Chapter 15: The Plasma Membrane Flashcards
what are the 5 functions of membrane?
- boundary & selective permeability
- organization & localization
- proteins for transport processes
- receptors for signal detection
- cell-to-cell communication
what are the main components of membranes?
phospholipids
what are phospholipids made of?
- hydrocarbon tails
- glycerol
- phosphate
- PC, PS, PE, PI, sphingomyelin
the phospholipid tails are
nonpolar & hydrophobic
the phospholipid heads are
polar & hydrophilic
membranes are impermeable to all
hydrophilic molecules
membranes are permeable to small…
such as
hydrophobic molecules
steroid hormones
membrane bilayers are
viscous fluids NOT solids
the outer portion of the membrane is made of
PC, sphingomyelin, glycoproteins
the inner portion of the membrane is composed of
PS, PE, PI
the membrane is composed of about
50% proteins
where is sphingomyelin synthesized?
in the Golgi
what aids in creating the cell membrane potential?
- the negative charges of head groups
- ion concentration in the cytosol
define phase transition
changing from one state to the other
the plasma membrane state can change very quickly over a very
narrow temperature range
what stabilizes both stages of a membrane’s phase transition?
cholesterol
phase transition is aka
a cell’s fluidity
___________ determines the proper function of the membrane
fluidity
what are the 4 determinants of fluidity?
- fatty acid length
- type of fatty acid
- cholesterol
- lipid rafts
how is fatty acid length a determinate of fluidity?
gel state is stabilized by van der Waals interactions
- short chains have less surface area for interaction
- short chain lipids melt at lower temperatures
short-chain lipids are
more fluid
long-chain lipids are
less fluid
what are the 2 types of fatty acids?
- saturated fatty acids
- unsaturated fatty acids
do saturated fatty acids increase or decrease membrane fluidity?
decrease
do unsaturated fatty acids increase or decrease membrane fluidity?
increase
why do unsaturated fatty acids increase membrane fluidity?
b/c they contain = bonds which increase the amount of proteins in the membrane and the passage of molecules
cholesterol at high temperatures
decreases fluidity
cholesterol at low temperatures
increases fluidity
lipid rafts are semisolid patches of
sphingomyelin, glycolipids, & cholesterol
lipid rafts are
discrete membrane domains
lipid rafts are thought to move
laterally within the plasma membrane
lipid rafts are thought to associate w/
specific proteins
what is the fluid mosaic model?
the plasma membrane w/ all the proteins, phospholipids, cholesterol. lipid rafts, and carbohydrate components
membrane proteins move _____ along the surface of the membrane
laterally
who observed the migration of labeled proteins and demonstrated the fluid mosaic model?
Frye and Edidin
what are 2 addition features of the plasma membrane?
- glycocalyx
- caveolae
describe glycocalyx
the carbohydrate coat around carbohydrate portions of glycolipids found on the OPM
glycocalyx protects cells from
ionic/mechanical stress
the glycocalyx is a barrier to
invading microorganisms
oligosaccharides participate in
cell-cell interactions
caveolae are
small invagination of the plasma membrane
caveolae are organized by
caveolin
caveolae carry out
receptor-mediated & receptor-independent endocytosis
what are the 3 main categories of membrane proteins?
- integral
- peripheral
- lipid anchored
integral proteins either
completely or partially transect the membrane
what type of integral proteins DO NOT transect the mem. all the way?
monotopic
integral proteins are typically found on the
cytosolic side of the plasma membrane
peripheral proteins DO NOT interact w/
membrane lipids directly
peripheral proteins interact w/
integral proteins
protein that is already in the mem.
lipid anchored proteins are
covalently linked to membrane phospholipid
lipid anchored proteins are inserted to the
cytosolic side of the membrane after being translated & folded
what is the exception of lipid anchored proteins? why?
- GPI anchors
- they are inserted to the outside of the membrane
what are 4 ways in which membrane proteins can be connected to the membrane?
- amphipathic α-helix
- hydrophobic loop
- lipidation
- electrostatic/ionic interactions
amphipathic α-helix protein connection to the membrane sits __________ to the membrane
parallel
lipidation is when a protein is
covalently bound to membrane lipids
explain the electrostatic/ionic interactions which some proteins can use to be connected w/ the plasma membrane
+ charged R groups of aa interact with - charged phospholipids
transmembrane proteins that form channels typically use
α-helices to cross the membrane
other transmembrane proteins that form channels can use
β-barrels
β-barrels are common in
aquaporins
seen in kidney nephron
proteins are anchored to the plasma membrane by
lipids and glycolipids
what are the proteins involved in anchoring other proteins to the cytosolic side of the plasma membrane?
