Golgi Apparatus

The Golgi apparatus, shown in Figure 4-16, is another system of flattened, membranous sacs. The sacs nearest the nucleus receive vesicles from the ER containing newly made proteins or lipids. Vesicles travel from one part of the Golgi apparatus to the next and transport substances as they go. The stacked membranes modify the vesicle contents as they move along. The proteins get "address labels" that direct them to various other parts of the cell. During this modification, the Golgi apparatus can add carbohydrate labels to proteins or alter new lipids in various ways.

Cells contain several types of vesicles, which perform various roles. Vesicles are small, spherically shaped sacs that are surrounded by a single membrane and that are classified by their contents. Vesicles often migrate to and merge with the plasma membrane. As they do, they release their contents to the outside of the cell.

Lysosomes

Lysosomes (LIE-suh-soHMZ) are vesicles that bud from the Golgi apparatus and that contain digestive enzymes. These enzymes can break down large molecules, such as proteins, nucleic acids, carbohydrates, and phospholipids. In the liver, lysosomes break down glycogen in order to release glucose into the bloodstream. Certain white blood cells use lysosomes to break down bacteria. Within a cell, lysosomes digest worn-out organelles in a process called autophagy (aw-TAHF-uh-jee).

Lysosomes are also responsible for breaking down cells when it is time for the cells to die. The digestion of damaged or extra cells by the enzymes of their own lysosomes is called autolysis (aw-TAHL-uh-sis). Lysosomes play a very important role in maintaining an organism's health by destroying cells that are no longer functioning properly.

Peroxisomes

Peroxisomes are similar to lysosomes but contain different enzymes and are not produced by the Golgi apparatus. Peroxisomes are abundant in liver and kidney cells, where they neutralize free radicals (oxygen ions that can damage cells) and detoxify alcohol and other drugs. Peroxisomes are named for the hydrogen peroxide, H2O2, they produce when breaking down alcohol and killing bacteria. Peroxisomes also break down fatty acids, which the mitochondria can then use as an energy source.

Other Vesicles

Specialized peroxisomes, called glyoxysomes, can be found in the seeds of some plants. They break down stored fats to provide energy for the developing plant embryo. Some cells engulf material by surrounding it with plasma membrane. The resulting pocket buds off to become a vesicle inside the cell. This vesicle is called an endosome. Lysosomes fuse with endosomes and digest the engulfed material. Food vacuoles are vesicles that store nutrients for a cell. Contractile vacuoles are vesicles that can contract and dispose of excess water inside a cell.

Protein Synthesis

One of the major functions of a cell is the production of protein. The path some proteins take from synthesis to export can be seen in Figure 4-17. In step Q, proteins are assembled by ribosomes on the rough ER. Then, in step ©, vesicles transport proteins to the Golgi apparatus. In step ©, the Golgi modifies proteins and packages them in new vesicles. In step ©, vesicles release proteins that have destinations outside the cell. In step ©, vesicles containing enzymes remain inside the cell as lysosomes, peroxisomes, endo-somes, or other types of vesicles.

figure 4-17

The rough ER, Golgi apparatus, and vesicles work together to transport proteins to their destinations inside and outside the cell.

Nucleus

Other vesicles remain in the cell and become lysosomes and other vesicles.

Some vesicles release their proteins outside the cell.

Proteins are modified the Golgi apparatus and enter new vesicles.

Nucleus

Some vesicles release their proteins outside the cell.

Proteins are assembled by ribosomes on the rough ER.

Vesicles carry proteins from the rough ER to the Golgi apparatus.

Endoplasmic reticulum

Mitochondrion

figure 4-18

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