Ch. 8: Biological Membranes Flashcards
char: cell (plasma) membrane
a semipermeable phospholipid bilayer
why is “a semipermeable phospholipid bilayer” such an apt description of the cell/plasma membrane? + diagram
this describes both the FUNCTION and STRUCTURE of the cell membrane: as a semipermeable barrier, it chooses which particles can enter and leave the cell at any point in time
what is the selectivity of the cell membrane mediated by? (2)
- the various channels and carriers that poke holes in the membrane
- the membrane itself
what type of compounds enter the cell membrane easily? what type of a harder time? why?
cross easily: fat-soluble compounds
alternative entry: larger and water-soluble compounds
bc it is composed primarily of 2 layers of phospholipids
defn: fluid mosaic model
the theory that underlies the structure and function of the cell membrane
what other compounds compose the phospholipid bilayer?
- proteins
- distinct signaling areas within lipid rafts
what is a glycoprotein coat composed of?
carbohydrates associated with membrane-bound proteins
do the cell walls of plants, bacteria, and fungi contain higher or lower levels of carbohydrates?
higher
func (5): cell membrane
- to protect the interior of the cell from the external environment
- selectively regulate traffic into and out of the cell
- involved in both intracellular and intercellular communication and transport
- contain proteins embedded within the lipid bilayer that act as cellular receptors during signal transduction and play an important role in regulating and maintaining overall cellular activity
- a stable semisolid barrier between the cytoplasm and the environment, but is in a constant state of flux molecularly
how do phospholipids act in the cell membrane?
move rapidly in the plane of the membrane through simple diffusion
defn: lipid rafts
collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules and often serve roles in signaling
how do lipid rafts and proteins move within the cell membrane? (2)
- can both travel within the plane of the membrane, but more slowly than phospholipids
- lipids can move between the membrane layers, but is energetically unfavorable because the polar head group of the phospholipid must be forced through the nonpolar tail region in the membrane interior
defn + func: flippases
specialized enzymes
assist in the transition or “flip” between layers
how is the concentration of various membrane proteins mediated? (3)
how is the number of specific cellular receptors on the surface mediated? (1)
membrane proteins:
1. gene regulation
2. endocytotic activity
3. protein insertion
cellular receptors:
1. many cell, esp. those involved in biosignaling processes
what is the primary component of the cell membrane?
lipids!
what 4 types of lipids are involved in the cell membrane?
- a large number of phospholipids
- very few free fatty acids
- steroid molecules and cholesterol
- waxes
what is the main function of steroid molecules and cholesterol and waxes in the cell membrane? (together and individual)
help to maintain the structural integrity of the cell
steroid molecules and cholesterol: lend fluidity to the membrane
waxes: provide membrane stability
defn: fatty acids
carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group
defn + aka + struct: triacylglycerols
aka: triglycerides
storage lipids involved in human metabolic processes
structure: contain 3 fatty acid chains esterified
char (3) + impact of char on plasma membrane: unsaturated fatty acids
- “healthier”
- tend to have one or more double bonds
- exist in liquid form at room temperature
impart fluidity to the plasma membrane
do humans make unsaturated fatty acids? do they come from food?
humans can only synthesize a few
the rest come from essential fatty acids in the diet that are transported as triacylglycerols from the intestine inside chylomicrons
what are the two important essential fatty acids for humans?
- alpha-linolenic acid
- linoleic acid
char (4) + impact on membrane: saturated fatty acids
- the main components of animal fats
- tend to exist as solids at room temperature
- found in processed foods
- less healthy
decrease overall membrane fluidity
how is a glycerophospholipid formed + aka?
