module3 Flashcards
Plasma membrane functions:
Compartmentalization:
- Continuos, unbroken sheets that enclose compartments (the cell and organelles within the cell)
- Allows for specialized activities to proceed
- Cellular activities can be regulated independently
Sceffold for biochemical activities:
- Isolates distincts compartments
- Ordered for effective interactions
Providing a selectively permeable barrier:
- Prevents the unrestricted exchange
- Means of communication
- Promote movement of selected elements
Transport solutes:
- Machinery for physically transported substances (ex. Active transport)
- Allows a cell to accumulate substance
- Transport specific ions
- Est ionic gradient
Respond to ext stimuli:
- Role in responding to ext stimuli (signal transduction)
- Different cells->different receptors->recognize and respond to different stimuli
- Generate signal that stimulates/inhibits intern activities
Intercellular interaction:
- Mediates the interactions b/ cell and its neighbors
- Recognize and signal one another
- Adhere when appropriate
- Exchange materials
- Interactions b/ extracellular and intracellular materials
Energy transduction:
- Convert one energy type into another (ex. photosynthesis)
General structure of phospholipids:
Glycerol head+P+2FA held via ester linkage
How are phospholipids oriented in the bilayer?
- Comprise most of the lipids in the membrane
- Are amphipathic
- Hydrophobic FA are inside the bilayer
- Hydrophilic glycerol+P towards the exterior
How does lipid composition influence the biological properties of membrane?
- Influences the activity of particular membrane proteins
- Determining the physical state of emembranes
- Playing a role in health and disease such as Tay-Sachs disease
- Providing precursor for highly active chemical messengers that regulate cell function
What’s glycoprotein? Function?
- Proteins w/ attached carbs (short, branched oligosach <15 sugars per chain)
- Role in mediating the interactions of cells w/ other cells and their nonliving envi
- Sorting of membrane proteins to cell compartments
What’s glycolipid? Function?
- Lipids w/ attached carbs (short, branched oligosac)
- Determine blood type (they have different enzyme that attach sugars to the end chain)
- Role in certain infectious diseases (ex. cholera and influenza viruses bind to a glycolipid)
- Might function as receptors in normal cell functions
Why is membrane fluidity important?
- Moderate fluidity allows interactions to take place within the membrane
- Clusters of membrane proteins assemble at particular sites within the membrane
- Cell processes such as transport of substances into and out of the cell, cell movement, growth and division, signal transduction, intercellular junction formation, secretion, and endocytosis/exocytosis depend on membrane component movement and are impossible in rigid membrane
- Play a role in membrane assembly b/c membranes arise from pre-existing membranes
integral proteins
- Exhibit different types based on type of cell and conditions
- Diffuse randomly w/ limited rate
- Immobilized due to interaction w/ membrane skeleton/extracellular material
- Moved in particular direction due to motor proteins interaction
- Restricted movement due to other integral proteins
- Restricted by proteins of membrane skeleton, but can hop into adjacent compartments thru transcient opening in the fence
- If lack the portion that would normally project into extracellular space ->move faster than wild type version of the proteins
integral proteins of RBC
Band 3:
- Contains carbohydrate
- Membrane spanning proteins (multiple)
- High #
- Present as a dimer
- Channel for passive exchange of anions across the membrane
- HCO- and Cl- movement
Glycophorin A
- Contains carbohydrate
- Membrane spanning proteins (single)
- Numerous
- Present as a dimer
- Due to the presence of -ve charges of a sialic acid, they prevent RBC sticking to each other (RBC repel each other)
- Receptor utilized by protozoan that causes malaria, providing a path for entry into RBC
membrane skeleton of RBC
Supported by fibrillar skeleton composed of peripheral membrane proteins
- Role in determining the biconcave shape
- Flatten RBC as they circulare thru the capillaries
spectrin:
- Major component
- Flexible and elastic
- Attached to internal surface of membrane
Actin:
- Involved in contractile activity
Tropomyosin
- Involved in contractile activity
Ankyrin
Band 4.1
Spectrin-actin network:
- Give cell strength, elasticity and pliability
membrane proteins of RBC
Peripheral proteins are found in the inner surfaces of RBC and are part of fibrillar membrane skeleton
Compare the rate of lateral diffusion of lipid w/ that of flip-flop. What is the reason for the difference?
