Life
Understanding life and its
constituent enable us to know the structure of our body and brain for happy and
enlightened living. Therefore, we will begin with the primary unit cells that
constitute living beings.
Basic Units of Life
All the matter that exists in the universe is made of
atoms. Since atoms of many elements do not exist alone, the combinations of
atomic elements are called molecules. For example, water molecules consist of
hydrogen and oxygen atoms; the common salt is sodium and chlorine. Water and
salt are simple molecules. More complex molecules, or macromolecules like sugar
and oil, are made of a long chain of carbon, hydrogen, and oxygen atoms.
All the universe's matter is classified into living
and non-living entities. All animals, plants, and human beings are considered
to be living entities as these can grow and reproduce. Like animals, humans are
made of flesh, bones, and blood, while all plants consist of roots, stems, and
leaves. All the organs of living entities consist of four types of
macromolecules. These biological macromolecules are known as carbohydrates,
lipids, proteins, and nucleic acids. The smallest unit of life consisting of
these macromolecules is called a cell. We will explain a cell structure in more
detail here.
A photograph of a leaf is shown in Figure 2 1. If we examine a leaf, we can see it is made
of smaller structures.
Figure 2.1 A magnified image of a plant leaf showing smaller structures.
The tiny units making these structures are the basic
building blocks of plants known as plant cells. All other parts of plants are
also made of these cells. Similarly, for all animals, including humans, all the
organs are constituted by a basic unit called an animal cell.
Figure 2.2: A schematic showing the organelles of a plant cell.
Source: Credit to [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)].
The typical size of a cell is between 10 micrometers
to 100 micrometers. Various small specialized structures of a living cell are
known as organelles. Similarly, the various organelles of an animal cell are shown
in Figure 2.3. By examining Figure
2 2 and Figure 2.3, we
find that the animal and plant cells have many similarities and a few
differences. Animal and plant cells have a cytoskeleton, endoplasmic reticulum,
cytoplasm, and nucleus. The chloroplasts, vacuoles, and cell walls are found
only in plant cells. Within the cell's nucleus, there are chromosomes
containing deoxyribonucleic acid DNA.
A cell performs all life processes, such as survival,
growth, reproduction, etc. A cell's functioning can be easily understood by
comparing it to a mini factory producing other macromolecules, such as proteins
required for life. The cell membrane acts as the boundary with several gates to
control the flow of what goes in and out of the factory. The cytoskeleton
provides shape to the factory using the microfilaments and the microtubules.
The nucleus acts as a manager office, where DNA works as a chief architect to provide
the design for all the types of proteins or macromolecules to be manufactured.
All the units are maintained in a stable environment by cytoplasm, which acts
as a factory's central air-conditioning system. The mitochondria act as a
powerhouse for the factory. In addition, the plant cells have machinery in the
chloroplast for converting the Sun energy.
Figure 2.3: A schematic representation of the various parts of an animal cell.
Source: Credit to
[CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)].
For making body parts, each cell grows and divides
itself into two similar cells in a process known as mitosis. This process
continues, and thus we have a speedy way of growth, with each cell multiplying
by two. In this way, the various organs of the animal or plant are constituted
depending on the instructions expressed by the DNA. DNA is an essential
organelle that we will describe in more detail.
DNA Structure
Each living species has unique DNA, and it is the main
differentiator of the species. The DNAs carry the genetic code for animal and
plant cell growth, function, and reproduction. A gene is a region of the DNA
chain that encodes a particular function. Depending on the sequence of four base
elements in the gene, it can express different characteristics in plants or animals.
DNA has a typical double helix structure, as shown in Figure 2.4.
DNA consists of four basic nucleobases: cytosine,
guanine, adenine, and thymine. Further, these molecules are made of four base elements
carbon, hydrogen, oxygen, and nitrogen, as shown in Figure 2.5. For constituting
various proteins and the DNA itself, ribonucleic acid (RNA) is the base unit shown
in the figure. The same four basic nucleobases also comprise RNA, except uracil
substitutes for thymine.
Figure 2.4: A magnified representation of the double helix structure of DNA.
Source: Credit to CC0 Public domain.
Figure 2.5: Schematic diagram showing the chemical structure of DNA and RNA with their
nucleobases.
