Artificial selection is the process by which humans select animals and plants for breeding because of their useful characteristics e.g. high crop yield in cereal crops and meat quantity and quality in beef cattle. Artificial selection has been practiced for thousands of years to produce varieties of animals and plants with increased economic importance. At GCSE, students may not only be required to define the process of artificial selection (or selective breeding as it is also known) but also state the similarities and differences between natural and artificial selection, and outline the steps involved in ‘improving’ crop plants and domesticated animals over many generations.
I would actually suggest introducing artificial selection before broaching (or revisiting) natural selection, as students often find it easier to grasp the concept of humans acting as the selective agent, purposively picking and choosing which individuals survive to breed, than the environment. There are also many examples of weird and wonderful selectively-bred plants and animals with which the students will already be familiar. Indeed, I tend to open the lesson with a short quiz in which I display photographs of sausage dogs, Merino sheep, Belgian blue cows etc., and ask the students to guess why they look the way that they do.
However, once the students are familiar with both artificial and natural selection, it is useful to compare and contrast the two using a card sort activity, Venn diagram or double-bubble map. The students should identify that both processes require genetic variation and result in individuals with particular phenotypic traits (characteristics) surviving to breed and pass on their genes, while others do not. At this stage it may also be useful to reinforce the concept of evolution as being a change in frequency of particular alleles within a population over time and that, as such, evolution occurs through artificial as well as natural selection (one common misconception is that natural selection and evolution are one and the same).
Selective Breeding Game
This is a fun game in which the students, as farmers, aim to selectively breed sheep with both plentiful wool and high quality meat. It is a particularly effective activity for demonstrating that artificial selection occurs over successive generations and that the farmer does not actually create anything (the alleles for the favourable characteristics already exist) but simply decides which individuals can breed and which can not. The game is available to download for free here.
Wolf and Dog Handraising Project
This is a BBC documentary entitled ‘The Secret Life of the Dog’ which features an overview of a fascinating experiment carried out by researchers at Eötvöus Loránd University in Hungary between 2001 – 2003. The aim of the experiment was to investigate whether the relationship between humans and domesticated dogs could be replicated with wolf cubs if they were treated like puppies and raised in the home. It makes for a fantastic discussion point about ‘nature and nurture’ by highlighting the fact that artificial selection can result in changes to an animal’s temperament as well as their appearance. The relevant section begins at 32 minutes in.
The Ethics of Artificial Selection
The danger of artificial selection is that that there may be too much inbreeding between closely related individuals. This can result in harmful recessive alleles being inherited alongside the desired genes, and an overall reduction in genetic variation. Indeed, many breeds of dog suffer from the effects of inbreeding e.g. elbow and hip dysplasia, epilepsy and heart disease (further information is available from the Kennel Club, among other sources). Asking students to consider the ethics of artificial selection, can prove an engaging topic for debate, if carefully structured.
These working models of heart valves illustrate the structure and function of both atrioventricular (i.e. bicuspid and tricuspid valves) and semi-lunar valves (i.e. the aortic valve and pulmonary valve, as well as the valves located in veins). They are extremely quick and easy to make.
As blood flows through capillaries within tissues, some of the plasma leaks out through gaps between the cells in the wall of the capillary, and seeps into the spaces between the cells of the tissues. This leaked plasma is known as tissue fluid.
Tissue fluid is almost identical in composition to blood plasma. However, it contains far fewer plasma proteins as most are simply too big to pass through the tiny holes in the capillary endothelium. Red blood cells are also too big so tissue fluid does not contain these, but some white blood cells can squeeze through and move freely between the tissue cells.
The tissue fluid leaves the capillaries under high pressure at the arterial end of capillary beds. In order to demonstrate this I use the following:
Five or six small balloons
Permanent marker pen
Large glass bowl
Pestle and mortar
Use the permanent marker pen to draw nuclei on the balloons and then place them into the glass bowl. The balloons represent the tissue cells. Fill the glass jug with water and add a drop or two of yellow food colouring (blood plasma). Throw in some red beads (red blood cells) and rice (plasma proteins) – grind up the rice in the pestle and mortar beforehand so that you have different sized fragments.
Pour the ‘blood’ through the sieve, highlighting that the holes in the sieve represent the tiny gaps in the capillary wall. I like to pour it from a great height so that it sprays everywhere (showing that the blood is under high pressure) and covers the cells below. Highlight that none of the red blood cells have passed through the holes and nor have most of the plasma proteins (the students should see some of the smaller bits of rice floating about in the tissue fluid but most will remain trapped in the sieve).
By the end of Key Stage 3 pupils are expected to be able to describe the structure of an atom, relate atomic structure to information given for each element in the Periodic Table and show the arrangement of electrons in shells around the nucleus. It is vitally important that pupils develop a secure knowledge of these fundamental concepts in chemistry since a superficial understanding can result in misconceptions and pose significant difficulties in understanding higher-order content such as ionic and covalent bonding at GCSE and beyond.
Take your time, break down the topic into bitesize chunks and use plenty of diagnostics such as hingepoint questioning to gauge the level of understanding of the whole class before moving forward together. In addition to the following activities, provide pupils with plenty of practice in relating atomic number and mass number to the number of protons, neutrons and electrons in a neutral atom, and drawing electron shells.
Hula hoop competition
Explain that all atoms consist of electrons orbiting a tiny nucleus then have a hula hoop competition! Who can keep their electrons orbiting for the longest?
Provide pupils with laminated electron shells, an element symbol, a particle key and coloured plasticine. Ask them to build an atom of the element they have been given before taking a photograph and sharing it with the class via Padlet.
Build giant models of atoms using foam balls, craft straws, pipe cleaners and wire etc. These make excellent mobiles which can be hung in order of atomic number along the length of the science corridor.
Alternatively, bake (or buy) cookies and decorate them with proton and neutron M&Ms and silver ball electrons.
Facebook profiles or cubes
Facebook profiles or these simple cubes can be used to present information on all manner of things in science (e.g. famous scientists, types of nuclear radiation, specialised cells). Ask the pupils to investigate the element for which they built their plasticine atom and then complete a Facebook profile or cube for it. Who discovered it? When was it discovered? Is it a metal, non-metal or semi-metal? What are its properties?
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).
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 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.
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.
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!
This is a fun way to introduce the concept of limiting factors in photosynthesis at Key Stage 3.
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!
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:
Site of production
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.
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 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 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
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 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’).
One of my AS Level Biology students pointed out that the bilayer of a cell membrane resembled the two halves of a cake. So the next week we had a competition to bake and decorate fluid mosaic cakes. It was great fun and the cakes were delicious.
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
Boiling water bath
Raw liver, cut into 5 g cubes
Raw potato, skin removed and cut into 5 g cubes
Pestle and mortar
Label the boiling tubes A, B and C and the watch glasses B and C.
Measure out 5 cm3 of hydrogen peroxide solution into each boiling tube.
Place a 5 g cube of raw liver into the boiling water bath and leave for two minutes.
Use the forceps to carefully remove the cube from the water bath, and place it on watch glass B.
Grind one raw liver cube with the pestle and mortar, and transfer the paste to watch glass C.
Add the remaining raw cube of liver to boiling tube A and after one minute record the height of froth in the boiling tube.
Repeat with the boiled cube in boiling tube B and the raw liver paste in boiling tube C.
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).
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.