Chapter 8: Biological Membranes Flashcards
What is a semipermeable barrier? Give an example.
- It chooses which particles can enter and leave the cell at any point in time
- ex: cell (plasma) membrane
How is the selectivity of semipermeable barriers mediated?
- Various channels and carriers that poke holes in the membrane
- By the membrane itself
What is the cell membrane composed primarily of?
Two layers of phospholipids
What kind of compounds does the cell membrane let enter? What kind of compounds does it not let enter?
Permits: fat-soluble compounds
Does not permit: larger and water-soluble compounds
What is the theory that underlies the structure and function of the cell membrane called?
The fluid mosaic model
What are lipid rafts?
Aggregates of specific lipids in the membrane that function as attachment points for other biomolecules and play roles in signaling
The phospholipid bilayer also includes proteins and distinct signaling areas within ______
lipid rafts
Carbohydrates associated with membrane-bound proteins create a _________
glycoprotein coat
Which microorganisms contain higher levels of carbohydrates within their cell walls?
Plants, bacteria and fungi
What is the major role of proteins embedded within the lipid bilayer?
- Act as cellular receptors during signal transduction
- Play an important role in regulating and maintaining overall cellular activity
How do phospholipids move in the plane of the cell membrane?
Rapidly, through simple diffusion
Do lipid rafts and proteins travel within the plane of the membrane?
Yes, but more slowly than phospholipids
Can lipids move between membrane layers?
Yes
Why is it energetically unfavourable to move lipids between membrane layers?
Because the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane
What kind of enzymes assist in the transition or “flip” between layers?
Flippases
What are flippases?
- Specific membrane proteins that maintain the bidirectional transport of lipids between the layers of the phospholipid bilayer in cells
- Without flippases, this movement would be energetically unfavourable
List the following membrane components in order from most plentiful to least plentiful: carbohydrates, lipids, proteins, nucleic acids.
Lipids > Proteins > Carbohydrates > Nucleic acids (ABSENT)
What is the role of steroid molecules and cholesterol in the cell membrane?
Lend fluidity to the membrane
What is the role of waxes in the cell membrane?
Provide membrane stability
Are there free fatty acids in the cell membrane?
Very few
What is the major role of triacylglycerols and free fatty acids in the cell membrane? Are they found in large quantities?
- Act as phospholipid precursors
- Found in low levels in the membrane
How are essential fatty acids from the diet transported as triglycerides from the intestine?
Inside chylomicrons
What are the two essential fatty acids?
a-linolenic acid and linoleic acid
When incorporated into phospholipid membranes, do unsaturated fatty acids increase or decrease overall membrane fluidity? What about saturated fatty acids?
Unsaturated: increase membrane fluidity
Saturated: decrease membrane fluidity
Phospholipids spontaneously assemble into _____ or ______ due to hydrophobic interactions.
- micelles (small monolayer vesicles)
- liposomes (bilayered vesicles)
What are glycerophospholipids used for in terms of the cell membrane? What can it produce
- Membrane synthesis, primary component of cell membranes
- Can produce a hydrophilic layer on lipoproteins such as VLDL, a lipid transporter
How can glycerophospholipids serve as second messengers?
In signal transduction
The phosphate group on glycerophospholipids provide an attachment point for ____________ groups, such as ____.
water-soluble groups, choline
How do the various classes of sphingolipids differ?
Primarily in the identity of their hydrophilic regions
Apart from phospholipids, what other lipids are important for constituents of cell membranes?
Sphingolipids
The ratio of certain __________ to ___________ can help to identify particular membranes within the cell.
sphingolipids to glycerophospholipids
What are the 4 types of sphingolipids?
- Ceramide
- Sphingomyelin
- Cerebroside
- Ganglioside
What is the role of cholesterol in terms of membranes? What is its major role?
- Imparts fluidity to membranes
- Necessary in the synthesis of all steroids, which are derived from cholesterol
Is cholesterol hydrophilic or hydrophobic?
Contains both a hydrophilic and a hydrophobic region
How does cholesterol increase membrane fluidity?
