Download Chapter 6: Concept 6.4

Objectives
Identify the role of the nucleus in a cell.
Describe how the functions of ribosomes, the endoplasmic
reticulum, and the Golgi apparatus are related.
Distinguish between the functions of vacuoles and lysosomes.
Summarize the path of cellular products through membranes.
Key Terms
nuclear envelope
nucleolus
ribosome
endoplasmic reticulum
Golgi apparatus
vacuole
lysosome
Just as a factory has a number of different departments and equipment
specialized for specific jobs, a cell is similarly specialized. If you think
of a cell as a factory, then the nucleus is its executive boardroom. The
top managers are the DNA molecules that direct almost all the business
of the cell. The other organelles are the "departments" that carry out the
instructions of the executive board. They build, package, transport,
export, and even recycle products of the cell.
Structure and Function of the Nucleus
You read in Concept 6.1 that the nucleus of a eukaryotic cell contains
most of the cell's DNA. The information stored in the DNA directs the
activities of the cell. This DNA is attached to certain proteins, forming
long fibers called chromatin. Most of the time, the chromatin looks like
a tangled mess to anybody examining it with a microscope. But you will
read in Chapter 9 that chromatin becomes much more organized when
cells reproduce. A pair of membranes called the nuclear envelope
surrounds the nucleus (Figure 6-18). Substances made in the nucleus
move into the cell's cytoplasm through tiny holes, or pores, in the
nuclear envelope. These substances include molecules that carry out
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Wednesday, October 19, 2011 9:14:45 AM CT
the instructions from the DNA of the nucleus. In addition to the
chromatin, the nucleus contains a ball-like mass of fibers and granules
called the nucleolus (plural, nucleoli). The nucleolus contains the parts
that make up organelles called ribosomes.
Figure 6-18
A cell's nucleus contains DNA—information-rich
molecules that direct cell activities.
Ribosomes
The DNA in the nucleus contains instructions for making proteins.
Proteins are constructed in a cell by the ribosomes. These organelles
work as protein "assembly lines" in the cellular factory. Ribosomes
themselves are clusters of proteins and nucleic acids assembled from
components made in the nucleolus. In the classroom-cell scale model, a
ribosome would be about the size of a marble. Some ribosomes are
bound to the outer surface of a membrane network within the cytoplasm
(Figure 6-19). These ribosomes make the proteins found in membranes,
as well as other proteins that are exported by the cell. Other ribosomes
are suspended in the cytoplasm. The suspended ribosomes make
enzymes and other proteins that remain in the cytoplasm.
Figure 6-19
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A ribosome is either suspended in the cytoplasm or
temporarily attached to the rough endoplasmic reticulum
(ER). Though different in structure and function, the two
types of ER form a continuous maze of membranes
throughout a cell. The ER is also connected to the
nuclear envelope.
The Endoplasmic Reticulum
Within the cytoplasm of a cell is an extensive network of membranes
called the endoplasmic reticulum (ER). You could think of the ER as
one of the main manufacturing and transportation facilities in the cell
factory. The ER produces an enormous variety of molecules. It is a
maze of membranes, arranged as tubes and sacs that separate the inside
of the ER from the surrounding cytoplasm (Figure 6-19). There are two
distinct regions: rough ER and smooth ER. These two regions are
physically connected, but they differ in structure and function.
Rough ER The rough ER gets its name from the bound ribosomes
that dot the outside of the ER membrane. These ribosomes produce
proteins that are inserted right into (or through) the ER membrane.
Ribosomes bound to the ER also produce proteins that are packaged in
vesicles by the ER and later exported, or secreted, by the cell (Figure 620). Cells that secrete a lot of protein—such as the cells of your
salivary glands that secrete enzymes into your mouth—are especially
rich in rough ER.
Smooth ER This part of the ER lacks the ribosomes that cover the
rough ER. A number of different enzymes built into the smooth ER
membrane enable the organelle to perform many functions. One
function is to build lipid molecules. For example, cells in the ovaries
and testes that produce sex hormones contain an especially large
amount of smooth ER.
Figure 6-20
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Some proteins are made by
ribosomes (the red structure)
on the rough ER and packaged
in vesicles. After further
processing in other parts of the
cell, these proteins will
eventually move to other
organelles or to the plasma
membrane.
The Golgi Apparatus
Some products that are made in the ER travel in vesicles to the Golgi
apparatus, an organelle that modifies, stores, and routes proteins and
other chemical products to their next destinations. The membranes of
the Golgi apparatus are arranged as a series of flattened sacs that might
remind you of a stack of pita bread. A cell may contain anywhere from
just a few of these stacks to hundreds. In the classroom-cell scale
model, a Golgi stack is about the size of a bass drum. This organelle is
like the factory's processing and shipping center all in one. One side of
a stack serves as a "receiving dock" for vesicles transported from the
ER (Figure 6-21). Enzymes in the Golgi apparatus refine and modify
the ER products by altering their chemical structure. From the
"shipping" side of a stack, the finished products can be moved in
vesicles to other locations. Some of these vesicles travel to specific
targets within the cell. Others export cellular products by fusing with
the plasma membrane and releasing the products outside the cell by the
process of exocytosis.
Figure 6-21
Golgi stacks receive, modify, and dispatch finished
products.
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Vacuoles
The cytoplasm also contains large, membrane-bound sacs called
vacuoles (VAK yoo ohlz). Many vacuoles store undigested nutrients.
One type of vacuole, called a contractile vacuole, is found in some
single-celled freshwater organisms. The contractile vacuole pumps out
excess water that diffuses into the cell.
Many plant cells have a large central vacuole. It stores chemicals such
as salts and contributes to plant growth by absorbing water and causing
cells to expand. Central vacuoles in the cells of flower petals may
contain colorful pigments that attract pollinating insects. In leaf cells,
central vacuoles may contain poisons that protect against plant-eating
animals.
Lysosomes
Membrane-bound sacs called lysosomes contain digestive enzymes that
can break down such macromolecules as proteins, nucleic acids, and
polysaccharides (Figure 6-23). Lysosomes have several functions. They
fuse with incoming food vacuoles and expose the nutrients to enzymes
that digest them, thereby nourishing the cell. Lysosomes also function
like safety officers when they help destroy harmful bacteria. In certain
cells—for example, your white blood cells—lysosomes release
enzymes into vacuoles that contain trapped bacteria and break down
the bacterial cell walls. Similarly, lysosomes serve as recycling centers
for damaged organelles. Without harming the cell, a lysosome can
engulf and digest another organelle. This makes molecules available
for the construction of new organelles.
Figure 6-23
Lysosomes contain digestive
enzymes that break down food for
cell use.
Membrane Pathways in a Cell
Follow the pathway of activity in Figure 6-24 to see how some of a
cell's organelles function together. Vesicles bud from one organelle (1)
and fuse with another (2), transferring membranes as well as products.
The arrows show some of the pathways cell products follow on their
journey through the cell (3 and 4). You may notice that the internal side
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of a vesicle membrane can eventually turn up as part of the outward
face of the plasma membrane at the cell's surface (5). Exocytosis has
turned the vesicle inside out! Membranes are constantly being
transferred throughout the cell. An ER product can eventually exit the
cell without ever crossing a membrane.
Figure 6-24
Products made in the ER move through
membrane pathways in a cell.
Concept Check 6.4
1. In what way does the nucleus direct the activities of a cell?
2. Trace the path of a protein from the time it is produced by a ribosome
on the ER until it reaches its destination.
3. How are undigested nutrients in a vacuole made available to a cell?
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Wednesday, October 19, 2011 9:14:45 AM CT