Cell Biology Exam 4

membrane proteins can act as transporters, anchors, receptors, and enzymes

identify the primary functions that can be carried out by membrane proteins

transport amino acids, ions, proteins, and nucleotides across the membrane

membrane proteins acting as a transporter do what?

they are responsible for the attachment of the membrane to the cytoskeleton, extracellular matrix, and adjacent cells

membrane proteins acting as an anchor do what?

they are responsible for the detection of extracellular chemical signals and relay information inside of the cell

membrane proteins acting as a receptor do what?

they catalyze reactions of the membrane in response to a signal

membrane proteins with enzymatic functions do what?

they are directly attached to the membrane, their removal requires disruption of the membrane with detergents, they include transmembrane, membrane-associated and lipid-linked proteins

Describe integral membrane proteins

they are released by gentle treatment that leave the membrane intact, they include protein-attached proteins

Describe peripheral membrane proteins

Integral proteins are imbedded within or span the lipid bilayer while peripheral proteins are attached to the inner or outer surface. Integral proteins interact with the hydrophobic core of the plasma membrane while peripheral proteins interact with the hydrophilic heads of the bilayer and other membrane proteins. Integral proteins are more stably attached, meaning they are harder to remove. Integral proteins are amphipathic, while peripheral proteins are primarily hydrophilic.

what are the key differences between integral and peripheral membrane proteins

transmembrane proteins, membrane-associated proteins, and lipid-linked proteins

what are some types of integral membrane proteins?

protein-attached

what is a type of peripheral membrane protein?

transmembrane proteins

cross the lipid bilayer as an alpha helix or a beta sheet: contain hydrophilic and hydrophobic domains

membrane-associated proteins

entirely cytosolic localized: alpha helix integrated into only the inner leaflet of the bilayer

lipid-linked proteins

attached through one or more covalently-linked lipid groups (myristoyl group, palmitic acid, glycosylphosphatidylinositol)

protein-attached

attached to the bilayer through its association with another protein

alpha helix and beta sheet

identify the two types of common folds adopted by proteins that cross a lipid bilayer

alpha helix

polypeptide backbone forms hydrogen bonds with itself on the inside of the helix and positions hydrophobic amino acids on the outside of the helix in direct contact with the lipid hydrocarbon tails

beta sheet

Folding pattern found in many proteins in which neighboring regions of the polypeptide chain associate side by side with each other through hydrogen bonds to give a rigid, flattened structure. Can assemble to form a barrel, which forms channels with large openings and are often involved in the transport of nutrients and small molecules but are able to restrict the passage of larger molecules such as antibiotics and toxins

contains a single transmembrane alpha helical domain, it often acts as a receptor or enzyme, it facilitates simpler communication or signaling, it has one start and one stop transfer sequence into the ER, it is a simpler structure with one hydrophobic segment.

what denotes a single pass transmembrane protein?

it contains two or more transmembrane alpha helix or beta sheet domains, it forms complex structures like ion channels and carrier proteins to transport molecules across the membrane, it has multiple start and stop transfer sequences, and it is a more complex structure with the ability to form intricate pathways or pores.

what denotes a multi-pass transmembrane protein?

it is responsible for regulating the shape of the cell as well as the mechanical properties of the plasma membrane

what is the function of the cellular cortex?

it is the underlying framework of the plasma membrane made of proteins that attach to the membrane through its association with transmembrane proteins, just beneath the plasma membrane

what is the composition and location of the cellular cortex?

it is formed of spectrin (long, thin, flexible rod-shaped dimer) and the junctional complex (the cluster of proteins that include actin, attachment proteins, and transmembrane proteins). The spectrin dimers are secured to the cytosolic side of the plasma membrane through association with attachment proteins which in turn are linked to transmembrane proteins in the plasma membrane

describe the cell cortex as it is formed in red blood cells

it is important for maintaining the mechanical strength of the cell as it is pumped through blood vessels

what is the function of the cell cortex in a red blood cell

it helps protect the cell surface from mechanical and chemical damage, it absorbs water and gives a cell surface and prevents cell-cell sticking (lubrication and it mediates cell-cell recognition and adhesion

what is the function of the glycocalyx in a eukaryotic cell?

