Histology: Membrane Transport and Intracellular Trafficking Flashcards
Types of passive transport?
Simple diffusion, ion channels, facilitated diffusion (carrier protein)
Types of active transport?
Primary active transport, secondary active transport
The plasma membrane contains…
Integral membrane proteins (transmembrane, peripheral), cholesterol, ion channels, transport proteins/carriers
What substance can pass through the membrane?
Small, hydrophobic non-polar molecules
What does cholesterol do in the plasma membrane?
Stabilizes the membrane - restricts the movement of phospholipids, increasing the rigidity of the membrane.
Formula for ideal unidirectional flux from outside of membrane to inside:
F_o-i = K_p (permeability constant/partition coefficient) * C_o (concentration)
Formula for ideal net flux across membrane:
F_net = K_p (permeability constant/partition coefficient) * (C_o - C_i)
Fick’s Law represents…
More realistic simple diffusion
What is Fick’s Law?
F_net (or J_net) = (K_p * A * deltaC_s)/(deltax) where F_net is in mole/sec, K_p is partition coefficient in cm^2/sec, A is area in cm^2, x is distance in cm, and deltaC_s is solute concentration gradient in mol/cm^3
(T/F) Simple diffusion occurs between capillaries and cells through the interstitial fluid.
True
What is the relationship between the partition coefficient and the solute?
Directly proportional to lipid solubility of solute, inversely proportional to the size of the solute
What is the permeability of the lipid bilayer to small uncharged polar molecules?
Low
What is the permeability of the lipid bilayer to large uncharged polar molecules such as glucose, amino acids, etc.?
Almost none
What is the permeability of the lipid bilayer to ions?
Not permeable
What is the permeability of the lipid bilayer to small non-polar molecules (CO2, O2)?
Highly permeable
(T/F) In the lungs, O2 diffuses from the alveoli into the pulmonary capillaries by passive diffusion.
True
What is pulmonary edema and how does it affect diffusion capacity of O2 in lungs?
Accumulation of fluid in the interstitial space. Decreases capacity because it increases distance required for diffusion
What is HAPE?
When low O2 pressure at high altitudes is compensated for by increased blood flow (increased blood pressure) resulting in increased fluid leakage from the capillaries.
(T/F) Integral membrane transport proteins are 10% of all proteins.
True
(T/F) There are less than 200 protein families.
False, more than 200
(T/F) Integral membrane transport proteins utilize 1/3 of cell’s energy resources.
False, 2/3
(T/F) Solute composition of interstitial fluid is approximately equal to that of plasma.
True
Normal concentrations of K+
139 mM intracellular, 4.0 extracellular, 3.5-5.5 plasma
Normal concentrations of Na+
5-15 mM intracellular, 145 extracellular, 136-146 plasma
Normal concentrations of Ca2+
<0.0002 mM intracellular, 1.8 extracellular, 1.0-2.0 plasma
Normal concentrations of Cl-
4-15 mM intracellular, 116 extracellular, 96-116 plasma
Normal concentrations of HCO3-
12 mM intracellular, 29 extracellular, 22-30 plasma
Normal concentrations of protein
138 mM intracellular, 9 extracellular, 8-10 plasma
Normal osmolarity
287 mOsm intracellular, 287 extracellular, 280-295 plasma
Normal pH (arterial)
7.06-7.13 intracellular, 7.4 extracellular, 7.37-7.42 plasma
(T/F) In ion channels, hydrophilic proteins face outside.
False, inside
How do ion channels regulate passage of ions?
They undergo conformational changes (gated, open)
What is the average rate if ion transport through ion channels?
10^7 to 10^8 ions/sec
What are the types of gating in ion channels?
Ligand gated, voltage gated, mechanically gated
Ligand gated ion channels are activated by…
Binding of a chemical signaling agent
Voltage gated ion channels are activated by…
Changes in membrane potential (can also inactivate)
Mechanically gated ion channels are activated by…
Mechanical force that results in physical deformation of the surrounding plasma membrane
What is an ion filter?
Determines selectivity/specificity of ion channel
How does a carrier protein work?
Molecule binds gated protein, causes conformational change so the molecule is moved to the other side, affinity decreased so molecule is released
Structure of a GLUT
12 transmembrane domains, sugar moiety on extracellular side, glucose binding site
How many GLUT transporter family members are there?
13
What is primary active transport?
Mediated (by protein) transport that requires ATP hydrolysis as an energy source; goes against concentration gradient.
