Cell Membranes: Structure and Function PDF

Cell membranes are crucial to the life of the cell, especially the plasma membrane which have various functions: Barrier - maintains the essential differences between the cytosol and the extracellular environment; Protective - encloses the cell, defines its boundaries, determines shape of cells; C...

Cell membranes are crucial to the life of the cell, especially the plasma membrane which have various functions: Barrier - maintains the essential differences between the cytosol and the extracellular environment; Protective - encloses the cell, defines its boundaries, determines shape of cells; Communicative -- controls the interactions between cells; Transport -- is responsible for the selective transport of molecules. [Inside] the eukaryotic cells membranes also define the internal compartments including the nucleus and cytoplasmic organelles for example: Endoplasmic reticulum, Golgi apparatus, Mitochondria, Lysosomomes and others. These membrane maintains the characteristic differences between the contents of each organelle and the cytosol. Remember: Only two organelles are not membraneous: Ribosomes and Centrioles. Cells structure and function are critically dependent on their membrane organization. **Membrane structure** The fluid-mosaic model of membrane structure is widely accepted at this time. This model was introduced by Singer and Nickolson and proposes that the membrane is a bilayer of phospholipids, having a consistency of light oil in which protein molecules are either partially or wholly embedded. The proteins are scattered throughout the membrane, forming a mosaic pattern. Carbohydrates are also found as a components of glycolipids, glycoproteins and proteoglicans: Some oligosaccharide chains attach to outer monolayer lipid molecules, some of them bind with proteins exposed on the cell surface or with proteins embedded in membrane bilayer. A layer of these carbohydrates on the surface of the plasma membrane form cell coat called as a glycocalyx. It plays a role in cell-cell adhesion and in regulating the exchange of materials between a cell and its environment. The fundamental building blocks of all cell membranes are lipids, Major classes of membrane lipid molecules are phospholipids (glycerophosphates), sphingolipids (sphingomyelin), glycolipids (cerebroside and gangliosides) and cholesterol. [Phospholipids] are amphipathic molecules, consisting of two hydrophobic fatty acid chains linked to a phosphate-containing hydrophilic head group. Because their fatty acid tails are poorly soluble in water, phospholipids spontaneously form bilayers in aqueous solutions, with the hydrophobic tails buried in the interior of the membrane and the hydrophilic (polar) head groups exposed on both sides, in contact with water. Such phospholipid bilayers form a stable barrier between two aqueous compartments and represent the basic structure of all biological membranes. Cholesterol has both a hydrophilic and hydrophobic end. Cholesterol inserts into the bilayer with its polar hydroxyl group close to the [hydrophilic](http://www.ncbi.nlm.nih.gov/books/n/cooper/A2886/def-item/A3121/) head groups of the [phospholipids](http://www.ncbi.nlm.nih.gov/books/n/cooper/A2886/def-item/A3250/). The presence of cholesterol makes membrane more impermeable to most biological molecules. Glycolipids are like phospholipids except that the hydrophilic head is made up of a variety of simple sugars joined to form a straight or branching carbohydrate chain. This chain is always directed externally of the cell. Investigations suggest that glycolipids might mark a cell as belonging to a particular individual, accounting for such characteristics as specific blood group and why a patient's system sometimes rejects an organ transplant. The glycolipids also regulate the action of plasma membrane proteins involved in the growth of the cell, and in this respect they may have a role in the occurrence of cancer. [Lipids constitute] approximately 50% of the mass of most cell membranes, although this proportion varies depending on the type of membrane. Plasma membranes, for example, are approximately 50% lipid and 50% protein. The inner membrane of mitochondria, on the other hand, contains an unusually high fraction (about 75%) of protein, reflecting the abundance of protein complexes involved in electron transport and oxidative phosphorylation. [The lipid] composition of different cell membranes also varies. The plasma membrane of *E. coli* consists predominantly of phosphatidylethanolamine, which constitutes 80% of total lipid. Mammalian plasma membranes are more complex, containing four major phospholipids---phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin---which together constitute 50 to 60% of total membrane lipid. In addition to the phospholipids, the plasma membranes of animal cells contain glycolipids and cholesterol, which generally correspond to about 40% of the total lipid