Photosynthesis

[bindibadgi]

Does anyone have an alternative to the rates of photosynthesis prac using hole-punched leaf discs & bicarb solution?

I'd like to try something different....we've been doing the leaf discs for years.

Cheers,

Kiah

[RosalieM]

We do it with pond weed. I don't think it matters which one but we use Hydrilla. Put it in a large beaker under a funnel fully immersed in water. Invert a test tube full of water over the funnel so there is no air in the system. Do the same with a second beaker and plant, and wrap the second one in foil so no light can get in. I think this ends up demostrating respiration more than photosynthesis, but the one with the light will produce air bubbles and after a few days there is a fair bit of air in the top of the test tube. I just threw ours out yesterday otherwise I'd attach a photo for you. Not sure if it's what you're after but it gets results

[RosalieM]

Sorry - just re-read your post and you wanted 'rates of photosynthesis', not sipmly 'photosynthesis'... sorry maybe the above reply isn't so helpful! I'll leave it there anyway incase it's good for someone else...

[bindibadgi]

Re: a post I put up yesterday about sheets with coloured discs for the rates of photosynthesis prac........the idea actually came from a local Adelaide labbie in our lab manager's association, Gail from Seaford High, so she's the angel with the nifty idea!

Anyway, I made up a couple of sheets, plus another one substituting layers of tracing paper to test light intensity & photos are attached (somewhat blurry). The cut outs are the size of a 50ml beaker base.

Gail also says if the kids want to time one colour or tracing paper disc at a time, just cover the other holes with bits of black cardboard.

Cheers,

Kiah

Photosynth sheet - wavelength.pdf

Photosynth sheet - intensity.pdf

[Labbie]

Apologies that I can't remember who it was, but someone posted an idea for testing light wavelengths for the rates of photosynthesis prac a while ago.

You cut beaker sized holes out of black cardboard sheets & cover the holes with various colours of cellophane.
Thank you to the angel who posted the idea......very nifty! Anyway, I made up a couple of the sheets & one class used them today........well, attempted to use them.

The teacher came into the prep 10 minutes into the lesson & asked " the sheet with all the pretty coloured circles goes on top of the beakers doesn't it?"
I told her "no, it goes on the OHP glass & the beakers sit on top of the coloured discs". She then said "but the light comes from above, so why put it under the beakers?"

I had to take her back into the lab & point to one of the OHPs, which was on, & show her the lovely bright light coming from UNDER the glass!

Honestly........I shouldn't be surprised.....but sometimes I don't know how some of these teachers have survived natural selection.

Kiah

Went to the incorrect topic

[Whspa]

So how do you actually measure the rate of photosynthesis? I don't think we do it here - at least I've never been asked to set it up.

[bindibadgi]

For the rates of photosynthesis prac, I use leaves from a plant called Fatsia japonica (photo attached), but ivy or something in that family works too, a vacuum pump, sodium bicarb solution & overhead projectors.

Using a hole punch, cut out little discs from the edges of the leaf, avoiding the veins down the middle of each section (before & after photos attached).
Put the discs in about 100ml distilled water in a 500ml volumetric flask. The discs at this point will float. Pop a one holed rubber stopper in the top & attach it to a vacuum pump with PVC tubing. Run the pump for about 20 minutes to suck all the air out. The air trapped in the spaces between the cells inside the leaf tissue will be sucked out through the cut surfaces around the edges of the discs, & the discs should then sink & sit flat on the bottom of the flask when you pull out the rubber stopper & break the vacuum. If they don't, just run the pump for a bit longer. Sometimes it takes 30-40 minutes, depending on the power of your pump.

If you don't have a pump you can use a 50ml plastic syringe & turn it into a mini-vacuum pump. Hole punch the discs, put them into the syringe in distilled water. Push most of the water out so the discs are in about 10ml, put your thumb over the end hole, & pull back on the plunger. Slip your thumb off the end & break the vacuum (you'll hear a little 'fffff' sound). Repeat the plunging action 8 or 9 times & you should find that the leaf discs sink (prac method attached).

Put the discs into some fresh distilled water & keep them in the dark so that they don't start to photosynthesise until you do the prac. You can do them a few hours before each lesson, but not the day before. When you put them into bicarb solution on a light source they photosynthesise & start to float. The faster they float the greater the rate of photosynthesis.

I've attached the prac method using bicarb concentration as the variable, but you can also test light intensity using layers of tracing paper, or wavelength using coloured cellophane shown in photos in my other post.

Cheers,

Kiah

Leaf.jpg

Leaf 2.jpg

Rate of Photosynthesis Prac Method.pdf

[Whspa]

Thanks for that. I'm going to suggest it to the bio teachers.
Carol

[RosalieM]

"they will sink because they become less dense than the solution"

Don't they sink because they are MORE dense? Not meaning to be picky...

Looks like a great prac though. I don't think we do anything like that here. Maybe I will mention it to a Bio teacher too!