Plant: Cell Structure And Function
Plant Cell
Plant cells contain a variety of membrane-bound structures called organelles. These include a nucleus that carries genetic material; mitochondria that generate energy; ribosomes and rough endoplasmic reticulum that manufacture proteins; smooth endoplasmic reticulum that manufactures lipids used for making membranes and storing energy; and a thin lipid membrane that surrounds the cell. Plant cells also contain chloroplasts that capture energy from sunlight and a single fluid-filled vacuole that stores compounds and helps in plant growth. Plant cells are surrounded by a rigid cell wall that protects the cell and maintains its shape.
The tremendous variety of plant species is, in part, a reflection of the many distinct cell types that make up individual plants. Fundamental similarities exist among all these cell types, however, and these similarities indicate the common origin and the interrelationships of the different plant species. Each individual plant cell is at least partly self-sufficient, being isolated from its neighbors by a cell membrane, or plasma membrane, and a cell wall. The membrane and wall allow the individual cell to carry out its functions; at the same time, communication with surrounding cells is made possible through cytoplasmic connections called plasmodesmata.
Cell Wall
The most important feature distinguishing the cells of plants from those of animals is the cell wall. In plants this wall protects the cellular contents and limits cell size. It also has important structural and physiological roles in the life of the plant, being involved in transport, absorption, and secretion.
A plant's cell wall is composed of several chemicals, of which cellulose (made up of molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils, which form the structural framework of the wall. Other important constituents of many cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata penetrate both primary and secondary cell walls, providing pathways for transporting substances.
Protoplast
Within the cell wall are the living contents of the cell, called the protoplast. These contents are bounded by a cell membrane composed of a phospholipid bi-layer. The protoplast contains the cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the nucleus, which is the hereditary unit of the cell.
Vacuoles
Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water containing various dissolved sugars, salts, and other chemicals.
Plastids
Plastids are types of organelles, structures that carry out specialized functions in the cell. Three kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments; they are the site of photosynthesis, the process in which light energy from the sun is fixed as chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture carotenoids.
Mitochondria
Whereas plastids are involved in various ways in storing energy, another class of organelles, the mitochondria, are the sites of cellular respiration. This process involves the transfer of chemical energy from carbon-containing compounds to adenosine triphosphate, or ATP, the chief energy source for cells. The transfer takes place in three stages: glycolysis (in which acids are produced from carbohydrates); the Krebs cycle, also called the citric acid cycle; and electron transfer. Like plastids, mitochondria are bounded by two membranes, of which the inner one is extensively folded; the folds serve as the surfaces on which the respiratory reactions take place.
Ribosomes, Golgi Apparatus, and Endoplasmic ReticulumTwo other important cellular contents are the ribosomes, the sites at which amino acids are linked together to form proteins, and the Golgi apparatus, which plays a role in the secretion of materials from cells. In addition, a complex membrane system called the endoplasmic reticulum runs through much of the cytoplasm and appears to function as a communication system; various kinds of cellular substances are channeled through it from place to place. Ribosomes are often connected to the endoplasmic reticulum, which is continuous with the double membrane surrounding the nucleus of the cell.
NucleusThe nucleus controls the ongoing functions of the cell by specifying which proteins are produced. It also stores and passes on genetic information to future generations of cells during cell division. See Cell.
