Polysaccharides: starch, cellulose
Polysaccharides are high molecular weight compounds containing hundreds and thousands of monosaccharide residues. What is common to the structure of polysaccharides is that monosaccharide residues are linked by the hemiacetal hydroxyl of one molecule and the alcohol hydroxyl of another, etc. Each monosaccharide residue is linked to adjacent residues by glycosidic bonds.
Polyglycosides can contain branched and unbranched chains. The monosaccharide residues that make up the molecule can be the same or different. The most important of the higher polysaccharides are starch, glycogen (animal starch), fiber (or cellulose). All three of these polysaccharides are made up of glucose molecules linked together in different ways. The composition of all three compounds can be expressed by the general formula: (C 6 H 10 O 5) n
Starch
Starch belongs to the polysaccharides. The molecular mass of this substance has not been precisely established, but it is known that it is very large (about 100,000) and may vary for different samples. Therefore, the formula of starch, like other polysaccharides, is depicted as (C 6 H 10 O 5) n. For each polysaccharide n has different meanings.
Physical properties
Starch is a tasteless powder, insoluble in cold water. It swells in hot water, forming a paste.
Starch is widely distributed in nature. It is a reserve nutrient material for various plants and is contained in them in the form of starch grains. The richest grains in starch are cereals: rice (up to 86%), wheat (up to 75%), corn (up to 72%), and potato tubers (up to 24%). In potato tubers, starch grains float in cell sap, and in cereals they are tightly glued together with a protein substance called gluten. Starch is one of the products of photosynthesis.
Receipt
Starch is extracted from plants by destroying the cells and washing it with water. On an industrial scale, it is obtained mainly from potato tubers (in the form of potato flour), as well as from corn.
Chemical properties
1) Under the action of enzymes or when heated with acids (hydrogen ions serve as a catalyst), starch, like all complex carbohydrates, undergoes hydrolysis. In this case, soluble starch is formed first, then less complex substances - dextrins. The final product of hydrolysis is glucose. The overall reaction equation can be expressed as follows:
There is a gradual breakdown of macromolecules. Hydrolysis of starch is its important chemical property.
2) Starch does not give the “silver mirror” reaction, but the products of its hydrolysis do. Starch macromolecules consist of many molecules of cyclic a-glucose. The process of starch formation can be expressed as follows (polycondensation reaction):
3) A characteristic reaction is the interaction of starch with iodine solutions. If an iodine solution is added to a cooled starch paste, a blue color appears. When the paste is heated, it disappears, and when cooled, it appears again. This property is used in determining starch in food products. For example, if a drop of iodine is applied to a cut potato or a slice of white bread, a blue color appears.
Application
Starch is the main carbohydrate in human food; it is found in large quantities in bread, cereals, potatoes, and vegetables. Significant quantities of starch are processed into dextrins, molasses, and glucose, which are used in the confectionery industry. Starch is used as an adhesive, used for finishing fabrics and starching linen. In medicine, ointments, powders, etc. are prepared based on starch.
Cellulose or fiber
Cellulose is an even more common carbohydrate than starch. It consists mainly of the walls of plant cells. Wood contains up to 60%, cotton wool and filter paper - up to 90% cellulose.
Physical properties
Pure cellulose is a white solid, insoluble in water and in common organic solvents, highly soluble in concentrated ammonia solution of copper (II) hydroxide (Schweitzer reagent). From this acid solution, cellulose is precipitated in the form of fibers (hydrated cellulose). Fiber has quite high mechanical strength.
Composition and structure
The composition of cellulose, as well as starch, is expressed by the formula (C 6 H 10 O 5) n. The n value in some types of cellulose reaches 10-12 thousand, and the molecular weight reaches several million. Its molecules have a linear (unbranched) structure, as a result of which cellulose easily forms fibers. Starch molecules have both linear and branched structures. This is the main difference between starch and cellulose.
