Biochem Channels. Transporters, and Receptors

What part of a phospholipid is polar and which is nonpolar

• the head is polar

• the tail is nonpolar

name the percentages that phospholipids and proteins make up in general cell membranes

• phospholipids: 40%

• proteins: 60%

name the percentages that phospholipids and proteins make up in inner mitochondrial membranes

• phospholipids: 20-25%

• proteins: 75-80%

name the percentages that phospholipids and proteins make up in myelin membranes

• phospholipids: 75%

• proteins: 25%

name some of the proteins associated with membranes

• peripheral proteins

• integral proteins

• glycoproteins

• protein membrane channels

list 3 types of membrane transport

• simple diffusion

• facilitated diffusion 

• active transport

describe simple diffusion

• doesn’t require a channel protein, but can still use a channel protein

• high to low concentration

• doesn’t require ATP

• molecules move down the concentration gradient

describe facilitated diffusion

• requires a protein channel to help in diffusion

• high to low concentration

• doesn’t require ATP

describe active transport

• REQUIRES ATP

• low to high concentration

• doesn’t require a channel protein, but can use one

what molecules tend to diffuse faster in simple diffusion

• smaller particles

• nonpolar molecules

describe the electrical potential (Vm) at equilibrium and before equilibrium

• before equilibrium: Vm>0

• at equilibrium: Vm=0

describe primary active transport

• goes against the electrochemical gradient

• requires ATP

describe secondary active transport

• goes against the electrochemical gradient

• doesn’t require ATP

• driven by ions moving down its gradient 

describe ionophore-mediated ion transport

• molecules move down the electrochemical gradient 

describe carriers

• bind substrates with high stereospecificity

• transport at rates well below the limits of free diffusion

• saturable 

• passive transporters

• Ex: Glucose transporter (GLUT)

describe channels

• less stereospecificity than carriers

• rates approach the limit of unhindered diffusion

• not saturable

describe secondary active transporters

couples UPHILL transport of one substrate and DOWNHILL transport of another substrate

describe passive transporters

• facilitate diffusion down a concentration gradient

describe primary active transporters 

• uses energy provided directly by a chemical reaction

what are some aspects of glucose transport by GLUT1 into a cell

• high rates of diffusion down concentration gradients

• saturability

• specificity

where is GLUT4 when blood sugar and insulin levels are low

• GLUT4 is stored in membrane vesicles 

• non-functioning form

what does GLUT4 do

• transports glucose into myocytes

• regulated by insulin

list the steps that involve transport of glucose via GLUT4 and insulin

1. glucose transporters are stored inside the cell inside membrane vesicles 

2. when insulin reacts with its receptors on the plasma membrane, the vesicles that contain glucose move up to the membrane and fuse (this results in more glucose transporters in the membrane)

3. when insulin levels go down, glucose receptors are moved back into the cell in membrane vesicles via endocytosis)

4. smaller vesicles fuse with larger endosomes

5. pieces of the endosome with glucose transporters bud off to become small vesicles, ready for when insulin levels rise again

name 3 general classes of transport systems

• uniports

• symports (cotransporters)

• antiport (cotransporters)

what class of transporter is the Na K ATPase

• antiport

• co-transport 

how do cardiac glycosides like Digoxin work

1. cardiac glycosides inhibit the Na+ K+ ATPase pump by stabilizing it in a transition state

2. Na+ can’t be extruded, so intracellular Na+ levels increase 

how does NCX work

• NCX is responsible for pumping calcium ions out of the cell and sodium ions in the cell (3 Na+ in, 2 Ca2+ out)

1. raised intracellular sodium levels inhibit NCX, so Ca2+ ions can’t leave and build up in the cell

2. increased cytoplasmic Ca2+ concentrations result in increases calcium uptake into the sarcoplasmic reticulum via the SERCA2 transporter 

3. increased sarcoplasmic reticulum calcium stores allow for greater calcium release on stimulation

4. the myocyte can achieve faster and more powerful contraction by cross-bridge cycling

describe the co-transport of glucose within intestinal epithelial cells

1. glucose is co-transported with Na+ across the apical plasma 

2. glycose moves through the cell to the basal surface and moves into the blood via GLUT2 (a passive glucose uniporter)

3. the Na+ K+ ATPase keeps pumping Na+ outward to maintain the Na+ gradient that drives glucose uptake

what kind of transports is the Na-glucose transporter

• primary uniporter

what do ATP Binding Cassette (ABC) transporters do

• they are a superfamily of proteins

• they efflux (get rid of) xenobiotics such as caffeine from the small intestine and blood brain barrier