Microbial Nutrition and Cultivation Flashcards
(43 cards)
We introduced the major groups of macromolecules found in living cells. The raw materials from which these are synthesised are ultimately derived from the
organism’s environment in the form of nutrients. These can be conveniently
divided into those required in large quantities* (macronutrients) and those which are needed only in trace amounts (micronutrients or trace elements). Give examples.
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Note that Micronutrients are all metal ions, and frequently serve as cofactors for enzymes.
Carbon
Carbon is the central component of the biological macromolecule. Carbon incorporated into biosynthetic pathways may be derived from organic or inorganic sources; some organisms can derive it from CO2, while others require their carbon in ‘ready-made’, organic form.
Hydrogen
Hydrogen is also a key component of macromolecules, and participates in energy generation processes in most microorganisms. In autotrophs, hydrogen is required to reduce carbon dioxide in the synthesis of macromolecules.
Oxygen
Oxygen is of central importance to the respiration of many microorganisms, but in
its molecular form (O2), it can be toxic to some forms. These obtain the oxygen they need for the synthesis of macromolecules from water.
Nitrogen
Nitrogen is needed for the synthesis of proteins and nucleic acids, as well as for
important molecules such as ATP. Microorganisms range in their demands for nitrogen from those that are able to assimilate (‘fix’) gaseous nitrogen (N2) to those that require all 20 amino acids to be provided preformed. Between these two extremes come species that are able to assimilate nitrogen from an inorganic source such as nitrate, and those that utilise ammonium salts or urea as a nitrogen source.
Sulphur
Sulphur is required for the synthesis of proteins and vitamins, and in some types is
involved in cellular respiration and photosynthesis. It may be derived from sulphurcontaining amino acids (methionine, cysteine), sulphates and sulphides.
Phosphorus
Phosphorus is taken up as inorganic phosphate, and is incorporated in this form into nucleic acids and phospholipids, as well as other molecules such as ATP.
Metals such as copper, iron and magnesium
Are required as cofactors in enzyme reactions.
A cofactor is a nonprotein
component of an enzyme (often a
metal ion) essential for its normal functioning.
Growth factors
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Many microorganisms are unable to synthesise certain organic compounds necessary for growth and must therefore be provided with them in their growth medium. These are termed growth factors, of which three main groups can be identified: amino acids, purines and pyrimidines (required for nucleic acid synthesis) and vitamins. Vitamins are complex organic compounds required in very small amounts for the cell’s normal functioning. They are often either coenzymes or their precursors. Microorganisms vary greatly in their vitamin requirements. Many bacteria are completely self-sufficient, while protozoans, for example, generally need to be supplied with a wide range of these dietary supplements. A vitamin requirement may be absolute or partial; an organism
may be able, for example, to synthesise enough of a vitamin to survive, but grow more vigorously if an additional supply is made available to it.
Nutritional categories - introduction
Microorganisms can be categorised according to how they obtain their carbon and energy. As we have seen, carbon is the most abundant component of the microbial cell, and most microorganisms obtain their carbon in the form of organic molecules, derived directly or indirectly from other organisms. This mode of nutrition is the one that is familiar to us as humans (and all other animals); all the food we eat is derived as complex organic molecules from plants and other animals (and even some representatives of the microbial world such as mushrooms!).
Heterotrophs
A heterotroph must use one or more organic compounds as its source
of carbon. Include all the fungi and protozoans as well as most types of bacteria. Microorganisms as a group are able to incorporate the carbon from an incredibly wide range of organic
compounds into cellular material. In fact there is hardly any such compound occurring in nature that cannot be metabolised by some microorganism or other, explaining in part why microbial life is to be found thriving in the most unlikely habitats. Many synthetic materials can also serve as carbon sources for some microorganisms, which can have considerable economic significance.
Autotrophs
An autotroph can derive its carbon from carbon dioxide.
A significant number of bacteria and all of the algae do not, however, take up their carbon preformed as organic molecules in this way, but derive it instead from carbon dioxide. These organisms are called autotrophs, and again we can draw a parallel with higher organisms, where all members of the plant kingdom obtain
their carbon in a similar fashion.