The packing of DNA into a chromosome is illustrated in Figure 2.6. Certain
types of proteins help compel the chromosomal DNA into the tiny space of the
eukaryotic nucleus, as shown in Figure 2.6. These proteins are called histones, and the
resulting DNA-protein complex is chromatin.
Figure 2.6: An illustration to show how DNA is packed in chromosomes.
Source: Credit to [CC BY
4.0 (https://creativecommons.org/licenses/by/4.0)].
It may seem paradoxical that proteins are added to DNA
to make it more compact. For example, we know it is much easier to compactly
store a long rope by coiling it. But, of course, coiling requires extra work,
and some energy is needed to perform this work. Thus, within the nucleus,
histones provide the energy to fold DNA. As a result, chromatin is packaged
into a much smaller volume than DNA alone.
Knowing the structure of DNA and a basic cell, we can
understand the buildup of a human body from the cells. A basic cell can become
a specialized cell depending on its functionality. The special cells for
reproduction are a sperm cell in males and an ovum cell in females. The sperm
cell from a father is combined with the ovum cell from a mother in the mother's
womb.
The sperm and ovum cells divide into four cells through
a unique process known as meiosis. These sperm and ovum cells are known as
haploid cells as these have only one copy of 23 chromosomes. The haploid human
genome contains approximately 3 billion base pairs of DNA packaged into 23
chromosomes. Except for female ova and male sperm, most body cells are diploid
cells, each containing 23 pairs of chromosomes. Therefore, each diploid cell
has 6 billion base pairs of DNA. The chromosomes have all the coding for all
the characteristics the body will have when it is fully grown.
Comparing DNA size with the astronomical distances we
discussed earlier is interesting. Each base pair is around 0.34 nanometers long,
with 6 billion base pairs. Therefore, each diploid cell contains about 2 meters
of DNA. It is estimated that the human body has about 50 trillion cells. Thus, the
total DNA length for a human turns out to be 100 trillion meters. We mentioned
earlier that the Sun is 150 million kilometers away from Earth. It means that
each of us has enough DNA to go from Earth to the Sun and back more than 300
times. Earlier, we also mentioned that Earth's perimeter is 40000 kilometers. Therefore,
our DNA can go around Earth's equator 2.5 million times.
With the knowledge of the basic units of living
entities, we can define life.
Everything in this universe can be considered to be a
form of energy. Therefore, some ontological and philosophical studies have
attempted to define life
in terms of energy. In addition to animals and plants, there is another variety
of life on the Earth. There exists a large number of different single-cell
organisms.
All the forms of life existing on Earth are broadly
classified into three categories. These categories are the three lineages of
life known as the eukaryote, the bacteria, and the archaea. Bacteria are the
smallest living cells. A base cell of bacteria is classified as a prokaryote
type, while all the plants and animal cells are classified as a eukaryote type.
Some entities similar to an organelle or DNA strand,
such as viruses, prions, etc., can grow. However, standalone, any organelles of
a cell do not constitute life. Therefore, we will not consider such structures
as life as these cannot support life functions independently.
Our body is an organized system of a large number of
cells. We need to consume food for survival and growth. Many chemical processes
in our body, such as the conversion of food into energy, are known as metabolic
processes. For optimum performance, our body maintains a constant temperature,
blood pressure, and heartbeat through processes known as homeostasis. We tend to
reproduce and evolve with the changing environment. We respond to various
stimuli to perform desired actions. Therefore, life is defined in terms of
biological processes such as metabolism, homeostasis, adaptation, and
reproduction. While in philosophy,
life is often studied in contrast to non-living things. Non-living does not
respond to the environment or any stimulus. A non-living thing does not have
free will and cannot move itself. Thus, free will is considered an essential
feature of living entities.
Since we shall be dealing mainly with intelligent
life, we will define it now. We define intelligent life as an autonomous system
of cells that undergo birth and death processes while interacting with the
external environment for survival, growth, reproduction, and evolution. Thus,
intelligent life has a brain that can think independently and has free will. Therefore,
it includes all types of animals that move according to free will. This
definition excludes plants, bacteria, viruses, prions, etc., from intelligent life.
