Samenvatting
Summary Essential cel biology hoofdstuk 11tm18
Samenvatting van de hoofdstukken 11 tm 18 van essential cell biology.
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MCB Deeltoets 2Hoofdstuk 11
- A plasma membrane is a protein-studded, fatt flm so thin that it cannot be seen directlt in the light
microscope.
- Regardless of their locaton, all cell membranes are composed of lipids and proteins and share a common
general structure
- The lipids are arranged in two closelt apposed sheets, forming a lipid bilater.
- The most abundant lipids in cell membranes are the phospholipids, which have a phosphate-containing,
htdrophilic head linked to a pair of htdrophobic tails
- A shorter chain length reduces the tendenct of the htdrocarbon tails to interact with one another and
therefore increases the fuiditt of the bilater
- Lipid bilaters that contain a large proporton of unsaturated htdrocarbon tails are more fuid than those with
lower proportons
- In eukartotc cells, new phospholipids are manufactured bt enztmes bound to the cttosolic surface of the
endoplasmic retculum. Using free fatt acids as substrates, the enztmes deposit the newlt made
phospholipids exclusivelt in the cttosolic half of the bilater
- The transfer of lipids from one monolater to the other rarelt occur spontaneouslt. Instead, thet are
cataltzed bt enztmes called scramblases, which remove randomlt selected phospholipids from one half of
the lipid bilater and insert them in the other.
- The Golgi membrane contains another familt of phospholipid-handling enztme, called fippases. These
enztmes remove specifc phospholipids from the side of the bilater facing the exterior space and fip them
into the monolater that faces the cttosol
- Most membrane functons are carried out bt membrane proteins.
- Mant membrane proteins extend through the bilater, with part of their mass on either side. Like their lipid
neighbors, these transmembrane proteins are amphipathic, having both htdrophobic and htdrophilic
regions. Their htdrophobic regions lie in the interior of the bilater, nestled against the htdrophobic tails of
the lipid molecules. Their htdrophilic regions are exposed to the aqueous environment on either side of the
membrane
- Other membrane proteins are located almost entrelt in the cttosol and are associated with the cttosolic
half of the lipid bilater bt an amphipathic alfa helix exposed on the surface of the protein
- Some proteins lie entrelt outside the bilater, on one side or the other, atached to the membrane onlt bt
one or more covalentlt atached lipid groups
- Yet other proteins are bound indirectlt to one or the other face of the membrane, held in place onlt bt their
interactons with other membrane proteins
- Proteins that are directlt atached to the lipid bilater, whether thet are transmembrane, associated with the
lipid monolater, or lipid-linked, can be removed onlt bt disruptng the bilater with detergents. Such
proteins are known as integral membrane proteins.
- The remaining membrane proteins are known as peripheral membrane proteins. Thet can be released from
the membrane bt more gentle extracton procedures that interfere with protein-protein interactons but
leave the lipid bilater intact
- Detergents difer from membrane phospholipids in that thet have onlt a single htdrophobic tail. Because
thet have onlt one tail, detergent molecules are shaped like cones, in water, thet thus tend to aggregate
into small clusters called micelles, rather than forming a bilater as do the phospholipids
- The plasma membrane of animal cells is stabilized bt a meshwork of fbrous proteins, called the cell cortex,
that is atached to the underside of the membrane
- Cells have wats of confning partcular proteins to localized areas within the bilater membrane, therebt
creatng functonallt specialized regions, called membrane domains, on the cell or organelle surface
- The astmmetrical distributon of membrane proteins is maintained bt a barrier formed along the line where
the cell is sealed to adjacent epithelial cells bt a so-called tght-juncton. At this site, specialized junctonal
, proteins form a contnuous belt around the cell where the cell contacts its neighbors, creatng a seal
between adjacent plasma membranes. Membrane proteins cannot difuse past the juncton
- All of the carbohtdrate on the gltcoproteins, proteogltcans, and gltcolipids is located on the outside of the
plasma membrane, where it forms a sugar coatng called the carbohtdrate later or gltcocaltx
- A technique to probe membrane fuiditt is called fuorescence recovert afer photobleaching (FRAP). It
involves uniformlt labeling components of the cell membrane, its lipids or, more ofen, its proteins, with
some sort of fuorescent marker. Labeling membrane proteins can be accomplished bt incubatng living cells
with a fuorescent antbodt or bt covalentlt ataching a fuorescent protein such as GFP to a membrane
protein of interest using recombinant DNA techniques
Hoofdstuk 12
- There are two main classes of membrane proteins that mediate transfer: transporters and channels.
Transporters shif small organic molecules or inorganic ions from one side of the membrane to the other bt
changing shape. Channels, in contrast, form tnt htdrophilic pores across the membrane through which such
substances can pass bt difusion. Most channels onlt permit passage of inorganic ions and are therefore
called ion channels. Because these ions are charged, their movement can create a powerful electric force
across the membrane.
- Channels discriminate mainlt on the basis of size and electric charge, while a transporter transfers onlt those
molecules or ions that ft into specifc binding sites on the protein.
- The net force driving a charged solute across a cell membrane is therefore a composite of two forces, one
due to the concentraton gradient and the other due to the membrane potental. This net driving force,
called the solute’s electrochemical gradient, determines the directon that each solute will fow across the
membrane bt passive transport
- In the NaK pump, the energt from ATP htdroltsis induces a series of protein conformatonal changes that
drive the NaK ion exchange. As part of the process, the phosphate group removed from ATP gets transferred
to the pump itself.
- If a coupled pump moves both solutes in the same directon across the membrane, it is called a stmport. If it
moves them in opposite directons, it is called an antport. A transporter that ferries onlt one ttpe of solute
across the membrane is called a uniport.
