Topic: Sugars – starch and cellulose

Target group

Student of an eight‑year elementary school

Core curriculum:

Elementary school. Chemistry.

X. Chemical substances of biological importance. Pupil:

10) gives examples of the presence of starch and cellulose in nature; gives summary models of these compounds; lists differences in their physical properties; describes the meaning and uses of these sugars; designs and conducts experiments to detect the presence of starch with iodine solution in various food products.

General aim of education

The student discusses the occurrence, structure and properties of starch and cellulose

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• to describe the presence of starch and cellulose;

• to write down summary formulas of polysaccharides;

• what properties and applications are starch and cellulose;

• to discuss the properties of starch and cellulose with an indication of the differences;

• to detect the presence of starch in food products.

Methods/techniques

• activating

  • discussion.

• expository

  • talk.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

3. Health and safety – before starting the experiments, students familiarise themselves with the safety data sheets of the substances that will be used during the lesson. The teacher points out the need to be careful when working with them.

Realization

1. The teacher, using the reverse lesson method, realizes the issue of the occurrence and construction of polysaccharides.
   Going to several practical exercises, he asks:
   What are the sources of starch and cellulose?
   Why is the natural glucose polymer referred to as starch?
   The molecular weight of a certain sample of starch is 60426u and cellulose 899910u. What are the summary formulas of these polysaccharides?

2. Working in pairs, students perform experience 1 from the abstract - study of the physical properties of polysaccharides. They record observations in work sheets or in an abstract form.

3. Experience 2, included in the abstract, is carried out by the teacher in the form of a show (due to the safety of students). The pupils agree on the conclusions together and write them in the work sheets or in the abstract form.

4. The teacher divides the students into groups - they will perform a problem‑verification experiment, which is described in the abstract as experience 3. Before they start the experimental work according to abstract instructions, they formulate a research question and a hypothesis with the help of a teacher, which they then write in the work sheets or in the form in the abstract. During the experiment, students note observations and conclusions on the form in an abstract - the leader of one of the groups will present them on the class forum.
   The teacher then displays the starch hydrolysis reaction on the interactive whiteboard. He discusses it and then asks the students the question: „Why is a long‑chewed crust of bread sweet taste?”.

5. The lecturer displays a film from the abstract „Detecting the presence of starch in food products”, stopping it on the frame depicting the reaction of starch paste in the presence of iodine. He asks students how starch (in starch paste) reacted in the presence of iodine - students give answers. Participants of the classes, working in the same groups in which they performed previous experience, perform the task according to the instructions described in the methodical commentary „Detection of starch in food products”, but only for the test sample (here the teacher can assign to groups various food products). They formulate a research question and hypotheses with the teacher's help. They are listed in job cards or in an abstract form. They perform an experiment, observe changes and write them in work cards. At the end, group leaders present the results of their work on the forum. The teacher asks students to formulate their conclusions, and after jointly establishing them, to write them down in the work sheets..

Summary

1. The student indicated by the teacher sums up the lesson, telling what he has learned and what skills he/she has been practicing.

Homework

1. Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Sugars – starch and cellulose

Polysaccharides are complex chemical compounds composed of many radicals of simple sugars. The best known polysaccharides are starch and cellulose.

The generalized formula for polysaccharides is $\text{(}{\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}{\text{)}}_{\text{n}}$, where $\text{n}$ is a natural number dependent on the type of polysaccharide and its origin.
Complex sugars, such as starch and cellulose, are created mainly in plants as a result of glucose molecules joining together. We call them natural polymers.

Starch is a macromolecular compound. The number of glucose molecules that produce starch varies from several hundred to several thousand, and depends on the origin. This sugar is a very important component of human diet. It is mainly found in potatoes, cereal grains, seeds, rice and corn. Starch has a granular structure – the shape of grains depends on their origin.

Cellulose, like starch, is a compound composed of many glucose radicals (2 500 to 10 000). It is the main component of plants, as it provides them with durability, flexibility and protects against water loss. It occurs in large quantities in cotton and wood. Cellulose has a fibrous structure.

Cellulose is also called fiber. Humans do not digest this sugar, but it plays an important role in the proper functioning of the digestive system.

Potato flour  is starch obtained from potatoes. Cellucotton is pure cellulose.

We will use these substances to test the properties of polysaccharides.

• The most common polysaccharides are starch and cellulose, the molecular formula of which is $\text{(}{\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}{\text{)}}_{\text{n}}$.

• Starch has a granular structure and occurs mainly in potatoes, cereal grains, rice and corn.

• Cellulose is the primary building material in plants. It has a fibrous structure; it occurs, among others, in wood and cotton.

• Starch and cellulose are white solids. Starch is slightly soluble in cold water, and it forms starch gruel in hot water. Cellulose does not dissolve in water; it is flammable.

• To detect starch in food products,the iodine‑starch test is used – the reaction of starch with iodine, as a result of which a dark‑blue colour occurs.

• Starch undergoes hydrolysis in the human body; it decomposes into dextrin and then into glucose molecules. Humans are unable to digest cellulose.