Exam 3 Cell Bio Flashcards
Six Plasma Membrane Transport Systems
Na+/K+ Pump
Na+ Leak Channel
K+ Leak Channel
Voltage Gated Na+
Voltage Gated K+
Voltage Gated Cl2+
Na+/K+ Pump Facts
Active
Na+ in
K+ out
ATP Driven
Na+ Leak Channel Facts
Passive
Na+ in
Electrochemical gradient
Ungated
K+ Leak Channel Facts
Passive
K+ out
Electrochemical gradient
Ungated
Voltage Gated Na+ Facts
Passive
Na+ in
Open at -40 mV to +40 mV
Gated
Voltage Gated K+ Facts
Passive
K+ out
Open at +40 mV to -70 mV
Gated
Voltage Gated Cl2+ Facts
Passive
Cl2+ in
Electric signal
How does an ion fit through an ion channel?
An ion is coated in water molecules that act as a water shell.
The water shells are released in the vestibule of the channel protein and ions enter the selectivity filter through the membrane.
Outside of the membrane, ions are surrounded once more by water shells.
How does a membrane maintain its membrane potential?
When (+) and (-) charges are evenly spread in and out of the cell, the membrane potential is 0.
Positive charges can line up against one side of the membrane and negative charges can line up against the other side of the membrane.
This creates the membrane potential.
Three stimuli that influence ion channels opening
- Mechanically gated
- Ligand-gated (intra or extracellular)
- Voltage-gated
Voltage-gated Na+ channel conformation steps
- Depolarization: -40 mV: Na+ gate opens allowing ions to leave
- Propagation: Ions continue transferring increasing mV.
- Resting: Cell hits +40 mV and is inactivated.
- Repolarization: Cell slowly decreases to -70 mV due to K+ entering the cell, the channel is reactivated/closed at -60 mV.
- Depolarization eventually occurs again.
How do action potentials affect neurons?
- Ca2+ channels are closed in the presynaptic cell.
- Electric signal is received in the form of an action potential.
- This signal opens the Ca2+ channels.
- This triggers neurotransmitters (chemical signals) to be released.
- Neurotransmitters cross the axon.
- They are received by neurotransmitter-gated ion channels or receptors in the postsynaptic cell.
- They are then converted back to electric signals.
Endosome purpose
Sorting of endocytosed material
Peroxisomes
Oxidative breakdown of toxic molecules
Rough ER Purpose
Ribosomes are freely moving and embedded into the membrane for protein creation.
Smooth ER Purpose
Lipid synthesis and function
Golgi Apparatus
Receives proteins and lipids for modification, sorting and packaging for secretion or delivery to other organelles
How do proteins know where they should be transported?
Their mRNA encoded a signal sequence that is specific to where they need to be moved in the cell
Signal recognition particle
A particle in the ER that binds to a growing polypeptide and moves it to bind to an SRP receptor in the ER membrane.
SRP is then released for reuse.
Once in the membrane, the ER signal sequence is no longer required and cleaved off by signal peptidase.
Hydrophobic Signal Sequences
A sequence in a polypeptide.
SRP binds to a signal sequence generated by a ribosome and mRNA.
The short polypeptide enters a transmembrane protein translocator then the ribosome moves the polypeptide along in making it.
Once the translocator comes to the hydrophobic stop-transfer sequence, the protein translocator is removed and the polypeptide remains in the membrane where it is.
The signal sequence is cleaved off with signal peptidase
A polypeptide may be locked twice into a membrane with a start-transfer and stop-transfer signals. Or it may enter the ER membrane fully
Exocytosis
Vesicular transport out of the cell
Endocytosis
Vesicular transport into the cell
receptor-mediated endocytosis
Clathrin-coated vesicles are surrounded by coats made of two multimeric proteins, clathrin and adaptor protein, to bring macromolecules to a cell
provides the driving force to induce a flat membrane to form a spherical vesicle
triskelion
basic unit of clathrin lattices (triplet patterns emanating from a center)
-Vertex
-Clathrin light chain
-Clathrin heavy chain
-Terminal globular domain
Clathrin-coated vesicles transport process
- A cargo receptor in the plasma membrane receives an external cargo molecule
- Internal receptor binds to adaptin
- More cargo molecules and adaptin are bound
- Clathrin bind to adaptin and the membrane begins distorting inward
- Dynamin begins cutting off part of the membrane to contain the cargo molecules in a vesicle.
