Plasma Membrane (B.B) Flashcards

Question

What are lipid domains? Why are they needed?

Answer 1

Domains - WIth certain lipid mixtures, we can observe phase segregation in which different lipids come together to form domains. - Domains called lipid rafts --\> permanent large scale segregation is uncommon --\> specific proteins/lipids concentrate in a more temporary dynamic fashion --\> facilitated by protein-protein interactions. Why? Good for organising and concentrating membrane proteins for transport (vesicles) or for protein assemblies (important for conversion of extracellular to intracellular signals).

Answer 2

Lipid raft composed of sphingomyelin and cholesterol --\> thicker than the rest of the membrane. Useful --\> acomodates membrane proteins --\> i.e. GPI anchor proteins.

Answer 3

No! Lipid composition between monolayers varies greatly --\> composition changes a lot --\> not absolute. Example: - \> Platelets can only form blood clots when PS is expressed in the outer leaflet. - \> Animal cells undergo apoptosis (a form of programmed cell death, discussed in Chapter 18), phosphatidylserine, which is normally confined to the cytosolic (or inner) monolayer of the plasma membrane lipid bilayer, rapidly translocates to the extracellular (or outer) monolayer --\> signals neighbouring cells, such as macrophages, to phagocytose the dead cell and digest it. How does this translocation occur of PS? 1. The phospholipid translocator that normally transports this lipid from the outer monolayer to the inner monolayer is inactivated. 2. “scramblase” that transfers phospholipids nonspecifically in both directions between the two monolayers is activated.

Answer 4

Lipid asymmetry is functionally --\> especially in converting extracellular signals into intracellular ones. 1. Many cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer --\> hence these heads groups are required for the function of the proteins. The enzyme protein kinase C (PKC), for example, which is activated in response to various extracellular signals, binds to the cytosolic face of the plasma membrane, where phosphatidylserine is concentrated and requires this negatively charged phospholipid for its activity. 2. In other cases, specific lipid head groups must first be modified to create protein-binding sites at a particular time and place. One example is phosphatidylinositol (PI), one of the minor phospholipids that are concentrated in the cytosolic monolayer of cell membranes (see Figure 13–10A–C). Various lipid kinases can add phosphate groups at distinct positions on the inositol ring, creating binding sites that recruit specific proteins from the cytosol to the membrane. An important example of such a lipid kinase is phosphoinositide 3-kinase (PI 3-kinase), which is activated in response to extracellular signals --\> recruits specific intracellular signaling proteins to the cytosolic face of the plasma membrane

Answer 5

- Lipid droplets act as a storage of excess lipids. - They can be retrieved for membrane synthesis or as a food source. - Adipocytes are specialized for this purpose whereas other cells have less and smaller droplets. - Lipid droplets store neutral lipids --\> (triglycerides/cholesterol esters) --\> no hydrophilic group --\> form droplets --\> droplets are surrounded by a monolayer with a large number of proteins --\> formed in ER.

Answer 6

- Most specific tasks are performed by membrane proteins --\> respiration, photosynthesis, cellular response to environmental change (hormones). - Proteins make up roughly 50% of membrane mass --\> varies i.e. myelin on axon is 25% protein by mass whereas, membranes in mitochondria/chloroplasts are roughly 75% protein by mass. - Since lipids are tiny compared to proteins --\> roughly 50 lipid molecules for every protein. - At least 20% of all active DNA that can be used for protein synthesis encodes for membrane proteins.

Answer 7

Note --\> the region that crosses through the hydrophobic bilayer (hydrophobic region of protein) --\> called the transmembrane domain --\> the TM domains are linked by soluble regions (pass through the aqueous environment) 1. /2. --\> alpha helices --\> 1 is a single pass helix whereas, 2 has multiple helices passing through the membrane 3. Beta-barrel --\> made from curved Beta strands.

Answer 8

4. Monotropic membrane protein --\> passes through a single monolayer NOT the entire membrane, 5. /6. Proteins associated with phospholipids --\> covalently attached --\> known as lipid modification. 6. Is a GPI anchor --\> found on exterior leaflet always --\> linked to phospholipids via oligosaccharides linker. 7. /8. associated with other proteins (non-covalently) --\> can be released easily --\> due to environmental change. Note --\> protein 5 is made in the cytosol and anchored to the membrane via a covalent bond whereas, protein 6 is made in the ER --\> TM protein is cleaved off and GPI anchor added --\> then transported to the membrane in a vesicle.

