Cells and Tissues
The cell is the lowest level of structure capable of performing all the activities of life. All organisms are composed of cells, either singularly as a great variety of unicellular organisms, or they occur as the subunits of organs and tissues.
The plant is composed of cells differing in structure and function. All plant cells are surrounded by a rigid primary cell wall, and sometimes a secondary cell wall, that is produced by the cell.
Inside the wall is the cell membrane which selectively regulates the movement of materials into and out of the cells Internal membranes partition the protoplast into compartments/organelles and also participate directly in much of the cell's metabolism. The nucleus, chloroplast, ribosomes, and vacuole are examples of cell organelles.
Membranes of various kinds are fundamental to the organization of the cell. In general, the membrane consists of a double layer of phospholipids and other lipids with proteins embedded or attached to its surface. Molecules flow into and out of the cells by three methods:
- simple diffusion,
- facilitated diffusion via passive transport proteins,
- and active transport.
Osmosis occurs in response to a water potential gradient produced by differences in solute concentration on each side of a membrane. Cell survival depends on balancing water uptake and loss.
Parenchyma - Mature parenchyma cells are the least specialised cells having primary walls that are relatively thin and flexible, usually no secondary thickening and the protoplast has a large central vacuole. They perform most of the metabolic functions of the plant, synthesising and storing various organic products. For example photosynthetic mesophyll cells in the leaf, starch storing cells in the stem/root, flesh of fruit.
Collenchyma - These have uneven thicker primary walls, are grouped in strands or cylinders and function as flexible support allowing growth.
Sclerenchyma - Function also for support, but with thick secondary walls strengthened with lignin. Rigid, they cannot elongate so are found where growth is finnished, and may be dead at maturity. They form 2 types, fibres (long, slender and tapered) usually found in bundles, ie Cannabis, and sclereids (short and irregular) found in nut shells, seed coats, pears, etc.
Vascular - These are covered in the sections Xylem and Phloem.
Divided into three types:
- Epidermal tissue - The dermal tissue or epidermis is generally a single layer of tightly packed cells that covers and protects all young parts of a plant. It also has specific functions such as cuticle production and root hairs.
- Ground tissue - makes up the bulk of a young plant, filling the space between the dermal and vascular tissue systems. Predominantly parenchyma, but collenchyma and sclerenchyma are also present.
- Vascular tissue - the continuum of xylem and phloem through the plant makes up the vascular tissue system, which functions in transport and support.