Outline of a lesson in biology (grade 5) on the topic: Experience "Identification of a protein in a cell." Color reactions for proteins

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An ordinary kitchen and a scientific laboratory have a lot in common. In pots and pans, complex biochemical processes take place. Some of them can be reproduced at home and feel like a real chemist
Today we will study the composition of milk. All children know that milk is a very healthy product. True, not everyone likes milk. But this does not diminish its usefulness. Remember that mammals feed their young with milk. Hence, it is highly nutritious and easy to digest. Milk simultaneously contains all the main substances of food - these are proteins, fats and carbohydrates. Fats provide the most energy. The carbohydrate in milk is called lactose. But unlike glucose or sucrose (food sugar), lactose has almost no sweet taste.

People eat mostly cow's milk. But in animals, the composition of milk is different. Analyze the table and answer the questions.

Now let's do some experiments with milk.

You can take one sample of milk, but it is much more interesting to compare several samples from different manufacturers. If someone has the opportunity to compare milk from a store and milk from their own cow or cows from a farm, then this is very interesting.

Unfortunately, in many factories milk is often made from milk powder or added to normal fresh milk. From this, some properties of milk may change.

You will need: several samples of milk (be sure to write down the name of the manufacturer), several test tubes (you can ask a chemistry teacher, if this is not possible, then take small glasses or even glasses), acetic acid, blue vitriol, a little alkali solution or washing soda , pipette (available in pharmacies).

Experience 1. Determination of fat in milk.

Cow's milk usually contains 3-4% fat. You probably already know that fat does not mix with water. But in milk, this fat does not immediately separate from the water, it can be seen only if the milk stands for a long time. Then the fat will float to the surface. This will be the cream. Milk fat is found in milk in the form of globules that form an oil-in-water emulsion with water. Fat globules are protected by strong and elastic shells of protein, so they do not stick to each other during a collision.

Apply a drop of each milk sample to the filter paper. (Try to keep the drops the same.) When they dry, measure the diameter of each spot with a ruler. The larger it is, the higher the fat content in milk. Since the fat content of sold milk is usually almost the same, you can take cream for interest. They should have a lot more fat.

Experience 2. Detection of protein in milk

Pour a few ml (milliliters) of milk into a test tube and carefully add an equal volume of a weak solution of copper sulphate (pale blue) and a little alkali solution or washing soda along the walls and mix. A purple color appears. This indicates the presence of protein in the product under study.

Milk contains several types of proteins. The main protein is casein. It is from casein that cottage cheese is formed. When milk is fresh, all proteins are dissolved. But if the milk is sour, you notice that it has become thicker - it has turned into curdled milk. If it is heated, then a precipitate of protein precipitates. This is how curd is made. But if you do not want to wait until the milk turns sour, you can add food acid to it - acetic or citric acid and get a casein precipitate.

Experience 3. Determination of the presence of casein in milk.

Pour 5 tablespoons of milk into a glass, add 1 tablespoon of acetic acid (9%), or a few drops of vinegar essence, mix. You can see the formation of white flakes. It's casein.

Experience 4. Obtaining whey.

When casein precipitates, other proteins and lactose remain in the liquid part - whey. To obtain serum, the precipitate must be filtered. To do this, take a small glass. Put a funnel in it. Put a filter from several layers of gauze or bandage into it and pour milk with the resulting curd onto the funnel. Cottage cheese (casein) will remain on the filter, and we will do other experiments with whey.

Experience 5. Detection of protein in serum.

It is necessary, as in experiment 2, to add a solution of copper sulphate and alkali to several ml of serum and mix. The purple color indicates that many other proteins remain in the whey after the casein is isolated. Therefore, whey is also a useful and nutritious product.

Experience 6 . Detection of carbohydrates in milk.

A little whey should be poured into a cup or saucer, which can be heated over a fire, and the liquid evaporated. As the liquid evaporates, the whey chars and develops a sweet odor similar to that of burnt sugar. We have proven that milk contains carbohydrates.

Experience 7. Sympathetic ink.

This is the name of liquids that, when written, do not leave any colored trace on paper. The text can be read only after special processing - heating or wetting with a certain substance. Milk is great for secret writingAnd. Therefore, the last task will be this - write a review about our competition with milk, carefully iron this sheet of paper with a not very hot iron until the text appears. Take a photo of him and place the photo under the answer table.

Research work.

We propose to compare several types of milk according to a number of indicators - from different manufacturers, both homemade and store-bought.

Carry out the necessary experiments and complete table.