aka: phospholipid
formed by substituting one of the fatty acid chains of triacylglycerol with a phosphate group, a polar head group joins the nonpolar tails
what do phospholipids spontaneously assemble into and why? (2) + defn for these 2 things
- micelles (small monolayer vesicles)
- liposomes (bilayered vesicles)
due to hydrophobic interactions
func (4): glycerophospholipid
- used for membrane synthesis
- can produce a hydrophilic surface layer on lipoproteins such as very-low-density lipoprotein (VLDL), a lipid transporter
- primary component of cell membranes
- second messengers in signal transduction
func: phosphate group of a phospholipid + ex (2)
provides an attachment point for water-soluble groups
ex:
choline (aka phosphatidylcholine aka lecithin)
inositol (phosphatidylinositol)
diagram: triacylglycerol vs. glycerophospholipid
char (3) + func: sphingolipids
func: important constituents of cell membranes
char: 1. do not contain glycerol
2. similar in structure to glycerophospholipids
3. contain a hydrophilic region and two fatty acid-derived hydrophobic tails
how are various classes of sphingolipids differentiated? + diagram
mostly in the identify of their hydrophilic regions
4 classes of sphingolipids and their hydrophilic groups
- ceramide
- sphingomyelins
- cerebrosides
- gangliosides
func (2): cholesterol
- regulates membrane fluidity
- necessary in the synthesis of all steroids
char (1): cholesterol
similar structure to that of phospholipids bc it contains a hydrophilic and a hydrophobic region
func (3): cholesterol
- stabilizes adjacent phospholipids in the membrane (due to interactions with the hydrophilic and hydrophobic regions of the phospholipid bilayer)
- occupies space between adjacent phospholipids(preventing formation of crystal structures in the membrane, and increasing fluidity at lower temps)
- at high temps: limits movement of phospholipids within the bilayer, thus decreasing fluidity and hold the membrane intact
ratio of cholesterol to phospholipid + effect
by mass: cholesterol composes about 20% of the cell membrane
by mole fraction: cholesterol is about half of the cell membrane
this large ratio of cholesterol to phospholipid ensures that the membrane remains fluid
defn + char (4): waxes
a class of lipids
- extremely hydrophobic
- rarely found in the cell membranes of animals, sometimes found in plant cell membranes
- composed of a long-chain fatty acid and a long-chain alcohol
- high melting point
func (2): waxes
- provide stability and rigidity within the nonpolar tail region (when present in the cell membrane)
- extracellular function in protection or waterproofing
what are the 3 types of membrane proteins that the fluid mosaic model accounts for? + diagram
- transmembrane proteins
- embedded proteins
- membrane-associated (peripheral) proteins
relationship to membrane: transmembrane proteins
pass completely through the lipid bilayer
relationship to membrane: embedded proteins
associated with only the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane
defn + reason for name: integral proteins
transmembrane membrane proteins and embedded proteins together
called this because of their association with the interior of the plasma membrane
what assists the integral proteins?
one or more membrane-associated domains that are partially hydrophobic
relationship to membrane: membrane-associated peripheral proteins
bound through electrostatic interactions with the lipid bilayer, especially at lipid rafts, or to other transmembrane or embedded proteins, like the G proteins found in G protein-coupled recetors
what 3 things are typically considered transmembrane proteins?
- transporters
- channels
- receptors
char (2): carbohydrates
- generally attached to protein molecules on the extracellular surface of cells
- generally hydrophilic
what is the impact of carbohydrates hydrophilicity?
interactions between glycoproteins and water can form a coat around the cell
func(1): carbohydrates
can act as signaling and recognition molecules
how are membrane carbohydrates and membrane proteins taken advantage of?
our immune systems and some pathogens do this to target particular cells
func (2) + char (2): membrane receptors
char: 1. tend to be transmembrane proteins
2. there are some that are carbohydrates or lipids, especially in viruses
func: 1. activate or deactivate some of the transported for facilitated diffusion and active transport
2. biosignaling
why are cell-cell junctions formed? what are they typically formed of? what is the function of these?
cells within tissues can form a cohesive layer via intercellular junctions
generally comprised of cell adhesion molecules (CAM)
func: provide direct pathways of communication between neighboring cells or between cells and the extracellular matrix
defn: cell adhesion molecules (CAM)
proteins that allow cells to recognize each other and contribute to proper cell differentiation and development
what are the 3 types of cell-cell junctions?
