An Introduction to Enzymes

Enzymes first make an appearance in Year 9 and although most students at this level quickly grasp that these globular proteins speed up chemical reactions there are always a number of stubborn misconceptions about exactly what they are and how they work. Below are a few ideas for class practicals (tried and tested – enzyme experiments are notoriously fickle) and activities that can help at Key Stage 3 and beyond.

Practical 1 – Factors affecting the activity of catalase

Science is about discovery and students should be given opportunities to actually be scientists by discovering things for themselves. Too often teachers feel that they have to tell students everything, explaining exactly what will happen in an experiment and leaving nothing to be explored. So instead I turn the topic of enzymes on its head and start with this class practical investigating factors which affect the activity of catalase.

At the end of the lesson ask the students to describe what has happened and make some simple deductions; they will have seen that both liver and potato share a curious ability to break down hydrogen peroxide and release bubbles of gas, but that boiling them removes their ability to do so – why? By the time you start talking about enzymes, the lock and key theory and denaturing, the students’ curiosities will have been stirred and they will want to know how on Earth it all works.

You will need per group:

  • 6 boiling tubes in a test tube rack
  • 4 watch glasses
  • 30 cm3 hydrogen peroxide (20%) solution
  • 10 cm3 measuring cylinder
  • Forceps
  • Dropping pipette
  • Boiling water bath
  • Ruler
  • Raw liver, cut into  5 g cubes
  • Raw potato, skin removed and cut into 5 g cubes
  • Glass rod
  • Pestle and mortar
  • Stopwatch

Steps:

  1. Label the boiling tubes A, B and C and the watch glasses B and C.
  2. Measure out 5 cm3 of hydrogen peroxide solution into each boiling tube.
  3. Place a 5 g cube of raw liver into the boiling water bath and leave for two minutes.
  4. Use the forceps to carefully remove the cube from the water bath, and place it on watch glass B.
  5. Grind one raw liver cube with the pestle and mortar, and transfer the paste to watch glass C.
  6. Add the remaining raw cube of liver to boiling tube A and after one minute record the height of froth in the boiling tube.
  7. Repeat with the boiled cube in boiling tube B and the raw liver paste in boiling tube C.
  8. Repeat the experiment using potato cubes instead of liver (the potato cubes are difficult to grind in a pestle and mortar so you may need to cut them up into smaller pieces first).
  9. Record the results in a suitable table.

Modelling enzyme action

Follow the class practical by modelling the protein structure of enzymes, discussing the lock and key theory and demonstrating what happens when an enzyme is denatured.

Locked out

Begin the lesson by purposely locking yourself and the students out of the classroom. Produce a big handful of keys and make a fuss about finding the right key to fit the lock. Once inside, introduce the lock and key theory of enzyme action.

Complementary pairs

Another good starter activity is to cut large pieces of paper into complementary enzyme and substrate molecules then hand one out to each student as they enter the classroom and ask them to find their partner.

complementary sites

Amino acid necklace

Give each student a shoelace or a long piece of string and a handful of different coloured beads. Ask them to thread the beads onto the shoelace in any order they wish in order to make a colourful necklace. Explain that enzymes are large protein molecules made of many amino acids joined together in a long chain, a bit like the beads on their necklace.

beads

Ask the students to screw the necklace into a tight ball to make an ‘enzyme’. Highlight that everyone in the class has made a different type of enzyme because the sequence of ‘amino acids’ on their necklace and the 3D shape of the balls are all different.

necklace

Plasticine models

Make two or three enzymes with different shaped active sites using plasticine or modelling clay. Demonstrate that the substrate molecule only fits the active site of one type of enzyme, before reshaping the plasticine to show what happens when the enzyme is denatured.

Student enzyme models

Nominate two or three students to play the role of enzymes by standing up and putting out their hands in front of them to model an active site. Use a piece of scrap paper as the substrate molecule and move from one student to another until you find the complementary enzyme (in truth this can be any one of the students but it helps to reinforce the specificity of enzyme action).

Model digestive enzymes by gesturing for the chosen student to rip the paper in two before throwing the products dramatically into the air so that their active site is free to accept a second substrate molecule.

found-two-bits-of-paper-while-doing-some-sorting-out-yesterday-that-uEnPFm-clipart

You can also model the action of anabolic enzymes by asking the student to hold two ‘substrate molecules’ together in their active site while a bond forms between them (using sellotape). Again, encourage the student to throw the product dramatically into the air, leaving their active site free to repeat the process.

