cellular membranes and signalling

functions of membranes (3)

1. selective barrier for molecules entering and exiting cells

2. detecting chemical signals from other cells

3. anchoring cells to adjacent cells and extracellular matrix of connective-tissue proteins

cholesterol associates with plasma membrane phospholipids and proteins to… (3)

• form organised clusters that work together

• pinch off portions of the plasma membrane to form vesicles

• that deliver their contents to various intracellular organelles

membranes are ____ thick

6 - 10nm

integral membrane proteins (4)

• closely associated with membrane lipids, cannot be extracted without disrupting membrane structure

• amphipathic: polar amino acid side chains on one side and non polar side chains on other

• polar amino acid side chains associate with phospholipid polar regions, nonpolar regions associated with core fatty acid chains

• many move laterally, others cannot due to linkage with network of peripheral proteins, mainly located at cytosolic surface of membrane

peripheral membrane proteins (6)

• branch (most) of integral membrane proteins, span entire membrane

• can form ion or water channels

• or help with transmission of chemical signals across membrane

• some help anchor extracellular and intracellular filaments to CM

• many of their polypeptide chains cross lipid bilayer many times, therefore multiple nonpolar and polar segments

• nonpolar segments can extend far past surface of CM

peripheral membranes (3)

• not amphipathic, do not associate with nonpolar segment of membrane

• located on (usually cytosolic) MS

• bound to polar regions of integral proteins or charged polar heads of phospholipds

peripheral membrane function examples (3)

• enzymes that control metabolism of membrane components

• transport of small molecules along membrane or between membrane and cytosol

• associated with cytoskeletal elements that influence cell shape and motility

interstitial fluids (2)

• fluid that fills up space between cells

• provides pathway for substances to pass from cells to and from blood

integrins

• transmembrane proteins in plasma membrane that bind to specific proteins in the EXM, linking adjacent cells by the membrane protein

3 types of membrane junctions

1. desmosomes

2. tight junctions

3. gap junctions

desmosomes (4)

• characterised by accumulation of dense plaque proteins along cytoplasmic surface of PM

• dense plaques are anchoring points for cadherins

• adjacent cells PM separated by around 20nm

• can be secured by keratin filament, anchoring cytoplasmic surface of desmosomes to interior structures of the cell

• function: hold adjacent cells firmly together in areas with considerable stretching, e.g. skin

specialised membrane around desmosomes…

is usually disk shapes

cadherins (3)

• proteins

• extend from cell to extracellular space

• cadherins from adjcaent cells bind to each other, firmly attaching two cells

tight junctions (4)

• EXC surfaces of 2 adjacent PM join togther, no extracellular space between them

• occurs in band around entire circumference of cell

• ions and water can move through tight junctions better than organic molecules e.g. glucose

• joins most epithelial cells near their apical surface

tight junction function example (3)

• epithelial cells in inner surface of small intestine

• forces nutrients through cells, rather than inbetween them

• therefore PM can control type and amount of substances absorbed

gap junctions (3)

• protein channels linking cytosols of adjacent cells

• connexin proteins form small, protein lined channels linkng cells

• diameter1.5nm limits passage - only small molecules and ions

gap junction function example

• in heart, transmission of electrical activity between cells