Sunday, July 28, 2013

Plant Class #4: Leaves and Photosynthesis

For this class on leaves and photosynthesis, I first asked the kids what a leaf was for. What does a leaf do? Most of them were able to answer at least within the correct ballpark. 

Leaves are the food factories of the plant. They make food from the sun and store it, sometimes moving it to stems and roots by osmosis.

I gave them a hand-out at this point, the contents of which I will duplicate here. Handouts are good in terms of giving them something to look at during and after class and also to let the parents know what we have covered when all is said and done. 

I also gave them each a leaf from the lamb's-ear plant. They are so soft and fuzzy, it's fun to hold them, and it served as an example of what we were talking about. 



While they were holding their leaves, I pointed out the parts of a leaf: tip, base, midrib, lateral veins, petiole and margins. I also showed them a picture from a plant key (How to Identify Plants) and explained that there is great deal of language wrapped up in identification of the parts of the leaf, though they need not worry about this right now.


Next, we looked at a diagram of the inside of a leaf and I pointed out some major features. There is the upper epidermis (skin) covered by a cuticle, or waxy coating, to limit water loss. Under that are the palisade cells, and under the palisade cells is where the xylem and phloem will be in a matrix of spongy mesophyll cells. Finally, the bottom is where the stoma are located, with their guard cells, and this is what regulates the rate of water loss for the leaf and lets oxygen in and out.

The stoma are very interesting. When there is plenty of water, the guard cells get very full and open up, letting moisture and oxygen out. When there is less water, they go limp and close up, conserving water.


The process of water being lost by the leaves and pulled up from the roots is called transpiration, and can be a very important process for various habitats. Sometimes in moist forests, you can even see the moisture in the air as a form of fog above the trees.


I had tried to do an experiment at home demonstrating the production of oxygen by some plants, but it didn't work very well for me. I brought it in for them to see anyway. In this experiment, some aquatic plants or algae (conveniently gotten from our fish tank at home) are put in a glass jar with water. A funnel is placed over the plants and a test-tube is inverted on top of this. If you assemble it all in a bucket, the test tube and funnel will begin full of water. Over time, you should notice an air bubble forming at the top of the test tube. This would be a clear example of oxygen being given off by the plant. 
Again, this didn't work for me, but I encouraged them to try it at home if they wanted too.

Next, I needed to try to explain photosynthesis. Photosynthesis is how plants make food. They need Carbon Dioxide (what we breathe out), water and light to make sugars and oxygen. I went ahead and showed them the chemical formula for this and explained each part. 

6 CO2 + 6 H2O → 6(CH2O) + 6 O2
                     glucose

The sugars are the food the plants make for themselves. Plants also use some of the sugars in their cells, reversing the process, but overall they produce more oxygen than carbon dioxide.

Plants have various pigments that help them to absorb light and use it. Chlorophyll is by far the most common of these. It is green because it reflects green light and absorbs mostly blue and red wavelengths.

Light is energy of a certain wavelength, and our eyes can see only a small range of this. Plants can only use light of a particular wavelength. Many plants have additional pigments to help broaden their ability to absorb energy from the sun.


Some other pigments include:

Caroteniods: carotine (reflects orange), lutein (reflects yellow), lycopene (reflects red - tomatoes), and xanthophyll (yellow)

Anthocyanins: reflect red or blue (in many flower petals)

Betalains: red or yellow (the color of beets)

Many of these pigments are what you see in the fall when the leaves turn colors. The chlorophyll is the first to go away and the other pigments are left behind as the leaf starts to shut down and die.


You can do an experiment to see the different pigments in a leaf. This is a very simple version of a test that is used in laboratories, called chromatography. I got this experiment from here.

In this experiment you take samples from various plants, mash them up with some acetone and then use a coffee filter strip. If you dip one end of the strip in the leaf/acetone solution and let the capillary action migrate the solution up the strip, in the end you will get a series of stripes. Large molecule pigments will migrate slower than small molecule pigments. Here is a picture of my results. I think the best one was the beet greens. You can see the red pigments at the bottom, followed by a band of green, and at the very top, a brown.


One warning for anyone trying this. First of all, let those mashed leaves soak for a while (at least a day). Also, when you start your experiment, make sure there is plenty of ventilation. That acetone is not good for the brain cells.

Finally, I did a brief overview of some interesting leaf adaptations. Some plants have minimized their leaf surface to save water in dry places (cacti), while others have big wide leaves to catch more light in moist and shady places (under a rain forest canopy). Because it is hard to pull water up from the ground when it is frozen, many deciduous plants drop their leaves and essentially go to sleep until Spring. Pine leaves solve this problem by making their leaves small, so that rain, snow and wind falls off or goes thru them. 





As a final fun activity, we made some Sunprints. I got a packet of the 8x12 size sheets and brought in a large selection of leaves and flowers for them to choose from. If you had time, you could have your kids go out and collect leaves of their own to use. Each person was able to arrange leaves and flowers on a sheet and we took them outside to develop. This was a bit of a tricky process to do with 8 kids, but we did manage it with some extra cardboard backing and the use of some plastic sheets used for overhead transparencies. They really enjoyed this.




If we had had more time, we could have also collected leaves and did rubbings of our favorites. I suggested this to them for when they got home. In the meantime, I had given them enough information for one day. Again, there are many things I could have delved into deeper, but for a one hour class for a bunch of 9-10 year olds, this was more than enough!

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