Leaves and starch…

If, like me, you’re a fan of the SAPS website (Science and Plants in Schools), you’ll already be familiar with this variation on a theme. If not, read on, bated breath optional…

I think it’s a brilliant idea. Rather than testing an entire geranium leaf for the presence/absence of starch, the students cut leaf discs with a cork borer. This is an improvement for lots of reasons.

leaf discs boil

Firstly, as you can see from the picture,  the whole starch testing procedure is much easier if you’re working with small tissue samples, rather than a whopping great leaf. There’s no forcing the leaf into the testtube, no trying to delicately unfold it on the tile without ripping it. You can do repeats, it’s easier to see what’s going on, and so on.

It also means one geranium plant goes a lot further….

Better still, it opens the possibility for lots of different experimental treatments – because the cells in the cut discs remain alive for several days afterwards and will happily photosynthesize if given the right conditions.

Here’s what I got my students to do.

Destarch your geranium plant as per usual.

The pairs of students then cut 12 discs (per group) and floated them, top side up, in petri dishes containing either distilled water OR 5% glucose solution. Then one dish from each treatment (i.e. one water, one glucose) went under the light bank, and one dish from each treatment went into the dark.

Got that? OK, like the students, make some predictions. What would you expect?

It’s a great thinking exercise – testing their understanding of photosynthesis, the significance of the starch test, the structure of starch and the structure of a leaf.

24 hours later, they carry out their starch test on all 4 sets of discs.

leaf disc results

What’s really neat is that this is not an Either/Or result – you can clearly see different amounts of starch in each set of discs.

So, can you figure out which of A, B, C and D were…

  • in the dark on water
  • in the dark on glucose solution
  • in the light on water
  • in the light on glucose solution…?

Can anyone suggest other experiments you could carry out with these leaf discs?

 

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