There are differences in the structure of these substances: starch macromolecules consist of residues of a-glucose molecules, and cellulose macromolecules consist of residues of b-glucose. The process of formation of a fragment of a cellulose macromolecule can be represented by the diagram:
Chemical properties. Applications of cellulose Small differences in the structure of molecules cause significant differences in the properties of polymers: starch is a food product, cellulose is unsuitable for this purpose.
1) Cellulose does not give a “silver mirror” reaction (no aldehyde group). This allows us to consider each C 6 H 10 O 5 unit as a glucose residue containing three hydroxyl groups. The latter in the cellulose formula are often distinguished:
Due to hydroxyl groups, cellulose can form ethers and esters.
For example, the reaction to form an ester with acetic acid is:
[C 6 H 7 O 2 (OH) 3 ] n +3nCH 3 COOH® [C 6 H 7 O 2 (OSOCH 3) 3 ] n +3nH 2 O
When cellulose reacts with concentrated nitric acid in the presence of concentrated sulfuric acid as a water-removing agent, an ester is formed - cellulose trinitrate:
This is an explosive substance used to make gunpowder.
Thus, at ordinary temperatures, cellulose reacts only with concentrated acids.
2) Like starch, when heated with dilute acids, cellulose undergoes hydrolysis to form glucose:
(С 6 Н 10 0 6) n +nН 2 O®nС b Н 12 O 6
Hydrolysis of cellulose, otherwise called saccharification, is a very important property of cellulose; it makes it possible to obtain cellulose from sawdust and shavings, and by fermenting the latter, ethyl alcohol. Ethyl alcohol obtained from wood is called hydrolytic.
At hydrolysis plants, up to 200 liters of ethyl alcohol are obtained from 1 ton of wood, which can replace 1.5 tons of potatoes or 0.7 tons of grain.
Crude glucose obtained from wood can be used as livestock feed.
These are just some examples of the use of cellulose. Cellulose in the form of cotton, flax or hemp is used to make fabrics - cotton and linen. Large quantities of it are spent on paper production. Cheap grades of paper are made from coniferous wood, the best grades are made from linen and cotton waste paper. By subjecting cellulose to chemical processing, several types of artificial silk, plastics, film, smokeless powder, varnishes and much more are obtained.
22.Polysaccharides (starch, cellulose, glycogen): structure, distinctive biological functions.
Polysaccharides are high-molecular-weight polycondensation products of monosaccharides linked to each other by glycosidic bonds and forming linear or branched chains. The most common monosaccharide unit of polysaccharides is D-glucose. Components of polysaccharides can also include D-mannose, D- and L-galactose, D-xylose and L-arabinose, D-galacturonic and D-mannuronic acids, D-glucosamine, D-galactosamine, etc. Each monosaccharide included in the composition of the polymer molecule can be in pyranose or furanose form. Polysaccharides can be divided into 2 groups: homopolysaccharides and heteropolysaccharides.
Homopolysaccharides consist of only one type of monosaccharide unit. Heteropolysaccharides contain two or more types of monomer units.
Homopolysaccharides. According to their functional purpose, homopolysaccharides can be divided into 2 groups: structural (glycogen and starch) and reserve (cellulose) polysaccharides.
Starch. This is a high-molecular compound containing hundreds of thousands of glucose residues. It is the main reserve polysaccharide of plants.
Starch is a mixture of two homopolysaccharides: linear - amylose (10-70%) and branched - amylopectin (30-90%). The general formula of starch is (C 6 H 10 O 5)n. As a rule, the amylose content in starch is 10-30%, amylopectin – 70-90%. Starch polysaccharides are built from D-glucose residues connected in amylose and linear amylopectin chains by α-1,4 bonds, and at the branch points of amylopectin by interchain α-1,6 bonds.
Rice. Starch structure. a - amylose with its characteristic spiral structure, b - amylopectin.
In the amylose molecule, 200-300 glucose residues are linearly linked. Due to the α-configuration of the glucose residue, the polysaccharide chain of amylose has a helical configuration. In water, amylose does not give true solutions; in solution, when iodine is added, amylose turns blue.
