AQA Specification:

Differences between unipolar depression, bipolar depression and sadness. The use of International Classification of Diseases in diagnosing unipolar depression: number and severity of symptoms including low mood, reduced energy levels, changes in sleep patterns and appetite levels, decrease in self-confidence. 

Biological explanation (influence of nature): imbalance of neurotransmitters, eg serotonin in the brain. Psychological explanation (influence of nurture): negative schemas and attributions. Use of antidepressant medications. Cognitive behaviour therapy (CBT). How these improve mental health, reductionist and holistic perspectives. Wiles’ study of the effectiveness of CBT.