Page 63 - Reading Success B3

All animals and plants need food so that they can live and grow. Foods such as
sugar, bread, and potatoes contain substances called carbohydrates, the main

source of energy in our food.
Carbohydrates take part in a complicated series of chemical changes in the

cells of living things. These changes produce the energy that enables living things
to grow. Green plants make their own carbohydrates, in the process called

photosynthesis. Animals obtain carbohydrates by eating green plants or other
animals that have eaten green plants.

Carbohydrates are composed of just three elements: carbon, oxygen and
hydrogen. The principal carbohydrates are sugar, starch, and cellulose. Cellulose

forms the strong walls of plant cells. The most important carbohydrate for humans
is the sugar called glucose, which is found in fruit. Glucose does not break down

in digestion. It passes through the wall of the small intestine into the blood, which
carries it to all of the cells of the body. Other, more complex carbohydrates must

be digested and changed into glucose before they can be carried to the cells. One
such carbohydrate is called sucrose. It can be found in sugarcane, beets, and

maples.
Other carbohydrates that humans use are vegetable and animal starches. The

vegetable or animal starches we eat are changed into glucose during digestion.
Sometimes the body gets more glucose than it needs for food. The liver then

changes the glucose into glycogen, a kind of animal starch, which is stored in the
liver and muscles. Glycogen is changed back into glucose when the body needs

extra energy. Plants store their extra carbohydrates as vegetable starch.

Main Idea

What is the main idea of this story?

a. Cellulose forms the strong walls of plant cells.

b. Carbohydrates take part in a complicated series of chemical changes in the
cells of living things.

c. Carbohydrates are composed of just three elements.
d. All animals and plants need food.

66_Reading Success B 3