Cellular Respiration

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Cellular Respiration in Yeast
Adapted from “Alcoholic Fermentation in Yeast Investigation” in the School District of Philadelphia Biology Core Curriculum © 2009 by Dr. Jennifer Doherty and Dr. Ingrid Waldron, University of Pennsylvania Biology Department[1]

All living cells, including the cells in your body and the cells in yeast, need energy for cellular processes such as pumping molecules into or out of the cell or synthesizing needed molecules. ATP is a special molecule which provides energy in a form that cells can use for cellular processes.

Cellular respiration is the process that cells use to transfer energy from the organic molecules in food to ATP. The following equation summarizes the chemical changes that occur in cellular respiration of the monosaccharide glucose when oxygen is available. C6H12O6 + 6 O2 (6 CO2 + 6 H2O + ATP

glucose oxygen carbon water energy gas dioxide gas

The chemical reactions in cellular respiration are similar to the chemical reactions when organic compounds are burned, but of course no ATP is produced. Instead energy is released in the form of light and heat. The following equation shows the chemical changes that occur when the monosaccharide glucose is burned. C6H12O6 + 6 O2 (6 CO2 + 6 H2O + light + heat

glucose oxygen carbon water energy gas dioxide gas

What are the similarities between this equation for burning glucose and the equation for cellular respiration of glucose when oxygen is available?

What is the difference between these equations?

There is another important feature of cellular respiration which is not shown in these equations. Cellular respiration involves many small steps; these multiple steps allow the cell to use the energy from each glucose molecule efficiently in order to make as many ATP molecules as possible. The multiple steps of cellular respiration are described in your textbook. Our description will focus on some major steps and how these steps differ, depending on whether oxygen is available or not.

The first major step in cellular respiration is glycolysis (see the figure on the top of page 2): 1 glucose (2 pyruvate + 2 ATP

What happens next depends on whether or not oxygen is available to the cells. When oxygen is available, cells can use the Krebs cycle and the electron transport chain to make up to 36 ATPs (see the right side of the figure).

2 pyruvate + 6 O2 (6 CO2 + 36 ATP

Cellular respiration that uses O2 is called aerobic respiration. Most of the time, the cells in our bodies use aerobic respiration:


When oxygen is not available, cells use anaerobic processes to produce ATP. (The "an" in front of aerobic means "not aerobic".)

Under anaerobic conditions, many cells use a process called fermentation to make ATP. As shown in the figure above, there are two types of fermentation: lactate fermentation (e.g. in muscles when an animal exercises hard) and alcoholic fermentation (e.g. by yeast to make wine and beer).

Fermentation has two disadvantages compared to aerobic respiration. Fermentation produces much less ATP than aerobic respiration, and fermentation produces a toxic byproduct (either lactate, which becomes lactic acid, or alcohol). However, fermentation is very useful if oxygen is not available.

Use the above information to complete the figures below. Fill in the ovals with the appropriate molecule. On the blank lines write the name of the appropriate process. In the boxes at the bottom of the figure write how much ATP is made in each pathway.


Humans use yeast every day. What is yeast? What are some common uses of yeast?

If you want to make your own bread, you can buy yeast in the grocery store....
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