human photosynthesis

Well... there is a sea slug that does it (one of the only known animals to obtain energy via direct photosynthesis if memory serves)
http://blog.mbl.edu/2015/02/03/sea-...o-photosynthesize-like-a-plant-study-reports/

How a brilliant-green sea slug manages to live for months at a time “feeding” on sunlight, like a plant, is clarified in a recent study published in The Biological Bulletin.

The authors present the first direct evidence that the emerald green sea slug’s chromosomes have some genes that come from the algae it eats.

These genes help sustain photosynthetic processes inside the slug that provide it with all the food it needs.

Importantly, this is one of the only known examples of functional gene transfer from one multicellular species to another, which is the goal of gene therapy to correct genetically based diseases in humans.

“Is a sea slug a good [biological model] for a human therapy? Probably not. But figuring out the mechanism of this naturally occurring gene transfer could be extremely instructive for future medical applications,” says study co-author Sidney K. Pierce, an emeritus professor at University of South Florida and at University of Maryland, College Park.
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That said... plants run at about 3 to 6% efficiency when performing photosynthesis:

https://en.wikipedia.org/wiki/Photosynthetic_efficiency

For actual sunlight, where only 45% of the light is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In actuality, however, plants do not absorb all incoming sunlight (due to reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels) and do not convert all harvested energy into biomass, which results in an overall photosynthetic efficiency of 3 to 6% of total solar radiation.
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By compare, a human is estimated to be around 20% efficient in terms of metabolic efficiency:
https://www.quora.com/How-efficient-is-the-human-body-at-converting-food-energy-into-work-energy

Studies on athletic motion suggest that the human body can turn about 20% of the food energy it absorbs into actual mechanical energy (like turning a generator or lifting objects). The rest goes into the normal energy of body processes, or is lost to inefficiency. The thing is, like any energy user, all of the energy that's lost ends up as waste heat. That means that, if you don't produce any other form of energy, then all the calories you burn end up as body heat.
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To take it further, and make a comparable figure, the human body needs about 80-100watts of energy just to keep itself alive (no physical activity, et al).
https://physics.stackexchange.com/questions/46788/how-efficient-is-the-human-body
The MET (Metabolic Equivalent Task) readout on your gym equipment is your body doing 1Kcal/kg/h = 4184 J/kg/h and can be reasonably accurately measured by how much oxygen a test victim uses.

Sitting still is roughly 1 met and cycling at 100 Watts is around 5.5 Mets.

So taking a man of 75kg, cycling at 100Watts (100J/s) he is having to do 5.5 * 4184 * 75 / 3600s = 480Watts so an efficency of 20%

Remember though that the person is spending 80-100Watts just staying alive doing nothing - unlike your car. There is an interesting experimental fit to how much energy you need to just stay alive, calculated about 100 years ago, the Harris-Benedict equation
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A plant, on the other hand, averages at the maximum roughly .7 Watts per square meter:
https://en.wikipedia.org/wiki/Photosynthetic_efficiency#Efficiencies_of_various_biofuel_crops

Popular choices for plant biofuels include: oil palm, soybean, castor oil, sunflower oil, safflower oil, corn ethanol, and sugar cane ethanol.

An analysis[original research?] of a proposed Hawaiian oil palm plantation claimed to yield 600 gallons of biodiesel per acre per year. That comes to 2835 watts per acre or 0.7 W/m2.[7][irrelevant citation] Typical insolation in Hawaii is around 5.5 kWh/(m2day) or 230 W/m2.[8] For this particular oil palm plantation, if it delivered the claimed 600 gallons of biodiesel per acre per year, would be converting 0.3% of the incident solar energy to chemical fuel. Total photosynthetic efficiency would include more than just the biodiesel oil, so this 0.3% number is something of a lower bound.

Contrast this with a typical photovoltaic installation,[9] which would produce an average of roughly 22 W/m2 (roughly 10% of the average insolation), throughout the year. Furthermore, the photovoltaic panels would produce electricity, which is a high-quality form of energy, whereas converting the biodiesel into mechanical energy entails the loss of a large portion of the energy. On the other hand, a liquid fuel is much more convenient for a vehicle than electricity, which has to be stored in heavy, expensive batteries.
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So, if we made a person photosynthesize at the .7 Watts / m^2 maximum across the average human surface area:
http://www.medicinenet.com/script/main/art.asp?articlekey=39851
Average body surface area for adult men: 1.9 m^2
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So, call it 2 square meters... that gives us 1.4 watts. That gives us roughly 98.6 watts to make up, just to stay alive.

Factor in clothing, shade cast by parts of the body casting shadow on other parts, places where the sun doesn't shine (unless you plan to stick a gro-lamp up your bum!), etc... and I think we can safely say that, even if we managed to up the efficiency of photosynthesis to current photovoltaic levels, which would give us about 44 watts, that a person simply could not produce enough energy via photosynthesis to survive... or, honestly, really make any difference in the amount of food they would have to eat to sustain themselves through even a mild jog.
 
ethernos said:
are there scientist who would want to perform human experiments realated to photosynthesis ?
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Sure. Several have won Nobel Prizes for it.

http://ar.iiarjournals.org/content/36/3/1345.long

The human body synthesizes several important chemicals using energy from sunlight, most notably vitamin D.

