Write plant cells in 500 words
Answers
Explanation:
Like all organisms, plants have cells. Plant cells have a nucleus with chromosomes and DNA, and they have mitochondria. Those are common to all eukaryotic cells. But in some ways they are different from animal cells and the cells of other eukaryotes.
1. They have cell walls composed of cellulose and some other compounds. Pectin, and sometimes lignin, is secreted on the outside of the cell membrane. This is different from the cell walls of fungi (which are made of chitin), and of bacteria (which are made of peptidoglycan). The cell wall gives the cell a definite shape.
2. There are special cell-to-cell communication pathways known as plasmodesmata. These are pores in the cell wall through which the cell content of adjacent cells (including endoplasmic reticulum) are continuous.
3. They have plastids. The most notable are the chloroplasts, which contain chlorophyll. This green-colored pigment absorbs sunlight, and allows the plant to make its own food by photosynthesis. Other types of plastids are the amyloplasts, which store starch, elaioplasts for fat storage, and chromoplasts for making and storing pigments. Like mitochondria, plastids have their own genomes of about 100–120 unique genes.[3] It is thought they started as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.
4. They have a large central vacuole, a water-filled volume enclosed by a membrane. The vacuole keeps the cell's turgor (stiffness), controls movement of molecules between the cytosol and sap, stores useful material and digests waste proteins and organelles.
5. Cell Division by construction of a 'cell plate' late in cytokinesis is characteristic of land plants and a few groups of algae.
6. The sperm of bryophytes and pteridophytes, cycads and Ginkgo have flagella similar to those in animals However, higher plants (including gymnosperms and flowering plants) lack the flagella and centrioles that are present in animal cells.
Here is an essay on the plant cell.
If a thin slice is taken from any part of a plant and observed under a microscope, it is found to be composed of many chambers or compartments resembling the chambers of a honeycomb. Each of these chambers is called a cell. A plant cell has usually an outer firm non-living boundary called the cell wall and a cavity which contains granular jelly-like protoplasm.
The organised mass of protoplasm of each cell is called protoplast. Protoplasm, as we knew, is the essence of life. It has been very aptly described by the famous nineteenth century biologist Thomas Henry Huxley as the ‘physical basis of life’.
Due to the presence of the distinct non-living wall the plant cells appear to be separate from one another. But this separation is by no means complete. Protoplasm is a continuous living system. Through the small pores on the cell wall fine fibres of protoplasm, called plasmodesmata, pass to the contiguous cells, establishing the organic continuity of protoplasm.
A cell is defined as the unit of structure and function. It is a unit of structure more or less in the same sense as a piece of brick is a unit in a building. Plant-body is built up structurally of cells-. It is called a unit of function, because a cell contains a unit mass of protoplasm which is solely responsible for all the vital activities.
In 1665 an Englishman Robert Hooke observed a thin slice of bottle cork under a microscope improved by himself. He noted many chambers and suggested the term ‘cell’, on the basis of their resemblance with the so-called cells or chambers of a bee-hive.
As bottle cork is a dead thing Hooke certainly noticed only the cell wall and not protoplasm. Italian Professor Marcello Malpighi (1628-94) and English physician Nehemiah Grew (1641-72) also carried on studies on cellular structures.
A theory, known as ‘cell theory’, was postulated by Matthias Jacob Schleiden (1804-81). a German botanist, and Theodor Schwann (1810-82), a German zoologist (Fig. 115), in 1838-39, who claimed that all living things, plants and animals, are cellular in nature, i.e. they are essentially made up of cells.
Matthias Jacob Schileiden and Theodore Schwann
The announcement of cell theory was an incident of paramount importance in the history of our knowledge about cell. Early workers concentrated their attention practically on the distinct cell wall, possibly because the transparent protoplasm escaped their notice.
A Frenchman Felix Dujardin appreciated the importance of cell contents to which he applied the term ‘sarcode’. Hugo von Mohl, a German botanist, was, however, the first man to recognise the importance of the living contents in the middle of the nineteenth century, and he gave it the name protoplasm.
Thomas Henry Huxley in 1868 described it as ‘the physical basis of life’. Hanstein in 1880 proposed the term protoplast for a cell.
Though even today we use the term cell as proposed by Hooke, present-day conception of a cell is that it is essentially a mass of protoplasm (protoplast) with or without the wall. In contradistinction to animal cells, the plant cells are usually walled.
Even in plant kingdom ‘naked’ cells, i.e. cells without walls are not altogether absent. Reproductive cells are usually naked. A cell is called dead, when it has lost its protoplasm like the cells of bottle- cork noted by Hooke.
Cells are so numerous and diverse that it is rather difficult to give a precise account of their shape and size. All shapes between round or circular on one hand and ‘elongate needle-like on the other, are possible.
The majority of them, however, are more or less isodiametric—round, elliptical, square, pentagonal, hexagonal, etc. (Figs. 116, 117). The average diameter of these cells ranges between 0.01 mm. to 0.1 mm., though there are cells much smaller and much larger in size; and average length of elongate cells ranges between 8 to 10 mm.
Plant Cell in Perspective
Few Cells from the Epidermis of Onion Scale
Some lower organisms are made up of single cells. They are called unicellular. But the vast majority of the plants have bodies built of innumerable cells. They are multicellular organisms.