Answer 9

1, Alpha helix 2. Beta-barrel --\> arrows show the direction from the N (Start) to the C terminus (end) When shown in diagram --\> polypeptide backbone is shown --\> side chains are dismissed (convention).

Answer 10

**Alpha helix structure** - Alpha helix starts from the amino terminus and ends at the carboxyl terminus with the R-groups sticking out away from the helix. - Peptide bonds are polar/hydrophilic and cause there is no H₂O --\> A.A is forced to form H-bonds. - Helix is held together by a network of hydrogen bonds between the Carboxyl group (N residue) and the primary amine group (-N-H) (N+4 residue) - All hydrogen bonds are intrahelical. - Structure maximizes H-bonds --\> H-bonds stabilise helix --\> helix is a very stable conformation. - Normally there are 20-30 residues within one helix. - In the TM domain --\> the residues need a hydrophobic R-group.

Answer 11

- Large red peaks --\> indicate a high probability of those residues being located within the membrane. - The top layer of the graph --\> indicates the topology --\> Top line = aq/soluble region (extracellular) --\> Thick block = TM domains --\> Bottom line = aq/soluble region (intracellular).

Answer 12

The strong drive to maximize hydrogen-bonding in the absence of water means that a polypeptide chain that enters the lipid bilayer is likely to pass entirely through it before changing direction since chain bending requires a loss of reg- ular hydrogen-bonding interactions.

Answer 13

Beta-barrels - Formed by multiple curved Beta-strands --\> creates rigid structures --\> used for porins - Found in bacteria, chloroplasts and mitochondrial membranes. - Beta-barrel --\> Hydrophobic residues point towards the lipids --\> Hydrophilic residues point towards the core fo the barrel. - Loops of the polypeptide often protrude into the lumen of the channel --\> narrows the channel --\> controls the solutes that can pass --\> allows porins to be highly selective. - Can also be used as an anchor for cytosolic loops that act as a binding site.

Answer 14

Alpha helices are able to slide against each other/allow for conformational change --\> useful characteristic for membrane proteins (channels, carrier, hormone receptors, etc. Whereas, Beta-barrels to rigid for this.

Answer 15

The topology/orientation of proteins relative to the membrane doesn't change ---\> basically the proteins orientation within membranes stays constant --\> essential for function.

Answer 16

- Functional and structural analysis requires soluble proteins --\> but the lipid environment must be maintained --\> essential for structure and function of the membrane protein. - Detergent --\> sodium dodecyl sulphate is used --\> much more soluble in water than lipids. 1. Use detergent monomers (at high concentrations --\> monomers join together to form micelles --\> point called critical micelle concentration) to extract membrane proteins (displace lipids) --\> micelles have a hydrophilic head + hydrophobic tail --\> hydrophobic ends of detergents bind to the hydrophobic regions of the membrane proteins. 2. Forms water soluble protein-lipid detergent complex which is stable in solution Problem? --\> strong detergents, however, unfold (denature) proteins by binding to their internal “hydrophobic cores,” Solution? ---\> some cases, removal of the SDS allows the purified protein to renature, with the recovery of functional activity. 3. This can be used for testing --\> when using mild detergents the protein can be solubilized and then purified in an active --\> detergent concentration decreased in presence of phospholipid --\> membrane protein reincorporates into small liposomes.

Answer 17

The plasma membrane is semi-permeable - Hydrophobic nature of the membrane prevents the passage of charged/polar molecules --\> hence only allowing non-polar molecules through. - This is important as it allows cells to control and maintain different concentrations of solutes between the cytosol and the extracellular space --\> important for building up concentration gradients. - However, this requires membranes to have mechanisms to move solutes across --\> uptake essential nutrients, excrete metabolic waster, regulate intracellular ion concentrations.

Answer 18

1. Non-polar (no charge distribution) --\> O₂, CO₂, N₂, steroids hormones, etc --\> can move across via passive diffusion. 2. Small uncharged polar molecules (H₂O, urea, glycerol) --\> can move across slowly but at a rate which is insufficient for cell function. 3. Large uncharged polar molecules (glucose, sucrose) --\> can move across but at a rate which is insufficient for cell function. 4. Ions (H⁺, Na⁺, HCO₃^-, K⁺, Ca²⁺, Cl^-, Mg²⁺ ) cannot pass the bilayer --\> highly energetically unfavourable, plus they associate with water (hydration shell) --\> further inhibits movement.

