Red blood cells release oxygen, which passively diffuses through the membrane. Now we'll feature some more complex membrane dynamics by looking, in some detail, at the activities of an intestinal epithelial cell. Specifically, we'll look to see how this cell absorbs glucose from the lumen of the small intestine. It passes, unchanged, from mouth, through esophagus, through the stomach, into the small intestine.
Epithelial cells line the intestine.
I will draw pictures, here. The luminal and contraluminal membranes have very different functions. One must take up nutrients from the intestine, one must deliver them to the blood. There is a problem - there is usually much more glucose within the epithelial cell than in the lumen of the intestine. Glucose, therefore, will have to travel 'up' a gradient.