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The Typical Cell
• typical cell: 1. nucleus
2. cell membrane
3. cytoplasm
-cytosol
-cytoskeleton
4. cytoplasmic organelles
-membranous
-non-membranous
Cytoplasm
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semi-fluid-like jelly within the cell
division into two subdivisions: cytosol & organelles
also contains a supportive framework of proteins:
cytoskeleton
Cytosol
Cytosol
-intracellular fluid of the cell
-about 55% of the cell’s volume
-about 70-90% water PLUS
dissolved nutrients
ions
soluble & insoluble proteins
waste products
glucose, ATP
amino acids, fatty acids
•higher K+
•lower Na+
•higher concentration
of dissolved and
suspended proteins
(enzymes, organelles)
•lower concentration
of carbohydrates
(due to catabolism)
•larger reserves of amino
acids (anabolism)
ECF
•lower K+
•higher Na+
•lower concentration
of dissolved and
suspended proteins
•higher concentration
of carbohydrates
•smaller reserves of amino
acids
Cytoskeleton:
•internal framework of the cell
•gives the cytoplasm flexibility and strength
•three major components
1. microfilaments
2. intermediate filaments
3. microtubules
microfilaments = made of thin filaments called actin
-forms a dense network immediately under the PM + scattered
throughout the cytoplasm (most prevalant at the cell periphery)
-function: 1. anchor integral proteins and attaches them to the cytoplasm
2. interacts with larger filaments made up of myosin - results
in active movements of the cell (e.g. muscle cells)
or changes in cell shape
-provide much of the mechanical strength of the cell + give the cell its shape
-also provide support for cellular extensions called microvilli (small intestines)
intermediate filaments = made up of vimentin, desmin or keratin
-function: 1. impart strength
2. stabilize organelles
3. transport materials
-some IFs are made of specialized proteins found in specific cell types
e.g. neurofilaments in neurons - for transport of synaptic vesicles
containing neurotransmitters
•five major groups of IFs
1. Type I: acidic keratins (epithelial cells)
2. Type II: basic keratins (epithelial cells)
3. Type III: vimentin (bone, cartilage)
-desmin (muscle)
4. Type IV: Neurofilaments (neuronal)
5. Type V: Lamins A, B, C (all cells)
microtubules = repeating units of tubulin
-assembly is controlled in the MTOC (microtubule organizing center)
- located near the nucleus – region of tubulin proteins
-for the assembly of tubulin into microtubules, centrioles and
cilia
-also known as a centrosome
-role in MT assembly
-found near the Golgi apparatus during interphase - called the
centrosome (pair of centrioles at right angles)
-MTs grow out from the centrosome
-microtubule – a cylinder = 13 rows of tubulin arranged as a “straw”
-straws can be glued together to form a doublet or a triplet
-function: 1. cell shape & strength
2. organelles: anchor & movement
3. mitosis - form the spindle (chromosome movement)
4. form many of the non-membranous organelles
(cilia, flagella, centrioles)
Non-membranous Organelles
A. Centrioles: short cylinders of tubulin – arranged
as 9 short microtubule triplets
-called a 9+0 array (9 peripheral triplets, 0 in the center)
-each centriole is made of a pair – arranged perpendicular to one another
-role in mitosis - spindle and chromosome alignment
B. Cilia & Flagella
•cilia = contain 9 groups of microtubule doublets surrounding a central pair
-called a 9+2 array
-anchored to a basal body just beneath the cell surface (9+0 array)
-exposed portion of the cilia covered by PM – but the cilia itself is non-membranous
-beat rhythmically to transport material
-found in linings of several major organs covered with mucus
-function in “cleaning”
cilia
9+2
basal body
9+0
•flagella = resemble cilia
-much larger
-found singly
-functions to move a cell through the ECF
??? What is the only human cell to possess a flagella???
Ribosomes = can be considered a nonmembranous organelle
•2 protein subunits in combination with RNA
-large 60S subunit = 28S rRNA (ribosomal RNA) + 50 proteins
-small 40S subunit = 18S rRNA + 33 proteins
•actual site of mRNA translation -> peptide strand
•in association with the ER = where the peptide strand
is fed into from the ribosome
•also float freely within the cytoplasm as groups = polyribosomes
Membranous Organelles
•completely surrounded by a phospholipid bilayer similar to the PM
surrounding the cell
•allows for isolation of each individual organelle - so that the components
of each organelle does not mix with the cytosol
•therefore requires a well-coordinated system of transport for organelles
to communicate and function together
-”vesicular transport” - small transport vesicles pinch off one
organelle, travel and then fuse to another organelle
Overview of the secretory pathway
1. Endoplasmic reticulum (ER) = series of membrane-bound, flattened
sacs in communication with the nucleus and the PM
-each sac or layer = cisternae
-inside or each sac = lumen
-two types:
Rough ER - outside studded with ribosomes
-continuous with the nuclear membrane
-protein synthesis, phospholipid synthesis
-the peptide strand as it is being translated by the ribosome
is fed into the RER
-initial site of processing (attachment of carbohydrates) and sorting for
transport to the Golgi
-three functions: 1. synthesis
2. storage
3. transport
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transport from the ribosome across the
ER membrane requires the presence of a
signal sequence
16-30 amino acids at the beginning of the
peptide sequence (N-terminal)
this signal sequence will vary from protein
to protein by will have a few
characteristics in common – starts with
one or two positively charges amino acids
and is followed by 6-12 hydrophobic AAs
A complex of proteins will bind this signal
in the cytoplasm = signal recognition
particle/SRP
the ER membrane has receptor for the
SRP – docks the translating ribosome
(yellow protein in figure)
actual entrance of the protein requires the
formation of a “hole” in the ER membrane
(blue protein in figure) = translocon
(series of multiple transmembrane
proteins)
this process also requires energy which is
provided by GTP
Translocation
• this animation might be a bit complicated –
but give it a try
• http://www.rockefeller.edu/pubinfo/proteint
arget.html
Modifications in the RER
• 1. formation of disulfide bonds
– help stabilize the tertiary and quaternary structure of proteins
• 2. folding of the peptide chain
– misfolded proteins remain in the cytoplasm and are degraded
– only properly folded proteins get transported to the Golgi for additional
processing and transport
– many proteins located in the ER which stimulate this folding
• 3. addition and processing of carbohydrates
• 4. breakage of specific peptide bonds – proteolytic cleavage
• 5. assembly into multimeric proteins (more than one chain)
for an animation go to
http://sumanasinc.com/webcontent/animations/content/protei
nsecretion_mb.html
Smooth ER – extends from the RER but is free of ribosomes
- many enzymes on the surface of the SER for:
- 1. lipid and steroid (e.g. estrogen and testosterone) biosynthesis for
membranes
- 2. detoxification of toxins and drugs
- 3. vesicle formation
- 4. cleaves glucose so it can be released into the bloodstream
-the amount of SER per cell can increase with drug use – cell accomadates
to increase protection
3. Golgi Apparatus = stack of 3 to 20 flattened membrane sacs/cisternae
•site of protein modification, and final packaging of the
finished protein into secretory vesicles -> exocytosis or for use in the cytosol
•movement of protein through the stacks via transport vesicles
-definite direction: first stack = cis-face (from the RER, protein modification)
middle stack = medial-face (adds carbohydrates)
last stack = trans-face (modification and packaging into vesicles)
Modifications in the Golgi
• glycosylation = glycoprotein
– most plasma membrane and secreted proteins have one or more
carbohydrate chains that help target them to the correct location
– some glycosylation occurs in the ER, others in the various sacs of the
Golgi
– in the Golgi are the addition of N- and O-linked oligosaccharides –
specific sugar residues
– O-linked are added one at a time in the Golgi to the amino acids serine,
threonine or lysine (one to four saccharide subunits total)
• added on by enzymes called glycosyltransferases
• human A, B and O antigens are sugars added onto proteins and lipids inserted in the PM of the RBC
• everyone has the glycosyltransferase needed to produce the O antigen
– N-linked saccharides are linked together (about 14 sugars!) in the ER
• transported out of the ER and imported into the Golgi
• are attached specifically to asparagine amino acids within a protein
• specific sugars can be removed or new ones added in the Golgi – changes
the composition of the resulting glycoprotein
– the sugars are found in the cytoplasm but are transported into the Golgi
by specific transporters for their addition
Modifications in the Golgi
• some PM proteins and most secretory proteins are
synthesized as larger, inactive proproteins that will
require additional processing to become active
– e.g. albumin, insulin, glucagon
• this processing occurs very late in maturation = trans
face
– processing is catalyzed by protein-specific enzymes called
proteases
– some proteases are unique to the specific secretory protein
– occurs in secretory vesicles that bud from the trans-Golgi face
– processing could be at one site (albumin) others may require
more than one peptide bond (insulin)
-proteins processed in
the Golgi will have
unique sequences of
amino acids that tell
the protein where to go
-these are called
sorting signals
-the lack of this signal
means you will be
secreted
targets:
1. secretory vesicles for
exocytosis
2. membrane vesicles for
incorporation into PM
3. transport vesicles for
intracellular destinations
e.g. digestive enzymes to
the lysosome
-requires unique sorting
signals
4. Mitochondria = site of energy production (ATP production)
-via Cellular Respiration
- breakdown of glucose into water and CO2 results in the
production of ATP
-initial glucose breakdown occurs in the cytosol
-terminal stages occur in the mitochondria = Oxidative
Phosphorylation
-has its own DNA - maternal
-reproduce themselves via dividing
-consists of an outer
mitochondrial membrane, an
inner mitochondrial membrane
and a fluid-filled space =
mitochondrial matrix
(contains ribosomes!)
