What Makes Us Alive?
What do a bacterium, a tiger, and fish have in common? They share the characteristics of life, without which there can be no life [1].
There is an obvious difference between a real duck and a wooden decoy duck. The real one is alive while the latter is not. It truly is not hard to distinguish the living from the nonliving in this case. Just look at it [1].
Does the wooden decoy duck respond to stimuli in its environment? No. Can it grow and develop? No. Is it able to procreate? No. Does it have homeostasis or the ability to keep its internal environment in constant stability? Well, perhaps that is not very obvious unless you know the meaning of homeostasis and its mechanisms. Does it have complex chemistry? Maybe that is not crystal clear right away. Is it composed of cells, which are the basic building blocks of the structure and function of something living? Perhaps that is not realizable right away, either [1].
Well, in case it is not clear to you yet as to what characterizes life, an organism must have these things to be alive: 1) Response to environmental stimuli, 2) Growth and Development, 3) Procreation, 4) Homeostasis, 5) Complex chemistry, and 6) Cells. [1]
Cells are essential to who you are, but they are not the only essentials. Many chemical reactions must incessantly occur inside your cells to keep you alive. Chemical reactions occur not only with chemists in a laboratory [2].
Chemical Reactions
Chemical reactions involve substances. A substance is matter with uniform and definite arrangement, type, and the ratio of atoms, which changes into a different substance, prompted by a reactant that makes a product within you, whether quickly or slowly. Compare your chemical reactions to a fire extinguisher expelling foam to kill a fire. That is a quick reaction. Many years of an iron tool becoming rusty and brittle is a slow reaction [2].
Although we know you are not a fire extinguisher or an iron tool, their chemical reactions are like yours in the sense that your reactions give you the same atoms as before but with different arrays and combinations due to the breaking of bonds in the reactants and the making of new bonds in the products [2].
When oxygen and hydrogen engender water, bonds break from the two oxygen atoms and four hydrogen atoms to unite them all. The arrangements are different in water while four hydrogen atoms and two oxygen atoms are still present in the reactants and products. This is an example of a synthesis reaction wherein two or more reactants synthetically compose one single product [2].
There are other types of reactions like decomposition, single replacement, double replacement, and combustion. But I like the synthesis reaction as it relates to water, and we humans are largely composed of water [2].
The chemical reactions that happen within you use energy to break bonds in reactants, and that energy moves or flows when new bonds ensue with the products. There are two types of chemical reactions important for discussion here: 1) exothermic reactions-- these discharge energy into the environment, usually in the form of heat or light, and 2) endothermic reactions--these swallow up energy from the environment [2].
Humans as endothermic organisms
You and I are endothermic organisms and are not crucially dependent on external environmental temperatures, unlike the ectothermic and poikilothermic animals, such as fishes and reptiles, with body heat that is hardly efficacious for regulating body temperature. Higher than the exothermic animals’ rate is our energy expenditure rate per unit of time that sustains an unwavering body temperature within a great dynamic of environmental temperatures. The brain, heart, liver, and gastrointestinal tract have thermoregulation that sustains a core body temperature of 37 degrees Celsius under ambient environmental conditions (immediate surroundings). Meanwhile, there are temperature differences in the limbs and appendages (28-36 degrees Celsius). It uses peripheral perfusion (passage of fluid through the bloodstream) and evaporation through sweat glands to satisfy the body’s thermal needs. Radiation is your body’s pathway for heat loss and evaporation is the cooling mechanism in hot environments [3].
‘Endo’ means ‘in’ while ‘exo’ means ‘out’ and ‘thermic’ refers to ‘heat’ during the whole breaking of bonds and releasing of energy amidst chemical reactions [4].
The products are more energized than the reactants in endothermic reactions, taking the heat from the external environment to cool the system where the reaction is going. The enthalpy (the change in heat energy) increases at the end of the reaction [4].
How we are connected to the world of photosynthesis
It is interesting to compare us, endothermic organisms, to other endothermic reactions such as photosynthesis (the transformation of water and carbon dioxide into glucose and oxygen with the involvement of chlorophyll) [4].
Photosynthesis is represented by:
6 CO2 + 6 H2O + light --> C6H12O6 + 6 O2 [4]
A test tube becomes colder when ammonium chloride is dissolved in water. Heat is taken from its test tube environment. Therefore we have another endothermic reaction represented by the following:
NH4Cl(s)+H2Ol —> NH4Cl(aq) — heat [4]
Transformation of ice into water through boiling, melting, or evaporation is another kind of endothermic reaction [4].
Respiration and food digestion are other examples of endothermic reactions. Perhaps you would find it interesting to use those features to relate us to the nonliving endothermic world around us while still setting us apart from the nonliving [4].
All living things must respire. Energy from chemical bonds of biomolecules, such as sugars and lipids, must be perpetually supplied to us so we can grow, repair ourselves, move, defend ourselves, and procreate. Plants and algae are autotrophic and self-nourishing, while we humans are heterotrophic and dependent on them. Plants and algae get their inorganic nourishment through photosynthesis and chemosynthesis, while humans must get their nourishment through consumption of the carbon compounds generated by those autotrophs [7].
This cellular respiration that occurs in both autotrophs and heterotrophs transitions adenosine diphosphate (ADP) into adenosine triphosphate (ATP). Our respiration is the aerobic kind that requires oxygen for making the ATP, wherein carbon dioxide and water are generated through a series of enzymatic reactions oxidizing glucose (C6H12O6) [7].
