Exam 2 Flashcards
Basic difference between cell membranes of bacteria vs eukaryotic cells
In some bacteria, the plasma membrane is the only membrane
Eukaryotic cells also have internal membranes that enclose individual organelles
Examples of internal membranes in eukaryotic cells
ER, vesicles, peroxisome, lysosome, endosome, golgi apparatus
Enclosed by 2 membranes: nucleus, mitochondria
Structure of cell membrane
lipid bilayer with proteins embedded
Phospholipids: hydrophilic head + hydrophobic tails
Where do kinks form in phospholipids?
In one of the hydrocarbon chains where there is a double bond between two carbon atoms
Most abundant phospholipid in the cell membrane
phosphatidylcholine:
hydrophilic head - choline linked to a phosphate group
hydrophobic tails - two hydrocarbon chains w/ a carboxyl
A molecule of glycerol links the head to the tails
Movement of lipid molecules in lipid bilayer
Membrane phospholipids move within the lipid bilayer.
Behaves as a two-dimensional fluid, in which the individual lipid molecules are able to move in their own monolayer. (lateral diffusion)
Note that lipid molecules do not move spontaneously from one monolayer to the other. (flip-flop)
Role of cholesterol in fluidity
cholesterol stiffens membranes by filling in gaps between phospholipids, making the bilayer less flexible and less permeable
Relation of kinks in phosopholipids with fluidity
The more unsaturated the hydrocarbon, the more kinks, the more fluid as it makes it harder for the tails to pack against one another
do colder environment animal cells want more or less fluidity?
more fluidity so they dont freeze
What is the role of flippases (what do they do and what is the result)
Flippases help to establish and maintain the asymmetric distribution of phospholipids
They selectively remove specific phospholipids (phosphatidylserine and phosphatidylethanolamine) from the side of the bilayer facing the exterior space + flip them into the monolayer that faces the cytosol
The resulting curvature of the membrane may help drive subsequent vesicle budding.
Name the phospholipids and glycolipids that are distributed asymmetrically in the lipid bilayer and which side they lay on
Due to flippase: phosphatidylcholine and sphingomyelin concentrated in the noncytosolic monolayer.
Phosphatidylserine and phosphatidylethanolamine are found mainly on the cytosolic side.
Phosphatidylinositols are in the cytosolic monolayer (participate in cell signaling)
Glycolipids found exclusively in the noncytosolic monolayer of the membrane.
Cholesterol is distributed almost equally in both monolayers.
What are extracellular vesicles and what is their role?
Extracellular vesicles are cell-derived membrane particles involved in signalling: exosomes, microvesicles, and apoptotic bodies.
Released under physiological conditions, but also upon cellular activation, senescence, and apoptosis.
Important role in intercellular communication.
May maintain cellular integrity by ridding the cell of damaging substances.
What are the various types of plasma membrane proteins
+ examples
Transporters: e.g Na+ pump which actively pumps Na+ out of cells and K+ in
Ion channels: e.g K+ leak channel allows K+ ions to leave cells, influencing cell excitability
Anchors: e.g integrins which link intracellular actin filaments to extracellular matrix proteins
Receptors: binds extracellular molecule and generates intracellular signals
Enzymes: e.g adenylyl cyclase catalyzes production of intracellular cAMP in response to extracellular signals
Role of the cell cortex: In blood cells
Cortex made largely of spectrin
Spectrin dimers are linked end-to-end to form longer tetramers.
Spectrin tetramers + actin molecules = a mesh
This network is attached to the plasma membrane by the binding of at least two types of attachment proteins to two kinds of transmembrane proteins
How can a cell restrict the movement of its membrane proteins
Membrane proteins are restricted to particular domains of the plasma membrane of epithelial cells in the gut.
Proteins are prevented from entering other domains by tight junctions which separate the domains
(polarity is also achieved by this)
What is the carbohydrate-rich layer coating the cell surface made of
Oligosaccharide side chains attached to membrane glycolipids and glycoproteins
And polysaccharide chains on the membrane of proteoglycans
Glycoproteins that have been secreted by the cell and then adsorbed back onto its surface can also contribute.
All the carbohydrate is on the external (noncytosolic) surface of the plasma membrane.
Role of neutrophils in recognition
Recognition of cell-surface carbohydrates on neutrophils allows these immune cells to begin to migrate out of the blood and into infected tissues.