-the inner membrane is folded
into folds called cristae
(increase the membrane surface
area for the enzymes of
Oxidative Phosphorylation)
-2 membrane layers: •outer - 50% lipid & 50% protein
-very permeable - contains pores
•inner - 20% lipid & 80% protein
-less permeable
-folded extensively to form partitions = cristae
-contains proteins that transport H+ out of the
lumen of the mitochondria -> electrochemical gradient
-contains enzymes that use this gradient for the synthesis
of ATP
-also contains pumps to move ATP into the cytosol
•matrix - lumen of the mitochondria
-breakdown of glucose into water and
CO2 ends here and results in the
production of ATP
Cellular respiration
-glycolysis
-citric acid cycle
-electron transport chain
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.sp.uconn.edu/%7Eterry/images/anim/ETS.html
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.biocarta.com/pathfiles/krebPathway.asp
http://vcell.ndsu.nodak.edu/animations/etc/movie.htm
Glycolysis
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literally means “splitting sugar”
conversion of glucose (6 carbon
sugar) into 2 molecules of pyruvate
(3 carbon sugar)
pyruvate will be converted into
acetyl-coenzyme A (Acetyl-CoA)
which then enters the citric acid cycle
under aerobic conditions glucose is
oxidized into pyruvate and then
continues on to the citric acid cycle
under anaerobic conditions glucose
is oxidized into pyruvate then
converted into lactate (lactic acid)
reactions of glycolysis take place in
the cytosol
this pathway also creates the building
blocks that are used for the synthesis
of long chain fatty acids
phosphorylation
isomerization
phosphorylation
cleavage
2 ATP consumed
no energy created
key enzyme in controlling
the rate of glycolysis =
phosphofructokinase
-liver enzyme that is inhibited
when ATP levels are high
http://web.indstate.edu/thcme/mwking/glycolysis.html
http://science.nhmccd.edu/biol/glylysis/glylysis.html
Glycolysis
• we consume sugars other than glucose
• two other abundant sugars are fructose and galactose
• fructose can be converted into glyceraldehyde using the fructose-1phosphate pathway which utilizes different enzymes but still creates
glyceraldehyde 3-phosphate
• alternatively fructose can be phosphorylated to fructose-6phosphate by the same enzyme that phosphorylates glucose
(hexokinase) – then is phosphorylated again = fructose-1,6phosphate
• galactose can be converted into glucose-6-phosphate in a series of
four steps using four different enzymes
Glycolysis
• the cells in all kinds of organisms convert glucose to
pyruvate using similar reactions
• however pyruvate can be processed in many ways to
produce
– 1. ethanol – by yeast
• pyruvate – acetylaldehyde – ethanol
• regulated by aldehyde dehydrogenase (DH = removes H+
from one substrate and adds it to another)
• the opposite (alcohol to aldehyde) is catalyzed by alcohol
dehydrogenase (ADH)
– 2. lactate – by cells in the absence of oxygen
– 3. acetyl CoA – the majority of pyruvate is processed by this next
phase
Citric Acid cycle
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pyruvate is transported from the cytosol across
the membranes of the mitochondria into its
matrix
interacts with coenzyme A (mitochondrial
enzyme) to produce acetyl-CoA
forms the waste product carbon dioxide (2
molecules)
Acetyl-CoA is converted into oxaloacetic acid ->
citric acid
the citric acid is converted into a series of
compounds that eventually regenerates OA acid
takes place in the matrix and inner membrane of
the mitochondria
cycle results in the formation of NAD+
(nicotinamide adenine dinucleotide) – capable
of storing high energy electrons
as the cycle runs - NAD+ is reduced to form
NADH and H+ (gains two electrons)
NADH = electron carrier
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NAD+ is capable of adding one H+ and 2 electrons
GTP hydrolysis required.
