Learning Objectives By the end of this section, you will be able to: Describe the extracellular matrix List examples of the ways that plant cells and animal cells communicate with adjacent cells Summarize the roles of tight junctions, desmosomes, gap junctions, and plasmodesmata. Extracellular Matrix of Animal Cells Most animal cells release materials into the extracellular space. The extracellular matrix consists of a network of proteins and carbohydrates.
Intercellular Junctions Cells can also communicate with each other via direct contact, referred to as intercellular junctions. Plasmodesmata In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell [link] b.
A plasmodesma is a channel between the cell walls of two adjacent plant cells. Plasmodesmata allow materials to pass from the cytoplasm of one plant cell to the cytoplasm of an adjacent cell. Tight Junctions A tight junction is a watertight seal between two adjacent animal cells [link]. Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction adherence. Desmosomes Also found only in animal cells are desmosomes , which act like spot welds between adjacent epithelial cells [link].
A desmosome forms a very strong spot weld between cells. It is created by the linkage of cadherins and intermediate filaments. Gap Junctions Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate [link]. A gap junction is a protein-lined pore that allows water and small molecules to pass between adjacent animal cells.
Link to Learning. You already know that a group of similar cells working together is called a tissue. As you might expect, if cells are to work together, they must communicate with each other, just as you need to communicate with others if you work on a group project. Most animal cells release materials into the extracellular space. The primary components of these materials are proteins, and the most abundant protein is collagen.
Collagen fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other.
How can this happen? Cells have protein receptors on the extracellular surfaces of their plasma membranes. When a molecule within the matrix binds to the receptor, it changes the molecular structure of the receptor. The receptor, in turn, changes the conformation of the microfilaments positioned just inside the plasma membrane. Given the important role of gap junctions in animal cells, you might wonder if they also exist in plant cells. However, gap junctions are absent in plant cells.
Plant cells contain channels called plasmodesmata. Edward Tangl first discovered these in Animal cells do not harbor any plasmodesmata per se, but scientists have discovered a similar channel that is not a gap junction.
There are a number of structural differences between plasmodesmata and gap junctions. So what are plasmodesmata plasmodesma if singular? Plasmodesmata are tiny channels that bridge plant cells together. However, in plant cells, plasmodesmata must cross primary and secondary cell walls to allow signals and materials across. Animal cells do not possess cell walls.
So plants need a way to get through cell walls, since plant plasma membranes do not directly contact each other in plant cells. Plasmodesmata are generally cylindrical and lined with plasma membrane. They possess desmotubules, narrow tubes made from smooth endoplasmic reticulum. The newly-formed primary plasmodesmata tend to cluster together. Secondary plasmodesmata develop as cells expand. Plasmodesmata allow the passage of specific molecules between plant cells.
Without plasmodesmata, necessary materials could not pass between the rigid cell walls of plants. Important materials that pass through plasmodesmata include ions, nutrients and sugars, signaling molecules for immune response, occasionally larger molecules like proteins and some RNAs.
They also generally serve as a kind of filter to prevent much larger molecules and pathogens. However, invaders can force the plasmodesmata to open up and override this defense mechanism of plants.
This change in the permeability of plasmodesmata is just one example of their adaptability. Plasmodesmata can be regulated. One prominent regulatory polymer is callose. How can this happen? When the cells lining a blood vessel are damaged, they display a protein receptor, which we call tissue factor. Cells can also communicate with each other via direct contact, or intercellular junctions.
There are differences in the ways that plant and animal and fungal cells communicate. Plasmodesmata are junctions between plant cells; whereas, animal cell contacts include tight junctions, gap junctions, and desmosomes. In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because the cell wall that surrounds each cell separates them Figure.
How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues xylem and phloem primarily. A tight junction is a watertight seal between two adjacent animal cells Figure.
Proteins predominantly two proteins called claudins and occludins tightly hold the cells against each other. This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space.
Also only in animal cells are desmosomes , which act like spot welds between adjacent epithelial cells Figure. Cadherins, short proteins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins connect two adjacent cells and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles. Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for transporting ions, nutrients, and other substances that enable cells to communicate Figure.
Structurally, however, gap junctions and plasmodesmata differ.
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