Transpiration Lab Essay Research Paper Water is

Transpiration Lab Essay, Research Paper

Water is essential to plants in many ways. It first provides the major substance

for living, to keep cells from shriveling up and dying. The second major

function is to keep the plants rigidity. As plant cells become turgid, full of

water, the cells expand, filling the extent of their cell walls, which are kept

taught with turgor pressure. If the cells lose water, two problems occur. First,

the cells dehydrate, causing the organism to die. Second, turgor pressure is

lost as cells become flaccid, limp and unfilled, causing a loss of support for

the plants structure which makes it appear wilted. As aquatic plants evolved

into large complex land plants, an adaptation occurred in the center of plants

to allow full growth without the problem of water loss. A system of vascular

bundles extending from the tips of the furthest leaves to the deepest roots of

each plant developed, carrying water in xylem sap and sugar in phloem. While

phloem can transport sugar in any direction within the plant, xylem can only

move water up, from root to leaf. Once in the leaf, the water evaporates through

stomata?tiny gaps in the lower epidermis of each leaf, which are regulated by

guard cells?a process called transpiration The movement of water into and out

of the xylem involves water pressure factors in different sections of the plant.

As water slips into the roots through osmosis, a positive water pressure gently

pushes the water into the plants roots and supplies a jumpstart for the

water?s journey up the vascular bundle. However, it is not this pressure that

supplies a great force towards the upward movement of water; it is the

evaporation of water from the stomata that pulls water upward and out. When the

stomata are open to take in carbon dioxide for carbohydrate production, water

begins to evaporate and seep out of the tiny holes in each leaf. With a constant

pull of water outward, other water molecules are pulled up to replace it. The

pull is provided by the cohesive properties of water molecules as each leaving

molecule pulls on another molecule which is hydrogen bonded to it. The process

continues as a series of movements until all the water molecules in the xylem

sap are being pulled upward by their hydrogen bonds to the water molecules ahead

of them. Thus the slight negative pressure occurs. Different environmental

factors can have impacts on the intensity of water evaporation, and thus the

rate of plant transpiration. Just like water in an open environment, a dry

environment would increase the evaporation of water, and the rate of

transpiration. A hot or very bright environment would do the likewise.

Conversely, moist, dark, or cool environments would allow for a slower rate of

transpiration because water would not be as readily evaporative. When testing

the rate of transpiration for any given plant, I hypothesize that plants exposed

to copious quantities of light will transpire more rapidly than those in a

regular environment. Methods We selected a bean plant on which to test varied

environmental factors on transpiration. The different environments included

excessive sunlight?a floodlight one meter from the plant, wind/dry air?a

stationary fan approximately one meter away from the plant on low speed,

humid/rainy climate?leaves misted, then covered with a clear plastic bag (open

at the bottom for air exchange). Normal room conditions were also tested for the

control. One bean plant was used for each simulated environment. To set up the

experiment, four pieces of Tygon clear plastic tubing were cut to sixteen

inches. Inside each was placed the tip of a 0.1-mL pipette. Taking four ring

stands, one paired with each tube/pipette set, each end of the tubing was

clamped, so that the tubing made a ?U? shape. Next the tubing was filled

with water so that no air bubbles were present and that water completely filled

the tubing and pipette. The four bean plants were each placed into the open end

of their respective tubing, then sealed with petroleum jelly around the sides

(to prevent accidental water evaporation). The plants were allowed to sit for

ten minutes before the initial reading was made, to allow for equilibration.

After recording levels of water for all plant environment simulations, readings

were made in ten minute increments until thirty minutes elapsed. After this, the

leaves were cut off of each plant to be weighed and measured. With these

figures, we found the total surface area of each plant, after which we could

calculate the rate of transpiration for each climate. Results To determine the

rate of transpiration for each tested bean plant, the cumulative water loss (in

milliliters) was divided by the leaf surface area of each plant (in meters

squared). This rate was figured for each time increment: initial, ten minutes,

twenty minutes, and thirty minutes. Table 1 shows these calculations for the

control, group a, floodlight, b, fan, c, and mist, d. The relationship among the

data is shown on Figure 1. The lines for test plants b and c both show high

rates for transpiration, while control plant a is at a moderate rate of

transpiration and test plant d has a relatively low rate of transpiration

compared to the other plants. Conclusion As Figure 1 shows, the plants tested in

dryer climates, b and c, showed higher rates of transpiration. This is due to

the greater potential for evaporation in their environments. The extra exposure

to light adds heat which dries up water vapor around the plant and inside the

leaves, as it leaves through the stomata. The water in the tube was then pulled

by the negative pressure created by the evaporation of water, increasing the

transpiration rate. With plant c, the fan dried water vapor around the plant and

in the leaves, causing the area to be dry, thus creating a negative pressure for

water in this plant as well. Plant d had a very low rate of transpiration

because its environment was very moist. Water was very unlikely to evaporate in

the misted enclosure, therefore causing the plant only to need to replace the

water which it used to maintain its turgor pressure. The environment for plant a

provided a normal room climate. Although evaporation was likely, it did not seem

to be a large factor in the plant?s functions. So, as water did escape from

the stomata of the plant?s leaves, the slow rate created enough negative

pressure to replace the water being lost to the air and being used by the plant,

which wasn?t very much. When this experiment was initially done in our

classroom, many faults occurred. Without prior experience handling plants and

petroleum jelly, the experiment is difficult. While it is a good idea to see the

experiment in order to understand it, the book provided the best data.


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