- gap junctions
- tight junctions
- desmosomes
func + char + aka + diagram: gap junctions
func: 1. allow for direct cell-cell communication
2. permit movement of water and some solutes directly between cells
3. do not generally transfer proteins
aka: connexons
- often found in small bunches together
- formed by the alignment and interaction of pores composed of 6 molecules of connexin
func (2) + char (2): tight junctions
func: 1. prevent solutes from leaking into the space between cells via a paracellular route
2. a physical link between the cells as they form a single layer of tissue
char: 1. found in epithelial cells
2. must form a continuous band around the cell
what happens when tight junctions limit permeability significantly enough?
to create a transepithelial voltage difference based on differing concentrations of ions on either side of the epithelium
why must tight junctions form a continuous band around the cell?
this is necessary for them to be effective, else fluid could leak through spaces between tight junctions
func + how are they formed + where are they found: desmosomes
func: bind adjacent cells by anchoring to their cytoskeletons
formed by interactions between transmembrane proteins associated with intermediate filaments inside adjacent cells
primarily found at the interface between two layers of epithelial tissue
func: hemidesmosomes
similar function to desmosomes but their main function is to attach epithelial cells to underlying structures, especially the basement membrane
summary: selectivity of different substances into and out of the cell via the cell membrane
transport of small nonpolar molecules –> rapid through cell membrane via diffusion
ions and larger molecules –> more specialized transport processes
summary of different membrane traffic processes differ (2)
- active or passive
- driven by concentration gradients or intracellular energy stores
what is ALL transmembrane movement driven by? what does this tell us?
concentration gradients
the gradient will tell us whether this process will be passive or active
what are transport processes classified as active or passive depending on?
their thermodynamics
char (thermodynamics): passive vs. active transport
PASSIVE = spontaneous process that do not require energy
ACTIVE = nonspontaneous and require energy
how are passive and active transport affected by temperature?
PASSIVE = generally increase in rate as temperature increases
ACTIVE = may or may not be affected by temperature, depending on the enthalpy of the process
what is the primary thermodynamic motivator in most passive transport?
an increase in entropy
defn: passive transport processes
those that do not require intracellular energy stores but rather utilize the concentration gradient to supply the energy for particles to move
defn + char (3): simple diffusion
substrates move down their concentration gradient directly across the membrane
char: 1. the most basic of all membrane traffic processes
2. only particles that are freely permeable to the membrane are able to undergo simple diffusion
3. there is potential energy in a chemical gradient, some of which is dissipated as the gradient is utilized during simple diffusion
analogy: energy used in simple diffusion
there is potential energy in the ball when it sits at the top of the hill
as the ball spontaneously rolls down the hill, and some of the energy is dissipated
defn: osmosis
a specific kind of simple diffusion with water – water will move from a region of lower solute concentration to one of higher solute concentration
aka: water will move from a region of higher WATER concentration (more DILUTE solution) down a gradient to a region of LOWER water concentration (more CONCENTRATED solution)
when is osmosis most important? + what happens in these cases?
when the solute itself is impermeable to the membrane
in this case: water will move to try to bring solute concentrations to equimolarity
defn + effect: hypotonic solution
defn: if the concentration of solutes INSIDE the cell is HIGHER than the surrounding solution
effect: such a solution will cause a cell to swell as water rushes in, sometimes to the point of bursting (lysing)
defn + effect: hypertonic solution
defn: a solution that is more concentrated than the cell
effect: water will move out of the cell
defn + effect: isotonic solution
defn: if the solutions inside and outside are equimolar
effect: does not prevent movement, rather prevents the NET movement of particles
diagram: osmosis
mnemonic: hypotonic
water flows into a cell placed in hypOtonic solution, so imagine the cell swelling to form a giant letter O
defn + char: osmotic pressure
char: one method of quantifying the driving force behind osmosis
char: 1. colligative property
defn: colligative property
a physical property of solutions that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles
eqn: osmotic pressure
where M = molarity of the solution
R = ideal gas constant
T = absolute temperature (in kelvins)
i = the van’t Hoff factor
defn + ex: i (van’t Hoff factor)
the number of particles obtained from the molecule when in solution
ex: glucose remains one intact molecule, so iglucose = 1, sodium chloride becomes 2 ions (Na+ and Cl) so inacl = 2
does osmotic pressure depend on the identity of the particles in solution?
no, just the presence and number of particles in solution
what is the relationship between osmotic pressure and the cell membrane?
the osmotic pressure is maintained against the cell membrane, rather than the force of gravity
if the osmotic pressure created by the solutes within a cell exceeds the pressure that the cell membrane can withstand, the cell will lyse
analogy: osmotic pressure
a “sucking” pressure –> drawing water into the cell in proportion to the concentration of the solutin
unless otherwise specified, what does a semipermeable membrane refer to?
