This chapter covers the “Energy Release” phase of plant life—how the sugars made during photosynthesis are broken down to power the plant’s growth and survival.

The Cellular Furnace: Mastering Respiration in Plants

While photosynthesis “stores” energy in the form of glucose, Respiration is the process that “releases” it. Every living cell in a plant—from the tip of the root to the surface of the leaf—must breathe to stay alive.

In this chapter, we look at how plants break down carbon-carbon bonds to produce ATP, the universal energy currency. We will travel from the Cytoplasm (Glycolysis) to the Mitochondrial Matrix (Krebs Cycle) and finally to the Inner Membrane (ETS).

The Core Pillars of Cellular Respiration

1. Glycolysis: The Universal First Step

This occurs in the Cytoplasm and does not require Oxygen. It is the only form of respiration in anaerobic organisms.

• The Process: One molecule of Glucose (6C) is broken down into two molecules of Pyruvic Acid (3C).
• The Gain: A net profit of 2 ATP and 2 NADPH + H⁺.

2. The Fate of Pyruvate

Depending on the presence of Oxygen, Pyruvic acid takes two very different paths:

• Anaerobic (Fermentation): Occurs in yeast (producing Ethanol) or muscle cells (producing Lactic Acid).
• Aerobic: Pyruvate enters the Mitochondria for the “Link Reaction” and the Krebs Cycle.

3. The Krebs Cycle (TCA Cycle)

This occurs in the Mitochondrial Matrix.

• Link Reaction: Pyruvate is converted to Acetyl CoA.
• The Cycle: Acetyl CoA is completely oxidized to CO₂. For every turn, we get 3 NADH, 1 FADH₂, and 1 GTP (ATP).

4. Electron Transport System (ETS) & Oxidative Phosphorylation

The “payday” of respiration. The NADH and FADH₂ produced earlier are used to create a proton gradient across the Inner Mitochondrial Membrane, which drives the synthesis of ATP via the enzyme ATP Synthase.

The Gauntlet: 10 Challenging Aptitude Questions

Question 1: The “Do Plants Breathe?” Mystery

Unlike animals, plants do not have specialized respiratory organs like lungs. How do they manage gas exchange in bulky parts like the woody stems of large trees?

Question 2: The EMP Pathway

Glycolysis is also known as the EMP Pathway. What do the letters E, M, and P stand for, and why is this pathway considered the most “primitive” form of energy extraction?

Question 3: The Energy Investment

In Glycolysis, the cell actually spends 2 ATP molecules before it starts making any. At which two specific steps of the 10-step process does this ATP consumption occur?

Question 4: Fermentation vs. Aerobic Respiration

In terms of energy efficiency, how much of the total energy available in a glucose molecule is released during Fermentation? (Hint: It is less than 10%!)

Question 5: The “Link” Reaction

Before entering the Krebs Cycle, Pyruvate undergoes Oxidative Decarboxylation. Which enzyme complex facilitates this, and what are the three products of this single reaction?

Question 6: The “Hub” Molecule

Why is Acetyl CoA often called the “Central Hub” of metabolism? Can molecules other than carbohydrates (like fats or proteins) enter the respiratory pathway through this molecule?

Question 7: Oxygen’s True Role

In the Electron Transport System (ETS), Oxygen is the “Final Electron Acceptor.” What would happen to the Krebs Cycle if Oxygen was removed from the system?

Question 8: The Respiratory Quotient (RQ)

The RQ is the ratio of CO₂ evolved to O₂ consumed. Calculate the RQ for a Carbohydrate (like Glucose) and a Fat (like Tripalmitin). Why is the value for fats less than 1?

Question 9: Amphibolic Pathway

We usually call Respiration a “Catabolic” (breaking down) process. Why do modern biologists prefer the term Amphibolic? Give an example of an “anabolic” (building up) use of a respiratory intermediate.

Question 10: ATP Synthase Geometry

The F₀-F₁ particle (ATP Synthase) is the machinery that makes ATP. Which part is the “peripheral membrane protein” that actually synthesizes ATP, and which part acts as the “proton channel”?

Detailed Explanations & Solutions

1. Plant Breathing

Plants use Stomata in leaves and Lenticels in the woody bark of stems.

Result: Lenticels are small openings that allow gases to reach the living cells inside the trunk.

2. EMP Pathway

Stands for Embden, Meyerhof, and Parnas.

Result: It is “primitive” because it happens in the cytoplasm and doesn’t require complex organelles or oxygen.

3. Energy Investment

ATP is used in:

1. Glucose → Glucose-6-Phosphate
2. Fructose-6-Phosphate → Fructose-1,6-Bisphosphate.
3. Result: These “prime” the molecule for splitting later.

4. Fermentation Efficiency

Result: Less than 7% of the energy in glucose is released. Most of the energy remains trapped in the bonds of Ethanol or Lactic Acid.

5. Link Reaction

The enzyme is Pyruvate Dehydrogenase.

Result: The products are Acetyl CoA, CO₂, and NADH + H⁺.

6. The Hub Molecule

Yes. Fats are broken into fatty acids and glycerol; fatty acids are then converted to Acetyl CoA.

Result: Acetyl CoA is the entry point for almost all organic substrates into the respiratory furnace.

7. Oxygen’s Role

Without Oxygen to “clear out” the electrons at the end of the ETS, the NADH and FADH₂ cannot be recycled.

Result: The whole system (including the Krebs Cycle) grinds to a halt.

8. Respiratory Quotient (RQ)

• Carbohydrates: RQ = 1.0
• Fats: RQ ~0.7
• Result: Fats require more oxygen for complete oxidation because they are oxygen-poor compared to carbohydrates.

9. Amphibolic Pathway

“Amphi” means both. Respiration involves catabolism to release energy, but its intermediates are often pulled out to build other things.

Result: For example, Acetyl CoA is pulled out to synthesize fatty acids when the cell has excess energy.

10. ATP Synthase Geometry

Result: F₁ is the “head” where ATP is made; F₀ is the “base” embedded in the membrane that allows protons to flow through.

Pro-Tip: The “ATP Payday” Summary

• NADH = 3 ATP
• FADH₂ = 2 ATP
• Total from 1 Glucose = 36 or 38 ATP (depending on the shuttle used).