Transpiration

get water to the top of very tall plants, like sequoia trees. This is called the cohesion theory.

Food, messenger RNA, plant hormones, and certain organic substances made by the plant cells are transported in the phloem of the plant. The main sugar made in plants is sucrose. This, along with amino acids and small organic molecules enter the cells and pass through plasmodesmata that connect cells to adjacent cells. Once in the phloem, the molecules can be transported to any part of the cell. The process of girdling a tree (removing a band of bark around the tree) removes the phloem. The tree will live for a period of time but will ultimately die because the roots are starved.

Transfer of food content through the phloem is dependent on the metabolism of the phloem cells, which is completely different from the xylem. Sugar will leave the sieve tubes near the leaves and stems (pumped out through active transport), with water following by osmosis. This increases the turgor (or pressure) in these areas, which causes the food and water to be pushed through the phloem under this high pressure. The sugar is used by the plant for nutrition and any that is left over becomes starch. Starch is insoluble in water and doesn’t affect the concentration of sugar in the phloem.

TRANSPIRATION

This is the evaporation of water from primarily the leaves. Leaves have stomata that are open for CO2 and O2 to pass as part of photosynthesis. The surrounding air doesn’t have a hundred percent humidity, making it drying for the leaves and resulting in substantial evaluation. This transpired water needs to be replaced by the transport of water from the roots to the leaves through the xylem. This provides the engine for pulling water up from the roots, bringing minerals along with it. The process also cools the leaves, similar to the way that the evaporation of water from human skin will cool the body.

There are several factors that impact the amount of transpiration that goes on, including the following:

• The amount of light—there is greater transpiration during light hours. This is because light will warm the leaf and will stimulate the opening of the stomata.

• Temperature—water will evaporate more rapidly at high temperatures so transpiration will happen at a higher rate.

• Humidity—transfusion happens during drier periods of time versus when the environment is more humid.

• Wind—increased wind will increase transpiration.

• Soil—during times of dry soil, the turgor of the plant decreases and this results in closing of the stomata and decreased transpiration.

In order to participate in photosynthesis, green plants need carbon dioxide and a way to get rid of the waste product, which is oxygen. On the other hand, plants need to carry on cellular respiration, which requires oxygen and gives off carbon dioxide. Both of these must take place in a plant. Unlike animals, there is no gas transport system in plants. The roots, leaves, and stems of the plant respire at lower rates than is seen in animals. Because plants have most of their cells close to the surface, gases can diffuse across cells. Remember, the interior of most cells is dead and structural; the outer surface can participate in gas exchange through diffusion.

The leaves have stomata that engage in the transpiration process and in gas exchange. Stomata will open when light strikes on them, which changes the turgor of guard cells that line the stomata walls. Increased turgor in the guard cells will cause the stomata to open, while decreased turgor will close the stomata. Light is absorbed by phototropin in the guard cell, causing a pump to turn on in the cell membrane, increasing the potassium concentration in the guard cell. This raises the osmotic pressure in the cells, opening the stomata.

Open stomata are crucial for gas exchange in photosynthesis, which is why they open during the light hours. They also increase water loss through transpiration. There is a hormone called ABA or abscisic acid that triggers closing of stomata when the soil cannot keep up with transpiration. ABA will bind to guard cells, resulting in a rise in guard cell cytosol pH. This causes the loss of potassium in the guard cells, closing the stomata. ABA will also close the stomata when bacteria are present so as to avoid bacterial invasion.