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Gardening

Plants are the original solar collectors. They self-build their own solar panels (leaves). It doesn't cost anything to speak of, since the plant itself mines materials needed from soil and air to build these solar panels. Plants can be used to grow many different types of fruits, vegetables, and building materials, directly from sunshine (or indirectly from electric light bulbs).

Gardening also provides a natural means for recycling. Used bathwater can be re-used on trees to provide needed water and fertilizer. Compostable materials such as paper, cardboard, peels, nut/egg shells, leftovers, etc can be turned into fertilizer. One particular material, wood, provides extremely good soil texture, moisture retention capability, and fertility when chipped and composted:


Interestingly, there are at least three different versions of photosynthesis mechanisms that have evolved in nature so far: C3, C4, and CAM. 

C3 is the oldest -- about 85% of all plant species use C3. Under CO2 concentrations of 50 PPM and below, C3 plants become very inefficient at photosynthesis because the catalyst "rubisco" that fixes carbon fixes oxygen instead. Instead of building up carbohydrates, peroxide is produced, wasting water and energy. The CO2 concentration of the atmosphere as of 2004 was about 380 PPM, which should be sufficient. However, during hot and dry conditions, the stomata of the plant's leaves close to prevent excessive water loss. Fixation of carbon in the Calvin cycle continues on, but with the stomata closed, relative concentration of CO2 inside the leaf eventually drops below 50 PPM and photosynthetic productivity suffers. C3 photosynthesis is thus only about 1% to 2% efficient.

C4 is the newest evolution, with only 0.4% of the known plant species using C4 photosynthesis. C4 solves the problem of low CO2 concentration by a two-stage process that keeps CO2 concentration high and oxygen low in the chloroplast where the Calvin cycle operates. Active pumping of CO2 into the bundle sheath cell and blocking of oxygen produces an environment with 10 to 120 times as much CO2 concentration. The drawback to C4 photosynthesis is extra energy is used to pump the 4-carbon acids to the bundle sheath cell and the pumping of the 3-carbon compound back to the mesophyll cell for conversion to PEP. As a result of this energy expenditure, C3 plants have the potential to outperform C4 plants if conditions are cool and moist, or if atmospheric CO2 concentrations are doubled (~780 PPM) from today's levels (see this article). Corn, sugarcane, and millet are the most common food crops that using C4. Other C4 plants include certain members of grass, bamboo, and brassicas. Brassicas may include kale, broccoli, cabbage, kohlrabi. Many weeds use C4.. Sugarcane has the highest photosynthetic efficiency of around 7%. 

About 10% of plant species use CAM photosynthesis. CAM plants store up CO2 at night  when atmospheric concentrations are high and temperatures are low so water lost to opening their stomata is minimized. The stored CO2 is used to avoid opening their stomata during the day. This results in very high water efficiency. Most CAM plants are cactus. The only food crops using CAM are pineapples and prickly pears. 
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