…every living thing is a package of consumable energy

[The Leaven – exploring the relationship between science and religion (cont)]

Although it’s meaning still remains a mystery, Life, in itself, is hard work and requires a lot of energy. It’s now well established that the initial source of this energy is provided by the Sun in the form of light, which is absorbed by a photosensitive pigment called chlorophyll found in plants and other photosynthetic organisms. The energy is then trapped in molecules of glucose, a carbohydrate compound composed by a series of chemical reactions involving carbon dioxide and water. Plant consumers then transfer the energy stored within the glucose carbon source along the food chain. When the glucose is broken down it produces adenosine triphosphate (ATP), the compound required to release the energy that powers most cellular functions. The most efficient way for an organism to synthesise ATP, thereby releasing energy, is by an oxygen requiring process called cellular respiration. In humans oxygen is transferred into the body from the surrounding atmosphere by respiring, it is extracted from air in the lungs by haemoglobin, which is then circulated around the system in the blood.

Chloroplasts visible in the cells of Thyme-moss. Image by Kristian Peters.

As it contains the oxygen required for anaerobic energy production humans cannot survive without blood. Blood was therefore considered of extreme importance in the Biblical era, as it was the substance thought to contain an animal’s character and life force.

Every living thing is a package of consumable energy but not every organism can boast a sophisticated circulatory system that enables cellular respiration. Microbes and other lower life forms have to adopt fairly basic means to generate their energy. The energy generating processes of yeast produces by-products that have been exploited by human civilisations for centuries. One of the ways yeast requires its solar produced energy is by fermentation; a biochemical transformation that converts carbon sources such as glucose or sucrose into energy, producing alcohol, giving off carbon dioxide as a by-product.

Fermentation is not as efficient in producing ATP as aerobic respiration but enables yeast to convert glucose into energy without the aid of oxygen. Scientifically defined, fermentation is a catabolic process that makes a limited amount of ATP from glucose without an electron chain (supplied by oxygen) producing a characteristic end product, such as, ethyl alcohol or lactic acids. During fermentation, yeast not only generates energy from the carbon source but it also breaks it down into an industrially and socially important commodity, namely alcohol. Yeast also has the ability to perform aerobic respiration to give off carbon dioxide but this process does not produce alcohol. Being able to live with or without oxygen is undoubtedly ecologically advantageous to this microbe. Certainly explaining why it inhabited the Earth long before humans did and why it will still be here long after our fragile species has disappeared.

The mysteries surrounding fermentation were once, and to some extent still are, the subject of great scientific endeavour. It was mainly assumed that the reaction was chemically induced because investigators were unaware that miniscule creatures unseen by the human eye could exist. The yeast commercially responsible for transforming carbohydrate rich ingredients, like flour and fruit juice, into loaves of bread or alcoholic drinks is predominately Saccharomyces cerevisiae also known as baker’s, brewer’s or budding yeast. When sugar is plentiful the metabolic route that this type of yeast chooses is fermentation. During fermentation cells multiply rapidly by budding, when all carbon resources are depleted cells either enter a stationary phase of non-division or produce spores. Budding yeasts can also reproduce sexually. Adjacent cells of opposing mating types fuse together, in response to pheromones, by forming protruded structures called shmoos. The end product is a slightly larger round diploid cell that contains two sets of chromosomes; this is a way in which genetic variability is introduced into the cell. This diploid cell can either continue budding or enter meiosis to produce four ascospores.

Performing meiosis is a risky business to budding yeast as it has to temporarily stop increasing population size therefore it only faces this challenge when nutrients are low and its survival is threatened. In this state cells become resistant to stress and can remain dormant for several months, years, decades or even centuries. While dormant they lie at the bottom of the fermenting vessel to form a thick layer of pale brown sediment. Some yeast cells die but many retain the ability to begin dividing again when conditions improve, for instance when more sugar becomes available. This mode of survival allows them to remain viable in the face of adversity.  They are well suited to harsh industrial conditions and, also,  the arid  environment that forms the backdrop of the Biblical Testaments.


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