- Eventually, the vesicle moves to transport the cargo and clathrin-adaptin is released
LDL meaning
Low-density lipoproteins
Hydrophilic on the outside and hydrophobic on the inside
Type of cholesterol
LDL process
- LDL binds to LDL receptors
- Receptor changes confirmation
- Adaptin binds to receptor
- Clathrin binds to adaptin and triskelion formation distorts membrane
- Dynamin pinches off plasma membrane to a clathrin-coated vesicle
- Uncoating ATPase removes clathrin-coat
- The naked vesicle binds to endosome
- The pH of endosomes are about 6.5 and breaks hydrogen bonds
- Receptor breaks free of cargo as it enters endosomes due to lower pH
- Endosome sorts and send to the lysosome
- pH continues breaking down lipoproteins with hydrolytic enzymes that break down peptide bonds
- Proteins are broken down to amino acids
- The lipids are then available for free use as cholesterol
- It will then travel as other lipoproteins to other parts of the body
Transcytosis
the transfer of molecules to a new part of the membrane
Explain how membrane potential is governed by the permeability of a membrane to specific ions.
The more permeable the membrane is to a particular ion, the more that ion will contribute to the overall electrical charge across the membrane
Understand how transmitter-gated ion channels in the postsynaptic membrane convert the chemical signal back into an electrical signal.
when a neurotransmitter binds to them, allowing specific ions to flow across the cell membrane, which in turn changes the membrane potential and generates a localized electrical signal in the postsynaptic neuron
Explain how signal sequences direct proteins to the correct compartment
They are directed by ribosomes who have “read” the signal sequence unique to the next location of the protein
Full mRNA to rough ER peptide process
–mRNA transcription in cytoplasm on ribosome
–Signal sequence transcribed and ribosome stops transcribing
–Signal recognition particle (SRP) binds to signal sequence and molecule moves to rough ER
–SRP binds to SRP receptor in rough ER membrane
–Signal sequence growing peptide chain moved to protein translocator
–SRP and receptor displaced for reuse
–Ribosome continues transcription
Understand how proteins are modified in the ER and Golgi apparatus and further sorted in the Golgi.
–Protein built in rough ER and dedicated for new organelles are glycosylated in the ER lumen
–A sugar/mannose is added to arginine in a signal sequence
–The mannose on the protein binds to a receptor in the ER membrane
–A vesicle buds and the vesicle is directed to the cis-Golgi
–Phosphate is added to the mannose of the protein to create mannose-6-phosphate in the protein
–The protein is transferred through the medial Golgi to the trans-Golgi
–The protein binds to the mannose-6-receptor and a vesicle buds
Illustrate how lysosomal proteins get to the lysosome.
–The vesicle transports a protein+mannose+phosphate from the rough ER to the endosome
–The vesicle fuses with the endosome and due to the lower pH of 6.5, the phosphate group is broken off of the protein
–The phosphate is returned to the Golgi via a new vesicle
–The protein with mannose binds to a new receptor that brings it to a lysosome with a pH of 4.5
–This lower pH and hydrolytic enzymes break down the bond between the sugar and protein, along with any peptide bonds in the protein
Explain the process of vesicle docking including the role of tethers and SNAREs.
–A Rab protein on a vesicle acts as a docking station
–The Rab protein uses energy (ATP) to bind to two tethering SNARES
–Conformational changes of the t-SNARE occur to bring the vesicle closer to the plasma membrane
–T-snares of the target membrane bind to v-snares to create coiled coils (alpha helices structures binding together)
–The vesicle is pulled so close to the plasma membrane that cytoplasm is pushed out of the way and the plasma membranes fuse
–Cargo particles are released into the cell
–Vesicle cell membrane components are merged into the target cell
–ATP unsnares t- and v-SNARES for reuse
–v-SNARES are returned to the originating plasma membrane via vesicles
Exocytosis - Secretory
–Vesicles store concentrated secretory proteins
–As the action potential signal is received, the vesicle can bind to receptors in the plasma membrane
–The vesicle is able to release the secretory proteins into the extracellular space
Exocytosis - Unregulated
–Vesicles are constantly moving to a plasma membrane and releasing cargo proteins from their cargo
