Physical limits on the flow of into and of the cell prevent most cells from being too large.

materials/out

Explanation:

Physical limits on the flow of materials into and out of the cell prevent most cells from being too large.

The limitation of the kind and amount of materials that flow into and out of the cell prevents the cell from being too large. The function of selectively allowing materials to enter and leave the cell is carried out by the cell membrane. While being permissible to some materials, it shuts some materials off and, thus, the size of the cell is maintained.

it provides an alternative energy pathway for the reaction to proceed.

a catalyst is a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction.

catalysts increase the rate of reaction without being used up. they do this by lowering the activation energy needed. with a catalyst, more collisions result in a reaction, so the rate of reaction increases. different reactions need different catalysts.

a catalyst works by providing an alternative reaction pathway to the reaction product. the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst.