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Absorbtion in the small intestine - contradictions!!

Reply 20

Comp_Genius

4

you do need a glucose gradient, glucose goes down the gradient (as is sodium)

Reply 21

Revenged

15

darkenergy

you do need a glucose gradient, glucose goes down the gradient (as is sodium)

If you need a concentration gradient for glucose, why is all glucose absorbed?

Reply 22

Comp_Genius

4

if you are talking about the intestine - transport maximum of the transporter is not exceeded (i.e. concentration of glucose is not so high that all cannot be absorbed) - so the transporter has time to absorb all

Reply 23

Revenged

15

darkenergy

if you are talking about the intestine - transport maximum of the transporter is not exceeded (i.e. concentration of glucose is not so high that all cannot be absorbed) - so the transporter has time to absorb all

that doesn't mean there is a higher concentration in the intestinine than in the blood... there surely would be a lower concentration of glucose in the intestine than in the blood...

Reply 24

Comp_Genius

4

no tbetween the intestine and the blood - it's the difference in concentration across a membrane - in this case on either side of the luminal membrane.

Reply 25

Revenged

15

darkenergy

no tbetween the intestine and the blood - it's the difference in concentration across a membrane - in this case on either side of the luminal membrane.

"The sodium-glucose cotransport system establishes a sodium ion gradient based on the hydrolysis of ATP (see above). The transport protein binds both glucose and Na+; glucose moves up its concentration gradient as sodium moves down its."

http://dwb.unl.edu/Teacher/NSF/C10/C10Links/mills.edu/RESEARCH/FUTURES/JOHNB/structurefunction/725.html

There you go... glucose moves against the concentration gradient... thus secondary active transport is not facillitated diffusion....

Reply 26

Comp_Genius

4

sorry my mistake - you are right

Reply 27

j00ni

9

Revenged

glucose and galactose = sodium cotransport / secondary active transport

(I don't agree with j00ni for once - i don't think secondary active transport is another name for facilitated diffusion...)

fructose = facilitated diffusion

(an example of simple diffusion is steroids through a cell membrane... e.g. something fat soluble that doesn't need to travel through a channel protein)

Yeah, normally I wouldn't agree with myself either on that comment, but I (rightly or wrongly) assumed this was A level, so I didn't want to completely confuse matters and 2ndry active transport and fac diffn are the same thing in A level textbooks IIRC (I think at A level faccilitated diffusion is defined as diffusion requiring protein channels/transporters, but not ATP - i could be wrong though, in which case I have only served to further the confusion, oops :redface:

)

Absorbtion in the small intestine - contradictions!!

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