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
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