Students should learn:
- that protein molecules are made up of long chains of amino acids
- that proteins act as structuralcomponents, hormones, antibodies and catalysts
- that an enzyme is a biological catalyst
- how enzymes work.
- describe how long chains of amino acids form protein molecules
- state the roles of proteins in theformation of muscles, hormones,antibodies and enzymes
- describe the structure and mode of action of an enzyme.
able to:
- explain in detail the concept of the active site of the enzyme.
Catalysts increase the rate of chemical reactions … [B2.5.1 b)]
The shape of an enzyme is vital for the enzyme’s function … [B2.5.2 a)]
Different enzymes work best at different pH values. [B2.5.2 b)]
Controlled Assessment: AS4.3 Collect primary and secondary data [AS4.3.2 a) b) c) d) e) f)]; AS4.4 Select and process primary and secondary data [AS4.4.1 a) b)], [AS4.4.2 b)]; AS4.5
Analyse and interpret primary and secondary data. [AS4.5.4 a)] / Chapter map: Enzymes
Teacher notes: Enzymes
Animation: Enzyme action
Bump up your grade: Nearly everything about enzymes
Lesson structure / Support, Extend and Practical notes
Starter
Biological stains – Bring in a cheap, clean white T-shirt and allow students to smear it with selected food and drink (tomato ketchup, mustard, egg). Discuss with the students how they could remove the stains and get the T-shirt clean. Show the students a box of biological washing powder and a box of non-biological washing powder and get them to say which one would be best to use with reasons. (Care needed if washing powders are handed around – some people can have sensitive skin). Support students by prompting as to the nature of the stains and how they could be broken down. Extend students by asking them to compare the contents of the two washing powders and to say what the enzymes are, breaking down, e.g. starches, proteins and fats. (10 minutes)
Main
Enzymes in action – The experiment ‘Breaking down hydrogen peroxide’ shows the action of manganese(IV) oxide, an inorganic catalyst, and a piece of liver, which contains the enzyme catalase, on hydrogen peroxide.
Use a PowerPoint presentation to build up a picture of how enzymes are composed of long chains of amino acids folded and coiled into special shapes. Introduce the concept of the active site, enzyme specificity, how they work and what they can do. Introduce them to the convention of naming enzymes – the ‘-ase’ suffix for many – and give some examples.
Provide students with a worksheet that they can fill in as the presentation proceeds.
Catalase is present in living tissue. The more active the tissue, the greater the catalase activity (see Practical support ‘Catalase in living tissues’ for full details). The reactions can be described or they can be measured. (This links to ‘How Science Works’ – making observations and measurements.) If the experiment is to be a qualitative one, i.e. just a simple comparison of the activity by observation, then written descriptions or comparative statements can be made.
It is possible to make this experiment more quantitative by using the same quantities of each tissue, and then measuring the activity when placed in the same volume of hydrogen peroxide. Simple heights of froth up the tube in a given time can be measured. A more accurate measurement is given by collecting the gas evolved in a given time. (This demonstrates many ‘How Science Works’ concepts.)
There are many variations of the catalase experiments:
- Investigate the volume of gas released when different quantities of fresh liver are used in the same volume of hydrogen peroxide, i.e. varying the amount of enzyme with a fixed quantity of substrate.
- The converse of this is to use the same quantity of liver and vary the concentration of hydrogen peroxide used, i.e. varying the quantity of the substrate with a fixed quantity of enzyme.
Find the substrate for the enzyme – Using thin card, make sets of ‘enzymes’ of different shapes and with differently shaped ‘active sites’, and a corresponding set of ‘substrates’ that fit into the enzymes’ ‘active sites’. (You could adapt very simple jigsaw pieces.) Support students by making the pieces very simple. Extend students by using more complex shapes and making the ‘substrates’ consist of two parts which fit together into the active site. Students need to find the ‘enzyme’ and ‘substrate’ that fit together.(10 minutes) / Support
Use toy building blocks to represent large molecules, such as starch, proteins and fats. Label each one on one side with the name of the substrate (‘starch’, ‘protein’) then label the individual bricks with the name of the products (‘sugars’, ‘amino acids’). Use plastic knives with the word ‘Enzyme’ on to cut up the blocks.
Extend
Ask students to research the structure of proteins and use a length of Bunsen tubing to demonstrate the differences between the primary, secondary and tertiary structure. Different sequences of amino acids can be marked with a pen and the tubing can be coiled and twisted into a C shape to illustrate the active site.
Practical support
Breaking down hydrogen peroxide
Equipment and materials required
Manganese(IV) oxide, fresh liver, tiles and knives for cutting, test tubes, hydrogen peroxide solution, eye protection, some method of collecting the gas given off (syringes/inverted test tubes, rulers if height of froth to be measured), water bath if liver is to be boiled and denatured.