High five collisions

This is a very simple yet effective way of demonstrating the effect of temperature on an enzyme-catalysed reaction. With reference to a graph of enzyme activity against temperature explain that at low temperatures the average kinetic energy of the enzyme and substrate molecules is low and as such they move very slowly and collide only infrequently. Model this by asking the students to trudge slowly around the classroom and to high five one another on the odd occasion that they meet.

Highfive

Now turn up the temperature. Ask the students to move around the room a little more quickly, again high fiving when they collide. The students should be able to hear that the number of successful collisions has now increased.

Increase the temperature further still. The students will now be whizzing around the room (careful!) and high fiving almost constantly. The noise of substrate molecules and enzymes colliding will be deafening. This is of course the optimum temperature and the enzyme’s catalytic activity is at its greatest.

Finally, raise the temperature beyond the optimum, denaturing the enzyme and inhibiting the formation of enzyme-substrate complexes. Ask the students to lower their hands so that they can no longer high five. They will still be whizzing about (in fact, faster than before) and will certainly collide but the collisions will no longer be successful. The classroom will fall silent, the reaction has stopped.

Flowmap donuts, zoetropes and flickbooks

Instead of using linear flowmaps to illustrate the stages of an enzyme-catalysed reaction use flowmap donuts, zoetropes or flickbooks to highlight that enzymes remain unchanged by the reaction and can be used again. The students could even animate their plasticine models using stop motion applications such as Stop Motion Studio.

zoetrope 2

Practical 2 – Investigating the effect of temperature on the activity of lipase

A simple protocol which provides reliable, unambiguous results. The investigation can be carried out as a demonstration at two different temperatures, or in groups of five or six students with each student working at a different temperature, allowing enough time to collect repeat data. A nice extension is to add washing-up liquid to the solution in order to emulsify the fats and provide a larger surface area for enzyme action (demonstrating the effect of bile salts in the digestive system).

Full teaching notes and student sheets are available to download from the Nuffield Foundation.

Practical 3 – Investigating the effect of pH on amylase activity

Another reliable class practical from the Nuffield Foundation, this time measuring the time taken for amylase to completely break down starch at different pHs. Again, students can work in groups of five or six with each student working at a different pH before pooling results.

Virtual lab

However, if time is tight, one alternative is to use the excellent Virtual Lab from McGraw-Hill Education, in which students can investigate both the effect of pH and substrate concentration on an enzyme-catalysed reaction from their computer or tablet.

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BBC Bitesize

A nice video from BBC Bitesize which can be used to summarise much of the Key Stage 3 and Key Stage 4 content on enzymes.

LEGO and M&M Half-Life

The half-life of a radioactive substance is the time it takes for the number of parent nuclei in a sample to halve, or for the count rate from the original substance to fall to half its initial level. Half-life is random and it is impossible to know which individual parent nucleus will be the next to decay. LEGO and M&Ms can be used to model this random decay while also negating the need for students to handle radioactive materials.

M&Ms

791px-Plain-M&Ms-Pile

Students start with 100 M&Ms (other sweets can be used so long as there are two distinct sides e.g. Skittles) and tip them into a tray. Record the number of M&Ms which have landed face-up (these represent parent nuclei which have decayed). Remove these ‘decayed’ nuclei and tip the remaining M&Ms into a second tray. Once again count the ones that have ‘decayed’ and repeat until all of the M&Ms have gone. Use the data to plot a half-life curve.

LEGO

Lego_Color_Bricks

Students throw 60 2×2 LEGO bricks into a tray and remove all of the bricks that land studs-up (these represent parent nuclei which have decayed). Stack these bricks together to show the activity i.e. the number of decays per throw. Throw the remaining LEGO bricks and again remove those that have ‘decayed.’ Stack these into a second column and place this next to the first to quite literally build an activity vs. throws bar chart. Repeat until all of the LEGO bricks have gone.

 

Modelling Electric Circuits

Electricity is something that students encounter every day of their lives. However, there tend to be lots of misconceptions and these are best addressed at Key Stage 3 and GCSE by using models and analogies to explain what are otherwise abstract concepts.

Below are four methods of modelling electric circuits but it is important to remember that not all of them need to be used at once and that their value lies not only in students identifying the ways in which they work well but also in evaluating their limitations.

