© 2011 Pearson Education, Inc. Chapter 8 – Cell-cell interactions© 2011 Pearson Education, Inc. Review of Plasma Membrane Structure and Function • The plasma membrane is made up of a phospholipid bilayer with many interspersed proteins. – Proteins may be integral or peripheral. • The primary function of the plasma membrane is to create an environment inside the cell that is different from conditions outside. – The selective permeability of the membrane controls the flow of materials into and out of the cell.© 2011 Pearson Education, Inc. The Structure and Function of an Extracellular Layer Structure • Most cells possess a protective layer or wall that forms just beyond the membrane. This layer generally consists of a “fiber composite” – a cross-linked network of long filaments surrounded by a stiff ground substance. Function • The rods or filaments protect against tension, and the ground substance protects against compression.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. The Primary Plant Cell Wall • When new plant cells form, they secrete a fiber composite called a primary cell wall. – This wall is composed of long strands of cellulose bundled into microfibrils that form a crisscrossed network. – The network is filled with hydrophilic gelatinous polysaccharides such as pectin, which keep the cell wall moist. • The primary cell wall defines the shape of the plant cell and counteracts the turgor pressure it experiences.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. The Secondary Plant Cell Wall • Some cells secrete a secondary cell wall inside the primary cell wall. – Secondary cell wall structure correlates with the specific cell’s function. • In cells that form wood, the secondary cell wall contains lignin.© 2011 Pearson Education, Inc. The Extracellular Matrix in Animals • Most animal cells secrete a fiber composite called the extracellular matrix (ECM). Like the extracellular materials found in other organisms, one of the ECM’s most important functions is structural support. • The amount and composition of the ECM vary depending upon the cell type.© 2011 Pearson Education, Inc. Extracellular Matrix Structure and Function • The ECM consists of a ground substance formed of gelatinous polysaccharide and a network of protein fibers. – The most common ECM protein fiber is collagen, which is more elastic than cellulose and forms a flexible extracellular layer. • In addition to structural support, the ECM also helps cells stick together, and forms protein-protein attachments that link the ECM directly to the cell’s cytoskeleton.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. The ECM and Cytoskeleton Are Directly Linked • The ECM is strengthened by connections to transmembrane proteins. • Actin protein filaments in the cytoskeleton bind to transmembrane integrin proteins. Integrins bind to ECM proteins such as fibronectins, which then bind to collagen. • Direct linkage between the cytoskeleton and ECM keeps individual cells in place and helps adjacent cells adhere to each other. – Breakdown can lead to metastasis of cancerous cells.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. How Do Adjacent Cells Connect and Communicate? • Unicellular organisms do not usually connect to one another; physical connections between cells are the basis of multicellularity. • Cells of multicellular organisms adhere to one another and have specific, distinct structures and functions. – Groups of similar cells performing similar functions comprise tissues.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. Cell-Cell Attachments • The structures that hold cells together vary among multicellular organisms.© 2011 Pearson Education, Inc. Connections between Plant Cells • The extracellular space between adjacent plant cells comprises three layers. • Plant cells are glued together by the middle lamella, which is continuous with the adjacent plant cells’ primary cell walls. – The middle lamella is comprised of gelatinous pectins.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. Connections between Animal Cells • A middle-lamella-like layer, made of gelatinous polysaccharides, exists between cells in many animal tissues. – The polysaccharide glue may be reinforced by cable-like proteins that span the ECM to connect adjacent cells. • Epithelial tissue is composed of sheets of cells that cover organs and line body cavities. – Many types of structures connect neighboring epithelial cells, including tight junctions and desmosomes.© 2011 Pearson Education, Inc. Tight Junctions • Tight junctions are composed of specialized proteins in the plasma membranes of adjacent animal cells. – These proteins line up and bind to each other, stitching the two cells together to form a watertight seal between the two plasma membranes. • Tight junctions are usually found between cells in tissues that form a barrier, such as the tissue lining the stomach or bladder. • Tight junctions are dynamic and variable.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. Desmosomes • Desmosomes are made of proteins that link the cytoskeletons of adjacent cells. • Desmosomes are common in epithelial and muscle tissue. • These proteins bind to each other and to the proteins that anchor cytoskeletal intermediate filaments.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc. Selective Adhesion • Experiments using adult sponges demonstrated selective adhesion, in which dissociated cells could aggregate and adhere to cells of the same tissue type, eventually re-forming functional adult sponges.© 2011 Pearson Education, Inc. The Molecular Basis of Selective Adhesion • Animal cells attach to each other selectively because there are several classes of cell adhesion proteins; each major cell type has its own cell adhesion proteins. – These cell-cell connections are also species and tissue specific. • Cadherins are the adhesion proteins in desmosomes.© 2011 Pearson Education, Inc. Cell Communication via Cell-Cell Gaps • Direct connections between cells in the same tissue allow cells to communicate and work together in a coordinated fashion. • Plant cells are connected by plasmodesmata, gaps in the cell wall where the plasma membranes, cytoplasm, and smooth ER of two cells connect. • In most animal
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