We can also categorise microorganisms nutritionally by the way they derive the energy they require to carry out essential cellular reactions.
Autotrophs thus fall into two categories. Chemoautotrophs obtain their energy
as well as their carbon from inorganic sources; they do this by the oxidation of inorganic molecules such as sulphur or nitrite.
Photoautotrophs have photosynthetic
pigments enabling them to convert light energy into chemical energy.
Chemoautotrophs
A chemotroph obtains its energy from chemical compounds.
Phototrophs
A phototroph uses light as its
source of energy.
Chemoheterotrophs
The great majority of heterotrophs obtain energy as well as carbon from the
same organic source. Such organisms release energy by the chemical oxidation of organic nutrient molecules, and are therefore termed chemoheterotrophs.
Photoheterotrophs
Those few heterotrophs which do not follow this mode of nutrition include the green and purple non-sulphur bacteria. These are able to carry out photosynthesis and are known as photoheterotrophs.
Lithotrophs and Organotrophs
A lithotroph is an organism that uses inorganic molecules as a source of electrons.
An organotroph uses organic molecules for the same purpose.
Whether organisms are chemotrophs or phototrophs, they need a molecule to act as a source of electrons (reducing power) to drive their energygenerating
systems. Those able to use an inorganic electron donor such as H2O, H2S or ammonia are called lithotrophs, while those requiring an organic molecule to fulfil the role are organotrophs. Most (but
not all) microorganisms are either photolithotrophic autotrophs (algae, blue-greens) or chemo-organotrophic
heterotrophs (most bacteria). For the latter category, a single organic compound can often act as the provider of carbon, energy and reducing power. The substance used
by chemotrophs as an energy source may be organic (chemoorganotrophs) or inorganic (chemolithotrophs).
Having found a source of a given nutrient, a microorganism must:
- have some means of taking it up from the environment
- possess the appropriate enzyme systems to utilise it.
The plasma membrane represents a selective barrier, allowing into the cell only those substances it is able to utilise. This selectivity is due in large part to …
This selectivity is due in large part to the hydrophobic nature of the lipid bilayer.
A substance can be transported across the cell membrane in one of three ways, known as:
simple diffusion
facilitated diffusion
active transport
Simple diffusion
In simple diffusion, small molecules move across the membrane in response to a
concentration gradient (from high to low), until concentrations on either side of the
membrane are in equilibrium. The ability to do this depends on being small (H2O,
Na+, Cl−) or soluble in the lipid component of the membrane (non-polar gases such as
O2 and CO2).
Facilitated diffusion
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Larger polar molecules such as glucose and amino acids are unable to enter the cell
unless assisted by membrane-spanning transport proteins by the process of facilitated diffusion. Like enzymes, these proteins are specific for a single/small number of related solutes;
another parallel is that they too can become saturated by too much ‘substrate’. As with simple diffusion, there is no expenditure of cellular energy, and an inward concentration gradient is required. The transported substance tends to be metabolised rapidly once inside the cell, thus maintaining the concentration gradient from outside to inside.
Active transport
Diffusion is only an effective method of internalising substances when their concentrations are greater outside the cell than inside. Generally, however, microorganisms find themselves in very dilute environments; hence the concentration gradient runs in the other direction, and diffusion into the cell is not possible. Active transport enables the cell to overcome this unfavourable gradient. Here, regardless of the direction of the gradient, transport takes place in one direction only, into the cell. Energy, derived from hydrolysis of adenosine triphosphate is required to achieve this, and again specific transmembrane proteins are involved. They bind the solute molecule with high affinity outside of the cell, and then undergo a conformational change that causes them to be released into the interior.
Active transport in Procaryotic cells
Procaryotic cells can carry out a specialised form of active transport called group translocation, whereby the solute is chemically modified as it crosses the membrane, preventing its escape. A well-studied example of this is the phosphorylation of glucose in E. coli by the phosphotransferase system. Glucose
present in very low concentrations outside the cell can be concentrated within it by this
mechanism. Glucose is unable to pass back across the membrane in its phosphorylated
form (glucose-6-phosphate), however it can be utilised in metabolic pathways in this form.
