Cells, Specialized Types

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Cells, Specialized Types

The specialized cell types found in plant stems, leaves, roots, flowers, and fruits are organized into three tissue systems: the ground tissue system, the dermal tissue system, and the vascular tissue system. Each tissue system carries out a different general function: the vascular tissue system transports water and solutes over long distances within the plant, the dermal tissue system provides protection and gas exchange at the surface of the plant, and the ground tissue system provides cells that carry out photosynthesis, storage, and support. Each tissue system has many specialized cell types, and a few cell types are found in more than one tissue system.

The different types of specialized plant cells are distinguished by cell shape and by properties of the cell wall and protoplast . The plant cell wall is one of the most important distinguishing features of the different kinds of specialized cells. All plant cells have a thin and flexible primary wall, made of the polymer cellulose and other carbohydrates. Other cell types have, in addition to a primary wall, a thick, rigid secondary wall, made of cellulose impregnated with lignin.

Cells of the Ground Tissue System

Parenchyma cells are the generalized, multipurpose cells in the plant. Most parenchyma cells have thin primary walls and range from spherical to barrel-like in shape. Parenchyma cells often store food reserves, as in the starch-storing parenchyma cells of a potato tuber or the sugar-storing parenchyma cells of an apple. The parenchyma cells of green leaves are specialized for photosynthesis; these cells contain numerous large chloroplasts and are called chlorenchyma cells. Other parenchyma cells called transfer cells are specialized for the transport of solutes across the cell membrane. These cells have a greatly enlarged surface area due to the highly convoluted inner surface of the cell wall. Transfer cells are found in nectaries where the extensive cell membrane houses transport channels that secrete sugars and other nectar components to the exterior of the cell.

Collenchyma cells function in the support of growing tissues. Individual collenchyma cells are long and narrow and have an unevenly thickened primary wall. Collenchyma cells form a long cable of thousands of cells that together can provide mechanical support while a stem or leaf elongates. Collenchyma is common in the veins of leaves and forms the strings of celery stalks.

Sclerenchyma cells function in the support of tissues that are no longer expanding. Individual sclerenchyma cells are long and narrow with a thick, hard, rigid secondary wall. Unlike parenchyma and collenchyma cells that are living cells, sclerenchyma cells are dead at maturity. The cell's function in mechanical support is carried out by the strong cell walls; a living protoplast is unnecessary. Sclerenchyma fibers make up the bulk of woody tissues and also form long strands in the leaves and stems of many plants. Natural fiber ropes such as those made from hemp or sisal plants are made up of thousands of sclerenchyma fibers. Some sclerenchyma cells called sclereids are much shorter than fibers; these form the hard layers of walnut shells and peach pits, and small clusters of sclereids form the grit in pear fruits.

Cells of the Dermal System

Epidermal cells form the surface layer of the plant, the epidermis. Typical epidermal cells are flat and form a continuous sheet with no spaces between the cells. Each epidermal cell secretes a layer of the hydrophobic polymer cutin on the surface, which greatly reduces the amount of water lost by evaporation. Most epidermal cells also secrete waxes on the surface of the cutin, which further reduces transpiration , as well as wettability of the leaf surface. When you polish an apple, you are melting these surface waxes through friction. Epidermal cells of green leaves lack pigmented chloroplasts, allowing light to penetrate to the photosynthetic tissues within. Epidermal cells of petals often contain blue or red anthocynanin pigments within the vacuole or orange carotenoid pigments within the plastids, giving rise to the bright colors of many flowers.

Guard cells are specialized epidermal cells that function to open small pores in the plant surface, allowing the carbon dioxide needed for photosynthesis to diffuse from the external atmosphere into the chlorenchyma tissue. Guard cells are usually crescent-shaped, contain green chloroplasts, and are able to rapidly change their shape in response to changes in water status. As guard cells take up water, the pore opens; as they lose water the pore closes. The two guard cells and pore are termed a stomate.

Trichomes are hairlike cells that project from the surface of the plant. They function to reduce water loss by evaporation by trapping water vapor near the plant surface. In some plants, trichomes are glandular and secrete sticky or toxic substances that repel insect herbivores.

Cells of the Vascular Tissue System

The vascular tissue system is composed of both xylem and phloem tissue. Xylem functions to carry water and mineral nutrients absorbed at the root tips throughout the plants roots, stems, and leaves. Vessel elements are the major cell type involved in the transport of water and these solutes. Vessel elements are elongate cells with thick secondary walls. Xylem sap travels under a negative pressure or vacuum, and the strong rigid walls keep the vessel elements from collapsing, much like the steel coil in a vacuum-cleaner hose. Like sclerenchyma fibers, vessel elements are dead at maturity, so that each cell forms an empty tube. Before vessel elements die, however, the cell's protoplast releases enzymes that degrade the cell wall at both ends of the cell, forming a perforation. Individual vessel elements are joined end to end at the perforations, thus forming a long, continuous pipe, the vessel. Other xylem cells called tracheids also function in transport of water and solutes, but are less efficient because they lack perforations and do not form long vessels. Xylem tissue also contains sclerenchyma cells that function in support and parenchyma cells that function in storage or as transfer cells. When transfer cells are found in the xylem, they function to recover valuable solutes such as nitrogen compounds from the sap traveling in the xylem vessels.

Phloem tissue functions to transport the products of photosynthesis from green tissues to parts of the plant where energy-rich carbohydrates are required for storage or growth (a process called translocation). Sieve elements are the conducting cells of the phloem. Sieve elements are elongated cells, with a thick primary wall. Phloem sap travels under a positive pressure, and the thick, elastic cell walls allow the cells to adjust to the fluctuations in pressure over a day-night cycle. Sieve elements have large, conspicuous pores on the end walls, forming a sieve plate. The sieve plate pores allow the phloem sap to travel from cell to cell along the file of cells called a sieve tube. Each sieve element is living, with an intact plasma membrane; the differential permeability of the membrane prevents solutes from leaking out of the sieve tube. Sieve-tube elements lack a nucleus and some other components of the cytoplasm; this feature functions to keep the pores unplugged. Companion cells are small parenchyma cells associated with each sieve element. The nucleus of the companion cell must direct the metabolism of both the companion cell itself and of its sister sieve element.

see also Anatomy of Plants; cell Cycle; Cells; Fiber and Fiber Products; Tissues; Translocation; Trichomes; Vascular Tissues; Water Movement.

Nancy G. Dengler