all of the carbohydrates on the glycoproteins, proteoglycans, and glycolipids located on the outside of the plasma membrane form a sugar coating called the carbohydrate layer (the glycocalyx)

what is the composition of the glycocalyx in a eukaryotic cell?

the exterior of the plasma membrane of most animal and bacterial cells

what is the cellular location of the glycocalyx in a eukaryotic cell?

the ABO antigens on human red blood cells are examples of extracellular oligosaccharides, different sugar molecules within determine which blood type a person has

What is the link between oligosaccharide composition and the ABO blood groups

A antigen

contains N-Acetyl galactosamine, lacks galactose transferase enzyme

B antigen

lacks N-Acetyl galactosamine, contains galactose transferase enzyme

O antigen

lacks both N-Acetyl galactosamine and galactose transferase enzyme

AB antigen

contains both N-Acetyl galactosamine and galactose transferase enzyme

when a bacterial infection is detected by endothelial cells that link blood vessels, the lectin are preserved on the cell surface. Lectin are transmembrane proteins that bind to specific groups of sugars. Neutrophils express on their cell surface specific glycoproteins and glycolipids that are recognized by lectins on the endothelial cells. With lectin-glycoprotein/glycolipid binding, the neutrophil migrates out of the bloodstream and into the infected tissue

Explain how specific cell surface oligosaccharides are used to select a specific cell type out of a population using neutrophils as an example

because a membrane is a two dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer

how does the fluidity of the lipids within a membrane impart movement to the embedded transmembrane proteins?

A mouse cell and human cell were fused together to form a double-sized hybrid cell and the distribution of certain mouse and human plasma membrane proteins was monitored. Within a half hour, the two sets of proteins became evenly mixed over the entire cell surface.

How was the generation of a mouse - human hybrid by cell-cell fusion used to experimentally demonstrate transmembrane protein fluidity

tethering to the cell cortex, tethering to the extracellular matrix proteins, tethering to proteins on the surface of another cell, diffusion barriers within the membrane

Identify ways in which transmembrane proteins within the plasma membrane can have their movement restricted

for cells with polarity, the asymmetrical distribution transmembrane proteins helps to establish the directional movement of material

Explain how restricting membrane proteins to specific domains can be used in the process of transcellular transport

cell cortex

located inside the cell, beneath the plasma membrane: composed of protein filaments (e.g.) actin: primary function = mechanical support and structural integrity

glycocalyx

located outside the cell, on the surface: composed of carbohydrate-rich molecules (glycoproteins, proteoglycans): function = cell recognition, protection, and filtration

integral membrane protein

located embedded into the lipid bilayer, tightly associated with the membrane via hydrophobic interaction, difficult to remove: requires disrupting the lipid bilayer with detergents, functions include transport, signaling, enzymatic activity, and has structural roles like channel, carrier, and receptor: ampipathic

peripheral membrane protein

located loosely attached to the outside leaflet of the plasma membrane, attached to the membrane via electrostatic/non-covalent interaction, usually with integral proteins or lipid heads, easily removed by changing pH or ionic strength, functions include regulatory and structural roles, usually links cytoskeleton to membrane, hydrophilic and usually globular

oligosaccharide

a carbohydrate whose molecules are composed of a relatively small number of monosaccharide units.

glycoprotein

A protein with one or more carbohydrates covalently attached to it.

glycolipid

a lipid with one or more covalently attached carbohydrates

phospholipid

a lipid that contains phosphorus and that is a structural component in cell membranes

lectin

A protein that binds a carbohydrate, commonly an oligosaccharide, with very high affinity and specificity, mediating cell-cell interactions.

tight junctions

Membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

transcellular transport

transport of materials through the cell; requires interaction with the cytoplasm and may require transport proteins

they have difficulty crossing the lipid bilayer due to their strong electrical attraction to water

Why do water-soluble molecules and ions have difficulty crossing a lipid bilayer

CO2, O2, N2, steroid hormones

List examples of molecules that can freely move across the lipid component of a cell membrane

the smaller the molecule and the more hydrophobic (nonpolar) it is, the more rapidly the molecule will diffuse across the lipid bilayer

What are the properties that govern the rate at which a given solute can cross a protein-free lipid bilayer (give the relationship)

they provide for the selective transport of macromolecules across a lipid bilayer, water soluble molecules can only pass through the lipid membrane through membrane transport proteins

