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During photosynthesis water molecules are stripped of hydrogen and oxygen forming molecular oxygen. My students want to know if a tiny atom like Hydrogen needs a special structure to move through thylakoid membrane, how does an oxygen molecule, a much larger structure, move through the thylakoid membrane?
Question Date: 2019-01-09
Answer 1:

That is an excellent question! The lipid bilayer of the cell membrane is selectively permeable to some small molecules, including carbon dioxide and oxygen. These molecules can cross the membrane passively via diffusion. Ions in solution, however, can’t pass through the membrane because they are hydrated. Although the bare radii of the ions are smaller than O2 and CO2, the charge of the ions causes water molecules to arrange around them in hydration shells. The arrangement of water molecules around an ion occurs because water is a dipolar molecule, with a positively charged end and a negatively charged end. Around a positively charged ion in solution, water arranges itself so that the negative end of the water molecule is close to the positive charge of the ion. This arrangement of bound water molecules around an ion is called solvation, and greatly increases the effective size of the ion.

For example, a water molecule has an approximate radius of 0.14 nanometers and O2 has an approximate radius of 0.18 nanometers (approximate because these are not spherical molecules). For comparison, a sodium ion, Na+, has a bare radius of only 0.095 nanometers, smaller than both water and O2. A bare hydrogen ion is even smaller, 1 femtometer, or 0.000001 nanometers. However, hydrogen ions in solution exist as hydronium ions (H3O+), which are thought to be solvated by between 2 and 4 water molecules, yielding a hydrated radius of 0.28 nanometers. Sodium is thought to be solvated by 3-5 water molecules, yielding a hydrated radius of 0.36 nanometers. More highly charged ions like are solvated by even more water molecules and are even larger. For example, Ca2+ is solvated by 5-7 water molecules and has a hydrated radius of 0.41 nanometers (much larger than its bare ionic radius of 0.099 nanometers). Therefore, the solvated sizes of the ions are greater than the sizes of water molecules and oxygen molecules, and so the ions cannot freely pass through the membrane.


Answer 2:

Oxygen gets passed through proteins in thylakoid membranes and released into the stroma.

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