To begin this question, I will quote Molecular Biology of the Cell (page 38):
... Biological systems are, ..., full of feedback loops, and the behavior of even the simplest of systems with feedback is remarkably difficult to predict by intuition alone; small changes in parameters can cause radical changes in outcome. To go from a circuit diagram to a prediction of the behavior of the system, we need detailed quantitative information, and to draw deductions from that information we need mathematics and computers.
... You might think that, knowing how each protein influences each other protein, and how the expression of each gene is regulated by the products of others, we should soon be able to calculate how the cell as a whole will behave, just as an astronomer can calculate the orbits of the planets, or a chemical engineer can calculate the flows through a chemical plant. But any attempt to perform this feat for anything close to an entire living cell rapidly reveals the limits of our present knowledge. The information we have, plentiful as it is, is full of gaps and uncertainties. Moreover, it is largely qualitative rather than quantitative.
(Johnson, A. D., Roberts, K., Lewis, J., Morgan, D., Raff, M. C., Walter, P., Alberts, B. (2015). Molecular Biology of the Cell. United States: Garland Science, Taylor and Francis Group.)
Thus comes a fundamental question: what does it mean when cell biologists want to quantitatively describe a cell?
In my understanding, a cell is a complex system, and a quantitative description of it invokes treatment of the cell as a mechanistic, mathematically determinate system. For example, a projectile in the context of Newtonian physics is a product of such treatment. Physicists use a series of equations to represent motion of the projectile. They also plug in initial values such as velocities and positions to supplement the equations.
As an analogy, quantifying a cell might mean finding a series of mathematical expressions for every chemical reaction or every biochemical inside a cell, and using a set of initial conditions to calculate the overall state of the cell at its very beginning. In this way, cell biologists can theoretically calculate figures of interest for whatever chemical reaction at any point during the cell's life. Of course, such an effort would be astronomically large and complex, and not feasible at present.
However, if this is what quantification of a cell means, why do Bruce Alberts and other authors cite feedback loops as a hurdle that stands in the way of quantifying a cell? How do feedback loops affect quantification of a cell?