Organisms that live in fresh water are almost always hypertonic to their environment. In what way is this a serious problem? How could some organisms cope with this problem?

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When an organism’s cells are hypertonic relative to its freshwater environment, its cells have a higher concentration of solutes (like salts and other molecules) compared to the surrounding water. This causes water to continuously move into the cells by osmosis. Over time, this influx of water can lead to swelling and potentially to cellular damage or even bursting (lysis). In multicellular organisms, the osmotic imbalance can disrupt overall cell function and homeostasis, making it difficult to maintain proper internal conditions.

Organisms have evolved several strategies to cope with this problem:

1. Contractile Vacuoles: Many single-celled freshwater organisms (such as protozoa) possess contractile vacuoles. These specialized organelles periodically collect excess water from the cell and actively expel it to the external environment, keeping the cell’s water balance in check.

2. Excretion of Dilute Urine: Freshwater fish, for example, have developed efficient kidney functions that produce large volumes of very dilute urine. This adaptation allows them to eliminate excess water while retaining essential ions.

3. Selective Membrane Permeability and Ion Transport: Cells may also use active transport mechanisms to adjust the concentration of solutes inside the cell. By actively pumping ions in or out, cells can reduce the osmotic gradient and minimize water influx.

Overall, these mechanisms help the organisms control their internal water and solute balance, ensuring that cells neither swell excessively nor burst, and maintaining the stable internal environment necessary for survival.
When organisms live in freshwater, their cells can become too full of water, which can damage them. To handle this, some single-celled organisms have special structures called contractile vacuoles that pump out excess water. Fish have kidneys that make very watery urine to get rid of extra water while keeping important salts inside. Cells also use special transport systems to control the balance of water and salts, preventing their cells from swelling too much.

Organisms that live in fresh water are almost always hypertonic to their environment. In what way is this a serious problem? How could some organisms cope with this problem?

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