molecules. [The] [lipid](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5399/) compositions of the two monolayers of the [lipid bilayer](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5400/) in many membranes are strikingly different. In the human [red blood cell](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5726/) [membrane](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5438/), for example -phosphatidylcholine and sphingomyelin are in the outer monolayer, whereas phosphatidylethanolamine and phosphatidylserine are in the inner monolayer. Lipid asymmetry is functionally important for signal response processes. [Animals exploit] the phospholipid asymmetry of their plasma membranes to distinguish between live and dead cells. When animal cells undergo programmed cell death, or apoptosis, phosphatidylserine, which is normally confined to the cytosolic monolayer of the plasma membrane lipid bilayer, rapidly translocates to the extracellular monolayer. The phosphatidylserine exposed on the cell surface serves as a signal to induce neighboring cells, such as macrophages, to phagocytose the dead cell and digest it. [Although the] [basic](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A4877/) structure of biological membranes is provided by the [lipid bilayer](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5400/), [membrane](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5438/) proteins perform most of the specific functions of membranes. It is the proteins, therefore, that give each type of membrane in the cell its characteristic functional properties. Accordingly, the amounts and types of proteins in a membrane are highly variable. In the myelin membrane, which serves mainly as electrical insulation for [nerve cell](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5520/) axons, less than 25% of the membrane mass is [protein](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5688/). Because [lipid](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5399/) molecules are small compared with [protein](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5688/) molecules, there are always many more lipid molecules than protein molecules in membranes---about 50 lipid molecules for each protein [molecule](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5486/) in a [membrane](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5438/) that is 50% protein by mass. [By contrast], in the membranes involved in ATP production (such as the internal membranes of mitochondria and chloroplasts), approximately 75% is protein. A typical [plasma membrane](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5642/) is somewhere in between, with protein accounting for about 50% of its mass. **[Membrane proteins]** are divided into two general classes, based on the nature of their association with the membrane. Integral membrane proteins are embedded directly within the lipid bilayer. Peripheral membrane protein are not inserted into the lipid bilayer but are associated with the membrane indirectly, generally by interactions with integral membrane proteins. Many integral membrane proteins(called transmembrane proteins) span the lipid bilayer, with portions exposed on both sides of the membrane. The membrane-spanning portions of these proteins are usually α-helical regions of 20 to 25 nonpolar amino acids. Like the phospholipids, transmembrane proteins are amphipathic molecules, with their hydrophilic portions exposed to the aqueous environment on both sides of the membrane. Some transmembrane proteins span the membrane only once; others have multiple membrane-spanning regions. Most transmembrane proteins of eukaryotic plasma membranes have been modified by the addition of carbohydrates, which are exposed on the surface of the cell and may participate in cell-cell interactions. [According] to their biological role, membrane proteins are: **[Channel] and p[umps], [receptors] and l[inker protein,]** [enzymes], [structural] [proteins]. **[Channel] and [Pumps]: allow the passage of small ions, molecules, and water across the plasma membrane in either direction, provide active and passive transport.** Many of molecules pass across membranes via the action of specific transmembrane proteins, which act as transporters. Such transport proteins determine the selective permeability of cell membranes and thus play a critical role in membrane function. Channel proteins form open pores through the membrane, allowing the free passage of any molecule of the appropriate size. Ion channels, for example, allow the passage of inorganic ions such as Na^+^, K^+^, Ca^2+^, and Cl^-^ across the plasma membrane In contrast to channel proteins carrier proteins selectively bind and transport specific small molecules, such as glucose. Rather than forming open channels, carrier proteins act like enzymes to facilitate the passage of specific molecules across membranes. **[Receptors] and [Linker protein]: allow recognition and localized binding of ligands in processes such as hormonal stimulation, coated-vesicle endocytosis, and antibody reactions; anchor the intracellular cytoskeleton to the extracellular matrix.