Bacterial Plasma membrane:

Why do bacteria have plasma membrane apart from having cell wall?

Apart from having cell wall, Bacteria are also surrounded by several layers below it which have their own roles and plasma membrane is an important one among all. The presence of the plasma membrane is so special because it gives  protection and shape to the cytoplasm and without which the cytoplasmic constituents would have been spilled out into the environment. Despite the presence of the outer layers surrounding the plasma membrane, the interaction of the cell with its environment, uptake of nutrients and elimination of wastes from the cell is carried out in a selective manner by the plasma membrane. The selectively permeable nature makes them capable of allowing only few ions and molecules to enter the cell that are necessary in the growth and nourishment of the cell thereby preventing the entry of unwanted ones. There are also several other activities such as the photosynthesis, respiration and synthesis of lipids and other cell wall constituents which take place within the plasma membrane. The most widely accepted model suggesting the structure of the plasma membrane is the Fluid Mosaic model according to which the protein members of this membrane float within the lipid bi layer.

The Fluid Mosaic Model:

The plasma membrane is a double layer of lipids with some proteins floating within it. Singer and Nicholson suggested the Fluid Mosaic model representing the structure of plasma membrane which was trustworthy, with a few drawbacks and is accepted widely. The two lipid layers appear as thin lines when viewed by the transmission microscope and the lipids are of the type, phospholipids having two regions, namely the hydrophobic and the hydrophilic regions. Another term describing these phospholipids is "Amphiphatic" which means that one of their regions is polar or water loving and the other region is non-polar or water repellent. The polar regions form the head portion with the non-polar regions acting as the tails of these lipids and the arrangement of the lipids within the bi layer is in such a way that the polar heads lie outside the membrane and the non-polar tails buried inside, away from the external environment. The membranes of the Bacteria are similar to the eukaryotic membranes with few differences such as the absence of cholesterol which is replaced by another sterol like molecules called, Hopanoids. Since bacteria have to survive in extreme conditions where the temperatures may be very high or the opposite to that, the fatty acids are different in environments. Bacterial membranes have saturated fatty acids with single covalent bond between their carbon atoms of the hydrocarbon chain in case of high temperatures and at low temperatures, the fatty acids are unsaturated with one or more double covalent bonds between the carbon atoms.

Apart from lipids, there are also some  proteins lying within the plasma membrane which are classified into two types based on the ease with which they can  be removed out of the membrane. Peripheral proteins are loosely connected to the membrane which are mainly involved in the communication of the cell and can be removed easily from the surface. They get associated with the inner proteins and help in communicating with the external environment by the bacteria. They constitute about 20-30% of the total membrane proteins and can be removed easily from the membrane as they are loosely bound to it. The second type of membrane proteins are the intrinsic ones which carry out important functions such as the transport of materials into or out of the cell and some of them participate in the electron transport chain, which is an important metabolic process that generates energy for the survival of the cell. The intrinsic proteins are also amphiphatic like the membrane lipids which have both the hydrophobic ends buried deep inside and the hydrophilic ends which project outside the membrane.

Structurally plasma membrane is similar to the membranes which surround the cell organelles of the eukaryotes. According to the fluid mosaic model, the proteins are free to move laterally within the lipid bi-layer and the lipid bi-layer is not moving, but there are some micro-domains within the membrane enriched for certain lipids and also some integral proteins are confined to a particular area, which are the drawbacks of this model and research is still going on in this particular area.The role of plasma membrane is to help the bacteria in acquiring important nutrients which are required for the survival and the reproduction of these bacteria. Different methods are adopted in the uptake of nutrients by these bacteria which include, the passive diffusion, facilitated diffusion, primary and secondary active transport, group translocation. The nutrients required by the bacteria are classified into the following different types:

• Micro nutrients or trace elements: These are the substances which are required by the bacteria in low amounts such as the zinc, cobalt, manganese etc which are required for the catalytic activity of the enzymes and are obtained from nature by these bacteria.
• Macro-elements: Carbon, Hydrogen, Oxygen etc which form the bio molecules of the cells constitute a major portion in their body and are called as the macro elements. Some of the important macro-molecules include the calcium, magnesium etc which are required for the stabilization of the structures like the ribosomes etc.
• There are also some growth factors such as the purines, pyrimidines, amino acids etc which also need to be obtained from the environment if they are not being synthesized by them.

As is said above, different methods are adopted in nutrient uptake which are explained in brief as following:

• Passive diffusion: This is the simple type of diffusion where, the movement of molecules takes place from the region of higher concentration to the region of lower concentration.The gaseous molecules such as the oxygen, carbon dioxide etc takes place through this process which can enter the cell freely down the concentration gradient. Water molecules also enter the cell by this process as they are required for the cellular activities.
• facilitated diffusion: This type of diffusion is carried out by special type of membrane proteins called the channels or the carriers which transport the nutrients across the membrane and the process is driven by the concentration gradient which does not require any energy input. when the solute molecule binds to the carrier protein on the outside of the membrane, there is a change in the protein conformation which transports it into the cell's interior and come back to their original form. Even though this is not an energy driven mechanism, Bacteria cannot adopt this as the important nutrient uptake mechanism as they mostly live in nutrient deprived areas where the concentration of the nutrients is very low and they have to rely on other mechanisms to carry out this process.
• Primary and secondary active transport: The transport of nutrients to a region of higher concentration at the expense of metabolic energy is called as the active transport. The movement of nutrients occurs against the concentration gradient and active transport is of two types namely the primary active transport and the secondary active transport. The use of energy produced by the hydrolysis of ATP to drive nutrient uptake is called as the primary active transport which usually involves the carrier proteins. ATP-binding cassette transporters(ABC transporters). E-coli transports sugar molecules amino-acids by this mechanism. The secondary active transporters use the energy generated by the ion gradient with the transport of the nutrients into the cell. They are usually called co-transporters as they simultaneously transport the ion molecules with the nutrient molecule and when the molecules move in the same direction, it is called symport and when they move in opposite directions, it is called antiport. Usually the proton gradient generated during the process of electron transport is coupled with either the generation of another gradient such as the sodium gradient and this is used to drive nutrients inside. The lactose permease of the E-coli is an example of the secondary active transporter which transports lactose molecules into the cell coupling the transport of protons outside the cell.
• Group translocation: This is a process where the molecule is modified as it is transported into the cell. The best studied process is the PTS (Phosphoenolpyruvate sugar phosphotransferase system) system where the nutrient molecule(example: sugar) is phosphorylated by accepting phosphate from the phosphoenol pyruvate molecule(PEP). Many sugar molecules such as mannitol, glucose, fructose etc are transported by this process and PTS system is adopted by various bacteria such as the bacteria of the species salmonella, staphylococcus, clostridium.

References:
Joanne M. Willey, Linda M. Sherwood, Christopher J. Woolverton. Prescott's Microbiology Ninth Edition. United states of America: McGraw-Hill, 2014.Print.