Answer 19

1. Size --\> smaller --\> more likely to pass 2. Charge/polarity Note --\> Given enough tie virtually any molecule with diffuse --\> the rate is the variable factor --\> but ions are completely unable.

Answer 20

1. Transporter --\> mainly for large molecules and export of waste/toxins 2. Channel --\> mainly for small molecule and ions. Play a role in the intake of essential nutrients + ions as well as removing waste + toxins.

Answer 21

Most transporters will only transport one molecule or molecules that are closely related Some transporters can transport a particular class of molecules --\> larger range of species.

Answer 22

Transporters often called carriers or permeases - Process --\> solute binds to a protein on one side of the membrane (binding site) --\> leads to a reversible conformational change --\> at one point the substrate will be in an occluded state --\> eventually results in the release of the solute on the other side. - Protein has a high-affinity solute binding site (only one binding site exposed at one time) --\> this is important as the molecule needs to remain bonded throughout the conformational change. - Systems work for export and import. - Two key characteristics --\> High-affinity binding and a radical conformational change. \ - Transportation can be blocked by a competitive and non-competitive inhibitors.

Answer 23

Yes, transporters may have pseudosymmetry Pseudosymmetry --\> Chemical and physical symmetry. Means that they can work in reverse directions.

Answer 24

Channels - Form much weaker interactions with solutes - Form aqueous pores across the membrane - Pore opens and allows the movement from one side to the other. - Much faster transport than transporter --\> 10⁹s^-1

Answer 25

1. Passive transport --\> Driven by the difference in the concentration gradient. 2. Active transport --\> Using energy to transport substances against the concentration gradient.

Answer 26

Passive transport --\> facilitated diffusion - Applies to all channels and some transporters - Driven by the difference in the concentration gradient - If solute is charged --\> 2 driving forces --\> Conc. gradient + electrical potential difference (membrane potential) --\> also known as electrochemical gradient --\> normally outside in +tive and inside is -tive --\> favours entry of positive charge.

Answer 27

Active transport - Cells transport solutes against the concentration gradient. - Channels can NOT perform active transport --\> only transporters. - Movement is unidirectional. - This is achieved by coupling with a source of metabolic energy (ATP/ion gradient) --\> provide energy.

Answer 28

Energy --\> needed to move substances against the concentration gradient. 1. **Coupled transporters** --\> energy from the electrochemical gradient of one substance is used to transport another substance against the gradient --\> link uphill transport in one direction with downhill transport in the other --\> uses an antiport. 2. **ATP-driven pump** --\> hydrolysis provides energy 3. **Light-driven pump** --\> only in bacteria cells --\> rare

Answer 29

1. Uniport --\> one molecular species move across 2. Symport --\> Movement of substance in the same direction 3. Antiport --\> movement in the opposite direction of two substances. 2/3 --\> examples of co-transport which involves 2 substrates --\> Transport of one solute depends on a second solute --\> harvesting energy from the electrochemical gradient of on to use for the other --\> Usually Na⁺ acts as a large driving force. Plus only when both binding sites are filled will the protein change conformation.

Answer 30

Types of active transport 1. Primary active transport (ATP driven) --\> the ATP hydrolysis and transport mechanism are located on the protein. 2. Secondary active transport --\> first transporter creates ion gradient (i.e. pumping out x) --\> electrical gradient drives uptake of another solute through another protein --\> note this may or may not involve ATP hydrolysis.

Answer 31

1. H⁺ is directly transported out --\> Na⁺-H⁺ exchanger 2. Transporting HCO₃^- in --\> neutralize excess H⁺ ---\> Na⁺-driven-Cl^--HCO₃^- exchanger --\> couples influx of Na⁺ with HCO₃^- to an efflux of Cl^- and H⁺ --\> NaHCO₃^- in and HCL out. --\> cell gets too alkaline. The second method is more effective --\> pump out H⁺ and pump in HCO₃^-which will neutralize H⁺ . Note --\> Na⁺-independent-Cl^--HCO₃^---\> adjusts cystolic pH.