** **[Enzymes]: ATP synthase is the major protein of the inner mitochondrial membrane, and digestive enzymes such as disaccharidases and dipeptidases are integral membrane proteins.** **[Structural proteins]: form junctions with neighboring cells and are building material for the membrane.** Cells membrane functional differences are result of various quantity composition of these proteins. Like phospolipids membrane proteins are also distributed asyimmetrically in bilayer. For instance, Inner monolayer of red blood cells are more abundant with membrane proteins than outer monolayer. The **cell cortex** is a specialized layer of cytoplasm on the inner face of the plasma membrane that functions as a mechanical support of the plasma membrane. In animal cells it is an actin-rich layer responsible for movements of the cell surface. In some animal cells, the protein spectrin may be present in the cortex. Spectrin helps to create a network by cross-linking actin filaments. The proportions of spectrin and actin vary in different cell types. Spectrin proteins and actin microfilaments are attached to transmembrane proteins by attachment proteins between them and the transmembrane proteins. The cell cortex is attached to the inner (cytosolic) face of the plasma membrane in cells where the spectrin proteins and actin microfilaments form a mesh-like structure much like a fishnet except that it can be broken and reformed. This breakage and reformation is referred to as \"dynamic instability.\" One more distinction between cell membranes is that they grow in different ways. Plasma membrane growth depends on insertion of membrane vesicles formed from endoplasmic reticulum. This process is very fast (during 1 hour the plasma membrane becomes 65 times enlarged). For mitochondrion, lipid and protein molecules synthesized in cytoplasm are transported independently towards the organelle membrane where it becomes inserted in membrane. In addition to plasma membrane, plant cells are surrounded by a cell wall, that varies in thickness, depending on the function of the cell. The plant cell wall is an elaborate extracellular matrix that encloses each cell in a plant. It was the thick cell walls of cork, visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. **[The walls of neighboring]** plant cells, cemented together to form the intact plant, are generally thicker, stronger, and, most important of all, more rigid than the extracellular matrix produced by animal cells. In evolving relatively rigid walls, which can be up to many micrometers thick, early plant cells forfeited the ability to crawl about and adopted a sedentary life-style that has persisted in all present-day plants. All plant cells have a primary cell wall whose main component is cellulose polymers united into thread like microfibrils and even larger fibrils. Pectin, a sticky substance which occurs in region called the middle lamella, keeps adjacent plant cells bound together. Some cells in woody plants have a secondary cell wall that forms inside the primary cell wall. The secondary wall has alternating layers of cellulose fibrils reinforced by the addition of lignin, a substance that adds strength. The most common additional polymer in secondary walls is **lignin**, a complex network of phenolic compounds found in the walls of the xylem vessels and fiber cells of woody tissues. The plant cell wall thus has a "skeletal" role in supporting the structure of the plant as a whole, a protective role as an enclosure for each cell individually, and a transport role, helping to form channels for the movement of fluid in the plant. When plant cells become specialized, they generally adopt a specific shape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified. All cell walls in plants have their origin in dividing cells, as the cell plate forms during cytokinesis, to create a new partition wall between the daughter cells. The new formed cells are generally small in comparison with their final size. To accommodate subsequent cell growth, their walls, called **primary cell walls**, are thin and extensible, although tough. Once growth stops, the wall no longer needs to be extensible: sometimes the primary wall is retained without major modification, but, more commonly, a rigid, **secondary cell wall** is produced by depositing new layers inside the old ones. These may either have a composition similar to that of the primary wall or be markedly different. **[Most bacteria]** secrete a covering for themselves which we call a **cell wall**, However, bacterial cell walls are a totally different thing than the cell walls we talk about plants having. Bacterial cell walls do not contain cellulose like plant cell walls do. Bacterial cell walls are made