Active transport is the moving of molecules across the membrane of the cell against the concentration gradient with the use of ATP.
Low to high concentration. Concentration gradient is the diffusion (movement of molecules from regions of low concentration) from high to low with the gradient. Active transport is from low to high, against the gradient.
The best answer is D.
Hormones are chemical messengers secreted mostly into blood and also extracellular fluid by specialized tissue of the endocrine system and affect the functioning of other tissues or cells.
Most hormones circulate in blood and come in contact with essentially all cells, but a particular hormone will only affect a limited number of cells, which are termed as target cells to that hormone.
A target cell responds to a particular hormone because it bears receptors for that hormone.
For example the red blood cells have receptors for the hormone called insulin which is produced by the pancreas. The red cells are able to take in glucose when their receptors bind to insulin.
