Tracy Staedter

Aug 28th 2019

Is Human Photosynthesis Possible?


Of all life on Earth, plants are remarkable for their ability to turn sunlight into food in a process called photosynthesis. The next time you look at a tree, appreciate its ability to thrive on sunlight. Human photosynthesis doesn’t exist; we must farm, slaughter, cook, chew and digest — efforts that require time and calories to accomplish.

As the human population grows, so does the demand for agricultural goods. Not only are our bodies expending energy, but so are the farm machines we use to make food. This leaves a considerable footprint. The United States’ agricultural industry is responsible for 10% of greenhouse gas emissions, according to the USDA.

But what if scientists could engineer human photosynthesis by changing our biology to evolve this plant-centric skill? If they could, just imagine how much time and energy people would save. We’d free up time for other pursuits. We’d eliminate food’s carbon footprint. We’d do away with starvation, malnutrition, and food-related allergies and illnesses.

Human evolution predictions are impossible to make. But perhaps using modern tools from biotechnology and genetic engineering, we could modify our cells to photosynthesize like plants. What would it take?

Some Animals Photosynthesize

Admittedly, human photosynthesis seems a bit far-fetched. But plants aren’t the only lifeforms that photosynthesize. There are a handful of examples of photosynthesis in the animal kingdom. For example, the pea aphid uses pigments to harvest sunlight and transfer it to their cells for energy production, according to Nature.

The Oriental hornet relies on a pigment in its exoskeleton, called xanthopterin, to turn sunlight into electrons. Recently, a team of students won a science competition with their idea to extract the pigment and use it as a form of renewable electricity to power a car’s battery.

When the emerald green sea slug, Elysia chlorotica, eats algae, it acquires the plant’s cellular components, called chloroplasts, that produce chlorophyll. Voilá, the slug is able to photosynthesize light. Experiments have shown that these slugs can go without eating for nine months, surviving only on the energy it gets from sunlight, reports National Geographic.

If other members of the animal kingdom can photosynthesize like plants, can humans?

Not Easy Being Green

Unfortunately, getting humans to photosynthesize sunlight is next to impossible. Unlike the tiny pea aphid, the hornet or the slug, humans demand an enormous amount of energy to survive. A full-grown human needs between 1,600 and 2,400 calories per day, according to the U.S. government’s Office of Disease Prevention and Health Promotion. The human digestion system breaks down food into glucose and our cells store the energy as a molecule called adenosine triphosphate (ATP).

Our body’s demand for glucose is higher than photosynthesis can accommodate. Associate professor Lindsay Turnbull of the University of Oxford determined that if the surface area of an adult woman contained chlorophyll like a leaf, it would produce only 1% of the daily energy requirements for the person to survive. To live by photosynthesis alone, the woman would need a green body with the surface the size of a tennis court.

There are other issues, as well. Let’s say we had green skin with cells with that contained chloroplasts. We would also need porous skin to take in carbon dioxide, a chemical in the air required during photosynthesis, says the American Chemical Society. Scientific research has not yet reached a level where genetic engineers can manipulate the human body enough to make all of these biological requirements possible.

Although human evolution predictions don’t include photosynthesis, people do have the capacity to solve other problems that threaten our survival, such as starvation, malnutrition and food borne illnesses.