READ: Respiration Stage 1

Overview of Respiration Stage 1

Powering the Cell: Introduction to Cellular Respiration

*Use the READING GUIDE as you read to help you identify and understand important concepts.

Powering the Cell: Cellular Respiration

Lesson Objectives

• Name the three stages of cellular respiration.
• Give an overview of glycolysis.
• Explain why glycolysis probably evolved before the other stages of aerobic respiration.
• Describe the mitochondrion and its role in aerobic respiration.
• List the steps of the Krebs cycle, and identify its products.
• Explain how electron transport results in many molecules of ATP.
• State the possible number of ATP molecules that can result from aerobic respiration.

Vocabulary

Introduction

You have just read how photosynthesis stores energy in glucose. How do living things make use of this stored energy? The answer is cellular respiration. This process releases the energy in glucose to make ATP, the molecule that powers all the work of cells.

An introduction to cellular respiration:

Stages of Respiration

Stages of Cellular Respiration

Cellular respiration involves many chemical reactions. As you saw earlier, the reactions can be summed up in this equation:

C₆H₁₂O₆ + 6O₂ ? 6CO₂ + 6H₂O + Chemical Energy (in ATP)

The reactions of cellular respiration can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure below gives an overview of these three stages, which are also described below.

Cellular Respiration Stage 1: Glycolysis

Cellular Respiration Stage I: Glycolysis

The first stage of cellular respiration is glycolysis. It takes place in the cytosol of the cytoplasm.

Splitting Glucose

The word glycolysis means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in Figure below.

You can watch an animation of the steps of glycolysis:

Results of Glycolysis

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD⁺ to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

Here is a summary of glycolysis:

Anaerobic and Aerobic Respiration

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.

Structure of the Mitochondrion

Structure of the Mitochondrion: Key to Aerobic Respiration

Before you read about the last two stages of aerobic respiration, you need to know more about the mitochondrion, where these two stages take place. A diagram of a mitochondrion is shown in Figure below.

As you can see from Figure above, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.

Lesson Summary

Lesson Summary

• Cellular respiration uses energy in glucose to make ATP. Aerobic (“oxygen-using”) respiration occurs in three stages: glycolysis, the Krebs cycle, and electron transport.
• In glycolysis, glucose is split into two molecules of pyruvate. This results in a net gain of two ATP molecules.
• Life first evolved in the absence of oxygen, and glycolysis does not require oxygen. Therefore, glycolysis was probably the earliest way of making ATP from glucose.
• The Krebs cycle and electron transport occur in the mitochondria. The Krebs cycle takes place in the matrix, and electron transport takes place on the inner membrane.
• During the Krebs cycle, pyruvate undergoes a series of reactions to produce two more molecules of ATP and also several molecules of NADH and FADH2.
• During electron transport, energy from NADH and FADH₂ is used to make many more molecules of ATP.
• In all three stages of aerobic respiration, up to 38 molecules of ATP may be produced from a single molecule of glucose.