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Small hydrophobic molecules
O2, CO2, N2, and benzene
Small uncharged polar molecules
H2o, glycerol, and ethanol
Larger uncharged polar molecules
Amino acids, glucose, nucleosides
H, Na, HCO3, K, Ca, Cl, Mg
Simple diffusion is the net movement of substances from a region of high concentration to a region of low concentration so its overall net movement is along the concentration gradient, simple diffusion does not require energy therefore it is 'passive', substances are diffused across the membrane between the phospholipids.
Transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration by means of a carrier molecule. Since the substances move along the direction of their concentration gradients, energy is not required.
The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
movement of molecules down their electrochemical gradient, both channels and transporters can be passive
undergo conformational changes to transfer small water-soluble molecules across the lipid bilayer
form a hydrophilic pore in which ions/small molecules diffuse. Channels can be in either an opened or closed conformation.
3 main mechanisms of active transport
- ATP driven pump
- Light driven pump
- Coupled transport
- Are pores that allow ions to cross membranes
- They have selectivity filters (i.e. CO2-) which filters what ions may enter.
Test of rates of ion flow through a channel, recorded as electrical current
Types of gated ion channels
- Voltage gated-neurons
- Ligand gated (extracellular ligand)-neurotransmitters
- Ligand gated (intracellular ligand)
- Stress gated
- Mediates conduction of action potentials along neuron axis
- Channel opens when gate senses a change in voltage potential
- located on the sarcolema (cell membrane) of skeletal muscle cells
- when Acetylcholine binds to its receptor, the resulting influx of Na depolarizes the cell, initiating muscle contraction.
consists of six transmembrane-spanning domains (cylinders) joined by connecting loops, which form a pore. The pore allows water to pass across the biological membrane.
Example of Kidney function and AQP2
ADH binds to receptor on kidney cell surface, releases cAMP which turns on Protein kinase A. This guides vesicles with AQP2 to fuse with cell membrane to become H2O channel.
3 types of transporters
- all 3 can be active or passive depending on ATP consumed (directly or indirectly), and gradient of ions.
The Glucose-Na transporter
- This is a symporter, as glucose uses the help of Na to go against gradient.
- It is a secondary active transport, bc it uses ATP to create the Na electrochemical gradient.
not a GLUT, but conformational changes are the same as GLUT 1. Requires all three molecules to function though.
The GLUT family
- Glucose uniporters, mediate the transport of glucose into cells.
- GLUTs have a range of tissue distributions
- GLUTs have a range of substrate preferences (Some of the GLUT proteins transport glucose in preference to fructose while others preferentially transport fructose and some rarer sugars)
Requires insulin to bind to its receptor for vesicles with GLUT4 to bind to cell surface for glucose transport
Type 2 Diabetes
Insulin binds to receptor, but there is a defect in signaling to GLUT4 which prevents glucose uptake.
Transepithelial transport of nutrients across the intestine
- NA/K ATPase
ATP powered pumps
- All ATP pumps are transmembrane proteins
- All have ATP-binding sites on the cytosolic side
- They can only hydrolyze ATP when other ions/molecules are ptesent and simultaneously transported
- The energy stored in ATPs phosphoanhydride bond is used to move ions/molecules against an electrochemical gradient
4 Classes of ATP pumps
- V-class proton
- F-Class proton
- ABC superfamily
- Become Phosphorylated
- All have 2 identical catalytic alpha subunits
- At least one of the alpha subunits becomes phosphorylated during transport
- Most have 2 beta subunits (regulatory function)
- Ca++ATPase pumps
- Cell membrane type (plasma membrane of all Euk cells)
- Sarcoplasmic reticulum (muscle cells)type
- Na/K-ATPase Pumps (plasma membrane of all Euk cells)
- H+/K+ ATPase pumps (stomach)
How can Ca++ be released into the SR lumen against its gradient???
Soluble proteins in the SR lumen bind Ca++; this reduces the concentration of free Ca++ in the lumen, which therefore reduces the Ca++ concentration gradient and the energy needed to pump Ca++ into the SR lumen
Na/K ATPase pump
- 3 Na out, 2 K in, for every 1 ATP
- drugs can bind to ATP site to inhibit the pump
H/K ATPase pump
- 1:1 ratio, per 1 ATP
- acidify the stomach in parietal cells
V class pumps
- Do not become phosphorylated (but use ATP)
- Transport only H
- Contain two discreet domains, each with multiple subunits: cytosolic hydrophilic domain (V1); transmembrane domain (V0)
- Found in membranes of lysosomes, endosomes, and plant vacuoles
- Maintain ~ 100-fold proton gradient between lysosomal lumen (pH~4.5-5.0) and cytosol (pH ~7.0)
- Establish a net movement of electric charge, this prevents too many H from moving into the lumen.