mostly of a chemical called **peptidoglycan** (made of polypeptides bonded to modified sugars), but the amount and location of the peptidoglycan are different in the two possible types of cell walls, depending on the species of bacterium. One of the most widely used methods of identifying bacteria is the gram stain, and the cell wall plays an important role in the identification process. In the gram stain method, bacteria are first stained with gentian violet iodine, followed by a solution of iodine in potassium iodide. The slides are then washed with ethyl alcohol, and if viewed under the microscope will either obtain a strong violet color, or be completely decolorized. The difference in color of the two types of bacteria is relative to the cell wall of each type. Bacteria that remain violet are classified as gram positive bacteria and bacteria from which the crystal-violate escapes when the decolorizer is added is defined as gram negative. The cell wall of the gram positive bacteria are relatively thick, and contain a dense layer of peptidoglycan. It is the thickness of the cell wall that traps the dark violet color and resists damage from the solvent. The gram positive cell wall also contains teichoic acids, which are polymers of glycerol or ribitol joined by phosphate groups. Amino acids, such as D-alanine are attached. Teichoic acid is covalently linked to muramic acid and links various layers of the peptidoglycan mesh together. Gram negative bacteria, on the other hand, have a much thinner cell wall. The wall is high in lipid content and low in peptidiglycan. The crystal-violate escapes from cell because of the lack of peptidglycan. Gram negative cell walls are more complicated than gram positive because there are two separate areas with an additional membrane besides the cellular membrane. The outer membrane of gram negative bacteria is composed of a high concentration of lipids, polysaccharides and proteins. Outside of the central membrane is a open area called the periplasmic space. Beyond this is a thin layer of peptidoglycan. Finally, external to the peptidoglycan is an additional membrane called the outer membrane. **Cell-Cell Interactions** Direct interactions between cells, as well as between cells and the extracellular matrix, are critical to the development and function of multicellular organisms. Some cell-cell interactions are transient, such as the interactions between cells of the immune system and the interactions that direct white blood cells to sites of tissue inflammation. In other cases, stable cell-cell junctions play a key role in the organization of cells in tissues. For example, several different types of stable cell-cell junctions are critical to the maintenance and function of epithelial cell sheets. Plant cells also associate with their neighbors not only by interactions between their cell walls, but also by specialized junctions between their plasma membranes. Cell-cell adhesion is a selective process, such that cells adhere only to other cells of specific types. This selectivity was first demonstrated in classical studies of embryo development, which showed that cells from one tissue (e.g., liver) specifically adhere to cells of the same tissue rather than to cells of a different tissue (e.g., brain). Such selective cell-cell adhesion is mediated by transmembrane proteins called cell adhesion molecules (CAMs), which can be divided into four major groups: I -the **selectins**, II - the **integrins**, III - the **immunoglobulin (Ig) superfamily (**intercellular adhesion molecules (ICAMs) and nerve cell adhesion molecules (N-CAMs) belong to this group) and IV - the cadherins. Cell adhesion mediated by the selectins, integrins, and cadherins requires Ca^2+^ or Mg^2+^, so many adhesive interactions between cells are Ca^2+^- or Mg^2+^-dependent. There are two types of interactions between CAM protein molecules: **Heterophilic interaction**, in which an adhesion molecule on the surface of one cell recognizes a different molecule on the surface of another cell. **Homophilic interactions**, in which an adhesion molecule on the surface of one cell binds to the same molecule on the surface of another cell. Such homophilic binding leads to selective adhesion between cells of the same type. Selectins and ICMs occur on the surface of the endothelial cells lining blood vessels, where they help to tether passing leucocytes at sites of inflammation. N-CAMs are important in ensuring proper cell-cell contacts during development of the nervous system and muscle tissue. Cadherins are glycoproteins found abundantly in junctions between epithelial cells and also in desmosomes. The integrins bind cells to components of the extracellular matrix, such as collagens and laminins, and can form cell-matrix junctions, such as hemidesmosomes. [Specialized c]ell junctions occur at points of cell-cell and cell-matrix contact in all tissues, and they are particularly plentiful in epithelia. Cell