Structural orientation of some proteins in the cell membrane.
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A, Membrane-associated protein with noncovalent attachment to plasma lipids. B, Membrane protein with noncovalent attachment to another membrane protein. C, Transmembrane protein extending through the lipid bilayer. D, Covalently attached peripheral membrane protein.
CHAPTER 3 Cell Structure and Function 31
gather the chromosomes and then fuse together to form a new nuclear membrane. Nuclear proteins and pore structures are then recruited back to their normal nuclear locations.
Endoplasmic Reticulum The ER is a membrane network that extends throughout the cytoplasm and is present in all eukaryotic cells (Fig. 3.12). The ER is thought to have a single continuous membrane that separates the lumen of the ER from the cytosol. The ER plays a central role in the synthesis of membrane components, including proteins and lipids, for the plasma membrane and cellular organelles, as well as in the synthesis of products to be secreted from the cell. The ER is divided into rough and smooth types based on its appearance under the electron microscope. The rough ER is coated with ribosomes along its outer surface. Ribosomes are complexes of protein and RNA that are formed in the nucleus and transported to the cytoplasm. Their primary function is the synthesis of proteins (see Chapter 5). Depending on the destination of the protein to be created, ribosomes may float free in the cytosol or may bind to the ER membrane. Proteins synthesized by free-floating ribosomes are released within the cytosol of the cell. Proteins to be transported into the ER have a special sequence of amino acids that directs the ribosome responsible for its synthesis to the ER membrane. Special proteins called signal recognition particles bind to the leading sequence of the protein and then bind to a receptor on the ER membrane. As the ribosome adds amino acids to the growing protein chain, it is pushed into the lumen of the ER through a pore in the ER membrane called a translocon. After being processed in the ER and Golgi apparatus, the protein is
FIG 3.8 Transmembrane proteins have complex folded structures, often with several twists and turns through the membrane. Cytochrome C oxidase. (From The European Bioinformatics Institute. http://www.ebi.ac.uk/. March 20, 2017.)
Kidney tubule
Transport proteins
Tubule epithelial cells
Tight junctions
N N
N N N
FIG 3.9 Transport proteins may be confined to a particular portion of the cell membrane by tight junctions. Segregation of transport proteins is important for the absorptive functions of the kidney epithelial cells. N, Nucleus.
and other proteins that define the cell’s type and behavior. (The structure and function of DNA are discussed in Chapter 5.)
During mitosis, the complex structure of the nuclear membrane is retracted into the endoplasmic reticulum membrane. After cell division is complete, pieces of nuclear membrane reemerge to surround and
32 UNIT II Cellular Function
Golgi Apparatus The Golgi apparatus, or Golgi complex, is composed of a stack of smooth membrane-bound compartments resembling a stack of hollow plates or pita breads (see Fig. 3.12). These compartments, or cisternae, are organized in a series of at least three processing compartments. The first compartment (cis face) lies next to the ER and receives newly synthesized proteins and lipids by way of ER transport vesicles. These transport vesicles are outgrowths that bud off from the ER membrane and diffuse to the Golgi, where they bind and become part of the Golgi apparatus membrane. The proteins and lipids then move through the middle compartment (medial) to the final compartment (trans face), where they depart for their final destination. As the lipid and protein molecules pass through the sequence of Golgi compartments, they are modified by enzymes that attach or rearrange carbohydrate molecules. After specific arrangement of these carbohydrates has occurred, the lipids and proteins are packaged into Golgi transport vesicles (secretory vesicles). The particular configuration of carbohydrate molecules on the lipid or protein is believed to serve as an “address label,” direct- ing them to the correct destination within the cell. Golgi vesicles transport their contents primarily to the plasma membrane and to lysosomes.
eventually transported to the appropriate organelle or secreted at the cell surface. Free-floating and rough ER ribosomes are identical and interchangeable; their location depends on the amino acid structure of the protein they are producing at the time.