Q&A table

1.	Whose milk is more nutritious - dogs or cows and why?
2.	Why the fattest milk (up to 40% fat) in whales and dolphins
3.	Children often ask, "Why is milk white when a cow eats green grass?" Try to answer this question.
4.	Did you manage to get casein sediment from all milk samples, that is, curd?
5.	What other proteins are in milk besides casein?
6.	Why do many people, as adults, not be able to consume milk, as it causes them indigestion? Find the answer to this question in books or the Internet.
7.	What products are made from milk?

As soon as a person began to cook his own food, so he, albeit unconsciously, became a chemist. In frying pans and braziers, in barrels and earthenware chemical And biochemical processes. By the way, not all of them have received a full explanation even today, which, however, does not prevent people from cooking, baking, salting and marinating. However, much has already been well studied. And something - of course, not the most difficult - can be reproduced even in home laboratory.

The experiments presented in this section have at least one indisputable advantage: the necessary substances (more precisely, products) can be found in the kitchen cupboard or in the refrigerator. Or you can buy them at the grocery store. You will need small amounts of substances, but if you buy more of this or that product than is required for the experience, the rest will not be wasted.

The most important ingredient in food is protein , the basis of all living things, the building material of any organism. Thousands of researchers around the world work with protein and study its properties. Of course, in our experiments we will not discover anything new. But, they say, the beginning is a dashing misfortune ... The first experience - qualitative reaction on protein, i.e., such a reaction that will allow us to confidently judge whether the protein is in front of us or not. There are several such reactions. The one we'll spend is called biuret. For her, we need solutions washing soda(or caustic soda) And blue vitriol.

Prepare several solutions that are likely to contain protein. Let it be meat or fish broth (preferably strained through gauze), a decoction of some vegetables or mushrooms, etc.

Pour the solutions into test tubes approximately half way. Then add a little alkali solution - caustic soda or washing soda (it is advisable to boil the soda solution and cool it). Finally, add the blue copper sulfate solution. If there is indeed protein in the test broth, then the color will immediately turn purple.

Such reactions are said to be characteristic. They only go if does the solution contain protein?. For control, put an experiment with lemonade or mineral water.

Everyone knows that protein coagulates when heated and passes into an insoluble form - a raw egg becomes steep. This phenomenon is called protein denaturation. Every housewife knows: to prepare a delicious broth, you need to put chopped meat in cold water. And when they want to cook boiled meat, then large pieces are dipped in boiling water. Is there in this chemical sense? Let's try to figure it out.

Pour a test tube of cold water into it, dip some raw minced meat into it and heat it up. As it heats up, gray flakes form (and in large quantities). This clotted protein, foam, which is removed with a slotted spoon so as not to spoil the look and taste of the broth. With further heating, water-soluble substances gradually pass from the meat into solution. These substances are called extractive, because they are extracted from the meat when it is extracted with boiling water (in other words, when boiling the broth). First of all, they give the broth a characteristic taste. And meat, having lost these substances, becomes less tasty.

Boil water in another test tube in advance and put the raw meat in boiling water already. As soon as the meat comes into contact with water, it will instantly turn gray, but very few flakes form. The protein that was on the surface, under the influence of high temperature, immediately curled up and clogged the numerous pores that permeate the meat. Extractives, including proteins, can no longer go into solution. This means that they remain inside the meat, giving it a good taste and aroma. And the broth, of course, turns out a little worse.

Protein is denatured(coagulates) not only when heated. Pour a little fresh milk into a test tube and drop one or two drops vinegar or solution citric acid. Milk immediately turns sour, forming white flakes. It coagulates milk protein. By the way, without such a reaction, you cannot cook cottage cheese, and it is no coincidence that cottage cheese is so useful - almost all milk protein passes into it.

When milk is left in a warm place, its protein also coagulates, but for a different reason - it works lactic acid bacteria. There are a lot of them, and they all produce lactic acid, even if they do not eat milk, but, say, cabbage juice. Filter some sour milk and add to the whey a few drops of some homemade indicator. The color of the indicator will show that there is acid in the solution. This acid is dairy, it can also be found in cabbage and cucumber brine.

Some protein molecules contain, in addition to carbon, hydrogen, oxygen And nitrogen, also sulfur. This can be verified by experience. Place a little egg white in a test tube with a solution of caustic soda or washing soda and, heating the test tube, add a little solution to it lead acetate Pb (CH 3 COO) 2 . 3H 2 O - lead lotion, which is sold in pharmacies. If the contents of the tube turn black, then sulfur is: it is formed lead sulfide PbS, black substance.