Answer 32

Channels - Form hydrophilic core region (differs physically and chemically between different channels) across the membrane --\> allows polar/charged molecules to pass. - Tends to be narrow and highly selective. - Open and close rapidly - Very efficient/quick --\> 100 million ions/second - Only allow for passive transport --\> movement down a concentration gradient. - Specific role in the transport of inorganic ions and H₂O (aquaporin).

Answer 33

Gating refers to the opening of the channel in response to a specific stimulus. 1. Voltage-gated channels --\> channel opens when there is a change in membrane potential. 2. Ligand-gated --\> an intracellular or extracellular mediator (different types of ligands --\> transmitter, ion, nucleotide) binds to the binding site --\> results in a conformational change --\> opens channel --\> when the ligand is released --\> protein reverts to its original shape. 3. Mechanically gated --\> responsive to changes in pressure (osmotic pressure) ---\> aquaporin

Answer 34

- Channels only allow some selective ions/molecules to pass through --\> molecules/ions with the appropriate size and charge. - Pores are made with particular dimensions and have specific residues (charge) which permit the passage of specific ions/molecules to pass. - Note --\> ions have a hydration shell --\> only when this shell is shed can the ions pass through --\> acts as a selectivity filter.

Answer 35

- Essential to cellular life --\> i.e. needed for the electrical excitability of muscle cells or the leaf closing response. - Transport of water --\> via osmosis - A lot of channels are K⁺ channels which are used for, but not limited to, maintaining the membrane potential --\> positive outside/negative inside.

Answer 36

K⁺channels --\> one of the most common channels. - Made of 4 identical polypeptide chains --\> each chain is made of 2 long helices that cross the membrane and 1 smaller helix (known as the pore helix) --\> example from bacteria (S.Lividas) --\> they are the first K⁺ channels that has its structure studied. - The 2 long helices are TM domains whereas the shorter one is not. - The opening of the channel --\> is filled with many negative residues to attract positively charged K⁺ . - The selectivity loop/filter allows K⁺ (1.33Å) but nothing else i.e. Na⁺(0.95Å) through. - The short helix is polar with a positive end and a negative end closer to the channel.

Answer 37

First of all negatively charged residues create a negative charge near the pore --\> this attracts K⁺ ions to enter the channel. Secondly, in the core of the channel, you have a selectivity loop which is lined with carbonyl oxygens (C=O) --\> these carbonyl oxygen are located perfectly to accommodate the passage of K⁺ ions. Note that the carbonyl oxygens are located so perfectly that the K⁺ ions prefer to lose the hydrating shell and associate with the carbonyl oxygen --\> results in the shedding of the H₂O which is needed for the passage (energetically favourable)

Answer 38

Na⁺ is too small to form interactions with carbonyl oxygen --\> not energetically favourable --\> prefers to keep its hydrating shell.

Answer 39

In K⁺channel there are 4 potential binding sites which are stabilized by carbonyl oxygens --\> 4 regions where K⁺ can pass. However, only two are occupied at one time because ions each other --\> note this repulsion also serves as another driving force for K⁺ ions --\> repel each other upwards.

Answer 40

Voltage-gated channel - voltage-gated K⁺ channels --\> Senses membrane potential --\> changes conformation in response to changes in potential. - Composed of a single polypeptide with 4 domains. - Each polypeptide contains two transmembrane alpha helices --\> that surround the central ion conducting pore. - There are four additional alpha helices --\> one of which is a voltage sensor (shown in red on the diagram). - Voltage-gated senors --\> abundant in arginine residues --\> positively charged.

Answer 41

1. S4 helix (positively charged) is attracted to negatively charge on the inside of the membrane. 2. As S4 is connected to the S5 subunit --\> so this ensures that the pore remains closed. 3. Membrane potential reverse --\> outside is negative and inside is positive. 4. S4 helix is attracted to the negative charge on the outside now --\> moves up 5. This then pulls down on the S5 subunit --\> changes the conformation of the protein. Note ---\> reverse process for closing. 6. This then opens the pore allowing for the movement of K⁺ ions.

