Executive Summary

The paper examines the characteristics of life. Literature is provided regarding the origin of research of the components of life. Early Greeks including Aristotle developed their theories that have been discussed in the paper. The three early theories, materialist, hylomorphism and vitalism had their bases, but were not enough to become conclusive in the ideas they lobbied for. The second theory that was developed by Aristotle was the first scientific approach to determine what life is. Since there has not been a clear cut definition and explanation on what life really is, modern scientists have come up with characteristics that define what a living organism is. These seven characteristics have been explained in details and their significance in life and to organisms is elaborated. Further, examples are given in some of these characteristics where multicellular and single cellular are discussed. Further, different origins to each of the seven characteristics are discussed in brief. The characteristics of life have been identified as homoeostasis, organization, metabolism, growth, adaptation, response to stimuli and reproduction. Finally, a short conclusion has been made to further appreciate the complexity of the issue on what life really is. The paper has also appreciated the worthy efforts that biologists have put on the research on life. Their efforts are regarded as better than those from theologists whose theory can not be proven as compared to scientific theories. One recommendation given is to improve the known level of biological sciences in order to come up with an explanation on what life is, and not describe is using its characteristics.

Characteristics of life

Introduction

Life can be defined as the characteristic that differentiates between animate and inanimate objects. It allows the animate objects to respond to stimuli and at the same time self sustain themselves. Inanimate objects can either have lost life or may not have possessed it at all. Study of life is known as biology. Lots of studies have been carried out by different people at different levels and periods. Some gave their theories that have become plausible while other theories have been discarded. The origin of life remains a mystery and apart from biologists, theologists as well as philosophy gurus have come up with theories that explain the origin of life (Stewart-Williams, 2010). None of the methods or theories has been fully accepted across all people and the search still continues.

With the many definitions and puzzles surrounding life, there has been no strong definition that has been accepted to define what life really is. Though it looks very obvious when one looks at living organisms, there sill lacks a correct scientific definition. Scientists have therefore come up with a way to describe those objects that possess life. This is done using seven characteristics. This paper will examine each of the seven characteristics that an object that has life must possess.

Literature Review

The search for plausible definition of life has been in existence for hundreds of years since education came into being. Early scholars gave their views and supported them as they thought appropriate (Thagard, 2012). Different cultures have their different believes regarding the origin of life as well as different experts across various fields. For instance, most theologists believe that the earth was created by a Supreme Being who up to date controls the universe. They also believe that all the life on earth belongs to the Supreme Being and He can terminate the lives any time He wishes (Stewart-Williams, 2010). Some even go further to explain and believe that even after death, there still is another form of life. They however base their claim on faith and provide no evidence regarding the same.

The first theory that was put forward to describe life was developed in 430 BC by Empedocles (Thagard, 2012). It was known as materialist theory. It was based on the facts that life was an arrangement of matter in a very complex design. He argued that the universe was made up of four unending elements that include water, air, fire and the earth. Every change that is experienced was as a result of differences in arrangement of matter. He also believed that the different life forms were as a result of different mixtures of matter. It was in this theory that Democritus who was Leucippus’ student explained that the soul was the source of life in all organisms. He noted that all the mannerism that all living organisms possess is as a result of their possession of the soul. According to Leucippus, the soul comprised of eternal atoms and of the four eternal elements that Democritus had developed, he chose fire atoms to be the main component of the soul (Marietta, 1998). Though the materialist theory was developed by the ancient Greeks, it was revisited and revived by Rene Descartes in the 17th century. In the19th century, a sound base for the model was developed through the cell theory.

The second early theory of life was developed by Aristotle in 322BC. It was known as hylomorphism and was the first approach to explain life though scientific means, especially biology (Thagard, 2012). Aristotle believed that there were three forms of souls: vegetative, animal and rational. Vegetative soul was found in plants and helps plants grow, flourish and eventually rot. It however did not provide the plant with the ability to move by itself. The second form of soul was found in animals. It enabled the animals to move, grow and perform all the things they need to do. Finally, rational soul was meant to enable the organism to reason and make decisions that considered different circumstances and situations. Aristotle believed that this form of soul was only found in human beings. The more developed soul possessed the elements of the less developed one, thus man has all the three forms of soul. Aristotle believed that matter can exist without form, but form could not exist without matter.