What it doesn't do is synthesize sugars (food) like glucose C6H12O6 and long-chain polysaccharides like cellulose (plant body structural material) from CO2, H2O and light energy the way green plants do. (Plants actually feed themselves from water and air! Humans think that CO2 sucks, but plants love it.)
 
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Yazata said:
Humans think that CO2 sucks, but plants love it.)
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No one thinks CO2 sucks (with the possible exception of the handful of people who have been killed by CO2 asphyxiation, that is.) It has some pretty important uses. Like almost everything, there are some problems with it in large quantities, though.
 
Kittamaru said:
Well... there is a sea slug that does it (one of the only known animals to obtain energy via direct photosynthesis if memory serves)
http://blog.mbl.edu/2015/02/03/sea-...o-photosynthesize-like-a-plant-study-reports/



That said... plants run at about 3 to 6% efficiency when performing photosynthesis:

https://en.wikipedia.org/wiki/Photosynthetic_efficiency



By compare, a human is estimated to be around 20% efficient in terms of metabolic efficiency:
https://www.quora.com/How-efficient-is-the-human-body-at-converting-food-energy-into-work-energy



To take it further, and make a comparable figure, the human body needs about 80-100watts of energy just to keep itself alive (no physical activity, et al).
https://physics.stackexchange.com/questions/46788/how-efficient-is-the-human-body


A plant, on the other hand, averages at the maximum roughly .7 Watts per square meter:
https://en.wikipedia.org/wiki/Photosynthetic_efficiency#Efficiencies_of_various_biofuel_crops



So, if we made a person photosynthesize at the .7 Watts / m^2 maximum across the average human surface area:
http://www.medicinenet.com/script/main/art.asp?articlekey=39851

So, call it 2 square meters... that gives us 1.4 watts. That gives us roughly 98.6 watts to make up, just to stay alive.

Factor in clothing, shade cast by parts of the body casting shadow on other parts, places where the sun doesn't shine (unless you plan to stick a gro-lamp up your bum!), etc... and I think we can safely say that, even if we managed to up the efficiency of photosynthesis to current photovoltaic levels, which would give us about 44 watts, that a person simply could not produce enough energy via photosynthesis to survive... or, honestly, really make any difference in the amount of food they would have to eat to sustain themselves through even a mild jog.
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You made an interesting analysis was it you or your dad. ?
 
billvon said:
No one thinks CO2 sucks (with the possible exception of the handful of people who have been killed by CO2 asphyxiation, that is.) It has some pretty important uses.
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Like regulating our breathing.

If CO2 in your system drops too low, your brain will tell your lungs to stop breathing.
 
DaveC426913 said:
Like regulating our breathing.

If CO2 in your system drops too low, your brain will tell your lungs to stop breathing.
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Which is why hyperventilating is so bad, if I'm not mistaken... And why the whole "breathing into a paper bag" thing came to be?
 
Kittamaru said:
Which is why hyperventilating is so bad, if I'm not mistaken... And why the whole "breathing into a paper bag" thing came to be?
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And is also why oxygen starvation is NOT obvious. It does not make you gasp for air. What makes you gasp for air is excess CO2 in the blood.

In the 1980s a shift supervisor at Shell Haven refinery died by putting his head into a column that was being purged with nitrogen. Very foolish thing to do - and there was a big inquest into safety procedures of course. He would have got no feeling of asphyxiation to warn him, because his lungs would have been able to get rid of CO2 as normal.

When my wife was anaemic (due to chemotherapy) she became ill on the flight home from holiday: nauseous, light-headed, sweating, incontinent. It was oxygen starvation, due to the reduced cabin pressure at high altitude. But no gasping for air at all, just feeing lousy.
 
Not sure if people are aware, but it's possible to become infected with a form of algae. I knew someone with green patches on the skin, doctors said it probably wasn't a problem unless they spread.
 
exchemist said:
And is also why oxygen starvation is NOT obvious. It does not make you gasp for air. What makes you gasp for air is excess CO2 in the blood.

In the 1980s a shift supervisor at Shell Haven refinery died by putting his head into a column that was being purged with nitrogen. Very foolish thing to do - and there was a big inquest into safety procedures of course. He would have got no feeling of asphyxiation to warn him, because his lungs would have been able to get rid of CO2 as normal.

When my wife was anaemic (due to chemotherapy) she became ill on the flight home from holiday: nauseous, light-headed, sweating, incontinent. It was oxygen starvation, due to the reduced cabin pressure at high altitude. But no gasping for air at all, just feeing lousy.
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hope she is well.....
 
ethernos said:
if we could use our hair for photosynthesis that be interesting ....
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Well, I mean, as the quick n dirty calculations I ran above point out... it would do nearly nothing in the face of our daily caloric burn.
 
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