junctions can be classified into three functional groups: 1.Occluding junctions (tight junctions belong to it) seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other. - The cell-cell interactions mediated by the selectins, integrins, and members of the Ig superfamily are **transient** adhesions in which the cytoskeletons of adjacent cells are not linked to one another. - **Stable** adhesion junctions involving the cytoskeletons of adjacent cells are instead mediated by the cadherins. All epithelia have at least one important function in common: they serve as selective permeability barriers, separating fluids on either side that have a different chemical composition. This function requires that the adjacent cells be sealed together by occluding junctions. [Tight junctions](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5870/) have this barrier role in vertebrates. When tight junctions are visualized by freeze-fracture electron microscopy, they seem to be composed of a branching network of *sealing strands* that completely encircles the apical end of each cell in the epithelial sheet Each [tight junction](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5870/) sealing strand is composed of a long row of transmembrane adhesion proteins embedded in each of the two interacting plasma membranes. The extracellular domains of these proteins join directly to one another to occlude the intercellular space. The major transmembrane proteins in a tight junction are the *claudins,* which are essential for tight junction formation and function and differ in different tight junctions A second major transmembrane protein in tight junctions is *occludin,* the function of which is uncertain. Claudins and occludins associate with intracellular peripheral membrane proteins called *ZO proteins* (a tight junction is also known as a *zonula occludens*), which anchor the strands to the actin cytoskeleton. 2.**Communicating junctions** (gap junctions and plasmodesmata in plants only) mediate the passage of chemical or electrical signals from one interacting cell to its partner. With the exception of a few terminally differentiated cells such as skeletal muscle cells and blood cells, most cells in animal tissues are in communication with their neighbors via gap junctions. Cell coupling via gap junctions also seems to be important in embryogenesis. In early vertebrate embryos, beginning with the late eight-cell stage in mouse embryos, most cells are electrically coupled to one another. Each gap junction appears in conventional electron micrographs as a patch where the membranes of two adjacent cells are separated by a uniform narrow gap of about 2--4 nm. The gap is spanned by channel-forming proteins *(connexins).* The channels they form *(connexons)* allow inorganic ions and other small water-soluble molecules to pass directly from the cytoplasm of one cell to the cytoplasm of the other, thereby coupling the cells both electrically and metabolically. **Connexins** are four-pass transmembrane proteins, six of which assemble to form a channel, a [connexon](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5022/). When the connexons in the plasma membranes of two cells in contact are aligned, they form a continuous [aqueous](http://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A4840/) channel that connects the two cell interiors. The connexons hold the interacting plasma membranes at a fixed distance apart---hence the gap. Gap-junction channels do not remain continuously open; instead, they flip between open and closed states. Moreover, the permeability of gap junctions is rapidly (within seconds) and reversibly reduced by experimental manipulations that decrease the cytosolic pH or increase the cytosolic concentration of free Ca^2+^ to very high levels. Thus, gap-junction channels are dynamic structures that can undergo a reversible conformational change that closes the channel in response to changes in the cell. large influx of Ca^2+^ into the damaged cell causes its gap-junction channels to close immediately, effectively isolating the cell and preventing the damage from spreading to other cells. Dye-injection experiments suggest a maximal functional pore size for the connecting channels of about 1.5 nm, implying that coupled cells share their small molecules (such as inorganic ions, sugars, amino acids, nucleotides, vitamins, and the intracellular mediators cyclic AMP and inositol trisphosphate) but not their macromolecules (proteins, nucleic acids, and polysaccharides). The tissues of a plant are organized on different principles from those of an animal. This is because plant cells are imprisoned within rigid *cell walls* composed of an extracellular matrix rich in cellulose and other polysacharides, The cell walls of adjacent cells are firmly cemented to those of their neighbors, which eliminates the need for anchoring junctions to hold the cells in place. But a need for direct cell-cell communication