Plant cells differ structurally from the cells of most other organisms in a few key ways. Specifically, they are usually larger than animal cells and are surrounded by a rigid cell wall made from cellulose. They also often have a large central vacuole that takes up most of the cell, and if they carry out photosynthesis, the cells will have chloroplasts. This does not mean that all such cells are the same, and in fact, there are a number of different types of cells found in most plants.
Plants basically have three types of tissues, which are made up of different types of cells. Surface tissue forms the protective outer layer covering the plant. Fundamental, or simple tissues, are usually only composed of one type of cell and are normally grouped based on the level of thickness of the cell wall. Vascular tissues are complex tissues that consist of more than one type of cell. There are only two types of vascular tissue: xylem and phloem.
The surface tissue, or epidermis, of a plant is often only one cell thick, although it can be much thicker if the plant lives in a very dry environment and protection from water loss is crucial. It is made up of epidermal cells, which often have a very large vacuole. The cell wall that faces the outside of the plant is often thicker than cell wall that faces into the plant.
Epidermal cells in the leaves may be specialized as guard cells. These cells control the opening and closing of small holes in the leaves, called stomata. In this way, they regulate the movement of gases into and out of the plant. The function of epidermal cells that line the roots is water absorption from the soil. To increase the surface area, many epidermal cells grow long hair, or filaments, from their surface.
There are several types of fundamental tissues, including parenchyma, collenchyma and sclerenchyma. Parenchyma is made up of parenchyma cells and occurs in the roots, leaves and stems of plants. These plant cells are relatively unspecialized and contain large vacuoles and a thin cell wall. Within the leaves and stems, most of the chloroplasts are found in parenchyma cells. They give the cells their green color and allow photosynthesis to take place.
Collenchyma cells are longer than parenchyma cells, and their cell walls are much thicker. Their function is to provide support in young plants and in the stems and leaves of non-woody older plants. Sclerenchyma cells also provide support to plants, and they are far more specialized than collenchymas cells. They have a thick secondary wall that is hardened to strengthen the plant, and these cells are usually dead at maturity.
Xylem and phloem are the two types of vascular tissue found in a plant. Xylem is made up of parenchyma cells and two specialized cells called tracheids and vessel elements. Both tracheids and vessel elements are dead, and their function is to provide support and water transport from the roots up to the rest of the plant.
Phloem tissue is alive and is made up of parenchyma and sclerenchyma cells. In addition, it contains specialized plant cells called sieve tube cells and companion cells. The function of phloem is the transport of material throughout the plant in all directions, and they contain no nucleus and very little cytoplasm. Companion cells are closely associated with sieve tube cells and are thought to carry out cellular functions for both types of cell.
