The plant cell has some peculiarities that allow it to be distinguished from the animal; these include highly specific structures, such as the cell wall, vacuoles and plastids.
The cell wall constitutes the outer covering of the cell and represents a sort of rigid envelope formed essentially of cellulose; its particular robustness protects and supports the plant cell, but the reduced permeability hinders the exchanges with the other cells. This problem is remedied by the tiny holes, called plasmodesmata, which cross the wall and the underlying membrane, connecting their cytosolves.
In general, the walls of plant cells have a wide variability in appearance and composition, thus responding to the functional needs of the tissue that houses them (cutin, for example, opposes excessive transpiration and is therefore abundant on the outer surface of the parts epigee of plants that live in particularly dry environments).
Very often, in the plant cell we find a large vacuole, that is a vesicle delimited by a membrane similar to the cellular one (called tonoplasto ), containing water and substances that the cytoplasm contains in excess (anthocyanins, flavonoids, alkaloids, tannins, essential oils, inulin, organic acids etc. in relation to the type of cell). The vacuoles therefore act as a repository for reserve and waste substances, and play an important role in preserving the osmotic balance between the cell and the external environment; small and numerous when young, they increase in size as they decrease in number as they age.
Plastids and Chloroplasts
In the cytoplasm of the plant cell, in addition to the characteristic organelles of the animal (mitochondria, nucleus, endoplasmic reticulum, ribosomes, Golgi apparatus, etc.), we find organelles of variable number and size, called plastids . Inside they contain particular pigments, that is, colored substances, such as carotenoids and chlorophylls; the former have a color ranging from yellow to red, while the emerald shades of chlorophyll give many vegetables the typical green color.
The presence of chlorophyll in some plastids, for this reason called chloroplasts, gives the plant cell the ability to operate chlorophyll photosynthesis, that is the autonomous synthesis of the organic substances it needs; for this purpose it uses the light energy of the Sun and inorganic compounds absorbed by the atmosphere (carbon dioxide) and by the soil (water and mineral salts). Overall, the series of biochemical steps that govern chlorophyll photosynthesis can be summarized in the classic reaction:
12H 2 O (water) + 6CO 2 (carbon dioxide) → C 6 H 12 O 6 (glucose) + 6O 2 (oxygen) + 6H 2 0 (water)
If the mitochondria are comparable to "power plants" to which to entrust the demolition of nutrients, the chloroplasts of the plant cell are comparable to "factories" appointed to build the same substances. Mitochondria and chloroplasts represent the only cellular structures endowed with their own DNA, capable of autonomous replication and of being transmitted from one generation to another through the female gametes.
Chloroplasts are delimited by a double membrane, whose innermost portion folds into an elaborate system of flattened and interconnected membranes, called thylacoids, immersed in an amorphous substance, the stroma, where the enzymes of the Calvin cycle are found (dark phase). of photosynthesis).
In addition to chloroplasts, in the plant cell we also find plastids rich in yellow-red pigments (called chromoplasts ) and others containing reserve substances ( leucoplasts, specifically amyloplasts if they are members of starch accumulation).