Details
By adding hydrogen peroxide, students can compare the activity of the inorganic catalyst with cubes of fresh liver and liver in which the enzymes have been denatured by heating. The denatured liver shows that the enzyme is present in living tissue and is destroyed by heating. Include a test tube containing hydrogen peroxide as a control. An additional control using a piece of boiled and cooled liver would show that the enzyme from the living tissue can be denatured.
Safety: CLEAPSS Hazcard 33 – disposal of organic waste.
CLEAPSS Hazcard 50 Hydrogen peroxide.
Catalase in living tissue
Equipment and materials required
Fresh liver, potato tuber tissue, apple, etc., tiles and knives for cutting, test tubes, hydrogen peroxide solution, eye protection, some method of measuring the gas given off (syringes/inverted test tubes or manometers; rulers if height of froth to be measured), stopwatches or stop clocks, water bath if tissues are to be boiled and denatured.
Details
Drop small cubes of different tissues, such as liver, muscle, apple and potato, into test tubes containing hydrogen peroxide (10 cm3 to 15 cm3 depending on the size of the tubes). If the experiment is to be qualitative, students should record their observations, make comparisons and write statements about the activity of the enzyme in the different tissues. If it is to be quantitative, then the same quantities of tissue and hydrogen peroxide should be used and measurements taken of the activity.
Safety: Wear eye protection, CLEAPSS Hazcard 50 Hydrogen peroxide. Take care with tubes, which can become hot.
Course / Subject / Topic / Pages
Additional science / Biology / B2 3.1 Proteins, catalysts and enzymes / Pages 32–33
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- that enzymes are vital to all living cells
- that changes in temperature affect the rate at which enzymes work
- that different enzymes work best at different pH values.
- describe experiments that show the effect of changes in temperature and pH on the rate of enzyme-controlled reactions
- describe how changes in temperature and pH affect enzyme action.
- explain in detail how changes intemperature and pH affect the active site of an enzyme.
Different enzymes work best at different pH values. [B2.5.2 b)]
Controlled Assessment: AS4.4 Select and process primary and secondary data [AS4.4.2 a) b) c)]; AS4.5 Analyse and interpret primary and secondary data. [AS4.5.2 a) b) c)], [AS4.5.3 a)] / How Science Works: Does temperature affect the speed of an enzyme reaction?
How Science Works: Lines of best fit and error bars
Lesson structure / Support, Extend and Practical notes
Starters
What happens to milk when it goes off? – Show the students fresh and sour milk. If possible, have one that is really solid but careful risk assessment is necessary. Discuss what has happened to the milk and why putting milk in the refrigerator stops it going off.
(5 minutes)
Denaturing eggs – Crack raw eggs (or get students to do this) into three beakers: one beaker at room temperature, one at a temperature where visible changes to the egg white just occur, and one at boiling point. Support students by asking them to describe the visible and textural changes to the egg white. Extend students by asking them to explain the changes that are happening to the shape of the protein. Are the changes irreversible?
Introduce the concept of denaturation. (10 minutes)
Main
Investigating the effect of temperature on enzymes – Students can use their own saliva to carry out this experiment on the action of amylase on starch (see Practical support ‘Investigating the effect of temperature on enzymes’).
A graph can be plotted of the rate of disappearance of starch (1/time taken in seconds) against the temperature. Many concepts of ‘How Science Works’ can be developed in the investigative work, e.g. hypotheses are formulated, predictions are made, variables are controlled and conclusions drawn. Concentrate on one or two of these, e.g. drawing conclusions from the graph plotted.
Other enzymes could be used for investigations into the effect of temperature. If the use of the students’ saliva is not possible, commercial amylase could be used, but it is usually derived from fungi and can give odd results.
If pepsin or trypsin (protein digesting enzymes) are used, the substrate to use is the white of hard-boiled eggs or an egg-white suspension made by adding 5 g of egg white to 500 cm3 of very hot water and whisking briskly. The rate at which the egg-white suspension clears can be timed at the different temperatures. More ‘How Science Works’ concepts are introduced here too.
Investigating the effect of pH – The effects of varying pH can also be investigated by modifying the experiments described above. Keep the temperature constant and vary the pH by using a range of buffer solutions.
The effect of varying pH on catalase. Potato discs can be added to hydrogen peroxide and buffer solutions and the quantity of oxygen evolved in a set time can be measured at each pH. A graph can be plotted of volume of oxygen evolved against pH and the optimum pH for catalase determined.
