Transport in Plants Test - 40

 Xylem vessel system is mainly used for translocation of water in an angiosperm plant. The vessel elements are mainly traced in xylem which is the water conducting tissue of plants. Vessel elements are typically found in flowering plants (angiosperms) but absent from most gymnosperms such as conifers. Vessel elements are the main feature distinguishing the "hardwood" of angiosperms from the "soft wood" of conifers.

• Phloem is the vascular tissue responsible for the transport of sugars from source tissues (e.g., photosynthetic leaf cells) to sink tissues (e.g., non-photosynthetic root cells or developing flowers). 
• Other molecules such as proteins and mRNAs are also transported throughout the plant via phloem. Phloem is composed of several cell types including sclerenchyma, parenchyma, sieve elements and companion cells. 
• The sieve element and companion cell are found closely associated with each other in what is referred to as the sieve element/companion cell complex. One or more companion cells may be associated with a single sieve element. 

When a living cell is placed in a sufficiently concentrated solution of a suitable agent (e.g. cane sugar), plasmolysis occurs, and the stage in which the plasm just begins to recede from the completely relaxed cell wall is termed the condition of incipient plasmolysis. The return of the protoplast of plant cells to its initial state, characterized by normal turgor, from a state of plasmolysis. Deplasmolysis occurs when plasmolyzed cells are transferred to water or hypotonic solutions. 

Answer is option A i.e. "A special type of diffusion called imbibition"
Imbibition is the physical adsorption of water or moisture. A common example is swelling of wooden furniture, doors, windows and frames of doors. The adsorption of water by the solid particles of a substance without forming a solution is called imbibition. Jamming of wooden frames during rains is caused by swelling of wood due to imbibition.

The phloem is composed of sieve tubes, companion cells, phloem parenchyma and fibres. The sieve tubes are the tube-like cells without the nucleus. These cells have lignified cell wall, broad lumen and perforated walls for the conduction of food. The companion cells are adjacent cells of sieve tube which control the function of sieve cells. 

Water potential is the potential energy of water in a system, a solution or in a living cell, compared to pure water at atmospheric pressure and same temperature. It is sum of solute potential, pressure potential and gravity potential. The amount by which water potential is reduced due to presence of solute is termed as solute potential. It is due to clustering of water molecules around the solute that decrease the water potential. This makes options A and B incorrect as solute potential for water in solution has negative value. Pressure potential is attribute of physical pressure. The pressure of cell wall contributes positive pressure potential which is called as turgor pressure. Water in solution does not experience pressure potential provided no other external pressure is applied on it. Effect of gravity on water potential is termed as gravity potential. It is considered negligible for water present at the height of less than 5 meters. Values of three component of water potential are zero for pure water which makes its water potential zero. Since, addition of solute reduces water potential, its value for water in a solution or living cell is always less than zero, i.e., negative. This makes option C incorrect and option D correct. 

The sugar is synthesized as glucose and stored as starch in the plants. Flowering plants translocate the carbohydrate in the form of sucrose because sucrose is more energy efficient in storage and transferring. It is less reactive than glucose and water soluble which helps in the transportation through the sieve tubes of phloem.