Page 115 - Fundamentals of anatomy physiology
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102 Unit 1 Levels of Organization
3 Rough Endoplasmic Reticulum it travels from the ER to the Golgi (GO■ L-je. ) apparatus, or
Golgi complex, an organelle that looks a bit like a stack of
The rough endoplasmic reticulum (RER) functions as a com- dinner plates (Figure 3–6). This organelle typically consists of
bination workshop and shipping warehouse. It is where many five or six flattened membranous discs called cisternae. A single
newly synthesized proteins are chemically modified and pack- cell may contain several of these organelles, most often near
aged for export to their next destination, the Golgi apparatus. the nucleus.
The fixed ribosomes on the outer surface of the rough The Golgi apparatus has three major functions (Spotlight
endoplasmic reticulum give the RER a beaded, grainy, or Figure 3–7). It
rough appearance (Figure 3–5). Both free and fixed ribo-
somes synthesize proteins using instructions provided by 1. modifies and packages secretions, such as hormones or
messenger RNA. enzymes, for release from the cell;
The new polypeptide chains produced at fixed ribosomes 2. renews or modifies the plasma membrane; and
are released into the cisternae of the RER. Inside the RER,
each protein assumes its secondary and tertiary structures. 3. packages special enzymes within vesicles (lysosomes) for use in
the cytoplasm.
p. 78 Some of the proteins are enzymes that will function
inside the endoplasmic reticulum. Other proteins are chemi- Lysosomes
cally modified by the attachment of carbohydrates, creating
glycoproteins. Cells often need to break down and recycle large organic
molecules and even complex structures like organelles. The
Most of the proteins and glycoproteins produced by the breakdown process requires powerful enzymes, and it often
RER are packaged into small membranous sacs that pinch generates toxic chemicals that could damage or kill the cell.
off from the tips of the cisternae. These transport vesicles Lysosomes (LI■ -so. -so. mz; lyso-, a loosening + soma, body)
then deliver their contents to the Golgi apparatus (Spotlight are special vesicles that provide an isolated environment for
Figure 3–7). potentially dangerous chemical reactions. These vesicles, pro-
duced by the Golgi apparatus, contain digestive enzymes.
The amount of endoplasmic reticulum and the proportion Lysosomes are small, often spherical bodies with contents that
of RER to SER vary with the type of cell and its ongoing activi- look dense and dark in electron micrographs.
ties. For example, pancreatic cells that manufacture digestive
enzymes contain an extensive RER, but the SER is relatively Lysosomes have several functions (see Figure 3–8 on p. 106).
small. The situation is just the reverse in the cells of reproduc- One is to remove damaged organelles. Primary lysosomes contain
tive organs that synthesize steroid hormones. inactive enzymes. When these lysosomes fuse with the mem-
branes of damaged organelles (such as mitochondria or frag-
The Golgi Apparatus ments of the ER), the enzymes are activated and secondary lysosomes
are formed. The enzymes then break down the contents. The
When a transport vesicle carries a newly synthesized pro-
tein or glycoprotein that is destined for export from the cell,
Figure 3–6 The Golgi Apparatus.
Secretory
vesicles
Secretory
product
Transport Golgi apparatus TEM × 42,000
vesicles
a Here is a three-dimensional view of b This is a sectional view of the Golgi apparatus
the Golgi apparatus with a cut edge. of an active secretory cell.
