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Cell Membrane (cont.)

©2001 Timothy Paustian, University of Wisconsin-Madison

Functions (cont.)

Transport

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

Passive diffusion

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.

Facilitated Diffusion

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.

Group translocation

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.

Active transport

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.

Above is a movie of some active transport mechanism. Three separate type of transport are shown. A symporter, moves a small molecule inside the cell during transport of the target molecule. An antiporter moves a small molecule outside the cell membrane during target molecule transport. A uniporter, binds and transports the target molecule only. Energy is required for these processes and the cell can accumulate molecules inside the cell using this mechanism.

Active transport proteins may be highly specific and carry only one molecule or may be more general and carry one class of molecules. The above animation shows several different types of transport molecules. An example of a general transport protein is the branch chain amino acid transporter of Pseudomonas aeruginosa, which transports leucine, valine, and isoleucine.

Table. A comparison of the four transport systems present in cells.

Energy generation

Many cells use respiratory processes to obtain their energy. During respiration, organic or inorganic compounds that contain high energy electrons are broken down, releasing those electrons to do work. These electrons find their way to the membrane where they are passed down a series of electron carriers. During this operation, protons are transported outside the cell. The outside of the membrane becomes positively charged; the inside becomes negatively charged.

Figure 1 - Generating the proton motive force in a membrane.

This proton gradient energizes the membrane, much like a battery is charged. The energy can then be used to do work directly, a process known as the proton motive force, or can be channeled into a special protein known as ATP synthase. ATP synthase can convert: ADP -->ATP, and the ATP can do work itself.

Photosynthetic cells also have a membrane system. Here light excites electrons and the electrons are again passed down through a series of electron carriers, a proton motive force is generated and ATP is synthesized. All the photosynthetic machinery is situated in the membrane.

These systems are discussed in the chapter on metabolism

Synthesis

Membranes also contain specialized enzymes which carry out many biosynthetic functions. These functions include:

1. Membrane synthesis
2. Cell wall assembly
3. Secretion of many proteins

Mesosomes and Infoldings of the Membrane

Mesosomes

Mesosomes are found in both G+ and G- organisms. Their function is not precisely known. Electron micrographs of mesosomes reveal a spherical, swiss cheese structure.

Figure 2 - An electron micrograph of a mesosome.

Mesosomes are often found near septa or dividing lines in bacteria and seem to be involved with segregation of newly replicated chromosomes.

Other infoldings

Although the width of the cell membrane is fixed, the area is not. Some bacteria have complex infoldings. These are attached to the cytoplasmic membrane but serve to increase its area. Below is a picture of the intercytoplasmic membrane of Rhodobacter sphaeroides

Figure 3 - The intercytoplasmic membrane of Rhodobacter sphaeriodes

These infoldings are found in photosynthetic and rapidly respiring bacteria. The infolding provides additional surface area and contact with the environment needed for high metabolic rates.

Membranes and Antibiotics

Several antibiotic are targeted at the membrane, including Polymyxin B and Gramicidin. Remember though that all living things have membranes and these antibiotics are dangerous to use internally because they affect host functions (including yours!). To get around this, these antibiotics are used topically or in very low doses. Neosporin is a common topical ointment that contains antibiotics.

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