Amylopectin has a branched structure. Individual linear sections of the amylopectin molecule contain 20-30 glucose residues. In this case, a tree-like structure is formed. Amylopectin is stained red-violet with iodine.
Starch has a molecular weight of 10 5 -10 8 Da. With partial acid hydrolysis of starch, polysaccharides of a lower degree of polymerization are formed - dextrins, with complete idolysis - glucose.
Glycogen. This is the main reserve polysaccharide of higher animals and humans, built from D-glucose residues. The general formula of glycogen is the same as that of starch (C 6 H 10 O 5) n. It is found in almost all organs and tissues of animals and humans, but the largest amount of glycogen is found in the liver and muscles. The molecular weight of glycogen is 10 5 -10 8 Yes or more. Its molecule is built from branching polyglucosidic chains, in which glucose residues are connected by α-1→4-glycosidic bonds. At branching points - α-1→6 bonds. Glycogen is characterized by a more branched structure than amylopectin; linear segments in the glycogen molecule include 11-18 α-D-glucose residues.
During hydrolysis, glycogen, like starch, is broken down to first form dextrins, then maltose and glucose.
Cellulose (fiber) – the most widespread structural polysaccharide of the plant world. It consists of β-glucopyranose monomers (D-glucose) linked by β-(1→4) bonds. With partial hydrolysis of cellulose, cellodextrins, a disaccharide of cellobiose, are formed, and with complete hydrolysis, D-glucose. The molecular weight of cellulose is about 10 6 Da. Fiber is not digested by enzymes in the digestive tract, because the set of these enzymes in humans does not contain hydrolases that cleave β-bonds.
Polysaccharides. Starch and cellulose Philon M.V. chemistry teacher MBOU secondary school No. 266
Comparative characteristics of starch and cellulose
Signs of comparison
Starch
Formula
Cellulose
Structural link
Molecule structure
Physical properties
Chemical properties
Application
Structural formula of starch
α-Glucose residues
Structural formula of cellulose
β-Glucose residues
Physical properties
starch
cellulose
- hard, fibrous white substance
- white amorphous powder
- does not dissolve in cold water
- does not dissolve in water
- swells in hot water
- does not have a sweet taste
- does not have a sweet taste
Chemical properties of starch
- Qualitative reaction
(C 6 H 10 O 5) n + I 2 → blue color
2. Hydrolysis
Starch → dextrins → maltose → glucose
Chemical properties of cellulose
1. Hydrolysis
(C 6 H 10 O 5) n + nH 2 O → nC 6 H 12 O 6
2. Formation of esters
Let's check ourselves
1. The starch macromolecule consists of molecular residues...
α - glucose
fructose
β - glucose
Let's check ourselves
2. Qualitative reaction to starch - interaction with...
copper(II) hydroxide
ammonia solution of silver oxide
Let's check ourselves
3. The hydrolysis of cellulose produces...
Let's check ourselves
4. Cellulose trinitrate is used as...
medicine
explosive
for extinguishing fires
Starch and cellulose are the most important representatives of polysaccharides
Lesson using development technology
critical thinking grade 10
The technology for developing critical thinking through reading and writing allows students to develop critical thinking when organizing their work with various sources of information (specially written texts, textbook paragraphs, videos, teacher lectures). Students are motivated to learn new material by involving them in independent goal setting and reflection, as well as by organizing collective, paired and individual independent work in the classroom. The use of this technology makes it possible to take into account the individual characteristics of students’ cognitive interests and to train everyone in the zone of proximal development*.
In accordance with this technology, the learning process consists of three stages. First - call stage ; it consists in updating and summarizing existing knowledge on the topic being studied, arousing interest in it, and motivating students for active learning activities.
At the second stage - stages of comprehension – the tasks are different: obtaining new information, comprehending it and relating it to one’s own knowledge.
