When a cell builds a new phospholipid molecule, that molecule can be either used to construct the plasma membrane for an organelle within the cell, or added to the existing plasma membrane surrounding the cell.

If the phospholipid molecule is added to the existing plasma membrane surrounding the cell, then the plasma membrane and the cell will both grow larger. This has certain ramifications within the cell model in Petri Dish.

Each phospholipid in a plasma membrane occupies a certain amount of two-dimensional space. This means that the area of a plasma membrane is proportional to the number of phospholipids it contains. Adding a phospholipid to the cell membrane increases the area of the membrane and the surface area of the cell. Increasing the surface of the cell means increasing the volume of the cell as well.

Increasing the surface area of the cell means that molecules will diffuse into and out of the cell at a faster rate. Increasing the volume of the cell will lower concentrations within the cell, which will slow down chemical reactions.

In Petri Dish, cells will have to grow to some minimum size before they can reproduce. Cells will also be limited in the number of organelles they can construct based on their size (organelles may have fixed volumes). Beyond that, there are no requirements for cell growth in terms of surface area or volume. We are not accounting for either molecule density or water in our model, which means that a player could cram a billion proteins into a cell without having to increase the size of that cell at all.