A-LEVEL BIOLOGY AQA NOTES
principles of homeostasis and negative feedback
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- The neuronal system uses neurones to carry signals very rapidly through the body to produce short-term responses
- The hormonal system uses blood to carry hormones from endocrine glands to target cell with the specific receptors. This usually produces long-term responses.
- Peptide hormones are made of amino acids and must bind to receptors on the cell surface, activating second messengers which control transcription.
- Steroid hormones are formed from lipids and soluble in the plasma membrane, therefore entering cells and binding to proteins to enter the nucleus and have an effect on the DNA.
- Homeostasis is the maintenance of a constant internal environment despite internal or external changes.
- Temperature & pH are important to regulate to allow optimum enzyme activity and rate of metabolic reactions.
- Water potential is important to regulate to prevent cells bursting or shrinking.
- Glucose concentration is important to regulate to allow cells to have access to the substrate for respiration, whilst preventing cell damage by dehydration caused by high concentrations.
- Negative feedback is the body’s mechanism for reversing a change so that it returns back to the optimum. The stages involve
- Positive feedback is a deviation from the optimum which causes changes resulting in an even greater deviation from the norm. This is usually harmful due to the large, unstable change in the body.