As electrons flow from one photosystem to another, the amount of energy present in the electrons decreases. how does the plant cell use the energy lost by the electrons?

The plant cell use the energy to maintain and establish a proton gradient.

The electrons flow through the elecron transport chain in the inner mitochondrial membrane producing a build-up of proton in the inner membrane space. This proton gradient is the direct source of energy for generating ATP.

it would be c. xylem.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products