READ: Chemistry of Life

Objective

• Identify the function of the four major macromolecules (i.e. carbohydrates, proteins, lipids, nucleic acids).

Introduction 

Organic compounds (compounds that contain the element carbon) are essential to all known life on Earth. Organic compounds are chemical substances that make up organisms and carry out life processes. Without carbon, life as we know it could not exist. 

The Significance of Carbon 

Why is carbon so important to organisms? Because carbon atoms can form a tremendous variety of very large and complex molecules. Nearly 10 million carbon-containing organic compounds are known. Large organic molecules known as macromolecules, are composed of smaller organic molecules linked together. There are four classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates (organic compounds such as sugars and starches that provide quick energy) contain only carbon, hydrogen, and oxygen and are the most common of the four major types of macromolecules.

Carbohydrates and Diet 

Carbohydrates include the sugars and starches. Sugars include sucrose (table sugar), fructose (the sugar found in fruit) and lactose (the sugar found in milk). Starches like potatoes and pasta are common foods in our diet. Long distance runners utilize carbohydrates to give them a reserve of energy for their race. Runners may “carb load” which means that they will eat a lot of carbohydrates (such as pasta) before a big race.

Lipids (organic compound that includes the fat, oils and waxes and provides a long-term energy source for organisms contain carbon (C), hydrogen (H) and oxygen (O). Examples of lipids in diet include cooking oils such as sunflower and olive oil, butter, margarine and lard. Many nuts and seeds also contain a high proportion of lipids. As the main long-term energy storage of an organisms they contain about twice the energy per gram compared to a gram of protein or carbohydrates.

Types of Lipids and their Functions 

Lipids may consist of fatty acids alone or in combination with other compounds. Several types of lipids consist of fatty acids combined with a molecule of alcohol: 

• Triglycerides are the main form of stored energy in animals. This type of lipid is commonly called fat. A triglyceride is shown in the figure above. 

• Phospholipids are a major component of the membranes surrounding the cells of all organisms. 

• Steroids (or sterols) have several functions. The sterol cholesterol is an important part of cell membranes and plays other vital roles in the body. Other steroids are male and female sex hormones.

Lipids and Diet 

Humans need lipids for many vital functions, such as storing energy and forming cell membranes. Lipids can also supply cells with energy. In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins. Lipids are necessary in the diet for most of these functions. Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids that must be consumed in food.

Proteins 

Proteins (organic compounds composed of amino acids and includes enzymes, antibodies and muscle fibers) contain carbon, hydrogen, oxygen, nitrogen, and, in some cases, sulfur. Proteins are made of smaller units called amino acids. There are 20 different common amino acids needed to make proteins. Amino acids can bond together to form chains of differing lengths. The sequence of amino acids in a protein’s chain(s) determines the overall structure and chemical properties of the protein.

Functions of Proteins 

Proteins are an essential part of all organisms. They play many roles in living things. Certain proteins provide a scaffolding that maintains the shape of cells. Proteins also make up the majority of muscle tissues. Many proteins are enzymes that speed up chemical reactions in cells. Other proteins are antibodies. They bond to foreign substances in the body and target them for destruction (see the Immune System and Disease chapter). Still other proteins help carry messages or materials in and out of cells or around the body. For example, the blood protein hemoglobin bonds with oxygen and carries it from the lungs to cells throughout the body.

Proteins and Diet 

Proteins in the diet are necessary for life. Dietary proteins are broken down into their component amino acids when food is digested. Cells can then use the components to build new proteins. Humans are able to synthesize all but nine of the twenty common amino acids. These nine amino acids, called essential amino acids, must be consumed in foods. Muscle tissues are composed of proteins; a good source of protein can be found in meat sources such as chicken, fish and beef.

Nucleic Acids

Nucleic acids (organic compounds that carry genetic information) contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. They are made of smaller units called nucleotides. Nucleic acids are named for the nucleus of the cell, where some of them are found. Nucleic acids are found not only in all living cells but also in viruses. They are the building blocks of DNA and RNA.

Types of Nucleic Acids 

• deoxyribonucleic acid (DNA) 

• ribonucleic acid (RNA) 

Functions of Nucleic Acids 

Nucleic acids contain the information needed for cells to make proteins. This information is passed from a body cell to its daughter cells when the cell divides. It is also passed from parents to their offspring when organisms reproduce. DNA and RNA have different functions relating to the genetic code and proteins. Like a set of blueprints, DNA contains the genetic instructions for the correct sequence of amino acids in proteins. RNA uses the information in DNA to assemble the amino acids and make the proteins.

Lesson Summary 

• Carbon’s exceptional ability to form bonds with other elements and with itself allows it to form a huge number of large, complex molecules called organic molecules. These molecules make up organisms and carry out life processes. 

• Carbohydrates are the starches and sugars. They provide cells with quick energy. • Lipids are the fats, oils and waxes. They provide cells with stored energy, and help form cell membranes. 

• Proteins provide structure for cells and help speed up chemical reactions (enzymes).

• Nucleic acids contain genetic instructions for proteins, help synthesize proteins, and pass genetic instructions on to daughter cells and offspring.