LESSON 2: CLASS AND CHARACTERISTICS OF MACROMOLECULES IN CELLS

Macromolecules refers to a class of (usually) large molecules that are very important biologically. These molecules are involved in all of the structures and processes of cells and organisms. The four different types of macromolecules – proteins, lipids, carbohydrates and nucleic acids – each perform specific functions for cells, although each type of macromolecule usually has more than one function. Proteins, for example, can serve as enzymes, help move other molecules around the cell, form structural supports for the cell and even protect the cell or organism in a variety of ways.

As you learn about the macromolecules, make sure you take a step back and see the important, overlying themes that are presented. Make note that all macromolecules are polymers that are built and broken down in the same way – by adding or subtracting monomers through condensation and hydrolysis reactions, respectively. All proteins are made of the same twenty amino acids. All nucleic acids are made of the same five nucleotides. These patterns should help you make sense of the material and avoid becoming lost in the potentially myriad details.

Learning Objectives

• Understand the concepts of monomer and polymer, and how they relate to macromolecules.
• Understand hydrolysis and condensation reactions, and how they are important in the metabolism of macromolecules.
• Become familiar with the four types of macromolecules, and how they are used in cells.

Topics covered in this Lesson

• Macromolecules
• Proteins
• Lipids
• Carbohydrates
• Nucleic Acids

Macromolecules

Macromolecules are typically BIG molecules, and they truly are the building blocks of cells. Macromolecules are generally built by combining many single units, or monomers, into larger units, called polymers. All cells are composed of the four general types of macromolecules, although each type can serve a cell in different ways. In this lesson you will learn the four different types of macromolecules, how macromolecules are formed and broken down and how they are used in cells. There are several activities and exercises included in this module that will help you understand and visualize how different macromolecules are formed. Be sure to try them all out.

Learning Objectives

• Understand that polymers are formed when monomers are joined together by condensation reactions.
• Be able to describe how polymers can be broken down by hydrolysis.
• Proteins are polymers of amino acids joined together by peptide linkages.
• Proteins have primary, secondary, tertiary, and sometimes quaternary structure.
• Lipids are joined together by ester linkages.
• Carbohydrates are simple sugars and sugar polymers consisting of monosaccharides joined together by glycosidic linkages.
• Nucleic acids are polymers of nucleotides joined together by phosphodiester linkages.

Proteins

Proteins are found everywhere – inside of cells, in membranes, and outside of cells – and play many roles for organisms. Many proteins act as enzymes, and catalyze very specific chemical reactions. Other proteins have roles in the transport of substances, self-defense and structure. Of the literally millions of different types of proteins used by living organisms, all proteins are made from the same 20 amino acids, and all are made in the same way.

• Know that proteins consist of one or more polymers called polypeptides, which are made by linking amino acids together with peptide linkages.

• Understand how peptide linkages are formed through condensation reactions.

• Know that all proteins are made from the same 20 amino acids.

• Different amino acids have different chemical properties.

• Understand how a protein’s primary structure largely determines its secondary, tertiary (and quaternary) structure.

• Understand that proteins subjected to extreme conditions (large changes in pH, high temperatures, etc.) often denature.

• Many proteins act as enzymes, and catalyze very specific chemical reactions.

Lipids

Lipids are a diverse group of molecules that play diverse roles for cells and organisms. Some lipid types, such as phospholipids, are essential components of membranes. Other types of lipids serve as energy storage molecules, signaling molecules, or even pigments. One characteristic that all lipids share, however, is that they are all hydrophobic. This means that lipids are not soluble in water. The hydrophobic nature of lipids has important consequences for how lipids are used, transported, and metabolized in organisms.

• Know that lipids constitute a very diverse group of molecules that all share the property of being hydrophobic.

• Fats and oils are lipids generally associated with energy storage.

• Fatty acids, which make up fats and oils, can be saturated or unsaturated, depending on the absence or presence of double bonded carbon atoms.

• Fatty acids are synthesized using two-carbon units derived from acetyl-CoA.

• Understand that phospholipids are the main component of biological membranes, and why their structure makes this possible.

• Other types of lipids are used for a other purposes, including pigmentation (chlorophyll, carotenoids), repelling water (cutin, suberin, waxes) and signaling (cholesterol and its derivatives).

Carbohydrates

Carbohydrates are biological molecules that are always composed of carbon, hydrogen and oxygen. Like other macromolecules, carbohydrates play a number of roles for organisms. They are involved in energy storage and production, structure and signaling. The fundamental monomer of carbohydrates is called a monosaccharide. Monosaccharides can be linked together by glycosidic linkages, which are covalent bonds formed through condensation reactions. Monosaccharides are linked together to form disaccharides, slightly larger oligosaccharides, or the largest class of carbohydrates, the polysaccharides.

• Know that carbohydrates are always composed of carbon, hydrogen and oxygen molecules, and often exist in the generalized formula CnH2nOn

• Monosaccharides typically have five or six carbon atoms.

• Monosaccharides can, such as the ribose and deoxyribose of RNA and DNA, can serve very important functions in cells.

• Understand the significance of the monosaccharide glucose is important in the metabolic pathways of many organisms.

• Understand how condensation reactions form covalent bonds between monosaccharides, called glycosidic linkages.

• Know that monosaccharides are the monomers for the larger oligosaccharides and polysaccharides.

• Oligosaccharides often play important roles in signaling.

• Know that polysaccharides play various roles, from energy storage (starch, glycogen) to structure (cellulose).

• Carbohydrates may be chemically modified, for instance by attachment of other functional groups (amines, acetyl group, etc.)

Nucleic Acids

The nucleic acids DNA and RNA are responsible for storing and transmitting the genetic code of all organisms. DNA is a huge polymer that stores information in the sequence of its monomers, called nucleotides. The information in DNA is used to produce proteins. RNA is used to transfer the information of DNA to sites of protein synthesis and to translate the information into the amino acid sequences of proteins.

DNA also serves as a partial record of the history of life, and allows us to peer into the past to discern evolutionary trends and relationships.

• Know that there are two types of nucleic acids: DNA and RNA.

• DNA stores the genetic information of organisms; RNA is used to transfer that information into the amino acid sequences of proteins.

• Know that both DNA and RNA are polymers composed of subunits called nucleotides.

• Nucleotides consist of a five-carbon sugar, a phosphate group and a nitrogenous base.

• Know the five nitrogenous bases found in nucleotides, the purines adenine and guanine, and the pyrimidines cytosine, thymine (DNA only), and uracil (RNA only).

• Understand how hydrogen bond between purines and pyrimidines is important in the structure of DNA, and the transcription and translation of RNA

• Know that DNA and RNA have several key structural and functional differences in cells.

• DNA is transmitted from generation to generation with high fidelity, and therefore represents a partial picture of the history of life.

• Be aware that the nucleotides of nucleic acids may be used for other purposes in cells, such as energy transfers (ATP, NAD, NADP).