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Lecture

lecture8 for BGYA01

by OC4

Department
Biological Sciences
Course Code
BIOA01H3
Professor
Clare Hasenkampf

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BGYA01 LECTURE 8 OCTOBER 4, 2007
WE HAVE NOW LOOKED AT STRUCTURES INSIDE THE CELL NOW LETS LOOK
OUTWARD.
Cells often secrete important materials outside the cell, these materials can be very important to
the structure of the cell and to the health of the organism. [Remember just because something is
outside the cell, it does not mean it is outside the organism.]
One important structural feature outside of cells of many eukaryotes is the cell wall.
Animals do not have cell walls but plants, fungi, and most groups of protists do.
The cell wall is a semi-rigid structure outside of the plasma membrane.
Cell walls are made of polysaccharides and proteins, but the different kingdoms use different
polysaccharides.
Plant cell walls have cellulose as the dominant polysaccharide,
while fungal cell walls have the modified polysaccharide chitin as its major component.
Lets take a quick look at a plant cell wall. Figure 4.24, page 91 and figure 4.7, page 77
The cell wall provides support for the cell, and limits the cells volume by its relative rigidity.
The cell wall also acts as a barrier against infection [by fungi and other organisms that can cause
plant diseases].
Even though cell walls surround the cells the wall does not isolate the cell from its neighbor
cells.
The neighboring cells are not isolated, because adjacent cells are connected by channels within
the cell walls.
The cell walls have gaps in them that are 20-40 nm in width. These organized gaps are called
plasmodesmata. In Figure 4.7, page 77. You can see the cell walls and the plasmodesmata.
A plasmodesmata is a gap in the cell walls of neighboring cells that allows the cytoplasm of
neighboring cells to be continuous.
Not only is there a gap in the cell wall at plasmodesmata, but the cells plasma membranes fuse
in this region so that the cytoplasm of the neighbors is continuous.
Thus the neighboring plant cells are very intimately associated. Similarly the neighboring cells
of fungi and multicellular algae are also intimately associated.
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What about animal cells?
Often the individual cells of multicellular animals are NOT directly touching each other.
Animal cells are usually embedded in an extracellular matrix (called the ECM).
The ECM consists of fibrous proteins interspersed in a proteoglycan matrix. (A proteoglycan is a
polysaccharides to which amino acids have been added).
How is the ECM different than a cell wall?
The ECM does not completely surround the cell in three dimensions. Animal cells may sit in
the ECM.
One major function of the extracellular matrix is to hold animals cells together in a tissue.
Figure 4.25, page 91 shows an electron micrograph of the kidney. The kidney cell is not
attached directly to a blood vessel. Rather both are adjacent to a layer of ECM.
The dominant fibrous protein of the ECM is collagen. An example of a region of extracellular
matrix with collagen can be seen in Figure 4.25 bottom right side.
Therefore one role of the ECM is to hold cells together into a tissue.
Once materials are outside of the cell they can move relatively freely through the extracellular
matrix through that region of the body. The signal molecules move through the ECM. Thus
another role of the ECM is in chemical signaling from one cell to another.
The ECM is also important for filtering materials that pass through it (Figure 4.28 top right
side.), and in programmed cell movements during development.
In some parts of our body like the brain, there is very little ECM. But in other parts of our
bodies, like our bones and cartilage, most of the tissue is actually ECM, and the exact proteins
and polysaccharides of the ECM can give it unique properties.
Thus our bones are quite different from our cartilage because the protein and polysaccharide
components of the two ECMs are different.
So many animal cells are not in direct contact with each other; they are separated from each
other by the extracellular matrix, but materials move freely between the cells.
Sometimes, for special purposes animal cells are in direct contact; we call these sites junctions.
Overhead Figure 5.7, page 104 Junctions are regions within animals where the cells are
actually touching.
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There are three general categories of junctions between animal cells.
tight junctions, desmosomes, and gap junctions.
Tight junctions bind the two cells very closely. Tight junctions are barriers to prevent materials
from moving between the cells. Figure 5.7a.
In the example, of the epithelial cells the tight junctions are used at the top surface of the cells to
form a wall to block movement IN BETWEEN the cells. Thus the only way for molecules to
move from one side of the cell layer to the other is through the cells, and this is tightly regulated.
Desmosomes are the next type of connection. Figure 5.7 middle panel. Desmosomes are
designed to keep the two cells linked, but not tightly linked. This type of linkage, allows the
passage of material in between the cells. Thus there are passage-ways still in between the cells.
Gap junctions are the third type of connection. Figure 5.7 lowest panel. Gap junctions are
designed for intimate direct communication between the two cells. Here there is a small gap
between the two cells that is spanned by a channel. Specific chemicals or electric impulses can
move quickly through these channels.
This completes our look at each eukaryotic cell structure.
CLICKER QUESTION
Which type of junction is most similar in function to the plasmodesmata?
a) Tight junction
b) Desmosome
c) Gap junction*****
d) ECM
LETS GET AN OVERVIEW OF ACTIVITY WITHIN A EUKARYOTIC CELL
The hereditary material of the cell is kept within the nucleus and never leaves the nucleus.
Therefore DNA replication (duplicating the genetic material) and transcription(the creation of
RNA) takes place in the nucleus.
But most RNA moves to the cytoplasm. Ribosomal subunits are assembled at the nucleolus
within the nucleus, but the subunits move out to the cytoplasm.
Translation (the synthesis of proteins) occurs in the cytoplasm, either on free ribosomes, or on
the rough ER.
Photosynthesis takes place within the chloroplast. Starches are made from simple sugars in the
leucoplasts of plant cells.
Cellular respiration occurs with the mitochondria, here sugars are broken down to produce ATP.
ATP is the main energy molecule used by proteins to do work.
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