a membrane governed by the same permeability rules as biological membranes: small, nonpolar, lipid-soluble particles (and water) can pass through freely, while large, polar, or charged particles cannot
defn + char: facilitated diffusion
simple diffusion for molecules that are impermeable to the membrane (large, polar, or charged) because the energy barrier is too high for these molecules to cross freely
classic examples involve a carrier or channel protein
what does facilitated diffusion require?
integral membrane proteins to serve as transporters or channels for these substrates
defn + analogy: carriers
only open to one side of the cell membrane at any given point
analogy: similar to a revolving door because the substrate binds to the transport protein (walks in), remains in the transporter during a conformational change (spins), and then finally dissociates from the substrate-binding site of the transporter (walks out)
defn + cause + char: occluded state
cause: binding of the substrate molecule to the transporter protein induces a conformational change
char: brief time
defn: the carrier is not open to either side of the phospholipid bilayer
func: channels
viable transporters for facilitated diffusion
how do channels act in open conformations? can they also be in a closed conformation?
OPEN = channels are exposed to both sides of the cell membrane and act like a tunnel for the particles to diffuse through, thus permitting more rapid transport kinetics
can also be in a closed conformation
result: active transport
results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill
does active transport require energy?
always, but the source varies
how does primary active transport work?
uses ATP or another energy molecule to directly power the transport of molecules across a membrane
typically involves the use of a transmembrane ATPase
how does secondary active transport work + aka
aka: coupled transport
uses energy to transport particles across the membrane, however there is no direct coupling to ATP hydrolysis
harnesses the energy released by one article going DOWN its electrochemical gradient to drive a different particle UP its gradient
defn: symport vs. antiport
SYMPORT = when the particles flow in the same direction across the membrane
ANTIPORT = when the particles flow in opposite directions
defn: endocytosis
occurs when the cell membrane invaginates and engulfs material to bring it into the cell
why is it important that the material is encased in a vesicle in endocytosis?
cells will sometimes ingest toxic substances
defn: pinocytosis
the endocytosis of fluids and dissolved particles
defn: phagocytosis
the ingestion of large solids such as bacteria
process: endocytosis
- substrate binding to specific receptors embedded within the plasma membrane initiates endocytosis
- invagination will then be initiated and carried about by vesicle-coating proteins, most notably clathrin
defn: exocytosis
occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment
what 2 situations are exocytosis important in?
- the nervous system
- intercellular signaling
what 2 factors lead to an electrochemical gradient between the exterior and interior of cells?
- the impermeability of the cell membrane to ions
- the selectivity of ion channels
defn: membrane potential, Vm
the difference in electrical potential across cell membranes
value: resting potential
for most cells: between -40 and -80 mV
although, the potential can rise as high as +35 mV during depolarization of the cell
why does maintaining membrane potential require energy?
because ions may passively diffuse through the cell membrane over time using leak channels
func: ion transporter or pump such as sodium-potassium pump (Na+/K+ ATPase)
regulates the concentration of intracellular and extracellular sodium and potassium ions
chloride ions also participate in establishing membrane potential
func: Nernst equation
used to determine the membrane potential from the intra- and extracellular concentrations of the various ions
func: Na+/K+ ATPase
to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping three sodium ions out for every two potassium ions pumped in
this movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell
why are cell membranes more permeable to K+ ions than Na+ ions at rest?
because there are more K+ leak channels than Na+ leak channels
how is a stable resting membrane potential maintained? (2)
the combination of 1. Na+/K+ ATPase activity and 2. leak channels together
why are mitochondria referred to as the powerhouse of the cell?
because of their ability to produce ATP by oxidative respiration
char (2): outer mitochondrial membrane
- highly permeable due to many large pores that allow the passage of ions and small proteins
- completely surrounds the inner mitochondrial membrane, with the presence of a small intermembrane space in between the two layers
char (4): inner mitochondrial membrane
- has a much more restricted permeability compared to the outer one
- contains numerous infoldings (cristae) which increase the available surface area for the integral proteins associated with the membrane that are involved in the ETC and ATP synthesis
- encloses the mitochondrial matrix
- contains a high level of cardiolipin and does not contain cholesterol
func: mitochondrial matrix
where the citric acid cycle produces high-energy electron carriers used in the electron transport chain