Specialized transmembrane proteins (called lectins) are made by the endothelial cells (lining the blood vessel) in response to chemical signals from a site of infection.
Lectins recognize particular sugar groups carried by glycolipids and glycoproteins on the surface of neutrophils circulating in the blood.
Neutrophils stick to the endothelial cells that line the blood vessel wall.
Neutrophil rolls along blood vessel wall.
Much stronger protein–protein interaction helps the neutrophil slip between the endothelial cells, so it can migrate out of the bloodstream and into the tissue at the site of infection
How can one measure the rate of lateral diffusion of a membrane protein
Using photobleaching techniques such as FRAP
A specific type of protein can be labeled with a fluorescent antibody or tagged with a fluorescent protein, such as GFP.
A small area of the membrane containing these fluorescent protein molecules is then bleached using a laser beam.
As the bleached molecules diffuse away, and unbleached, fluorescent molecules diffuse into the area, the intensity of the fluorescence is recovered.
The diffusion coefficient is then calculated from a graph of the rate of fluorescence recovery: the greater the diffusion coefficient of the membrane protein, the faster the recovery.
To which molecules is the membrane quite permeable to
small nonpolar molecules diffuse rapidly
small uncharged polar molecules diffuse readily if they are small enough
To which molecules is the membrane hardly/not permeable to?
large uncharged polar molecules hardly cross
highly impermeable to ions
Ion concentrations inside and outside mammilian cell
Na+
K+
Mg2+
Ca2+
H+
Cl-
Inside vs outside
5-15 : 145
140 : 5
0.5 : 1-2
10^-4 : 1-2
7x10^-5 : 4x10^-5
5-15 : 110
How can inorganic ions and small, polar organic molecules cross a cell membrane ?
through either a transporter or a channel
A channel forms a pore across the bilayer through which specific inorganic ions or polar organic molecules can diffuse. (based on size and charge)
Ion channels can exist in either an open or a closed conformation
Channel opening/closing is usually controlled by an external stimulus or by conditions within the cell.
A transporter undergoes a series of conformational changes to transfer small solutes across the lipid bilayer. Transporters are very selective for the solutes that they bind, and they transfer them at a much slower rate than do channels.
Outline the difference in passive vs active transport
Passive transport:
- allows solutes to move down their concentration gradients
- occurs spontaneously
Active transport
- against a concentration gradient
- requires an input of energy (ATP from hydrolysis, transmembrane ion gradient, or sunlight)
Only transporters can carry out active transport, and they are called pumps
What are the main ways pumps carry out active transport
Transports solute against its electrochemical gradients
1) Gradient driven pumps: link the uphill transport of one solute across a membrane to the downhill transport of another (transmembrane ion gradient)
2) ATP-driven pump: uses the energy released by hydrolysis of ATP to drive uphill the transport of the solute
3) light-driven pump: use energy from sunlight to drive uphill transport of solute (found mainly in bacterial cells)
How does the Na+ pump work?
ATP-driven Na+ pump transports Na+ out of the cell (against its electrochemical gradient) and carries K+ into the cell
3 binding sites for Na+ and two for K+
Conformational changes occuring during:
1) The binding of cytosolic Na+
2) phosphate group removed from ATP and transferred to the cytosolic face of the pump
3) high-energy linkage of the phosphate to the protein induces conformational changes that transfer the Na+ across the membrane and release it outside the cell
4) Binding of K+ from the extracellular space
5) Dephosphorylation of pump
6) Protein returns to original conformation = transfers the K+ across the membrane and releases it into the cytosol
What inhibits the Na+ pump? and how?
Ouabain inhibits the pump by preventing K+ binding
What is the net result of the Na+ pump?
The net result of one cycle of the pump:
3 Na+ out and 2 K+ in
What happens in the Ca2+ pump in the sarcoplasmic reticulum?
When a muscle cell is stimulated, Ca2+ floods into the cytosol (which originally has a low Ca2+ conc) from the sarcoplasmic reticulum
Influx of Ca2+ stimulates the cell to contract
To recover from the contraction, Ca2+ must be pumped back into the sarcoplasmic reticulum by this Ca2+ pump.
The Ca2+ pump uses ATP to phosphorylate itself, inducing a series of conformational changes (similar to the ones of the Na+ pump); when the pump is open to the lumen of the sarcoplasmic reticulum, the Ca2+-binding sites are eliminated, ejecting the two Ca2+ ions into the organelle
Ca2+ pumps return to their original conformation w/o the requirement of a second ion
What are symports and antiports?
