Consider three journeys of molecules inside one mesophyll cell of a leaf.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Consider three journeys of molecules inside one mesophyll cell of a leaf. Each molecule takes the shortest route and stays inside the cell. Every membrane on the way is one lipid bilayer, that is, two layers of phospholipid molecules.
- A molecule of carbon dioxide made in the matrix of a mitochondrion moves into the stroma of a chloroplast. How many membranes does it cross, and how many layers of phospholipid molecules is that?
- A molecule of oxygen released into the lumen of a thylakoid moves into the matrix of a mitochondrion. How many membranes does it cross, and how many layers of phospholipid molecules is that?
- A molecule of mRNA made in the nucleus leaves through a nuclear pore, a passage where the two membranes of the nuclear envelope are fused, and reaches a ribosome on the outer surface of the rough endoplasmic reticulum. Through how many lipid bilayers does it pass?
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Write the membranes that wrap each compartment, from the cytoplasm inwards. A molecule must leave through every membrane around its starting compartment and enter through every membrane around its end compartment.
(a) Matrix to stroma. Leaving the mitochondrion: the inner membrane, then the outer membrane. Entering the chloroplast: the outer membrane, then the inner membrane. That is membranes, and with two layers of phospholipid in each, layers.
(b) Thylakoid lumen to matrix. Leaving: the thylakoid membrane, the inner chloroplast membrane and the outer chloroplast membrane. Entering: the outer and the inner mitochondrial membranes. That is membranes, or layers.
(c) Nucleus to a ribosome on the RER. Nuclear pores are passages through which RNA and protein molecules move in both directions between the nucleus and the cytoplasm. A pore is formed where the two membranes of the envelope fuse, so going through it is not going through a bilayer. The ribosomes of the rough ER sit on its outer surface, facing the cytoplasm. So the mRNA passes through lipid bilayers.
Answer: (a) membranes, layers of phospholipid. (b) membranes, layers. (c) lipid bilayers.
Check. Count from the other end. In (a), the stroma is membranes in from the cytoplasm and the matrix is membranes in, so the route is . In (b), the lumen is membranes in and the matrix , so .
Watch out. The lumen of a thylakoid is not the stroma. It lies behind one more membrane, which adds one membrane and two layers to the journey.
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