Page 24 - 新思维科学学生用书9 样章

1.3 The carbon cycle

Carbon and living organisms

Carbon is an element. The symbol for carbon is C.

Carbon is a non-metal. It occurs naturally in different forms. (You will find out more about carbon in
Topic 2.4.)

Diamonds are made of carbon.

The ‘lead’ in a pencil is not lead at all – it is another form of the element carbon,
called graphite.

Living organisms do not need diamonds or pencil leads, but they do need
carbon. Organisms cannot use carbon in the form of an element. They can only
use it when it is part of a compound.

Carbon is part of many different compounds that make up cells. Carbohydrates,
proteins and fats are all compounds that contain carbon.

We rely on plants to make these substances. Plants take carbon dioxide from the air and use it in
photosynthesis to make carbohydrates. Carbon dioxide is a compound that contains carbon atoms
combined with oxygen atoms. (You will find out more about this in Unit 2.) The carbohydrates in
plants contain carbon atoms that were originally part of the air.

Plants use the carbohydrates to make proteins and fats. All of these nutrients are compounds that

contain carbon atoms.

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