Download 7 Structural components of eucaryote cells

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The structural bits and pieces of the eucaryote cell
Investigating this
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0.2 micrometer structures can be resolved with the aid of a light microscope
Organelles can be centrifuged:
 First, you puree the cells
 Then, you spin them down
 The heaviest things - the NUCLEI- sediment first
 The mitochondria are next heaviest
 Microsomes, such as ribosomes and peroxisomes, sediment last
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7.5 nanometers thick
Semipermeable, bridged by proteins
Made of AMPHIPATHIC (both hydrophilic and hydrophobic) phospholipids; the hydrophilic
phosphate heads all line up on the outside.
Proteins spanning the membrane are linked to it in a number of ways, which prevent them from
floating away.
The cytosol
has a pH of 7.2
Cell membranes
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Mitochondria
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Domesticated aerobic bacteria
Have an outer membrane, and an inner membrane which is folded up into cristae (“shelves”)
The enzymes responsible for oxidative phosphorylation are lined up along the cristae
Mitochondria have their own DNA, as independent organisms should; but it doesn’t code for
much – mainly the oxidative phosphorylation enzymes and intramitochondrial ribosomes.
99% of the proteins in mitochondria are actually encoded for by the nucleic DNA of the cell.
Mitochondria are crap at repairing their own DNA. Mutations accumulate 10 times faster than in
normal nuclear DNA
Mitochondria are derived from the ovum, their inheritance is maternal
Lysosomes
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Large irregular membrane sacks full of hideous acid
H+ ATPase keeps the inside acidic
The inside is also full of hydrolytic enzymes
The enzymes function best at an acidic pH; so, if a lysosome does break open somehow,
the enzymes wouldn’t be functioning at their murderous best, because the cellular
interior has a relatively neutral pH of 7.2
If a lysosome enzyme is absent, the lysosome fills up with whatever that enzyme was
meant to degrade. Examples of this are
FABRY DISEASE – alpha-galactosidase deficiency
GAUCHER DISEASE – beta-galactocerebrosidase deficiency
TAY-SACHS DISEASE – hexosaminidase A deficiency
Peroxisomes
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Obviously, these have to have something to do with peroxide
They contain either enzymes which produce H2O2 (OXIDASES) or ones which break it down
(CATALASES)
The function is a variety of anabolic or catabolic reactions, for example the degradation of lipids
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Cytoskeleton
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Microtubules, intermediate filaments and microfilaments
o MICROTUBULES:
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Made of alpha-tubulin and beta-tubulin
Make tracks along which transport proteins drag various organelles, such as secretory
granules or mitochondria
They form the spindle which aligns the chromosomes during mitosis
They are constantly being dynamically rearranged; gamma-tubulin is involved in this
process
DRUGS TARGET MICROTUBULES:
COLCHICINE prevents tubule assembly, thus decreasing leucocyte mobility
VINBLASTINE does something very similar
PACLITAXEL binds to micrtubules and makes them sos stable that the
organelles cannot move; thus cell death is caused by the fact tat the mitotic
spindle cannot form
o INTERMEDIATE FILAMENTS:
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MAKE UP A FLEXIBLE SCAFFOLD which prevents the cell from collapsing
These are usually cell-specific and can be used as tissue markers, eg. cytokeratin for
skin and vimentin for fibroblasts
o MICROFILAMENTS
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Made of ACTIN
Actin is actually present in all types of cells! It’s the MOST ABUNDANT PROTEIN IN
MAMMALIAN CELLS making up as much as 15% of the cell protein
Actin filaments connect to the integrin receptors and act as points of traction when a cell
is crawling along a surface
Molecular motors
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These are all 100-500 kDa ATPases
Their job is to move stuff like proteins and organelles around the cell
With one end, they attach to the cargo, and with the other, to the microtubules or actin
polymers along which they crawl
They convert the energy of ATP into movement along the microtubule
3 major families: kinsins, dyneins, and MYOSIN (the very same one which contracts
muscle)
Centrosomes
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Near the nucleus of a mammal cell is a CENTROSOME
Its made up of two centrioles and surrounding amorphous pericentriolar material
The centroles are two short cylinders arranged at right angles
They contain gamma-tubulin, and are involved in organizing the growth of microtubulesespecially during cell division
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Functionally indistinct from the flagella of sperm cells
Made of microtubules; contains axonemal dynein, which coordinates its movement
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Abbreviated as CAMs
Pass though the cell membrane and anchor to the cytoskeleton
They either bind to other cells (homophilic binding), or to extracellular membrane
proteins (heterophilic binding)
Apart from serving as anchors, they also serve to transmit intercellular messages, and
trigger apoptosis when the cell for some reason loses its anchorage and tries to float
away.
Cilia
Cell adhesion molecules
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Intercellular connections
o GAP JUNCTIONS vs TIGHT JUNCTIONS:
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TIGHT JUNCTIONS: let very little through; only some ions and solute. No protein
movement is allowed
GAP JUNCTIONS: narrow spaces between cells which permit the movement of
substances eg ions, sugars, amino acids
The X-linked Charcot-Marie-Tooth disease is a failure of connexins to form a gap
junction channel, thus impairing the conduction of simple molecules between
cells
GAP JUNCTION CONTENTS IS NOT EXTRACELULAR FLUID!
Nucleus and friends
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Made up of chromosomes
DNA is 2 metres long, but is wrapped around histone proteins in a NUCLEOSOME
The whole of the genetic material is called CHROMATIN
The nucleus has an outer membrane, which has selective small pores for small molecules, and
larger ones for transmitting things like mRNA
Endoplasmic reticulum
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The ROUGH ER contains ribosomes, and the smooth does not
ROUGH ER is responsible for protein synthesis
SMOOTH ER is responsible for steroid synthesis
There is also SARCOPLASMIC RETICULUM in muscle cells: its job is calcium sequestration
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Made up of two subunits, named after the rates of sedimentation when centrifuged: the 60S and
the 40S subunit
These are the sites of protein synthesis
General rule is, the ribosomes in the endoplasmic reticulum synthesise ll the exported proteins
like hemoglobin, and all the transmembrane protines, while free ribosomes synthesise all the
cytoplasmic proteins
Ribosomes
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Golgi apparatus and vesicular traffic
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The Golgi apparatus is a collection of membrane-enclosed sacks that are stacked like dinner
plates, around six sacks in all.
The purpose of it is to coordinate the glycosylation of proteins and lipids
It receives protein-containing vesicles from the endoplasmic reticulum, and eventually produces
vesicles of its own, which can release their contents via exocytosis
References: Ganong's Review of Medical
Physiology, Chapter 1