Human life starts with the fertilization
of sperm with egg and ends when all the body functions cease to work. This
whole time span is a complex combination of chemical reactions and processes,
taking place frequently and continuously. A very important thing to be kept in
mind before reading the article is that the terms steroids, carbohydrates,
lipids, proteins, enzymes, hormones, DNA, RNA etc., wherever used in the
article, are all chemical compounds or macromolecules composed of chemical entities,
in other words, it’s all about chemistry.
The very first step in human life is the
fertilization. Only about 200 sperms, out of the 30 million released, reach the site where fertilization
takes place. It is believed that some chemicals in the follicle cells release
signals to attract the sperms. As soon as the sperm finds an egg it must bind
to it, but it can do so once it is modified, a process known as capacitation. Bicarbonate
ion play key role in this process, by entering the sperm and activating the
adenylyl cyclase enzyme, which in turn produces the cyclic AMP thus helping the
initiation of capacitation. The lipid and glycoprotein composition is altered
during the process. The mammalian egg itself is composed of glycoproteins. As
soon as fusion takes place, the concentration of Ca+2 ions increases
in the cytosol, induced by an unknown enzyme. Development of the egg begins and
as a result of release of different enzymes by the cortical reaction, the egg
hardens so that the sperm no longer binds to it, thus preventing polyspermy and
ensuring only one sperm fertilizes one egg.
Morphogenesis
The fertilized
egg develops into a zygote which starts dividing and through a process, induced
by several hormones, called morphogenesis, develops into an embryo. The embryo
grows and with the passage of time several organs start to develop with the
help of different hormones and enzymes. Morphogens are soluble molecules which
carry signals related to cell differentiation and division. This signaling is
accomplished by binding to specific protein receptors. The fate of the cells is
decided by the transcription factor proteins through interaction with DNA. The
process goes on and on and on till the cells divide and differentiate into
different body organs. The fetus takes nutrients through the placenta from the
mother.
Birth
The next stage
is the birth. Oxytocin is the hormone that helps the uterus to contract and
push the baby outwards. But there are other chemical compounds, known as
catecholamines, which are responsible for providing that energy and feel to the
mother to push harder and help the baby step into the world. Oxytocin is also
responsible for the emotional, physical and motherly interaction of the mother to
the child. Beta-endorphin, another hormone released by brain during stress,
helps to keep the mother relaxed and keeps her emotionally attached to the
newborn.
Body Development
The newborn baby
starts feeding and body development commences, with a rate depending upon
various factors which vary from case to case. But there are some common things
happening in every human body with the passage of time. For example, the body
is made up of cells which comprises of water, nucleus and cell organelles, the
division of cells is controlled by proteins and changes in lipids inside the
cells. The hereditary information is relayed from parents to off springs
through genes and deoxyribonucleic acid (DNA), which themselves are made up of nitrogenous
bases (adenine, guanine, cytosine, thiamine, and uracil) composed of purine and
pyrimidine rings, ribose sugar and phosphates (nucleotide). The DNA itself
needs to be replicated which is done by the DNA polymerase enzyme. DNA has the
information for production of ribonucleic acid (RNA), thus it provides codes
for RNA synthesis, which in turn provides codes for protein synthesis, and the
cycle goes on (The Central Dogma). The messenger RNA carries information from
the DNA to the ribosome where the code is translated into a protein. Respiration,
another important function of the body is concerned with the intake of oxygen
and removal of CO2 from the body. The oxygen taken in through
breathing is taken up by the blood in the lungs and distributed throughout the
body through the bloodstream. The exchange of CO2 also takes place
in the lungs which is exhaled out. This oxygen serves as a source of energy for
the body organs. Brains is an important organ of the human body which controls
it. Brain works through a network of nerves spread all across the body. The
brains receives signals from every part of the body through these neurons and
sends signal back to the relevant organ through the same network. But several
chemical neurotransmitters are involved here too, e.g., acetylcholine is
involved in learning and memory, serotonin is involved in pain, sleep and
emotions, while norepinephrine is concerned with depression and emotions.
Imbalance of these chemicals may lead to several diseases. Steroids such as
estrogen, testosterone, progesterone, estradiol, and prolactin are involved in
the development of primary and secondary sex characteristics, which differentiate
between male and female. Digestion and metabolism takes place in the stomach,
liver, and intestine. Strong hydrochloric acid (HCl) is sprayed on the food
items that enter the stomach. The cytochrome P450 enzyme system in the liver is
responsible for metabolism. The urease enzyme, secreted by the bacteria, H.
pylori in the stomach is responsible for stomach ulcer, thus it needs to be suppressed
or inhibited to avoid the disorder. The blood itself is made of cells. The hemoglobin
in red blood cells contains iron. The deficiency of heme (or iron) may lead to
several health problems.
Aging is a process in
which the body organs and functions start deteriorating, culminating into full
stoppage
when death occurs. It has been proved that glycosylation of proteins is the
major process which contributes to aging. Another important factor that affects
aging and life span is oxidative stress caused by reactive oxygen species
produced inside the body. In brain, aging starts with DNA damage. Oxidative
stress plays an important role in aging of brains in addition to the low level
of production of serotonin, glutamate, and dopamine. Death occurs when all body
functions cease to work. The chemistry behind death may be understood in simple
words as the stoppage of oxygen intake, which provides energy to the cells and
without which the body organs cannot work. Another important compound associated
with death is dimethyltryptamine (DMT), which is produced in the skull and we
see dreams by the release of this compound in small amounts, but the whole lot
is released during occurrence of death. The production of Adenosine
triphosphate (ATP), another energy source, is also affected during this
process, leading to death and “THE END
OF LIFE”. 
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