Cell bio: Membrane Transport Flashcards
MEMBRANE TRANSPORT- how do cells organize what comes in and what goes out?
What do the K and Na channels do?
What do the NA+/K+ channels do? =
- they maintain the resting membrane potential
- ions go against the conc. gradient so from lowest to highest conc. which requires energy (active transport)
- Leak channels K+ and Na+ do the opposite, ions go from higher to lower conc. thus no energy needed (passive transport)
MEMBRANE TRANSPORT
MEMBRANE PERMEABILITY AND TRANSPORT
- The interior of the lipid bilayer is * ? *, thus the passage of “ ? “
- This allows cells to maintain ** ? ** in its ? that differ from those in the ? and in each of the ?
- However, cells must transfer molecules and ions across their membranes to maintain the homeostasis
- 15-30% of all membrane proteins are ?
CYTOSOL (ICF):
liquid ? surrounding ?
CYTOPLASM:
All the materials inside a cell except for the cell ?
MEMBRANE TRANSPORT
MEMBRANE PERMEABILITY AND TRANSPORT
- The interior of the lipid bilayer is * hydrophobic *, thus the passage of “ polar molecules is restricted “
- This allows cells to maintain ** concentration of solutes ** in its cytosol that differ from those in the extracellular fluid and in each of the intracellular compartments
- However, cells must transfer molecules and ions across their membranes to maintain homeostasis
- 15-30% of all membrane proteins are transport proteins
CYTOSOL (ICF):
liquid matrix surrounding organelles
CYTOPLASM:
All the materials inside a cell except for the cell transport proteins
The ? the molecule and the ? strongly associated with ?
→ the more ? the molecule diffuses across the membrane
Would these easily diffuse through?
1.Hydrophobic molecules (O2, CO2, N2, steroid hormones)
2. Small uncharged polar molecules
3. Large uncharged polar molecules
4. Ions
The smaller the molecule and the less strongly associated with water
→ the more rapidly the molecule diffuses across the membrane
Would these easily diffuse through?
1.Hydrophobic molecules (O2, CO2, N2, steroid hormones) - YES!
2. Small uncharged polar molecules (H2O, urea, glycerol): no but if they stay long enough near the lipid bilayer then some will diffuse through
3. Large uncharged polar molecules (glucose, sucrose): even fewer than small uncharged polar molecules will diffuse through as its larger
4. Ions: (H+, Na+, K+..): NOT AT ALL as they are fully charged
MEMBRANE TRANSPORT
[movement of ions will change the membrane voltage and membrane voltage change the way the cell will respond to diff. stimuli arriving there]
TRANSPORT PROTEINS:
* transfer specific ? molecules across the plasma membrane
MEMBRANE POTENTIAL
Membrane potential
(resting membrane potential = the membrane potential of an ? cell)
- A ? in the electrical charge on the two sides of a membrane due to a slight excess of positive ions over negative ones on one side and a slight deficit on the other
normally inside of the cell where more of K + is present relative to Na+, its more ANIONIC (-) INSIDE the cell
note: CAT RED; AN OX so reduction is gain of electrons thus + so cations = +
MEMBRANE TRANSPORT
[movement of ions will change the membrane voltage and membrane voltage change the way the cell will respond to diff. stimuli arriving there]
TRANSPORT PROTEINS:
* transfer specific hydrophillic molecules across the plasma membrane
MEMBRANE POTENTIAL
Membrane potential
(resting membrane potential = the membrane potential of an unstimulated cell)
- A difference in the electrical charge on the two sides of a membrane due to a slight excess of positive ions over negative ones on one side and a slight deficit on the other
normally inside of the cell where more of K + is present relative to Na+, its more ANIONIC (-) INSIDE the cell
note: CAT RED; AN OX so reduction is gain of electrons thus + so cations = +
MEMBRANE POTENTIAL
The resting membrane potential of cells is the result of an active transport
(electrogenic) and a passive diffusion, as follows:
- Na +,K +-ATPase pumps ? out of the cell and draws K+ ions into the cell
- K+ tends to diffuse out of the cell through ? (? channels specifically) to reach an equilibrium whereas ? charged ions (phosphates and proteins) stay inside the cell
- The ? of the cell will turn more negative
(- ? to -90 mV)
conc. of K inside more so leak channel allow K to flow out TOWARDS its conc. gradient (low to high) so inside more - as anions stay inside the cell.