This theory was then supported by the teleological approach to explain life. Teleological approach explains that an organism possess its features in order to live in a certain environment. However, it has been refuted by the modern scientists who believe that the environment leads to an organism developing its traits, unlike the teleological approach that an organism possessed the traits in order to live in a certain environment. Modern scientists attribute traits such as the white color of the polar bear to camouflage in snow as a result of the natural selection.

Finally, the third early theory was known as vitalism. It was developed in the 17th century by Stahl and remained until the 19th century (Thagard, 2012). It had been supported by anatomists, chemists as well as philosophers. It refuted the ideas that organic material could only be obtained from living organisms. It was the basis of the current organism chemistry since it was supported by Friedrich Wohler who prepared an organic substance (urea) from inorganic compounds (Wilkinson, 1998). It was however abandoned in the 1850s when it was determined that there is no energy lost during the movement of the muscles.

Modern science suggests that life may be 3.3 billion years old (Thagard, 2012). There has been, however, no clear-cut explanation as to how life arose. Scientists believe that the earth is about 3.7 billion years old and life started to exist 4 billion years after the formation of the earth (Thagard, 2012). The biggest mystery is the explanation on where life originated from. Since the cell is the fundamental source of life, scientists have strived to determine its source, since it later developed to become the millions of organisms we have today. The main contention is the belief that cells could only have developed from metabolism, while metabolism only occurs in cells (Schwartz, 2009). Whatever emerged first is yet to be determined. However, with the improvement in technology and knowledge, some evidence suggests that cell development and metabolism could have occurred at the same time.

The most plausible models regarding the development of the cell are derived from the hypothesis that the primitive earth had conducive conditions that could have encouraged chemical reactions that could synthesize organic compounds and amino acids into organic compounds. The second hypothesis is that phospholipids form lipid bilayers spontaneously, which eventually forms the fundamentals structure of the cell membrane (Schwartz, 2009).

From all the previous studies, the complete understanding of life has been difficult. The origin, development and what life actually is remains a puzzle to most people. However, there is one thing that all the experiments suggest regarding the issue; organisms with life possess most or all of the seven characteristics that will be discussed in this paper.

Characteristics of Life

Homeostasis

This refers to the ability of an organism to balance its internal environment. The main elements that organisms balance within their bodies are body temperature, blood volume, pH balance and internal water balance. With this regard, living organisms have mechanisms that regulate the substances that affect these conditions at cellular level. When the body temperature is high, the organism has a mechanism to cool it down to the desired levels. High temperatures have negative effects to the cells and if not controlled, they may result to undesired consequences such as failure of the metabolic processes (Morgan, 2007). If metabolic proceses are stopped in a cell, it does not produce energy thus may lead to the death of the cell. One of the common ways that organisms use to regulate temperature is through electrolyte concentration. In mammals, for example, sweating is a way to reduce the body temperatures. There are organs that aid in the process such as the kidney and this ensures that there is a constant temperature. Most organisms have distinct body temperatures with a good example being human whose body temperature should be retained at around 370C. At the same time, high pH levels in the cells affect the metabolic processes (Alberts, et al, 2002). Cell pH has to be maintained at a certain level to ensure that the metabolic processes are executed in accordance to the cell requirements. High acidity or alkalinity in the cell may lead to the destruction of the cell through the stoppage of all cellular activities. Water is essential to all cells, and they consist of over 60% water. This high amount of water allows it to become a solute to most of the solvents in the cell. It also acts as the medium where the chemical reactions take place. The water level in the cell has to remain at the preferred level, failure to which most of the cellular activities would be affected (Alberts, et al, 2002). The cell has its way to expel excess water and at the same time a mechanism to increase the internal water if the level goes down. The cell wall and the cell membrane are able to discharge or allow inflow of water since they are permeable or semi-permeable respectively (Morgan, 2007). The process of diffusion, osmosis and active transport ensures that there is water balance in the cells to allow the necessary processes to continue.

Blood balance is also vital in organisms that have blood as the main transport medium of oxygen and nutrient. They have a regulatory system that ensures that there is a rapid increase in blood cells whenever there is an injury that results to profuse blood loss. This balance is important for the maintenance of energy flow within the organism.

Generally, it is through homeostasis that organisms are able to ensure that the metabolic processes are not affected by changing external conditions such as excessive cold or heat.

Organization

All the animals that have life have an organized form. Those from the same family have their cells organized into similar units and they can be differentiated from their physical appearance. Different organisms are differentiated from the organization of their cells (Alberts, et al, 2002). The cells are organized into tissues and the tissues into organs. The organs are finally organized into organ systems which eventually constitute of the organism.

