22 terms

MB 6.7

What is the cell wall?
It is an extracellular structure of plant cells that distinguishes them from animal cells. The wall protects the plant cell, maintains its shape, and prevents excessive uptake of water.

On the level of the whole plant, the strong walls of specialized cells hold the plant up against the force of gravity. Prokaryotes, fungi and some protists also have cell walls.
What are some characteristics of plant cell walls?
Plant cell walls are much thicker than the plasma membrane, ranging from 0.1 micrometers to several micrometers. The exact chemical composition of the wall varies from species to species and even from one cell type to another in the same plant, but the basic design of the wall is consistent.

Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose synthase and secreted to the extracellular space, where they become embedded in a matrix of other polysaccharides and proteins. This combination of materials, strong fibers in a "ground substance" (matrix), is the same basic architectural design found in steel-reinforced concrete and in fiberglass (Simply amazing, I wonder if we learned to build buildings by looking at nature?).
What is a primary cell wall?
In plants, it is a relatively thin and flexible layer that surrounds the plasma membrane of a young cell. A young plant cell first secretes this. In actively growing cells, the cellulose fibrils are oriented at right angles to the direction of cell expansion.
What happens to the plant cell wall in actively growing plant cells?
In actively growing cells, the cellulose fibrils are oriented at right angles to the direction of cell expansion.

Observations by researchers strongly support the idea that microtubules in the cell cortex guide cellulose synthase as it synthesizes and deposits cellulose fibrils. By orienting cellulose deposition, microtubules thus affect the growth pattern of the cells.
Where is the middle lamella located and what is its function?
Between the primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins. (Pectin is used as a thickening agent in jams and jellies.)The middle lamella glues adjacent cells together.
How do plant cells strengthen their walls?
When the cell matures and stops growing, it strengthens its wall. Some plant cells do this simply by secreting hardening substances into the primary wall. Others add a secondary cell wall between the plasma membrane and the primary wall.
What are some characteristics of the secondary cell wall?
The secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support.

Wood for example, consists mainly of secondary walls. Plant cell walls are perforated by channels between adjacent cells called plasmodesmata.
What are plasmodesmata's?
They are open channels through the cell wall that connects the cytoplasm of adjacent plant cells, allowing water, small solutes, and some larger molecules to pass between the cells.
What is extracellular matrix (ECM)?
ECM is the meshwork surrounding animal cells, akin to the walls of plant cells. The main ingredients of ECM are glycoproteins and other carbohydrate containing molecules secreted by the cells. (Recall that glycoproteins are proteins with covalently bonded carbohydrate, usually short chains of sugars.)
What is the most abundant glycoprotein in the ECM of most animal cells?
The most abundant glycoprotein in the ECM of most animal cells is collagen.
What is collagen?
It is a glycoprotein in the ECM of animal cells that forms strong fibers, found extensively in connective tissue and bone; the most abundant protein in the animal kingdom.

In fact collagen accounts for about 40% of the total protein in the human body. The collagen fibers are embedded in a network woven out of proteoglycans secreted by cells.
What is a proteoglycan?
It is a large molecule consisting of a small core protein with many carbohydrate chains attached, found in the extracellular matrix of animal cells. A proteoglycan may consist of up to 95% carbohydrate.
What is a proteoglycan complex consisted of?
It is consisted of hundreds of proteoglycan molecules attached non covalently to a single long polysaccharide molecule.
What are integrins?
Integrins are membrane proteins with two subunits, that bind to the ECM on one side and to associated proteins attached to microfilaments on the other side. This linkage can transmit signals between the cell's external environment and its interior and can result in changes in cell behavior.

The name integrin is based on the word integrate.
What is fibronectin?
It is an extracellular glycoprotein secreted by animal cells that help them attach to the extracellular matrix.
What is current research on fibronectin, other ECM molecules, and integrins revealing about the influential role of the extracellular matrix in the lives of cells?
By communicating with a cell through integrins, the ECM can regulate a cell's behavior. For example, some cells in a developing embryo migrate along specific pathways by matching the orientation of their microfilaments to the "grain" of fibers in the ECM.

Researchers have also learned that the ECM around a cell can influence the activity of genes in the nucleus (Awesome).

Information about the ECM probably reaches the nucleus by a combination of mechanical and chemical signaling pathways. Mechanical signaling involves fibronectin, integrins, and microfilaments of the cytoskeletin. Changes in the cytoskeleton may in turn trigger chemical signaling pathways inside the cell, leading to changes in the set of proteins being made by the cell and therefore changes in the cell's function.
How are cells usually organized in animals or plants?
Cells in animals and plants are organized into tissues, organs, and organ systems. Neighboring cells often adhere, interact, and communicate via sites of direct physical contact.
What are the functions of the plasmodesmata?
Cytosol passes through the plasmodesmata and joins the internal chemical environment of adjacent cells. These connections unify most of the plant into one living continuum. The plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous.

Water and small solutes can pass freely from cell to cell, and recent experiments have shown that in some circumstances, certain proteins and RNA molecules can also do this. The macromolecules transported to neighboring cells appear to reach the plasmodesmata by moving along fibers of the cytoskeleton. (Analogous to traveling to the border of states and going through the border control - How interesting).
What are the three main types of cell junctions in animals?
The three main types are "tight junctions", "desmosomes", and "gap junctions". (Gap junctions are most like the plasmodesmata of plants, although gap junction pores are not lined with membrane.) All three types of cell junctions are especially common in epithelial tissue, which lines the external and internal surfaces of the body.
What role do tight junctions play in animal cells?
At tight junctions, the plasma membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continuous seals around the cells, tight junctions prevent the leakage of extracellular fluid across a layer of epithelial cells. For example, tight junctions between skin cells make us watertight by preventing leakage between cells in our sweat glands.
What role do desmosomes play in animal cells?
Desmosomes (also called anchoring junctions) function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm.

Desmosomes attach muscle cells to each other in a muscle. Some "muscle tears" involve the rupture of desmosomes.
What role do gap junctions play in animal cells?
Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to adjacent cell and in this way are similar in their function to the plasmodesmata in plants.

Gap junctions consist of membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissue.