Biology
August 18, 2026 Lead Investigator: Simon Frye
What if humans could photosynthesize?
Could you sit on your windowsill and skip breakfast? Biochemistry isn’t the problem, but the surface area of humans may be a challenge.
Field explored: Biology, Chemistry, Mathematics
But before we explore
MAKE YOUR PREDICTION.
Will humans…
A.) Become green?
B.) Not survive?
C.) Explode?
D.) Do something else?
Our predictions
“Humans’ skin would turn green”
“Humans will not get enough energy to survive”
“They’ll need a whole lot of leaves”
THE RULES
1.) Human skin will serve as the leaves and will adopt the same properties of a typical houseplant’s leaves. (Around 2% efficient)
2.) The human body will be altered to run on photosynthesis as opposed to chemosynthesis.
3.) The change will happen instantly.
4.) The mechanism that could do this will be ignored.
WHAT WE NEED TO KNOW FIRST
Plant efficiency
Typical plants convert around 0.2-2% of sunlight into energy.
That’s about 3-4 Joules of energy per second, or about 3-4 watts. We’ll give our plant human the benefit of the doubt and have them produce 4 watts of energy. That’s about the same energy that a phone charger produces.
Mosteller body surface area formula
BSA = √(( Height (cm) x Weight (kg)) / 3600)
This formula will help us calculate the surface area required for our photosynthesizing human.
Formula for BMI (Body Mass Index)
BMI = Weight (Kilograms) / (Height (cm) / 100)²
This formula will also help us calculate the height and weight that our new human will have to be to live sustainably off of photosynthesis. We will assume they have an average BMI of 22.5
Calories to joules
1 calorie = 4184 Joules
We will assume our human needs about 2,200 calories a day, but since they wont need a digestive system anymore, we will subtract 200 calories from our total. This is because your body burns around 200 calories a day digesting food. This totals out to 8,368,000 Joules per day.
THE PROCESS
How much energy would our human get throughout the course of a day?
E = 4J x 60 × 60 × 12 × 2 = 345600J of energy
Things aren’t looking good for our human. That is around 24.2 times less energy than what we need to sustain this human. They would have to sit in direct sunlight for about 290 hours for just the amount of energy our human needs day to day.
How much more surface area do we need?
A = 2 × 24.2 = 48.4m²
Our human will need to grow incredibly larger for this to occur, let’s explore just how big that is:
First we need to find their weight with our BMI formula:
Weight = .00225 x Height²
Now that we have what weight is equal to, we will just have to plug it into our body surface area formula to solve for height.
And let’s do that:
.00225H³ = 8433216 — H = 1553cm = 15.53m
If we plug that back into our BMI equation, we get a weight of 5425kg. The problem with this is that our new human now requires significantly more energy to sustain. It is important to note that this calculation should only be treated as an illustration, as a human scaled to this extreme would not maintain ordinary human proportions or biology, which is what the BMI is scaled on. (It is a major extrapolation). So lets switch to leaves instead:
How much do our leaves weigh?
48.4 × 1lb = 48.4lbs
Not only would our leaves cover the size of a standard classroom, but they would also weigh 48.4 pounds when full of water. This adaptation is possible in theory, but pretty inconvenient for our human.
So…What happens?
Photosynthesizing humans are in fact possible, assuming all of the biological adaptations have been automatically applied. Our human would need to have leaves that cover a surface area of roughly 48.5 square meters, which is about the size of a standard school classroom. This mass of leaves would weigh down our human by an additional 48.5 pounds, likely more as they would be filled with water. This version of humans’ lives would also become much more inconvenient, as they would have to spend almost every waking moment basking in the sunlight, though some people may enjoy that. There are always grow lights as well.
Our photosynthesizing human is unable to function from leaf skin, though. The surface area of human skin is around 24 times too small to support a comfortable life for our human. Even if we tried to scale our human’s size up to make the 48.5 square meter size required, the energy it would then need far exceeds the energy that the enlarged leaf skin can provide. A human with 48.5 square meters of skin would be 15.5 meters tall, which is funnily enough the estimated length of the largest carnivorous dinosaur: The Giganotosaurus. Our leaf skin simply cannot keep up with any size we scale up or scale down our human to, so external leaves were the only option left.
As for the predictions, our humans would very likely not explode. There are a vast amount of chemical and biological changes that had to happen to our human to make this experiment work in the first place, and perhaps somewhere in this process our human exploded?
Our human’s skin could be much darker than the color green for optimal sunlight absorption, but then they would be a very great risk of overheating. The color green in leaves has been optimized by plants over millions of years of evolution in order for sunlight to penetrate the canopy of leaves while still absorbing some sunlight, ensuring sunlight reaches every layer. Our leaf skin humans would likely appear green if we assumed our human with leaf skin had multiple layers of photosynthetic tissue. But as we saw in the experiment, external leaves appeared to be the solution, so no green humans for us. With our external leaves, they would very likely appear green, especially with 48.5 square meters of leaf. This would not necessarily maximize total absorption, but it could help distribute light through several layers of leaves.
Humans might create powerful grow lights to continue photosynthesizing indoors or during cloudy weather. However, this would be extremely energy-intensive because electricity must first be converted into light and then, with relatively low efficiency, into chemical energy. Photosynthetic humans would therefore need to store carbohydrates produced during sunny periods, much as plants store sugars and starches, rather than depending upon uninterrupted sunlight.
What surprised us?
Although I figured that humans would not have enough surface area of skin to body their bodies in the first place, I did not expect for them to need as much surface area as they did to survive. I was even more surprised to learn that plants are surprisingly inefficient at capturing the Sun’s energy.
WEIRD QUESTIONS TO REAL RESEARCH
Scientists actually study…
Photosynthetic animals and photosymbiosis
Endosymbiosis and the evolution of chloroplasts
Increasing photosynthesis efficiency
Artificial photosynthesis
Want to keep exploring?
RELATED CONCEPTS
Photosymbiosis
Energy conversion efficiency
Autotrophs and heterotrophs
Endosymbiotic theory
SOURCES AND FURTHER READING
NASA: Evaluating Plants as Energy Stores
FAO: Human energy requirements
National Library of Medicine: Effect of secondary interventions on patency of vascular access sites for hemodialysis (for the surface area of humans).
Perinatology: Adult Body Mass Index (BMI) and Body Surface Area (BSA) Calculator
National Geographic: Photosynthesis
USDA: Plants versus Solar Cells
UC Berkeley: Green Light and Photosynthesis
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