Membrane Structure and Transport Flashcards
describe lipid rafts, what the are rich in and what they facilitate
- An area with unique proteins and lipid composition which separates them from other fluid membrane areas
- Lipid rafts are rich in:
- Cholesterol
- Glycosphingolipids
- Sphingomyelin
- Lipid rafts can facilitate signal transduction and virus infection
describe the composition of the leaflets in the PM
- The outer leaflet contains
- mainly PC
- sphingomyelin
- some PE
- glycosphingolipids
- The inner leaflet contains
- mainly PE
- some PC
- PS
- PI
- PIP2
- Glycolipids and glycoproteins are only found in the outer layer and form a glycocalyx (2-10% of the weight)
what determines the fluidity of the PM
Fluidity of the PM is determined by the FA composition of phospholipids and also by the amount of free cholesterol
what role does cholesterol play in the PM and where is it in the PM
- Cholesterol is present in both layers
- Cholesterol decreases the fluidity of the PM close to the polar head groups and increases the fluidity inside of the bilayer
- Cholesterol binds in the space that is generated by cis-double bonds of FA
- Cholesterol prevents drastic changes in fluidity due to high or low temps
how does temp affect fluidity of PM
- Low temp: the fatty acyl groups are stiffer and cholesterol increases the membrane fluidity by separating them and prevents close packing
- High temp: the fatty acyl groups are more fluid and cholesterol decreases the membrane fluidity with its steroid ring system and slows down their movement
describe the composition of other biomembranes
- Membranes inside the cell do not contain cholesterol. The fluidity is mainly regulated by the FA composition
- The inner mitochondrial membrane contain cardiolipin which is not found in other membranes
- Lipoprotein membranes are composed of a phospholipid monolayer which allows transport of nonpolar lipids (TAGs and cholesteryl ester) in the blood and lymph. Free cholesterol is found in monolayer
what determines membrane fluidity?
it is determined by the FA found int he polar lipids in the membrane
why is the FA composition especially important for intracellular membranes?
This is because intracellular membranes have little or no cholesterol is help regulate fluidity
what characteristics of phospholipids make the membrane more fluid? Name 2.
Shorter FA and unsaturated fatty acids make the membrane more fluid. Especially arachidonic acid (20:4; w-6) and DHA (22:6, w-3) increase fluidity
glycerophospholipids contain normally ____ fatty acid at position 1 and _____ fatty acid at position 2
- Saturated
- Unsaturated
transport of glucose into cells: name the sodium independent glut transporters, facilitated diffusion transport
- GLUT 1: brain, RBC
- GLUT 2: liver, kidney, B-cells of pancreas, intestine
- GLUT-3: neurons
- GLUT 4: muscle and fat
- GLUT 5: intestine, seminal vesicles
transport of glucose into cells: name the sodium-dependent glucose transporters, secondary active transport
SGLT-1: intestine (epithelial cells at the luminal side)
SGLT-2: kidney (epithelial cell in renal tubules)
High affinity GLUT transporters are
GLUT1, GLUT3 and GLUT4
Low affinity GLUT transporters are
GLUT2 and GLUT5
describe the glucose uptake with GLUT1 and GLUT3
The uptake with GLUT1 and GLUT3 remains at a constant rate with normal and lower blood glucose concentrations
GLUT1 is abundant in RBC, and it is also found in the blood-brain barrier as well as in the kidneys
GLUT3 is dominant in neurons and brain
which GLUT transporter is insulin-dependent?
- GLUT4 is insulin-dependent.
- GLUT4 is abundant in fat cells, skeletal muscle and the heart
- Insulin mobilizes the transport of GLUT4 from the endosome to the PM. Once the GLUT4 arrives at the PM, it allows the influx of blood glucose into the cell
- Insulin injection can lead to hypogylcemia due to rapid uptake of blood glucose into skeletal muscle and fat cells via the mobilized GLUT4
what mobilizes GLUT4?
