Endoplasmic Reticulum

A typical plant cell is so small that diffusion, the random movement caused by molecular motion, carries a molecule to many different parts of the cell every second. Diffusion may be the only means by which small molecules like monosaccharides and cofactors move around the cell, but some large molecules such as proteins are carried by the endoplasmic reticulum (ER), a system of narrow tubes and sheets of membrane that form a network throughout the cytoplasm (Fig. 1).

A large proportion of a cell's ribosomes are attached to the ER, giving it a rough appearance; consequently, this ER is called rough ER, or RER. As an attached ribosome synthesizes a protein, it passes through the membrane and collects in the lumen. If the protein is a storage product, as in seeds of legumes, it merely remains in the ER, which may become quite swollen. But if the protein is to be secreted (digestive enzymes, mucilages, adhesive proteins, certain nectars), then its accumulation causes regions of the ER to form vesicles. These detach, move to the plasma membrane, and fuse with it, releasing their contents to the cell's exterior by exocytosis (see Fig. 3.9). In many cases the protein must be modified before export; if so, the ER pinches off only very small vesicles from regions where the ER is close to another organelle, the dictyosome, which carries out the protein modification.

Endoplasmic reticulum that lacks ribosomes is smooth ER, or SER, and it is involved in lipid synthesis and membrane assembly. The lipids range from simple to extremely complex; as they are produced, the lipids are inserted into the membrane, then vesicles form and pinch off, carrying the new membrane to other parts of the cell. Once the ER-derived vesicles reach the correct organelle, they fuse with it and the vesicles become a new patch of membrane in the organelle. SER is abundant only in cells that produce large amounts of fatty acids (cutin and wax on epidermal cells), oils (palm oil, coconut oil, safflower oil), and fragrances of many flowers.

FIGURE 1:  Electron micrograph of a cross-section of a hair cell that secretes protein-rich mucilage. The mucilage is synthesized in the endoplasmic reticulum, which is unusually abundant for a plant cell (X 3000).