Most multicellular organisms rely on groups of cells working together to carry out life's functions. A tissue is an integrated, cooperative assembly of cells working together. For example, humans have muscle tissue, epithelial (skin) tissue, and nervous tissue, among others. Tissues do not have to be thin or flexible—bone tissue is often thick and rigid, yet it is composed of many integrated cells working together.
To maintain their integration and cooperation, the cells within tissues secrete an extracellular matrix, a molecular framework that supports them structurally and biochemically. The extracellular matrix is a collection of mostly water and protein that holds cells together and assists in cellular signaling. This structure makes wood and bone difficult to break and skin difficult to tear. The extracellular matrix also helps ensure that the cells within a tissue perform the functions of that tissue—for example, it helps nerve cells behave like nerve cells and not muscle or bone cells.
As a collection of functioning cells, tissues must be supplied with nutrients and have a mechanism for removing waste. Tissues of one type can interact with and support tissues of another type. For example, vascular tissue supplies muscle tissue with oxygenated blood, which delivers oxygen to individual muscle cells and removes carbon dioxide during cellular respiration.
Different organisms need different tissue types to carry out their characteristic functions. Animal cells are similar to plant cells, but there are key differences, such as the rigid cell wall of the plant. Animal and plant tissues are also similar in some ways and different in others, mostly because of some key differences between how animals and plants function. Animals generally need to be able to move quickly to react to their surroundings, while plants remain relatively motionless, potentially withstanding high winds or flowing water. Thus, the tissues of animals and plants vary both in their composition and in the manner in which they are formed.
Plants, for the most part, do not rely heavily on rapid movement to survive, as animals do. They tend to have fairly rigid structures that change slowly. Plant cells contain a cell wall, a rigid carbohydrate structure that provides overall support and protection for the cell. It is found in some, but not all, cells. Adjacent plant cells are in direct contact by their cell walls. Plant cells can control the rigidity of the extracellular matrix by secreting varying amounts of substances, giving rise to tough, woody stems or soft, pliable leaves and petals as required. Plants can then alter the shape of a tissue either by growing more cells in a particular location or by adjusting the water content within the vacuoles of the cells of a certain tissue. The water pressure within the cell is known as turgor pressure.When a plant cell newly forms, its cell wall is thin and flexible. This allows the cell to grow to the proper size. Once the cell has reached the proper size, it thickens and toughens the cell wall with cellulose microfibrils. A cellulose microfibril is a bundle of polysaccharide chains that contributes to plant cell wall structure. These fibers are actually synthesized outside of the cell membrane by enzymes embedded in the cell membrane itself. Cellulose microfibrils interweave with other polysaccharides and structural proteins, forming a dense, tough fiber network. This makes the cell wall resistant to bending, stretching, and breaking and also adds strength to the tissue. Because of this resistance, the plant cell must carefully align the network to allow for growth. The network of cellulose microfibrils orients in the direction of growth, allowing the plant cell to grow as needed without rupturing.
Growth Based on Orientation of Cellulose Microfibrils
Plants can also add to rigidity by retaining a network of dead cells held together by cellulose microfibrils. Large spaces between the cells are packed with a network of cellulose microfibrils, yet the cells are no longer living. The dead cells do not require access to vascular tissue, which provides nutrients and waste removal for living cells. Many of the dead cells are desiccated, making them hard and dry. The spaces within the dead cells are packed with lignin, an organic molecule that makes the cell very hard and difficult to deform. This adds to the rigidity of the tissue. This type of tissue, which is dense and tough, makes up a large portion of the trunks of trees, as well as other woody tissues in other plants.To move water and nutrients through their tissues, plants employ small channels that connect cells together. A plasmodesma (plural, plasmodesmata) is a small channel connecting the cytoplasm of adjacent plant cells through which molecules pass. Plasmodesmata are lined with cell membrane, making the cell membrane actually continuous between cells.
Unlike plants, animals tend to move within their environments, responding more quickly to changes and stimuli. Thus, their tissues must be pliable enough for them to react and move.
There is a major distinction at the cellular level between connective tissues and the muscle, nervous, and epithelial types of animal tissues. Connective tissues are distinctive by having a ubiquitous extracellular matrix that carries the mechanical load. Other types of tissues have comparatively little extracellular matrix, and the cells themselves carry much of the mechanical load and are especially adapted for the transmission of electrical signals.Collagen is the protein that makes up the bulk of the extracellular matrix in animal tissues. It is a protein that is unique to animals. A collagen fiber, a rope-like structure found in some tissues, is composed of a bundle of collagen fibrils, which are themselves composed of three single collagen chains twisted together. This protein structure is tremendously strong yet also highly flexible, similar to rope. In other tissues, the triple-helical chains of collagen peptides spread out in a matrix, similar to fiberglass. This sheetlike matrix provides a tough, flexible material that can deform without being damaged.
Fibronectin is an extracellular protein that attaches cell membrane proteins to collagen. This connection between animal cells and fibronectin is mediated by an integrin, a receptor protein embedded in the cell membrane. Integrin is attached to actin filaments inside the cell, allowing for linkage between the extracellular matrix and the cytoskeleton. By using integrin proteins to bind with fibronectin molecules on opposite sides of the cell simultaneously, cells are able to pull themselves along the extracellular matrix.Collagen provides elasticity in animal tissues, but it does not resist compression. That role is filled by glycosaminoglycans. A glycosaminoglycan (GAG) is a polysaccharide chain of repeating disaccharides, usually linked to a core protein, that forms part of the extracellular matrix. These units often form a chain together, resulting in an enormous aggregate with a bottlebrush shape and a very high molecular weight.