across the membrane. Simple diffusion happens with CO2, oxygen, and other gases that are nonpolar and small. Glucose, sodium, and chloride are passed through facilitated diffusion. Filtration involves the movement of water plus solute because of the differences in hydrostatic pressure and oncotic pressure. This happens in the kidneys. The hydrostatic pressure is generated by the heart, which raises the pressure in the capillaries of the kidneys. Depending on the size of the holes in the glomerulus of the kidneys (which are the filtration components of the kidneys), small substances, including albumin, are passed through the pores. It happens to a degree in the liver as well; however, the pores are larger in the liver.
ACTIVE TRANSPORT There are two types of active transport. Primary active transport makes use of ATP energy in order to allow for the transport of substances against their concentration gradient across a membrane. The ATP energy comes from the cytoplasm and will not be on the outside of the cell. The system requires a pump that has at least one binding site for ATP. There are four different classes of ATP-dependent ion pumps related to the cell membrane. These include the following: •
P-class pumps
•
F-class pumps
•
V-class pumps
•
ABC pumps
Of these, the F, P, and V-class pumps only transport ions, while the ABC pumps can transport small molecules. These each take a great deal of ATP energy so that about a quarter of the ATP produced by the cell is used in ion transport among kidney cells. In electrically active cells, like nerve cells, about two-thirds of the ATP energy is used to pump just sodium and potassium through the cell membrane.
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