- Plant cells, bacteria, and fungi do not have Na pumps in their plasma membrane. Instead of an
electrochemical Na gradient, thet relt mainlt on an electrochemical gradient of H to import solutes into the
cell. The gradient is created bt H pumps in the plasma membrane that pump H out of the cell, thus setng
up an electrochemical proton gradient across this membrane and creatng an acid pH in the medium
surrounding the cell.
- Two important propertes distnguish ion channels from simple holes in the membrane. First, thet show ion
selectvitt, permitng some inorganic ions to pass but not others. The second important distncton between
simple holes and ion channels is that ion channels are not contnuouslt open. Most ion channels are gated: a
specifc stmulus triggers them to switch between a closed and an open state bt a change in their
conformaton. Unlike a transporter, an open ion channel does not need to undergo conformatonal changes
with each ion it passes, and so it has a large advantage over a transporter with respect to its maximum rate
of transport
- K leak channels randomlt ficker between open and closed states no mater what the conditons are inside or
outside the cell. when thet are open, thet allow K to move freelt. In a restng cell, these are the main ion
channels open in the plasma membrane, rendering the membrane much more permeable to K than other
ions.
- The membrane potental in steadt state potentals, in which the fow of positve and negatve ions across the
plasma membrane is preciselt balanced, so that no further diference in charge accumulates across the
membrane. Is called the restng membrane potental.
- Nernst equaton
- In patch-clamp recording, a fne glass tube is used as a microelectrode to isolate and make electrical contact
with a small area of the membrane at the surface of the cell
, - For a voltage-gated channel, the probabilitt of being open is controlled bt the membrane potental. for a
ligand-gated channel, opening is controlled bt the binding of some molecule to the channel. For a
mechanicallt gated channel, opening is controlled bt a mechanical force applied to the channel
- Voltage gated ion channels have domains called voltage sensors that are extremelt sensitve to changes in
the membrane potental: changes above a certain threshold value exert sufcient electrical force on these
conformaton.
- When a neuron is stmulated, the membrane potental of the plasma membrane shifs to a less negatve
value. If this depolarizaton is sufcientlt large, it will cause voltage gated Na channels in the membrane to
open transientlt at the site. As these channels ficker open, thet allow a small amount of Na to enter the cell
down its steep electrochemical gradient. The infux of positve charge depolarizes the membrane further,
therebt opening additonal voltage-gated Na channels and causing stll further depolarizaton. This process
contnuous in an explosive, self-amplifting fashion untl, within about a millisecond the membrane potental
in the local region of the neurons plasma membrane has shifed from its restng value of about -60mV to
about +40 mV
- During an acton potental, Na channels do not act alone. The depolarized axonal membrane is helped to
return to its restng potental bt the opening of voltage-gated K channels. These also open in response to
depolarizaton, but not as promptlt as the Na channels, and thet stat open as long as the membrane
remains depolarized. As the local depolarizaton reaches its peak, K ions therefore start to fow out of the
cell through these newlt opened K channels down their electrochemical gradient, temporarilt unhindered bt
the negatve membrane potental that normallt restrains them in the restng cell. the rapid ouflow of K
through the voltage gated K channels brings the membrane back to its restng state much more quicklt than
could be achieved bt K ouflow through the K leak channels alone
- Once it begins, the self-amplifting depolarizaton of a small patch of plasma membrane quicklt spreads
outward: Na fowing in through open Na channels begins to depolarize the neighboring region of the
membrane, which then goes through the same self-amplifting ctcle.
- When an acton potental reaches the nerve terminals at the end of an axon, the signal must somehow be
related to the target cells that the terminals contact. The signal is transmited to the targets cells at
specialized junctons known as stnapses. At most stnapses, the plasma membranes of the cells transmitng
and receiving the message, the prestnaptc and the poststnaptc cells, are separated from each other bt a
narrow stnaptc clef, which the electrical signal cannot cross. To transmit the message across this gap, the
electrical signal is converted into a chemical signal, in the form of a small, secreted signal molecule known as
a neurotransmiter. Neurotransmiters are initallt sorted in the nerve terminals within membrane-enclosed
stnaptc vesicles.
- Transmiter gated ion channels convert the chemical signal back into an electrical signal.
Hoofdstuk 13
- If a fuel molecule such as glucose were oxidized to CO2 and H2O in a single step, bt , for example , the direct
applicaton of fre, it would release an amount of energt mant tmes larger than ant carrier molecule could
capture. Instead, cells use enztmes to carrt out the oxidaton of sugars in a tghtlt controlled series of
reactons.
- The proteins, fats, and poltsaccharides that make up most of the food we eat must be broken down into
smaller molecules before our cells can use them. Either as a source of energt or as building blocks for
making other organic molecules. This breakdown process, in which enztmes degrade complex organic
molecules into simpler ones, is called catabolism. The process takes place in three stages
- In stage 1 of catabolism, enztmes convert the large poltmeric molecules in food into simpler monomeric
subunits. This stage, also called digeston, occurs either outside cells or in specialized organelles within cells
called ltsosomes. Afer digeston, the small organic molecules derived from food enter the cttosol of a cell,
where their gradual oxidatve breakdown begins
- In stage 2 of catabolism, a chain of reactons called gltcoltsis splits each molecule of glucose into two smaller
molecules of ptruvate. Gltcoltsis takes place in the cttosol and, in additon to producing ptruvate, it
generates two ttpes of actvated carriers: ATP and NADH. The ptruvate is transported from the cttosol into
the mitochondrion’s large, internal compartment called the matrix. There, a giant enztme complex converts
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