McDougal Littell Science Cells and Heredity 

Table of Contents
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All cells release energy.

All cells must have energy to function. Glucose and other sugars are cell food—they are the power source for cell activities in almost all living things. When glucose is stored as glycogen or taken in as starch, it must be broken down into individual sugar molecules before cells are able to use it. Chemical energy is stored in the bonds of sugars. When a sugar molecule is broken down, a usable form of energy is released for the cell's life functions.

Cells can release energy in two basic processes: cellular respiration and fermentation. Cellular respiration requires oxygen, but fermentation does not. In addition, cellular respiration releases much more usable energy than does fermentation.


What is released when a sugar molecule is broken down?

Cellular Respiration

In cellular respiration, cells use oxygen to release energy stored in sugars such as glucose. In fact, most of the energy used by the cells in your body is provided by cellular respiration.

Just as photosynthesis occurs in organelles called chloroplasts, cellular respiration takes place in organelles called mitochondria. Remember that mitochondria are in both plant cells and animal cells, so both kinds of cells release energy through cellular respiration.

Like photosynthesis, cellular respiration is a process that changes starting materials into new products.

  1. 1 The starting materials of cellular respiration are sugars—such as glucose—and oxygen.
  2. 2 The process begins when glucose in the cytoplasm is broken down into smaller molecules. This releases a small amount of energy. These molecules then move into the mitochondria. At the same time, oxygen enters the cell and travels into the mitochondria. As the smaller molecules are broken down even further, hydrogen is released in a way that allows cells to capture energy in a usable form. The hydrogen combines with oxygen to make water.
  3. 3 The products are energy, carbon dioxide, and water.

    Some of the energy released during cellular respiration is transferred to other molecules, which then carry the energy where it is needed for the activities of the cell. The rest of the energy is released as heat. Carbon dioxide formed during cellular respiration is released by the cell.


What are the three products of cellular respiration?

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Cellular Respiration
Animal cell
  1. 1 The starting materials Glucose and oxygen enter the cell. Glucose is split into smaller molecules.
  2. 2 The process Inside the mitochondria more chemical bonds are broken in the smaller molecules. Oxygen is needed for this process.
  3. 3 The products Energy is released, and water and carbon dioxide are produced.


Where in the process is energy released?

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Photosynthesis and Respiration Cycle

You may find it interesting to compare cellular respiration with photosynthesis. The diagram above highlights the cycle that occurs between photosynthesis and cellular respiration. Notice that the starting materials of one process are also the products of the other process. This cycle does not necessarily occur in the same cell, or even in the same organism.


Fermentation is the process by which cells release energy without oxygen. Recall that in cellular respiration the cell first breaks glucose into smaller molecules. This releases a small amount of energy. Without oxygen, cellular respiration cannot continue. In eukaryotic cells, instead of entering the mitochondria, these smaller molecules stay in the cytoplasm, where fermentation occurs.

There are two main types of fermentation: alcoholic fermentation and lactic acid fermentation. Both types of fermentation break sugars down to small molecules. In the absence of oxygen, different reactions occur that produce either alcohol and carbon dioxide or lactic acid. In both cases, a small amount of energy is released.


Use a Venn diagram to compare and contrast fermentation and cellular respiration.


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The production of many foods that people eat every day involve either alcoholic fermentation or lactic acid fermentation. Three important foods are bread, yogurt, and cheese.

Bread is often made by mixing flour, milk, and sugar with a microorganism you know as yeast. Yeast runs out of oxygen and uses fermentation to convert the sugar into alcohol and carbon dioxide. Bubbles of carbon dioxide gas forming inside the dough cause it to rise. When the dough is baked, the small amount of alcohol evaporates, the yeast is killed, and the carbon dioxide bubbles give the bread a light, spongy structure.

Some bacteria release energy through lactic acid fermentation. These bacteria convert the sugar found in milk into lactic acid and are used to make yogurt, cheese, and sourdough bread. Lactic acid changes the acidity of a bread mixture to give it a slightly sour flavor. In yogurt and cheese, the buildup of lactic acid causes the milk to partially solidify, producing the creamy texture of yogurt. If fermentation continues for a long time, the milk eventually turns into cheese.

INVESTIGATE Fermentation

How can you tell if fermentation releases material?


  1. 1 Add 1/2 teaspoon of yeast to the empty water bottle.
  2. 2 Fill the bottle about three-quarters full with the sugar solution.
  3. 3 Place the balloon tightly around the mouth of the bottle.
  4. 4 Gently swirl the bottle to mix the yeast and sugar solution.
  5. 5 After 20 minutes, observe the balloon and record your observations.


  • What changes did you observe? What do you think is the source of energy that caused these changes?
  • What accounts for the change in the amount of gas inside the balloon?

CHALLENGE Design an experiment to answer the following question. How might the temperature of the sugar solution affect the process?

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Energy and Exercise

Your muscle cells, like some organisms, are able to release energy by both cellular respiration and fermentation. While you are at rest, your muscle cells use specialized molecules to store both energy and oxygen.

During hard or prolonged exercise, your muscle cells may use up all their stores of energy and oxygen. Then your muscle cells rely on fermentation to break down sugars. There is much less energy available to cells that use fermentation, which is why you cannot continue to run rapidly for long distances. When your cells use fermentation to release energy, one of the waste products is lactic acid, which can cause a burning sensation in your muscles.

When you stop after this type of exercise, your muscles continue to hurt and you continue to breathe hard for many minutes. During this time, your muscles are playing catch-up. They use the oxygen brought into your blood by your heavy breathing to finish breaking down the byproducts of fermentation. As the lactic acid is converted into carbon dioxide and water, the burning sensation in your muscles goes away. Your muscles build back up their stores of energy and oxygen until the next time they are needed.


APPLY Why might these students feel a burning sensation in their arm muscles while doing pull-ups?