Theory of Intelligent Design, the best explanation of Origins

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Theory of Intelligent Design, the best explanation of Origins » Molecular biology of the cell » How the uptake of nutrients in the cell points to design

How the uptake of nutrients in the cell points to design

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How the uptake of nutrients in the cell points to design 1

To obtain energy and construct new cellular components, organisms must have a supply of raw materials or nutrients. Nutrients are substances used in biosynthesis and energy production and therefore are required for microbial growth. 2 Analysis of microbial cell composition shows that over 95% of cell dry weight is made up of a few major elements: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium,
and iron. These are called macroelements or macronutrients because they are required by microorganisms in relatively large amounts. The first six (C, O, H, N, S, and P) are components of carbohydrates, lipids, proteins, and nucleic acids. The remaining four macroelements exist in the cell as cations and play a variety of roles. For example, potassium (K) is required for activity by a number of enzymes, including some of those involved in protein synthesis. Calcium (Ca2), among other functions, contributes to the heat resistance of bacterial endospores. Magnesium (Mg2) serves as a cofactor for many enzymes, complexes with ATP, and stabilizes ribosomes and cell membranes. Iron (Fe2 and Fe3) is a part of cytochromes and a cofactor for enzymes and electron-carrying proteins. 

All organisms, including microorganisms, require several micronutrients or trace elements besides macroelements. The micronutrients—manganese, zinc, cobalt, molybdenum, nickel, and copper—are needed by most cells. However, cells require such small amounts that contaminants in water, glassware, and regular media components often are adequate for growth. Therefore it is very difficult to demonstrate a micronutrient requirement.
In nature, micronutrients are ubiquitous and probably do not usually limit growth. Micronutrients are normally a part of enzymes and cofactors, and they aid in the catalysis of reactions and maintenance of protein structure. For example, zinc (Zn2) is present at the active site of some enzymes but is also involved in the association of regulatory and catalytic subunits in E. coli aspartate carbamoyltransferase. Manganese (Mn2) aids many enzymes catalyzing the transfer of phosphate groups. Molybdenum (Mo2) is required for nitrogen fixation, and cobalt (Co2) is a component of vitamin B12.

Besides the common macroelements and trace elements, microorganisms may have particular requirements that reflect the special nature of their morphology or environment. Diatoms  need silicic acid (H4SiO4) to construct their beautiful cell walls of silica [(SiO2)n]. Although most bacteria do not require large amounts of sodium, many bacteria growing in saline lakes and oceans depend on the presence of high concentrations of sodium ion (Na). Finally, it must be emphasized that microorganisms require a balanced mixture of nutrients. If an essential nutrient is in short supply, microbial growth will be limited regardless of the concentrations of other nutrients.

Uptake of the required nutrients by the microbial cell is important. Since microorganisms live in nutrient poor habitats, they must be able to transport nutrients from dilute solutions into the cell against concentration gradient. Finally, they must pass through a selectively permeable plasma membrane. Microorganisms use different transport mechanisms like facilitated diffusion, active transport and group translocation. Eukaryotic microorganisms do not employ group translocation but take up nutrients by endocytosis.

Movement of materials across the plasmamembrane is mostly done by two processes:

Passive processes : Substances cross the area from an area of high concentration to an area of low concentration without any expenditure of energy (ATP). Example, simple diffusion, osmosis and facilitated diffusion.

Active process: The cell must use energy (ATP) to move substances from areas of low concentration to areas of high concentration. Example, Group translocation.

Passive processes:
Passive or simple diffusion :Often called diffusion, is the process in which molecules move from a region of higher concentration to one of lower concentration. The rate is dependent on the size of the concentration gradient between a cell's exterior and its interior. Very small molecules such as water and oxygen and carbon dioxide move across membranes by simple or passive diffusion. Larger molecules, ions, and polar substances do not cross membranes by this method.

Osmosis: Is the net movement of solvent molecules across a selectively permeable membrane from an area in which the solvent molecules are highly concentrated to an area of low concentration until equilibrium is reached. In living systems the chief solvent is water. The three types of solutions which are normally found are isotonic, hypotonic and hypertonic.

Facilitated diffusion: The rate of diffusion across selectively permeable membrane is greatly increased by using carrier proteins, sometimes called permeases which are embedded in the plasma membrane. Because a carrier aids the diffusion process, it is called as facilitated diffusion. Carrier proteins also resemble enzymes in their specificity for the substances to be transported; each carrier is selective and will transport only closely related solutes. Because there is no energy input, molecules will continue to enter only as long as their concentration is greater on the outside. Two widespread major intrinsic protein channels in bacteria are aquaporins that transport water and glycerol facilitators which aid glycerol diffusion. The carrier protein complex spans the membrane (Figure 1). After the solute molecule binds to the outside, the carrier may change conformation and release the molecule on the cell interior. The carrier would subsequently change back to its original shape and be ready to pick up another molecule. The mechanism is driven by concentration gradients and therefore is reversible.

Over the past decade, several liposome-based models for protocells have been developed. For example, liposome systems composed of polymerase enzymes encapsulated with their substrates have demonstrated that complex compartmentalized reactions can be carried out under conditions in which polymeric products are protected from degradation by hydrolytic enzymes present in the external medium. However, such systems do not have nutrient uptake mechanisms, which would be essential for primitive cells lacking the highly evolved nutrient transport processes present in all contemporary cells. In this report, we explore passive diffusion of solutes across lipid bilayers as one possible uptake mechanism. We have established conditions under which ionic substrates as large as ATP can permeate bilayers at rates capable of supplying an encapsulated template-dependent RNA polymerase. Furthermore, while allowing the permeation of monomer substrates such as ATP, bilayer vesicles selectively retained polymerization products as small as dimers and as large as a transfer RNA. These observations demonstrate that passive diffusion could be used by the earliest forms of cellular life for transport of important nutrients such as amino acids, phosphate, and phosphorylated organic solutes.

2. Prescotts microbiology, 5th edition, page 109

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