Lego Telomeres

telomeres

Telomeres are located at the ends of chromosomes. They consist of multiple repeat sequences and their main function is to ensure that when DNA is replicated, the ends of the molecule are included in the replication and not left out. When teaching the significance of telomeres, I use Lego to build giant chromosomes as this allows me to demonstrate the loss of a short section of each telomere during cell division (by removing Lego bricks) and the role of telomerase in replenishing it (by adding Lego bricks).

Stem Cell Potency

A stem cell is a cell that can divide an unlimited number of times by mitosis. When it divides, each new cell has the potential to remain a stem cell or to differentiate into a specialised cell. The extent of the power of a stem cell to produce different cell types is variable and referred to as its potency. A simple yet enjoyable way to demonstrate the potency of different types of stem cell is to use Plasticine or modeling clay.

Start by giving each student an identical ball of Plasticine (apologies for the different colours in the photographs) and ask them to model it into an animal of their choosing. As you can see, in today’s lesson we had a snail, a penguin, a pig, a cat, two fish and a snake. In other words, the Plasticine has the potential to be absolutely any animal in the world. As such, it can be described as having high potency, much like the embryonic totipotent stem cells which can differentiate into any type of cell.

any

Next, explain that some of the totipotent stem cells differentiate into specialised cells in the placenta (demonstrate this by removing a few of the now ‘specialised’ animals but provide their sculptors with a new ball of Plasticine in order for them to continue with the activity) whereas others become a second type of stem cell, called pluripotent stem cells.

Pluripotent stem cells have lower potency than the totipotent cells but can still form all of the cells that will lead to the development of the embryo and later the adult. Demonstrate this reduced potency by asking the students to roll their Plasticine back into a ball before modeling it into an animal of their choosing but stipulating that it must now be an animal with four legs. In case you were wondering, we now have an elephant, two pigs, two lizards, a tortoise, and a cat.

animals-four-leg

Again, explain that many of the pluripotent stem cells differentiate into specialised cells (as before, remove some of the now ‘specialised’ animals and replace with a new ball of Plasticine) but that some become multipotent stem cells, found in the organs and tissues of adults. Multipotent stem cells have far lower potency than embryonic stem cells and can typically only differentiate into a very small number of specialised cell types. So, once again, ask the students to roll their Plasticine back into a ball before modeling it into an animal of their choosing…so long as that animal is either a dog or a cat!

animals-four-legs

Limiting Factors and Super Cheesy Burgers!

This is a fun way to introduce the concept of limiting factors in photosynthesis at Key Stage 3.

Burgers 2

Explain to the students that they are working at McBoulton’s (please feel free to change the name!), a popular hamburger fast food restaurant. It is a particularly busy day in the restaurant and the students are working in teams to prepare the most popular item on the menu, the McBoulton’s Super Cheesy Burger. Each Super Cheesy Burger consists of a sesame seed bun, a 100% pure beef patty, a slice of cheddar cheese, and a crunchy lettuce leaf (simply print and laminate for durability). Delicious!

burger

To begin, provide each team with 12 sesame seed buns, 8 beef patties (the second limiting factor), 12 lettuce leaves, and just two slices of cheddar cheese. Challenge the teams to make as many Super Cheesy Burgers as they can in one minute. Go!

Of course, after just 10 seconds or so, the production line will ground to a halt. Ask the students to record the number of complete Super Cheesy Burgers they have made (i.e. two) and to discuss why they made so few (i.e. they ran out of cheese slices). Now repeat the challenge with four, six, eight, ten and finally twelve slices of cheese. Each time ask the students to record the total number of burgers they managed to make in one minute and to discuss exactly what stopped them from making more.

Ask each team to plot a line graph of the number of complete burgers against the number of cheese slices they were given. Next, ask the students to describe and explain the graph (i.e. at first, the number of cheese slices governed the rate at which Super Cheesy Burgers could be made but eventually, when there were plenty of cheddar cheese slices available, the amount of beef patties limited production instead).