- Src
- Ras
Ras proteins makes connections through
disulfide bonds
Src proteins make connections through
ionic interactions
detergents solubilize
integral mem. proteins
detergents separate what from membranes?
proteins
detergents are synthesized to be
amphipathic
detergent that is considered to be ‘mild’ stringency
SDS
detergent that is considered to be ‘high’ stringency
Triton X-100
what are the 3 types of transport of molecules across the plasma membrane?
- passive/simple diffusion
- facilitated diffusion (passive transport)
- active transport
what are the 2 types of active transport
- indirect
- direct
passive/simple diffusion require NO
energy input
what are the 3 main factors that affect the passive diffusion of molecules?
- size
- polarity
- ion permeability
how can size affect the passive diffusion of molecules?
b/c the mem. is more permeable to small rather than large molecules
how can polarity affect the passive diffusion of molecules?
the mem. is permeable to nonpolar & less permeable to polar molecules
in general lipid bilayers are VERY
impermeable to ions
phospholipid bilayers are ONLY permeable to
- gases
- small hydrophobic molecules
facilitated diffusion is aka
passive transport
does facilitated diffusion require energy?
NO
facilitated diffusion can be described as movement in the direction of
relative concentrations
in facilitated diffusion molecules do NOT dissolve
in the membrane
facilitated diffusion is mediated by
proteins
facilitated diffusion typically transports
charged or polar molecules
give some examples of the charged or polar molecules that are transported through facilitated diffusion
- aa
- nucleosides
- sugars
- ions
what are the 2 classes of proteins that mediated facilitated diffusion?
- carrier proteins
- channel proteins
what are the 2 types of transporters?
- uniport
- cotransport
what are the 2 types of cotransport?
- symport
- antiport
describe the symport cotransport
the transport of two molecules in the SAME direction
describe the antiport cotransport
the transport of two molecules in OPPOSITE directions
what is an example of facilitated diffusion by carrier proteins?
the glucose transporter
the glucose transporter transports molecules in both directions and is dependent on where
[] of glucose is the highest
list the steps of the glucose transporter
- glucose binds to the binding site on the outside of the cell
- transporter changes conformations
- glucose is released to the inside of the cell
- transporter changed to original conformation
most ion channels are NOT permanently ___________, but are regulated by ________ that open in response to __________.
- open
- gates
- stimuli
regulated opening of ion channels mediates transmission of
electrical signals
what are the 2 types of ion channels?
- voltage gated
- ligand gated
voltage gated ion channels open via
membrane depolarization
ligand gated ion channels open via
the binding of a key
which type of transport is extremely rapid?
facilitated diffusion through ion channels
ligand-gated channels open in response to
binding of neurotransmitters/ signaling molecules
voltage-gated ion channels open in response to
changes in electrical potential across the plasma mem.
Na+ channels select Na+ atoms that are
bound to H2O
K+ channels select
dehydrated K+ atoms
isotonicity relies on
total solute concentration
________ is pumped out of the cell while _______ is pumped in
Na+
K+
which ion has the greatest effect on resting membrane potentials? why?
- K+
-b/c some K+ channels remain open
Na+ channels are open during which period of the action potential?
depolarization
what causes hyperpolarization?
the increase amount of open K+ channels
Ca2+ is stored where?
in the ER
list the 5 steps in the
- arrival of action potential
- voltage-gated Ca2+ channels open
- Ca2+ enters axon terminal causing vesicles to bind to synaptic membrane
- neurotransmitter is released into synaptic cleft
- neurotransmitter binds to ligand-gated ion channels
- post-synaptic membrane gets depolarized
what is an example of a unique pore?
porins
porins cross the membrane as
beta-barrel structures
porins enclose an
aqueous pore
porins do NOT allow
charged ions to cross
which types of porins are only found in some unique tissues?
aquaporins
aquaporins allow
rapid passage of large volumes of H2O
aquaporins are found in
- proximal tubules of kidneys
- erythrocytes
what is known as the transport of molecules against their concentration gradient?
active transport
facilitated diffusion is energetically
favorable
active transport is energetically
unfavorable
what are the 3 major functions made possible by active transport?
- nutrient uptake
- substance removal
- maintain non-equilibrium intracellular []
active transport is based on an
energy source
active transport is regarded as either
direct or indirect
direct active transport is aka
primary active transport
indirect active transport is aka
secondary active transport
direct active transport is coupled directly to an
exergonic chemical reaction
indirect active transport depends on the
co-transport of 2 solutes
describe indirect active transport
the movement of a solute down its concentration gradient drives the movement of another solute against its concentrations gradient
with indirect active transport one of the solutes is most likely an
ion
which type of active transport is directly coupled to an exergonic chemical reaction?
direct/ primary active transport
what is the exergonic chemical reaction that direct active transport is coupled with?
hydrolysis of ATP
what is the name for the transport proteins that are driven by ATP hydrolysis?
