tissues - highly specialized cell groups found only in multicellular organisms 

• each tissue cell performs only the functions of that tissue
• cells gain their identities by controlling the expression of the genes
• only specific sets of genes are turned on
• tissue-specific identity markers - mark cell surfaces as a particular type
  • cells of the same tissue type form connections when they recognize each other
• glycolipids - lipids w/ carbohydrate heads
  • accounts for majority of tissue-specific surface markers
  • responsible for differences between blood types
• MHC proteins - distinguishes cells of the organism from foreign cells
  • single-pass proteins anchored in the plasma membrane
  • immune system cells destroy cells w/o the correct identity markers

intercellular adhesion - cells usually in physical contact w/ each other at all times 

• cell junctions - permanent/long-lasting connections between cells
• tissue functions depend on how the cells connect
• 3 main types of connections - tight junctions, anchoring junctions, communicating junctions

tight junctions - aka occluding junctions  

• connect plasma membranes of adjacent cells in a sheet
• prevent small molecules from leaking between cells
• digestive tracts only 1 cell thick, but still prevents food from passing through due to tight junctions
• prevents certain proteins from drifting from 1 side to another
• food enters the blood stream by going through the transport proteins

anchoring junctions - mechanically attach the cytoskeletons 

• most common in muscles and skin
• desmosomes - connect cytoskeletons of adjacent cells
• hemidesmosomes - connect epithelial cells to basement membrane
• connections between proteins not tethered to intermediate filaments not as strong as connections between tethered proteins
• cadherins - mostly single-pass transmembrane glycoproteins
  • forms the link in the anchoring junction
  • can also connect actin filaments of adjacent cells
  • may have a role in determining where migrating cells go during development
• adherens junctions - connects actin filaments of neighboring cells or to extracellular matrix
  • integrins - proteins that bind to a protein part of extracellular matrix

communicating junctions - direct connections between adjacent cells used for communication 

• chemical/electrical signals pass directly from 1 cell to another
• some small molecules/ions can also pass through
• gap junctions - communicating junctions in animals
  • made up of connexons (complexes of 6 transmembrane proteins arranged in a circle)
  • forms when connexons line up perfectly
  • small enough to prevent large molecules like proteins from passing through
  • holds plasma membranes of adjacent cells about 4 nm apart
  • can open/close in response to environment
• plasmodesmata - communicating junctions in plants
  • occurs at holes/gaps in the cell wall
  • more complex than gap functions
  • lined w/ plasma membrane
  • contains a central tubule that connects the ER of 2 cells