MEMBRANE POTENTIAL
The resting membrane potential of cells is the result of an active transport
(electrogenic) and a passive diffusion, as follows:
- Na +,K +-ATPase pumps Na+ out of the cell and draws K+ ions into the cell
- K+ tends to diffuse out of the cell through potassium channels (leak channels specifically) to reach an equilibrium whereas ( - ) ly charged ions (phosphates and proteins) stay inside the cell
- The interior of the cell will turn more negative
(- 70 to -90 mV)
MEMBRANE TRANSPORT
The ? of a charged solute affects its transport
(Electrochemical gradient = combination of ? and ? of the solute)
Normally inside of cell is more ? than outside so when electrochemical gradient with membrane potential positive inside then v few ions will go in as + (present on the outside) will not want to go inside where + lies as + and + dont attract
Thus the MOST ions transport inside the membrane occurs when electrochemical gradient with membrane potential negative inside and positive outside as inside is - so the + ions outside (usually more + outside) will want to go into the membrane as + of the outside is attracted to - of the inside (+ attracts -)
The electrochemical gradient of a charged solute affects its transport
(Electrochemical gradient = combination of membrane potential and conc. gradient of the solute)
MEMBRANE TRANSPORT PROTEINS - TOPOLOGY
Proteins can ? in different ways (proteins contain hydrophilic and hydrophobic regions)
transporters are proteins and they are made of these shapes:
- Single alpha helix; (more ? as connected to inner membrane where ? tails are present)
- Multiple ? helices
- Rolled-up ? (beta barrel) - beta sheets are usually ? -> associated with transporter and channels
- Attached only to ? layer (with one hydrophobic face) - could be inner or outer side of membrane
- Attached to the membrane by a ? bound lipid chain
- Via an ?
- Attached to other ?
diff. ways we can find transporter structures given above:
Proteins can associate with plasma in different ways (proteins contain hydrophilic and hydrophobic regions)
transporters are proteins and they are made of these shapes:
- Single alpha helix; (more hydrophoic as connected to inner membrane where hydrophobic tails are present)
- Multiple alpha helices
- Rolled-up beta sheet (beta barrel) - beta sheets are usually channels -> associated with transporter and channels
- Attached only to one layer (with one hydrophobic face) - could be inner or outer side of membrane
- Attached to the membrane by a covalently bound lipid chain
- Via an oligosaccharide
- Attached to other proteins
TRANSPORT PROTEINS
TRANSPORTERS SHARE COMMON STRUCTURAL FEATURES:
- They typically consist of ? that span the membrane (transmembrane domains)
- Substrate ? sites are located ? through the membrane
- They show two different states:
- ? OR
- ? conformation
- The binding sites are ? of the
membrane at one time - They would be able to work in the ? direction if ion and solute gradients were adjusted
TRANSPORTERS SHARE COMMON STRUCTURAL FEATURES:
- They typically consist of 10 or more alpha helices that span the membrane (transmembrane domains)
- Substrate binding sites are located midway through the membrane
- They show two different states:
- inward-open OR
*outward-open conformation
(when binds to solute it transfers the solute up, closes the lower part and then opens the upper part to release it) - The binding sites are accessible by passageways from only side of the
membrane at one time - They would be able to work in the reverse direction if ion and solute gradients were adjusted
TRANSPORT PROTEINS
Most membrane proteins cross the lipid
bilayer in an ? conformation
(Absorption by enterocytes of the MONOSACCHARIDE products of carbohydrate digestion by the two below:
GLUT = glucose transporter,
SGLT-1 = sodium (Na+)-dependent
glucose cotransporter)
Na+/glucose cotransporter SGLT
Glucose transporter GLUT
TRANSPORT PROTEINS
Na+/Ca2+ exchanger (NCX)
Bidirectional (aka ?) transporter → 1 x Ca2+ out of the cell and 3 x Na+ into the cell. (this exchanger is really imp. in maintaining the balancing the levels of calcium inside and out of cell)
-> Is an ? that removes calcium from cells
-> It uses the energy that is stored in
the ? of sodium (Na+) by allowing Na+ to flow ? its gradient across
the plasma membrane in exchange for the countertransport of ? ions (Ca2+)
*** NCX exists in many different cell types and animal species
- is considered one of the most important cellular mechanisms for
? Ca2+ - found in the ? and the ?
