Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert, Mary Jean Holland & Joan Japha


ENERGY PRODUCTION BY CATABOLISM

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Introduction

Catabolism is a multistep process by which cells break down complex, energy-rich compounds such as glucose to form smaller, less energy-rich combounds such as carbon dioxide and water. Some of the chemical energy from the original energy-rich molecules is transferred to ATP (adenosine triphosphate), the common energy carrying molecules in cells. The two major types of catabolism are respiration and fermentation.

In respiration electrons are transported from an organic molecule, such as sugar, to an inorganic molecule (e.g., oxygen or nitrate). ATP is produced during electron transport. In aerobic respiration electrons are transported to oxygen, whereas in anaerobic respiration electrons are transported to some other inorganic molecule (not oxygen). Most of the cells in the human body produce ATP by aerobic respiration.

Aerobic respiration of carbohydrate involves a sequence of chemical reactions. The first set, called glycolysis, takes place in the cytoplasm, and does not require oxygen or electron transport. At the end of glycolysis sugar such as glucose, C6H12O6, has been transformed into two molecules of pyruvate, C3H3O3. Pyruvate enters mitochondria where the oxygen requiring reactions, the Krebs Cycle followed by electron transport, occur, and a large quantity of ATP is made. Water is formed at the end of transport, when oxygen receives electrons and hydrogen ions from the original glucose molecule. Oxygen must be present or the mitochondria shut down. The overall equation for the catabolism of glucose and transfer of energy from it to ATP in aerobic respiration is:

C6H12O6 + 6O2 + 36ADP + 36Pi 6CO2 + 6H2O + 36ATP

Energy-rich compounds, such as carboydrates from food, are broken down to form energy poor compounds (carbon dioxide and water). If you have ever wondered where the oxygen that your lungs extract from the air goes and where the carbon dioxide that your lungs put into the air comes from, now you know: mitochondria.

Fermentation, like aerobic respiration, also involves a sequence of chemical reactions, and commonly begins with glycolysis. But here the similarity ends: Fermentation does not involve electron transport. The pyruvate molecules (C3H3O3), produced by glycolysis, are converted into different organic molecules, and the kind formed depends upon the type of cell carrying out the fermentation. Yeast cells (Kingdom Fungi) form ethanol and carbon dioxide; we use yeast to make bread, beer and wine. Some bacteria (Kingdom Eubacteria) form acetic acid and are used to make vinegar. In humans and other animals strenuous muscular exercise can create an oxygen need greater than the rate of supply from blood; with oxygen in short supply, aerobic ATP production by mitochondria is replaced by simple glycolysis. The muscle cells transform pyruvate into lactic acid, and its build-up is sometimes painful.

Catabolism of a molecule of glucose by fermentation produces much less ATP than catabolism of a molecule of glucose by respiration. This result is not surprising because, the final products of fermentation—ethanol, acetic acid, lactic acid, etc.—have much more energy than the final products, carbon dioxide and water, produced by aerobic respiration.

 

Experiment I: Aerobic Cellular Respiration

In the demonstration, three experiments have been set up some hours before class. All contain peas, a common seed that you eat. Seeds contain a rich store of nutrients, and, when they germinate, the nutrients are broken down and used as an energy source for making ATP; carbon dioxide gas is a waste product of this aerobic cellular respiration.

Experimental setup 1 contains live, germinating pea seeds.
Experimental setup 2 contains germinated then boiled pea seeds.
Experimental setup 3 contains dry pea seeds that have not begun to germinate.
Experimental setup 4 is a tube of phenol red and glass pipette.

Gasses have accumulated in the space above the seeds. You will see a sealed thistle tube that penetrates the lid and another tube that leads from the experimental container into the phenol red solution in a test tube. The accumulated gas can be pushed out and bubbled through the phenol red by unstopping the thistle tube and filling the space up with water.

One of the obvious products of cellular respiration that can be observed is the build up of the gas carbon dioxide. When the gas is bubbled through water, carbonic acid is formed:

CO2 + H20 H2CO3 HCO3- + H+

Acid can be detected with phenol red solution. Phenol red solution has the property that it is yellow if the pH is acidic (<7) and pink if basic (>7).

What do you predict the color of phenol red will be in each of the setups.

Experiment 1

Experiment 2

Experiment 3

Experiment 4: Bubble a person’s breath directly through the indicator solution.

Your instructor will add the water so all can observe the results.

 

Experiment 2: Fermentation

Brewers' yeast (Saccharomyces) is a strain bred for anaerobic production of ATP. A by-product of this reaction is alcohol (ethanol):

C6H12O6  +  2ADP  +  2Pi 2(CH3CH2OH)  +  2CO2  +  2ATP

Brewers' yeast is also used in making bread. It is the production of carbon dioxide gas that makes the dough rise and the bread have a spongy texture when cooked.

You will be doing three experiments. Ingredients should be at room temperature, not chilled as in other experimental procedures. Stir each mixture with a clean rod (don't use the same one unless you clean it each time).

Pour the contents into three fermentation tubes. Cover the opening with parafilm, and tip the tube so that the blind compartment is filled and contains no air. You can then observe the production of carbon dioxide by the appearance of gas bubbles in this blind compartment. Incubate the tubes at 37 degrees Celsius. Incubators are in the back of the room.

Experiment

Length of tube filled with gas

Volume filled with gas

Minutes:

15

30

45

60

V  =  p(radius)2(length)

Beaker 1
15 mL 10% glucose +
0.5 grams fresh yeast*

         

Beaker 2
15 mL 1% starch +
0.5 grams fresh yeast*

         

Beaker 3
15 mL H2O +
0.5 grams fresh yeast*

         
*If cake yeast is not available, mix yeast in water so it has a pasty consistency, and use 5 ml.

What are your predictions for the amounts of gas that will be produced? You know from the equation above that yeast digests sugar and produces ethanol and carbon dioxide gas. Starch is a complex polysaccharide that is made of sugars covalently bonded end to end.

If you put yeast and sugar in a beaker of water and monitored the pH, would you expect the pH to change over time?


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Last updated 13 August 2000 (JHW/JJ)