Viability of animal plant? (Energy calculation)

Question

I'm trying to design a humanoid or anthropomorphic organism that would be classified as a plant. The main argument people mention is that it wouldn't be able to produce enough energy to survive. I've done some basic calculations but I want to check it with someone else.

The average amount of sunlight in a square meter is equal to $120W$ or $120$ x $24$ x $60$ x $60 = 10368000 Jm^{-2}$ in 24h.

To increase photosynthesis, they need more surface area. These creatures would be slightly larger than a person. Average area of an adult human male is $1.9m^2$. Slightly larger with tail and features, lets say it's area is $2.5m^2$. Since only about part of them, lets say 35%, can be in the sun at any given time, photosynthetic area is only $0.81^2$. This wouldn't be enough. Instead, if they had grass-like fur the area would increase exponentially. Average $30mm$ length, $2mm$ thick base, $200mm^2$ area per blade, $250,000 blades/m^{2}$, photosynthetic area x $50 = 40.5m^2$ x $120W = 4860W$. Photosynthetic efficiency is at least 2% so they would produce $97.2W$, the same as what the average person uses. But ps efficiency can get higher, if it was 5% they would produce $243W$.

Did I make a mistake in my calculations?

Note that if this is not a sufficient energy source, they are mobile and are capable of digestion, meaning that they could get energy from external starch and glucose.

(Edit: I recalculated some things, using the sunlight duration by WMO and lowering the amount of sun exposure to 35% not 50%.)

Answer 1

Not sure if this belongs in discussion... I'm attempting to answer the titular question about whether or not a animal-plant is viable at all, and I'm going to do it using napkin math. I'm not a biologist.

Some plants eat bugs or other kinds of meat; they just digest it very slowly, and they move very slowly. Venus flytraps, for example, spend a couple of weeks generating the necessary potential to snap shut. So if your plant creature is to move around quickly enough to be mechanically competitive with animals, it's going to need to digest its environment much more quickly.

So, I'm going to defer to XKCD for the ammount of energy a green cow would get in a day from sunlight (less than 2 million joules), and assume that the plant man is taking in about as much sunlight as a cow. This website: http://sustainability.blogs.brynmawr.edu/2012/07/31/understanding-energy-part-1/ says that a person might take in about 2000 calories per day (8.4 million joules). So since the plant-man is pretty big, we might say that it needs triple what a person needs. So, 25 million - 2 million = 23 million joules, or about 5500 calories.

The plant man will have to digest quite a bit of food per day, so he's going to need a strong solvent in his gut. Also, as I understand it, plant cells are adapted to store energy for long-term slow-release, rather than the way animals do it using sugars for quick release. However, if we can accept some limitations, a plant can nonetheless achieve locomotion as long as it is constantly digesting food at a steady rate; it would just have to pull simultaneously from a large quantity of slow-release energy stores to make its motions, and the motions would have to be done quite differently. So I'll defer again to my understanding of venus flytraps. Rather than a complex network of sinews which contract to produce motion, such as what we see in animals, the plant would have an array of lobes which can be snapped to either concave or convex. So the plant man can move slowly by snapping his lobes in small quantities, or quickly by snapping them all at once in a location. So, rather than having muscles which pull at a joint internally, it is the surface of the creature which deforms to produce motion, and so it would look quite a bit different externally (the joints would be arranged around these changes, so when it bends its arm, the arm would become a rounded concavity around the joint rather than display a bulging bicep).

So, the big limitation is that the motion is done by storing energy prior to snapping lobes, which means that there is a fixed limitation on how frequently a lobe can be snapped, directly dependent on the rate of energy release from the cells, and the number of cells available to draw from. If you pluck one of the leaves from your plant person, he won't die, but he'll move slower.

Well, that was fun speculation for me, but I hope it leans towards what you were looking for!