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Biology exam question help aqa a level Please help

Following digestion and absorption of food, the undigested remains are processed to form
faeces in the parts of the intestine below the ileum.
The faeces of people with constipation are dry and hard. Constipation can be treated by
drinking lactulose. Lactulose is soluble, but is not digested or absorbed in the human
intestine.
Use your knowledge of water potential to suggest why lactulose can be used to help people
suffering from constipation.


The answer is :
1. (Lactulose) lowers the water potential of faeces / intestine / contents of the intestine;
Accept Ψ for water potential
2. Water retained / enters (due to osmosis) and softens the faeces;



I got the second mark ,but i said it would increase the water potential.Cuz the faeces has a low water potential due to being hard, water enters boom water potential increases.But i guess i was wrong.Can someone explain why???
Original post by fa9089
Following digestion and absorption of food, the undigested remains are processed to form
faeces in the parts of the intestine below the ileum.
The faeces of people with constipation are dry and hard. Constipation can be treated by
drinking lactulose. Lactulose is soluble, but is not digested or absorbed in the human
intestine.
Use your knowledge of water potential to suggest why lactulose can be used to help people
suffering from constipation.
The answer is :
1. (Lactulose) lowers the water potential of faeces / intestine / contents of the intestine;
Accept Ψ for water potential
2. Water retained / enters (due to osmosis) and softens the faeces;
I got the second mark ,but i said it would increase the water potential.Cuz the faeces has a low water potential due to being hard, water enters boom water potential increases.But i guess i was wrong.Can someone explain why???
Osmosis - movement of water from a high water potential to a low water potential.
Highest water potential is 0 - pure water. Lower water potential than this are negative numbers, due to solutes.
The water enters the undigested food from the small intestine.
By lowering the water potential of the undigested food, there would be a greater water potential gradient, so more water would flow from the small intestine to the undigested food.

If lactulose increased the water potential, it would make the constipation worse, as less water would go to the undigested food, as the water potential gradient would be smaller.
(edited 1 month ago)

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