Question

Photosynthesis in plants can be represented by the following overall reaction:

$\mathbf{6}{\mathbf{CO}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{6}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{l}\mathbf{\right)}\stackrel{\mathbf{Light}}{\mathbf{\to }}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{6}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{6}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

Algae grown in water containing some radioactive${}^{\mathbf{18}}\mathbf{O}$(in${{\mathbf{H}}_{\mathbf{2}}}^{\mathbf{18}}\mathbf{O}$) evolve oxygen gas with the same isotopic composition as the oxygen in the water. When algae growing in water containing only role="math" localid="1663892405978" ${}^{\mathbf{16}}\mathbf{O}$ were furnished with carbon dioxide containing${}^{\mathbf{18}}\mathbf{O}$, no ${}^{\mathbf{18}}\mathbf{O}$was found to be evolved from the oxygen gas produced. What conclusions about photosynthesis can be drawn from these experiments?

Short answer

The required oxygen gas comes from water and not from carbon dioxide.

Step-by-step solution

Photosynthesis.

Photosynthesis is the method of conversion of energy (light to chemical) through green plants and other organisms.

Light energy is collected and utilized by green plants during photosynthesis. Water, carbon dioxide, and minerals transform into oxygen and energy-rich organic molecules.

Conclusions from the experiment

The reaction of photosynthesis is:

$6{\mathrm{CO}}_2\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\text{Light}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{s}\right)+6{\mathrm{O}}_2\left(\mathrm{g}\right)$

When water contains radioactive oxygen-18, the oxygen gas will contain oxygen-18 after photosynthesis.

When carbon dioxide$\left({\mathrm{CO}}_2\right)$ contains radioactive oxygen-18, no oxygen gas will contain radioactive oxygen-18 after photosynthesis.

$6{\mathrm{C}}^{18}{\mathrm{O}}_2\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\text{Light}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{s}\right)+6{\mathrm{O}}_2\left(\mathrm{g}\right)$

As a result of this experiment, photosynthesis occurs because all oxygen gas comes from water and not from carbon dioxide.