Why must all reactions be reversible according to thermodynamics? I am reading the book "Models of Calcium Signalling" by Dupont et al., and on pages 31-32 the statement is made with regard to some previously written chemical reaction equations

We have written each of these equations as unidirectional, even though we know from thermodynamics that every reaction must be reversible. However, in many biological reaction networks unidirectional reactions are a useful approximation, as in the physiological regime the reverse reactions are often very slow.

How does thermodynamics lead to all chemical reactions being reversible? Certainly the underlying dynamics of particles are described by time-reversible equations, but there exist irreversible processes in thermodynamics. I am sure I am overlooking something simple here.
 A: Processes that are irreversible thermodynamically are reversible in detail, but (by definition) not in a bulk statistical sense.
The underlying processes are reversible in the sense that if you had the Magic Molecular Tweezers of Fate then you could put things back together -- it's just that doing so would decrease entropy, and global decreases of entropy are statistically unfavorable to the point that saying they're just impossible is a fair way to put it.
A: Edit: To avoid confusion, the authors aren’t referring to thermodynamic reversibility (i.e., zero entropy generation). This never occurs in real life. They’re referring to whether a strongly exothermic reaction can be run in reverse.
Broadly, Nature prefers both strong bonding (low enthalpy $H$) and many possibilities (high entropy $S$). (This is why we see spontaneous minimization of the Gibbs free energy $G = H-TS$.)
Consider the chemical reaction
$$A+B\to AB,$$
where AB has an extraordinarily low enthalpy. I mean, really great bonding there. Some would describe this reaction as occurring completely—as being irreversible. If you have A and B around, putting aside any kinetic limitations, it's inevitable that they're going to permanently form AB, they might say.
But this isn't truly the case. In a universe with AB only, regardless of how strongly it's bonded, the entropy increase from a single dissociation of an AB molecule into A and B would increase total entropy enormously, as the A and B components could uniquely occupy any location in the cosmos. This entropy increase provides a sufficient driving force for some degree of dissociation (or reaction reversal).
The dissociation tendency increases with increasing temperature; on the molecular level, you could interpret this as more surrounding kinetic energy tending to break the AB bond by chance, or on the system level, you could interpret this as temperature mediating the strength of the $TS$ term above.
The upshot then is that all reactions are reversible, in the sense that we could reverse them by removing enough of the reactants. (This is one implication of Le Chatelier's principle.)
A: In thermodynamics equilibrium is understood where all the relaxation processes that are fast in the time scale of interest have already ended, whereas the slow processes are too slow to produce noticeable changes on the time scales of interest. Only in this case the arguments producing the laws of equilibrium thermodynamics and statistical physics apply. Thus, if we have a chemical reaction, say
$$
A+B\leftrightarrow C,
$$
we consider that each of the components, $A,B$ and $C$ are in thermodynamic equilibrium, and then calculate the rate of change of the concentration of these components. The change of the concentration, of course, means that the system is not really in equilibrium - the true equilibrium is achieved only when the amount of newly produced component $C$ equals to the amount of $C$ that disintegrates in the reverse reaction, i.e., when the concentrations do not change anymore. However, as should be clear from before, we can assume each component to be in thermodynamic equilibrium, provided that it equilibrates much faster than the concentration changes.
This approach typically works for the reactions used in biology. Note however that it is manifestly not the case in other situations - such as burning or explosions: these require description using non-equilibrium thermodynamics (=kinetic theory).
