After plants absorb energy from the sun, the energy gets converted into energy for the plant to carry out life activities.

The energy from the sun is known as solar energy. Solar energy is helpful to the plant, but not in that stage so what the plant does is to convert it to chemical energy so that the plant can get a better use of it.

Further info: primary consumers also convert the chemical energy obtained fron eating plants into another form called kinetic energy, which is also known as the energy of movement. But since plants don't move, they don't do the transformation.

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answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.