And in conclusion, we will prepare a real protein glue - casein, which is still used today, despite the abundance of synthetic adhesives. Casein- this is the basis of cottage cheese, and if so, then we will make glue from milk, more precisely, from its protein substances.

Filter yogurt from whey. What remains on the filter, rinse several times with water to remove soluble impurities, and dry. Then rinse the resulting mass with gasoline and dry again; this is necessary in order to get rid of milk fat (it dissolves in gasoline). When the mass becomes completely dry, grind it in a mortar and mortar - you will get casein powder.

Making glue out of it is quite simple - mix the powder with ammonia and water in a 1:1:3 ratio. Of course, you'll want to test the glue. Try gluing some wooden or ceramic objects with it, because for these materials casein glue especially good.

Carbohydrates are one of the "three pillars" of our diet (the other two are proteins and fats). Glucose And fructose, starch And cellulose, dozens of other carbohydrates are formed continuously and "burn" (oxidized) in plant and animal cells, serve as the body's most important energy material.

For all the dissimilarity of individual representatives of carbohydrates, they, of course, have common properties that are mandatory for all. This makes it possible to detect carbohydrates even in very small quantities. A true and also beautiful way to recognize them - Molisch color reaction.

Pour about 1 ml of water into a test tube and drop in a few grains of granulated sugar ( sucrose), part of a glucose tablet, or piece of filter paper ( fiber). Now add 2-3 drops of alcohol solution resorcinol or thymol(these substances are sold in pharmacies). Tilt the tube and carefully pour 1-2 ml of concentrated sulfuric acid. Be careful with acid, make sure that it does not get on the skin!

Fix the test tube in a vertical position. Heavy acid will sink to the bottom, and a bright beautiful ring will appear on its border with water - red, pink or purple.

If a substance whose composition is unknown gives such a ring in the Melisch reaction, you can be sure that carbohydrate on the face. Just remember that this reaction is so sensitive that even a speck of dust and a fiber on the walls of the test tube can cause it. Therefore, the dishes in which the reaction is carried out must be washed very carefully, and it is better to rinse with distilled water.

Now, having learned to recognize carbohydrates, let's move on to starch, one of the most famous carbohydrates. First, let's learn how to cook starch paste- a colloidal solution of starch in water. Pour some cold water into a saucepan and stir in the starch, at the rate of about two teaspoons per glass (including any water you add later). Stir the mixture well - you get the so-called starchy milk. While stirring, add boiling water to it and, continuing to stir, heat over a fire until the solution becomes transparent. Cool it down. This is the starch paste that sticks paper together so well; therefore it is often used, for example, for wallpapering.

You already know that starch turns blue in the presence of free iodine. This property of his is still useful to us; note only that the iodine solution must be very weak. By the way, using such a solution (and to prepare it, it is enough to dilute the pharmacy solution with water), you can examine various food products for starch content. Having prepared a test tube with a weak solution of iodine, we will observe the transformations of starch. Let's try make glucose from starch paste.

Huge starch molecules under the action of water hydrolyzed are broken down into smaller molecules. First, a soluble starch, then smaller "stumps" - dextrins, then disaccharide but not familiar to everyone sucrose, and the other - maltose, or malt sugar. Finally, during the breakdown of maltose, glucose, grape sugar. The finished product of hydrolysis often contains all transitional substances; in this form it is known as molasses.

To half a glass of starch paste, add 1-2 teaspoons of diluted, approximately 10% sulfuric acid. Do not forget: when diluting sulfuric acid, be sure to pour acid into water, and not vice versa!

Set the mixture of paste and acid to boil in a saucepan, gradually adding water as it evaporates. From time to time take samples of the liquid with a spoon and, after cooling slightly, drip dilute iodine solution on them. Starch, as you remember, gives blue staining, but dextrins are red-brown. As for maltose and glucose, they do not stain at all. Samples will change color as the hydrolysis progresses, and when the iodine stain disappears, heating can be stopped. However, for a more complete decomposition of maltose, it makes sense to boil the mixture for a few more minutes.

After boiling, the liquid should be cooled slightly and gradually add about 10 g of powder to it with stirring. chalk to completely neutralize sulfuric acid. At the same time, the mixture will foam, because during the reaction of acid with chalk, carbon dioxide. As soon as the foaming stops, put the resulting yellowish liquid on a low fire so that it evaporates by about two-thirds, then filter it still hot through several layers of gauze, then evaporate the liquid again, but now more carefully, not over an open fire, but in a water bath (the mixture burns easily). You will get thick sweet syrup, which is based on glucose. In much the same way, molasses is obtained in large quantities at starch factories.

