Lesson 5.03 – Chemical Equation for Cell Respiration

Standard:  B1.g Students know the role of the mitochondria in making stored chemical-bond energy available to cells by completing the breakdown of glucose to carbon dioxide.

INTRODUCTION

When you are hungry, how do you feel? If you are like most people, your stomach may seem empty, you might feel a little dizzy, and above all, you feel weak. The sensations produced by hunger may vary, but the bottom line is always the same. Our bodies have a way of telling us when we need food.

Food provides living things with the chemical building blocks they need to grow and reproduce. Food serves as a source of raw materials for the cells of the body. Most of all, food serves as a source of energy.

INSTRUCTION

How much energy is actually present in food? Quite a lot, although it varies with the type of food, since our cells can use all sorts of molecules as food, including fats, sugars and proteins. One gram of the sugar glucose (C₆H₁₂O₆), when burned in the presence of oxygen, releases 3811 calories of heat energy. A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius. The Calorie (capital “C”) that is used on food labels is a kilocalorie, or 1000 calories. Cells, of course, don't “burn” glucose. Instead, they gradually release the energy from glucose and other food compounds.  This process begins with a pathway called glycolysis (gly-KAHL-ih-sis). Glycolysis releases only a small amount of energy.

Overview of Cellular Respiration

Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen. The equation for cellular respiration is:

As you can see, cellular respiration requires oxygen, a food molecule such as glucose, and gives off carbon dioxide, water, and energy. Do not be misled, however, by the simplicity of this equation. If cellular respiration took place in just one step, all of the energy from glucose would be released at once, and most of it would be lost in the form of light and heat. Clearly, a living cell has to control that energy. It can't simply start a fire—it has to release the explosive chemical energy in food molecules a little bit at a time. The cell needs to find a way to trap those little bits of energy by using them to make ATP.

At the end of glycolysis, about 90 percent of the chemical energy that was available in glucose is still unused, locked in the high-energy electrons of pyruvic acid. To extract the rest of that energy, the cell turns to one of the world's most powerful electron acceptors—oxygen. Oxygen is required for the final steps of cellular respiration. Because the pathways of cellular respiration require oxygen, they are said to be aerobic.

As you know, the word respiration is often used as a synonym for breathing. This is why we have used the term cellular respiration to refer to energy-releasing pathways within the cell. The double meaning of respiration points out a crucial connection between cells and organisms: The energy-releasing pathways within cells require oxygen, and that is the reason we need to breathe, to respire.

The Krebs Cycle (also called Citric Acid Cycle)

In the presence of oxygen, pyruvic acid produced in glycolysis passes to the second stage of cellular respiration, the Krebs cycle. The Krebs cycle is named after Hans Krebs, the British biochemist who demonstrated its existence in 1937.  During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. Because citric acid is the first compound formed in this series of reactions, the Krebs cycle is also known as the citric acid cycle.

PRACTICE

1. Take notes on the above information.

2. Click here (http://videos.howstuffworks.com/hsw/12580-the-science-of-life-cellular-respiration-video.htm) to watch a video on Cell Respiration

3. Click here (http://videos.howstuffworks.com/hsw/8547-the-living-cell-mitochondria-video.htm) to watch a video on Mitochondria where Cell Respiration takes place.

ASSESSMENT

1. Turn in your notes.
2. Take the 5.03 Quiz.

Demo of Four Tango Dancing Steps -- powered by eHow.com