Asseration: plant cell have a definite shape and size. Reason: cell wall is an additional outer covering in a plant cell
Answers
Explanation:
The plant cell wall is an elaborate extracellular matrix that encloses each cell in a plant. It was the thick cell walls of cork, visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. The walls of neighboring plant cells, cemented together to form the intact plant (Figure 19-68), are generally thicker, stronger, and, most important of all, more rigid than the extracellular matrix produced by animal cells. In evolving relatively rigid walls, which can be up to many micrometers thick, early plant cells forfeited the ability to crawl about and adopted a sedentary life-style that has persisted in all present-day plants.
Figure 19-68. Plant cell walls.
Figure 19-68
Plant cell walls. (A) Electron micrograph of the root tip of a rush, showing the organized pattern of cells that results from an ordered sequence of cell divisions in cells with relatively rigid cell walls. In this growing tissue, the cell walls are still (more...)
Go to:
The Composition of the Cell Wall Depends on the Cell Type
All cell walls in plants have their origin in dividing cells, as the cell plate forms during cytokinesis to create a new partition wall between the daughter cells (discussed in Chapter 18). The new cells are usually produced in special regions called meristems (discussed in Chapter 21), and they are generally small in comparison with their final size. To accommodate subsequent cell growth, their walls, called primary cell walls, are thin and extensible, although tough. Once growth stops, the wall no longer needs to be extensible: sometimes the primary wall is retained without major modification, but, more commonly, a rigid, secondary cell wall is produced by depositing new layers inside the old ones. These may either have a composition similar to that of the primary wall or be markedly different. The most common additional polymer in secondary walls is lignin, a complex network of phenolic compounds found in the walls of the xylem vessels and fiber cells of woody tissues.The plant cell wall thus has a “skeletal” role in supporting the structure of the plant as a whole, a protective role as an enclosure for each cell individually, and a transport role, helping to form channels for the movement of fluid in the plant. When plant cells become specialized, they generally adopt a specific shape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified (Figure 19-69; see also Panel 21-3).
Figure 19-69. Specialized cell types with appropriately modified cell walls.
Figure 19-69
Specialized cell types with appropriately modified cell walls. (A) A trichome, or hair, on the upper surface of an Arabidopsis leaf. This spiky, protective single cell is shaped by the local deposition of a tough, cellulose-rich wall. (B) Surface view (more...)
The plant cell wall is an elaborate extracellular matrix that encloses each cell in a plant. It was the thick cell walls of cork, visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. The walls of neighboring plant cells, cemented together to form the intact plant (Figure 19-68), are generally thicker, stronger, and, most important of all, more rigid than the extracellular matrix produced by animal cells. In evolving relatively rigid walls, which can be up to many micrometers thick, early plant cells forfeited the ability to crawl about and adopted a sedentary life-style that has persisted in all present-day plants.
Figure 19-68. Plant cell walls.
Figure 19-68
Plant cell walls. (A) Electron micrograph of the root tip of a rush, showing the organized pattern of cells that results from an ordered sequence of cell divisions in cells with relatively rigid cell walls. In this growing tissue, the cell walls are still (more...)
Go to:
The Composition of the Cell Wall Depends on the Cell Type
All cell walls in plants have their origin in dividing cells, as the cell plate forms during cytokinesis to create a new partition wall between the daughter cells (discussed in Chapter 18). The new cells are usually produced in special regions called meristems (discussed in Chapter 21), and they are generally small in comparison with their final size. To accommodate subsequent cell growth, their walls, called primary cell walls, are thin and extensible, although tough. Once growth stops, the wall no longer needs to be extensible: sometimes the primary wall is retained without major modification, but, more commonly, a rigid, secondary cell wall is produced by depositing new layers inside the old ones. These may either have a composition similar to that of the primary wall or be markedly different. The most common additional polymer in secondary walls is lignin, a complex network of phenolic compounds found in the walls of the xylem vessels and fiber cells of woody tissues.The plant cell wall thus has a “skeletal” role in supporting the structure of the plant as a whole, a protective role as an enclosure for each cell individually, and a transport role, helping to form channels for the movement of fluid in the plant. When plant cells become specialized, they generally adopt a specific shape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognises and classified (Figure 19-69; see also Panel 21-3).
Figure 19-69. Specialized cell types with appropriately modified cell walls.
Figure 19-69
Specialized cell types with appropriately modified cell walls. (A) A trichome, or hair, on the upper surface of an Arabidopsis leaf. This spiky, protective single cell is shaped by the local deposition of a tough, cellulose-rich wall. (B) Surface view (more...)