This organization is vital to the organism since the organs ensure that the necessary processes are carried out effectively to ensure that the organism maintains its life. During the formation of the organism, the individual cells develop and then differentiate into distinct forms that allow them to meet perform a specific process. This means that cells in an organism are not all similar. Reproductive cells, or gametes, are different from muscle cells in mammals (Morgan, 2007). At the same time, excretory organs are different between species and organisms.

The process of differentiation of cells start from the period the cells start multiplying after a new organism is created either asexually or sexually after fertilization. Different organs require different features in their cells, For instance, muscle cells require more mitochondria than the reproduction cells due to the difference in energy requirements. Likewise, the leaf cells of a green plant get many more chloroplasts than the root hair cells of the same plant. Cells of the same organ differentiate in a similar way (Mohr & Shopfer, 1995).

Organization in single celled animals is different because they do not have organs or organ systems. Therefore, their organization is between the cells organelles since despite being a single cell, the organism carries out almost all the processes that other multi cellular organism do. The single celled organisms feed, respirate and others even move. The process of feeding, in most cases, is through phagocytosis (Morgan, 2007). They are able to sense the right food particle to engulf and they then move towards the food. A good example of these organisms is the bacteria which locomotes, ingests, respirates and excretes. The cell organelles allow the organism to do all the processes effectively and to ensure that their life continues.

The same process of phagocytosis is also used in the multicellular organisms but in engulfing and killing pathogens. It is used in the immune system. Cells in the immune system are therefore differentiated to deal with pathogens and this results to killing of the pathogens before they colonize the organism. Therefore, all organisms, either the simple single celled animals or more complex multi cellular animals must possess organization within the cells or within the organs and organ systems.

Metabolism

This refers to the chemical reactions that take place in an organism. They are vital in the survival and every activity since it is through the process of metabolism that organisms are able to produce the energy they use to maintain their lives. Metabolism is divided into Anabolism and catabolism (Taiz & Zeiger, 2010).

Anabolism is the process where the organism builds up complex substances from simple substances. The inputs are simple and they may include water, carbon dioxide and sunlight. The products are however not as simple since they comprise of oxygen and glucose. At the same time, the process could lead to the increment of cell or the cellular components both in size and number (Alberts, et al, 2002).

Catabolism on the other hand refers to the process that complex substances are broken down to simpler substances that the organism body can take up. The breakdown is meant to avail the nutrients in a process known as nutrient extraction. This process can be digestion in multicellular organisms or cellular respiration. Digestion involves both the mechanical and chemical breakdown of food material (Morgan, 2007). Different enzymes are involved throughout the process and different foods are broken down by different enzymes. Once the food is broken down the unwanted material is passed out through egestion or through excretion (egestion does not involve the metabolic waste while excretion refers to the elimination of metabolic waste from the body).

The products of metabolism in organisms are energy, water and carbon dioxide. The excess water is passed out through the excretory system while carbon dioxide is passed out through the cell walls and into the atmosphere. The main chemical reaction in the cell is respiration. It occurs in the cell mitochondria and releases the metabolic waste. The process involves the breakdown of nutrients to release energy (in the form of Adenosine Triphosphate [ATP]), water and carbon dioxide. The process requires oxygen in its atomic state to take place. It acts as the oxidizing agent for the process to complete. However, single celled organism such as nitrobacters (a form of bacterium), can use other inorganic molecules such as sulphur, hydrogen as the oxidizing agent for the process of respiration (Alberts, et al, 2002).

Respiration can either be aerobic or anaerobic. Aerobic respiration is experienced when there is abundant oxygen for respiration to take place. Anaerobic respiration occurs when the oxygen available is not enough. The nutrients that are availed to the cells from the process of digestion in animals and uptake in plants are broken down in a series of reactions (Mohr & Shopfer, 1995). These nutrients include fatty acids, sugar and amino acids and they are broken down into ATP, the form in which energy is stored in the body. The whole process of breakdown is known as the Kreb’s cycle. The energy stored in the form of ATP is availed to the muscles and the body every time it is needed (Morgan, 2007).

