- found in all eukaryotic cells except mammalian RBCs and platelets
- some cell types are multinucleate (e.g., osteoclasts, skeletal muscle cells)
- others with distinctly shaped nucleus (e.g., adipose cells, neutrophil) - (see handout)

Nuclear Membrane (Envelope) = double membrane; too thin to be viewed w/ light
microscope, light microscope appears to show n.m. but this is actually peripheral
chromatin adhering to n.m.; outer surface studded with ribosomes
Chromatin = genetic material; condensed strands of DNA + associated proteins; stains
purple (basophilic) in H&E
- Different types of chromatin:

1) Heterochromatin = inactive, condensed DNA (this is what stains purple)

a) Constitutive = always condensed
b) Facultative = condensed only under certain conditions (e.g., when not actively
transcribing)

2) Euchromatin = genetically active DNA in extended form, not visible w/ light
microscope
3) Peripheral Chromatin = heterochromatin associated with nuclear membrane
4) Nucleolus-associated Chromatin = associated with nucleolus (= dark-staining
spherical structure in nucleus; composed of ribosome precursors specifically
binding to each other + RNA + associated DNA; nucleoli are prominent and usually
multiple in cells active in protein synthesis)

II. Cell Membrane
- composed of lipid bilayer (all biological membranes have this structure) +
associated proteins + cholesterol

- functions to regulate influx and efflux of materials to cell
a) permeable to some molecules (small molecules, ions, dissolved gases - O2, CO2)
b) impermeable to larger molecules, some ions
- 7.5-10 nm thick, too thin to visualize with light microscope

- lipid bilayer with hydrophilic (water-loving) heads and hydrophobic tails
- membrane proteins with similar hydrophilic and hydrophobic regions
Two types:
Intrinsic (integral) = pass entirely through membrane
Extrinsic = regulated to outer or inner half only

- membrane components free to move laterally w/in membrane (e.g., G-protein
binding to adenylate cyclase in protein hormone mechanism of action - see handout)

- threadlike to granular structures in cytoplasm under light microscopy
- under E.M. has double membrane: inner membrane folded in on itself (folds =
cristae); outer membrane surrounds inner and is generally oval-shaped
- functions in ATP production ("energy factory" of cell)
- Cells active in metabolism (energy usage) have many mitochondria (e.g., muscle),
inactive cells don't (e.g., adipose)
- mitochondrial matrix contains DNA (circular), RNA, ribosomes, proteins; can
direct synthesis of some mitochondrial proteins; enzymes for Beta-oxidation and
Krebs Cycle (except for succinate dehydrogenase are found in mitochondrial matrix
- enzymes for oxidative phosphorylation (ATP production) are located on inner
membrane; also succinate dehrydrogenase located here
- other Krebs Cycle enzymes may be associated with this in a loosely bound complex.
- ATP produced in matrix, must be transported across double-membrane for use in
cellular activities
- position of mitochondria in cell varies - in muscle cells they lie near contractile
elements, in kidney cells they lie basally in cytoplasm to provide ATP for active
transport; General Rule = mitochondria are located near where ATP is most needed

- formed as extensions of nuclear membrane into the cytoplasm
- 2 types:

1) RER = studded with ribosomes (stain blue in H&E due to high rRNA content,
blue areas of cytoplasm indicative of RER; example - basal cytoplasm of pancreatic
acini
2) SER = lack ribosomes
- RER (Rough Endoplasmic Reticulum) active in protein synthesis

1) Proteins synthesized from cytoplasmic precursors (amino acids)
2) Inserted, while being synthesized, through membrane into E.R. lumen
3) When completed, proteins released into E.R. lumen
4) Proteins released from E.R. lumen by fusion of secretory vesicles with golgi
or plasma membrane

- SER (Smooth E.R.) = not involved in protein synthesis; in secretory cells SER
represents portion of E.R. membrane from which secretory vesicles bud off; SER
active in steroid and lipid synthesis, detoxification of metabolites, glycogenolysis,
Ca2+ sequestering in skeletal muscle
- ribosomes free in cytoplasm also active in protein synthesis of intracellular
proteins, etc.

V. Golgi Apparatus = flattened sacs composed of membranes arranged in stacks in cytoplasm

- visible in light microscope (H&E) as a pale area in contrast to surrounding basophilic cytoplasm in cells active in protein synthesis (= negative image)
- Golgi functions: modification of secretory products, packaging of secretory vesicles and lysosomes, membrane recycling
- Mechanism of Protein Secretion/Modification

1) transfer vesicle from E.R. reaches forming face
2) fusion with golgi membrane
3) modification of secretory protein (cleavage, glycosylation)
4) secretory granule with modified protein buds off maturing face

VI. Lysosomes = small spherical vesicles (0.2-0.4_m in diameter) - (see handout)

- contain hydrolytic enzymes that degrade cellular constituents
Primary Lysosome = inactive (pale in electron microscope)
Secondary Lysosome = active (formed by fusion of primary lysosome with
membranous vesicle, usually brought into cell by phagocytosis; dark in E.M.)
- after digestion of phagocytosed vesicle, nutrients pass through lysosome membrane
to cytoplasm for use in the cell
- lysosomes also digest functionless organelles and kill infecting microbes

VII. Cytoskeleton

- Composed of:

1) Microtubules (polymers of tubulin) = form main supporting structure of cell
2) Microfilaments = slender rods composed of actin and associated tropomyosin
(same proteins as found in muscle; contractile and active in alterations in cell
shape, cytokinesis during mitosis, and stress-bearing and supportive functions
- microtubules and microfilaments are only visible with the E.M.




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