The internal energy reserve in plants is starch while animals have glycogen as internal energy reserve.
Justification: Glycogen is much more highly branched structure than glucose. Glycogen has shorter strands as compared to starch so it can be metabolized very quickly to provide energy to the body. Plants have a much lower metabolic rate than animals and also they don't require sudden bursts of energy. So, plants store glucose in the form of starch as compared to animals where glycogen is the storage form of carbohydrates.