BIO 101: Characteristics of life (1)

INTRODUCTION

The study of biology as a subject has everything to do with life. Even though there are many branches the subject is divided, every of those branches has to do with life or living organisms. The study, however, cannot kickstact without discussing life itself. What really makes a life? Or better put, what makes an organism a living one?

The basic characteristics that puts 'life' in any object can be summarized under 8 topics. In elementary biology, these topics were collectively given the acronym of 'MR NIGER D' which in completion means: movement, reproduction, nutrition, irritability, growth, excretion, respiration, and death. Our task for today is to summarily examine these characteristics one after the other.

Movement

Movement is one of the key attributes of any living organism. The movement, however, might not be in the sense that most people think or the conventional ways that we are used to. Both plant and animal move. There are different types of movement in living organsisms. These include:

1. Total movement which requires that the whole organism moves from one place to another. This type of movement is usually found in animals and protozoans and involves organs or organelles such as the legs, cilia, pseudopodia, flagella, etc. Movement is usually in response to external stimuli - towards or away from it.
2. Continuous movement happens in the individual cells that make up an organism. This type of movement is necessary for cellular activities that are important for sustaining life. An example is the cytoplasmic streaming.
3. Positional movement takes place in certain organs or organelles of living organisms in response to homeostatic processes.
4. Touch movement takes place in some plants where their leaves collapse in response to touch. The touch triggers some homeostatic responses in the cells of the leaves which lead to their folding. Example is found in the Mimosa pudica plant.
5. Tropism: the movement of plant parts in response to external stimuli such as light, gravity, water, etc. Tropism in response to light is termed phototropism, that of water is termed hydrotropism, while that of gravity is termed gravitropism.

Reproduction

There can never be continuity of life without reproduction. In a layman's language, reproduction is the process of giving birth to young ones who will eventually grow to replace the old ones that die off from the population. There are two types of reproduction in living organisms:

1. Sexual reproduction: this invloves the sex gametes and fertilization. The male organism produces the male gamete while the female produces the female gamete. Both gametes contain half of the genetic materials of each parent. They eventually unite during fertilization to form a zygote with complete genetic material of a normal organism.
2. Asexual reproduction: this does not invlove any sex gametes and hence, the word 'asexual'. Only a part of an organism is required to produce a new organism in this type of reproduction. Fragmentation, budding, stem cutting, layering, etc. are examples of asexual reproductive processes found in living organisms.

Nutrition

No life can survive without food and this is an undisputable fact. Although the process of feeding may vary from one living organism to another. Some organisms are naturally equipped to manufacture their own foods using freely available inorganic materials with the assistance of energy from the sun - photosynthesis/chemosynthesis while others depend on an already made foods. The latter are referred to as heterotrophic while the former are known to be autotrophic. Heterotrophic organisms can be:

1. holozoic: those that ingest their foods, digest it internally, and then expel the undigested ones from their bodies.
2. saphrophitic: those that externally secrete chemicals to digest their foods once it is located and then absorb the digested materials into their bodies. They are often referred to as saprobes.
3. parasitic: those (parasites) that obtain the necessary nutrients from another organism - the host.
4. mixotrophic: those that can utilize more than one process of nutrition. E. g. euglena
5. phagocytosis and pinocytosis.

Irritability

From the word 'irritable'- a feeling of being irritated, irritability simply means the characteristic of being able to respond to changes in one's environment. All organisms must be able to respond to changes in their environment in order to be considered 'living', otherwise, they will be referred to as 'zombies' which are generally considered to be non-living.

Practice Questions

1. *You went on an excursion with your classmate to a forest nearby and saw a transparent object on the floor. Some of your mates said it is a living organism while others disagree. write a short essay (not more than 200 words) to convince your friend that the object is indeed a living organism.