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.

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.”
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

BFM074

Simple sugars have to be broken down into smaller molecules before they can enter the cells of the body.

52%

47%

BFM076

Fatty acids have to be broken down into smaller molecules before they can enter the cells of the body.

41%

47%

BFM058

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

42%

40%

BFM075

Amino acids have to be broken down into smaller molecules before they can enter the cells of the body.

36%

34%

BFM072

Proteins do not have to be broken down into smaller molecules before they can enter the cells of the body.

28%

26%

BFM071

Complex carbohydrates do not have to be broken down into smaller molecules before they can enter the cells of the body.

29%

19%

BFM059

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

19%

17%

BFM013

The carbohydrates, fats, and proteins from the food people eat are used by the body unchanged; they are not broken down into smaller molecules (AAAS Project 2061, n.d.).

13%

11%

BFM060

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

6%

5%

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.