Sugar, starch or cellulose?
What sort of carbohydrates do plants make?
Technical & Teaching Notes
Introduction
This resource tackles the learning objective ‘“Carbohydrates can be found in a range of plant organs”. This is activity 4 in the ‘Photosynthesis: A Survival Guide’ scheme, and follows on from Activity 3, ‘Let’s talk about carbohydrates’, in which students learn about the range of carbohydrates that plants produce.
Pupils can test a variety of plant products for different sorts of carbohydrates, including glucose, starchand cellulose. These plant products can be fresh fruit and vegetables, but it is also interesting to test materials such as cotton wool and egg boxes, which contain a high proportion of cellulose.
This is another good opportunity to get students engaged in practical work related to photosynthesis and will stretch their understanding of the carbohydrates involved beyond the simple standard equation given at KS3 that photosynthesis results in glucose.
This resource is designed for 11-14 pupils but could be extended for use with older students as appropriate.
Safety
•Water bath at 85 – 90°C
•Use of scalpel / knife
•Schulze’s solution should be made up fresh,contained in a small stoppered bottle andadministered only by the teacher. Label thebottle CORROSIVE
Teaching notes
Learning Objective
“Carbohydrates can be found in a range of plant organs”
Suggested approaches
Pupils can test a variety of plant products for glucose, starchand cellulose. It isuseful for pupils to think about where in the plant the sampleis taken from, once again asking the question: “Where do theatoms in the starch, sugars and cellulose come from?”
Plants which give interesting results includeonion, apple, pear, celery, potato and grapes.
Photos showing some expected results are given in the accompanying Powerpoint.
It is also interesting to test materials such ascotton wool, paper towel, egg boxes andother materials made from recycled paper.These show up clearly the high proportionof cellulose fibres.
Benedict’s reagent will not give a colouredproduct with all simple sugars.All monosaccharide sugars such as glucose,fructose, and galactose will give a positiveresult as well as some disaccharide sugarssuch as maltose and lactose. Sucrose will notgive a positive result with Benedict’s reagent.
Apparatus
Equipment andmaterials required foreach working group:
•White tile
•Knife/scalpel
•5–6 boiling tubes – one for each type of food
•Small pestle and mortar
•Small bottle of Benedict’s solution
•Small bottle of iodine solution
•Paper towels
Equipment and materialsrequired centrally
•Access to water bath set at 85 – 90°C
•Food samples containing carbohydratestores e.g. onion, apple, pear, celery,potato, grapes.
•Optional extras: cotton wool, paper towels,egg boxes, flour, rice, powdered glucose.
•Buckets/plastic containers for food remains
•1 bottle of Schulze’s solution (to be administered by the teacher)
Preparation of materials
To make the solutions, see guides such as the CLEAPPS recipe book (in England and Wales).
Benedict’s qualitative reagent –CLEAPSS recipe book No 11
Iodine solution – CLEAPSS recipe card No 50
Schulze’s solution –CLEAPSS recipe book No 93, ‘Stains for Plant Material’
Ideally the samples fortesting should be choppedinto small pieces about1 cm3 maximum so thatstudents can easily selectthree pieces to carry outtheir tests.
Disposal of materials:In the laboratory have one ortwo large plastic containersin which students place theirwaste so that sinks do notget blocked.
Suppliers:All regular scientific suppliersprovide chemicals neededfor preparation of solutions.
Acknowledgements
This resource is based on an original by Debbie Eldridge in ‘Photosynthesis: A Survival Guide’ (2009).
Science & Plants for Schools:
Sugar, starch or cellulose : p. 1
This document may be photocopied for educational use. Revised 2012.