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CONCEPT: CELL-CELL ADHESION Cells must be able to bind and interact with nearby cells in order to have functional and strong tissues Cells can in two main ways - Homophilic interactions occurs when two cells use the same molecule type to interact (common) - Heterophilic interactions occurs when two cells use different molecules to interact Homophilic Heterophilic Cell adhesion molecules (CAMs) are the main proteins used to adjacent cells Cadherins are glycoproteins found in the plasma membrane that connect cells - Require calcium - There are three classical types: Endothelial (E), Placenta (P), and Neural (N) - But there are many more non-classical types Cadherins are extremely important for the epithelial-mesenchymal transition which occurs during development - Leads to creation of mesodermal tissue (blood, muscle, and bone) Page 2
There are other types of CAMs too - Lectins promote cell-cell adhesion by binding to sugars on the plasma membrane - Selectins are glycoproteins that mediate interactions between leukocytes and endothelial cells - Important during inflammation - Has three classes: Endothelial (E), Platelet (P), and Leukocyte (L) - But there are many more non-classical types Page 3
PRACTICE: 1. Which of the following is not a type of cell adhesion molecules? a. Lectins b. Selectins c. Mectins d. Cadherins 2. Which of the following CAMs work by binding to sugars on the plasma membrane in order to promote cell-cell adhesion? a. Lectins b. Selectins c. Mectins d. Cadherins Page 4
CONCEPT: CELL-CELL JUNCTIONS Cell junctions connect together Adherens (Anchoring) junctions are cell-cell adhesions that anchor cells together - Cadherins are proteins that tether to actin filaments Page 5
Desmosomes bind epithelial cells to each other - Cadherins that tether to intermediate filaments - Hemidesmosomes link epitheial cells to the basal lamina Page 6
Tight (occluding) junctions form a seal between cells in order to prevent leakage between the cells - Claudins and occludins are proteins that make up tight junctions - Also forms polarized regions in cells (apical/basal surfaces) Page 7
Gap Junctions are regions of the plasma membranes that align in parallel to connect - Connexons are cylindrical proteins that join the adjacent plasma membranes - Allow water and inorganic ions to cross between the cytosols Plasmodesmata connect plant cells together Page 8
PRACTICE: 1. Which of the following is not a cell cell junction? a. Adherens junctions b. Tight junctions c. Gap junctions d. Loose Junctions 2. Which of the following cell-cell junctions forms a seal between cells that prevents leakage? a. Adherens junctions b. Tight junctions c. Gap junctions d. Plasmodesmata Page 9
3. Connexons are proteins used in which of the following junctions? a. Adherens junctions b. Tight junctions c. Gap junctions d. Plasmodesmata Page 10
CONCEPT: EXTRACELLULAR MATRIX The extracellular matrix (ECM) is a collection of fibers and protein which provide support to cells and tissues Collagen fibrils are long bundles of which make up the ECM - Forms connective tissues - Fibroblasts are cells that secrete collagen and other extracellular matrix components Elastin is a protein that provides the ECM with elasticity Fibronectins are cellular glycoproteins that connect cells to collagen in the ECM GAGs are carbohydrates that can be bound to proteins in the ECM - Hyaluron is a simple GAG that is a space filler in the ECM - Provides the ECM with a gel consistency Integrins are cellular transmembrane proteins that help attach the ECM to the cell s cytoskeleton Integrins are made up of two alpha and two beta Integrins have active and inactive conformations Anchorage dependent growth, is when cells depend on attachment for cell growth, proliferation, and survival - Integrins control this process Page 11
Integrins help control signaling between the extracellular and intracellular environment - MAPK pathway Active Integrin Inactive Integrin PRACTICE: 1. Which of the following proteins are not found in the ECM a. Fibronetics b. Elastin c. Collagen d. Bip Page 12
2. Which of the following ECM proteins connect collagen to the ECM? a. Fibronetics b. Elastin c. Collagen d. GAGs Page 13
CONCEPT: BASAL LAMINA The basal lamina acts as a thin, but strong extracellular matrix under certain cell types Mainly found epithelial cells - But can also be found under certain muscle, fat, and schwaan (surround a neuron) cells The basal lamina is about 40-120mm thick Basal lamina The basal lamina is composed of two Laminin is an organizer of various fibrous proteins making up the basal lamina Type IV Collagen provides tensile strength to to the basal lamina - Links with laminin, and other proteins to form a rope-like helix Laminin protein Page 14
PRACTICE: 1. The basal lamina supports mainly which cell type? a. Epithelial cells b. Endothelial cells c. Connective tissue 2. Type IV collagen is an important protein in the basal lamina because it provides what? a. Organization to the proteins in the basal lamina b. Connections between multiple proteins in the basal lamina c. Tensile strength to the basal lamina Page 15
CONCEPT: PLANT TISSUE Plant cells use as the primary means of supportive cell matrix Primary cell wall is the newly formed cell wall - These are thin and flexible, but still fairly tough - Contains pectins which are branched polysaccharides that can crosslink with Ca 2+ to form a semisolid gel Secondary cell walls form later, after the cell has matured a bit - These walls are thicker, and less flexible - Contain ligin which is a polymer of alcohols that acts as a waterproof filler (Ex: woody tissue) Cell walls contain cellulose microfibrils which are long cellulose (with glucose) - Provide the cell wall with great tensile strength (acts as a skeleton) - Hemicellulose refers to polysaccharide chains with different sugar subunits than cellulose Page 16
The cell wall provides certain to plant cells The cell wall prevents cell growth - Cellulose microfibrils limit growth The cell wall provides a supportive matrix to maintain cell shape and osmotic pressure - Turgor pressure is due to differing concentrations of water on either side of the cell Plasmodesmata are connections between the cytoplasms of adjacent plant cells (even go through cell walls) Page 17
PRACTICE: 1. The secondary cell wall is composed of: a. Pectins b. Ligin c. Integrins d. GAGs 2. The primary cell what is composed of what? a. Pectins b. Ligin c. Integrins d. GAGs Page 18
3. Plasmodesmata are important to plant cells because they do what? a. Connect adjacent plant cells together b. Provide tensile strength to plant cells c. Control turgor pressure of plant cells d. Prevent plant cell growth Page 19