Test 2 Flashcards
What’s the structure of a cell membrane? (plasma membrane)
Is made up of the amphipathic (charged and uncharged) phospholipid bilayer. Contains proteins & certain steroid lipids embedded in it
What are the functions of the cell membrane?
-provides structural basis for metabolic order (keeps together groups of enzymes of the same metabolic pathway) and separates other enzymatic systems
-separates living cells from their surroundings
-takes up required substances and disposes of the unwanted waste
What’s the fluid mosaic model?
-It describes the structure of the plasma membrane as a mosaic of components
-both phospholipids & proteins move laterally (rarely vertically)
-embedded & surface proteins interspersed throughout the bilayer
Can you briefly describe a
key experiment that helped elucidate the model?
-Plasma membrane proteins on both mouse and human cells were labelled differently then the cells were fused together
-Results: hybrid cells with mixed proteins which proved that some plasma membranes can move around the plasma membrane
What does membrane fluidity depend on, and what happens to the membrane when it’s hot or cold?
-fluidity of membrane depends on lipid components
-more double carbon bonds = less fluid
-Hotter temps means too fluid and won’t hold shape
-Colder temps means rigid, not as flexible and may possibly break
What’s the the relationship between membrane fluidity and
membrane permeability?
Lower membrane fluidity will reduce permeability the reducing it’s ability to take in a push out molecules
What’s the relationship between membrane fluidity and temperature?
-When temperature increases (hotter), the membrane becomes too fluid/flexible and it won’t hold shape
-When temperature decreases (colder), the membrane becomes rigid, not as fluid and it may break
What’s the relationship between membrane fluidity and saturation of fatty acids in phospholipids?
-Some organisms will alter the fatty acid content of their membrane lipids to compensate for temp changes
-Homeoviscous adaptation; temp decreases then the proportion of fatty acids increases so the membrane remains fluid
What’s the relationship between membrane fluidity and fatty acid length in phospholipids?
-Longer fatty acid chains makes the membrane less fluid because of the increase of the van der waals interactions between chains
Practice question: What will happen to membrane fluidity if # of double bonds in fatty acids is increased?
-decreases fluidity
Practice question 2: What will happen to the membrane fluidity if the length of fatty acid chains is increased?
-Becomes less fluid because of increase in van der waal interactions b/w chains
What’s the relationship between membrane fluidity and the amount of cholesterol?
-cholesterol has different affects on the membrane depending on the temperature
-high temps –> stabilizes membrane by binding cholesterol to the hydrophilic head of an adjacent phospholipid
-low temps –> acts as spacer to prevent van der waal interactions thus increasing fluidity
What’s a fluidity buffer?
-Fluidity buffer has different affects on the membrane fluidity depending on the temperature
-cholesterol stabilizes the membrane at high temps by bind OH group in cholesterol to adjacent phospholipid head
-cholesterol creates space between membranes at cold temps to prevent van der waal interactions making it more fluid
Why have different membrane compositions evolved as adaptations in organisms?
Because different organisms live in different environments; different temperatures require different adaptations (homeoviscous adaptation - fatty acids)
What are the two classes of membrane proteins?
1) integral proteins (inside)
2) peripheral proteins (outside)
Describe the integral protein
-firmly bound to the inside of the membrane & they penetrate the core of the lipid bilayer
-amphipathic (hydrophilic extends outside of cell or into cytoplasm & hydrophobic interacts w/ fatty acid tails)
-some don’t extend all the way through the membrane others do, these are transmembrane proteins (some can even span the membrane many times)
What are some examples of an integral proteins?
1) Aquaporins: they transport water in out out of cell down its gradient (osmosis)
2) Glycoproteins: embedded in plasma membrane and the oligosaccharide faces the ECM (A,B,O blood types)
describe the peripheral protein
-not embedded in the lipid bilayer
-located on the inner or outer surface of the membrane
-may be receptors on the surface of the cell or enzymes associated w/ the inner membrane
What are some examples of peripheral proteins?
Cytochrome c, cupredoxins, high potential iron protein, adrenodoxin reductase, some flavoproteins
What’s a transmembrane protein and explain why they’re amphipathic?
