Ions, vitamins & minerals Flashcards
How can molecules cross the epithelium to enter the bloodstream
Paracellular transport - through tight junctions and lateral intercellular spaces
Transcellular transport - through the epithelial cells
How can solutes cross membranes
Diffusion
Facilitated diffusion (channel carrier)
Primary active transport (linked directly to cellular metabolism)
Secondary active transport (derives energy from concentration gradient of another substance that is actively transported)
What percentage of water presented to the GI tract is absorbed
99%
How much water is absorbed in the small intestine and large intestine
Small intestine - 8L
Large intestine - 1.4L
How much water is ingested and where is it used
2L ingested per day 1.2L in saliva 2L in gastric secretions 0.7L in bile 1.2L in pancreas 2.4L in intestines
Where is vitamin B12 stored
Liver contains a large store of B12 (2-5mg)
What does impaired absorption of B12 cause
Impaired absorption of vitamin B12 retards maturation of RBCs - pernicious anaemia
Most B12 in food is bound to proteins
How does B12 survive in the stomach
Low pH and digestion of proteins in the stomach releases free B12, B12 is easily denatured by HCl. To avoid this it binds to R protein (haptocorrin) from the saliva and parietal cells.
How is digestion of B12 in the small intestine avoided
Intrinsic factor is secreted from parietal cells, but has a lower affinity than R protein so doesnt bind until R protein is digested in the duodenum. The B12-IF complex is resistant to digestion and binds to cubilin receptors and is taken up in the distal ileum.
What dose B12 do in the cell
The B12-IF complex is broken down, B12 binds to TCII and this complec crosses the basolateral membrane into the liver. TCII receptors allowe uptake of the complex and proteolysis breaks down the TCII inside the cell.