Plenaries
What temperature do I work best at? – Discuss what might be the optimum temperature for the enzymes in the human body. What happens if we get a fever? Why do parents worry when you get too hot? Contrast our body temperature with that of other organisms
– include some fish, reptiles and invertebrates. Do all enzymes have the same optimum temperature? (5 minutes)
Definitions – Write up a list of the key words and phrases used in this topic so far. Support students by providing a list of definitions which they need to match with the words. Extend students by asking them to write their own definitions and using them to compose a short passage which they could use as a revision card. (10 minutes) / Support
Tell students that bits of milk need to be joined together by enzymes to make yoghurt. Make some yoghurt in a vacuum fl ask, a water bath or preferably a commercial yoghurt maker. Set up controls in the refrigerator and at room temperature. Prepare a work sheet with a results table for time taken for it to run through a funnel. Adjust the bore so that some yoghurt will very slowly flow through. Try it with boiled yoghurt (risk assessment).
Extend
Get students to research some of the organisms that live in hot springs, very cold conditions and conditions of extreme pH.
Practical support
Investigating the effect of temperature on
enzymes
Equipment and materials required
Test tubes and racks, water baths for different temperatures, 2% starch solution, fresh saliva, boiled saliva, iodine solution, white tiles, glass rods, eye protection.
Details
Each student will need at least 2 cm depth of saliva in a test tube. Test tubes should be set up containing equal volumes of saliva and starch solution, shaken and then placed into water baths at different temperatures. Drops of the mixtures are then tested at 30 second intervals for the presence or absence of starch by dipping a glass rod into the mixture and then into a drop of iodine solution on a white tile. Note the colour each time and record how long it takes for the starch to disappear at each temperature. A control could be set up using boiled saliva.
Safety: CLEAPSS Hazcard 54B Iodine solution. Dispose of saliva in disinfectant. CLEAPSS Hazcard 33 Enzymes.
Course / Subject / Topic / Pages
Additional science / Biology / B2 3.2 Factors affecting enzyme action / Pages 34–35
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- that during digestion, the breakdown of large molecules into smaller molecules is catalysed by enzymes
- that these enzymes, which areproduced by specialised cells in glands, pass out into the gut
- that the enzymes include amylases that catalyse the breakdown of starch, proteases that catalyse the breakdown of proteins and lipases that catalyse the breakdown of lipids.
- explain how enzymes are involved in the digestion of our food
- describe the location and actionof the enzymes which catalyse the breakdown of carbohydrates (starch), proteins and lipids.
- explain digestion in terms of the molecules involved.
Some enzymes work outside the body cells … . [B2.5.2 c)]
The enzyme amylase is produced in the salivary glands … . [B2.5.2 d)]
Protease enzymes are produced by the stomach … . [B2.5.2 e)]
Lipase enzymes are produced by the pancreas and small intestine … . [B2.5.2 f)]
The stomach also produces hydrochloric acid. The enzymes … . [B2.5.2 g)]
The liver produces bile, which is stored in the gall bladder … . [B2.5.2 h)]
Controlled Assessment: AS4.5 Analyse and interpret primary and secondary data. [AS4.5.2 a) b) c) d)], [AS4.5.3 a)], [AS4.5.4 d)]
Lesson structure / Support, Extend and Practical notes
Starters
What we know about enzymes so far, a quick quiz –
Ask 10 questions on enzyme structure and factors affecting their action. Support students by making the questions simple and straightforward. Extend students by asking more difficult questions and expecting more detailed answers. (5 minutes)
The fly – Show photographs of a fly’s mouthparts and talk through how they function, or how a spider sucks the juice out of its victims. (For a taster, search the internet for ‘The Fly watch trailer’). (10 minutes)
Main
Introduce the different types of digestive enzymes by reviewing the different components of the diet. Get the students to realise that complex carbohydrates, proteins and lipids have to be digested before they can be absorbed. Introduce the groups of digestive enzymes and what they do. Reference to carbohydrases, proteases and lipases, their substrates and their products is required. Project a diagram of the human digestive system and its associated glands and indicate where the different enzymes work in the gut. Also indicate on this diagram where the enzymes are produced as well as where they act. It could be helpful to provide the students with an outline of the digestive system, so that they can fill in the information for themselves.
Making a model gut – each group of students will need two 15 cm lengths of dialysis (Visking) tubing to model the gut (see ‘Practical support’). If desired, the experiments can be left for 24 hours at room temperature before testing.
If there is not time for the students to carry out their own experiments, then a length of dialysis tubing can be filled with a mixture of 30% glucose solution and 3% starch solution and placed in a test tube of distilled water. If this is left for about 15 minutes, the water can be tested for starch and glucose.
Some glucose should have diffused through the tubing into the water, but the starch should not. Tests for starch and glucose will confirm this. Note: This only demonstrates that glucose will pass through the tubing but starch will not; it does not show that the enzyme catalyses the breakdown of the starch.
The model gut can be used to show the effect of changes in temperature and pH on the activity of saliva or amylase on starch. The tubing should be placed in boiling tubes, and samples of the water surrounding the tubing can be tested for starch and sugars at intervals to determine whether or not digestion has taken place.