- Cholesterol stabilizes adjacent phospholipids
- Cholesterol also occupies space between them; prevents the formation of crystal structures in the membrane; increasing fluidity
- Cross-linking adjacent phospholipids through interactions at the polar head group and hydrophobic interactions at the nearby fatty acid tail
By mass, cholesterol composes about ___ percent of the cell membrane; by mole fraction, it makes up about _____.
20%, half
Are waxes hydrophilic or hydrophobic? Are they found in the cell membranes of animals or plants?
- Extremely hydrophobic
- Rarely in the cell membranes of animals, sometimes in those of plants
When present within the cell membrane, waxes can provide both _____ and ______ within the ___________ region only
- stability
- rigidity
- nonpolar tail
Do most waxes serve an intracellular or extracellular function? How?
- Extracellular function
- Protection or waterproofing
What are the major roles of proteins located within the cell membrane?
Act as transporters, cell adhesion molecules, and enzymes
What are the three types of membrane proteins?
- Transmembrane proteins
- Embedded proteins
- Membrane-associated proteins
What are transmembrane proteins? What is their function? How many hydrophobic domains do they contain?
- Pass completely through the lipid bilayer
- Can have one or more hydrophobic domain and are more likely to function as receptors or channels
What are embedded proteins?
- Associated with only the interior or exterior surface of the cell membranes
- More likely part of a catalytic complex or involved in cellular communication
What are membrane-associated proteins?
May act as recognition molecules or enzymes
Which membrane proteins are considered integral proteins? Why?
- Transmembrane and embedded proteins
- Because of their association with the interior of the plasma membrane
What is the association of membrane proteins with the interior of the plasma membrane assisted by?
By one or more membrane-associated domains that are partially hydrophobic
How are membrane-associated (peripheral) proteins bound to the lipid bilayer?
- Electrostatic interactions with the lipid bilayer, especially at lipid rafts
- Other transmembrane or embedded proteins (G proteins found in G protein-coupled receptors)
Transporters, channels, and receptors are generally ________ proteins
transmembrane
What are carbohydrates generally attached to? Where?
- Generally attached to protein molecules
- On the extracellular surface of cells
Are carbohydrates generally hydrophilic or hydrophobic? What does that help to form?
- Hydrophilic
- Interactions between glycoproteins and water can form a protective glycoprotein coat
Apart from the protective glycoprotein coat, what is the other major role of carbohydrates in the cell membrane?
Can act as signaling and recognition model
Some of the transporters for facilitated diffusion and active transport can be activated or deactivated by ____________
membrane receptors
What are membrane receptors usually? What can they also be?
- Usually proteins
- There are some carbohydrate and lipid receptors, especially in viruses
Cells within tissues can form a cohesive layer via _________________
intercellular junctions
What do cell-cell junctions provide?
- Direct pathways of communication between neighboring cells or between cells and the extracellular matrix
- Regulate transport intracellularly and intercellularly
What are cell-cell junctions generally comprised of?
Cell adhesion molecules (CAMs)
What are CAMs?
- Cell adhesion molecules
- Proteins that allow cells to recognize each other and contribute to proper cell differentiation and development
What can extracellular ligands bind to in order to function as channels or enzymes in second messenger pathways?
To membrane receptors
What are the four types of cell-cell junctions?
- Gap junctions
- Tight junctions
- Desmosomes
- Hemidesmosomes
What do gap junctions allow? What is not generally transferred?
- Allow for the rapid exchange of ions and other small molecules between adjacent cells (water and some solutes)
- Proteins are generally not transferred
What do tight junctions prevent? What don’t they provide?
- Prevent paracellular transport
- Do not provide intercellular transport
In terms of cell-cell junctions, __________ allow for direct cell-cell communication and are often found in small bunches together.
Gap junctions
What are gap junctions also called? What are they formed of?
- Connexons
- Formed by the alignment and interaction of pores composed of six molecules of connexin
In terms of cell-cell junctions, __________ prevent solutes from leaking into the space between cells via a paracellular route.
tight junctions
Define paracellular transport.
The transport of materials through the interstitial space without interactions with the cytoplasm or cell membrane
In which cells are tight junctions usually found?