Growth

This is the increase in both size and mass of the cells and eventually the whole organism. This process accelerates at some point and then stagnates once the organism has attained the maximum size during its lifetime. Growth is facilitated by the process of metabolism where the cells increase in number and size (Alberts, et al, 2002). The multiplication involves the division of cells in processes known as either mitosis or meiosis. Mitosis occurs in the eukaryotic cells that have a distinct nucleus. It results into the division of the parent cell to form a similar cell as the mother cell. Mitosis involves five main stages which ensure that the new cell will be complete and would resemble the mother cell (Alberts, et al, 2002). These stages are interphase, prophase, metaphase, anaphase and telophase. They are then followed by cytokinesis that involves the final nourishment of the new cell. The number of chromosomes is equal to that of the mother cell but the size is normally smaller. However, the size increases after sometime to attain the normal size of the cells of that particular organism. The resultant cell has double chromosomes (2n). This form of cell division is normally found in somatic cells of multicellular organisms (Morgan, 2007).

Meiosis occurs in eukaryotic gamete cells (Alberts, et al, 2002). It involves the cell division of organisms that reproduce sexually. Meiosis involves the first five stages of mitosis but they are followed by another four stages where the interphase stage is omitted. When the stage is omitted, the chromosomes are not given the time to align in the nucleus. Therefore, the number of chromosomes that the daughter cell ends up with is half that of the mother cell (n). This later helps in the reproduction.

Growth is important to organisms. For multicellular organisms that give birth to young ones, lay eggs or reproduce vegetatively, increase in size enables the organism to perform all the tasks that is required to including giving birth and attending to the younger ones.

Adaptation

Different organisms live in different habitats. These habitats range from salty water, fresh water, dry land, snow caps among others. Different organisms can not live under similar conditions due to their adaptive capability. Every Organism in any part of the world is adapted to the environment it lives in. No organism can thrive in a habitat it is not adapted to. This is because the organs of the organisms develop differently due to the conditions they are exposed to. For instance, organisms that are found in regions that have little water are adapted to those conditions. Plants, for instance, develop deep roots as well low stomata on the leaves to ensure that minimum water is lost through respiration (Taiz & Zeiger, 2010). The stomata are at times in the form of thorns where the tip of the thorn has the stomata. The leaves are also succulent to reduce water loss and some plants have a reversed stomatal rhythm where the stomata open at night when the heat from the sun has little intensity and closes them during the day when the rate of respiration would be high (Morgan, 2007). These are known as drought endurers since they are persist in the dry region even during the dry spells of the year. There are other plants that germinate, grow and reproduce seeds within a short time when the dry areas receive the small rainfall. These are known as drought escapers. They only germinate their seeds when there is rainfall.

Organisms in the polar region where there is little water available have almost similar characteristics. They do not have adequate water since most of the water is held in the form of ice. Animals in these regions have a thick skin fatty that shelters it from the excessive cold. This ensures that they retain their body temperatures to ensure that their metabolic processes are not affected by low temperatures.

Birds are also adapted differently with their beaks and claws well adapted to the food they take. Birds that feed on flesh have short and strong beaks that are hooked. The claws are also strong to hold the flesh. On the other hand, a bird that feeds on nectar has a long and slim beak that allows it to reach to the nectarines of the big flowers.

There are other forms of adaptation that involves the cells. For instance, organisms in the marine environment could lose a lot of water through osmosis due to the high solvent concentration around them if they were not adapted to the environment (Morgan, 2007).

The adaptive capability of an organism provides the basis for the theory of natural selection. The organisms that are better adapted for a certain environment thrive better and reproduce to continue their genetic lineage. Those which are weak are eliminated.

Response to Stimuli

Various organisms have different reactions to the surrounding environment. The responses are initiated as a result of complex reactions that happen in the organisms. In mammals, the central nervous system controls the stimuli along with several other hormones (Morgan, 2007). An animal subconsciously moves to shade when the sun gets hot. This is as a result of reaction to stimuli. Movement is the main response that animals respond to stimuli where the muscles are compelled to move the organism when the present conditions get non-conducive. Hormones such as adrenaline are found in most animals and they enable the animals to respond with utmost agility whenever there is danger. Further, reflex reactions ensure that the animal gets away from danger subconsciously and quickly enough. Organisms that can not move fast enough from their predators are eliminated trough the process of natural selection.

In plants, where movement is limited, the growth and development brings forth the stimuli. It is mainly a hormonal process where the growth is directed towards a certain direction. If a green plant was grown in an extreme corner of a dark cave, it will grow towards the opening of the cave where there is enough light. This phenomenon is known as phototropism. Plant hormones will compel it to move towards the light (Mohr & Shopfer, 1995). Another phenomenon is the plant roots growing towards the water even when it means that the roots grow horizontally (Taiz & Zeiger, 2010). This is referred to as hydrotropism. Plants roots always grow into the ground as per the law of gravity, a response known as geotropism. At the same time the shoot grows against the gravity and even when a shoot is permanently bent to face the ground, it develops to oppose the gravity. Other forms of stimuli that are experienced in plants include the falling off of dry leaves and ripe fruit along the abscission layer, healing of a wound, among others (Mohr & Shopfer, 1995).