Glucose a person needs it, it is one of the main suppliers of energy. But bread, potatoes, pasta contain mainly starch, and in the body it turns into glucose under the action of enzymes.

In our experience sulfuric acid was not consumed during the reaction. She played the role of a catalyst, i.e., a substance that sharply accelerates the course of the reaction. The catalytic action of natural enzymes is much stronger, it is more targeted. There are a lot of enzymes, and each of them has its own, narrow area of ​​\u200b\u200bwork. For example, contained in saliva enzyme amylase can convert polysaccharide starch to the disaccharide maltose. Let us follow experimentally the action of this enzyme.

Lesson - practical work

Theme “Analysis of food quality. Detection of protein in the test sample”.

prepared by a chemistry teacher of the Moscow Autonomous Educational Institution of Lyceum No. 28 named after N.A. Ryabov

Popova S.I.

Purpose: “To determine the quality of food products. Investigate the proposed samples for protein content.

Tasks:

1.Educational:

to form the ability to solve experimental problems of an applied nature to establish the content of proteins in food products;

to establish intersubject connections of the studied theoretical basis with close, "home" problems.

2.Educational:

to continue the formation of a responsible, creative attitude to the performance of tasks, accuracy, observation, perseverance;

continue local history education;

fostering a positive attitude towards a healthy lifestyle; education of tolerance, cooperation, independence.

3.Developing:

develop the ability to transfer knowledge to new conditions;

develop the ability to conduct, observe and describe a chemical experiment;

develop mental operations (analysis, synthesis, establishing cause-and-effect relationships, hypothesizing, classifying, drawing analogies, generalization, ability to prove, highlighting the main thing);

to improve the communication skills of students in joint activities (the ability to conduct a dialogue, listen to an opponent, substantiate one's point of view with reason).

Skills:

1. General laboratory - conduct heating; filtration.

2. Organizational - to observe accuracy and clarity in work, PTB, work according to instructions, maintain a clean workplace, draw up records, exercise self-control.

(introductory speech of the teacher)

Proteins in food play the role of the main building material for the body, without which its vital activity, growth and restoration of cells is impossible.

As you probably already know, any product consists of proteins, fats and carbohydrates, and our main task is to skillfully combine them in our daily diet.

One ofthe main principles of a healthy dietjust says that the food should be balanced, and contain the optimal amount of proteins, fats and carbohydrates.

Protein food is one of the most important components of the diet of any person. With a lack of protein, the process of splitting fats is impossible, in turn, a satisfying protein product allows you to increase the absorption time of carbohydrates, helping to maintain a stable blood sugar level and suppressing hunger.

What is the structure of protein? (Working with the “Visual school” complex - protein structures)

Qualitative reactions to protein discuss

(Switch to new topic)

Evaluation of food quality, determination of microbiological contamination are very responsible and time-consuming processes. The approach to each sample is made strictly individually.

There are several types of food quality research:

organoleptic - smell, taste, color, turbidity, temperature, foreign films, precipitation;

physical and chemical - the qualitative composition of the declared sample, the presence of impurities (both physical and chemical);

microbiological - determination of the presence of foreign microorganisms, infections in the test sample (a particularly important type of research recommended by the Analytic Company for baby food);

comprehensive – examination for compliance of the product with certain GOST, GOST R, TU, SaNPin and other standards that establish safety criteria for humans.

The guys are divided into 4 groups and conduct 2 experiments according to a previously issued instruction. . Then all students fill out a laboratory journal, followed by a discussion of the results.

BEFORE practicing, repeat the safety rules !

Experience No. 1. Determining the quality of milk pasteurization.

Pasteurization provides for the destruction of pathogenic microorganisms that degrade the quality of milk. The protein content should not decrease.

1. To 3 ml of milk add 3 ml of distilled water.

2. Drop by drop add 0.1 N solutionH 2 SO 4 casein flakes. Filter.

3. Filtrate. Heat to a boil.

KEY. Casein flakes reappear in raw milk .

4. Assume the degree of milk pasteurization in the studied samples.

5. Suggest the reasons for the falsification of various samples.

6. Add a few drops of concentrated nitric acid to the sample obtained. Heat up. Yellow staining indicates the presence of aromatic rings in the protein (xantoprotein reaction).

Experience number 2.