Gradient-driven pumps can act as symports or antiports.
symports: transfer solutes in the same direction
antiports: transfer solutes in the opposite directions
Uniports: only facilitate the movement of a solute down its concentration gradient. Because such movement does not require an additional energy source, uniports are NOT pumps but are still transporters.
Example of a symport
Glucose-Na+ symport: uses the electrochemical Na+ gradient to drive the active import of glucose
Fluctuates between outward-open (extracellular) and inward-open (cytosolic) states
Pump can only transition between the states when both Na and glucose are bound or neither are bound (so not when ONLY one is bound)
Because Na+ conc is high in extracellular, Na+ binding site is readily occupied in outward-open state and is waiting for a rare glucose molecule to bind > flips to occluded-occupied state (both molecules bound)
Now it can either flip back to outward-open state = solutes dissociate + nothing happens
OR
flips to inward-open state = Na+ dissociates and then glucose eventually dissociates > occluded-empty state > outward-open state
REPEAT
What maintains the steep Na+ gradient?
When Na+ is pumped into the cytosol it is also pumped back out of the cell
What are the diff types of glucose transporters that enable gut epithelial cells to transfer glucose across the epithelial lining of the gut?
Glucose-Na+ symport:
In apical domain of plasma membrane = faces gut lumen = import glucose into epithelial cell, creating high conc of sugar in cytosol
Na+ is pumped out by Na+ K+ pumps
Passive glucose uniports:
basal + lateral domains of plasma membrane
release glucose down its conc gradient to be used by other tissues
gut lumen (low glucose conc) > epithelial cell (high glucose conc) > extracellular fluid (low glucose conc)
What is the basis of electrical signalling in many cells?
changes in membrane potential
What plays a major role in generating the resting membrane potential across the plasma membrane?
K+ conc gradient and K+ leak channels
How does K+ conc gradient and K+ leak channels generate a resting membrane potential?
When K+ leak channels open, K+ will tend to leave the cell (down its conc gradient)
K+ will cross the membrane but -ve ions will be unable to flow = charge imbalance > a membrane potential that tends to drive K+ back into the cell
At equilibrium the effect of K+ conc gradient is exactly balanced by the effect of the membrane potential = no net movement of K+ across the membrane
How can we monitor ion channel activity ?
Patch-clamp recording
To expose the cytosolic face of the membrane, the patch of membrane held in the microelectrode can be torn from the cell. This technique makes it easy to alter the composition of the solution on either side of the membrane to test the effect of various solutes on channel activity.
Structure of a neuron + how it works?
a cell body, a single axon, + multiple dendrites
The axon conducts electrical signals away from the cell body toward its target cells, while the multiple dendrites receive signals from the axons of other neurons.
How does membrane potential influence voltage-gated Na+ channels
A voltage-gated Na+ channel can flip from one conformation to another, depending on the membrane potential.
When membrane at rest + highly polarized, +vely charged amino acids in the voltage sensors of the channel are oriented by the membrane potential in a way that keeps the channel in its closed conformation.
When membrane is depolarized > voltage sensors shift = change in the channel’s conformation so the channel has a high probability of opening (when threshold is reached Na+ channel opens = Na+ influx further depolarizing membrane)
But in the depolarized membrane, the inactivated conformation is more stable than the open conformation so after a while in the open conformation (when peak mV is reached),
Refractory period:
the channel becomes temporarily inactivated and cannot open. After the membrane has repolarized the channel returns to its original conformation
Voltage-gated Na+ channels: mV and conformation of the channel
AP is triggered by a brief pulse of electric current = partially depolarizes the membrane, reaches threshold = Na+ channel opens > mV reaches peak and starts to depolarise = inactivated > hyperpolarization > resting membrane potential > return to original closed confirmation
Even if restimulated, the plasma membrane cannot produce a second action potential until the Na+ channels have returned from the inactivated to the closed conformation. Until then, the membrane is resistant, or refractory, to stimulation.
