Eukaryotic Cell Structure: The Endoplasmic Reticulum

III. THE EUKARYOTIC CELL

THE ENDOPLASMIC RETICULUM

The overall purpose of this Learning Object is to learn the chemical makeup and the functions associated with the endoplasmic reticulum in eukaryotic cells.

LEARNING OBJECTIVES FOR THIS SECTION

The Endoplasmic Reticulum (def) (see Fig. 31, Fig. 30, Fig. 32, and Fig. 33).

The endoplasmic reticulum or ER is a maze of parallel membranous tubules and flattened sacs surrounding the nucleus that connects with the nuclear membrane and runs throughout the cytoplasm (see Fig. 33). The ER functions to:

1) provide a surface area for protein and lipid synthesis;

2) form a pathway for transporting molecules within the cell; and

3) provide a storage area for molecules the cell has synthesized.

The endoplasmic reticulum connects to the pores of the nuclear envelope. In this way, ribosomal subunits and mRNA transcribed off genes in the DNA can leave the nucleus, enter the endoplasmic reticulum, and participate in protein synthesis.

There are two distinct regions of the ER: the rough ER and the smooth ER.

1. Rough Endoplasmic Reticulum

ER with ribosomes attached is called rough endoplasmic reticulum (see Fig. 31, Fig. 30, and Fig. 33) and is involved in protein synthesis, production of new membrane, modification of newly formed proteins, and transport of these proteins and membrane to other locations within the cell.

Ribosomal subunits and mRNA molecules transcribed off genes in the DNA leave the nucleus through pores in the nuclear membrane, enter the cytoplasm, and participate in protein synthesis. Ribosomes attached to mRNA molecules coding for proteins to be secreted from the cell or enter lysosomes attach to receptors on the ER. The ribosomes are tightly attaced to the rough ER and contain a tunnel that connects to a pore in the ER called a translocon. The proteins that are synthesized by the ribosomes can then pass through the translocon and enter the lumen of the ER where they can be transported to other locations within the ER. Proteins secreted from the cell by exocytosis or destined for lysosomes are synthesized by the ribosomes on the surface of the rough ER. Proteins for use within the eukaryotic cell or intended for organelles such as mitochondria, chloroplasts, and peroxisomes are synthesized by mRNA molecules attached to ribosomes in the cytoplasm.

2. Smooth Endoplasmic Reticulum

ER without ribosomes is called smooth endoplasmic reticulum (see Fig. 31 and Fig. 33) and contains enzymes for lipid biosynthesis, especially the synthesis of phospholipids and steroids. The smooth endoplasmic reticulum forms transition vesicles to transfer molecules produced in the rough ER to the Golgi complex (see Fig. 31 and Fig. 33).

Animation showing the endomembrane system.

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