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# 2.24: Anaerobic vs Aerobic Respiration

Difficulty Level: At Grade Created by: CK-12
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Practice Anaerobic vs Aerobic Respiration

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How long can you hold your breath?

With or without air? In terms of producing energy, that is the key question. Can cellular respiration occur without air? It can, but it does have limitations.

### The Presence of Oxygen

There are two types of cellular respiration (see "Cellular Respiration I: Introduction" concept): aerobic and anaerobic. One occurs in the presence of oxygen (aerobic), and one occurs in the absence of oxygen (anaerobic). Both begin with glycolysis - the splitting of glucose.

Glycolysis (see "Cellular Respiration II: Glycolysis" concept) is an anaerobic process - it does not need oxygen to proceed. This process produces a minimal amount of ATP. The Krebs cycle and electron transport do need oxygen to proceed, and in the presence of oxygen, these process produce much more ATP than glycolysis alone.

Scientists think that glycolysis evolved before the other stages of cellular respiration. This is because the other stages need oxygen, whereas glycolysis does not, and there was no oxygen in Earth’s atmosphere when life first evolved about 3.5 to 4 billion years ago. Cellular respiration that proceeds without oxygen is called anaerobic respiration.

Then, about 2 or 3 billion years ago, oxygen was gradually added to the atmosphere by early photosynthetic bacteria. After that, living things could use oxygen to break down glucose and make ATP. Today, most organisms make ATP with oxygen. They follow glycolysis with the Krebs cycle and electron transport to make more ATP than by glycolysis alone. Cellular respiration that proceeds in the presence of oxygen is called aerobic respiration.

### Summary

• Cellular respiration always begins with glycolysis, which can occur either in the absence or presence of oxygen.
• Cellular respiration that proceeds in the absence of oxygen is anaerobic respiration.
• Cellular respiration that proceeds in the presence of oxygen is aerobic respiration.
• Anaerobic respiration evolved prior to aerobic respiration.

### Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology\begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Metabolic Strategies
1. What is the role of oxygen in aerobic metabolism?
2. What is the role of oxygen in anaerobic metabolism?
3. Aerobic metabolism involves what processes?
4. Anaerobic metabolism is also known by what name?
5. How is water formed in the presence of oxygen?
6. Which form of metabolism produces more energy?

### Review

1. Define aerobic and anaerobic respiration.

2. Why do scientists think that glycolysis evolved before the other stages of cellular respiration?

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### Vocabulary Language: English Spanish

aerobic

In the presence of oxygen.

aerobic respiration

Type of cellular respiration that requires oxygen.

anaerobic

In the absence of oxygen.

anaerobic respiration

Type of cellular respiration that does not require oxygen.

glycolysis

First stage of cellular respiration in which glucose is split to form two molecules of pyruvate (pyruvic acid) and two (net) molecules of ATP.

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