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Summary Molecular cell biology: Answers to the previous asked exam questions
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- Katholieke Universiteit Leuven (KU Leuven)
This document contains the previous asked exam questions and their answers.
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Laura van den EndMolecular cell biology
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Chapter 7: Biomembrane structure
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1. Is the following statement true or false (and why): "Integral membrane proteins always
consist of alpha-helices across the membrane.”
• False, most integral membrane do, but porin consists of β sheets that form a barrel like
structure
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Chapter 11: Transmembrane transport of ions and small molecules
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2. FRAP ( uorescence recovery after photobleaching)
• Can be used to follow membrane proteins and to identify the low pH of for example the
lysosome, due to V-class pumps
• Label particles with pH-sensitive uorescent dye and modify the DNA so it encodes a
natural uorescent protein. The lysosome is then treated with a laser so it is bleached. The
emitted recovered uorescent can be measured as the proteins are targeted to the
lysosome
3. How does bone resorption occur? And how is its defect related to osteoporosis?
• Bone resorption occurs to heal bones or to extract the Ca2+ so it can be used elsewhere
• It is done in osteoclasts → they form an enclosed EXT space between themselves and the
bone, they secrete HCl in here to dissolve the bone into Ca2+ and phosphor
• HCl production is highly homologous with that in the stomach, though here a V class
pump is used instead of a P class pump to pump H+ out of the cell and into the enclosion
• Cl is brought in via a HCO3-/Cl- antiporter where HCO3- is leaving
the cell.
• Water dissociates in H+ and OH-, where H+ is transported via the
pump and OH- combines with CO2 (di uses in) to form HCO3- via
carbonic anhydrase (CA)
• When Cl- enter the cell, it leaves the cell and goes into the enclosed
space via a Cl- channel where it combines with H+ to form HCl
• In osteoporosis Cl- can’t enter the cell and no HCl can be formed, therefore no bone
resorption occurs and very dense bones are formed.
4. Researchers injected a concentrated solution of salt and sugar into the blood of mice.
Will the salt and sugar be transported into the epithelial intestinal and lumen?
• There is an Na+/K+ ATPase (in the basolateral membrane facing the
blood) that pumps Na out of the cell and K into the cell, this creates
the concentration gradient needed for the 2Na/glc symporter (in the
apical membrane facing the intestinal lumen) to import glucose into
the cell. Glucose will then leave the cell via a GLUT2 uniporter. Since
water follow Na, it will also go out of the cell (into blood) via the tight
junctions. This can cause diarrhea, can be treated with a solution of
high water and salt → the water goes back
• No, the direction of the transport is the other way.
Molecular cell biology —Examenvragen— 1/8
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, Laura van den End
5. Transport of CO2 in the blood
• RBC: transport CO2 in the form of HCO3-.
• At the tissues: water dissociates into OH- and H+. H+ is taken up by Hb
and OH- combines with CO2 to HCO3- via CA. The HCO3- is then
transported out of the cell in exchange of Cl- by AE1 = antiporter
• At the lungs: opposite happens. HCO3- is taken up in the exchange of a
Cl- and CA converts it back into OH- and CO2. OH- can form water again
with the H+ of Hb and CO2 di uses out of the cell
6. Na+/K+ ATPase uitleggen + andere ATPasen en voorbeeld
• De Na+/K+ ATPase is a proton pump that uses ATP hydrolysis to actively pump 3Na out
and 2K in, this is eg used in the epithelial cells of the intestine to create a concentration
gradient for the Na/glc symporter to import glucose into the cell (see Q4)
• There are 4 classes of proton pumps
• P class: generate and maintain PM electric potential in plants, fungi, and bacteria
• H+/K+ ATPase involved in the acidi cation of the stomach lumen (see Q10).
• Muscle relaxation: depends on Ca2+ ATPases that pump Ca2+ from the cytosol
into the SR: rise in cytosolic Ca → binding to calmodulin → activation ATPase →
low free Ca = relaxation
• V class: generate low pH of acidic compartments (e.g. lysosome) by pumping H+ from
cyt to exopl face against gradient P class
• Relatively few H are required for acidi cation of intracellular vesicles
• Can never acidify the lumen on their own: the more H+ in, the higher the (+)
charge on the other side of the membrane, (-) charge is left behind. They will
attract each other → electric potential will work against pump, which makes it
di cult to bring in more H+ and the process will stop before any lowering of pH
is done
• The H+ pump used in osteoclasts (see Q3)
• F class: reverse proton pumps: E released by energetically favored movement of
protons from exo to cyto face of membrane down H+ electrochemical gradient is used
to power E unfavorable synthesis of ATP
• Important in the ATP synthesis in mitochondria and chloroplasts
• ABC superfamily: rst identi ed as multidrug-resistance protein. Common substrates
are: toxins, drugs, PL, peptides, and proteins
• Cystic brosis: caused by a mutation in gene for CFTR (member of ABC → is
chloride channel, re-uptake of Cl- is inhibited).
7. How to investigate a new ion channel
• Ion channels can be investigated using patch clamping: the in- or outward movement of
ions across a patch of membrane is quanti ed from the amount of electric current needed
to maintain the membrane potential at a particular “clamped” value. This value is speci c
for di erent ions since they have a unique e ect on the membrane potential
• Frog oocytes do not produce ion channels, so injecting the proteins into the oocytes let
you investigate these without the presence of other channels in the membrane. The
oocytes are also large, this makes it easier.
Molecular cell biology —Examenvragen— 2/8
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