Dr. Claire OIander of Appalachian State University demonstrates
the structure of cellulose
|Introductory Science Concepts||Hands-On Activities|
In this session, students first learn about the molecular structure of cellulose and starch. Then they work with these materials by making paper from natural, cellulose containing, plant materials and decorating construction paper with a mixture made from starch and acrylic pigments. Finally, the students assemble these creations into a book which they will use as their journal for the program.
Introductory Science Concepts
At the beginning of the session, students are given a plastic bag that contains 12 black and 6 red angular plastic pieces from a molecular model kit produced by Darling Models. An outline of one of the angular pieces is shown below.
The angle between the peg and the tube is 109.5o, known as the
A tetrahedron is a geometric solid which has four triangular faces. Each face is an equilateral triangle. A diagram of a tetrahedron is shown below.
Students use the plastic pieces to construct models of the alpha and beta forms of the glucose molecule. Glucose is the most prevalent molecule in the biosphere. The glucose molecule, itself, is the chief energy source for living organisms. It is synthesized by plants during photosynthesis. Glucose molecules are also linked together, chemically (polymerized), to form molecules which serve as stored energy sources ( starch or glycogen ) and as structural materials (cellulose ).
The structure of the glucose molecule, chemical formula C6H12O6, in water solution, is a six-membered ring, a structure that students learn is very prevalent in nature. With a little guidance, students can easily assemble the pieces of their model kit to form the hexagonal glucose nucleus which consists of five carbon atoms and one oxygen atom. The remaining carbon atom and five oxygen atoms are connected to the glucose nucleus to form the glucose molecule with the understanding that hydrogen atoms are bonded so as to fulfill the chemical valence for each carbon and oxygen ( See the diagrams below.).
There are actually two common forms of glucose, beta and alpha. They
are shown below. Each unlabeled point on the puckered hexagons represents
a carbon atom. Four of these carbon atoms are additionally bonded to an
H and an OH group. The fifth is bonded to an H and a CH2OH group. The difference
in the structures is at the extreme right hand carbon in each structure.
In beta-glucose, the OH group is on a bond pointing up and out, while in
alpha glucose, the OH group is pointing straight down.
The hydrogen atoms in the structure above have to be imagined by the students. They learn that a hydrogen is connected to every peg or tube on their model that is not already connected to another atom. (The model kits contain only a few pieces that could represent hydrogen - not nearly enough for a molecule this large.) Students learn that the placement of atoms in the glucose molecule is very specific. A slight difference in the placement of atoms results in a model of a substance different from glucose.
Beta glucose molecules are constructed first. Then students begin to link individual glucose molecules together. The linking process also occurs in living plants. During this chemical reaction, a molecule of water splits out or "condenses" from between each two glucose molecules that link together. Thousands of Beta-glucose molecules linked together form a molecule of cellulose.
The cellulose molecule is chain-like. Many chains of cellulose molecules twist together to form a fiber. The linear shape of the cellulose molecule is reflected in the visible linear strands of cellulose that are tangled together to make paper.
Students take the cellulose molecule apart and rearrange the bonds on the glucose molecules to form alpha glucose. Then they link the alpha glucose molecules together, again splitting out a molecule of water with each linkage. The new molecule formed is a portion of a starch molecule.
The apparently slight difference in the structures of cellulose
and starch is actually a large difference from the perspective of the digestive
enzymes in mammals' intestines. Enzymes (the chemical catalysts contained
in all living organisms) are very particular about the shapes of the molecules
whose reactions they catalyze. The enzymes in mammals' intestines will
break apart starch molecules so that the individual glucose units can be
used as an energy source by the mammal. The cellulose molecule is not broken
down by the mammals' enzymes. It passes through the digestive system intact.
Starch is digestible while cellulose is not. Cellulose is often referred
to as "fiber" by nutritionists, while starch is referred to as "complex
At this time, students begin to work with the craftsperson. They mix rice flour and wheat flour (common sources of starch) with water to make a paste. Glycerin and liquid soap are added to increase the paste's flexibility. The paste is then colored with acrylic pigments, each color in its own container. The craftsperson then demonstrates the techniques of applying the paste to the surface of construction paper and creating designs using various tools and brushes. Each student has enough space and material to create several paste paper art pieces. Students place their paste paper creations on a drying rack to dry for several days.
Meanwhile, the craftsperson teaches the students some of the fundamental techniques of making paper from natural plant materials. She/he shows the students how to "cook" the plant materials in a solution of sodium carbonate (washing soda) to free the cellulose fibers in the plant from the matrix of other plant constituents in which they are trapped. Once the mixture has cooled, it is mashed by hand or placed in a blender to shorten the fibers and complete the process of separation of cellulose from other plant constituents. The cellulose fibers are separated from the solution, washed, and then placed in water and mixed to make a slurry. These slurries made from the natural plant materials are somewhat colored since the extraction of cellulose from the matrix is usually not complete. The color, however, lends character to the paper. Each plant makes a paper with individual characteristics of color and texture.
Students use specially framed screens called "moulds" and "deckels" to collect fibers from the slurry. If they wish, they may decorate the slurry with small flowers and leaves. They allow the excess water to drain and then invert the tangled mass of fibers onto a porous surface, such as a scrap of wool blanket. The scraps of blanket containing the incipient papers are then placed in an hydraulic press and the water is pressed out of the paper under significant pressure. The papers are then placed on absorbent blotters and allowed to dry in a well ventilated place for several days.
During a subsequent session, students construct a book cover by folding
their decorated paste paper around pieces of cardboard. The handmade paper
is then glued to the inside cover of the book. Several pieces of blank,
white paper are folded, placed in between the covers, and the entire construction
is sewn along the spine to form a book. Each student's book is an individual
and unique work of art and craft. The book may also serve as the journal
in which students record their experiences during Science in the Studios.
Dr. Claire OIander, Appalachian State University