Figure Q.S A portion of a leaf of the water weed Elodea. A. Normal cells. B. Plasmolyzed cells.
example, that a pressure of 42.2 kilograms per square centimeter (600 pounds per square inch) is needed to break the seed coat of a fresh walnut from within and that water being imbibed by a cocklebur seed develops a force of up to 1,000 times that of normal atmospheric pressure. Yet when water and oxygen reach walnut and cocklebur seeds, they germinate readily, as do seeds that fall into the crevices of rocks or have boulders roll over on them.
Figure Q.6 Black-eyed pea seeds before and after imbibition of water.
The huge stone blocks used in the construction of the pyramids of Egypt are believed to have been quarried by hammering rounded wooden stakes into holes made in the face of the stone and then soaking the stakes with water. As the stakes swelled, the force created by imbibition split the rocks.
Return for a moment to our two rooms with the tennis balls. Suppose that, besides the 100 tennis balls, we drop in 50 slightly underinflated basketballs; these basketballs are also extraordinary in having perpetual-motion motors that propel them in any direction at 12 MPH. They should also become randomly distributed throughout the room shortly after they are introduced. Assume, however, that the hole in the wall (which is large enough for the passage of a tennis ball) is not quite large enough to allow a basketball to pass through freely. The basketballs will then remain in the first room. However, if we were to install a mechanical arm next to the hole in the second room, and if this arm could grab basketballs that come near the hole and squeeze them through in one
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Water in Plants 159
Figure Q.7 A live oak that grew from an acorn lodged in a small crack in the rock. When it rained, the acorn imbibed water, and the force of the swelling split the rock. A root is now slowly widening the split.
direction, basketballs would be transported into the second room through the expenditure of energy. The basketballs obviously would gradually accumulate in the second room in greater numbers.
Plants expend energy, too. Plant cells generally have a larger number of mineral molecules and ions than exist in the soil immediately next to the root hairs. If it were not for the barriers imposed by the differentially permeable membranes, these molecules and ions would move from a region of higher concentration in the cells to a region of lower concentration in the soil.
Figure Q.8 A mangrove tree. Mangroves flourish in tropical tidal zones where the salt content of the water is high enough to plasmolyze the cells of most plants. The mangroves still obtain water via osmosis, which takes place because the mangrove cells accumulate an unusually high concentration of organic solutes; some are also able to excrete excess salt.
Most molecules needed by cells are polar, and those of solutes may set up an electrical gradient across a differentially permeable membrane of a living cell. To pass through the membrane, molecules require special embedded transport proteins (see Fig. 3.11). The transport proteins are believed to occur in two forms, one facilitating the transport of specific ions to the outside of the cell and the other facilitating the transport of specific ions into the cell.
The plants absorb and retain these solutes against a diffusion (or electrical) gradient through the expenditure of energy. This process is called active transport. The precise mechanism of active transport is not fully understood. It apparently involves an enzyme and what has been referred to as a proton pump. The pump involves the plasma membrane of plant and fungal cells and sodium and potassium ions in animal cells. Both pumps are energized by special energy-storing ATP molecules (discussed in Chapter 10).
Mangroves, saltbush, and certain algae thrive in areas where the water or soil contains enough salt to kill most vegetation. Such plants accumulate large amounts of organic solutes, including the carbohydrate mannitol and the amino acid proline. The organic solutes facilitate osmosis, despite the otherwise adverse environment (Fig. 9.8). The leaves of some mangroves also have salt glands through which they excrete excess salt.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.