Key Idea: Most of the carbohydrates, fats, and proteins from the food humans eat must be broken down into smaller molecules before they can enter cells to be used for energy and building materials.
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Molecules from food are broken down into smaller molecules in the digestive tract and then enter the circulatory system by way of capillaries located in the lining of the digestive tract.

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  • Most of the carbohydrates, fats, and proteins from the food humans eat must be broken down into smaller molecules before they can enter cells to be used for energy and building materials.

    Students are expected to know that:

    1. Most of the food that humans eat is made up of large molecules (fats, proteins, and complex carbohydrates) that are too big to get to the cells of the body where they can be used for energy and building materials.
    2. These large molecules are made up of subunits, which are smaller molecules of the same type that are linked together.Fatty acids are subunits of fats, amino acids are subunits of proteins, and simple sugars are subunits of complex carbohydrates.Carbon dioxide is not a subunit of fats, proteins, or complex carbohydrates.
    3. Some molecules from food (e.g., simple sugars, fatty acids, and amino acids) are already small enough to get to the cells of the body without being broken down into smaller molecules.
    4. Digestion is the process of breaking larger molecules into their subunits. The subunits are called the products of digestion.
    5. Digestion does not include the breaking down of food into molecules of carbon dioxide and water.
    Boundaries:
    1. Students are not expected to know that glycerol is also a subunit of fats.
    2. Students are not expected to know the chemical formulas of complex carbohydrates, fats, proteins, simple sugars, fatty acids, or amino acids.
    3. Students are not expected to know the term “macromolecules.”
  • Molecules from food are broken down into smaller molecules in the digestive tract and then enter the circulatory system by way of capillaries located in the lining of the digestive tract.

    Students are expected to know that:

    1. Digestion takes place in body organs (e.g., mouth, stomach, intestines) that collectively are known as the digestive tract.
    2. The food humans eat moves from the mouth to the stomach by way of a tube that is separate from the tube that carries air to and from the lungs.
    3. The breakdown of food into smaller molecules usually involves a combination of mechanical processes (mechanical digestion) and chemical reactions (chemical digestion).
    4. During mechanical digestion, more of the carbohydrate, fat, and protein molecules from food come in contact with digestive enzymes, which increases the number of carbohydrate, fat, and protein molecules that are chemically broken down.
    5. Examples of mechanical digestion include chewing food in the mouth and mixing food in the stomach.
    6. Molecules that result from the digestion of carbohydrate and protein molecules leave the digestive tract and enter the circulatory system by way of the capillaries located in the lining of the digestive tract.
    7. Not everything that we eat is digested: In some cases mechanical digestion is incomplete (e.g., inadequate chewing of food) so that digestive enzymes cannot come in contact with the molecules from food. In other cases, the body does not have the digestive enzymes needed to break down the molecules that we eat (e.g., cellulose). In still other cases, some of the molecules from food do not have to be digested because they are already small enough to enter the circulatory system and cells of the body.
    8. Undigested fats, proteins, complex carbohydrates or other undigested material that is too large to be used by the cells of the body leave the body at the end of the digestive tract.

    Boundaries:

    1. Students are not expected to know he terms “alimentary canal,” “esophagus,” “small intestine,” or“large intestine.”
    2. Students are not expected to know that most chemical digestion and absorption occurs in the small intestine.
    3. Students are not expected to know the different types of digestive enzymes or the specific role they play in chemical digestion.
    4. Students are not expected to know when digestion involves primarily chemical reactions or primarily mechanical processes.
    5. Students are not expected to know that stomach acid and bile are also molecules involved in chemical digestion.
    6. Students are not expected to know the role of microorganisms in digestion.
    7. Students are not expected to know that fatty acids enter the blood stream through the lymphatic system.
    8. Students are not expected to know that dipeptides are molecules that are small enough to enter capillaries.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number
Knowledge Being Assessed Grades
6–8
Grades
9–12
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BF003005

If a person could not digest food anymore, the person would lose weight because most of the molecules from food could not be used for building materials.

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Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8
Grades
9–12

BFM060

Digestion does not affect the way food is used by the body (AAAS Project 2061, n.d.).

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BFM059

Digestion helps us use food but it is not necessary for us to use food (AAAS Project 2061, n.d.).

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BFM058

If food could not be digested, it would stay in the body and would not be eliminated (AAAS Project 2061, n.d.).

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Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.