Among the most important plant hormones that control their response to stimuli are cytokinins, abscisic acid and gibberellins (Morgan, 2007). They are responsible for the healing of plant wounds, ripening of fruits among many other processes that demand plant stimulus response.

Stimuli response is also present in unicellular organisms such as bacteria. Those who feed through phagocytosis sense the presence of the food particle, develops false legs (pseudo legs) and move towards the food. Once the cell reaches the food particle, the cell membrane develops a mouth-like fold where the food is engulfed. It is then passed into the body of the organism where it is digested. The movement of the cell results from the stimulus that alerts the organism that there is food within its reach.

Reproduction

This refers to the process where new organisms are formed from the existing ones. All organisms that have life reproduce and since they all have a lifespan and they die, they leave their young ones for gene and species continuity. Different organisms reproduce differently. The two main forms of reproduction are sexual and asexual means. Sexual reproduction refers to the process where male and female gametes from organisms of a similar species meet in a process known as fertilization to form a new organism (Morgan, 2007).

On the other hand, asexual reproduction involves the formation of a new organism from part of the parent. The most widely known forms of asexual reproduction are binary fission and vegetative propagation. Binary fission mainly occurs in single celled organisms. The cell simply divides to form a new one, and they new cell again divides to form a new organism. The population of these single celled organisms grows exponentially when placed in a conducive environment until the resources such as (among others) food starts to get exhausted due to the exponential increase in population (Morgan, 2007). The population then stagnates before starting to reduce.

Vegetative propagation occurs when part of the mother organism detaches from the main plant to develop into an independent organism (Taiz & Zeiger, 2010). Any vegetative part of the organism would develop into a new entity if subjected to favorable conditions.

Sexual reproduction starts with the gamete cells (Alberts, et al, 2002). Organisms that reproduce sexually could be in two categories: where they exist as two different organisms or as a heterosexual, where one organism possesses both sex organs which is common in earthworms. However, most of them can not reproduce on their own, but they must pair up in order to get the gametes together. In most plants, the flowers possess both the male and female parts. However, some flowers have a way of reducing the chances of self pollination where the organs mature at different times. By the time the late organ matures for fertilization, the fertilization is perceived to have already taken place. This way, chances of self pollination are reduced thus better springs are obtained (Morgan, 2007). In other flowers, self pollination is avoided by the arrangement where the anthers and the stigma are strategically arranged such that there is little chance that pollen grains reach the stigma of the same flower. There are other plants where male flower are not found on the same plant with the female flowers such as pawpaw tree (Taiz & Zeiger, 2010).

The most common form of sexual reproduction in animals is where a species has both male and female animals. Some animals have internal fertilization while others have external fertilization. Only organisms from the same species can produce a viable off spring.

Conclusions & Recommendation

Understanding life is a complex issue that requires more knowledge and deeper understanding of organisms. Though cultural and philosophical approaches are good for the social understanding of life, scientific approach is the best since it is practical and one can test its viability. Since the periods of around 430 BC when Empedoclesdeveloped the first theory to explain the origin of life, there has not been any concrete explanation on what life is or where it originated from. The scientific approach is however the best since it first examines the state of affairs today in the living organisms to try and understand the actual origin of life. The characteristics of life discussed above gives a deep knowledge on what life causes organisms to do and react to different situations. They also give the cellular reactions and these leads such as the processes like respiration which have been used to explain the origin of life.

There have been a lot of discoveries made in the recent past due to the large innovations in technology and other fields. These new attributes should be employed to dig deeper into the quest for knowledge regarding life. However, this is a complex parameter to understand since there is nothing to measure. In 460 BC Democritus introduced the word soul to define the factor that allows organisms to behave the way they do. Therefore, what the scientists are simply looking for is the origin of soul and what it is comprised of. It is in this fact, that there has not been any explanation of what soul is, that religious and theology groups believe that there is a supernatural being that blows the soul to organisms. Scientists should therefore determine whether there is a possibility that soul or life, or in more scientific terms the cell, can develop on its own. If it can not develop on its own, then there is course to believe that on top of science, there is a strong being that blow life to new organisms.

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