They are a type of integral protein that span the entirety of the cell membrane and they’re amphipathic because they extend to the aqueous part of the environment and they interact with the hydrophobic tails
why is the bilayer asymmetrical?
-Because one of the sides of the membrane has more proteins attached attached to it (more proteins on cytoplasmic side)
-this asymmetry is because of the way each protein is inserted into the bilayer
-each side has specific characteristic due to the proteins attached
what is meant by the selective permeability of a membrane?
-the membranes ability to differentiate b/w molecules and only letting certain ones in
-this allows them to maintain their internal composition
how do proteins and lipids play a role in selective permeability?
-transport proteins can move substances in or out of the cell
-longer fatty acid chains makes the membrane more rigid and less permeable
What’s the function of an enzyme? And what does its function depend on?
-accelerate chemical rxs (organic catalysts)
-function depends on its ability to recognize and bind to some other molecules
What’s the function of a storage protein?
-provides amino acids to developing embryos
-example: ovalbumin, casein, plants store proteins in seeds
What’s the function of chemical messenger proteins?
–help coordinate hormones and neurotransmitters
-examples: insulin (hormone excreted by pancreas causing tissues to take up excess glucose), acetylcholine (neurotransmitter used to signal other cells)
What’s the function of contractile proteins?
-function in movement
-examples: myosin and actin (contraction of muscles)
What’s the function of defensive proteins?
-defensive substances within the body
-examples:when there’s a foreign substance antibodies are made to fight it
What’s the function of transport proteins?
-move substances within the body or in and out of cells
-example: hemoglobin (moves oxygen from lungs to other tissue)
What’s the function of receptor proteins?
-regulates response to chemical stimuli
-example: receptors built into membrane of nerve cell used to detect chemical signals from other nerve cells
What’s the function of structural proteins?
-provides support
-function in cell membrane in muscle tissue, tendons, and ligaments
-examples: collagen and elastin
What is passive transport?
-very simple, requires no energy as it goes along the gradient (high to low)
What is active transport?
ATP is required as it goes against the gradient (low to high)
How are plasma membrane proteins made?
-proteins destined to be peripheral proteins on inner surface are made by free ribosomes in cytoplasm
-proteins destined to be integral or peripheral proteins on outer surface are made by ribosomes in the rough ER
How do proteins aid in cell-cell recognition?
-they provide identification tags to cells to distinguish them.
-allows for immune system to recognize & reject foreign cells
-enables cell sorting
What is the function of plasmodesmata?
-channels allowing for communication, and transport of water and small molecules b/w plant cells
-plants can dilate these channels
-bridges the cell membranes and cell walls of adjacent cells
What is the function of desmosomes?
-attaches animal cells together without stopping flow of materials
-used to absorb mechanical stress by distributing throughout the tissue
what is the function of tight junctions? Where are they used?
-junctions b/w animal cells that block passage and leaks of materials
-holds cells together in actual physical contact
-Used in: stomach, capillaries, & bladder
What is the function of gap junctions?
-bridges space b/w animal cells (narrower than desmosomes)
-contains channels connecting cytoplasms of adjacent cells (allows communication)
-used in: pancreas (coordinated response of insulin), adn heart muscle cells (permit flow of ions to synchronize contractions)
Practice question 3: Is diffusion a spontaneous process?
YES
What is the end result of diffusion in a closed system?
Equilibrium of molecules on both sides (most cells do not want eqm because
What is net movement, net diffusion and dynamic eqm?
-Net movement: one direction - movement in other direction
-net diffusion: when the flow of solute particles is greater in one direction than the flow of solute particles in the other direction
-The cells operate to make sure that we don’t deviate from a narrow range of internal balance (dynamic eqm, but not total eqm)
what are the 4 factors that affect the rate of diffusion of a substance?
concentration gradient, membrane permeability, temperature, and pressure.
What’s the difference between simple and facilitated diffusion?
-simple: direct transport transport of molecules across plasma membrane (examples: water, oxygen, carbon dioxide, ethanol and urea)
-facilitated: requires a transmembrane protein (carrier, channel, or aquaporins) (examples: carbohydrates, amino acids, nucleosides, and ions)
What are transporters and what is their structure and function?