Determination of milk freshness

To 3 ml of 1% phenol solution add 3 ml of 1% iron chloride solution (III), note the purple coloration, add 3 ml of milk.

KEY. Sour milk gives a yellow-green color.

Set the degree of freshness, the reasons for the falsification of different samples.

Experience number 3.

Determination of impurities in sour cream.

Pour 10 ml of hot water into a beaker. Place a teaspoon of sour cream in hot water.

KEY. In the presence of impurities (cottage cheese, kefir, starch), fat will float, and casein will settle to the bottom.

Knowing that normally sour cream should not have sediment, suggest the reasons for the falsification of various samples.

Experience No. 4

Determination of meat freshness.

1. Prepare meat broth. (Heat 4 ml of water + a piece of meat to a boil).

2. Filter into a flask using a funnel.

3.Add 5 drops %H 2 SO 4 and after a few minutes note the result.

KEY. In fresh broth, the solution is clear, with doubtful freshness, the solution becomes cloudy, and in definitely stale meat, a jelly-like sediment with flakes.

Determine the degree of sample falsification. Compare several different samples. Suggest reasons for the different results.

Experience number 5.

Determination of protein in the proposed products.

1Pour 4 ml of water into 3 test tubes. In 1 add 0.5 bouillon cube, in the 2nd piece of meat, in the 3rd piece of sausage. Boil.

2.Filter.

3. Add concentratedHNO 3 drop by drop and heat up a bit.

KEY. In a solution in which protein is present, a yellow color will appear.

Make a judgment about the quality of the samples.

Goal of the work: prove the presence in biological objects of such important organic compounds as proteins, lipids, carbohydrates

Progress:

1 Determination of glucose in grape juice

To 2 cm3 of grape juice, add 1 cm3 of sodium hydroxide solution and 3 drops of copper sulfate solution. Heat to a boil.

Analytical effect: the formation of a yellow precipitate of Cu(OH)2 is observed, which, upon further heating, turns into a red precipitate of Cu2O.

2 Reaction of starch with iodine

To 2 ml of starch solution add 1-2 drops of Lugol's solution. The solution turns blue. Then they heat up. The color disappears, but the color reappears on cooling.

3. Physico-chemical properties of fats

Take 2 test tubes. Pour 2 ml of distilled water into the first, alcohol into the second. Pour 5 drops of vegetable oil into each test tube. Shake all tubes well and note the dissolution of fat in different substances.

4. Fat emulsification

Take 2 test tubes. Pour 3 ml of distilled water and fresh vegetable oil into each of them. Add a few drops of 10% Na2CO3 soda solution to the second tube. All test tubes are closed and shaken thoroughly. Leave for 5 min to stand and observe the stability of the emulsion.

5. Definition of fat unsaturation

Add 0.5-1 g of oil to a test tube, add 2 drops of KMnO4 and shake thoroughly. Discoloration of permanganate occurs.

6. Biuret reaction to determine the peptide bond

Add 5 drops of 1% egg white solution, 3 drops of 10% NaOH solution and one drop of 1% CuSO4 solution to a test tube and mix. The contents of the tube acquire a blue-violet color.

7. Xantoprotein reaction

To 5 drops of a 1% solution of egg white, add 3 drops of concentrated nitric acid (carefully!) And heat it up. A yellow precipitate will appear. After cooling, add 5-10 drops of a 10% NaOH solution to the test tube until an orange color appears (it is associated with the formation of the sodium salt of these nitro compounds).

8. Protein coagulation on heating

Pour some protein solution into a test tube. Heat the solution to a boil. The solution becomes cloudy, the protein precipitates in the form of flakes. Protein coagulation during boiling is an irreversible precipitation process, protein molecules change their structure.

9. Precipitation of proteins with salts of heavy metals, alcohol, sulfuric acid

Pour the protein solution into two test tubes. Add a solution of copper (II) sulfate to the first test tube, and a solution of silver nitrate to the second. Precipitation occurs in both test tubes. Heavy metal salts precipitate proteins from solutions, forming water-insoluble salt-like compounds with them.

Add a little ethyl alcohol to the protein solution. The solution becomes cloudy. Under the action of alcohol, protein denaturation occurs - the destruction of its structure.

Add a little sulfuric acid to the protein solution. A precipitate falls out. Under the action of sulfuric acid, protein denaturation occurs - the destruction of its structure.

Prepare a report on laboratory work in the form of a table (each experiment separately)

Starting substancesWhat did they do? What did they observe? Conclusions, reaction equations