All flesh is grass.

After molecules have been absorbed they go on to be assimilated. Molecules are transported around the body by the blood. Eventually the blood will pass through the capillaries. Here interchange of molecules takes place, between the blood and the tissue fluid. Tissue fluid is the fluid that surrounds the cells in the body. The process of forming tissue fluid is similar to the process in the Bowman’s capsule in the kidney; it is formed through ultra-filtration.

At the arteriolar end of the capillary the blood pressure is approximately 40mm Hg, at this pressure water is forced out of the capillary. However, this is opposed by the osmotic effect of the plasma proteins, which is approximately 25mm Hg. As a result the outward force is the difference, about 15mm Hg. At the venular end of the capillary the blood pressure has dropped to about 10mm Hg, but the osmotic pressure has remained at 25mm Hg. Therefore a net inward of pressure of 15 mm Hg is created. This draws water back into the capillaries from the tissue fluid by osmosis. This process means that new tissue fluid is always being formed at the arteriolar end of the capillary, carrying glucose, amino acids, fatty acids, glycerol, minerals, dissolved gases and vitamins. Also waste from the cells is taken away at the venular end of the capillary. Some tissue fluid drains into the lymphatic system instead of going back into the blood.

Once these molecules have been transported into the tissue fluid they can be taken up by the cells. For the development of muscle specific molecules are needed. Muscle growth is a specific from of protein synthesis that is brought about by the diffusion of testosterone into the muscle cell. The testosterone combines with a receptor in the cell and stimulates the protein synthesis process.

Protein synthesis takes place in the ribosomes. Amino acids are brought to the ribosome by tRNA molecules. The enzyme peptidyl transferase catalyses the formation of the peptide bond and the polypeptide begins to form. Once the polypeptide is complete the chain is released. As proteins for muscle development are formed in the muscle cell they stay within the cell.

Muscle cells are different to other cells. During muscle development the individual muscle cells, myofibrils, do not divide; they become thicker and longer. A myofibril may become up to 28 times larger than its original size before mitosis begins. Muscle cells are also multinucleated. It is thought that muscle cells benefit from being multinucleated as it allows them to carry out protein synthesis at a faster rate.

This process from ingestion to muscle development continues, at varying rates, throughout an animal’s life. However just as important to the relationship between plants and flesh is what happens once the animal is dead.

Dead animals contain organic nitrogen compounds as do faeces and urine. All plants need nitrogen as it is essential to the formation of nucleic acids and protein. However plants can only take up nitrogen in the form of inorganic ions, in the forms of NO3- (nitrate) or NH4+ (ammonia).

The organic compounds left in the detritus are converted to inorganic ions by saprophytic bacteria and fungi; these are referred to as decomposers. These decomposers break down the organic compounds to release NH4+. When there is enough oxygen in the soil the decomposers will oxidise the ammonia to nitrates such as NO3- and NO2-. This process is known as nitrification. Nitrate ions produced in this way are available for uptake by plants.

There is another way in which ammonia and nitrates are made available to plants. In the soil there are nitrogen fixing organisms known as diazotrophs. These are able to convert nitrogen gas into ammonia. This is a biological version of the Haber-Bosch process; however it is far more efficient and occurs at low temperatures and at atmospheric pressure, whereas the chemical equivalent requires temperatures of 300° to 500°, high pressures and an iron catalyst. The reaction for nitrogen fixation is catalysed by nitrogenase, an enzyme containing iron and molybdenum.

 

These nitrate and ammonia ions are taken up by the plants through their roots. Many of the ions will be incorporated into organic compounds and used to synthesis amino acids.

The plant is again forming a part of the food chain. It is primary producer; this means that it is viewed as food by both herbivores and carnivores. As the plant is ingested by the herbivores or carnivores, or as the herbivore is ingested by the carnivore the nitrogen and other essential molecules are being transferred on again. This is known as the nitrogen cycle.

Although at initial thought it was difficult to understand how plants and flesh were related, or how the bible had anything relevant to say about modern science, it has become clear that the symbiotic relationship between plants and animals is crucial to the survival of not just human life on earth but of all life. Animals cannot survive without muscle, without the ability to move, and we cannot generate that muscle without nitrogen. We could not obtain nitrogen without plants, which in turn would not be able to obtain enough nitrates and ammonia if animal detritus was not converted to inorganic ions that they can use. Perhaps this interdependence should serve as a reminder to humans that we are not as powerful as we think we are, and we are still fundamentally part of a very complex web, that not one of us could survive without. Indeed “all flesh is grass”, we could not survive if it were not.