Osmosis in Animals

    If a cell uses up water in its chemical reactions, the cytoplasm becomes more concentrated and more water will immediately be drawn in by means of osmosis from outside the cell membrane. Similarly if the cytoplasm becomes too dilute because water is produced during the chemical reactions taking place, water will leave the cell by osmosis restoring the water balance.

    However osmosis can also cause some very serious problems in animal cells. If the solution outside the cell is more dilute than the cells contents, then water will move into the cell by osmosis, causing the cell to swell and in some serious cases even burst.

    On the other hand, if the solution outside the cell is more concentrated than the cell contents, then water will move out of the cell by osmosis. The cytoplasm will become too concentrated and the cell will shrivle up. Once you understand homeostasis and maintaining constant internal conditions becomes very clear.

    Osmosis in Plants

    Plants rely on well-regulated osmosis to support their stems and leaves. Water moves into plant cells by osmosis, makeing the cytoplasm swell and press against the plant cells walls. The pressure builds up until no more water can physically enter the cell, this makes the cell hard and rigid.

    This swollen state keeps the leaves of and stems of the plant rigid and firm. So for plants there isn't too much of a problem when the water supply is abundant because the plant will also only draw the water from the ground that it needs. The problem comes when there isn't enough water for the plant.

    When the water supply is very sparse water is drawn out of the the plant cells to support other processes in the plant for which water is vital. As this happens the cells of the stems ond leaves become flaccid and the plant begins to droop and wilt. If the water supply does not return then the plant will wilt and die.

    動物體內(nèi)的滲透作用

    如果細胞在化學反應(yīng)中用完了水份，細胞質(zhì)就會變得更濃，更多的水份將從細胞膜外通過滲透作用馬上進入細胞。類似地，如果細胞質(zhì)因為化學反應(yīng)中產(chǎn)生水份而變得太稀釋，水份會通過滲透作用離開細胞，保持水份平衡。

    但是，滲透作用也會引起一些動物體內(nèi)的嚴重問題。如果細胞外的溶液比細胞內(nèi)稀釋，那么水份會通過滲透作用進入細胞，導致細胞膨脹，嚴重的時候甚至爆炸。

    另一方面，如果細胞外的溶液比細胞內(nèi)濃度高，那么水份就會通過滲透作用流出細胞。細胞質(zhì)會變得太濃，細胞也會萎縮。一旦你理解了內(nèi)環(huán)境，保持持續(xù)的內(nèi)部環(huán)境就很清楚了。

    植物體內(nèi)的滲透作用

    植物依賴有條理的滲透作用供養(yǎng)枝干和樹葉。水份通過滲透作用進入細胞，細胞質(zhì)膨脹并擠壓細胞壁。直到?jīng)]有更多水份可以完全地進入細胞的時候，壓強就建立起來，細胞變得堅硬。

    這種膨脹的狀態(tài)讓植物的樹葉和樹干堅硬牢固。所以對植物來說，當水份供給充足的時候，沒有太多的問題，因為植物也只會從地下吸取所需水份。當沒有植物所需的水份的時候，問題才會出現(xiàn)。

    當水份供給非常稀少的時候，水份會被從植物細胞中抽出，用于其他特別需要水份的過程。這種情況發(fā)生之后，枝干和樹葉的細胞變得軟弱，植物開始下垂枯萎。如果水份供給還沒有回來，那么植物會枯萎死亡。