Answer 42

Aquaporin --\> water channels - Responsible for water secretion in tears, sweat, water uptake (kidney --\> collecting duct) and saliva production. - Flow rate --\> 10⁹ S^-1 --\> low activation energy. - The direction of flow depends on the osmotic gradient. - Aquaporin is a tetramer made up of 4 identical polypeptide chains --\> each monomer contains 6 transmembrane alpha helices + 2 half helices. - Asparagine-proline-alanine residues are key in preventing the passage of hydronium ions (H₃O⁺) --\> positive charges repel --\> act as a selectivity filter plus pore is 2.8Å --\> prevents larger molecules from passing.

Answer 43

H₂O molecule move single file through the aquaporin protein --\> form transient interactions with the amino acid residues to move through.

Answer 44

Aquaporin objective --\> transport of neutral water - Positively charged Arg residues prevent the passage of + charged ions (H₃O⁺) - Hydronium ions diffuse through water extremely rapidly, using a molecular relay mechanism that requires the making and breaking of hydrogen bonds between adjacent water molecules --\> strategically placed asparagines --\> bind to the oxygen atom of the central water molecule in the line of water molecules traversing the pore, imposing a bipolarity on the entire column of water molecules --\> impossible for the “making and breaking” sequence of hydrogen bonds central Oxygen in H₂O can't form H-bonds --\> H₃O⁺ can't get past central asparagine. - OH^- cannot pass because it is always associated with other molecules/ions.

Answer 45

Receptor function - Important for neurotransmitters, hormones, light, odorants, and drugs --\> which cause an effect by interacting with the cell surface receptors. - Undergo conformational change --\> results in a transmission of information to the cell --\> but the ligand does not pass.

Answer 46

Three types of receptors 1. Ion-channel coupled receptors --\> ligand-gated ion channel which acts as a receptor + transport protein. 2. Enzyme coupled reactions --\> this is when one signal molecule binds two receptors (inactive) bring them together to form an active catalytic domain or active complex. 3. G-protein coupled receptor --\> receptor that has an exposed ligand binding site on the extracellular membrane --\> active receptor binds to g-protein --\> activated receptor and G protein activate an enzyme which initiates a cascade of reactions.

Answer 47

G-protein coupled receptors There are many types --\> all share a similar architecture - Always 7 T.M.D - Specific membrane topology - N terminus is extracellular / C terminus is intracellular - Within the helical bundle, one can find the ligand binding site. - Ligand binds which induces a conformational change --\> activate the G-protein - Ligand has a high affinity for receptor --\> affinity measured using molar constant --\> measurement that measures the minimum concentration needed to see the desired response in the cell.

Answer 48

G-protein --\> protein modified to lipids in the membrane (receptors can pull specific G-proteins from the cytosol). - Note there are more receptors than G-proteins --\> G-proteins can bind to multiple receptors. - They are heterotrimeric proteins (made of 3 polypeptides that are different --\> denoted as α, β, γ) - α and γ is modified so that they are attached to lipids --\> β subunit is very important as it holds the two other subunits together and also mediates all interactions conducted by the other proteins.

Answer 49

G-proteins coupled receptors 1. In the unactivated form, the alpha subunit of the G-protein binds to GDP. 2. However, when the ligand binds --\> it causes a conformational change in the receptor. 3. This conformational change catalyzes the exchange from GDP to GTP 4. This results in the alpha subunit dissociating from the beta and gamma subunits. 5. The released alpha subunit is free to interact with other proteins --\> adenylate cyclase --\> results in the activation of downstream signalling For example: In response to epinephrine --\> Adenylate cyclase --\> converts ATP into cAMP --\> cAMP activates protein kinase --\> results in increased ATP production, degradation of food stores, etc.. --\> effects also extend to the nucleus --\> increases/decreased protein expression.

Answer 50

Important to shut down system --\> many metabolic pathways involve a signalling cascade which amplifies the response --\> hence it is important not to waste resources. Alpha subunits have GTPase activity --\> Overtime the GTP bound to the alpha subunit breaks down into GDP --\> once broken down it spontaneously reassociates with the Beta gamma dimer --\> reforms the inactive heterotrimer. This breakdown process is quick enough to minimise waste but slow enough that the Alpha subunit can impact the effector before breaking down.

Answer 51

It is important to make the ligand dissociate and to prevent other ligands from binding in order to prevent an excessive response. However, ligand dissociation is difficult due to high affinity (ligand does dissociate by itself but not quick enough) and we need to prevent other intracellular ligands from binding. How? G-protein kinase --\> Phosphorylation of the serine and threonine residues on the C-terminal tail --\> this provides a platform for arrestin molecule to bind to the receptor --\> blocks any other G-proteins from binding --\> this method shuts down the system.