- Lipid bilayer is relatively fluid
- Movement of polar lipid heads can be monitored under the microscope if they are linked to the gold particles
- If lipids flip-flop to the other leaflet, their mobility is highly restricted
- The reason for this difference:
- Lateral movement within the leaflet is easy for phospholipids
- Do not flip-flop often since it is thermodynamically unfavourable for polar head to move thru hydrophobic membrane
- Enzymes (flippases) can move certain lipids b/ leaflets
Mechanisms to transport materials thru the pl.membr:
Passive diffusion thru bilayer (ex. O2):
- Down the conc. Grad
- Membrane must be permeable to the substance
- Solute can pass directly thru the bilayer
Passive diffusion thru channel (ex. Na+)
- Solute diffuses thru the pore spanning the membrane that prevents contact w/ lipids of the bilayer
Facilitated diffusion by protein transporter (ex. glucose)
- Substances bind selectively to membrane-spanning protein that facilitates movement
Active transport (can move against concentration gradient) (ex. Na/K ATPase)
- Uses E
- Against concen gradient
- Est of the gradient dep on integral proteins that selectively bind solute
hyper/hypotonic soluations and what happens to the cells
- Hypertonic - have higher solutes concentration
- Hypotonic - have lower solutes concentration
- Cell in hypotonic solution (less solutes in solution than cell): water moves into the cell->swelling (in plant cell: normal turgor pressure)
- Cell in hypertonic solution (more solutes in solution than in cell): water moves out of cell -> cell shrinks (in plant cell: no turgor pressure)
- If the solution is slightly hypo/hypertonic- osmosis is temporarily and cell returns to its original state
facilitated diffusion
- Diffusion during which a substance binds selectively to membrane-spanninf protein that facilitates movement
- Kinetics are similar to enzymes:
- Both are specific to the molecules they transport
- Saturation-type kinetics: if they already operate at max velocity, increasing concentration of the solutes will not change anything
- Activity can be regulates
Na/K ATPase mechanism
- Electrogenic - contributed directly to the separation of charge across the membrane
- Shows sidedness by transporting 3Naout, but 2K in
- steps:
- E1 conformatino (ion binding sites accessible inside the cell; binds 3Na and ATP)
- gate within the protein is closed
- hydrolysis of ATP
- E2 conformation (ion binding sites are accessible outside the cell, binds 2K)
- gate within the protein is closed
- dephosphyrylation
- ATP binding
Membrane potential:
- The inside and outside of the plasma membrnae shows difference in voltage (electric potential) due to the different ion concentration inside and outside the cell
- Resting membrane potential of nerve cell = -70mV
- Can be measured w/ electrodes
- One electrode records V outside the cell
- Another electrode records V inside the cell
Secretory pathway:
- Proteins are synthesized in ER, modified at golgi and transported to various destination outside the cell
- Aka exocytosis, biosynthetic pathway
- Controlled protein trafficking is required to target proteins to the appropriate sites
- Starts in RER->Golgi->further destination (most are secreted out of the cell)
- Include constitutive secretion and regulated secretion
constitutive secretion
- Materials are transported in secretory vesicles from their sites of synthesis and discharged into extracellular space in continual manner
- In most cells
regulated secretion
- Materials are stored as membrane-bound packages and discharged only in response to an appropriate stimuli
- Ex. in endocrine cells, nerves that release neurotransmitters
Endocytic pathway:
Material move from the outside to the inside the cell
How are particular proteins targeted to particular subcellular compartments?
- Via specific sorting (import) signals that are encoded in protein AA sequence or in attached oligosaccharide
- Sorting is facilitated either by specific membrane receptors or by coats that form the outer surface of transport vesicles
pulse-chase method
- part of autoradiography
- to determine the intracellular path
- steps
- Incubate w/ radioactive AA (pulse period) to allow incorporation into proteins
- Wash of isotope access
- Put in medium w/ unlabeled AA (chase period) to allow protein synthesis from unlabeled AA
- Longer chase->further radiolabeled protein travel away from the site of synthesis
- Observe the movement of radioactive material within the cell (from the wave of radioactivity moving thru the cell)
SES
- No ribosome
- Tubular
- Well developed in skeletal muscles, kidney tubules, endocrine glands
- Consists of network membranes with luminar space different from the cytosolic space
- tubular and forms an interconnecting pipeline system.