remains. Thus, plant cells have only one class of intercellular junctions, **plasmodesmata** (singular, plasmodesma). Like gap junctions, they directly connect the cytoplasms of adjacent cells. Every living cell in a higher plant is connected to its living neighbors by these structures, which form fine cytoplasmic channels through the intervening cell walls. As shown in Figure, the plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma, and the cytoplasm of the two cells is connected by a roughly cylindrical channel with a diameter of 20--40 nm. Thus, the cells of a plant can be viewed as forming a syncytium, in which many cell nuclei share a common cytoplasm. Running through the center of the channel in most plasmodesmata is a narrower cylindrical structure, the *desmotubule,* which is continuous with elements of the smooth endoplasmic reticulum in each of the connected cells. Between the outside of the desmotubule and the inner face of the cylindrical channel formed by plasma membrane is an annulus of cytosol through which small molecules can pass from cell to cell. In spite of the radical difference in structure between plasmodesmata and gap junctions, they seem to function in remarkably similar ways. Plasmo-desmata allow the passage of molecules with a molecular weight of less than about 800, which is similar to the molecular-weight cutoff for gap junctions. As with gap junctions, transport through plasmodesmata is regulated. 3.Anchoring junctions (cell-cell adherens and cell-matrix adhesions) mechanically attach cells (and their cytoskeletons) to their neighbors or to the extracellular matrix. The lipid bilayer is flimsy and cannot by itself transmit large forces from cell to cell or from cell to extracellular matrix. Anchoring junctions solve the problem by forming a strong membrane-spanning structure that is tethered inside the cell to the tension-bearing filaments of the cytoskeleton. Anchoring junctions are widely distributed in animal tissues and are most abundant in tissues that are subjected to severe mechanical stress, such as heart, muscle, and epidermis. The prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt (or *zonula adherens)* just below the tight junctions, encircling each of the interacting cells in the sheet. The adhesion belts are directly apposed in adjacent epithelial cells, with the interacting plasma membranes held together by the cadherins that serve here as transmembrane adhesion proteins. Within each cell, a contractile bundle of actin filaments lies adjacent to the adhesion belt, oriented parallel to the plasma membrane. The actin is attached to this membrane through a set of intracellular anchor proteins, including *catenins*, *vinculin*, and *α-actinin*, Desmosomes are buttonlike points of intercellular contact that rivet cells together. Inside the cell, they serve as anchoring sites for ropelike intermediate filaments, which form a structural framework of great tensile strength. Through desmosomes, the intermediate filaments of adjacent cells are linked into a net that extends throughout the many cells of a tissue. The particular type of intermediate filaments attached to the desmosomes depends on the cell type: they are *keratin filaments* in most epithelial cells, for example, and *desmin filaments* in heart muscle cells. Some anchoring junctions bind cells to the extracellular matrix rather than to other cells. The transmembrane adhesion proteins in these cell-matrix junctions are *integrins*---a large family of proteins distinct from the cadherins. **Focal adhesions** enable cells to get a hold on the extracellular matrix through integrins that link intracellularly to actin filaments. When cultured fibroblasts migrate on an artificial substratum coated with extracellular matrix molecules, they grip the substratum at focal adhesions, where bundles of actin filaments terminate. At all such adhesions, the extracellular domains of transmembrane integrin proteins bind to a protein component of the extracellular matrix, while their intracellular domains bind indirectly to bundles of actin filaments via the intracellular anchor proteins talin, α-actinin, filamin, and vinculin. Hemidesmosomes, or half-desmosomes, resemble desmosomes morphologically and in connecting to intermediate filaments, and, like desmosomes, they act as rivets to distribute tensile or shearing forces through an epithelium. Instead of joining adjacent epithelial cells, however, hemidesmosomes connect the basal surface of an epithelial cell to the underlying basal lamina. The extracellular domains of the integrins that mediate the adhesion bind to a *laminin* protein in the basal lamina, while an intracellular domain binds via an anchor protein *(plectin)* to keratin intermediate filaments. Whereas the keratin filaments associated with desmosomes make lateral attachments to the desmosomal plaques, many keratin filaments associated with hemidesmosomes have their ends buried in the plaque.