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mammary gland, red blood cells

Amino acid synthesis and breakdown

liver

Urea synthesis

liver

Cholesterol and bile acid synthesis

liver

Steroid hormone synthesis

adrenal cortex, gonads

Figure 1-2

chemical concentration gradient. However, the energy supplied for this does not come directly from ATP; instead, energy comes from the movement of another molecule along its electrochemical gradient. For instance, glucose, amino acids, phosphate, and other molecules undergo secondary active transport from the renal tubular lumen into renal tubule cells against their concentration gradients (with the assistance of carrier molecules), using energy derived from simple diffusion of sodium into the cell along its electrochemical gradient. The situation is somewhat analogous to energy from falling water (sodium diffusion) turning a water wheel (glucose transport). In the case of glucose, this normally results in virtual removal of all glucose in the proximal renal tubule. This does not mean that all the glucose is always going to be reabsorbed in the kidney. If there is too great a concentration of glucose, the system is over whelmed (i.e. it has reached its transport maximum, or "Tm") and some may not be reabsorbed. Glucose, normally absent from the urine, commonly appears in the urine when its blood concentration reaches about 180 mg/dL.

Cotransport is secondary active transport in which the transported molecule (moving uphill against its gradient) and the molecule supplying the energy (moving downhill along its gradient) are moving in the same direction across the membrane. In countertransport, they move in opposite directions (Fig. 1-3).

5) Endocytosis. Endocytosis involves an invagination of the plasma membrane to encompass and take in the transported material. It is a way of transporting macromolecules, such as proteins. Endocytosis is actually a form of active transport as it uses ATP as an energy source.

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