A-Level Biology OCR Notes

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Communication Systems

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

Homeostasis is the maintenance of a constant internal environment despite internal or external changes.

Receptors detect a change; coordinators transmit the information and effectors bring about a response
Negative feedback is the body’s mechanism for reversing a change so that it returns back to the optimum.
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.

Control of Temperature

Internal body temperature needs to be regulated to ensure reactions can happen as efficiently as possible without denaturation of proteins.
Ectotherms are organisms that rely on external sources of heat to regulate its body temperature
- If its too cold, they can move into the sun, move to a warm surface or expose more surface area to the sun
- If its too hot, they can, move into the shade, move underground or expose less surface area to the sun
Endotherms are organisms that rely on metabolic reactions to regulate its body temperature
- If its too cold, they can shiver, increase respiration rate, vasoconstrict and erect hairs or feathers
- If its too hot, they can sweat, pant, vasodilate.
Endotherms can be active in colder habitats however they need more food and use more energy than ectotherms.

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