The three characteristics components of plant cell are:
- Cell wall.
Plastids are a family of cell organelles that are closely related to each other and are double membranous structures. Plastids develop from precursor preplastids which distributed throughout the cell. Depending upon the function, plastids developed in to its various types. Three types of plastids are main: chloroplast, chromoplast and leucoplast. Chloroplast is the site of photosynthesis in plant. It contains a green color pigment chlorophyll which is essential for photosynthesis.
A Uni-cellular algae may have only one to two chloroplast, where as a leaf of higher plant may have 20-100 chloroplast. Plastids containing carotenoid pigments impart yellow-orange-red color of petals and fruits. In many species, are called as chromoplasts. While leucoplast are not pigmented hence are colorless, found in roots and tubers where they store starch. All plastids contain DNA and produce new plastids. In darkness plastids develop in to etioplast and contain a yellow precursor of chlorophyll called proto chlorophyll. It exposed to sunlight, the etioplast develop into chloroplast which are most common type.
The plant cell have a fixed shape and size due to the presence of a cell wall, a specialized forms of extra cellular matrix, which is closely associated with the external surface of plant cell membrane, cell wall is sufficiently thick, strong and rigid. The composition and appearance of cell wall differs according to cell type and the function it performs. In higher plants, the cell wall fibers are made up of the polysaccharide cellulose and are embedded in a highly cross linked matrix of polysaccharide such as pectin, lignin and hemicelluloses, such as layer of pectin present in between the two cells is known as the middle lamella.
The cell wall of young growing cells is thin and semi rigid and comprises the primary cell wall. The fully grown cells may either retain the primary cell wall structure with some more thickening, is superimposed on the inner surface of primary wall called as secondary cell wall. In some plants there is another innermost layer of the cell wall, which is relatively thin and lacks cellulose micro fibrils and is called tertiary wall.
In young dividing cells there are many small vesicles which fuse to form one or more large vesicle called vacuoles. The vacuoles are separated from cytoplasm by a single vacuolar membrane called the tonoplast. The vacuoles are similar in structure and function like plasma membranes, and they are filled with all sap which is acidic in nature and play a role in maintaining pressure with in the cell. Plant cell uses their vacuoles for transporting and storing nutrients. The vacuoles regulate the flow of water and solute of the cell such as osmotic regulation, and help in digestion and recycling of certain materials with in the cell, and help in breakdown of organelles such as mitochondria and plastids.
Some vacuoles participate in defense mechanisms: the substance of the vacuoles differ in quality as well as quantity from those of the cytoplasm. The plasma membrane and tonoplast have different permeability properties, the permeability of these two membranes is regulated by turgar pressure.