Nutritional needs

The four elements required by organisms in the largest quantity (gram amounts) are carbon, hydrogen, oxygen and nitrogen. This is because these are the elemental constituents of the key cellular components of carbohydrates (Fig. 1.3), lipids (Fig. 1.4), proteins (Fig. 1.5) and nucleic acids (Fig. 1.6). Phosphorus and sulphur are also important in this regard. Calcium, magnesium, potassium, sodium and iron are demanded at the milligram level, while microgram amounts of copper, cobalt, zinc, manganese, molybdenum, selenium and nickel are needed. Finally, organisms need a preformed supply of any material that is essential to their well-being, but that they cannot themselves synthesise, namely vitamins (Table 1.2). Micro-organisms differ greatly in their ability to make these complex molecules. In all instances, vitamins form a part of coenzymes and prosthetic groups that are involved in the functioning of the enzymes catalysing the metabolism of the organism.

As the skeleton of all the major cellular molecules (other than water) comprises carbon atoms, there is a major demand for carbon.

Hydrogen and oxygen originate from substrates such as sugars, but of course also come from water.

The oxygen molecule, O2, is essential for organisms growing by aerobic respiration. Although fermentation is a term that has been most widely applied to an anaerobic process in which organisms do not use molecular oxygen in respiration, even those organisms that perform metabolism in this way generally do require a source of this element. To illustrate, a little oxygen is introduced into a brewer's fermentation so that the yeast can use it in reactions that are involved in the synthesis of the unsaturated fatty acids and sterols that

CH2OH

CH2OH

CH OH

CH2OH 6CH2OH

OH H

CH2OH

OH a-D-Glucose

OH H HO OH

OH a-D-Glucose

CH2OH

CH2OH

CH2OH

CH OH

CH2OH

Maltose

Maltose

CH2OH

HO CH2OH

Sucrose HO

CH2OH

OH O

CH2OH

Isomaltose

Lactose CH2OH

jHOH

Lactose CH2OH

Cellobiose

Cellobiose

Fig. 1.3 Carbohydrates. (a) Hexoses (sugars with six carbons), such as glucose, exist in linear and cyclic forms in equilibria (top). The numbering of the carbon atoms is indicated. In the cyclic form, if the OH at Cj is lowermost, the configuration is a. If the OH is uppermost, then the configuration is f. At Cj in the linear form is an aldehyde grouping, which is a reducing group. Adjacent monomeric sugars (monosaccharides, in this case glucose) can link (condense) by the elimination of water to form disaccharides. Thus, maltose comprises two glucose moieties linked between Cj and C4, with the OH contributed by the Cj of the first glucosyl residue being in the a configuration. Thus, the bond is a1 ^ 4. For isomaltose, the link is a1 ^ 6. For cellobiose, the link is f 1 ^ 4. Sucrose is a disaccharide in which glucose is linked f 1 ^ 4 to a different hexose sugar, fructose. Similarly, lactose is a disaccharide in which galactose (note the different conformation at its C4) is linked f 1 ^ 4 to glucose. (b) Successive condensation of sugar units yields oligosaccharides. This is a depiction of part of the amylopectin fraction of starch, which includes chains of a1 ^ 4 glucosyls linked by a1 ^ 6 bonds. The second illustration shows that there is only one glucosyl (marked by •) that retains a free C1 reducing group, all the others (o) being bound up in glycosidic linkages.

are essential for it to have healthy membranes. Aerobic metabolism, too, is necessary for the production of some of the foodstuffs mentioned in this book, for example, in the production of vinegar.

All growth media for micro-organisms must incorporate a source of nitrogen, typically at 1-2gL-1. Most cells are about 15% protein by weight, and nitrogen is a fundamental component of protein (and nucleic acids).