and ? of excitable cells ***
TRANSPORT PROTEINS
Most membrane proteins cross the lipid
bilayer in an a-helical conformation
-> Is an antiporter membrane protein that removes calcium from cells
-> It uses the energy that is stored in
the electrochemical gradient of sodium (Na+) by allowing Na+ to flow down its gradient across
the plasma membrane in exchange for the countertransport of calcium ions (Ca2+)
*** NCX exists in many different cell types and animal species
- is considered one of the most important cellular mechanisms for
removing Ca2+ - found in the plasma membrane and the mitochondria and ER of excitable cells ***
TRANSPORT PROTEINS
NCKX2 Na+/Ca2+-K+ exchanger
(so we don’t only have Na+ and Ca+ but we also have K+ which is imp. for transport)
- Located on ? cell membranes and constitutes a ? mechanism, with key roles in ? plasticity
- It is associated with ?, ?, and ? functions
MEMBRANE TRANSPORT
The ? is present in the plasma membrane of almost all animal cells and
maintains Na+ and K+ concentration differences across the plasma membrane
TRANSPORT PROTEINS
NCKX2 Na+/Ca2+-K+ exchanger
(so we don’t only have Na+ and Ca+ but we also have K+ which is imp. for transport)
- Located on neuronal cell membranes and constitutes a Ca2+ mechanism, with key roles in synaptic plasticity
- It is associated with motor learning, memory, and cognitive functions
MEMBRANE TRANSPORT
The Na+, K+-ATPase is present in the plasma membrane of ALMOST ALL animal cells and maintains Na+ and K+ concentration differences across the plasma membrane
(it transfers 3 Na+ outside the channel and 2 K inside )
TWO MAIN CLASSES OF MEMBRANE TRANSPORT PROTEINS: CHANNELS AND TRANSPORTERS (CARRIERS)
- Channels form pores for ? solutes (ions, water, ammonia) so Na+ channels would not allow K+ ions to pass through it
They interact with the solute (yellow things in pic - 1st one) much more ? compared to transporters
- Transporters ? the specific substrate (solute) to be transported and undergo a series of ? changes that alternately expose ?-binding sites on one side of the membrane and then to the other to transfer the ? across it
TWO MAIN CLASSES OF MEMBRANE TRANSPORT PROTEINS: CHANNELS AND TRANSPORTERS (CARRIERS)
- Channels form pores for specific solutes (ions, water, ammonia) so Na+ channels would not allow K+ ions to pass through it
They interact with the solute (yellow things in pic - 1st one) much more weakly compared to transporters
- Transporters bind the specific substrate (solute) to be transported and undergo a series of conformational changes that alternately expose substrate-binding sites on one side of the membrane and then to the other to transfer the solute across it
GATED ION CHANNEL TYPES
Gated/K+ channel, Nicotinic Ach-receptor Mechanosensitive channels
VOLTAGE GATED CHANNELS:
needs a ? as its gated channels
(v imp in ? - AP: gated channels)
LIGAND-GATED (extracellular ligand)
ligand-gated mostly towards the ? part of cells and imp. for cellular communication
MECHANICALLY-GATED CHANNELS Mechanical will respond to mechanical stimuli
** Mechanosensitive channels: convert mechanical stimuli to ? or ? signals thereby modulating sensation
Skin: sensing ?, ? sensation, stretch, touch, and ? touch
Veins: ? pressure
Cells: ? pressure **
GATED ION CHANNEL TYPES
Gated/K+ channel, Nicotinic Ach-receptor Mechanosensitive channels
VOLTAGE GATED CHANNELS:
needs a trigger as its gated channels
(v imp in axon - AP: gated channels)
LIGAND-GATED (extracellular ligand)
ligand-gated mostly towards the inner part of cells and imp. for cellular communication
MECHANICALLY-GATED CHANNELS Mechanical will respond to mechanical stimuli
** Mechanosensitive channels: convert mechanical stimuli to chemical or electrical signals thereby modulating sensation
Skin: sensing vibration, pressure sensation, stretch, touch, and light touch
Veins: blood pressure
Cells: osmotic pressure **
K+ LEAK CHANNEL
The vestibule and the selectivity filter
- Channels are not ?, K+ flows through concentration gradient
- In the ? (chamber), the ions are hydrated (refer to pic)
- In the ?, they have lost their water and oxygens of the carbonyl groups and the channel accommodates the dehydrated solutes
- Since ? is smaller than potassium, it can not be successfully accommodated (see 2nd structure/middle - K fits perfectly but Na+ doesn’t) and will not be recognized in the ?
