Cell Metabolism & Energy

ATP • Glycolysis • Respiration • Fermentation • Yeast Metabolism

All living cells need energy to survive. This energy comes from metabolism, the set of chemical reactions that convert nutrients into usable cellular energy.
This page introduces the essential pathways — ATP production, glycolysis, respiration, and fermentation — with a special focus on yeast, one of the most studied organisms in energy biology.

1. What Is ATP?

ATP (Adenosine Triphosphate) is the cell’s primary energy currency.

Cells use ATP for:

• movement (motor proteins like myosin, kinesin)
• building molecules (protein, DNA, RNA synthesis)
• transporting ions across membranes
• cell division
• maintaining shape (cytoskeleton)

ATP is constantly produced and consumed.
A human cell recycles its entire ATP pool every ~1–2 minutes.

2. Glycolysis: The First Step of Energy Production

Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, producing a small amount of ATP.

2.1. Where it happens

• Cytoplasm (in both yeast and onion cells)
• Does NOT require oxygen (anaerobic)

2.2. What glycolysis produces

For each glucose molecule:

• 2 ATP
• 2 NADH
• 2 pyruvate molecules

2.3. Importance

Glycolysis is the foundation of:

• aerobic respiration
• fermentation (used heavily by yeast)

Optical visibility

None of glycolysis is visible under a light microscope.

3. Aerobic Respiration (with Oxygen)

When oxygen is available, cells convert pyruvate into a lot of ATP using the mitochondria.

3.1. Steps of aerobic respiration

1. Pyruvate oxidation → enters mitochondria
2. Citric Acid Cycle (Krebs Cycle) → produces NADH & FADH₂
3. Electron Transport Chain (ETC)
   - electrons flow through membrane proteins
   - oxygen is the final electron acceptor
   - generates a proton gradient
4. ATP Synthase uses the gradient to generate ~30–34 ATP

3.2. Total ATP per glucose (aerobic)

~36–38 ATP

Massive increase compared to glycolysis alone.

3.3. Yeast & respiration

Yeast can respire if oxygen is present, but often prefer fermentation (see below).

4. Fermentation (Anaerobic Energy Production)

When oxygen is absent (or limited), cells can still produce energy using fermentation.

4.1. In Yeast: Alcohol Fermentation

Yeast convert pyruvate into:

• Ethanol
• CO₂
• 2 ATP per glucose (very low)

Steps:

1. Pyruvate → acetaldehyde
2. Acetaldehyde → ethanol (regenerates NAD⁺)

Yeast use fermentation:

• when oxygen is low
• when sugar concentration is high
• when rapid ATP production is needed
• in brewing, baking, winemaking
• on solid media (agar plates)

4.2. Why Yeast Prefer Fermentation (Even With Oxygen!)

This is called the Crabtree Effect.

Yeast often ferment even when oxygen is available if:

• glucose is abundant
• they want to grow fast
• fermentation is simpler and faster, though less efficient

Respiration = efficient
Fermentation = fast

4.3. ATP comparison

• Respiration: ~36 ATP
• Fermentation: ~2 ATP

Fermentation is wasteful, but extremely rapid.

5. The Role of Mitochondria

Mitochondria are the cell’s “power plants”, essential for:

• aerobic respiration
• regulating reactive oxygen species (ROS)
• apoptosis (programmed cell death)

They contain their own DNA and replicate independently.

Optical visibility

• Typically not visible in onion or yeast with standard microscopy
• Require fluorescence or TEM to observe structure

6. NAD⁺ and NADH: The Cell’s Redox Currency

NAD⁺ and NADH are molecules that shuttle electrons during metabolism.

Key points:

• NAD⁺ = oxidized form
• NADH = reduced form
• Required for glycolysis and respiration
• Regenerated during fermentation

This is why fermentation is essential when oxygen is absent — without it, NAD⁺ would run out and glycolysis would stop.

7. Yeast Metabolism: Why It Matters

Yeast can switch between: - Respiration (when oxygen is high and glucose is low)
- Fermentation (when glucose is high or oxygen is low)

This flexibility makes yeast ideal for: - studying aging
- studying metabolism
- brewing & baking
- synthetic biology
- drug testing
- understanding mitochondrial diseases

Yeast lifespan is strongly influenced by: - mitochondrial function
- metabolic state
- availability of NAD⁺
- level of reactive oxygen species

8. What You Can See Under a Light Microscope

Even though metabolism is invisible directly, you can observe signs of metabolic state:

Yeast

• Rapid budding → high metabolic activity
• Large vacuoles → sometimes indicates aging or stress
• Chains of buds → active growth

Onion cells

• Not metabolically active once removed
• You can sometimes see cytoplasmic streaming in other plant cells (not onion epidermis)

But the actual biochemical reactions are too small to see.

9. Summary Table: Metabolic Pathways

10. Quick Beginner Summary

• ATP is the universal energy molecule.
• Glycolysis runs in all cells, even without oxygen.
• Respiration is efficient but requires oxygen and mitochondria.
• Fermentation is inefficient but fast — yeast love it.
• Yeast switch between fermentation and respiration depending on the environment.
• None of the metabolic pathways are visible under a light microscope,
  but their effects on cell behavior can be observed.