Biology / A2 level
Homeostasis regulates internal conditions within functional ranges rather than keeping every variable perfectly constant.
Homeostasis regulates internal conditions within functional ranges rather than keeping every variable perfectly constant. Receptors and control pathways alter effectors so deviations are opposed through negative feedback.
The kidney filters small molecules from blood and selectively reabsorbs useful substances. ADH affects collecting-duct water permeability, while insulin and glucagon influence glucose handling. Ultrafiltration is not the same as complete waste selection; valuable molecules can enter the filtrate and then be recovered.
A hypothetical filtrate contains 100 units of glucose and all 100 are reabsorbed. How many reach the final urine in this model?
Subtract reabsorbed amount from filtered amount: 100−100=0 units, under the model’s stated conditions.
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
No exact PhET model is claimed for this topic. Use diagrams and supplied-data reasoning instead.
Opposes a deviation in a regulated variable
A response tends to reduce the original disturbance, although time delays and fluctuations remain.
0 units
Subtract reabsorbed amount from filtered amount: 100−100=0 units, under the model’s stated conditions.
No
Filtrate includes useful substances such as glucose, ions and water, many of which are selectively reabsorbed.