Hula hoop model

pink-hoop

Students sit in small groups (4-5 students) and hold a plastic hula hoop (or loop of rope) loosely in their hands. One of the students acts as the cell / power supply and begins to turn the hula hoop in one direction. The main message here is that the current moves at all points at all times in a circuit (a common misconception is that the current starts at the cell and slowly makes its way in procession through the wire).

The students will also feel some heat from the friction of the plastic hula hoop as it passes through their hands. This demonstrates that energy is being transferred but that the electrons themselves are not being used up (another common misconception). This model can also be used to introduce resistance i.e. for a given power supply, a higher resistance (i.e. a tighter grip on the hula hoop) will result in a lower current.

Student electron model

A student plays the part of the cell / power supply with a big plus sign on their right shoulder (positive terminal) and a big minus sign on their left (negative terminal). The remaining students are electrons and should arrange themselves in a tight circle (the circuit) around the edge of the classroom. Remind the students that electrons are negatively charged and, as such, repel each other (so they need to spread out evenly rather than clump together).

The student (electron) nearest the ‘positive terminal’ is pulled into the ‘cell’ and then pushed (gently!) out of the ‘negative terminal.’ As a result, this student will bump into / move close to the student standing next to the ‘negative terminal’ who, in turn, will be repelled and move away. This repulsion is repeated all the way around the classroom until a new ‘electron’ is pulled into the ‘cell’ at the ‘positive terminal.’ The whole process should be repeated and sped up to create a giant electric circuit.

Highlight that the push or shove from the ‘cell’ represents the voltage. The more powerful the cell, the bigger the voltage it gives to each electron. Finally, model resistance by placing two rows of chairs, through which the students have to squeeze, along one side of the classroom. As with the hula hoop model, the students should see that in a series circuit, if they are slowed down in just one small section of the circuit the current is reduced everywhere.

Bank and shop model

As above, the students should arrange themselves in a tight circle around the classroom. One student is the ‘bank’ (cell / power supply) and another is the ‘shop’ (bulb) which is located someway further down the road. At the bank, each student is given ten pounds (use Monopoly money) which they must then spend in full at the shop and therefore return to the bank with nothing (herein lies one limitation of the model as some energy is required for the current to get back to the battery).

monopoly

Next, pretend there are two bulbs in series of equal brightness or, in other words, two neighbouring shops in which each student spends an equal amount of money (the ‘bank’ should give each student ten pounds in two £5 notes in order to model this). Extend the activity by asking the students to model what would happen if there were two bulbs of different brightness or how the model would differ in a parallel circuit.

Mini whiteboards and sweets

Before building electric circuits, it can be useful for students to draw circuit diagrams on mini-whiteboards and then use sweets to demonstrate what is happening at each component. For example, if each sweet represents 1 V and the students are using a 6 V cell then they should start with just six sweets. If there are two bulbs in series but one is twice as bright as the other, how many sweets (volts) does each bulb require? Again, extend this activity by asking students to consider what would happen in different series and parallel circuits.

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Oreo Plate Tectonics and Moon Phases

A couple of nice activities using Oreo cookies (or in my case, cheaper alternatives).

Plate tectonics

Explain that the upper cookie is the lithosphere, the creamy filling is the asthenosphere, and the lower cookie is the lower mantle. Begin by simulating the motion of the rigid lithosphere plate over the softer asthenosphere by sliding the upper cookie over the cream. Then break the top cookie in half and simulate a divergent plate boundary by sliding the two cookie halves apart.

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Push one cookie half under the other to make a convergent plate boundary.

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Finally, simulate a transform plate boundary by sliding the two cookie halves past one another. Students should feel and hear that the two ‘plates’ do not glide smoothly past one another (thus modelling the earthquakes that occur at transform fault lines such as San Andreas).

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Moon phases

Simply remove the top cookie to reveal the creamy filling beneath. Scrape away and shape the cream to show the phases of the moon. Students should draw the relative location of the Earth and label the phases. Great as a revision tool or plenary.

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Forces Dance Mat

This is a great activity for introducing students to drawing force diagrams and resultant force. I have taken the idea directly from TES (the hugely popular original is available here) but I have made my own version in order to emphasise that the length of the arrow shows the size of the force. Obviously any music can be used to accompany it but I have always found that Gangnam Style works well (some of the students even do the dance moves as they jump about!).

Start the music, start the presentation and then jump in the direction of the resultant force. Have fun!