Final stage – stage of reflection and reflection, implying a holistic understanding, appropriation and generalization of the information received, developing one’s own attitude to the material being studied, identifying what has not yet been learned - questions and problems for further work (“new challenge”), analysis of the entire process of studying the material.
What does this technology do for students? Firstly, responsibility for the quality of one’s own education increases. Secondly, they develop skills in working with texts of any type and with large amounts of information. Thirdly, creative and analytical abilities are developed, as well as the ability to work effectively together with other people.
The technology for developing critical thinking is most effective when studying material from which an interesting, educational text can be compiled. There are several possible forms (strategies) of using this technology: “Reading a text with notes”, “Filling out a table of ZKH (I know, I want to know, I found out)”, “Zigzag”, “Advanced lecture”.
Positive aspects of the proposed technology: independent acquisition of knowledge, understanding of one’s own activities in the educational process, increasing the responsibility of students. A full-fledged lesson is obtained with a double lesson. It is possible to organize a practical lesson and study new material. The difficulty lies in the unequal pace of reading and formatting of written work by students.
Lesson objectives. Summarize students' knowledge about the classification of carbohydrates and the differences between polysaccharides and monosaccharides; study the structural features, occurrence in nature, physical and chemical properties of starch and cellulose in comparison; consider the biological role of polysaccharides.
DURING THE CLASSES
Call stage
Teacher. In previous lessons, you studied the classification of carbohydrates and examined in detail the features of monosaccharides. Today you have to study the structure, occurrence in nature, physical and chemical properties of polysaccharides. But first, let's remember the main differences between polysaccharides and monosaccharides. For this purpose, you are asked to complete a test.(Sheets with the test are laid out in advance on the students’ tables.)
Test
Select from the given statements only those that are true:
I v a r i a n t – for monosaccharides;
Option II – for polysaccharides.
1. Their representatives are glucose, fructose, galactose, ribose, deoxyribose.
2. Their representatives are starch, glycogen, dextrins, cellulose, chitin.
3. Molecules are made up of many identical repeating groups of atoms.
4. They are divided into trioses, tetroses, pentoses, and hexoses.
5. They have a general formula (C 6 H 10 O 5) n .
6. The molar mass is small and usually does not exceed several hundred g/mol.
7. The molar mass is large and can reach several million g/mol.
8. They do not undergo hydrolysis reactions.
9. Capable of undergoing hydrolysis.
10. Residues of the molecules of some of them are part of DNA and RNA nucleotides.
| Answers. Option I: 1, 4, 6, 8, 10; Option II: 2, 3, 5, 7, 9. |
Students take the test and then check with each other in pairs.
Conception stage
The teacher asks students for 20 minutes. based on the textbook by O.S. Gabrielyan “Chemistry. 10th grade" (M.: Bustard, 2004) work through the text - § 24, p. 206–210, using special pencil marks:
“V” – I know this;
“+” – new information;
“–” – information that contradicts my knowledge;
"?" – information requiring explanation;
"!" - this is interesting.
Students work in groups of 3-4 people, exchange opinions on the issue being studied, help each other overcome difficulties that arise, making the necessary explanations.
Stage of reflection and reflection
Students return to pairs and make a table on the characteristics of starch and cellulose (table). In each pair, one student fills out a column about starch, and the other about cellulose, after which they exchange the results.
