# Structural Biochemistry/Properties of Living Organisms
## Definition of Living Organisms

living organism can be anything which can breath, respond to stimuli, motion or locomotion,reproduce, etc examples are humans,plants,viruses,bacteria etc

## Characteristics of Living Organisms
### 1. High degree of chemical complexity and microscopic organization

Living organisms are composed of cells with highly organized systems, starting from the smallest cellular unit to the largest. Some organisms are unicellular (composed of one cell), free-living and contain organelles to help carry out their life processes. More complex organisms are multicellular (composed of more than one cell). These cells are organized into specialized roles, like tissues or muscles, where they each live co-dependently. Tissues are organized into organs. The organ system in composed of multiple organs that function together. An organism is a complex series of organ systems.

### 2. Systems to extract, transform and use energy from the environment

Living organisms that convert organic matter from the environment and convert into energy that chemical material inside our cells use in metabolism are called heterotrophs. Other organisms such as algae and plants that use photosynthesis to extract energy from the sun along with carbon dioxide and water to produce sugars are called autotrophs. Certain types of bacteria use chemosynthesis to couple the transformation of organic molecules into from their energy with inorganic molecules. Without this ability to extract, transform and use energy, organisms will quickly become unable to power their life functions.

### 3. Self-replication and self-assembly

Living organisms have the ability to reproduce. In plants and simple animals, reproduction is through the process of asexual reproduction. Asexual reproduction is defined as a single individual reproducing without the help of another individual. Examples of asexual reproduction include splitting or budding. More complex organisms participate in sexual reproduction, where two individual contribute genetic information to create a new individual with a unique combination of traits.

### 4. Sensing and responding to changes in environment

Living organism have the ability to respond to stimuli in their environment. Changes may include light, heat, sound or chemical contact. In response to stimuli, an organism uses hormones to change their behavior in response to the surrounding environment.

### 5. Define function for each component and regulation among them

Living organisms possess specific conditions that they must maintain for survival. The ability to regulate conditions such as temperature, nutrients and water are essential for survival. Homeostasis, a stable balance of internal conditions against external forces is a condition that must be kept.

### 6. History of evolutionary change

Populations of living organisms adapt to changes in their environment through evolution. As changes occur in the population, the organism adapts, using their abilities to metabolize respond and reproduce. Characteristics that are more useful in survival are passed to offsprings and allows survival for the species.

### Growth of Living Organisms

All living organisms grow. An increase in mass and increase in number of individuals are twin characteristics of growth. A multicellular organism grows by cell division. In plants, this growth by cell division occurs continuously throughout their life span. In animals, this growth is seen only up to a certain age. However, cell division occurs in certain tissues to replace lost cells. Unicellular organisms also grow by cell division. One can easily observe this in an invitro cultures by simply counting the number of cells under the microscope. In a majority of higher ordered animals and plants, growth and reproduction are mutually exclusive events. One must remember that increase in body mass is considered as growth. Non-living objects also grow if we take an increase in body mass as a criterion for growth. Mountains, boulders and sand mounds grow, however this kind of growth exhibited by non-living objects is by the accumulation of material on the surface. In living organisms, growth is from inside. Growth, therefore cannot be taken as a defining property of living organisms. Conditions under which growth can be observed in living organisms have to be explained in order to understand that it is a characteristic of living systems. Dead organism do not grow.

### Reproduction

Reproduction is a characteristic of living organisms.--- In multicellular organisms, reproduction refers to the production of progeny possessing features more or less similar to those of parents. Invariably and implicitly we refer to sexual reproduction. Organisms reproduce by asexual means also. Fungi multiply and spread easily due to the millions of asexual spores they produce. In lower organisms like yeast and hydra, we observe budding. In Planaria (flat worms), we observe true regeneration, i.e., a fragmented organism regenerates the lost part of its body and becomes, a new organism. The fungi, the filamentous algae, the protonema of mosses, all easily multiply by fragmentation. When it comes to unicellular organisms like bacteria, unicellular algae or Amoeba, reproduction is synonymous with growth, i.e., increase in number of cells. We have already defined growth as equivalent to increase in cell number or mass. Hence, we notice that in single-celled organisms, we are not very clear about the usage of these two terms - growth and reproduction. Further, there are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc). Hence, reproduction also cannot be an all-inclusive defining characteristic of living organisms. Of course, no non-living object is capable of reproducing or replicating by itself.

### Metabolism

Another characteristic of life is metabolism. All living organisms are made of chemicals. These chemicals, small and big, belonging to various classes, sizes, functions, etc., are constantly being made and changed into some other biomolecules. These conversions are chemical reactions or metabolic reactions. There are thousands of metabolic reactions occurring simultaneously inside all living organisms, be they unicellular or multicellular. All plants, animals, fungi and microbes exhibit metabolism. The sum total of all the chemical reactions occurring in our body is metabolism. No non-living object exhibits metabolism. Metabolic reactions can be demonstrated outside the body in cell-free systems. An isolated metabolic reaction(s) outside the body of an organism, performed in a test tube is neither living nor non-living. Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things but surely living reactions.

### Adaptation of Environments

The most obvious and technically complicated feature of all living organisms is this ability to sense their surroundings or environment and respond to these environmental stimuli which could be physical, chemical or biological. We sense our environment through our sense organs. Plants respond to external factors like light, water, temperature, other organisms, pollutants, etc. All organisms, from the prokaryotes to the most complex eukaryotes can sense and respond to environmental cues. Photoperiod affects reproduction in seasonal breeders, both plants and animals. All organisms handle chemicals entering their bodies. All organisms therefore, are 'aware' of their surroundings. Human beings are the only organisms known to be self aware, i.e., have self-consciousness.

### Consciousness

Consciousness therefore, becomes the defining property of living organisms. When it comes to human beings, it is all the more difficult to define the living state. We observe patients lying in coma in hospitals virtually supported by machines which replace heart and lungs. The patient is otherwise brain-dead. The patient has no self-consciousness. Are such patients who never come back to normal life, living or non-living? In higher classes, you will come to know that all living phenomena are due to underlying interactions. Properties of tissues are not present in the constituent cells but arise as a result of interactions among the constituent cells. Similarly, properties of cellular organelles are not present in the molecular constituents of the organelle but arise as a result of interactions among the molecular components comprising the organelle. These interactions result in emergent properties at a higher level of organization. This phenomenon is true in the hierarchy of organizational complexity at all levels.

### Self-Replicating, and Self-Regulating

All living organisms are self-replicating, evolving and have self-regulating interactive systems capable of responding to external stimuli. Biology is the story of life on earth and is the story of evolution for living organisms on earth. All living organisms - present, past and future, are linked to one another by the sharing of common genetic material, but to varying degrees of assortments.

## References

http://wiki.answers.com/Q/What_are_the_8_characteristics_of_living_organisms_and_describe_each Viadiu, Hector. Chem 114A Lecture Notes 1. Fall, 2012.</ref> http://www.cliffsnotes.com/study_guide/Characteristics-of-Living-Things.topicArticleId-8741,articleId-8578.html</ref> http://www.essortment.com/six-characteristics-life-47733.html http://lisacruz2.tripod.com/id30.html http://www.ehow.com/facts_5535157_living-organisms.html
* ↑ living organisms