- Producing a change in the electrical potential of a cell.
F class pump
- Very closely related to V-class pumps
- Transport only H+
- Contain two discreet domains, each with multiple subunits: cytosolic hydrophilic domain (F1); transmembrane domain (F0)
- F-class pumps are NOT phosphorylated during proton transport
- Works in the opposite direction: brings H in, and synthesizes ATP.
- Found in bacterial plasma membranes, inner mitochondria membrane, thylakoid membrane of chloroplasts
- ATP Binding Cassette
- Contain 2 transmembrane domains and 2 ATP-binding domains
- ATP hydrolysis without phosphorylation allows transport of specific molecules across the membrane
- Bacteria plasma membranes: a.a., sugars, peptide transporters (e.g. vitamin B12)
- Mammalian cells: phospholipids, cholesterol, lipophilic drugs, other small molecules
the Multidrug-resistance transport protein (MDR1)
an ABC class transporter that EXPORTS a large variety of drugs OUT cells; many cancer cells OVERPRODUCE MDRs and are resistant to chemotherapy!!
- Chain of a.a. usually located at one end of a protein, but can be in middle to create a patch. (signal peptide vs. signal patch)
- Directs protein to correct location within cell.
Ribosomes can be located in two places within cells
free-floating in the cytosol, or bound to the RER
Proteins fated to be localized in mitochondria
Are synthesized by cytosolic ribosomes, and then imported into the mitos via protein translocators that recognize the mito signaling sequence
A model for how a soluble protein hormone is translocated across the RER membrane
- When the newly-translated RER signal sequence emerges from the ribosome, it directs the ribosome to a translocator protein on the RER membrane that forms a pore in the membrane through which the polypeptide is translocated.
- The signal sequence is clipped off during translation by a signal peptidase, and the mature protein is released into the lumen of the RER immediately after being synthesized.
Hormone Protein prior to cleavage of signal sequence
Hormone with signal sequence removed.
The molecular chaperone BiP binds to polypeptide chains as they cross the ER membrane and facilitates protein folding and assembly within the ER.
Additional protein modifications that take place in the RER lumen
- 1)Glycosylation: Addition of temporary oligosaccharides (carbohydrate chains) that denote the protein is fated to be secreted out of the cell
- 2)Addition of disulfide bonds (not all proteins)
- 3)Folding by chaperones (especially long proteins)
- Aids in proper protein folding.
- Provides protection against proteases (e.g. lysosomal membrane proteins)
- Employed for signaling
- Assists in protein dimerization
- Most soluble and membrane-bound proteins made in the ER are glycoproteins, in contrast to cytsolic proteins that are not glycoproteins
Where do the glycosylated proteins obtain their carbohydrates?
- Antibodies are made up of two heavy and two light chains, which assemble in the ER.
- The chaperone BiP is thought to bind to all incompletely assembled antibody molecules and to cover up an exit signal, preventing them from leaving the ER
- Antibodies also undergo glycosylation in the ER
removes the oligosaccharides for protein to be cleared by proteasomes. Must become tagged with ubiquitin.
- An N-terminus hydrophobic signal/start transfer sequence attaches to a translocation channel
- When an internal hydrophobic stop transfer sequence enters the channel, it is discharged and anchored into the cell membrane
- The signal peptide is cleaved off, creating a single-pass transmembrane protein
Double pass transmembrane protein
Both the signal/start transfer sequence and the hydrophobic stop transfer sequence are internal (not on either end of the protein)
Proteins are imported into organelles by three mechanisms
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles
- Method of protein translocation from RER to golgi, and vesicles for secretion.
- Uses transport vesicles to get to golgi
- Then secretory vesicles to activate and export proteins out of cell.
Opposite of secretory
How do vesicles move throughout a cell?
- Vesicular trafficking: movement of proteins within membrane-bound vesicles inside of cells
- Transport vesicles collect cargo proteins in buds arising from the donor membrane, then deliver the cargo proteins by fusing with the target membrane of another organelle
- Vesicles do NOT diffuse randomly in the cytoplasm, but are actively transported by motor proteins along microtubule highways