- Epithelial cells
- Physical link between the cells as they form a single layer of tissue
Tight junctions can limit permeability enough to create a ________________ difference on differing concentrations of ions on either side of the epithelium.
transepithelial voltage
To be effective, tight junctions must form a ______________________; otherwise, ___________________________________
- must form a continuous band around the cell
- fluid could leak through spaces between tight junctions
_______________ are found in the lining of renal tubules, where they restrict passage of solutes and water without cellular control.
Tight junctions
Which membrane protein is most likely to serve as channels or receptors?
Transmembrane proteins
Which membrane protein is most likely to have catalytic activity linked to nearby enzymes?
Embedded membrane proteins
Which membrane proteins is most likely to be involved in signaling or in recognition models on the extracellular surface?
Membrane-associated (peripheral)
Associate the cell-cell junction with the function:
1) Gap junctions
2) Tight junctions
A) Intercellular transport of material (Y/N)
B) Prevent paracellular transport of material (Y/N)
1) A) Intercellular transport of material
B) Do NOT prevent paracellular transport of material
2) A) NOT used for intercellular transport
B) DO prevent paracellular transport
Transport processes can be classified as active or passive depending on what?
Thermodynamics
Differentiate active and passive transport.
Passive: spontaneous, does not require E (negative deltaG)
Active: nonspontaneous, requires E (positive deltaG)
What transport processes increase in rate as temperature increases?
Diffusion, facilitated diffusion and osmosis
Is active transport affected by temperature?
May or may not be, depending on the enthalpy of the process
The primary thermodynamic motivator in most passive transport is an increase in _________
entropy (deltaS)
What tells us whether a transmembrane process is passive or active?
The concentration gradient
What does passive transport utilize to supply the energy for particles to move? In what direction do molecules move?
- Concentration gradient
- Molecules move down their concentration gradient or from an area with higher concentration to an area with lower concentration
What is simple diffusion? What kind of particles can use this type of transport?
- Substrates move down their concentration gradient directly across a membrane
- Only particles that are freely permeable to the membrane are able to undergo simple diffusion
What is osmosis?
The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration
What is a hypotonic solution? What does the cell look like?
- If the concentration of solutes inside the cell is higher than the surrounding solution
- Cause a cell to swell as water rushes in, sometimes to the point of bursting
What is a hypertonic solution? What does the cell look like?
- If the solution is more concentrated than the cell
- Water will move out of the cell
What is an isotonic solution?
If the solutions inside and outside are equimolar
Does isotonicity prevent movement?
- No, it only prevents the NET movement of particles
- Water molecules will continue to move; however, the cell will neither gain nor lose water
What is a method to quantify the driving force behind osmosis?
Osmotic pressure
What are colligative properties? Give an example.
- A physical property of solutions that is dependent on the concentration of dissolved particles, but NOT on the chemical identity of those dissolved particles
- Osmotic pressure, vapour pressure depression, boiling point elevation, freezing point depression
Do solutes move in osmosis?
No, only the water moves
What is the equation for osmotic pressure?
Osmotic pressure = iMRT M is the molarity of the solution R is the ideal gas constant T is the absolute temperature (in K) i is the van't Hoff factor
What is the van’t Hoff factor?
The number of particles obtained from the molecule when in solution
ex: NaCl = 2
In cells, the osmotic pressure is maintained against the ___________, rather than the force of gravity.
cell membrane
What happens if the osmotic pressure created by the solutes within a cell exceeds the pressure the cell membrane can withstand?
The cell will lyse
What is facilitated diffusion? What kind of molecules is it used for?
- Uses transport proteins to move impermeable solutes across the cell membrane
- For molecules that are impermeable to the membrane (large, polar or charged)
What does facilitated diffusion require?
Requires integral membrane proteins to serve as transporters or channels for these substrates (carrier or transport protein)
Differentiate carrier and channel proteins.
Carrier: only open to one side of the cell membrane at any given point (ex: revolving door)
Channel: may have an open or closed conformation (open = exposed to both sides of the cell membrane and act like a tunnel for particles to diffuse through)
What is an occluded state? Which transport protein is that associated with?
- In which the carrier is not open to either side of the phospholipid bilayer
- Carriers
What is active transport?
