So the short answer is that we don't 100% know but most physicists do not think so.
The reason that they do not think so comes down to two things: Ehrenfest’s theorem and decoherence.
Ehrenfest’s theorem is a bound on how weird quantum mechanics can be. It says that on average quantum mechanics is not weird: particular measurement outcomes get correlated in weird ways but the average picture looks always like classical mechanics would say it looks.
Decoherence says that quantum things start to average out as soon as they get entangled with some broader outside world. So for example a protein folding in water is constantly entangling with those water molecules which constantly entangle with each other, and so the interesting correlations cannot be measured on the protein itself anymore but we would have to involve all of the water molecules too.
Note that the actual physical size does not matter at all to QM: Quantum does not really mean “small” and we have created tests of QM spanning kilometers. It just requires “isolated” things, and small nanoscale systems and single atoms happen to be isolated from their surroundings more often than big things like baseballs flying through the many air atoms knocking them all out of the way.
When you combine those two together you get a result that once a system is immersed in constant interactions with an environment, quantum mechanics only has two sorts of effects:
- the system carves out a space inside of it which is isolated from the environment, and arbitrary quantum stuff happens in that space, or
- the system displays some big features of a bunch of little quantum "nudges" to the classical picture -- something doesn't happen in quite the way that you would have expected for example.
So for example the pigments that plants use to convert light into chemical energy only absorb certain wavelengths of light, and this is a little quantum nudge (quantum systems frequently have discrete energy transitions and preferentially absorb photons that have an energy between the two states), and there is a quantum "stickiness" that molecules have towards each other called the van der Waals interaction that is crucial for understanding lots of different chemistry.
Biological structures that would display deeply quantum features would therefore generally have to create a safe, non-interacting space for a quantum state to be preserved. This is why the slightly cooky among us like Penrose start from examples like cytoskeleton tubules: they are looking for quantum computation in cells and so they are very interested in the tiny little spaces that are walled off from the rest of the world. It is also why smart non-physicists like Searle are very careful to say something like “look I just want to import the bulk features of our quantum realm like nondeterminism but then explain things as classical physics+nondeterminism rather than getting super cooky for quantum mechanics,” he wants to use the bulk features that come from a lot of little nudges rather than make the appeal Penrose is making that somehow the brain is a quantum computer because its cells are quantum computers.
It's not that it's wrong to say that it's a quantum system: because undoubtedly it is, everything is! It's just that one might expect synapses for example to probably have a very good classical approximation with maybe a couple quantum nudges, because those synapses are coupled strongly with all of the warm, wet, noisy things around it.