|Index to this page|
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|
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
- to supply all its energy needs from the potential energy stored in glucose and
- manufactures all of its organic molecules using the carbon atoms in glucose.
- NH4Cl and MgSO4 supply the nitrogen and sulfur atoms needed to synthesize proteins.
- Na2HPO4 and KH2PO4 supply the phosphorus atoms needed for nucleic acid synthesis (as well as being needed for the functioning of many proteins).
- These ingredients also supply the Na+, K+, Mg2+, and Cl- needed by the cell.
- CaCl2 supplies the needed Ca2+ ions.
All these ingredients are dissolved in water and are taken up from this solution.
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:
- all 20 of the amino acids from which proteins are synthesized;
- a purine (hypoxanthine) and a pyrimidine (thymidine) for the synthesis of nucleotides, and their polymers DNA and RNA;
- 2 precursors (choline and inositol) needed to synthesize some of the phospholipids in the cell;
- 8 vitamins, all of which serve as parts of coenzymes;
- the coenzyme lipoic acid;
- glucose as a source of energy and carbon atoms;
- the inorganic ions Na+, K+, Ca2+, Cu2+, Zn2+, and Co2+ (E. coli may need some of these as well but in such tiny amounts that it can acquire them as impurities in the other ingredients of its medium.)
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.
This third recipe is used to culture green algae.
|Bristol's Medium for Algae|
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
- sources of nitrogen, sulfur, and phosphorus in order to synthesize proteins and nucleic acids;
- substantial amounts of magnesium (used to make chlorophyll), calcium, and potassium
- small amounts of Fe, Mo, Zn, B, and Cu.
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.
22 April 2014