ATPases ATPase Pumps
the largest family of membrane transporters consists of the
ABC transporters (ABC-type ATPases)
ABC transporters are characterized by a highly conserved ATP-binding domain known as
ATP-binding cassette
the Na+/K+ ATPase pump is used to regulate and maintain ________ in the cell
isotonicity
the Na+/K+ ATPase pump pumps _______ Na+ ____ and _________ K+ ________.
3
out
2
in
list the 9 steps in the sodium/potassium pump
- pump open to inside
- 3 Na+ bind
- ATP phosphorylates causing a conformational change
- 3 Na+ expel outside
- pump is open to the outside
- 2 K+ bind
- dephosphorylation causes a conformational change
- 2 K+ expel inside
- pump returns to original shape
the ion gradients that are maintained by Na+/K+ pumps are essential for
transmission of action potentials
there is a higher concentration of what ion inside the cell?
K+
there is a higher concentration of which ion outside the cell?
Na+ & Cl-
what other pump is also powered by ATP hydrolysis?
Ca2+ pump
Ca2+ is pumped into the
ER lumen
since Ca2+ is pumped into the ER lumen Ca2+ _______________ concentrations are extremely low
intracellular
transient increases in cellular Ca2+ are important in
cell signaling
tonicity is a comparison of
2 aqueous systems
tonicity is the measure of the
osmotic pressure of 2 solutes separated by a semipermeable membrane
tonicity commonly describes a
response of cells immersed in solution
like osmotic pressure, tonicity is influenced by only
solutes that CANNOT cross the membrane
- b/c these exert an osmotic pressure
why do solutes that can freely cross the membrane NOT affect tonicity?
b/c there will always be an equal concentration on both sides of the membrane
osmotic pressure is the pressure that must be applied to a solution to prevent the
inward flow of H2O across a semipermeable membrane
osmolarity (tonicity) is a term used to describe the concentration of a solute in terms of the
osmotic pressure it can exert
the Na+/K+ pump plays a crucial role in regulating
cell volume & maintaining osmolarity
what are the 3 sources of intracellular osmolarity?
- macromolecules
- small inorganic molecules
- small inorganic ions
macromolecules contribute little to osmolarity b/c
they count as a single molecule
macromolecules are highly charged and attract many
inorganic ions of the opposite charge
b/c most small inorganic molecules are charged they attract
counterions
examples of small organic molecules include
sugars, amino acids, nucleotides
small inorganic ions leak
slowly across membrane into the cell
small inorganic ions are attracted to
- macromolecules
- small organic molecules
why does water tend to move in the direction of the cell cytoplasm?
- b/c of the intracellular sources of osmolarity
the Na+/K+ pump increases external osmolarity by
pumping out excess Na+
ABC transporters transport in
1 direction
in bacteria ABC transporters are used to drive the ______________ of _____________.
import
nutrients
in eukaryotes ABC transporters are used to transport toxic substances _____________ of the cell
out
ABC transporters can be problematic in the
delivery of chemotherapeutics
ATP phosphorylation causes the ABC transporters to be open to the
outside of the cell
mutations affecting ABC transporters are the molecular cause of
cystic fibrosis
CFTR stands for
cystic fibrosis transmembrane-conductance regulator
lack of function of Cl- channel results in the
accumulation of mucus in the respiratory & digestive tracks
indirect active transport is aka
secondary active transport
describe indirect active transport
the transport of 2 solutes
- one w/ its concentration gradient
- one against its concentration gradient
what is an example of a transporter that transports molecules in an indirect active transport manner?
the Na+/glucose transporter
describe the indirect active transport mechanism of the Na+/glucose transporter
the downward flow of Na+ provides enough energy to take up glucose and create high intracellular glucose concentrations
list the 6 steps in the active transport of glucose
- pump open outside
- Na+ ion binds (stimulating glucose binding)
- conformational change
- pump opens to inside
- Na+ is released inside
- glucose is released inside
- pump returns to original position
with the Na+/ glucose pump how does the Na+ that’s transported into the cell return to the outside of the cell?
by the Na+/K+ pump
explain the transport of glucose in the intestines
- active transport takes glucose to the apical surface of the intestinal epithelial
- glucose is then transferred to connective tissue and blood via facilitated diffusion
the transport of glucose in the intestines is driven by the
N+/K+ pump
what 4 types of transport are seen in erythrocytes?
- simple diffusion
- facilitated diffusion by carrier proteins
- facilitated diffusion by channel proteins
- active transport
describe the simple diffusion that is seen in erythrocytes?
cellular respiration: diffusion of O2 & CO2
describe the facilitated diffusion by carrier proteins that is seen in erythrocytes?
-glucose
- Cl- and HCO3- by anion exchange protein
describe the facilitated diffusion by channel proteins that is seen in erythrocytes?
- aquaporins
describe the active transport that is seen in erythrocytes?
Na+/K+ pump driven by ATP hydrolysis