K+ LEAK CHANNEL
The vestibule and the selectivity filter
- Channels are not gated, K+ flows through concentration gradient
- In the vestibule (chamber), the ions are hydrated
- In the selectivity filter, they have lost their water and oxygens of the carbonyl groups and the channel accommodate the dehydrated solutes
- Since Na+ is smaller than potassium, it can not be succesfully accomodated and will not be recognized in the filter
AQUAPORINS: SPECIFIC WATER CHANNELS
* Facilitate ? water flow
* Cells that secrete ? amounts of ? (such as those lining ducts of exocrine glands, mammary gland, sweat glands) OR
* Cells that ? high volumes of water (in the kidney)
-> express ? on plasma membrane making water movement more efficient
Some aquaporins are ?-responsive and play an important role in the formation of ? in animals
Anti-diuretic hormone (ADH) stimulates AQPs in the collecting ducts
Water ? -> increase ? -> activation of ? (hypothalamus) -> ? secretion (posterior pituitary) -> increase water permeability in ?.
THEREFORE ADH acts on Kidneys; reabsorbs more water in plasma (resulting in INCREASED plasma volume)
AQUAPORINS: SPECIFIC WATER CHANNELS
- Facilitate osmotic water flow
- Cells that secrete high amounts of water (such as those lining ducts of exocrine glands, mammary gland, sweat glands) OR
- Cells that reabsorb high volumes of water (in the kidney)
-> express aquaporins on plasma membrane making water movement more efficient
Some aquaporins are hormone-responsive and play an important role in the formation of a concentrated urine in animals
Anti-diuretic hormone (ADH) stimulates AQPs in the collecting ducts
Water deficit -> increase extracellular osmolarity -> activation of osmoreceptors (hypothalamus) -> ADH secretion (posterior pituitary) -> increase water permeability in collecting ducts.
THEREFORE ADH acts on Kidneys; reabsorbs more water in plasma (resulting in INCREASED plasma volume)
-> the ones that live in desert will have aquaporin working v efficiently like Mexican red hair
MEMBRANE TRANSPORT - Transporters
Each transporter can have one or more specific binding sites for its solute (substrate)
- Outward-open state: The binding site for solutes is exposed to the ?
- Occluded state: binding sites are ?
- Inward-open state: binding sites exposed to the ?
MEMBRANE TRANSPORT - Transporters
Each transporter can have one or more specific binding sites for its solute (substrate)
- Outward-open state: The binding site for solutes is exposed to the outside
- Occluded state: binding sites are not accessible
- Inward-open state: binding sites exposed to the inside
TRANSPORT CAN BE ACTIVE OR PASSIVE
Passive transport down a concentration gradient occurs spontaneously by ? (through the ? or ? or ?).
ENERGY NEEDED OR NOT?
Active transport requires ? as it moves solutes against their concentration gradients
- it is always mediated by ?
Passive transport down a concentration gradient occurs spontaneously by diffusion (through the plasma mem. or channel or transporter).
