Re: Why do plants use chlorophyll?

Well, I think you win the debate.  Your surmise that the earliest 
autotrophs were marine aquatic organisms is absolutely correct, so 
chlorophyll makes use of wavelengths of light that penetrate water.  And 
why do land plants have such a poorly designed light capturing molecule?  
Simply, they have common ancestry with aquatic organisms, an hypothesis 
that makes sense out of many different observations.  Herein is a basic 
evolutionary lesson.  Life makes do with whatever works best at the time; 
organisms, and their parts, like chlorophyll, are not perfectly designed 
for their functions.

However, at least one part of your friends argument is correct as well.  
Chlorophyll is very similar in structure to porphyrin rings, a highly 
conserved molecular structure found in electron carrying pigments, the 
cytochromes, part of all electron transport chains.  If cytochromes were 
employed by ancestral, fermenting organisms to pump hydrogen ions out of 
the cell, chlorophyll probably arose as a derivative of cytochromes 
converted from an ion pump to photosynthesis.

Chlorophyll is actually only found in two places: embedded in the inner 
folded cell membranes of photosynthetic bacteria like the Cyanobacteria 
(blue-green algae) and embedded in the inner membranes of chloroplasts, 
which as it turns out is exactly the same place.  Many separate lines of 
evidence support the hypothesis that chloroplasts arose from free-living 
autotrophic bacteria via an endosymbiotic interaction between a host cell 
and what was originally probably a Cyanobacterial prey.  The result 
contributed to the evolution of eukaryotes, but was probably one of the 
later developments, relatively speaking.  


David Hershey adds the following:

One of the most common misconceptions about photosynthesis is that leaves 
reflect all the green light and do not use green light in photosynthesis. The 
truth is that leaves typically absorb half or more of the green wavelengths, and 
green light is used fairly efficiently in photosynthesis. Most leaves do reflect
more green light than other colors so leaves appear green to our eyes. 

The misconception about green light use in photosynthesis arises because of the 
chlorophyll absorption spectrum printed in many botany and biology textbooks. 
The chlorophyll absorption spectrum is made using a spectrophotometer and 
chlorophyll extracted into a test tube of an organic solvent such as acetone. 
The chlorophyll solution does absorb relatively little green light compared to 
red and blue, however, chlorophyll solution in a test tube behaves differently 
than chlorophyll in a leaf. In plants, chlorophyll occurs in highly structured 
chloroplasts, which are in complex plant cells. In a test tube, the light passes 
right through the chlorophyll solution so the chlorophyll has one chance to 
absorb a green light particle or photon. In leaves, a chlorophyll molecule in a 
highly structured chloroplast has many chances to absorb a green photon because 
the unabsorbed photon can be reflected repeatedly from chloroplast to 
chloroplast many times increasing its chances of being absorbed. There are also
accessary pigments, the carotenoids, which can absorb green photons and pass the 
energy to chlorophyll. 

Reference

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.