this NADH will then enter the electron transport
chain
Also generates the FADH2 electron carrier (for 4
electrons)
while this cycle only runs in the presence of
oxygen – no oxygen is used
For each glucose
this cycle has to “turn”
Twice!!
Electron transport chain
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protons are pumped from the
matrix into the space between the
inner and outer mitochondrial
cytochrome c cytochrome c
NADH
membranes by enzyme complexes
oxidase
dehydrogenase reductase
in addition, electrons are
transferred from these complexes
to oxygen eventually forming water
and carbon dioxide
protons are taken from NADH and
pumped across the inner
mitochondrial membrane by NADH
dehydrogenase
at the same time electrons are
protons
moved from NADH
electrons
dehydrogenase to cytochrome c
reductase (by Q = ubiquinone)
cytochrome c reductase pumps
more protons (from water) across
the membrane and more electrons
are transferred to cytochrome c
oxidase (by cytochrome c)
cytochrome c oxidase also pumps
more protons (taken from water)
this creates a proton gradient =
energy
as protons flow down their gradient
back into the matrix, they pass
through an enzyme called ATP
synthase – which synthesizes
ATP
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.sp.uconn.edu/%7Eterry/images/anim/ETS.html
ETC animations
• http://www.youtube.com/watch?v=xbJ0nbz
t5Kw&feature=relmfu
• http://www.youtube.com/watch?v=3y1dO4
nNaKY
• http://biology.about.com/gi/dynamic/offsite.
htm?site=http://www.sp.uconn.edu/%7Eter
ry/images/anim/ETS.html
5. Lysosomes = “garbage disposals”
-dismantle debris, eat foreign invaders/viruses taken in by
endocytosis or phagocytosis
-also destroy worn cellular parts from the cell itself and recycles the
usable components = autophagy
-form by budding off the Golgi
-vesicle sacs that contain powerful enzymes to breakdown substances
into their component parts
e.g. nucleases = breakdown RNA & DNA into nucleotides
proteases = breakdown proteins into amino acids
-over 60 kinds of enzymes within the lysosome
-these enzymes are collectively known as acid hydrolases
-acidic interior - critical for function of these enzymes
-created and maintained by a hydrogen pump that
transport H+ into the interior - Active transport
-chloride ions that diffuse in passively through a
chloride channel - forms hydrochloric acid (HCl)
6. Peroxisomes: abundant in liver and kidney cells
-main function is the breakdown of long chain fatty acids through their
oxidation
-oxidation is done by oxidases = enzymes that use oxygen to oxidize substances
- such as peroxidase
-oxidation reactions by these oxidases generate toxic oxidative chemicals
such as hydrogen peroxide (H2O2)
-therefore peroxisomes contain enzymes to break up these oxidative
substances
e.g. catalase to break this peroxide down
- catalyze the breakdown of H2O2 into water and oxygen
-outer membrane contains a variety of enzymes for
1. synthesis of bile acids
2. breakdown of lipids - oxidation of
fatty acids and amino acids
(formed during
normal metabolic processes)
3. detoxification of alcohol – converts
it into sugars
F-actin and peroxisomes
Diseases at the Organelle Level
1. Tay Sachs and lysosomes: lack one of the 40 lysosomal enzymes
-produces a lysosomal storage disease -> build up of fatty
material on nerve cells
-failure of nervous system communication
2. Adrenoleukodystrophy and peroxisomes: peroxisomes lack an
enzyme on the outer membrane which transports an essential
enzyme into the peroxisome
-leads to a build up of a long-chain fatty acid on cells of the
brain and spinal cord -> loss of the myelin sheath
-lethargy, skin darkens, blood sugar drops, altered heart rhythm
imbalanced electrolytes, paralysis, death
*** slowed by a certain triglyceride found in rapeseed oil
Lorenzo Odone = “Lorenzo’s Oil”