NO ENERGY NEEDED (goes down conc. gradient)
Active transport requires energy as it moves solutes against their concentration gradients
- it always mediated by TRANSPORTERS
PASSIVE - FACILITATED DIFFUSION
- It involves the movement of molecules in the direction determined by their relative ? inside and outside of the cell
- is external source of energy provided?, so molecules travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane
molecules -> conc. gradients
charged molecules -> electric potential
- It differs from passive simple diffusion in that the transported molecules do not ? in the phospholipid bilayer
- Their passage is mediated by proteins (channels or carriers) that enables ? and ? (?) molecules to cross the membrane without directly interacting with its ? interior
examples?
PASSIVE - FACILITATED DIFFUSION
(note: here going down its conc. gradient , its active transport that goes AGAINST its conc. gradient!)
- It involves the movement of molecules in the direction determined by their relative concentratoin inside and outside of the cell
- ** external source of energy is NOT provided!!!! **
so molecules travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane
molecules -> conc. gradients
charged molecules -> electric potential
- It differs from passive simple diffusion in that the transported molecules do not dissolve in the phospholipid bilayer
- Their passage is mediated by proteins (channels or carriers) that enables large and polar (charged) molecules to cross the membrane without directly interacting with its hydrophobic interior
e.g. GLUT transporters
- GLUT 5 - Fructose
- GLUT 2 - Glucose, fructose, galactose (monosaccharides)
FACILITATED DIFFUSION VS SIMPLE DIFFUSION
- ? and ? are directly proportional to the solute concentration
- Carrier/Transporter-mediated transport is * ? * (recall: enzyme kinetics and inhibition of activity – binding site)
FACILITATED DIFFUSION VS SIMPLE DIFFUSION
- simple diffusion and channel-mediated are directly proportional to the solute concentration
- Carrier/Transporter-mediated transport is * saturable * (recall: enzyme kinetics and inhibition of activity – binding site)
-> HYPERBOLIC CURVE
if increasing conc. on one side then will keep passing through so direction linear line but when talking about carrier-mediated transport the protein that needs to bind to solute then we can saturate those transporters on the plasma mem. then a hyperbolic curve seen
ACCORDING TO THE ENERGY SOURCE
- Primary active (Na, K, ATPase) -> antiporter
- Na out and K in with help of ATPase (converts ATP to ADP+Pi)
- ALWAYS USES ? - primary source of energy!! - ** IMP Secondary active (Na+, H+ exchanger, NHE) - antiporter or symporter?
- DRIVEN BY A GRADIENT THAT WAS GENERATED BY A ? ACTIVE TRANSPORTER
- ? goes inside the cell (due to gradient from Na+ going out of cell through primary active NA K ATPase pump & ? goes out - ** IMP Tertiary active (proton/peptides co-transporter, PEPT)
- DRIVEN BY A GRADIENT THAT WAS GENERATED BY A ? ACTIVE TRANSPORTER
- H+ due to H+ gradient from secondary active (H went out of cell in secondary active) so now in tertiary H goes ? the cell through tertiary active and since its a co-transporter/sympoter, H and Di/Tripeptides both are transferred inside or outside the cell?
ACCORDING TO THE ENERGY SOURCE
- Primary active (Na, K, ATPase) -> antiporter
- Na out and K in with help of ATPase (converts ATP to ADP+Pi)
- ALWAYS USES ATP - primary source of energy!! - ** IMP Secondary active (Na+, H+ exchanger, NHE) -antiporter
- DRIVEN BY A GRADIENT THAT WAS GENERATED BY A PRIMARY ACTIVE TRANSPORTER
- Na goes inside the cell (due to gradient from Na+ going out of cell through primary active NA K ATPase pump & H goes out -> antiporter) - ** IMP Tertiary active (proton/peptides co-transporter, PEPT)
- DRIVEN BY A GRADIENT THAT WAS GENERATED BY A SECONDARY ACTIVE TRANSPORTER
- H+ due to H+ gradient from secondary active (H went out of cell in secondary active) so now in tertiary H goes inside the cell through tertiary active and since its a co-transporter/sympoter, H and Di/Tripeptides both are transferred inside the cell
