Activity One

FROM THE TREE OF LIFE TO MACROMOLECULES

Activity Overview:

Key concepts:
All key components of every living cell are made of macromolecules. The four kinds of macromolecules are lipids, carbohydrates, nucleic acids, and proteins. Most macromolecules are polymers constructed of many organic molecules called monomers.

Students start with images of living organisms, from bacteria to plants and animals. They zoom into cells and tissues to discover that they are made of different macromolecules. Students observe that these macromolecules are polymers. They zoom into polymers to find that some are made from almost identical monomers, while others, such as proteins, are made from a set of different monomers. They discover that all monomers making up biological macromolecules are composed of just a few types of chemical elements: C, H, O, N, P and S.

Learning Objectives:

Students will be able to:
Identify typical molecular building blocks (monomers) that form biological macromolecules.
Determine the types of atoms that make up most biopolymers.
Reason about the uniformity on the atomic level of life's molecular building blocks

Macro to Micro Connection

Students connect living organisms to their atomic and molecular composition.

Conceptual Prologue

Life's Macromolecules All cells, organs and tissues of a living organism are made of atoms and molecules. A special class of very large molecules that determine the structural and functional properties of living cells are called biologically - active macromolecules. They are called "macro" for their very large size relative to other organic molecules.

Commonality in life's macromolecules The monomers that link together to form polymers are made of very few elements. Some of them, such as C, H, O, N, P and S (also called macro elements) make up biomolecules and are therefore the largest dry weight of all living organisms. Other elements are present in very small quantities, but can still play important roles (e.g. the iron in hemoglobin, which helps to carry oxygen, or the sodium and potassium ions that are responsible for nerve impulses.)

Here is one way to think of the commonalities in atomic composition: All carbohydrates such as wood or starch in every plant are made of just three chemical elements: C, H and O. All proteins of all organisms on earth are made of five chemical elements: C,H,O,N,S. All nucleic acids of all organisms on earth are made of C,H,O,N,P. Here we see a uniformity of living organisms at the most elemental level.

The uniformity continues at the level of monomers as well. In this activity students trace a number of biological structures from their appearance in our macro world to their monomers. When they compare proteins of different organisms (e.g. human hair, silk) , students discover that they are all made of a small diversity (about 20) of amino acids. They discover that there is even less diversity in carbohydrates, most of which are made from just one monomer, glucose. That is why all cellulose fibers tend close-up to look alike (whether taken from carrot or
baobab tree), while proteins can be different.

A special rule: Only one family of monomers is used to make a specific class of polymers; there are no polymer chains in which amino acids and nucleotides are interlinked.

Types of Polymers Polymers can be divided into those in which the building blocks are all the same (homopolymers), and those in which there is a variety of building blocks (heteropolymers). Some biological polymers such as cellulose are made of a large number of identical connected glucose molecules, monomers, and are considered homopolymers. Nucleic acids and proteins are both heteropolymers: DNA and RNA are made of four different monomers called nucleotides, while proteins are made of 20 plus** different monomers called amino acids. In heteropolymers, the variety in charge, polarity and other properties of their monomers leads to a variety of shapes, and therefore to a variety of their possible functions in living cells.

 Activity Design and Execution:

 Major Science Concepts  Monomer, polymer, macromolecule
 Assumed Previous Knowledge: Chemical elements, molecules, living cell
 Time: One 50-minute classes
Technology Requirement: Web access
 Supportive Materials Macromolecule Work Page, Tree of Life Worksheet (Student) [PDF version]
 Advanced preparation (if any) *Have web pages available on computer.
*Print student worksheets, if they cannot fill them out on computer.
*Print Prepost-Test

Investigative Question: What are the similarities and differences among molecular building blocks of living organisms?

STEPS

1. Give the Prepost-Test.

2. Open or distribute the Monomers of Macromolecule Work Page. If your students are not used to chemical notation, you might want to review the key atomic notation, but then encourage them to treat it as a straightforward puzzle. The molecules of life belong to four chemical groups, e.g. , such as sugars, organic acids, aromatic compounds etc. Ask students to scan the picture and see if they can find any characteristics useful for such a division. (They might notice the presence of rings, the large number of oxygen atoms in sugars, the long C-H tails of lipids, the presence of several nitrogens in the nucleotides etc.)

Ask students to describe the different methods they used for dividing the biomolecules. Explain to them that they will be returning to these molecules later to identify the functional molecular groups that determine the individuality of the monomers.

If students are unfamiliar with the variety of molecular representations, you might want to review them.

3. Zoom down theTree of Life The following can be done either serially by all students, or as teams that reconvene to teach each other what they have discovered. Explain to your students that they will zoom into organs and tissues of plants and animals in order to discover life's essential building blocks. The exercise they will do includes just a few of the many possible "zooms" into the structures of living organisms.

Record the Investigative Question on the board: What can you find out about macromolecules ­ building blocks of our organs and tissues ? How different are these building blocks from one another? Are there similarities between them?

Open and Distribute the Tree of Life Worksheet and have students begin their investigations.

Tree of Life Worksheet (Student)

4. Discussion: Work with your students to summarize that in the molecules of life a limited number of atoms (C, H, O, N, P and S) are organized into four groups of molecules (nucleotides, amino acids, sugars and fatty acids) from which key macromolecules (DNA, RNA, proteins, carbohydrates, and lipids) are built. Polymers, in turn, can make fibers or other associations that form the structural components of living cells. Ask them what makes living tissues, such as wood or skin so strong and flexible? They may discuss the structure of cellulose fibers that are major structural components of plant cells, or fibers of keratin in animal skin or hair.

Transition to next activity: How are the building blocks assembled?

Extensions and Connections

Go through zooms, determining whether a particular polymer is a heteropolymer or homopolymer. See Homopolymer/heteropolymer worksheet. Teacher version.
Develop new "zooms" from macro organisms to their atomic components.


Monomers to Polymers Index

*Photosynthesis provides plants with a broad spectrum of different sugars such as glucose or fructose. Then plants convert those sugars to amino acids or nucleotides using minerals from the soil (Nitrogen, Sulfur, Phosphate).
**There are a few additional amino acids present in some unique proteins.