Identify the functions carried out by membrane transport proteins

Carrier/Transporter/Pump Proteins and Channel Proteins

Identify the two broad classes of membrane proteins involved in solute transport

a solute binds to the transporter, mediated by change in protein confirmation

briefly describe how a carrier/transporter/pump operates

passage of material based on size and charge

briefly describe what a channel protein facilitates

Simple diffusion

uncharged, small solutes that directly pass through the membrane; no need for membrane localized transport proteins: CO2, ethanol, O2, fat soluble molecules: solute movement down concentration gradient

Passive/facilitated transport

solute movement down concentration gradient: does not require the input of energy: mediated by all channel proteins and many carrier proteins

Active transport

solute movement against concentration gradient: requires the input of energy: carried out only by carrier proteins that couple an energy source to the transport process

carrier proteins

all small organic molecules cross the cell membrane via...

confirmation

change in carrier protein _______________ triggers solute transport

glucose moving down its concentration gradient

what would be an example of passive transport using a carrier protein?

State A where the solute binding site is exposed to the outside of the cell, State B where the solute binding site is exposed to the inside of the cell, and Occluded where the solute binding site is not accessible to either side.

What are the three confirmations of the mammalian glucose carrier protein?

no, this can happen at random

is glucose binding to the carrier protein required to trigger the change in confirmation of the protein?

the concentration gradient

the direction of glucose being transported via carrier protein is solely dictated by what?

no, so long as transport moves down the concentration gradient, transport could happen either way

does the direction of transport of glucose via a carrier protein matter?

the conversion of glucose into glucose-6-phosphate

after a meal, what keeps the intracellular levels of glucose low?

it is responsible for the transport of glucose

what is the role of the GLUT2 transporter in the transport of glucose?

they discriminate mainly on the basis of size and electric charge: when the channel is open, only ions of an appropriate size and charge can pass through

in what way do channels discriminate what solutes they transport?

they transfer only molecules or solutes that fit into specific binding sites on that protein

in what way do transporters discriminate what solutes they transport?

carrier proteins

... can carry out both active and passive transport

channel proteins

... only carry out passive transport

carrier proteins

bind to a solute and change shape to move it across the membrane, which can be active (requiring energy) or passive (facilitated diffusion)

channel proteins

form pores through which substances move passively down their concentration gradient.

concentration gradient and membrane potential

what two factors govern the transport of charged molecules across a membrane?

membrane potential

the charge difference across a membrane

negative

For the plasma membrane, the cytoplasmic side of the membrane is at a _________ potential relative to the extracellular side

electrochemical gradient

The concentration gradient and the voltage across the membrane is referred to as the

carrier proteins change confirmation happen when the solute binds to it on one side of the membrane, the protein changes shape and moves the binding site and solute to the other side of the membrane, releasing the solute. The direction of the solute's movement is driven by the electrochemical gradient. The transporter itself does not change the gradient, it just facilitates movement along it

Link changes in transporter conformation to the solute concentration gradient and the direction of solute transport

gradient-driven coupled pump/transporters, ATP driven pumps, and light driven pumps

Identify the three sources of energy / three models of active transport used by transmembrane pumps to actively transport a solute against its concentration gradient

The Na+ pump uses ATP to move Na + out of the cell and K + into the cell. Both Na + and K + are being moved against their concentration gradient. This active transport process involves the pump binding to

Na+, using ATP to change its shape and release the ions outside, then binding to K and returning to its original state to release the ions inside the cell.

how does the sodium-potassium pump in animal cells uses ATP hydrolysis to maintain the concentration gradients of sodium and potassium ions across the plasma membrane

3 sodium ions out and 2 potassium ions in

Identify the direction of movement and the number of each ion moved for each transport cycle for a sodium potassium pump

Phosphorylation by ATP occurs after the pump binds three sodium ions, causing a conformational change that releases sodium outside the cell. Dephosphorylation happens after the pump binds two potassium ions from the outside, which reverts the pump to its original shape, releases the potassium inside, and primes it to start the cycle again

Identify the role of phosphorylation and dephosphorylation in sodium-potassium pump activity

Ouabain inhibits the pump by preventing potassium binding and Digitalis inhibits the pump by preventing dephosphorylation

