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This topic covers the function and structure of the cytoskeleton, extracellular components
of multicellular organisms, and cell connections in multicellular organisms.
Crisscrossing the cytoplasm of eukaryotic cells is the cytoskeleton. Just like the name
sounds, the cytoskeleton can give support and shape to the cell, like the human skeleton
does to the human body. Unlike the human skeleton, the cytoskeleton can also be involved in changing
the shape of the cell, transporting organelles and molecules within the cells, and so on.
The cytoskeleton is composed of three types of fibers: actin filaments, intermediate filaments,
and microtubules. Microtubules are the largest fibers. They
form the spindle that separates chromosomes during cell division, they form the internal
structure of flagella and cilia, and they help maintain the shape of cells that don't
have a cell wall. In animal cells, microtubules radiate out
from a region near the nucleus called the centrosome. It is thought of as a microtubules
organizing center. Within the centrosome is a pair of centrioles.
The centrioles are composed to nine sets of bundles of microtubules, three microtubules
per bundle. The centrioles function in cell division. When the cell is ready to divide,
it makes a copy of all of its DNA, so that both cells can have a copy. The centrioles
move to opposite sides, or poles, of the cells and a bunch of microtubules radiate out from
the centrioles. We call this the mitotic spindle. The mitotic spindle makes sure one copy of
the DNA ends up in each new cell, rather than two copies in one cell and none in the other.
Eukaryotes have structures for motility, or moving the cell through the environment. In
multicellular organisms, one of these structures, cilia, have been adapted to moving materials
in the environment pass the cell and its tissue, like the cilia here on a tracheal, or windpipe,
cell, but in this class we will be focusing on unicellular organisms.
Flagella in eukaryotes have a very different structure than prokaryotic flagella. Eukaryotic
flagella are composed of bundles of microtubules in a 9+2 arrangement, like in this micrograph.
Nine bundles of two microtubules are arranged in a circle around the core. ATP is used to
pull the bundles one way or the other. The result is a back and forth or rowing motion.
The flagella attached to a basal body within the cytoplasm. The structure of the basal
body resembles the structure of the centrioles. Cilia are composed of the same 9+2 arrangement
of microtubules. They are basically short flagella and organism that have cilia usually
have a lot. Actin filaments are the smallest of the cytoskeleton
fibers. They are found in highest concentration under the cytoplasm. They are involved with
cell division, contraction of the cell, cell extensions, and pinching off vacuoles and
lysosomes. Intermediate filaments are the medium-sized
fibers. Their main role is in structural reinforcement of the cell and of organelles, such as the
nucleus. In multicellular organisms they are involved in cell-to-cell connections.
Plant cells have a cell wall that protects the cell and helps the plant have rigidity,
sort of like how a human skeleton provides rigidity. The cell wall of plants is composed
of cellulose, the most common polysaccharide on the planet.
Animal cells do not have cell walls. Instead, they have what is called the extracellular
matrix, or ECM. The ECM protects the cells by cushioning them, keeping them moist, and
acting as a barrier to pathogens. The EMC also connects animal cells to each other,
helps with communication between cells, and helps with cell movement within developing
embryos. The ECM is composed of proteins like collagen, polysaccharides like proteoglycans,
and glycoproteins like fibronectin. Cells in multicellular organisms like plants
and animals are connected to each other. Let's go over this diagram and talk about the functions
of each of the cell connections. Tight junctions form very tight connections
between animal cells (that's why they are called tight junctions). They help form barriers
that don't allow fluids to leak between the cells. For example, in the intestinal, tight
junctions form between the intestinal mucosal cells so that the contents of the intestine
don't leak between those cells into the blood stream.
Desmosomes, or anchoring connections, are strong connections in animal cells which forms
them into sheets. Intermediate filaments pass between the desmosomes of different cells,
making them stronger. Gap junctions are little ports between cells.
When they open up, fluid and small molecules pass directly from one cell to another. This
helps will cell communication in tissues. Plant cells have a junction similar in function
to a gap junction. It is called a plasmodesmata. They are membrane-lined pores that pierce
the cell walls of neighboring plants, allowing cytosol and molecules to pass from cell to
cell. This aids in the communication of cells within plant tissues.