Chapter 8 – Cell-Cell InteractionsI. The cell surfacea. The structure and function of an extracellular layeri. All extracellular layers are made of fiber composites- the layers consist of cross-linked network of long filaments embedded in a ground substance- can withstand tension and compression.b. The cell wall of plantsi. Primary cell walls are made up of fibrous cellulose (carbohydrate) with gelatinous Pectin.Some have a secondary cell wall.c. Extracellular Matrix ( ECM) in animals (NO cell wall in animal cells)i. The fibrous component is mostly collagen (a protein), a cable like molecule that is made up of 3 intertwined polypeptide chains surrounded by gelatinous polysaccharides. Cells attach to ECM via transmembrane proteins (eg: Protein Integrin). ii. ECM also helps cells stick together, and forms protein-protein attachments that link the ECM directly to the cell’s cytoskeleton.II. How do adjacent cells connect and communicate? a. Multicellularity- Cell-Cell attachments in Eukaryotes [mechanical(structural) role]i. Plant cells are held together by a middle lamella that is made up of gelatinous Pectins.ii. In animal cells: cytoskeleton-ECM connections hold cells together.iii. Tight junctions are composed of specialized proteins in the plasma membranes of adjacent animal cells. These proteins line up and bind toeach other, stitching the two cells together to form a watertight seal between the two plasma membranes oftwo animal cells (“seal” tissue).iv. Desmosomes are rivets that link the cytoskeletons ofadjacent animal cells. They contain selective adhesionproteins, Cadherins (“hold” cells together).v. Selective adhesion – Expt with the sponges (cells adhereto cells of the same tissue type) and the discovery ofcadherins [cell adhesion proteins] using antibodies to block the interactions?b. Cells communicate via cell-cell gaps: [communication role]i. Plasmodesmata between adjacent plant cells ii. Gap junctions are protein-lined holes between some animal cells  Amino acids, Nucleotides and ions can pass through but the organelles cannot (example: ribosomes,Lysosomes cannot pass through the gap junctions).III. How do distant cells communicate?a. Cell-cell signaling in multicellular organismsi. Hormones (chemical signals) are released from one tissue and travel through the extracellular matrix or the blood stream to distant cellsii. Hormones have a variety of structures and functions in both plants and animals.b. Signal reception: Need a hormone specific receptor (a glycoprotein). Binding is very specific, reversible and favored. i. Lipid soluble (hydrophobic) signal  can diffuse through the lipid membrane  cytoplasmic receptor. ii. Water-soluble (hydrophilic) signal  integral membraneprotein receptor (on the cell surface).c. Signal Receptor: Cytoplasmic or membrane receptor depending on the signali. Receptors are restricted to particular cell types. ii. Receptors are dynamic, the # of receptors can vary [classified by location]iii. They will change conformation [shape] upon signal bindingd. Signal Processing:i. Steroid hormones enter the cell and bind to cytoplasmic receptors  translocate to the nucleus through nuclear pores to change gene expression  No intermediate steps. (Not all intercellular signals require transduction  a lipid-soluble signal would be processed without transduction) ii. Signal Transduction pathways are created when water-soluble hormones bind to cell surface receptors and convert an intercellular signal to an intracellular signal.iii. Three types: Ion Channel, G-protein and Enzyme-Linkediv. The intracellular signal can be amplified because many of the activated molecules are enzymes, which can activate many more molecules down the cascade.v. Signal Transduction via G-proteins: G proteins are intracellular peripheral membrane proteins that are closely associated with transmembrane signal receptors.- G proteins are associated with the cell membrane and integrate the signal from the cell surface to second messengers inside the cell. Fig 8.151. Hormone binds to the membrane receptor, which changes shape and activates G protein.2. G protein exchanges GDP for GTP and splitsinto two parts.3. One part of the G protein activates a membrane enzyme, which catalyzes the production of a second messenger. Examples: cAMP, Ca++, NO Epinephrine = Adrenalin (the “fight or flight” hormone) uses G protein receptor and cAMP second messengervi. Signal Transduction via enzyme-linked receptors: Fig 8.161. Hormone binds to a receptor (it is a protein Kinase  phosphorylation){Protein Kinases catalyze transfer of phosphate from ATP to a specific protein or molecule}2. The complex changes shape  phosphorylation of the receptor  phosphorylation cascade (amplification) Example: Insulinvii. Ion channel receptors : Some ion channel proteins, acting as “gates,” are signal receptors. The signal to open or close the channel can be chemical, light, sound, pressure, or voltage. An example of a gated ion channel is the acetylcholine receptor [acetylcholine is aneurotransmitter]. Venom from Black Widow Spider causes an explosive release of acetylcholine. Effect?  muscles unable to relaxe. Signal Response: (i) Change in gene expression (ii) Changes in protein functionf. Signal deactivation:IV. Cross talk: signal transduction pathways form a network and allow cells to respond to extracellular signals in an integrated way. Cross talk help regulate the flow of information.V. Quorum sensing in Bacteria: In bacteria, cell-cell communication is called quorum sensing. The response can range from bioluminescence to attack of multiple hosts by secreting proteins, to secretion of biofilms  in closely related