Active Transport

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.

P class pumps will use the phosphorylating activity of ATP to change the conformation of the alpha subunit of the pump. This changes its conformation so that transport is possible. The sodium-potassium ATPase pump to be discussed soon is one of these pumps. Figure 29 shows what this type of pump looks like:

Figure 29.

Other pumps that are of the P class include the calcium-ATPase pump. This pump is seen in the sarcoplasmic reticulum inside muscle cells and in the plasma membrane of some animal cells. The pump uses ATP to send calcium out of the cytoplasm—either out of the cell or into the sarcoplasmic reticulum lumen of the muscle cell. The sarcoplasmic reticulum is the same as the endoplasmic reticulum in other cells but it pumps calcium as a major function.

With the calcium-ATPase pump, calcium is bound to the pump proteins while they are unphosphorylated. ATP binds and releases phosphate that binds to the protein, resulting in a large conformational change. This change allows for calcium to pass through the membrane so it can be released on the other side.

Another related pump is the hydrogen-potassium ATPase pump. This is seen in the stomach, in the distal renal tubules, and in the renal collecting ducts. It is directly involved in acid secretion by the stomach, exchanging potassium for hydrogen ions. The same thing happens in the kidneys.

V-class pumps will only pump protons across the membrane. These pumps do not involve phosphorylation and dephosphorylation. The V-class pump is seen in plant vacuoles and in certain lysosomal membranes in animals. It keeps the pH level low inside lysosomes and endosomes of animal cells, which is necessary for the activity of the enzymes within these organelles.

F-class pumps only pump hydrogen ions and does not involve phosphorylation. These are found in certain bacterial plasma membranes and in both chloroplasts and mitochondria. These pumps are also known as ATP synthases because they act in the synthesis of ATP. The pumps use the electron transport chain as an energy source to cause oxidative phosphorylation, which makes ATP.

ABC pumps are a family of pumps that are also referred to as the ATP-binding cassette. There are specific ABC pumps that work for different substrates or related substrates. There are more than a hundred of these transport proteins in all types of cells. These will transport amino acids, sugars, ions, and even drugs. The CFTR protein is a protein of this family that is defective in patients who have cystic fibrosis; this is a chloride channel pump.

Secondary active transport is the transportation of molecules across the cell membrane, making use of energy that is not from ATP. It is instead energy that comes from the pumping of ions out of the cell and the electrochemical gradient that comes from this process. This type of energy is referred to as symport or antiport transfer.

The mechanism of action of these types of transport processes happens through the primary active transport of sodium. Sodium is pumped out of the cell, creating a high extracellular sodium concentration versus the concentration on the inside. This gradient produces energy because sodium is constantly trying to diffuse back into the cell.

The antiport system involves two separate molecules getting transported in opposite directions. Sodium is allowed to flow from a high concentration to a low concentration, while the other species get transported against their concentration gradient. Figure 30 shows both the symporter and antiporter mechanisms across the membrane.

Examples of antiport systems include the sodium-calcium counter-transport system, which has sodium bound on the outside of the membrane and calcium bound on the inside of the membrane. After both our respectively bound, there is a conformational change that allows both to cross over. Sodium ends up on the inside and calcium ends up on the outside. All cell membranes have this function.

An antiporter system occurs with the transport of hydrogen and sodium in the proximal renal tubules. This is not as powerful as the primary active transport of hydrogen ions but participates greatly in hydrogen atom homeostasis because it can transport a great many hydrogen atoms at a time.

Symport mechanisms involve the co-transport of a molecule from high to low concentrations, while pulling another molecule along in the same direction from a low concentration to a higher concentration. An example of this is the co-transport of sodium and glucose. In this system, there are two binding sites outside the cell membrane—one for glucose and one for sodium. They bind and create a conformational change that allows both to enter the cell. The same thing happens with amino acid entry into the cell. These symporter mechanisms are particularly active in the intestinal tract and kidneys.