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Degradation of extracellular substances

Enzymes are secreted that degrade macromolecules outside of the cell.The resulting small molecules are transported into the cell.

Macromolecules are brought into the cell by pinocytosis or, in the case of protozoa and phagocytes, phagocytosis. Lysosomes carry digestive enzymes.

Motility

Generally involves flagella, which are composed of protein subunits. Flagella rotate like propellers, using proton motive force for energy.

Involves cilia and flagella, which are made up of a 9 + 2 arrangement of microtubules. Cilia move in synchrony; flagella propel a cell with a whiplike motion or thrash back and forth to pull a cell forward. Both use ATP for energy.

Protein secretion

A characteristic signal sequence marks proteins for secretion by the general secretory pathway.The precise mechanisms of translocation are still poorly understood.

Secreted proteins are translocated to the lumen of the rough endoplasmic reticulum as they are being synthesized. From there, they are transported to the Golgi apparatus for processing and packaging.

Strength and rigidity

Peptidoglycan-containing cell wall.

Cytoskeleton composed of microtubules, intermediate filaments, and microfilaments. Some have a cell wall; some have sterols in the membrane.

Transport

Primarily active transport; mechanisms include major facilitator superfamily and ABC transport. Group translocation.

Facilitated diffusion and active transport; mechanisms include major facilitator superfamily and ABC transport. Ion channels.

an ABC transporter system to eject the antimalarial drug chloro-quine. These chloroquine-resistant strains are a global health concern. Some human cancer cells use an ABC transporter system that ejects therapeutic drugs intended to kill those cells. ■ malaria, p. 731 ■ major facilitator superfamily, p. 56 ■ ABC transporter, p. 56

Endocytosis and Exocytosis

Endocytosis is the process by which eukaryotic cells take material from the surrounding environment. They do this by enclosing those materials in a fluid-filled compartment (figure 3.50).

The type of endocytosis common to most animal cells is pinocytosis. In this process a cell internalizes and pinches off small pieces of its own membrane, bringing along a small volume of liquid and any material attached to the membrane. This endocytic vesicle becomes a membrane-enclosed, low-pH compartment called an endosome. This then fuses with a digestive organelle called a lysosome. The characteristics of lyso-somes will be discussed shortly.

In animal cells, pinocytosis is generally a type called receptor-mediated endocytosis. This process allows cells to internal-

Pinocytosis

Pinocytosis

Pseudopod

Phagocytosis

Figure 3.50 Endocytosis and Exocytosis Endocytosis includes pinocytosis (receptor-mediated endocytosis) and phagocytosis.

Pseudopod

Phagocytosis

Figure 3.50 Endocytosis and Exocytosis Endocytosis includes pinocytosis (receptor-mediated endocytosis) and phagocytosis.

ize extracellular ligands that bind to the cell's receptors. Certain regions of the cell membrane are lined with a protein called clathrin and studded with receptors. These regions are internalized to form an endocytic vesicle, bringing with them the receptors along with their bound ligands. The low pH of the endosome frees the ligands from the receptors, which are often recycled. The endosome then fuses with a lysosome. Many viruses, including those that cause influenza and rabies, exploit receptor-mediated endocytosis to enter animal cells. By binding to a specific receptor, they too are taken up during pinocytosis. ■ influenza, p. 586 ■ rabies, p. 680

Protozoa and phagocytes, both of which ingest bacteria and large debris, illustrate a specific type of endocytosis called phagocytosis. Phagocytes are important cells of the body's defense system. These cells send out armlike extensions, pseudopods, which surround and enclose extracellular material, including bacteria. This action envelops the material, bringing it into the cell in an enclosed compartment called a phagosome. These ultimately fuse with a lysosome to form a phagolysosome. Phagocytes have a greater abundance of lysosomes than do other animal cells, which reflects their specialized function. In addition, their lysosomes contain a wider array of powerful digestive enzymes. Thus, most microbes are readily dispatched within the phagolysosome. Those that resist the killing effects are able to cause disease. ■ phagocytes, p. 384 ■ survival within a phagocyte, p. 385

The process of exocytosis is the opposite of endocytosis. Membrane-bound vesicles inside the cell fuse with the plasma membrane and release their contents into the external medium. The processes of endocytosis and exocytosis result in the exchange of material between the inside and outside of the cell.

Secretion

Proteins destined for a non-cytoplasmic region, either outside of the cell or the lumen of an organelle, must be translocated

3.12 Protein Structures Within the Cytoplasm 73

across a membrane. Ribosomes synthesizing a protein that will be secreted attach to the membrane of the endoplasmic retic-ulum (ER). The characteristics of this organelle will be described shortly. As the protein is being made, it is threaded through the membrane and into the lumen of the ER. The lumen of any organelle can be viewed as equivalent to an extracellular space. Once a protein or any substance is there, it can readily be transported by vesicles to the lumen of another organelle, or to the exterior of the cell.

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