- Function
- Synthesis of steroid hormones and lipids
- Detox in the liver
Via oxidases (convert hydrohobic compounds into hydrophilic)
Ex. ctyrochrome P450
- Requesting Ca2+
Regulate its release
RER
- Ribosome
- Proteins synthesis
- Composed of network of cisternae
- Consists of network membranes with luminar space different from the cytosolic space
- extensive organelle with ribosomes attached on the cytosolic surface
- omposed mostly of cisternae, which are interconnected flattened sacs
- luminal or cisternal space is continuous with the nuclear envelope outer membrane
- Organelles are positoined in the way to produce a polarity:
- Reflect movement of secretory proteins
* Functions: - Synthesis of protein(for secretion, integral membrane proteins and soluble proteins residing in the endomembrane system)
- Synthesis of most lipids
- Addition of sugar to Asp
synthesis of proteins occurs
RER
free ribosomme
RER ribosime protein synthesis
- 1/3 of proteins
- Released into the ER lumen during co-translational traslocation
- Include
- Secreted proteins
- Integral membrane protien
- Soluble proteins that reside in the compartment of the endomembrane system
free ribosome proteins synthesis
Synthesis of:
- Protein that remain in the ctosol (ex. glycolytic enzymes)
- Peripheral prtoteins of cytosolic surface
- Proteins transported into nucleus
- Proteins to be incortopated into peroxisome, chloroplast, mito (completely synthesized and then imported)
- Directed specifically to peroxisome, mito, chlor by sorting signals and receptors that recognize these signals
Maintenance of the membrane asymmetry:
- Each protein is synthesized in RER
- Protein inserted into the bilyer in the fashion determined by AA sequence
- Orientation of the protien is maintained as it travels thru the endomembrane system
- Carbs chains that are added in the ER provide a way to assess membrane sidedness
- They are present on the cisternal side of cytoplasmic membrane-> becomes exoplasmic side upon fusion of vesicle w/ plasma membrane
* Domains position: - Cytosolic surface of ER membrnae
- Cytosolic surface of Golgi cisternae
- Cytoplasmic surface of plasma membrane
Steps during ribosome attaches to mRNA and protein leaves RER:
- The synthesis of a polypeptide begins on a free ribosome.
- As the signal sequence emerges from the ribosome, it binds the SRP (signal recognition particle), which stops further translation until the membrane of the RER is contacted.
- The ribosome interacts with the protein-lined membrane channel (translocon).
- The signal peptide binds to a component of the translocon, and the translocon fully opens.
- The SRP is released from its receptor, and the polypeptide translocates through the channel into the ER lumen.
- Carbohydrates are added to the protein by enzymes called glycosyltransferases.
- The proteins are folded properly by chaperones in the ER lumen.
Models to explain how material move thru Golgi:
Vesicular transport:
- Cargo carried in anterograde direction by transport vesicle
- Cisternae remain stable
Cisternal maturation model:
- Cisternae: cis->trans->disperse at TGN
- Transport vesicles are carried in retrograde direction
Movement thru Golgi:
Anterograde:
Cis->trans Golgi
Retrograde:
Trans->cis Golgi
Why is the Golgi complex not uniform in composition from the cis to the trans face?
A protein that moves along the Golgi complex is highly modified from one end to the other: it may be trimmed by proteolytic enzymes, and the amino acids and carbohydrates it contains may be modified
What determines the specificity of interaction between a transport vesicle and the membrane compartment with which it will fuse?
