How to Describe the Chemical Composition of Plants Including Percent of Dry Weight as Water

Plants grow by making food from chemicals and water.

Plants contain chemicals they use to make sugars. These sugars feed the plant. In addition to chemicals, plants need water. At any given moment, chemicals and water move through and out of the plant. When you describe the composition of plants, you are talking about that composition at one moment in time. You can describe the chemical composition and the amount of water in plants if you organize your thoughts according to how plants use these elements. Follow the process of making sugars and evaporation, and you will be describing the chemicals and water that make up plants.

Instructions

1. Describe the percentage of water in plants. Water can make up 85 percent of the weight of leaves. This means 15 percent of the leaf is dry weight made of plant fiber. Some plants can have as much as 95 percent of their weight as water. These measurements are for fresh plants. If you weigh a plant that is freshly picked, then wait for it to dry out and weigh it again, you will know how much water weight it was carrying.

2. Talk about carbon in plants. All living things on earth contain carbon. Plants take in carbon through their roots and store it. They also absorb carbon dioxide through leaves and convert that gas into a solid form of carbon. The part of the plant that is not made up of water contains about 45 percent carbon. When a plant dies, that carbon goes back into the soil as part of the decaying process.

3. Explain how oxygen works in plants. Plants combine oxygen with other elements to form cellulose, which is the material that makes up cell walls. If you put a leaf under a microscope, you will see that it is made from little compartments called cells. The walls of these cells are made from cellulose. Without oxygen, the plants could not make cell walls. Plants get rid of excess oxygen through their leaves.

4. Describe the hydrogen in a plant. Plants combine hydrogen and oxygen when they make cellulose. Hydrogen also is very important in making a substance called arabinoxylan. This is the substance that makes bread dough smooth. Bread dough is made from wheat, which is a plant. Without hydrogen, plants cannot make cellulose or arabinoxylan.

5. Explain that chemicals and water are always moving in plants. For example, plants take in carbon dioxide, change it into solid carbon by taking out the oxygen, and put some of that oxygen back in the atmosphere. When plants die, the carbon in them goes back into the soil. New plants absorb that carbon through their roots and put more oxygen into the atmosphere. At the same time, plants absorb hydrogen from the soil and use it to combine with oxygen and carbon to make cell walls.

References

• Enchanted Learning: Plant Cell Anatomy
• Biology Reference: Water Movement in Plants
• The Biology Corner: The Occurence of Water in Living Things
• Environmental Science Activities for the 21st Century: Trees and Carbon
• University of Maryland, Clark School of Engineering: Cellulose Degradation
• Scientific Psychic: Carbohydrates -- Chemical Structure