Plants and humans are said to live symbiotically with each other as the glucose is provided by the plants undergoing photosynthesis that can be conceptualized as a process opposite to cellular respiration [6]. The chloroplasts are the light-capturers and fuel-storers in plant tissues that connect with your mitochondria that break down the fuel molecules into a form that your cells can use [8].
You are connected to the carbon cycle
The nature of your atoms connects you to the ecosystem’s biogeochemical cycles that involve carbon and nitrogen moving around the planet [5]. A food chain is made wherein carbon moves from the atmosphere to plants and then to animals that devour them and then back into the ground where the animals are deceased and decomposing. Carbon leaves you every time your cellular respiration exhales carbon dioxide gas into the atmosphere [5].
You could not contribute to the carbon cycle without digestion
If you are an animal digesting plants, you need sugar molecules for your energy needs. Respiring, excreting, and decomposing the stuff discharges the stuff back into the soil and atmosphere. You could not accomplish this without digestion [19].
Your digestion is needed for your body to have proteins that reduce down to amino acids, fats that reduce down to fatty acids and glycerol, and carbohydrates that become simple sugars. These are indispensable for having energy, growth, and cell repair. The fatty acids are absorbed by the lymphatic vessels transporting white blood cells and battling against infection. Hormones that make digestive juices and signal to the brain about hunger and satiation work in tandem with your nerves and glands that make you salivate. Your enteric nervous system within the walls of your gastrointestinal tract unleashes many different substances that either expedite or impede food movements and digestive juices, depending on how the food stretches your GI tract walls [9].
So, the glucose that is so crucial for your energy-making is derived from the carbohydrates digested by the stomach’s food that has mixed with acids and enzymes. Glucose is unleashed into the bloodstream, but it cannot stay there for too long. Hence, we have insulin to store glucose for later use and prevent high sugar levels [10].
Your pancreas’ beta cells make insulin that is a hormone. Your beta cells monitor your glucose levels and insulin amounts. The insulin that is traveling through your bloodstream I liken to guards at the cell doors, dictating when they should open and close for inviting the glucose in. Sugar levels diminish with the movement of glucose from the bloodstream and into the cells [10].
Your insulin levels ebb and flow throughout the day and night, depending on when and how much you eat. Between 70 and 120 milligrams per deciliter is a normal blood sugar level. One hundred eighty during or immediately after a meal is an anomalous level that even non-diabetic people can experience. The standard/desired consequence is that your level decreases to under 140 two hours after a meal [10].
The right insulin amounts are crucial to our energy needs for staying alive, working, playing, and being who we are [10].
Our relationship with photosynthesis
As oxygen is important to photosynthesis and our cellular respiration, and plants need photosynthesis and carbon dioxide to survive and to keep the carbon cycle going, we all are therefore in a symbiotic relationship [18].
People have wondered if we will ever be able to manipulate our human cells so that we can photosynthesize.
A Vice article in 2015 discussed the sea slug, Elysia chlorotica [11]. The slug, being an animal, is alleged to do what many other animals cannot do: to get sustenance from algae, chloroplasts, and the sun’s energy alone, without necessitating oxygen from the environment for their respiration. The Marine Biological Laboratory in Woods Hole has been credited with research on this [12] [13].
Of course, you and I need Vitamin D sustenance from the sun and plants, but we need more than that. So, could there be a day when we survive based on only the sun? According to the science journal called nature, aphids capture sunlight and synthesize pigments called carotenoids for their energy requirements [14].
The Oriental hornet is said to use its xanthopterin pigment from its exoskeleton to convert sunlight into electrons [15]. A team of females from St. Margaret’s School won the Aberdeenshire prize for using the xanthopterin from the hornet as electrical energy for powering a car’s battery [16].
But experienced science writer, Tracy Staedter, believes that “getting humans to photosynthesize sunlight is next to impossible.” Our energy demands (glucose, adenosine triphosphate, and 1,600 to 2,400 per day) outweigh those of the aphid, the hornet, and the slug. The entire surface area of an adult holding chlorophyll like a leaf would only fulfill 1% of our energy standards for survival. A human would need chlorophyll the size of a tennis court. Our skin is too thick to absorb the right amount of carbon dioxide that makes photosynthesis possible [17].
Perhaps human survival by photosynthesis alone is worthy of more elaboration in a different post. But for now, I would like to leave you with the thought that humans still have a fighting chance against starvation, malnutrition, and foodborne illnesses by some other innovative means.
General Disclaimer: All sources are hyperlinked in this article. The author has made their best attempt to accurately interpret the sources used and preserve the source-author’s original argument while avoiding plagiarism. Should you discover any errors to that end, please email thecommoncaveat@gmail.com and we will review your request.
All information in this article is intended for educational/entertainment purposes only. This information should not be used as medical/therapeutic advice. Please seek a doctor/therapist for health advice.
Works cited:
[3]https://www.sciencedirect.com/topics/neuroscience/endothermic
[4]https://biodifferences.com/difference-between-endothermic-and-exothermic-reactions.html
[5] https://scied.ucar.edu/carbon-cycle
[6]https://msu.edu/~potters6/te801/Biology/biounits/respiration.htm
[7]https://www.jove.com/science-education/10567/cellular-respiration
[13]https://www.mvtimes.com/2018/09/05/celebrity-sea-slugs-prove-photosynthesis-isnt-just-plants/
[14]https://www.nature.com/news/photosynthesis-like-process-found-in-insects-1.11214
[15]https://phys.org/news/2011-01-physicists-outer-shell-hornet-harvest.html
[18] https://sciencing.com/photosynthesis-important-organisms-6389083.html
[19] https://tinyurl.com/y5yq8q42
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