I assume that definitions are a minor component in Biology. Based on this, I don't think it would be an issue to use textbook definitions. They also have the evident advantage of accuracy, so there's very little chance for error if you use it in the appropriate manner. Additionally, I don't think markers will be particularly interested in the source of the definition you provide, rather they will be interested in the quality of your response, including specific components relevant to the question and how well your response performs according to a specified marking criteria.