McDougal Littell Science Cells and Heredity 

Table of Contents
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Large molecules support cell function.

In living things, there are four main types of large molecules: (1) carbohydrates, (2) lipids, (3) proteins, and (4) nucleic acids. Thousands of these molecules work together in a cell. The four types of molecules in all living things share one important characteristic. They all contain carbon atoms. These large molecules are made up of smaller parts called subunits.



Carbohydrates are used for structure and energy storage. Carbohydrates, such as cellulose, are made of sugars.

Carbohydrates (KAHR-boh-HY-DRAYTS) provide the cell with energy. Simple carbohydrates are sugars made from atoms of carbon, oxygen, and hydrogen. Inside cells, sugar molecules are broken down. This process provides usable energy for the cell.

Simple sugar molecules can also be linked into long chains to form more complex carbohydrates, such as starch, cellulose, and glycogen. Starch and cellulose are complex carbohydrates made by plant cells. When a plant cell makes more sugar than it can use, extra sugar molecules are stored in long chains called starch. Plants also make cellulose, which is the material that makes up the cell wall. Animals get their energy by eating plants or other animals that eat plants.

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Lipids are the fats, oils, and waxes found in living things. Like carbohydrates, simple lipids are made of atoms of carbon, oxygen, and hydrogen and can be used by cells for energy and for making structures. However, the atoms in all lipids are arranged differently from the atoms in carbohydrates. Many common lipids consist of a molecule called glycerol bonded to long chains of carbon and hydrogen atoms called fatty acids. This structure gives lipids unique properties. One extremely important property of lipids is that they cannot mix with water.


Lipids make up the membranes surrounding the cell and organelles. Lipids are made of fatty acids and glycerol.


How do cells use carbohydrates and lipids?



Proteins are made up of amino acids. Proteins carry out most of the chemical activity in cells.

Proteins are made of smaller molecules called amino acids. Amino acids contain the elements carbon, oxygen, hydrogen, nitrogen, and sometimes sulfur. In proteins, amino acids are linked together into long chains that fold into three-dimensional shapes. The structure and function of a protein is determined by the type, number, and order of the amino acids in it.

Your body gets amino acids from protein in food, such as meat, eggs, cheese, and some beans. After taking in amino acids, your cells use them to build proteins needed for proper cell functioning. Some amino acids can be made by the body, but others must be taken in from an outside food source.

There are many types of proteins. Enzymes are proteins that control chemical reactions in the cells. Other proteins support the growth and repair of living matter. The action of proteins in your muscles allows you to move. Some of the proteins in your blood fight infections. Another protein in your blood delivers oxygen to all the cells in your body. Proteins are also important parts of cell membranes. Some proteins in the cell membrane transport materials into and out of the cell.

Nucleic Acids


Nucleic acids store and translate the genetic information a cell needs to function. Nucleic acids, such as DNA, are made up of nucleotides.

Nucleic acids (noo-KLEE-ihk) are the molecules that hold the instructions for the maintenance, growth, and reproduction of a cell. There are two types of nucleic acids: DNA and RNA. Both DNA and RNA are made from carbon, oxygen, hydrogen, nitrogen, and phosphorus. The subunits of nucleic acids are called nucleotides.

DNA provides the information used by the cell for making proteins a cell needs. This information takes the form of a code contained in the specific order of different nucleotides in the DNA.

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The pattern of nucleotides in DNA is then coded into RNA, which delivers the information into the cytoplasm. Other RNA molecules in the cytoplasm produce the proteins.


What is the function of DNA and RNA?