Results in the net movement of a solute against its concentration gradient
What does active transport require?
- Requires energy in the form of ATP or an existing favorable ion gradient
- Requires membrane proteins
What is primary active transport?
- Uses ATP or another energy molecule to directly transport molecules across a membrane
- Involves the use of a transmembrane ATPase
What is secondary active transport?
- Uses energy to transport molecules across the membrane
- There is NO direct coupling to ATP hydrolysis
- Harnesses the energy released by one particle going down its electrochemical gradient to drive a different particle going up its gradient
Differentiate primary and secondary active transport.
Primary: direct coupling to ATP hydrolysis
Secondary: no direct coupling to ATP hydrolysis
In terms of secondary active transport, what is a symport? What is an antiport?
Symport: when particles flow in the same direction across the membrane
Antiport: when particles flow in opposite directions
What kind of transport system maintains potential of neurons in the nervous system? What kind of transport system is used by the kidneys to reabsorb and secrete various solutes into and out of the filtrate?
- Primary active transport
- Secondary active transport
What is endocytosis? How is the material incased?
- Occurs when the cell membrane invaginates and engulfs material to bring it into the cell
- Material encased in a vesicle
What is pinocytosis?
The endocytosis of fluids and dissolved particles
What is phagocytosis?
The endocytosis of large solids, such as bacteria
What initiates the process of endocytosis?
Substrate binding to specific receptors embedded within the plasma membrane
What is exocytosis? When and how does it occur?
Occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment
Water will move toward the compartment with the highest or lowest osmotic pressure?
Highest
What is the function of the sarcolemma of muscle cells?
Must maintain a membrane potential for muscle contraction to occur
What is membrane potential?
- Vm
- The difference in electrical potential across cell membranes
What is the resting potential for most cells? What can it rise to during depolarization?
- -40 and -80 mV
- +35 mV
Why does maintaining membrane potential require energy?
Because ions may passively diffuse through the cell membrane over time using leak channels
What is the major function of ion transporters or pumps? Give an example of one.
- Regulates the concentration of intracellular and extracellular sodium and potassium ions
- Sodium-potassium pump
What can the Nerst equation determine?
Determines the membrane potential from the intra- and extracellular concentrations of various ions
The Goldman-Hodgkin-Katz voltage equation derives from the Nerst equation? What does it take into account?
Takes into account the relative contribution of each major ion to the membrane potential
The electrical potential created by one ion can be calculated by using which equation?
Nerst equation
The resting potential of a membrane at physiological temperature can be calculated using which equation?
Goldman-Hodgkin-Katz voltage equation
Learn the Nerst and the Goldman-Hodgkin-Katz voltage equation.
Learn it.
What is one of the main functions of the Na+/K+ ATPase? Do you want a high or low concentration of sodium/potassium? Where?
Maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly
How many sodium ions and potassium ions does the Na+/K+ ATPase pump? What does it cause?
- Pumping three sodium ions out for every two potassium ions pumped in
- Removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell
What do leak channels allow?
Ions, such as Na+ and K+, to passive diffuse into or out of the cell down their concentrations gradients
Are cell membranes more permeable to Na+ ions or K+ ions? Why?
- K+ ions
- Because there are more K+ leak channels than Na+ leak channels
What two factors contribute to maintaining a stable resting membrane potential?
- Na+/K+ ATPase activity
- Leak channels
What is the outer mitochondrial membrane permeable to?
- Highly permeable due to many large pores
- Allow for the passage of ions and small proteins
Integral proteins associated with the inner mitochondrial membrane are involved in which processes?
The electron transport chain and ATP synthesis
Which major process happens in the mitochondrial matrix?
- Citric acid cycle
- Produces high-energy electron carriers used in the electron transport chain
The inner mitochondrial membrane contains a very high level of ___________ and does not contain __________
cardioliping, cholesterol
How is resting membrane potential maintained?
Primarily by the sodium-potassium pump
What distinguishes the inner mitochondrial membrane from other biological membranes?
It lacks cholesterol
What is the pH gradient between the cytoplasm and the intermembrane space?
There is NO pH gradient because the outer mitochondrial membrane has such a high permeability to biomolecules