What is the process of primary active transport?
- Molecule binds on one side
- ATP phosphorylates on other side
- Causes a conformational shift where molecule switches to other side
- Phosphate group is released
- Affinity for molecule is decreased; molecule is released
What is the most important primary active transporter?
NaK-ATPase (sodium-potassium pump)
What does the sodium potassium pump do?
Pumps 3 Na ions from inside to outside, pumps 2 K ions from outside to inside (both against conc. gradient). Uses up to 30% of total cell ATP.
What are common examples of primary active transporters?
NaK-ATPase, Ca-ATPase, H-ATPase, HK-ATPase
What is the process of sodium-potassium pump transport?
- Binding of 3 Na+ ions inside
- Binding of ATP inside, phosphorylation of aspartate on pump
- Conformational change of pump, Na+ moved to outside
- Dissociation of Na+, binding of 2K+ on outside
- Hydrolysis of aspartyl phosphate
- Conformational change of pump, inward transport of K+
- Dissociation of K+ ions
What is secondary active transport?
Mediated (by protein) transport that uses energy linked with co-transport and electrochemical gradient. No ATP required.
What are the types of secondary active transport?
Co-transport/symport and counter-transport/antiport
What is co-transport/symport?
Energy produced by one molecule moving down its conc. gradient pumps another molecule against its concentration gradient in the same direction.
What is counter-transport/antiport?
Energy produced by one mlcl moving down its conc. gradient pumps another molecule against its conc. gradient in the opposite direction.
Examples of co-transport/symport?
- Na+ down gradient, glucose up (from intestinal lumen into the body)
- Na+ down, amino acids up
- Na+ down, K+ and Cl- up
Examples of counter-transport/antiport?
- Na+ down gradient, H+ up
- 3Na+ down, Ca2+ up
- HCO3- down, Cl- up
(T/F) Water flow across plasma membrane is both passive and facilitated.
True
Equation for flow of water across plasma membrane:
J_v = L_p * deltaP where J_v is flow of water volume, L_p is hydraulic conductivity/permeability, deltaP is pressure gradient (net force of hydrostatic, osmotic, and oncotic pressure).
Water will flow until…
deltaP = 0
How does hydrostatic pressure affect flow of water?
Water flows from high to low hydrostatic presure
Units of hydrostatic pressure
deltaP in mmHg, cmH2O
What is osmotic pressure?
deltaPi, pressure gradient formed by differences in solute osmolality.
How does osmotic pressure affect flow of water?
Water flows from low to high solute osmolality (down the concentration gradient of water).
What are the units of osmotic pressure?
1 mOsm = 19.3 mmHg = 26.2 cmH2O
What is the formula for osmotic pressure?
deltaPi = n * R * T * deltaC
What is the formula for osmolarity?
Pi (mOsm) = n * deltaC (mM) where n is # particles/mole (ex. n for NaCl is 2)
What are isotonic solutions?
Have same concentration of nonpenetrating solutes as normal extracellular fluid
What are hypotonic solutions?
Have lower concentration of nonpenetrating solutes as normal extracellular fluid (cell swells, eventually lysis).
What are hypertonic solutions?
Have higher concentration of nonpenetrating solutes as normal extracellular fluid (cell shrinks).
How can you explain the difference in how cells respond to being placed in water? (some burst, some never rupture)
Difference in permeability because of aquaporins, water channel proteins that facilitate water diffusion
(T/F) Lipid membranes are slightly permeable to water.
True
How many family members of aquaporins are there?
12
RBCs contain how many AQP1 molecules?
~200,000
An AQP1 molecule can transport water at what rate?
3 x 10^9 water mlcls/sec
What are high LP examples?
Capillary endothelium, most cell membranes, epithelium of the proximal renal tubule, epithelium of the proximal intestine
What are low or 0 LP examples?
Epithelium of thick ascending limb, epithelium of distal tubule, epithelium of distal colon
What is LP?
Lipid permeability to water
Fluid movement between capillaries and ISF is subject to:
Oncotic pressure gradient (higher protein conc. in plasma, causes fluid to move from ISF into plasma), blood pressure (higher in capillaries, causes fluid to move from capillaries into ISF)
What is the tendency of fluid movement between capillaries and ISF?
Somewhat greater tendency for fluid to move out of the blood into the ISF than the opposite. So normally there is some loss of fluid from plasma to surrounding tissues. This leaked fluid is taken up by vessels of the lymphatic system and returned to the blood.
