1979 A.P.

PLANT SYSTEMS QUESTION 1979:L. PETERSON/AP BIOLOGY

In relation to plants, describe in detail one way of:

a) measuring the rate of transpiration

b) measuring the rate of photosynthesis

c) separating pigments

Essay :

Transpiration is the loss of water vapor from land plants into the atmosphere, causing movement of water through the plant from the soil to the atmosphere via roots, shoot and leaves, which occurs mainly through the stomata. Factors which can affect the rate of transpiration are those such as temperature, humidity, wind, and light, and the water in the soil. A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts. To measure transpiration, you can artificially create the environment for a plant for which you want to transpire. For example, if you have a .1 mL pipette, plastic tubing and a leaf, you have almost all the materials you need to measure the rate of transpiration for that leaf and whatever environmental conditions you impose upon the leaf. One will be examining the amount of water in the tubing which is being used, and from there you can tell how much water has been used.

Photosynthesis is the process by which green plants, algae and some bacteria absorb light energy and use it to synthesize organic compounds. In green plants, photosynthesis occurs in chloroplasts, that contain the photosynthetic pigments. Photosynthesis occurs by slightly different processes in C3 and C4 plants. Factors which can affect this are the stomata. Plants can regulate the movements of water vapor, O2 and CO2 through the leaf surface. This is accomplished by opening and closing the stomata, usually found on the bottom side of the leaf. Opening and closing of stomata is controlled by specialized cells called guard cells. A possible method of determining the rate of photosynthesis could be to take a leaf or a stem of a plant and place it in water, using a tube or some other secure membrane which would also have water. One would then measure the amount of air bubbles released, which would be oxygen. Oxygen is a byproduct of photosynthesis, and whether or not it is happening quickly, you would then know whether the rate is high or low.

Plants use green pigments called chlorophylls to trap light energy. The

chlorophylls give a plant its green color. Inside the cells that have

chloroplasts, the light energy is used to make glucose. The process by which plants use light energy to make glucose is photosynthesis. Chlorophyll a's composition is C55H72MgN4O5, with a characteristic blue-green color to it. Chlorophyll b is comprised of C55H70MgN4O6, having a brilliant green coloration to it.

In aquatic ecosystems, oxygen is scarce. To be useful to aquatic organisms, oxygen must be in the form of molecular oxygen, O2. The concentration of oxygen in water can be affected by many physical and biological factors. Respiration by plants and animals reduces oxygen concentrations, while the photosynthetic activity of plants increases it. In photosynthesis, carbon is assimilated into the biosphere and oxygen is made available, as follows: 6 H2O + 6 CO2(g) + energy ® C6H12O6 + 6O2(g). One method of measuring the production of oxygen is the light and dark bottle method. In this method, a sample of water is placed into two bottles. One bottle is stored in the dark and the other in a lighted area. Only respiration can occur in the bottle stored in the dark. The decrease in dissolved oxygen (DO) in the dark bottle over time is a measure of the rate of respiration. Both photosynthesis and respiration can occur in the bottle exposed to light, however. The difference between the amount of oxygen produced through photosynthesis and that consumed through aerobic respiration is the net productivity. The difference in dissolved oxygen over time between the bottles stored in the light and in the dark is a measure of the total amount of oxygen produced by photosynthesis. The total amount of oxygen produced is called the net productivity.