At this stage I usually ask the students to compare their fast food production line with the process of photosynthesis (using bridge maps) by identifying the following in each:

  • Raw materials
  • Products
  • Site of production
  • Energy source

Finally, I show the students examples of limiting factor graphs in photosynthesis, highlight the similarities with their own graph, and then ask them to identify the limiting factor in each.

What’s the Door Policy? Modelling Cell Transport.

doorman

When explaining how substances enter and leave cells, I use the different door policies of (fictional) exclusive restaurants and nightclubs to model the processes of simple diffusion, facilitated diffusion, active transport and osmosis. I often ask the students to act out each scenario (using various props) before asking them to identify the process, explain their reasoning, and discuss the limitations of each model.

Explanations are given in italics beneath each description.

Luigi’s

Luigi’s is a simple Italian restaurant which allows anyone in; so long as they are not being too noisy (really boisterous people tend to go to Jack’s instead). There are no doormen and it is free to enter. However, it is quite small inside and so it quickly fills up and then, once full, no one else can get in. There are usually an equal number of people waiting outside as there are inside but as one person leaves, another can enter so the actual number of diners never changes.

Luigi’s represents simple diffusion. Small, non-polar molecules can diffuse across living cell membranes (i.e. do not require transport proteins or ‘doormen’) but large, polar molecules (‘noisy people’) can not. Diffusion is passive (‘free to enter’) and net movement continues until equilibrium is reached (‘the actual number of diners never changes.’)

Havanna’s

rooftop

Havanna’s is an exclusive rooftop restaurant. It is free to enter but there are doormen who are notoriously fussy about who they let in; usually only the big names in town (who would never dream of going to Luigi’s or Jack’s). In fact, Havanna’s is so strict that the doormen actually accompany you up in the elevator all the way from the ground floor to the restaurant. As with Luigi’s it is only small and once full, it is a one out, one in policy even if there are lots of people waiting downstairs.

Havanna’s models the facilitated diffusion of large, polar molecules (‘big names’) via carrier proteins (‘the doormen actually accompany you up in the elevator’). Again, facilitated diffusion is passive (‘free to enter’) and net movement continues until equilibrium is reached (‘once full, it is a one out, one in policy’).

The Oxford Club

doormen

You have to pay to get into The Oxford Club, an exclusive members club downtown. There are doormen and they are extremely fussy about who they let in. It is a very strange place though as it is always busy inside but you rarely see people outside waiting to get in.

The Oxford Club represents active transport. It requires energy (‘you have to pay to get into the Oxford Club’) and carrier proteins (‘doormen’). Active transport involves the accumulation of ions against a concentration gradient (‘it is always busy inside but you rarely see people outside waiting to get in’). 

Jack’s

Jack’s is free and tends to be full of the particularly lively people who were turned away from Luigi’s. There is a doorman but he just politely holds the door open and in you go. It does tend to fill up quickly though and once full, the policy is strictly one out, one in only.

Jack’s models facilitated diffusion through channel proteins (‘just politely holds the door open and in you go’). Facilitated diffusion is passive (‘free’) and net movement continues until dynamic equilibrium is reached (‘once full, the policy is strictly one out, one in only’).

The Penalty Spot

The Penalty Spot is free but only open to supporters of the local football team after a match. There are lots of entrances but you can only get through if you present your season ticket (it is very selective). Away fans certainly can not get in. By the way, did you know that the nickname of the local team is the H2Os because they play in blue?

The Penalty Spot represents the net movement of water molecules by osmosis (‘only open to supporters of the local football team’). Water moves from a region of higher water potential to a region of lower water potential (‘open to supporters of the local football team after a match’), through a partially permeable membrane (‘you can only get through if you present your season ticket’). 

 

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.

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.

Screen Shot 2559-08-13 at 12.52.26

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.