in plants, especially in oil-rich seeds (e.g., sunflower, castor, groundnut).
Unlike carbohydrates, fats are highly reduced molecules, yielding more energy per gram upon oxidation. During seed germination, when photosynthesis is not yet established, stored fats are mobilized and converted into sugars (mainly sucrose) via the glyoxylate cycle to support the growth of the seedling.

Learn how plants break down fats through β-oxidation in peroxisomes, why it’s called β-oxidation, and how energy balance differs from animals.

2. Initial Breakdown: Lipolysis

Enzyme: Lipase hydrolyzes triacylglycerols into glycerol and free fatty acids.
Glycerol: Enters glycolysis after conversion to glyceraldehyde-3-phosphate.
Fatty acids: Transported into peroxisomes (in plants, not mitochondria as in animals) where they undergo β-oxidation.

Breakdown of Fats with Special Reference To Beta-Oxidation and Its Energy Balance.

3. β-Oxidation of Fatty Acids

Location:

In plants, β-oxidation occurs primarily in peroxisomes (also called glyoxysomes in germinating oil seeds).
In animals, it occurs in the mitochondrial matrix.

Why is it called β-Oxidation?

The name β-oxidation comes from the fact that the β-carbon (the second carbon atom from the carboxyl end of the fatty acid) is the site of oxidation during each cycle. Each turn of the cycle shortens the fatty acid chain by two carbons, releasing acetyl-CoA.

Steps of the β-Oxidation Spiral (One Cycle)

Activation of Fatty Acid
Fatty acid + CoA + ATP → Fatty acyl-CoA (uses 2 ATP equivalents).
Dehydrogenation (Oxidation at the β-carbon)
Acyl-CoA dehydrogenase forms a double bond between the α and β carbons, producing FADH₂.
Hydration
Water is added across the double bond, forming a hydroxyl group at the β-carbon.
Second Dehydrogenation
The hydroxyl group at the β-carbon is oxidized to a keto group, generating NADH.
Thiolysis
Coenzyme A cleaves the bond, releasing acetyl-CoA and a shortened fatty acyl-CoA (2 carbons shorter).

This shortened acyl-CoA re-enters the cycle until the fatty acid is completely degraded into acetyl-CoA molecules.

4. Energy Balance of β-Oxidation

Let’s consider Palmitic Acid (C₁₆:0) as an example:

Number of β-oxidation cycles: 7
Products:
8 Acetyl-CoA
7 NADH
7 FADH₂

ATP Yield (in animal mitochondria for reference):

From 8 Acetyl-CoA (via Krebs + ETC): 80 ATP
From 7 NADH: 21 ATP
From 7 FADH₂: 14 ATP
Gross total: 115 ATP
Minus 2 ATP (activation): 113 ATP net

In Plants:

β-oxidation in peroxisomes does not directly yield ATP.
Instead, NADH and FADH₂ are used for H₂O₂ detoxification (via catalase), not for ATP production.
The main product, acetyl-CoA, enters the glyoxylate cycle → converted into succinate → malate → glucose (via gluconeogenesis).
Thus, in plants, the real energy gain is indirect: fats are converted into sugars that later enter glycolysis and respiration to generate ATP.

5. Regulation of Fat Breakdown in Plants

Fat mobilization is triggered during seed germination, when the seedling cannot photosynthesize.
Hormonal regulation: Gibberellins stimulate the synthesis of lipases and enzymes of the glyoxylate cycle.
Once photosynthesis begins, fat breakdown slows down, and carbohydrates become the main energy source.

🌱 Key Takeaways

Fats in plants are stored as triacylglycerols and broken down in peroxisomes.
β-Oxidation is named because the β-carbon undergoes oxidation.
Products: acetyl-CoA, NADH, FADH₂ → acetyl-CoA enters glyoxylate cycle.
In plants, unlike animals, β-oxidation does not directly generate ATP; instead, it produces substrates for sugar synthesis, ensuring energy supply during early seedling growth.

Interactive Biology Quiz: β-Oxidation