What is edema?
Presence of excess ISF in a tissue
What are the primary causes of tissue edema?
- Increased blood pressure in the capillaries
- Abnormally low plasma protein concentration leading to decreased colloid osmotic pressure
- Lymphatic obstruction
(T/F) Edema can cause decreased solute concentration in the urine.
True
What is the normal resting membrane potential?
-70 to -90 mV
What is the direction of movement in an electrical gradient?
+ to - potential
The driving force for ion flux is..
The sum of the electrical and chemical gradients.
What is the Nernst equilibrium potential?
E, potential when electrochemical gradient = 0 and chemical and electrical gradients are equal and opposite.
What is the Nernst potential for Na+?
+70 mV
What is the Nernst potential for K+?
-98 mV
What is the Nernst potential for Ca2+?
+150 mV
What is the Nernst potential for Cl-?
-30 to -65 mV
How do you calculate net electrochemical driving force?
Resting membrane potential minus equilibrium potential. + means driving force is from inside cell to outside.
How is the resting membrane potential established?
By balancing the Nernst potentials of the open channels
What are the steps of an action potential?
- Threshold reached (~-50 mV): Na+ channels open, Na+ enters cell
- Depolarization: K+ channels open, K+ begins to leave cell
- Peak (~+50 mV): Na+ channels close
- K+ leaves cell
- Dip below resting potential: K+ channels close
- Resting potential: excess K+ outside diffuses away
For an action potential to occur, the voltage-gated channels should be…
Highly selective, very fast, voltage sensitive, have a mechanism for rapid inactivation
Glucose transport occurs in…
Intestines, kidneys, fat, muscle, pancreas
Salt and water transport occur in…
Kidney - urine production
Cystic fibrosis and cholera both involve issues with…
Cell volume regulation and ionic balance
What membranes are excitable (can have action potentials)?
Skeletal muscle (neuromuscular junction), cardiac muscle, neurons (synaptic transmission)
What is the most common fatal childhood disease in Caucasian populations?
Cystic Fibrosis
What are examples of human diseases/problems caused by defects in membrane transport?
Cystic Fibrosis, Bile Salt Transport Disorders (leads to liver cirrhosis and failure), retinal degeneration (retinitis pigmentosa, macular dystrophy), Copper Homeostasis (Menkes syndrome, Wilson disease), Multidrug Resistance
Receptor-mediated endocytosis is associated with…
“Coated pits/vesicles” and clathrin, which later fall off once vesicle is separated from the membrane
What is adaptin?
Acts as linker between membrane proteins (receptors) and clathrin
What is dynamin?
Along with other proteins, wraps around the neck of a forming vesicle and aids in its pinching off from the source membrane
What are the steps of receptor-mediated endocytosis?
- Coat assembly (adaptin, clathrin) and cargo (molecule to bind) selection and binding
- Bud formation
- Vesicle formation with help of dynamin
- Uncoating, leaves a “naked” transport vesicle
Most cholesterol in the blood is transported as…
Lipid/protein particles known as low-density lipoproteins (LDL) - bad cholesterol. Since cholesterol itself has limited solubility in an aqueous environment.
What is the structure of LDL?
Core of cholesterol mlcls surrounded by a lipid monolayer and organized by a protein
How is cholesterol imported by cells?
Receptor-mediated endocytosis with LDL receptor protein and LDL as the “cargo” molecule
What is familial hypercholesterolemia?
Condition characterized by high cholesterol levels (particularly LDL). Leads to atherosclerosis and heart failure. Caused by mutations in LDL receptor - poor ability to bind LDL, failure of receptor to sort into coated pits (cytoplasmic domain missing)
What are the general features of receptor-mediated endocytosis of LDL?
- Clathrin-coated pit
- Clathrin-coated vesicle
- Uncoating of vesicle
- Reduction of endosomal pH from 7 to 5
- Disassociation of receptor and ligand at low pH
- Delivery of LDL particle to lysosome
- Degradation of LDL by lysosomal enzymes into amino acids, cholesterol, and catty acids
- Recycling of the receptor to the cell surface
How are different vesicles distinguished from each other?
By surface molecular markers (membrane-associated proteins) - act like bar code and tell vesicle where to go.
How is the plasma membrane repaired?
Vesicle pinches off from Golgi apparatus, joins the plasma membrane
The contents of transport vesicles are referred to as…
Cargo
What are the steps of vesicle fusion to a target?