Protein coats provide a mechanism for selecting the components to be carried by the vesicle. These selected components may be the cargo to be transported and the machinery required to target and dock a vesicle to an acceptor membrane.
lysosome function
- Digestion of materials brought into the cell
- Digestion of materials and organelles from inside cell (autophagy)
- Role in organelle turnover: regulated destruction of the cell’s own organelles and their replacement
- Ingested food particles (ingested by phagocytosis) are disassembled in the lysosome
- In sperm head: release enzymes during fertalization
vacuoles function
- Maintain turgor pressure (Exerts pressure to the cell wall and promoted cell growth and support for the soft tissue)
- Storage of toxic compounds of metabolism
- Storage of solutes and macromolecules
- Intracellular digestion
chaperons
aid in the unfolding of polypeptides in the cytosol and folding of the proteins in the chloro
What are the components of the ECM
collagen, proteoglycan, fibronectin, laminins
Functions of basement membranes:
- Mechanical support for attached celles
- Maintain epithelial cell polarity
- Serve as a substratum for cell migration and determine cell migration paths
- Separate adjacent tissues within an organ
- Act as a barrier to macromolecular passage
- Barrier to the invasion of tissue by CA cells
Function of hemidesmosomes:
- Anchor cells to the underlying basement membrane
- Tightest attachment between cells and its ECM
cell wall function
- Develop turgor pressure that pushes agains their surrounding wall
- Gives enclosed cell its characteristic polyhedral shape
- Support for individual cells
- Serve as skeleton for entire plant
- Protect against damage from mechanical abrasion and pathogens
- Mediate cell-cell interaction
- Source of signals that alter activities of cells that it contacts
- Mechanical support for individual cells and for the entire plant
- Primary barrier to the penetration of large molecular substances while allowing small ions and molecules to pass freely
Desmosomes:
- Numerous in tissues subjected to mechanical stress
- Form a network that gives structural continuity and tensile strength to the cell sheet
adhrens junction
- Occur as a belt that encircles the cell at its apical end
- Binds the cell to its neighbour
- May transmit signals b/ neighbouring cells
How does structure of tight junction contribute to its function?
- Integral tight junctions form continuous fibrils that encircle cells like gaskets and contact neighbouring cells on all sides
- Serve as barrier or seal to free diffusion of water and solutes from extracellular compartments on one side of epithelial sheet to the other side
Gap junction vs plasmodesmata
Gap in animals
Plasmodesmata in plants
trypsin as protease
a protein digestive enzyme (protease) that is unable to act inside the cell since it can not cross plasma membrane because it is hydrophilic enzyme. Therefore, in case of addition of trypsin to the i_ntact cell_, trypsin is unable to go through the bilayer into the cytoplasm and it can be seen that trypsin only digests parts of proteins 2 and 3 that are outside the bilayer (on the cell exterior).
You are studying a transporter. It appears to bind temporarily to the molecule to be transported. During normal transport, no energy is expended. The addition of a particular molecule that closely resembles the normally transported molecule inhibits transport. An increase in the concentration of the normally transported molecule in the presence of a constant concentration of the inhibitor increases the rate of transport.
- what kind of transporter it is
- what term would you use to describe such an inhibitor
- Facilitated diffusion is an example of passive transport that does not utilizes any energy. Facilitated diffusion occurs when a solute is moved across the membrane with the assistance of the membrane-spanning protein (the solute binds selectively to the transport protein). Since this situation involves a transporter (a transport protein that facilitates movement of another substance) that temporarily binds to the molecule (a solute to be transported) and does not use energy, this is a facilitated diffusion
- competitive inhibitor since the inhibitor resembles the substrate and this inhibition can be overcome with addition of additional substrates. Therefore, the max velocity of the reaction does not change while it does require more substrate to reach ½ of max velocity of the reaction (Vmax remains the same; Km increases).
You are characterizing an isolated membrane protein. The protein is exposed on both sides of the membrane. Over much of the central part of its surface, there are hydrophobic amino acids, while the ends are hydrophilic. It crosses the membrane a number of times. An opening exists in the center of this protein lined by hydrophilic amino acids. What kind of protein is this and what is its probable function?
- multi-membrane spanning protein (they belong to the integral membrane proteins
- common function of integral proteins:
- Receptor that binds a specific polar substrate;
- Channel that is involved in movement of ions;
- Agent that transfers electrons during process
Transition t and effect on membrane fluidity
- Transition temperature is the temperature at which the membrane fluidity changes between more liquid and solid states
- temperature above the transition temperature, the hydrophobic tails are not restricted and are free to move, so the membrane is more liquid
- below transition temperature in which the tails are more restricted, and the membrane is more solid.
