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The Nutrient Requirements of Cells

Introduction

In order to keep running, all living things need an unceasing supply of energy and materials. One useful way to discover the nutritional needs of cells is to attempt to culture them in the laboratory. We shall examine the ingredients needed in the medium for culturing
Minimal Medium for E. coli
(grams/ liter)
Glucose 5 g
Na2HPO4 6 g
KH2PO4 3 g
NH4Cl 1 g
NaCl 0.5 g
MgSO4 0.12 g
CaCl2 0.01 g

As these lists show, mammalian cells are far more fussy than is E. coli. At first, this may seem perfectly reasonable: E. coli cells seem simple and mammalian cells seem complex.

But actually, little difference exists between them in the range and complexity of the macromolecules of which they are built. Both must make carbohydrates, proteins, DNA, and RNA from which they assemble cell membranes, ribosomes, and so on.

In a sense, E. coli appears to be more complex. It is able to manufacture molecules as complex as those of the mammalian cell, but using a far more limited range of starting materials.

E.coli is able

All these ingredients are dissolved in water and are taken up from this solution.

Ham's Medium

The list of ingredients needed to grow human cells in culture is far longer.

Ham's Tissue Culture Medium for Mammalian Cells
(amounts dissolved in 1 liter of triple distilled water)
L-Arginine 211 mg Biotin 0.024 mg
L-Histidine 21 mg Calcium pantothenate 0.7 mg
L-Lysine 29.3 mg Choline chloride 0.69 mg
L-Methionine 4.48 mg i-inositol 0.54 mg
L-Phenylalanine 4.96 mg Niacinamide 0.6 mg
L-Tryptophan 0.6 mg Pyridoxine hydrochloride 0.2 mg
L-Tyrosine 1.81 mg Riboflavin 0.37 mg
L-Alanine 8.91 mg Thymidine 0.7 mg
Glycine 7.51 mg Cyanocobalamin 1.3 mg
L-Serine 10.5 mg Sodium pyruvate 110 mg
L-Threonine 3.57 mg Lipoic acid 0.2 mg
L-Aspartic acid 13.3 mg CaCl2 44 mg
L-Glutamic acid 14.7 mg MgSO4.7H2O 153 mg
L-Asparagine 15 mg Glucose 1.1 g
L-Glutamine 146.2 mg NaCl 7.4 g
L-Isoleucine 2.6 mg KCl 285 mg
L-Leucine 13.1 mg Na2HPO4 290 mg
L-Proline 11.5 mg KH2PO4 83 mg
L-Valine 3.5 mg Phenol red 1.2 mg
L-Cysteine 31.5 mg FeSO4 0.83 mg
Thiamine hydrochloride 1 mg CuSO4.5H2O 0.0025 mg
Hypoxanthine 4 mg ZnSO4.7H2O 0.028 mg
Folic acid 1.3 mg NaHCO3 1.2 g
This list includes:

Even when all these ingredients have been mixed together, most mammalian cells still fail to grow unless some blood serum (e.g., from a human or a calf) is added. Just what metabolic need is met by this supplement is uncertain, but trace amounts of hormones in the serum are probably important.

Why does a mammalian cell require such a complex broth compared to E. coli? It is the price of multicellularity. A mammal is made up of hundreds of different cell types, each specialized to perform one or a few functions. All the many other functions of life — including the synthesis of many of the organic molecules it needs, it delegates to other cells. The extracellular fluid, derived from the blood, supplies it with these. Ham's tissue culture medium is an attempt to recreate this extracellular fluid.

Bristol's Medium

This third recipe is used to culture green algae.

Bristol's Medium for Algae
(mg/liter)
NaNO3 250 mg
K2HPO4 75 mg
KH2PO4 175 mg
CaCl2 25 mg
NaCl 25 mg
MgSO4.7H2O 75 mg
FeCl3 0.3 mg
MnSO4.4H2O 0.3 mg
ZnSO4.7H2O 0.2 mg
H3BO3 0.2mg
CuSO4.5H2O 0.06 mg

The most striking difference from the other two media is the item that is missing: glucose. In fact, no carbon-containing (organic) compound is included. This is because green algae, like all green plants, make the organic molecules they need by photosynthesis.

Green algae use the energy of sunlight to assemble inorganic precursors, chiefly carbon dioxide and water, into the array of macromolecules of which they are made. Such organisms are called autotrophic ("self-feeders").

The rest of us, who depend on organic molecules taken in from our surroundings are "heterotrophic".

Although heterotrophs may feed partially (as most of us do) or exclusively on other heterotrophs, all the food molecules on which we depend come ultimately from autotrophs.

Link to discussion of food chains.
Although autotrophs can secure all their carbon from carbon dioxide in air or water, they still need

Most of these micronutrients or trace elements are used in enzymes. Trace amounts of copper, for example, are incorporated in one of the enzymes used in photosynthesis.

There is an irony and an important lesson here. Copper sulfate (CuSO4) is a widely used and potent algaecide; that is, an agent that kills unwanted algae (e.g., in municipal water supplies and swimming pools). But trace amounts of CuSO4 provide an element essential to the life of these organisms.

This is one of many examples of substances that are poisonous above a certain concentration but essential at a lower concentration. So poisons may not be intrinsically poisonous. It is a matter of dose.

Another example

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22 April 2014