Plant Cell Adhesions

Elastin is named for its high elasticity in connective tissue. It allows for the stretching or contracting of tissues, including the skin. It is encoded by the ELN gene in humans. If the gene is defective, there can be a genetic disorder, such as is seen in cutis laxa, which is an autosomal dominant disorder of elastin synthesis.

Elastic fibers are made from elastin, which is more amorphous in shape, and fibrillin, which is more fibrous. Elastin is particularly important in arteries, in the lungs, in the skin, and in the bladder, where elasticity is necessary.

Elastin is made by the linkage of many soluble tropoelastin molecules in order to make the durable elastin protein. The tropoelastin is cross-linked in order to make the whole elastin molecule. As mentioned, fibrillin is necessary to make elastic fibers; fibrillin is a glycoprotein secreted by the fibroblasts in the connective tissue. Fibrillin is made by the FBN genes.

There are three types of fibrillin. Fibrillin-1 is made into microfibrils. Mutations in the gene FBN1 leads to conditions like Marfan syndrome, which involves increased elasticity of some tissues in the body. There are more than 1500 mutations that are possible in the FBN1 gene. Fibrillin-2, fibrillin-3, and fibrillin-4 have been more recently isolated and are less commonly linked to known disease states.

PLANT CELL ADHESIONS

Plant cells have unique structures called plasmodesmata. These are also seen in some algae. The purpose of these structures is to allow transport of molecules and communication between the different cells. Almost all plant cells have a cell wall around the cell, keeping it effectively separate from other cells. Cell walls can be somewhat impermeable to certain substances so plasmodesmata are necessary to have transport between the cells. There are two types of plasmodesmata: primary plasmodesmata, which are made during cell division, and secondary plasmodesmata, which happen between already mature cells.

Primary plasmodesmata are made from endoplasmic reticulum, which can become trapped outside the cell at the time of cell division. These lead to cytoplasmic connections between the two sister cells. The cell wall is not made where the

plasmodesmata are formed, allowing for connection to happen between the cell membranes of the plant cells. Figure 24 shows what the plasmodesmata look like:

Figure 24..

There are thousands of plasmodesmata seen in each plant cell. There are three main layers: the plasma membrane, the cytoplasmic sleeve, and the desmotubule. They can get through very thick cell walls, up to 90 nanometers thick.

The purpose of the plasmodesmata is to allow sugars, amino acids, water, and ions to pass from one cell to another. There is no need for chemical energy to pass these things from cell to cell; they pass through via simple diffusion. Larger molecules can also sometimes get from cell to cell but the mechanism by which it does this is unknown. Signaling molecules, RNA, and transcription factors seem to be able to get from one cell to another but it isn’t known how this occurs.