Question

Which is not true about vacuole?
A. It is bounded by a single membrane called tonoplast
B. It has a hypotonic cell sap and transports a number of ions and other materials.
C. In plant cells, vacuoles occupy up to 90 percent of the volume of the cells
D. Contractile vacuole of amoeba is important for excretion

Answer 1

Vacuoles are membrane bound organelles present in cells. The organelle contains water along with organic and inorganic salts. Vacuoles play an integral role in maintaining the morphological, physiological and biochemical structure of the cell it is present in.

Complete answer:
Vacuoles are membrane bound organelles performing a variety of functions in plant cells and animal cells alike. It has no definite size or shape and varies according to cellular requirements. Vacuoles are known to have formed by fusion of vesicles are in essence, the larger forms of the same.
Vacuoles are surrounded by a membrane called the tonoplast. It helps to maintain the shape of the cell. The tonoplast comprises a lipid bilayer and is filled with cell sap. It is semi permeable in nature – which helps the cell to maintain a constant turgor pressure. The membrane, being selectively permeable allows select molecules and ions to pass through it.
The plant cells contain a central vacuole. The vacuole occupies a major portion of the cell, around 30 to 90 percent of the cell volume. The huge size of the vacuole is due to storage of large amounts of water and food particles, along with waste products.
The contractile vacuoles of Amoeba are capable of contracting and relaxing. During each cycle of contraction and relaxation, the organism expels and accumulates water, respectively. The contractile vacuoles are known to perform exocytosis like normal cellular vacuoles. The vacuoles excrete substances toxic to cells through the process of exocytosis.
The cell sap of the vacuoles contain water and dissolved organic and inorganic salts. Protons from the cytosol are transported in the vacuole to balance the pH of the cell cytoplasm. This transport of protons increases the acidic nature of vacuoles. Therefore, the vacuoles are hypertonic and acidic in nature.
Hence, option (b) is correct.

Note: Vacuoles are membrane bound organelles present in cells. Vacuoles are well developed and larger in plant cells than in animal cells. Cell sap of the vacuoles are acidic in nature due to the presence of protons in it. These organelles are known to store food particles, minerals, ions and water. The vacuoles help the plant cells to maintain turgidity. Vacuoles are also known to secrete waste products by the process of exocytosis.