-Carrier and channel proteins
-carrier; binds to the substance to make it change shape and cross membrane
-channel; forms small pores in membrane allowing small molecules to pass
-structure: alpha-helical structures of the membrane-spanning domains
What is facilitated diffusion via channels? What are two examples?
-contain hydrophilic corridors that allow specific molecules to pass (no change in shape, but can have a gate)
-Examples: Aquaporins (facilitated diffusion of water), ion channels (open and close w/ stimulus)
What is facilitated diffusion via carrier proteins?
-Binds to a solute and undergoes a subtle change in shape upon binding to a solute that translocates the solute-binding site across the membrane
-examples: GLUT4 (transports glucose), LacY (transports lactose)
-rate depends on # of carriers
What is osmosis?
-The diffusion of water across a permeable membrane
-Goes from low to high due to cohesion and adhesion
What drives osmosis?
It’s driven by impermeable solutes only
What is an osmotic gradient?
It is the difference in concentration between two solutions on either side of a semipermeable membrane (tonicity is the measure of the osmotic gradient)
What is osmotic pressure and how does it work?
-the hydrostatic pressure needed to stop the net flow of water across a membrane due to osmosis
-High solute concentration = high osmotic pressure
-Low solute concentration = low osmotic pressure –> this is important for keeping cells under homeostasis
Is osmosis spontaneous or non-spontaneous?
It is the spontaneous net movement
What is the end result of osmosis in a
closed system?
An equal concentration on both sides of the membrane
Does osmosis typically reach equilibrium in cells?
Define effective osmolarity
Osmolarity is the total concentration of all solutes
What is tonicity?
It is tonicity which is the ability to cause a cell to gain or lose water (effective osmolality)
Explain what happens to a cell in isotonic, hypertonic, and hypotonic solutions
-Isotonic: Same concentration inside and outside of cell and no net water movement. (normal)
-Hypertonic: solute concentration in solution is greater than inside the cell, and cell loses water making the cell shrivel
-Hypotonic: solute concentration in solution is smaller in solution than inside the cell and the cell gains water making it burst
Explain what determines whether a cell will gain or lose water by osmosis?
-Osmolarity is the total concentration of all solutes and it determines if the cell will gain or lose water.
-Iso-osmotic: particles of solute are the same on each side (cell does not gain or lose water)
-Hyper-osmotic: concentration of solutes is higher outside the cell so the cell loses water and becomes shriveled
-Hypo-osmotic: concentration of the solutes is greater inside the cell than outside so the cell gains water and bursts
What is turgor pressure? And how is it important in plants?
solution surrounding the cells is hypotonic so the vacuole becomes full providing pressure on the cell wall (makes plant stand up straight without it, the plant shrivels)
What is osmoregulation?
is the control of water balance
Why is osmoregulation important for some organisms (give examples (3))?
Examples:
1) Paramecium; is hypertonic to the pond water so it has a special organelle called the contractile vacuole which acts as a pump and pushes out water
2) urinary system
3)Plants; to help cell walls help maintain water balance (hypotonic, isotonic, & hypertonic)
What is active transport?
-Net movement is always against the gradient
-Cell must expend energy
-Transported molecules can be large or small
-Requires a carrier/transporter protein
-Allows cell to maintain different internal and external environments
Is active transport spontaneous or nonspontaneous?
If molecules have to be transporter from low to high the energy is expended and it is spontaneous
How does active transport differ from facilitated diffusion?
-Facilitated diffusion goes from high to low and doesn’t require energy
WHEREAS
-active transport requires energy as molecules go from low to high
what unifies active transport and facilitated diffusion?
The both use carrier proteins to assist in the transport of molecules
What is the mechanism of a pump?
Pumps, also called transporters, are transmembrane proteins that actively move ions and/or solutes against a concentration/gradient
Does the mechanism of a pump more closely resemble a channel or a carrier?
channel
What is an electrogenic pump?
Ion pumps are electrogenic pumps. These pumps are transport
proteins that generate a voltage across a membrane
What is the main pump in animals? Other organisms?
The Na+/K+ ATPase is the main electrogenic pump of animal cells and
is especially important in neuronal physiology
What are some important functions of electrochemical gradients in cells and organisms?