- Vesicle must recognize its target membrane (“docking”)
2. Vesicle and target membrane must fuse.
What are SNAREs?
Membrane-bound proteins that provide specificity for vesicle targeting and catalyze membrane fusion. At least 20 types.
What are the types of SNAREs and how do they work?
Vesicle SNAREs (v-SNAREs) and target SNAREs (t-SNAREs). Their alpha-helical rods wrap around each other to make a tightly bound complex.
What are Rabs?
Monomeric GTPases that facilitate and regulate vesicle docking and the matching of t-SNAREs and v-SNAREs. Over 30 forms.
What are endosomes?
Membrae-bound compartments in which endocytosed material is sorted, then sent to different regions of the cell. pH is lowered by proton pumps as they get closer to the nucleus. In some cases the low pH causes cargo to be released from the receptor.
What are the types of endosomes?
Early endosomes - just below plasma membrane, pH ~6
Late endosomes - closer to nucleus, pH ~5
What are the different fates for receptors?
Recycled back to the plasma membrane, degraded, or transported (with cargo) across the cell (transcytosis)
What is transcytosis?
Transport across a cell.
What is the primary site of intracellular digestion?
Lysosomes
Lysosomes contain…
Degradative enzymes/acid hydrolases: proteases, nucleases, lipases, phosphatases, glycosidases, sulfatases, phospholipases, etc. All require acid environment for optimal activity.
How do lysosomes stay acidic?
H+-ATPase uses ATP hydrolysis to pump protons across the membrane
Lysosomes have an internal pH of about…
5.0
(T/F) Lysosomal enzymes are optically active at pH 5.0
True
Which pathways deliver materials to lysosomes?
Endocytosis, phagocytosis, and autophagy
What is autophagy?
Process where damaged or senescent (no longer used) organelles are surrounded by membrane and delivered to lysosomes for degradation.
What causes lysosomal storage diseases?
Genetic defects that affect one or more of the acid hydrolases, resulting in accumulation of undigested material in lysosomes. Most critical effects generally in the nervous system.
What are examples of lysosomal storage diseases?
Neimann-Pick disease
Tay-Sachs disease
Hurler’s syndrome
Inclusion cell disease
What is Neimann-Pick disease?
Defect in sphingomyelinase (lysosomal enzyme), causing accumulation of sphingolipid, CNS defects, death < 18 months.
What is Tay-Sachs disease?
Defect in hexosaminidase A (lysosomal enzyme), which breaks down phospholipids. Causes accumulation of phospholipids in brain, mental retardation, death < 4 years.
What is Hurler’s syndrome?
Defect in alpha-L iduronidase (lysosomal enzyme), which breaks down mucopolysaccharides. Causes mental retardation, death < 10 years.
What is Inclusion cell disease (I-cell disease)?
Defect where almost all hydrolases are missing from the lysosome but found in blood. Lysosomes cannot function, and accumulation of material in lysosomes (inclusions occurs), causing death < 7 years.
What are the mechanisms that proteins made in the cytosol enter membrane-bound compartments?
- Nuclear proteins are transported through selective nuclear pores
- Proteins destined for mitochondria (or chloroplasts) are unfolded and guided across mitochondrial membranes.
- Proteins (ex. insulin) destined for the ER or other organelles enter the ER while being synthesized.
What are the functions of exocytosis?
- Replace portions of the plasma membrane that endocytosis has removed.
- Add new membrane components to the membrane.
- Provide a route by which membrane-impermeable molecules (ex. protein hormones) the cell synthesizes can be secreted into the ECF.
What is the difference between constitutive secretion and regulated secretion?
Constitutive is unregulated - vesicles fuse to membrane when they reach it.
In regulated - vesicles do not fuse with membrane right away. They store proteins, and when activated by a signal, then they fuse.
(T/F) Insulin decreases the efficiency of uptake of glucose into cells.
False, increases.
(T/F) Insulin storage granules are normally very close to Ca2+ channels.
True
Why are insulin storage granules very close to Ca2+ channels normally?
Ca2+ influx is coupled with secretion of insulin/exocytosis of insulin storage granules. Ca2+ influx is most important to induce fusion.
What are important examples of vesicular transport/trafficking?
Killing of invading microbes, glucose uptake (insulin release and signaling), cholesterol uptake, iron uptake, organelle homeostasis, development, neuronal function (release of neurotransmitters, platelet function)
What does “integrated transport system” mean?
Each transporter affects other transporters.
(T/F) Cells are normally polarized.
True