Table
Characteristics of starch and cellulose
Characteristic |
Polysaccharide |
|
Cellulose |
||
| Molecular formula | (C 6 H 10 O 5) n | (C 6 H 10 O 5) n |
| Structural features | The structural unit is the remainder of the cyclic glucose molecule. The degree of polymerization ranges from several hundred to several thousand. The molar mass reaches several hundred thousand g/mol. Structure of macromolecules: linear (amylose) and branched (amylopectin). In starch, amylose accounts for 10–20%, and amylopectin accounts for 80–90% | The structural unit is the remainder of the cyclic glucose molecule. The degree of polymerization ranges from several thousand to several tens of thousands. The molar mass reaches several million g/mol. Structure of macromolecules: linear |
| Occurrence in nature and biological functions | In the cytoplasm of plant cells in the form of grains of a reserve nutrient. Content (by weight): in rice - up to 80%, in wheat and corn - up to 70%, in potatoes - up to 20% | An essential element of the cell membrane of plants, performing a building, structural function. Content (by weight): in cotton fibers - up to 95%, in flax and hemp fibers - up to 80%, in wood - up to 50% |
| Physical properties | White amorphous powder, insoluble in cold water, swells in hot water and forms a colloidal solution - starch paste (while amylose, as a component of starch, dissolves in hot water, and amylopectin only swells) | Solid fibrous substance, insoluble in water |
| Chemical properties | (C 6 H 10 O 5) n + n H 2 O -> n C 6 H 12 O 6 . 2) Formation of esters due to hydroxy groups (no practical significance). 3) Qualitative reaction with iodine - blue color |
1) Formation of glucose as a result of complete hydrolysis: (C 6 H 10 O 5) n + n H 2 O -> n C 6 H 12 O 6 . 2) Formation of esters due to hydroxy groups: when interacting with nitric acid (in the presence of sulfuric acid) - mononitrates, dinitrates and trinitrates; when interacting with acetic acid (or acetic anhydride) - diacetates and triacetates. All esters are widely used. 3) Does not react with iodine |
Homework. Complete the table with the lines “Obtaining” and “Application”, using § 24 of the textbook and reference books; solve problem No. 1, p. 210.
Literature
Gabrielyan O.S., Maskaev F.N., Ponomarev S.Yu., Terenin V.I. Chemistry. Grade 10. Textbook for general education institutions. M.: Bustard, 2004, p. 206–210; Bessudnova N.V., Evdokimova T.A., Klochkova V.A.. Developing students' critical thinking in biology lessons. Biology at school, 2008, No. 3, p. 24–30.
A.S.GORDEEV,
teacher of chemistry and ecology
gymnasium No. 20
(Donskoy, Tula region)
* A concept introduced by L.S. Vygotsky, denoting the discrepancy between the child’s existing level of development and the potential that he is able to achieve under the guidance of a teacher and in collaboration with peers.
Starch is an amorphous powder with a characteristic crunch (potato starch), insoluble in water under normal conditions. When starch grains get into hot water, theyswell, their shells rupture, and a colloidal solution is formed.
Cellulose is a white fibrous substance that is insoluble in water. Unlike starch, cellulose does not react with water at all, even when boiled. Pure cellulose is found in our lives in the form of cotton wool.
The structure of starch molecules and cellulose
The simplest formula of starch (cellulose and) is (C 6 H 10 O 5) n . In this formula the value n - from several hundred to several thousand. So, starch is a natural polymer consisting of repeatedly repeating structural units C 6 H 10 O 5 . It consists of two types of molecules. For this reason, starch is even considered a mixture of two substances - amylose and amylopectin. Amylose (20% of it in starch) has linear molecules and is more soluble. Amylopectin molecules (80%) are branched, and it is less soluble in water. These molecules also differ in relative molecular weight: for linear molecules (amylose) it reaches hundreds of thousands, for branched molecules (amylopectin) - several millions.
The simplest and molecular formulas of cellulose are similar to those of starch. Obviously, with the same composition, these substances differ significantly in properties. Compared to starch, cellulose has a higher relative molecular weight. The reason cellulose is strong and insoluble is that it has a three-dimensional structure. However, cellulose not only does not have a three-dimensional structure, but also does not have a branched structure. But this is the reason why cellulose molecules are strong, because they have a linear structure, and the individual macromolecules are arranged tightly together in an orderly manner. As a result, the strength of intermolecular interaction between individual macromolecules increases significantly. Numerous hydrogen bonds are established between ordered cellulose macromolecules: the Oxygen atoms of the hydroxyl groups of one molecule electrostatically interact with the Hydrogen atoms of the hydroxyl groups of another molecule. For the same reason, cellulose forms strong fibers, which is not typical for starch. Meanwhile, in starch, most molecules have a branched structure, so there are possibilities for... less hydrogen bonding.