As well as being physically present in the growth medium, it is equally essential that the nutrient should be capable of entering into the cell. This transport is frequently the rate-limiting step. Few nutrients enter the cell by passive diffusion and those that do tend to be lipid-soluble. Passive diffusion is not an efficient strategy for a cell to employ as it is very concentration-dependent. The rate and extent of transfer depend on the relative concentrations of the substance inside and outside the cell. For this reason, facilitated transportation is a major mechanism for transporting materials (especially water-soluble ones) into the cell, with proteins known as permeases selectively and specifically catalysing the movement. These permeases are only synthesised as and

H2 H2 H2 H2

C Ho

Stearic acid Ci8:0

H3C\ /c\ /C\ /C\ /¡C=C9\ /C\ /C\ /C\ /P_OH C C C C 10 9 C C C C1

H2 H

Oleic acid Ci8:i

Linoleic acid Cis:2

ho-ch2 I 2

HO CH

HO CH2 Glycerol

H3C-(CH2) -c-o-ch2 h3c-(ch2)-c-o-ch2 h3c-(ch2)-c-o-ch2

HO CH

HO CH

Monoglyceride

Diglyceride

H3C-(CH2)Z- c-o-ch2 Triglyceride

Diglyceride

Ergosterol

Fig. 1.4 Lipids. Fatty acids comprise hydrophobic hydrocarbon chains varying in length, with a single polar carboxyl group at Ci. Three different fatty acids with 18 carbons (hence Cig) are shown. They are the 'saturated' fatty acid stearic acid (so-called because all of its carbon atoms are linked either to another carbon or to hydrogen with no double bonds) and the unsaturated fatty acids, oleic acid (one double bond, hence Cig:i) and linoleic acid (two double bonds, Cig:2). Fatty acids may be in the free form or attached through ester linkages to glycerol, as glycerides.

Ergosterol

Fig. 1.4 Lipids. Fatty acids comprise hydrophobic hydrocarbon chains varying in length, with a single polar carboxyl group at Ci. Three different fatty acids with 18 carbons (hence Cig) are shown. They are the 'saturated' fatty acid stearic acid (so-called because all of its carbon atoms are linked either to another carbon or to hydrogen with no double bonds) and the unsaturated fatty acids, oleic acid (one double bond, hence Cig:i) and linoleic acid (two double bonds, Cig:2). Fatty acids may be in the free form or attached through ester linkages to glycerol, as glycerides.

when the cell requires them. In some instances, energy is expended in driving a substance into the cell if a thermodynamic hurdle has to be overcome, for example, a higher concentration of the molecule inside than outside. This is known as 'active transport'.

An additional challenge is encountered with high molecular weight nutrients. Whereas some organisms, for example, the protozoa, can assimilate these materials by engulfing them (phagocytosis), micro-organisms secrete extracellular enzymes to hydrolyse the macromolecule outside the organism, with

Amino acid

L-Amino acid

Glycine

Alanine (Ala)

H3C CH3

Valine (Val)

CH2 I 2 CH

H3C CH3

Leucine (Leu)

Isoleucine Phenylalanine (Ile) (Phe)

CH2 OH

Threonine (Thr)

-Cysteine (Cys-SH)

Methionine (Met)

Tryptophan (Trp)

Tyrosine (Tyr)

Asparagine (Asn)

CH2 I 2

Glutamine (Gln)

CH2 I 2

CH2 I 2

Aspartic acid Glutamic acid Lysine Arginine Histidine

Proline (Pro)

-CHI

CH I

CH I

CH I

N C CH N C CH N C CH I II |H I II |H I II |H H O R2 H O R4 H O R6

Fig. 1.5 Proteins. (a) The monomeric components of proteins are the amino acids, of which there are 19 major ones and the imino acid proline. The amino acids have a common basic structure and differ in their R group. The amino groups in the molecules can exist in free (—NH2) and protonated (—NH+) forms depending on the pH. Similarly, the carboxyl groups can be in the protonated (—COOH) and non-protonated (—COO-) states. (b) Adjacent amino acids can link through the 'peptide' bond. Proteins contain many amino acids thus linked. Such long, high molecular weight molecules adopt complex three-dimensional forms through interactions between the amino acid R groups, such structures being important for the properties that different proteins display.

the resultant lower molecular weight products then being assimilated. These extracellular enzymes are nowadays produced commercially in fermentation processes that involve subsequent recovery of the spent growth medium containing the enzyme and various degrees of ensuing purification. A list of such enzymes and their current applications is given in Table 1.3.

The Miracle Of Vinegar

The Miracle Of Vinegar

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