# Anaerobic Respiration ## When Oxygen is Unavailable - Sometimes cells experience conditions with **little or no oxygen** - There are several consequences when there is not enough oxygen available for respiration: - There is **no final acceptor of electrons** from the electron transport chain - The electron transport chain **stops functioning** - **No more ATP is produced** via oxidative phosphorylation - Reduced NAD and FAD **aren't oxidised** by an electron carrier - No oxidised NAD and FAD are **available for dehydrogenation** in the Krebs cycle - The **Krebs cycle stops**

Without oxygen as the final electron acceptor, the electron transport chain halts and oxidative phosphorylation ceases

- However, there is still a way for cells to produce some ATP in low oxygen conditions through **anaerobic respiration** ## Anaerobic Pathways - Some cells are able to **oxidise the reduced NAD** produced during glycolysis so it can be used for further hydrogen transport - This means that **glycolysis can continue** and **small amounts of ATP** are still produced - Different cells use different pathways to achieve this: - **Yeast and microorganisms** use ethanol fermentation - **Other microorganisms and mammalian muscle cells** use lactate fermentation ## Ethanol Fermentation - In this pathway reduced NAD transfers its hydrogens to **ethanal to form ethanol** - In the first step of the pathway **pyruvate is decarboxylated to ethanal**: - Producing $\text{CO}_2$ - Then **ethanal is reduced to ethanol** by the enzyme alcohol dehydrogenase: - Ethanal is the **hydrogen acceptor** - **Ethanol cannot be further metabolised**; it is a waste product $$\text{Glucose} \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + \text{Energy (2 ATP)}$$

Ethanol fermentation is a two-step process: decarboxylation of pyruvate to ethanal, then reduction to ethanol

## Lactate Fermentation - In this pathway reduced NAD transfers its hydrogens to **pyruvate to form lactate** - **Pyruvate is reduced to lactate** by enzyme lactate dehydrogenase: - Pyruvate is the **hydrogen acceptor** - The final product **lactate can be further metabolised** $$\text{Glucose} \rightarrow 2\text{C}_3\text{H}_6\text{O}_3 + \text{Energy (2 ATP)}$$

Lactate fermentation is a one-step process where pyruvate is directly reduced to lactate

## Metabolization of Lactate - After lactate is produced two things can happen: 1. It can be **oxidised back to pyruvate** which is then channelled into the Krebs cycle for ATP production 2. It can be **converted into glycogen** for storage in the liver - The oxidation of lactate back to pyruvate **needs extra oxygen** - This extra oxygen is referred to as an **oxygen debt** - It explains why animals breathe deeper and faster after exercise ### Oxygen Debt - During intense exercise, muscle cells may not receive enough oxygen for aerobic respiration - They switch to lactate fermentation to continue producing ATP - Lactate accumulates in muscles (causing fatigue and the burning sensation) - After exercise stops, breathing rate remains elevated to: - Supply extra oxygen needed to oxidise lactate back to pyruvate - Replenish oxygen stores in muscles - "Repay" the oxygen debt

Think of oxygen debt like borrowing money - lactate fermentation lets you keep going, but you have to "pay back" the oxygen later

### Visualizing Oxygen Debt During Exercise and Recovery - The graph below shows what happens to lactate levels and oxygen consumption during a sprint followed by recovery: - Notice how **lactate accumulates rapidly** during the exercise period (0-2 minutes) - During recovery (2-12 minutes), **lactate gradually decreases** as it is oxidised back to pyruvate - **Oxygen consumption remains elevated** during recovery period - this is the oxygen debt being repaid

## Why Cells Respire Anaerobically - Anaerobic respiration allows cells to **continue producing ATP when oxygen is limited** - This is particularly important for: - **Athletes during intense bursts of activity** (sprinting, weightlifting) where oxygen delivery can't keep up with demand - **Organisms in low-oxygen environments** such as waterlogged soil or deep water - **Yeast in brewing and baking** where controlled anaerobic conditions are maintained ## Comparison of Pathways | Feature | Ethanol Fermentation | Lactate Fermentation | |---------|---------------------|---------------------| | Organisms | Yeast, microorganisms | Mammalian muscles, microorganisms | | Steps | Two-step process | One-step process | | Products | Ethanol + $\text{CO}_2$ | Lactate | | Hydrogen acceptor | Ethanal | Pyruvate | | Further metabolism | Ethanol is waste | Lactate can be oxidised | | Gas produced | Yes ($\text{CO}_2$) | No | ## Key Regeneration Mechanism - Both pathways achieve the same critical goal: **regenerating oxidised NAD** - Without this regeneration: - Glycolysis would stop (no NAD available to accept hydrogens) - No ATP would be produced at all - Cells would quickly run out of usable energy $$\text{Reduced NAD} \rightarrow \text{NAD}^+ + \text{Hydrogens}$$ - The hydrogens from reduced NAD are transferred to either: - **Ethanal** (in ethanol fermentation) - **Pyruvate** (in lactate fermentation) - This releases NAD to return to glycolysis and accept more hydrogens - The cycle can continue producing small amounts of ATP

Anaerobic respiration produces only 2 ATP per glucose compared to approximately 36 ATP from aerobic respiration

## Why Anaerobic Respiration Produces Less ATP - Only **glycolysis occurs** during anaerobic respiration - The link reaction, Krebs cycle, and oxidative phosphorylation **cannot proceed** - Glycolysis produces only **2 ATP per glucose molecule** (net gain) - Most of the energy in glucose remains **locked in the end products** (ethanol or lactate) - This is why anaerobic respiration is only used as a **short-term solution** when oxygen is unavailable