Starch molecules are made up of residuesα -glucose, and cellulose - from the remains of moleculesβ -glucose. This is also the reason for the differences in the chemical properties of starch and cellulose:
Starch
Cellulose
Chemical properties of starch and cellulose
1. Complexation of starch with iodine.
The property of starch to form a blue color with iodine is used as a qualitative reaction for the detection of starch. Mainly amylose reacts with iodine, forming a colored compound. The amylose molecule in the form of a spiral surrounds the iodine molecules, and around each iodine molecule there are six glucose residues. Heating destroys such a complex and the color disappears.
2.Hydrolysis.
Sucrose is characterized by a hydrolysis reaction. The same property is inherent in starch. When starch is boiled for a long time in the presence of an acid (usually sulfate), the molecules undergo hydrolysis. Moreover, the final product of hydrolysis is onlyα -glucose. However, the hydrolysis process occurs in steps with the formation of intermediate hydrolysis products. The stepwise hydrolysis process can be expressed in the following scheme:
Cellulose has a similar property. However, cellulose hydrolysis takes place under more severe conditions, and the final product of hydrolysis isβ-glucose.
The intermediate products of cellulose hydrolysis are not of particular interest, so they can be omitted and the reaction equations can be summarized:
3. Thermal decomposition.
When wood is heated to a high temperature without air access, a fairly large amount of products is released. In addition to carbon and water, liquid products are formed, including methyl alcohol (which is precisely why they are called wood alcohol), acetone, and acetic acid.
4. Esterification.
Since the glucose residues that make up cellulose retain hydroxyl groups, it is able to react with esterification with acids.
Each cellulose unit contains three hydroxyl groups. All of them can enter into ester formation reactions. In the usual formula of cellulose, these hydroxyl groups are separated as follows:
The most important are cellulose esters with nitrate acid (nitrocellulose) and acetic acid (acetylcellulose).
Starch is the main carbohydrate in our food; Like fats, it is not directly absorbed by the body. Hydrolysis of starch under the action of enzymes begins in the mouth when chewing food, and continues in the stomach and intestines. Formed as a result of hydrolysis, glucose is absorbed into the blood and enters the liver, and from there to all tissues of the body. Excess glucose is stored in the liver in the form of high molecular weight carbohydrate glycogen, which is again hydrolyzed to glucose as it is consumed in the body's cells.
To produce glucose, starch is heated with dilute sulfuric acid for several hours. When the hydrolysis process is completed, the acid is neutralized with chalk, the resulting precipitate of calcium sulfate is filtered off and the solution is evaporated. When cooled, glucose crystallizes from the solution.
If the hydrolysis process is not completed, the result is a thick sweet mass - a mixture of dextrins and glucose - molasses.
Dextrins, extracted from starch, are used as glue. Starch is used for starching linen; when heated with a hot iron, it turns into dextrins, which glue fabric fibers together and form a dense film that protects the fabric from rapid contamination. In addition, this makes the next wash easier, since dirt particles associated with dextrins are much easier to wash off with water.
Starch is used to produce ethyl alcohol. During this process, it is first hydrolyzed by an enzyme contained in the malt, and then the hydrolysis product is fermented in the presence of yeast into alcohol.
Ethyl alcohol, which is used for industrial needs (rubber synthesis), is produced synthetically from ethylene and hydrolysis of cellulose.
Due to its mechanical strength, cellulose in wood is used in construction; all kinds of joinery products are made from it. In the form of fibrous materials (cotton, flax, hemp), it is used to make threads, fabrics, and ropes. Cellulose isolated from wood (freed from accompanying substances) is used to make paper.
Cellulose esters are used for the manufacture of nitrovarnishes, film, medical collodion, artificial fiber and explosives.