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:

• 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.