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Cell Membrane (cont.)
©2001 Timothy Paustian, University of Wisconsin-Madison
Many of the proteins in the membrane function to help carry out selective transport. These proteins typically span the whole membrane, making contact with the outside environment and the cytoplasm. They often require the expenditure of energy to help compounds move across the membrane.
There are four basic types of transport systems
- Passive Diffusion
- Facilitated Diffusion
- Group Translocation
- Active Transport
For these molecules, transport is directed by laws of simple diffusion. The membrane is not a barrier with the molecule being soluable in both the lipid membrane and the surrounding aqueous environment. These types of molecules are uncommon since very few will dissolve in both the membrane and water. There is no transport protein, it is nonspecific, and energy is not required. A concentration gradient of these molecules cannot be generated.
This involves a protein that binds the molecule to transport and is therefore specific. However, solutes are not concentrated against a gradient nor is energy required. It is not a widely used strategy in procaryotes as far as we know.
|At the left is picture the migration of solutes in and out of the cell as regulated by a protein. Notice that the concentration of solute does not become higher inside the cell.|
A protein specifically binds the target molecule and during transport a chemical modification takes place. No actual concentration of the transported substance takes place, since as it enters the cell, it is now chemically different. Most group translocation requires energy. Catbolic pathways, those that degrade substances to produce energy and carbon, somtime use group translocation. This is an efficient way to both bring substrate into the cell and begin the breakdown process.
|At the left is an animation of Group Translocation.. The molecule that is being transported is modified from glucose to glucose-6-phosphate. In this case the phosphate comes from phosphoenolpyruvate so energy is required.|
In active transport the target is not altered and a significant accumulation occurs in the cytoplasm with the inside concentration reaching many times its external concentration. Active transport proteins are molecular pumps that pump their substrates against a concentration gradient. As in all pumps, fuel is necessary and in the case of cells, this fuel comes in two forms, ATP or an the proton motive force (PMF). Both pmf and ATP are made by central metabolism and we will cover their formation by the cell later in the chapter on metabolism.