How is a chemical signal converted into an electrical signal
Done by postsynaptic transmitter-gated ion channels at a synapse
Released neurotransmitter binds to + open transmitter-gated ion channels in the plasma membrane of the postsynaptic cell = resulting ion flows alter the membrane potential of the postsynaptic cell
give examples of water soluble molecules that diffuse freely across cell membranes
CO2 and O2
Thousands of ______ form on the cell body and dendrites of a motor neuron in the ________ synapses
synapses, spinal cord
What can control the activity of specific neurons in a living animal + study that discovered this
light-gated ion channels
Study of optogenetics (showed aggressive behaviour in rats was influenced by presence / absence of blue light as it activated rhodopsin channels to open
What is AE2
anion exchanger across plasma membrane
exchanging intracellular Cl⁻ with extracellular HCO₃⁻
Cl⁻ out of the cell
HCO₃⁻ is moved into the cell
What is the result of the absence of AE2 (from knockout study) + interpretations
results in more bone mass
AE2 is linked to osteoclast activity
Osteoclasts are responsible for bone resorption (breaking down bone).
Without AE2 (-/-), osteoclasts are inactive, and bone resorption does not occur as efficiently, leading to increased bone density.
Role of AE2 in osteoclast functioning?
anion exchange process is necessary for the proton pumps to work correctly
= help osteoclasts acidify the environment for bone breakdown
What are the general principles of cell signalling
Signals Can Act over a Long or Short Range
A Limited Set of Extracellular Signals Can Produce a Huge Variety of Cell Behaviors
A Cell’s Response to a Signal Can Be Fast or Slow
Cell-Surface Receptors Relay Extracellular Signals via Intracellular Signaling Pathways
Some Intracellular Signaling Proteins Act as Molecular Switches
Cell-Surface Receptors Fall into Three Main Classes
Ion-Channel-Coupled Receptors Convert Chemical Signals into Electrical Ones
What determines whether and how much gene is transcribed?
signals from outside, transcription factors, signal transduction, at DNA: repressor and enhancer sequences
What determines how much a functional protein is produced by the cell?
Efficiency of posttranslational modifications:
- cleavage of signaling peptides
- glycosylation
- phosphorylation
miRNAs
Ubiquitination
Repressors at the 3’UTR of mRNA
Stability of mRNA:
- CAP
- Poly A tail
The same signal molecule can induce different responses in different target cells. Give an e.g
acetylcholine reacts on pacemaker cells, salivary gland cells, skeletal muscle cells
Which type of receptors do extracellular signal molecules bind to?
cell-surface receptors (generate one or more intracellular signaling molecules in the target cell) or to intracellular receptors (pass through the target cell’s plasma membrane and bind to intracellular receptors—in the cytosol or in the nucleus—that then regulate gene transcription or other functions)
What are the 4 types of cell communication?
1) Endocrine: Hormones produced in endocrine glands are secreted into the bloodstream and are distributed widely throughout the body
2) Paracrine: paracrine signals are released by cells into the extracellular fluid in their neighborhood and act locally
3) Neuronal: Neuronal signals are transmitted electrically along a nerve cell axon. When this electrical signal reaches the nerve terminal, it causes the release of neurotransmitters onto adjacent target cells
4) Contact dependent: a cell-surface-bound signal molecule binds to a receptor protein on an adjacent cell.
Explain why certain extracellular signals act slowly or rapidly
Certain types of cell responses—such as cell differentiation or increased cell growth and division—involve changes in gene expression and the synthesis of new proteins; they therefore occur relatively slowly.
Other responses—such as changes in cell movement, secretion, or metabolism—need not involve changes in gene expression and therefore occur more quickly
Pathway of extracellular signals that alter the behavior of a target cell
extracellular molecule binds to receptor protein > intracellular signalling molecules > interact with specific effector proteins (e.g metabolic enzyme, cytoskeletal protein, transcription regulator) altering them > changes in target cell behavior (e.g gene expression, metabolism etc)
Components of intracellular signalling pathways perform one or more crucial functions. What are they?
1) they can RELAY the signal onward + help spread it through the cell
2) AMPLIFY the signal, making it stronger so that a few extracellular signal molecules are enough to evoke a large intracellular response
3) detect signals from more than one intracellular signalling pathway and INTEGRATE them before relaying a signal onward
4) DISTRIBUTE the signal to more than one effector protein
5) MODULATE the response to the signal by regulating the activity of components upstream in the signalling pathway (feedback regulation)
Explain further how feedback regulation within an intracellular signalling pathway works?
Positive feedback:
A downstream protein in a signalling pathway, protein Y, acts to increase the activity of the protein that activated it
Negative feedback:
protein Y inhibits the protein that activated it
What are 2 examples of an/off molecular switches
signaling by protein phosphorylation
signaling by GTP-binding proteins
