system
Given the following post, respond as if you're commenting on Reddit:
human
Can we make sugar, in vitro, out of CO2 and water--with sunlight or any other kind of energy input--any more efficiently than plants can?
gpt
The short answer is: No, we cannot make sugar out of CO2 and Water more efficiently than photosynthetic organisms.

There are a couple things to think about this question though. First off, we can't do it more efficiently than plants, but turning CO2 into sugars is something that *could* in principle be done with our current understanding of organic chemistry. 

As you may be aware, CO2 is a very stable molecule and consequently is not very reactive. Because of this, getting reactions with carbon dioxide typically involves "high energy" reactants (e.g. alkali metals or alkaline earth metals). It takes a fair amount of energy to obtain these reactive metals - so much so that generating them to react with carbon dioxide is not an efficient means of using carbon dioxide. Essentially, all reactions of carbon dioxide will lead to products that are higher in energy at than the reactants you used (i.e. you need to put energy in to get the reaction to go forward).

One of the main challenges in turning carbon dioxide into more useful things is putting in as little energy as you need to get to your products. For example, lets say carbon dioxide to product B takes 400 kcal/mol energy imput on paper. When you try to do this reaction (and lets assume you can) it takes you 800 kcal/mol to do it (i.e. you have wasted 400 kcal/mol to get to your products).  One of the principal causes of this wasted energy is having to go through very unstable (high energy) intermediates. This is where catalyst can help - they can change the reaction mechanism so that you do not have to go through such high energy intermediates, allowing you to get to your products with much much less (in theory) wasted energy so you can get really close to that predicted 400 kcal/mol of our example reaction.  The catalysts in plants and other photosynthetic organisms is a protein called RuBisCO. This enzyme *specifcally* turns a 5-carbon sugar into two 3-carbon sugars by the addition of carbon dioxide. The organism then goes on to make sugars using other chemistry.

Another thing that is worth considering is why even bother making sugar from CO2? Sugar is a very complicated molecule compared to carbon dioxide and consequently would take a lot of effort to make. At this point, you have to ask what your goal is. If your goal is to turn carbon dioxide into a fuel, then it would be better to turn carbon dioxide into methanol or carbon monoxide (carbon monoxide can be turned into liquid hydrocarbons).  Essentially, each transformation we make to our carbon dioxide and subsequent products is going to require an input of energy (and time). So ideally (for the purposes of energy storage) you would want to make the fewest transformations possible.