Extracellular components and connections between cells

Plasma membrane Regulates what goes in and out of the cell… Outside of cell, there is the ECM – Extracellular matrix. Glycoproteins Collagen – forms strong fibers outside cells, 40% of protein in body Integrins – cell surface receptor proteins, bind to associated proteins attached to microfilaments of the cytoskeleton. Transmit signals between the ECM and the cytoskeleton, integrate changes inside and outside of cell.

Figure 4.26a

Cell junctions in animal cells Especially common in epithelial cells Tight junctions: plasma membranes tightly pressed against each other, prevent leakage of extracellular fluid across a layer of epithelial cells. Ex. Between skin cells – watertight. Desmososmes – (anchoring junctions) – like rivets, fastening to cells into strong sheets. Attach muscle cells to each other Gap junctions (like plasmodesmata in plant cells) – Communicating junctions, cytoplasmic channels from one cell to an adjacent cell. Membrane protein surrounding a pore that ions, sugars and AA pass. Ec. Heart muscle, animal embryos.

Figure  m Intermediate filaments TEM 0.5  m TEM 0.1  m TEM Tight junction Tight junction Ions or small molecules Extracellular matrix Gap junction Desmosome Space between cells Plasma membranes of adjacent cells Tight junctions prevent fluid from moving across a layer of cells

Cell wall of plants Protection, maintains shape, prevents excessive uptake of water Thicker than cell membrane Contains cellulose Primary cell wall – in young plants Middle lamella – thin layer between primary walls of cells, contain pectins that are sticky and glues adjacent cells together. When mature, the cell may reinforce the primary cell wall or add a secondary cell wall between the plasma membrane and primary wall. Ex. Wood consists mainly of secondary walls

Figure 4.25 Secondary cell wall Central vacuole Primary cell wall 1  m Middle lamella Plasmodesmata Cytosol Plasma membrane Plant cell walls

Cell Junctions in plant cells Plasmodesmata – in plant cells Non-living cell walls are perforated with membrane-lined channels filled with cytosol. Make cells continuous with each other. Water and small solutes pass freely from cell to cell, proteins and RNA may as well.