- The transition temperature depends on the length of the fatty acid chain as well as the saturation of the fatty acids
- saturated FA: fatty acids to be tightly packed and the membrane will be more solid; would have higher transition temperature.
- unasaturated FA: and fatty acid does not align perfectly well with other chains creating more space and more fluidity. Unsaturated fatty acids would have lower transition temperature.
- the longer the chain, the higher the transition temperature would be b/c increased interactions
Trypsin is an enzyme that can digest the hydrophilic portions of membrane proteins, but it is unable to penetrate the lipid bilayer and enter a cell. Due to these properties, trypsin has been used in conjunction with SDS-PAGE to determine what proteins have an extracellular domain. Describe an experiment using trypsin to determine the sidedness of proteins of the erythrocyte membrane. Be sure to include any relevant control(s).
SDS-PAGE is technique used to separate proteins based on their molecular weight.
The other experiment that can be done to determine sidedness of the protein is as follows:
- In the intact cell, fractionate the extracellular proteins using SDS-PAGE (SDS would denaturate protein to the primary structure)
- Treat cell with trypsin and run SDS-PAGE
- Permealize the cell and treat it with trypsin. Run SDS-Page.
- Compare SDS-PAGE from previous steps.
- You also must have a control of SDS-PAGE analysis of membrane proteins from untreated cells
The rate of nerve impulse conduction is different in two nerve cell axons. The first cell exhibits a rate of conduction substantially higher (about 20 times faster) than the second cell. The two cells have axons of the same diameter. What is a possible explanation for the difference in the nerve conduction rate
Nerve impulse conduction in the axon can depend on a few factors including the diameter of the axon as well as the presence of myelin.
The larger the axon, the faster the conduction is (because the surface area is bigger and there are more ion channels). However, since the question stem says that the diameter of two axons are the same, the faster conduction must be due to the presence of myelin on the first cell.
The function of the myelin is to isolate the axon so that action potential “jumps” across those myelinated areas and arrive at the node of Ranvier where depolarization/repolarization occurs. Therefore, it takes less time for transmission of AP in the presence of myelin and the speed of conduction is greater.
You have undertaken a study of protein transport in live cultured eukaryotic cells, and your objective is to follow the in vivo movement of a secreted protein through the secretory pathway to the cell exterior. You have introduced a gene that encodes a mutant form of this protein tagged with Green Fluorescent Protein (GFP) into the cells. When the cells are grown at 42OC, the mutant protein is folded in a way that causes it to be retained in the rough ER, whereas at 32oC, the protein folds normally and is able to proceed through the pathway. Briefly describe an experiment that would allow you to accomplish your objective and be sure to mention the microscopic technique you would use.
You would take cells containing the gene for the fusion protein and maintain them for many cell generations at 42oC to ensure that all cells have the target protein retained in the RER. Then you would shift the cells to 32oC and use the appropriate microscopy (see above) to follow movement of the target protein through the secretory pathway and out of the cell.
the purpose of COPII-coated vesicles
are responsible for transport from the RER to the ERGIC and Golgi
COPI-coated vesicles purpose
- ERGIC-> RER;
- Golgi ->RER;
- Golgi enzymes in the retrograde direction (trans Golgi to cis Golgi).
What major difference in internal structure would you predict for the liver cells of someone who has consumed barbiturates for many years versus the liver cells of someone who does not take these drugs
The smooth ER in the liver cells would have undergone extensive expansion
a. Where in the cell is the assembly of N-oligosaccharides on the secretory and integral membrane proteins initiated?
Glycoprotein (proteins that have oligosaccharides attached to them) are formed during the process of glycosylation. The initiation of glycosylation occurs in the RER. However, those proteins that are glycosylated in the RER are likely to undergo additional glycosylation in the Golgi complex.
Briefly describe the mechanism of assembly and attachment of the core segment of the carbohydrate chain to a typical target polypeptide.