1) an electrical gradient (different net charges inside and outside the cell)
2) A concentration gradient For example, more Na+ outside the cell makes the gradient go from outside (high) to the inside (low) of the cell.
Why are Na+, K+, H+ gradients important?
-They create membrane potential through active transport (i.e. positive exterior/negative interior) which favours the passive transport of cations(+) into the cell and anions(-) out of the cell by ion channels
-it drives processes like cellular respiration, transmission of nerve impulses and muscle contraction
What is secondary active transport?
-can produce electrochemical gradients that store energy for cellular work
-it is the use of an existing gradient to drive the active transport of a solute
What are some examples of secondary active transport in plants & animals
1) Plants; Plants use ATP to pump H+ against its gradient out of the cell and the H+ gradient is coupled transport sucrose into the cell
2) Humans; Glucose is co-transported into intestinal epithelial cells (where glucose concentration is high) with Na+ as it moves down the gradient created by the Na+/K+ pump
What are the differences in activities of a uniporter, a symporter and an antiporter?
1) Uniporter: A single solute moves in one direction
2) Symporter: Two solutes move in the same direction
3) Antiporter: Two solutes move in opposite directions
Can you describe the process of bulk transport?
-Large molecules (polysaccharides and proteins), viruses and bacteria must cross the membrane in bulk transport via vesicles
-They leave a cell by exocytosis; they enter a cell by endocytosis
-Requires energy
Compare constitutive vs. regulated bulk transport of exocytosis.
1) Constitutive: macromolecules are secreted from the cell without having to await a specific signal
2) Regulated: contents of a vacuole are secreted in response to a specific signal
What is exocytosis?
-Vesicle fuses with plasma membrane and releases contents into extracellular space
-proteins and phospholipids are incorporated into plasma membrane
Question: Are the following regulated or constitutive processes?
-Example 1: Insulin release by pancreatic cells (REGULATED)
-Example 2: Neurotransmitter release by neurons (REGULATED)
What is endocytosis? (3 types)
1) Phagocytosis: Large particles engulfed into vacuole which fuses with lysosome (ie. Bacteria are ingested by white blood cells)
2) Pinocytosis: Nonspecific uptake of extracellular fluid
3) Receptor-Mediated Endocytosis: Triggered by binding of ligand to surface receptor
What makes receptor-mediated endocytosis different?
-A vesicle is formed through recruitment of a coated pit when specific molecules attach to their specific receptors on the membrane
-This is a specific mechanism meaning it depends on the molecule it’s taking in
-Cells take in LDLs via receptor-mediated endocytosis (they bind to specific surface receptors)
What is the purpose of cell-cell signaling in unicellular organisms? How is this achieved? What molecules are involved?
-It allows populations of cells to coordinate with one another and work like a team to accomplish tasks, this is called quorum sensing
-This is achieved when the concentration of an autoinducer reaches a certain level due to population density and the bacteria coordinate a response
What are examples of specific responses to cell-signaling in unicellular organisms (5)?
1) Sporulation (protists & bacteria) by quorum sensing
2) Exchange of plasmid DNA
3) Bioluminescence
4) Virulence: Secretion of proteins (help cause disease in multicellular organisms)
5) Production of biofilms: hard, polysaccharide-rich substances that encase the cells and attach them to a surface.
What are examples of molecules used in cell signaling in multicellular organisms?
-Happens (most often) through the release of chemical messengers
-Pheromones; Male luna moths detect pheromones with their antennae
-volatile compounds released by plants when under attack can attract another organism to help (caterpillar and parasitoid wasp)
What are the four types of local cell-signaling?
1) through cell junctions
2) cell surface molecules
3) paracrine signalling
4) synaptic signalling (neurotransmitters)
How do we communicate through cell junction?
-through the transfer of molecules through these junctions
-this is done through gap junctions between animal cells and between plasmodesmata between plant cells.
How do we communicate through cell surface molecules?
-GPCRs and RTKs are cell surfcace receptors.
- when a signal molecule binds to a cell surface molecule is changes the proteins shape which causes a change in the cells activity
How do we communicate through paracrine signalling?
A secretory cell releases local regulator molecules (with secretory vesicles) that travel through the extracellular matrix to nearby cells
