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9-1 Chemical Pathways

Chemical Energy and Food
There is a lot of energy in food. One gram of glucose, when burned in the presence of oxygen, releases 3,811 calories of heat energy.

Calorie – the amount of energy needed to raise the temperature of 1 gram of water 1 Celsius degree. Calories in our food gradually release the energy from glucose and other food compounds. This process begins with a pathway called glycolysis.

Glycolysis releases only a small amount of energy
Glycolysis releases only a small amount of energy. If oxygen is present, glycolysis leads to two other pathways that release a lot of energy. If oxygen is not present, glycolysis is followed by a different pathway.

Overview of Cellular Respiration
When oxygen is present, glycolysis is followed by the Krebs cycle and the electron transport chain. This makes up a process called cellular respiration. Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen.

Glucose + oxygen → carbon dioxide + water + energy

Glycolysis Glycolysis is the process in which one molecule of glucose is broken in half, producing two molecules of pyruvic acid, a 3-carbon compound

In the beginning, 2 molecules of ATP are used
In the beginning, 2 molecules of ATP are used. During glycolysis, 4 high-energy electrons are passed to NAD+ . The process is so fast that cells can produce thousands of ATP molecules in just a few milliseconds. Also, glycolysis does not require oxygen.

However, when a cell generates large amount of ATP from glycolysis, all of the cell’s NAD+ molecules are filled up with electrons. Without NAD+, the cell can’t keep glycolysis going and ATP production stops.

Fermentation When oxygen is not present, glycolysis is followed by a different pathway. This process is called fermentation. Fermentation releases energy from food molecules in the absence of oxygen. Because it doesn’t require oxygen, It is called anaerobic. The two main types of fermentation are alcoholic fermentation and lactic acid fermentation

Alcoholic Fermentation
Yeasts and some other microorganisms use alcoholic fermentation, forming ethyl alcohol and carbon dioxide as wastes. Pyruvic acid + NADH → alcohol + CO2 + NAD+ -causes bread to rise

Lactic Acid Fermentation
In many cells, the pyruvic acid that accumulates as a result of glycolysis can be converted to lactic acid. Pyruvic acid + NADH → lactic acid + NAD+

-produced in your muscles during rapid exercise when the body can’t supply enough oxygen to the tissues -the buildup of lactic acid causes painful, burning sensation. This is why muscles feel sore after only a few seconds of intense activity.

9-2 The Krebs Cycle and Electron Transport

The Krebs Cycle When oxygen is present, the second stage of glycolysis is the Krebs Cycle. During the Krebs Cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.

Electron Transport After the Krebs cycle, the electrons are passed to the electron transport chain. The electron transport chain uses the high-energy electrons from the Krebs cycle to convert ADP into ATP.

Energy and Exercise Quick energy – When your body needs lots of energy in a hurry, fermentation produces lactic acid. To get rid of the excess lactic acid, oxygen is needed. (heavy breathing after a sprint)

Long-term energy – When your body needs energy for a long time (long race) cellular is needed to generate a continuing supply of ATP. Cellular respiration releases energy more slowly than fermentation (athletes have to pace themselves during a long race).

Comparing photosynthesis and Cellular Respiration
Photosynthesis and cellular respiration are almost opposite processes. Photosynthesis removes carbon dioxide from the atmosphere, and cellular respiration puts it back. Photosynthesis releases oxygen into the atmosphere, and cellular respiration uses that oxygen to release energy from food.

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