What are Living Things?

Introduction

Anything that possess life is known as a living thing. A living thing is basically any organism that has the characteristics of being alive. Some of the characteristics of living organisms are that they have an organized structure, require energy to perform work, they respond to the stimuli and adapt to the environmental changes. They are also capable of reproduction, growth, movement, metabolization, and death.

Origin of Life

Abiogenesis

Abiogenesis or the origin of life theory was given by Thomas Henry Huxley. He proposed that life formed gradually from nonliving organic matter. The first life forms were simple but gradually through evolution more complex life forms came into existence.

Primordial Soup Theory

There is a famous hypothesis known as the primordial soup theory proposed by Oparin in 1924 and Haldane in 1929. They proposed that the Earth had a reducing atmosphere. In this atmosphere when a mixture of inorganic and organic compounds accumulated in water, they became the primordial soup. Further transformations lead to the formation of more complex polymers and ultimately life forms developed. This theory was supported by the findings of Miller and Urey. Their primordial soup experiment proved that organic molecules that the building blocks of life, can be made from inorganic materials.

RNA World Hypothesis

Another hypothesis known as the RNA world hypothesis suggests that primordial life was based on RNA because RNA also has ribozyme. Hence, it can act as both the genetic material and catalyst. Hence, it can replicate itself or make its own copies without needing the help of other molecules. The RNA preceded DNA and other proteins during the course of evolution.

Endosymbiotic Theory

Another theory known as endosymbiotic theory came to light. This theory attempts to explain that how eukaryotes came to life from prokaryotes. It explains how cell organelles like mitochondrion and chloroplast came to existence. It states that endosymbiosis between a larger cell and a prokaryote must have taken place which over time must have led to the formation of semiautonomous organelles and thus led to the formation of first photosynthetic eukaryote.

Multicellularity

Multicellular life forms probably began around 600 million years ago and occurred several times in biological history. The most popular theory of the origin of multicellularity is Haeckel’s Gastraea Theory. The Gastraea theory posits that the mode of germ layer formation in the ancestors were produced by the invagination of the cells which formed the gut.

Characteristics of Living Organisms

All living things show certain characteristics of being alive. These characteristics are given below −

• An organized structure.

• Requires Energy.

• Reproduction.

• Growth.

• Metabolism.

• Response to stimuli.

• Movement.

• Death.

Let us discuss each of these characteristics in detail.

An Organized Structure

Living things form an organized structure known as cell. They may be a single celled organism, such as a bacterial cell, or multicellular organisms such as animals and plants.

A cell is the fundamental unit of life. It consists of protoplasm surrounded by a plasma membrane and the cell organelles that performs specific roles and functions. A prokaryotic cell is different from a Eukaryotic cell in the same way a plant cell differs from an animal cell.

Requires Energy

All Living things need energy for survival. Energy acts as a fuel for numerous metabolic activities of a cell. Green plants get energy from the process of photosynthesis, where light energy from the Sun is converted into chemical energy.

Animals get energy from the food they eat. During cellular respiration biochemical energy is harvested from organic substances such as glucose from food. Certain organisms can store energy for later use.

Reproduction

All living organisms are capable of reproducing. There are two ways by which living organisms can reproduce.

• Sexual Reproduction: During sexual reproduction, male and female gametes fuse to form a zygote. The zygote develops into a new individual.

• Asexual Reproduction: During asexual reproduction, fertilization or fusion between gametes it does not take place. The offspring comes from only one parent. Examples of asexual reproduction are binary fission, budding, vegetative propagation, sporogenesis, fragmentation, parthenogenesis, apomixis, and nucellar embryony.

Growth

All living things grow. We can see the growth of an organism through naked eyes too. Growth is a uni-directional phenomenon.

Growth refers to an increase in the number or an increase in the size of the cell. An increase in the number of cells occurs through cell division. The stem cells of animals and meristematic cells of plants divide to give rise to new cells. The increase in cell size refers to the increase in cytoplasmic mass.

The cell undergoes a series of phases in the cell cycle. Most of the time, the new cell produced by mitosis undergoes interphase. It is the phase in the cell cycle wherein the cell grows in size. Unless fully differentiated, the cell could replicate its DNA to prepare for the next cell division. In plants, new cells increase in volume by taking in and storing water inside a vacuole.

The extent of regeneration varies among species. Regeneration capacities are mostly limited to plants. Only certain organisms can regenerate a part of themselves like a lizard has the ability to only regenerate its tail. Humans are capable of regenerating certain parts of their body, such as skin and parts of the liver.

Metabolism

All living things metabolize. Metabolism refers to the various processes that are responsible for keeping up of the living state of a cell or an organism. It is of two forms −

• Catabolism: In catabolism, complex molecules are broken into smaller units to obtain energy that is released during the process.

• Anabolism: In anabolism, smaller units combine together to form larger molecules.

Response to Stimuli

All living things respond to stimuli. Living things can detect changes in the environment, especially by cells that function as receptors and adapt to them. For example, humans have five fundamental senses: sight, hearing, smell, touch, and taste.

Plants can also respond to certain stimuli. For example, sunflowers turn according to the Sunlight.

Movement

All living things move. As a living thing can detect stimuli from its surroundings, it can respond accordingly. For example, animals move to search for food, escape predators, and seek a potential mate.

Plants have a limited form of movement, referred to as nastic movement (e.g. thigmonasty, nyctinasty). Thigmonasty or seismonasty is the nastic response of a plant to touch or vibration. Nyctinasty refers to the nastic movement of leaves or petals of higher plants in response to darkness like the closing of a flower at dusk, for instance, a biological rhythmic event.

Death

All living things eventually die. Senescence is the biological term for aging. It is when living things gradually deteriorate over the course of their life. The organism gradually loses its ability to function and deal with stressors. As such, it becomes more vulnerable to diseases and dysfunction.

At the cellular level, the cell no longer divides although it may still be metabolically active. One of the main causes of cellular senescence is the shortening of telomeres leading to DNA damage.

Conclusion

We can conclude that living things are organisms that are made up of cells. All living things require energy to perform their various life functions. They reproduce, grow, metabolize, respond to stimuli, move and eventually die.

FAQs

Q1. What are cells?

Ans. Cells are the fundamental units of life.

Q2. What are the characteristics of living things?

Ans. Living things are made up of cells. They obtain and use energy to survive. They have the ability to reproduce, to grow, to metabolize, to respond to stimuli, to adapt to the environment, to move, and last but not least an ability to respire.