Cell Biology

Cell and Tissue Design

Cells are fluid sacs surrounded by selectively permeable membranes, allowing for transport of specific fluids

important chemical reactions occur in the fluid phase
cannot create cell de novo – DNA template is important

Four predominant cell types

(1) epithelial cell – polarized – apical and basolateral surfaces; often over layers of connective tissue, muscle, glands

(2) connective tissue – cells synthesize structures that hold body together – include bone, cartilage, ligaments, adipose

extracellular matrix (ECM) – lots secreted by relatively few cells – often contain collagen and proteoglycans

(3) neural cell – extremely polarized – dendrite Þ cell body Þ axon

(4) muscle – related to nerve – three types: skeletal, smooth, cardiac – all use actin and myosin

Different cell types combine to form tissues and organs

Membranes, Matrix, and Junctions

Different types of membranes:

(1) Nucleus – many pores, very complex (more than 100 proteins) – membrane proteins A,B,C regulate architecture

where ribosomes are assembled; nuclear envelope has 2 membrane; inner membrane covered with nuclear lamina Þ B (membrane protein) anchors lamin A and C

(2) Mitochondria – cisternae – have two membranes; inner less permeable

(3) Cell Membrane – voltage gradient due to Na⁺/K⁺ pump – membrane proteins made by the ER

one ligand = one receptor; can make chimeric cell by exchange of receptors

Junctions – Three Types:

(1) Tight Junction – along epithelial cell sheet– seal between adjacent cells, like spot-welding – does not provide strength

mechanism not known – perhaps protein-protein or lipid-lipid
inhibit movement of material between cells – molecular tracers stay on one side or the other

(2) Adherens Junction – structural, provides strength – but not as tight, so allows diffusion of materials through

complex cell surface proteins (anchor filaments)

(3) Communication Junction (Gap Junction) – allow diffusion between cells by direct cytoplasmic communication

six subunits on each side – hole produced only if in register, so one can open independently of the other

Cytoskeleton – Three things facilitate cell structure and motility:

(1) Actin Filaments (microfilaments) – most abundant intracellular protein in all cells – many genes, different types

polymerize from monomeric (globular, G) to filamentous (F) forms – high conc. of G-actin activates polymerization Þ ATP is associated with G actin Þ Ý rate of F actin formation (not a necessity)

polarized – actin added at (+) end, removed from (-) end – added at (-) end too if G-actin concentration is very high

can form treadmill – used by cell for translocation and movement of cell processes
e.g. microvilli – (+) end at end of microvillus is capped with protein that prevents further assembly

actin-associated proteins: fimbrin and a -actinin bundle it, gelsolin cuts filaments, spectrin stabilizes cross-linkage

myosin – most important actin binding protein – binds actin toward (+) end and converts ATP to mechanical work

myosin has conformation-changing ATPase that works efficiently only in presence of actin
ATP binding also causes release of actin – then ATP hydrolysis to cock myosin head, rebind actin, release ADP, contraction of myosin head to move actin and do work, bind ATP and start over

acrisomal reaction – during fertilization – speratozoa acrisomes full of G-actin, does not polymerize due to profilin

influx of Ca⁺⁺ during fertilization causes breakage of association with profillin, polymerization, microvillus into egg

motility = ameboid movement – contraction of cortical actin gel; formation of cleavage furrow

(2) Microtubules – 25 different types – all from microtubulin – 55 kD dimer (a and b ) that acts as a single molecule

also polymerize into polarized tube 22.5 nm diameter, composed of 13 strands with a hole in the middle

assembly and disassembly occurs at both ends – five times more at (+) end

assembly activated by:

(1) high concentration of free tubulin

(2) low Ca⁺⁺ level

(3) high GTP level (GTP hydrolyzed during polymerization)

(4) higher temperature (37° C favors assembly, 4° C disassembly)

microtubule associated proteins (MAP) – e.g., tau (stabilization)

ATPase MAP’s used to move things: kinesin – move toward (+) end (away from cell); dynein – toward (-) end
used to transport things to end of long neuronal axon shaft (no protein synthesis in axon) – kinesin toward end

microtubules are arranged around a microtubule organizing center (MTOC) in the cell – (-) end closer to center

MAP’s control growth by stopping disassembly

cilia – 9+2 form (9 cilia form tube with 2 in the middle) – linked together with sidearms and dynein (causes movement)

basal body – point of cilia attachment – has its own DNA, can regenerate cilia

centrioles – during cell division, microtubules form spindle – kinesin and dynein cause movement along spindle

pull chromosomes from middle where they line up (kinetochore) toward poles; also push poles apart, divide cell

(3) Intermediate Filaments – give cell shape and strength – component of adherens junction

very stable – difficult to disassemble, so permanent

lamins hold nucleus in place, also keratins, neurofilaments (stabilize neurons), desmin/vimentin (in muscle)