Differences between Active transport, Diffusion and Osmosis
How does it occur
Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed.
Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient. 
- uptake of glucose by epithelial cells in the villi of the small intestine
- uptake of ions from soil water by root hair cells in plants 
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