Identify inhibitors of the sodium potassium pump and at what stage they block pump activity

the high sodium concentration gradient established by the sodium potassium pump helps the sodium-glucose symporter move sodium down its concentration gradient and glucose against it in the same direction, helps the sodium-calcium antiporter move3 sodium ions into the cell and 1 calcium out of the cell, helps the sodium-proton exchanger antiporter move 1 sodium ion in and 1 proton out by using the electrochemical gradient established by the pump, and helps with osmosis

what are some examples illustrating the importance of sodium-potassium pump activity in other cellular activities

a symporter moves solutes in the same direction while an antiporter moves solutes in a different direction

Differentiate between a symporter and an antiporter with regards to the direction of solute moved

the electrochemical gradient established by the sodium potassium pump in which the energy of sodium moving down its concentration gradient is used to power the movement of another molecule

Where is the energy coming from to drive the action of the symporter and antiporters

the sodium-glucose symporter moves sodium and glucose in the same direction into the cell, the sodium-calcium antiporter moves 3 sodium ions in and 1 calcium ion out, and the sodium-hydrogen antiporter moves 1 sodium in and 1 proton out

Give examples of co-transporters that use this mechanism of transport identifying the molecules transported and their direction of movement

the epithelial cells have a glucose-sodium symporter in the apical domain that takes up glucose from the gut lumen, and glucose-sodium uniports in the basal and lateral domains to passively release glucose down its concentration gradient to be used by other tissues

Explain how the differential localization of a symporter and a passive transporter can drive the transcellular transport of glucose in an intestinal epithelial cell;

the transporters are kept separated by a diffusion barrier formed by tight junctions around the apex of the cell. the barrier prevents mixing of membrane components between the two domains

what is the importance of cellular domains in the localization of the symporter and passive transporter in the intestinal epithelial cell

if the transporters were not restricted to their domains, glucose would be pumped out of the cell at the basal site rather than in, which in turn would cause glucose to be able to passively seep back into the gut lumen instead of out into the rest of the body

Recognize the impact on directional transport if the location of these transporters were not restricted to these plasma membrane domains

the solute concentrations established by the sodium potassium pump dictate the direction of water movement across the plasma membrane

Explain the role of sodium-potassium pump activity in maintaining the balance of water across the plasma membrane of a mammalian

plant cells use their cell wall to build up turgor pressure that prevents the cell from wilting when osmosis causes the plasma membrane to push against the cell wall. Protists use a contractile vacuole to eject water from the cell when the cell starts to swell

Explain the alternative mechanisms used by protists and plant cells to control the cytosolic concentration of water;

aquaporins make the lipid much more permeable to water by facilitating the bulk of water flow across the membrane. they are impermeable to charged ions or other small molecules

Describe the role of aquaporins in water flow across a membrane

the serca pump is found in the sarco/endoplasmic reticulum and moves 2 calcium ions at a time from the cytosol into the lumen using ATP hydrolysis. The PMCA pump is found in the plasma membrane and pumps 1 calcium ion out of and 1 proton into the cell at a time using ATP hydrolysis

Identify the transporters involved in controlling the cytosolic concentration of calcium, list their cellular location, the direction of calcium transport, and the source of energy used for transport

the proton pump

Identify the transporter that provides the energy for the transport of solutes across the plasma membrane of plants, fungi, and bacteria

they function in the lysosomes and transport protons from the cytosol to the inside of the lysosome. This maintains the cytosol at a neutral pH and the lysosome at an acidic pH

State where the proton pump functions in mammalian cells, the direction of solute transport, and the role of this transporter in mammalian cells

a photon from sunlight is absorbed by a retinal, which changes shape and induces a change in protein confirmation. The retinal transfers a proton to an aspartic acid on the extracellular side of the transporter, which releases its bound proton. An aspartic acid on the intracellular domain of the transporter transfers a proton to a retinal, which returns it to it's original state. A cytosolic proton binds to an aspartic acid on the intracellular side of the transporter.

Describe the transport pathway taken by protons through bacteriorhodopsin

light

Identify the source of energy for bacteriorhodopsin

it absorbs a photon, changes into confirmation that lets protons move from the cytoplasm to the extracellular side of the membrane, ends the process by regaining the original confirmation

Identify the role for retinal in the bacteriorhodopsin transport process

membrane potential

voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other

electrochemical gradient

driving force that determines which way an ion will move across a membrane: consists of the combined influence of the ion's concentration gradient and the membrane potential