Welcome to the "Backup Plan": Anaerobic Respiration
In your previous studies, you learned how cells use oxygen to get energy from food (aerobic respiration). But what happens when oxygen runs out? Whether you are sprinting for the bus or a plant is stuck in waterlogged soil, life doesn't just stop. Cells switch to anaerobic respiration.
Think of aerobic respiration as a high-efficiency power plant and anaerobic respiration as a small, portable backup generator. It isn't as powerful, and it's a bit "messy," but it keeps the lights on when the main power goes out! Let’s dive into how this works for Edexcel Biology B.
1. What is Anaerobic Respiration?
According to the syllabus, anaerobic respiration is the partial breakdown of hexoses (like glucose) to produce a limited yield of ATP in the absence of oxygen.
Prerequisite Check: Remember that Glycolysis is the first stage of all respiration and happens in the cytoplasm. It doesn't need oxygen to produce a tiny bit of energy (2 ATP) and some reduced NAD (NADH).
The Problem: In aerobic respiration, NADH drops its electrons off at the Electron Transport Chain. But if there’s no oxygen, the Electron Transport Chain shuts down. If NADH can't drop its electrons anywhere, the cell runs out of "empty" NAD. Without NAD, Glycolysis stops, and the cell dies.
The Solution: Anaerobic respiration provides a way to regenerate NAD so that Glycolysis can keep running. It’s not about the products (lactate or ethanol); it’s all about keeping the NAD recycling!
Quick Review: Why do it?
• To regenerate NAD.
• To allow Glycolysis to continue.
• To produce a small amount of ATP when oxygen is scarce.
2. Anaerobic Respiration in Mammals (The Lactate Pathway)
When you exercise intensely, your muscles use oxygen faster than your blood can supply it. Your muscle cells switch to the lactate pathway.
The Process Step-by-Step:
1. Glycolysis happens as usual in the cytoplasm, turning glucose into pyruvate.
2. This creates 2 molecules of ATP and 2 molecules of reduced NAD (NADH).
3. To get the NAD back, pyruvate acts as an "electron sponge." It takes the hydrogen from NADH.
4. Pyruvate is reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase.
5. The NADH becomes oxidized NAD again, which goes back to help with more Glycolysis.
The Consequences: Lactate and Muscle Contraction
Lactate is a bit of a double-edged sword. It keeps you moving, but it has side effects:
• pH Drop: Lactate is acidic. As it builds up, it lowers the pH of the muscle tissue.
• Enzyme Inhibition: The acidic environment can change the shape (denature) of enzymes involved in muscle contraction.
• Fatigue: This leads to muscle fatigue and that "burning" sensation, eventually stopping the muscle from contracting effectively.
Don’t worry if this seems tricky! Just remember: Pyruvate + NADH → Lactate + NAD. The goal is always the NAD!
Memory Aid: "Lactate makes you Late." (It slows down your muscles because of the pH drop/fatigue!)
3. Anaerobic Respiration in Plants and Yeast (The Ethanol Pathway)
Plants (especially in flooded roots) and microorganisms like yeast use a different "backup generator." They produce ethanol and carbon dioxide.
The Process Step-by-Step:
1. Glycolysis occurs, producing pyruvate, 2 ATP, and NADH.
2. Decarboxylation: A molecule of \(CO_2\) is removed from pyruvate to form ethanal.
3. Reduction: Ethanal then takes the hydrogen from NADH to become ethanol.
4. This regenerates the NAD needed for Glycolysis to continue.
Key Difference to Remember: In mammals, the lactate pathway is reversible (lactate can be turned back into pyruvate later). In plants and yeast, the ethanol pathway is irreversible—you can’t turn ethanol back into pyruvate easily, and if ethanol levels get too high, it actually becomes toxic to the organism!
Did you know? This process in yeast is exactly how we make bread rise (the \(CO_2\)) and how we make beer (the ethanol)!
4. Comparing ATP Yields: Aerobic vs. Anaerobic
In the exam, you might be asked why aerobic respiration is "better" for energy.
Aerobic Respiration:
• Yield: Approximately 30 to 38 ATP per molecule of glucose.
• Reason: Glucose is fully oxidized. The Link Reaction, Krebs Cycle, and Oxidative Phosphorylation all take place.
Anaerobic Respiration:
• Yield: Only 2 ATP per molecule of glucose.
• Reason: Glucose is only partially broken down. The only ATP produced comes from Glycolysis. The energy remaining in the glucose is "locked away" in the bonds of the lactate or ethanol.
Quick Review Box: The Numbers
• Aerobic: ~38 ATP (High efficiency)
• Anaerobic: 2 ATP (Low efficiency, but fast!)
5. Core Practical 9: Respirometers
You may study the rate of respiration using a respirometer. This piece of equipment measures the change in gas volume.
• When measuring aerobic respiration, we use soda lime or potassium hydroxide to absorb the \(CO_2\) produced. The drop in volume then shows us exactly how much Oxygen was consumed.
• When investigating anaerobic respiration, we ensure no oxygen is present (e.g., by using a layer of oil over a yeast suspension).
Common Mistake to Avoid: Students often forget that in the ethanol pathway, \(CO_2\) is produced. In the lactate pathway, no gas is produced or consumed. This is a favorite "trick" question in multiple-choice papers!
Summary Takeaway
• Purpose: To regenerate NAD so Glycolysis can keep making a small amount of ATP without oxygen.
• Mammals: Glucose → Pyruvate → Lactate (Reversible, causes muscle fatigue via pH drop).
• Plants/Yeast: Glucose → Pyruvate → Ethanal → Ethanol + \(CO_2\) (Irreversible).
• Energy Yield: Much lower than aerobic (only 2 ATP per glucose) because hexoses are only partially broken down.
You've reached the end of the notes for Anaerobic Respiration! Great job. Keep practicing those pathways, and you'll master this chapter in no time.