- First 7 sugars are transferred to the dolichol-PP on the cytosolic side of the ER
- Dolichol w/ sugars flipped across
- Remaining mannose sugars attached to the luminar side of the membrane
- Remaining glucose attached to the cytosolic side of the membrane to the end of dolichol-P
- Dolichol flips
- Dolichol donates sugars to the growing oligosaccharide chain
- Assembles oligosaccharide is transferred to the Asp of the polypeptide
- Dolichol-pp is flipped back and can accept sugars again
An organelle whose interior exhibits a low pH is identified in an animal cell. It is irregularly shaped and contains acid phosphatase. Which organelle is it most likely to be and why?
Lysosomes:
- are animal cell’s digestive enzyme
- contain acid hydrolases (depending on the type, the enzyme that breaks down macromolecule)
- function the best in the acidic condition
- in order to maintain acidic conditions lysosomal membranes use proton pump.
- contains glycosylated integral proteins that protect lysosomal membrane from being digested
- it is difficult to identify lysosomes on the microscopy because they have different size and shape (they are usually of irregular shape and have variable electron density).
which compartments of cell is/are associated with:
- clathrin and some vesicles
- Ca2+ ions in skeletal membrane
- O-linked glycosylation of proteins
- toxin compounds products of metabolism in plant cell
- unbound signal recognition particles
- trans Golgi,
- SER
- Golgi
- plant vacuoles
- cytosol
You are observing a cell process in which small vesicles continually merge with the cell membrane. A number of different treatments known to influence the secretion of specific materials seems to have no effect on the process. What type of secretion appears to be occurring
Because the regulatory treatment seems to have no effect on the secretion of the material, constitutive secretion is occurring. During the constitutive secretion materials are transported in secretory vesicles from their sites of synthesis and discharged into the extracellular space in the continual manner. This type of secretion (in comparison to the regulated secretion) occurs in most cells.
How are signaling receptors typically marked for endocytosis and subsequent destruction? What is the experimental evidence for the importance of this mark in receptor destruction?
- Endocytosis of the signaling receptors often leads to the destruction of the receptor (receptor down-regulation) -> reduces cell sensitivity to further stimulation by the hormone or growth factors.
- Signalling receptors are marker for the endocytosis by covalent attachment of tag (protein called ubiquitin) to the cytoplasmic tail of the receptor while it is at the cell surface
- The evidence is that membrane proteins that aren’t normally endocytosed and internalized if linked to ubiquitin
two major cellular acrivities integrins have been implicated
- Adhesion of the cells to their substratum
- Transmission of signals between external environment and the cell interior
adherens junctions and what are their functions
- : They are a type of specialized adhesive junction common in epithelia.
- Occurs as a belt that encircles the cell as its apical end
- Binds the cell to its neighbors
- May transmit signals b/ adjacent cells
- Have actin filaments binding to cadherin:
- Connect external environment to the actin cytoskeleton
- Provide path for signal to be transmitted from cell to exterior of the cytoplasm
- Held together by Ca-dep linkage b/ cadherin molecules of neighboring cells
- They perform multiple functions including cell-cell adhesion, regulation of actin skeleton, intracellular signaling and transcriptional regulation.
gap junction
- Are sites b/ animals cells specialized for intercellular communication
- Six connexin sububits form a connexon that forms a hydrophillic channel
- Permits movements of molecules b/ adjacent cells
- Contains channel that connect different cells
- Composed of connexin organized into complexes called connexons:
- Completely span the membrane
- Takes 6 connexin to form one connexon
- Form a complete intercellular channels (using wto opposing plasma membranes)
- Can allow free passage of molecules smaller than 1000 daltons
- Relatively nonselective
- Gated
tight junction
- Occluding junction
- Form continuous permeability barrier
- Not all junctions exhibit the same permeability properties
- May be leaky junctions (ex. proximal renal tubules)
- May be tighter junctions (ex. bladder and brain capillaries)
- Plasma membrane contain interconnected strands that run mostly parallel to one another and to the apical surface of the epithelium
- Integral proteins form continuous fibrils that completely encircle the cell
- Barrier for free diffusion
- Serves as a fence that maintains polarized character of epithelial cells
- Increase in number of parallel strands leads to more “tight” tight junction
- Prevent water loss