Answer 52

The same forces that drive phospholipids to form bilayers also provide a self-sealing property. A small tear in the bilayer creates a free edge with water; because this is energetically unfavourable, the lipids tend to rearrange spontaneously to eliminate the free edge. The prohibition of free edges has a profound consequence: the only way for a bilayer to avoid having edges is by closing in on itself and forming a sealed compartment

Answer 53

This inability to flip flop creates a problem for their synthesis. Phospholipid molecules are manufactured in only one monolayer of a membrane, mainly in the cytosolic monolayer of the endoplasmic reticulum membrane. If none of these newly made molecules could migrate reasonably promptly to the noncytosolic monolayer, new lipid bilayer could not be made. Solved using --\> phospholipid translocators, or flippases

Answer 54

Sugar-containing lipid molecules called glycolipids have the most extreme asymmetry in their membrane distribution: these molecules, whether in the plasma membrane or in intracellular membranes, are found exclusively in the monolayer facing away from the cytosol. - In animal cells, they are made from sphingosine, just like sphingomyelin

Answer 55

The asymmetric distribution of glycolipids in the bilayer results from the addition of sugar groups to the lipid molecules in the lumen of the Golgi apparatus. Thus, the compartment in which they are manufactured is topologically equivalent to the exterior of the cell. As they are delivered to the plasma membrane, the sugar groups are exposed at the cell surface

Answer 56

The glycolipid molecules tend to self-associate, partly through hydrogen bonds between their sugars and partly through van der Waals forces between their long and straight hydrocarbon chains, which causes them to partition preferentially into lipid raft phases

Answer 57

In these proteins, neighbouring transmembrane helices in the folded structure of the protein shield many of the other transmembrane helices from the membrane lipid. How come the helices are still hydrophobic? The answer lies in the way in which multi-pass proteins are integrated into the membrane during their biosynthesis --\> transmembrane α helices are inserted into the lipid bilayer sequentially by a protein translocator. After leaving the translocator, each helix is transiently surrounded by lipids, which requires that the helix be hydrophobic.

Answer 58

As in glycolipids, the sugar residues are added in the lumen of the ER and the Golgi apparatus. For this reason, the oligosaccharide chains are always present on the noncytosolic side of the membrane.

Answer 59

Another important difference between proteins on the two sides of the membrane results from the reducing environment of the extracellular environment. - The cytosolic environment decreases the likelihood that intrachain or interchain disulfide (S–S) bonds will form between cysteines on the cytosolic side of membranes. - Due to the reducing environments --\> intrachain or interchain disulfide (S–S) bonds form on the noncytosolic side, where they can help stabilize either the folded structure of the polypeptide chain or its association with other polypeptide chains

Answer 60

1. Protein --\> glycoproteins --\> oligosaccharide chain covalently bonded to the membrane protein. 2. Protein --\> proteoglycan --\> long chain polysaccharide chain linked covalently to a protein core. Note --\> protein core either extends across the lipid bilayer or is attached to the bilayer by a GPI anchor. 3. Lipids --\> glycolipids --\> oligosaccharide chain covalently bonded to the membrane lipid.

Answer 61

A term used to describe the carbohydrate-rich zone on the cell surface.

Answer 62

Protein functions as a light-activated H+ pump that transfers H+ out of the archaeal cell (Halobacterium salinarum). Function --\> converts solar energy into an H⁺ gradient, which provides energy to the archaeal cell.

Answer 63

Bacteriorhodopsin molecule is folded into **seven** closely packed transmembrane α helices and contains a single light-absorbing group or chromophore (retinal). Retinal is covalently linked to a lysine side chain of the bacteriorhodopsin protein.

Answer 64

When activated by a single photon of light, the excited chromophore changes its shape and causes a series of small conformational changes in the protein, resulting in the transfer of one H+ from the inside to the outside of the cell. Light-driven proton transfer establishes an H+ gradient across the plasma membrane, which in turn drives the production of ATP by a second protein in the cell’s plasma membrane.

