Lecture 9

Question 1

endomembrane system

Answer 1

Question 2

Cell membrane

Answer 2

Membranes prevent homogenization of the cell.

Question 3

The Ca2+ concentration in the ER is
103times higher than in the cytosol

Answer 3

[ Ca ] in ER = 100 uM
[ Ca ] in cytoplasm = 100nm.

Rule of thumb = delta-G = approx 2kbT per order of magnitude [ ] difference.
(the amount of free energy released when Ca moves out of ER into cytoplasm).

Question 4

Ca2+ membrane permeability

Answer 4

very very low.

Approximately how many calcium ions cross one
square micron (1 μm2) of ER membrane per second?

pCa2+ = ~10-9 μm/s
[Ca2+]ER = 100 μM
[Ca2+]cytoplasm = 100 nM
1 M = 10-15 mol/μm3
NA = 6x1023 molecules/mol
1 day = ~105 s

J = -p (Cin – Cout)
J = ~ - 6x10-5 ions/μm2 s

Question 5

How long would it take for Ca2+to equilibrate
across the ER membrane?

Answer 5

Question 6

How long would it take for Ca2+to equilibrate
across the ER membrane?

Answer 6

Passive - channel, transporter;
Active - requiring energy, moving against gradient.

Question 7

Transport through the
nuclear pore

Answer 7

Nuclear localization sequences (NLS) mediate
transport through the nuclear pore.

Question 8

NLS mediate transport across the nuclear pore:
how do we know?

Answer 8

Correlation - Proteins that contain NLS should localize to the nucleus; proteins that do notcontain NLS should notlocalize to the
nucleus.

Necessity - Removal of an NLS should prevent nuclear localization.

Sufficiency - Addition of an NLS should induce nuclear localization.

Question 9

A specific NLS is

Answer 9

necessary for nuclear localization

Question 10

Directionality in transport thru nuclear pore

Answer 10

RanGTPase. If import receptors reach the nuclear side of the pore complex, RanGTP binds them, and if the recetpors arrive loaded with cargo molecules, the RanGTP binding causes the receptors to release their cargo. RanGDP in the cytosol doesn’t bind to import or export receptors, so unloading only occurs on the nuclear side of the nuclear pore complex.

Question 11

Nuclear import receptor

Answer 11

Bind to nuclear pore and increase the local [ ] of those proteins, facilitating diffusion.

Question 12

ER signal sequences and a signal recognition
particle

Answer 12

direct ribosomes to the ER membrane

Question 13

ribosomes

Answer 13

Many ribosomes can
bind and translate a
single transcript