CK-12 Physical Science - For Middle School

9.3. Carbon and Living Things http://www.ck12.org

TABLE9.3:Classes of Biochemical Compounds

Class Elements Examples Functions

Carbohydrates carbon

hydrogen

oxygen

sugars

starches

cellulose

provide energy to cells

store energy in plants

makes up the cell walls of

plants

Proteins carbon

hydrogen

oxygen

nitrogen

sulfur

enzymes

hormones

speed up biochemical re-

actions

regulate life processes

Lipids carbon

hydrogen

oxygen

fats

oils

store energy in animals

store energy in plants

Nucleic acids carbon

hydrogen

oxygen

nitrogen

phosphorus

DNA

RNA

stores genetic information

in cells

helps cells make proteins

Carbohydrates

Carbohydratesare biochemical compounds that include sugars, starches, and cellulose. They contain oxygen in
addition to carbon and hydrogen. Organisms use carbohydrates mainly for energy.

Sugars

Sugars are simple carbohydrates. Molecules of sugar have just a few carbon atoms. The simplest sugar is glucose
(C 6 H 12 O 6 ). Glucose is the sugar that the cells of living things use for energy. Plants and some other organisms make
glucose in the process of photosynthesis. Living things that cannot make glucose obtain it by consuming plants or
these other organisms.

You can see the structural formula of glucose and two other sugars inFigure9.16. The other sugars in the figure
are fructose and sucrose. Fructose is an isomer of glucose. It is found in fruits. It has the same atoms as glucose,
but they are arranged differently. Sucrose is table sugar. It consists of one molecule of glucose and one molecule of
fructose.

Starches

Starches are complex carbohydrates. They are polymers of glucose. They consist of hundreds of glucose monomers
bonded together. Plants make starch to store extra sugars. Consumers get starch from plants. Common sources of
starch in the human diet are pictured inFigure9.17. Our digestive system breaks down starch to simple sugars,
which our cells use for energy.