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Cells Contain Various Biological Molecules Such as Sugars and Salts

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* Since cells contain various Biological Molecules, such as Sugars and Salts, they have a Water Potential lower then 0 kPa. Water may move in or out of a cell depending of the Water Potential Gradient between the inside of the cell and its environment.

* When water diffuses into a plant cell, when it is placed in a solution of higher Water Potential than inside it, the cell contents will expand. However, since plant cells are surrounded by a strong cell wall, they will not burst. The cell contents will push against the cell wall, and the cell will become Turgid.

* If a plant cell is placed in a solution of lower Water Potential, water will diffuse out. This causes the Cytoplasm to shrink and become Flaccid. If enough water leaves, the Cytoplasm will pull away from the cell wall. The cell will become Plasmolysed.

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The central vacuole in plant cells (see Figure 1) is enclosed by a membrane termed the tonoplast, an important and highly integrated component of the plant internal membrane network (endomembrane) system. This large vacuole slowly develops as the cell matures by fusion of smaller vacuoles derived from the endoplasmic reticulum and Golgi apparatus. Because the central vacuole is highly selective in transporting materials through its membrane, the chemical palette of the vacuole solution (termed the cell sap) differs markedly from that of the surrounding cytoplasm. For instance, some vacuoles contain pigments that give certain flowers their characteristic colors. The central vacuole also contains plant wastes that taste bitter to insects and animals, while developing seed cells use the central vacuole as a repository for protein storage.

Among its roles in plant cell function, the central vacuole stores salts, minerals, nutrients, proteins, pigments, helps in plant growth, and plays an important structural role for the plant. Under optimal conditions, the vacuoles are filled with water to the point that they exert a significant pressure against the cell wall. This helps maintain the structural integrity of the plant, along with the support from the cell wall, and enables the plant cell to grow much larger without having to synthesize new cytoplasm. In most cases, the plant cytoplasm is confined to a thin layer positioned between the plasma membrane and the tonoplast, yielding a large ratio of membrane surface to cytoplasm.

The structural importance of the plant vacuole is related to its ability to control turgor pressure. Turgor pressure dictates the rigidity of the cell and is associated with the difference between the osmotic pressure inside and outside of the cell. Osmotic pressure is the pressure required to prevent fluid diffusing through a semipermeable membrane separating two solutions containing different concentrations of solute molecules.

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