Insulin and vigorous exercise of the skeletal muscles
describe the efficiency and rate of glucose transport with GLUT2
GLUT2 (low-affinity transporter) is capable of transporting large amounts of glucose molecules at a high glucose concentration (1/2 maximal transport of glucose is reached at 10-15 mmol/L)
where is GLUT2 found?
- Intestinal mucosal cells and facilitates release of dietary sugars into portal vein
- Hepatocytes: GLUT2 takes up glucose when there are high levels of dietary sugars. During fasting, glucose is released into the blood from hepatocytes with the concentration gradient
- Renal tubular cells for the re-uptake of glucose
- B-cells of pancreas: GLUT2 transports large amounts of glucose and is used to “measure” high blood glucose levels
summarize the transport rates for GLUT3, GLUT4 and GLUT2 related to fasting blood glucose levels
why is GLUT5 different?
- GLUT5 prefers the transport of fructose over glucose
- GLUT5 transports mainly dietary fructose. Dietary glucose only at very high level of glucose concentration
where is GLUT5 found?
- Found in the intestinal mucosal cells on the luminal side of the intestinal membrane
- GLUT5 is also found in seminal vesicles which release fructose into semen. The seminal vesicles use blood glucose to form fructose to supply the energy metabolism of sperm cells
describe problems that arise from GLUT1 deficiency and what it causes
- Deficiency of GLUT1 leads to microcephaly and epilepsy-like seizures in the first few months of life
- As individuals mature they develop the follwing:
- Ataxia
- Delayed psychomotor development
- Movement disorders
- Impaired speech
how does the Na/K/ATPase pump work?
By cleaving ONE ATP, 3 Na+ are pumped out and 2 K+ ions are pumped into the cell.
describe what SGLT1 is
- It is a symporter that transports Na ions and glucose or galactose into the intestinal mucosal cell
- The transport of glucose or galactose is performed against a gradient and the Na/K ATPase has to be active
what are ABC transporters?
- ATP Binding Cassette
- In eukaryotes, ABC transporters allow the active transport of molecules from the cytosol to the extracellular space:
- This process needs hydrolysis of ATP
- The transported molecules are often lipids or lipid-related compounds
- Example: liver releases bile salts, cholesterol and conjugated bilirubin into the bile ducts using ABC transporters
what is the CFTR?
- It is a gated channel with a pore for chloride ions. This pore can be opened or closed following a stimulus.
- Although a few molecules of ATP are cleaved the transport is considered PASSIVE TRANSPORT which leads to many chloride ions flowing through the open channel
describe the CFTR
- Cystic fibrosis transmembrane conductance regulator is a special ABC transporter which is more a chloride ion channel than a transport protein
- CFTR does NOT pump Cl ions–they flow with the gradient once channel is opened
name the epithelial cells where CFTR is present
- Reproductive ducts
- Intestinal lumen
- Pancreatic ducts
- Airway ducts
- Skin
describe the structure of CFTR
- CFTR consists of 2 transmembrane domains
- 2 groups of 6 alpha helices
- Each is connected to an ATP-binding site
- One regulatory domain of the channel is intracellular
what are symptoms of deficient CFTR?
- In the newborn the early sign are salty skin
- Later in life
- Recurrent lung infections
- Chronic pancreatitis
- Steatorrhea (abnormal amount of fat in feces)
describe the normal process of sweating
- In a typical epithelia cell, Cl ions are first accumulated inside the cell which generates a higher Cl ion concentration than in the extracellular space
- These negatively charged Cl ions are then released with the gradient into the extracellular lumen via CFTR, where sodium ions and water follow
- In the sweat gland coil, large amounts of Na and Cl ions are released along with water
- The sweat formed in the coiled section of the sweat gland has a higher concentration of NaCl than the sweat actually released
- In the secretory duct, the Cl ions flow back into epithelial cells via CFTR.
- This allows the salvage and re-uptake of Cl ions by the epithelial cells using passive transport (with graident)