Answer 65

When the transporter is saturated (that is, when all solute-binding sites are occu- pied), the rate of transport is maximal. This rate, referred to as Vmax (V for veloc- ity), is characteristic of the specific carrier. Vmax measures the rate at which the carrier can flip between its conformational states.

Answer 66

Each transporter has a characteristic affinity for its solute, reflected in the Km of the reaction, which is equal to the concentration of solute when the transport rate is half its maximum value.

Answer 67

Primary active transport --\> free energy of ATP hydrolysis is used to directly drive the transport of a solute against its concentra- tion gradient. Secondary active transport --\> ion-driven coupled transporters (coupling of movement)

Answer 68

1. Transporters are typically built from bundles of 10 or more α helices that span the membrane. 2. Solute and ion-binding sites are located midway through the membrane, where some helices are broken or distorted and amino acid side chains and polypeptide backbone atoms form ion- and solute-binding sites. 3. In the inward-open and outward-open conformations, these binding sites are accessible by passageways from one side of the membrane but not the other. 4. Transitioning between the two conformations you usually have an occluded conformation --\> passageway is closed.

Answer 69

1. **P-type pumps** are structurally and functionally related multipass transmembrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes. 2. **ABC transporters** (ATP-Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes. 3. **V-type pumps** are turbine-like protein machines, constructed from multiple different subunits. The V-type proton pump transfers H+ into organelles such as lysosomes, synaptic vesicles, and plant or yeast vacuoles (V = vacuolar), to acidify the interior of these organelles. 4. **Extra** --\> F-type ATPases, more commonly called ATP synthases because they normally work in reverse --\> use the H+ gradient across the membrane to drive the synthesis of ATP from ADP and phosphate

Answer 70

After the release of calcium stores from the SR --\> Ca²⁺ pump moves Ca²⁺ from the cytosol back into the SR. **Structure** - 10 transmembrane α helices connected to three cytosolic domains. - Amino acid side chains protruding from the transmembrane helices form two centrally positioned binding sites for Ca²⁺--\> normally available on the cytosolic side. **Function** 1. Ca²⁺ binding triggers a series of conformational changes that close the passageway to the cytosol. 2. Activate a phosphotransfer reaction in which the terminal phosphate of the ATP is transferred to an aspartate. 3. The ADP then dissociates and is replaced with a fresh ATP --\> causing another conformational change that opens a passageway to the SR lumen through which the two Ca²⁺ ions exit. 4. Ca²⁺ Binding sites stabilized by two H+ ions and a water molecule --\> closes passageway. 5. Hydrolysis of the phosphoryl-aspartate bond returns the pump to the initial conformation

Answer 71

- The concentration of K+ is typically 10–30 times higher inside cells than outside, whereas the reverse is true of Na+. (High K⁺ conc. inside and high Na⁺outside) - Na⁺-K⁺ pump, or Na⁺- K⁺ ATPase maintains this concentration gradient --\> Like the Ca²⁺ pump, the Na⁺-K⁺ pump belongs to the family of P-type ATPases -\> functions as an ATP-driven antiporter, actively pumping Na+ out of the cell against the electrochemical gradient and pumping K+ in. - Na⁺-K⁺ pump drives three positively charged ions out of the cell for every two it pumps in --\> makes it electrogenic --\> it drives a net electric current across the membrane, tending to create an electrical potential --\> contributes more than 10% to the membrane potential

Answer 72

ABC transporters --\> each member contains two highly conserved ATPase domains --\> cytosolic side of the membrane. - ATP binding brings together the two ATPase domains --\> exposing the binding site --\> ATP hydrolysis leads to their dissociation (revert to original shape) --\> moving the substances across the membrane - Transport is directional toward inside or toward outside.

Answer 73

1. Ion selectivity, permitting some inorganic ions to pass, but not others. 2. Ion channels are not continuously open. Instead, they are gated, which allows voltage-gated ligand-gated (extracellular ligand) them to open briefly and then close again.

Answer 74

Mechanosensitive channel --\> proteins that are capable of responding to mechanical forces. Difficult to study --\> complex interactions with extracellular matrix.... but a well-studied class of mechanosensitive channels is found in the bacterial plasma membrane. These channels open in response to mechanical stretch- ing of the lipid bilayer in which they are embedded.

Answer 75

When the cell is in a hypotonic solution --\> it will absorb water from the surroundings and swell.

Plasma Membrane (B.B) Flashcards

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