Why do vacuoles in plant cell have an additional layer called as Tonoplast?
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Answer:The term means "empty space". But in the cell they are many membrane bound sacs with little or no inner structure. Plant cells have very large distinct vacuoles. In fact this organelles often dominates the inside if the plant cell crowding all other organelles toward the cell wall. The membrane surrounding the plant cell vacuole is called the tonoplast. This a very active, dynamic membrane.
Plant cells use their vacuoles for transport and storing nutrients, metabolites, and waste products. In a sense, the vacuole can be regarded as equivalent to the extra cellular space of animals. The simple space-filling function of the vacuole is of great importance to plants, which capture energy from the sun rather than move to capture food. The mechanical stability provided by the combination of a cell wall and turgor pressure allows plant cells to grow to a relatively large size, so they generally occupy a much larger volume than animal cells. However, producing large cells by filling them with cytoplasm would be costly both on terms of maintenance and initial synthesis. The majority if plant cells accumulate water in their vacuoles as they become larger through tugor driven cell-wall expansion (usually in many small vacuoles that then coalesce to form a large vacuole.
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