Standard #6
a)Carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms.

You could do the experiment/demo of mixing concentrated sulfuric acid with granulated white sugar and put a glass beaker over it. The students will see the sugar turn black (into carbon, C) and steam being produced (H₂O), showing that sugar is made up of carbon.

By extension you could burn paper or dried bread and show that they are also made of carbon chains. Since the students eat sugar and bread to stay alive they too are made of those elements.

Conservation of mass.
b) Living organisms are made of molecules largely consisting of carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. Try growing some plants form seeds - like peas or beans- in different pots and once they have sprouted to about the same height then transplant them to some sterile sandy soil. Then water each plant the same amount. In one pot only give water, so the only elements it gets are carbon, hydrogen and oxygen. In another pot just add ammonia fertilizer, supplying N (nitrogen) and in another supply sulfur and phosphor only and finally in the last pot a balance died. This is the usual science fair experiment but very effective.

c) Living organisms have many different kinds of molecules including small ones such as water and salt, and very large ones such as carbohydrates, fats, proteins, and DNA.