Does Glucose Cross Fenestrations in the capillaries in to enter the tissue fluid ?

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lhh2003
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Does it do this to enter the tissue fluid from the blood ,or does it pass through the small gaps between the cells in the capillaries ?
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RM_Asclepius
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(Original post by lhh2003)
Does it do this to enter the tissue fluid from the blood ,or does it pass through the small gaps between the cells in the capillaries ?
Fenestrations and the small gaps between the capillary endothelial cells are the same thing. The glucose passes through these fenestrations as well as via the carrier proteins and channel proteins in the cell-surface membrane of the capillary endothelial cells.
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lhh2003
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(Original post by RM_Asclepius)
Fenestrations and the small gaps between the capillary endothelial cells are the same thing. The glucose passes through these fenestrations as well as via the carrier proteins and channel proteins in the cell-surface membrane of the capillary endothelial cells.
Thank you. Why is it that some carrier proteins can transport large molecules and some ions, but channel proteins can only transport ions. How do you determine how a molecule will be transported across the phospholipid bilayer ?
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Reality Check
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(Original post by lhh2003)
How do you determine how a molecule will be transported across the phospholipid bilayer ?
You can't always, but there's some general rules you can use to make an educated guess about how a substance will be transported across a membrane.

• Small molecules like CO2, O2 etc can just diffuse across the membrane without any special mechanism.

• Steroid hormones like the sex hormones and corticosteroids can obvious easily cross the lipid bilayer because it is energetically favourable for them to insert themselves into the lipid bilayer rather than the ECF or ICF (the question there is how does the steroid hormone leave the membrane to the ICF...).

•Generally, the larger and more polar the molecule, the more likely it will need some sort of facilitated diffusion or active transport - examples here would be the obvious one of the SGLT for glucose, but many more exist. There are also shuttles for large charged molecules which need to cross membranes, such as the carnitine shuttle to move larger fatty acids across the innner mitochondrial membrane.

So, in very general terms, the larger and more charged the molecule in question, the greater the likelihood of its needing some sort of active transport or facilitated diffusion to move it across the membrane. The smaller/more hydrophobic the molecule, the more the chance it can move by simple diffusion.
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lhh2003
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(Original post by Reality Check)
You can't always, but there's some general rules you can use to make an educated guess about how a substance will be transported across a membrane.

• Small molecules like CO2, O2 etc can just diffuse across the membrane without any special mechanism.

• Steroid hormones like the sex hormones and corticosteroids can obvious easily cross the lipid bilayer because it is energetically favourable for them to insert themselves into the lipid bilayer rather than the ECF or ICF (the question there is how does the steroid hormone leave the membrane to the ICF...).

•Generally, the larger and more polar the molecule, the more likely it will need some sort of facilitated diffusion or active transport - examples here would be the obvious one of the SGLT for glucose, but many more exist. There are also shuttles for large charged molecules which need to cross membranes, such as the carnitine shuttle to move larger fatty acids across the innner mitochondrial membrane.

So, in very general terms, the larger and more charged the molecule in question, the greater the likelihood of its needing some sort of active transport or facilitated diffusion to move it across the membrane. The smaller/more hydrophobic the molecule, the more the chance it can move by simple diffusion.
Thanks for this.
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