Characteristics and Classifications of Living Organisms : Gate Academy

You can use the mnemonic MRS GREN to remember the characteristics.

M – Movement:
- Movement is an action that causes a change in the position of an organism or a part of an organism.
R – Respiration:
- Respiration describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
S – Sensitivity:
- Sensitivity is the ability of an organism to sense or detect changes in the external or internal environment and make appropriate responses to the change.
G – Growth:
- Growth is the increase in size and dry mass of an organism which is characterized by an increase in cell number or cell size or both.
R – Reproduction:
- Reproduction is the process by which new individual organisms are made from their parents.
E – Excretion:
- Excretion is a process by which metabolic wastes and other non-useful materials are eliminated from an organism.
N – Nutrition:
- Nutrition is the intake of materials required for growth and development.

An addition to the characteristics listed above are:

A – Adaptation:
- Adaptation is the ability of an organism to survive in a particular habitat.
C – Competition:
- Competition is the interaction between organisms or species in which both the organism or species are harmed. It dictates the survival of an organism/species.

A natural system, called taxonomy, uses important features shared by as large a group as possible is used to classify living organism.
There are seven levels of taxonomy;
The binomial system of naming species is an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and the species.
The first name gives the genus and the second gives the species.

Viruses are not true living things.
They are not considered to be alive, because they cannot do anything until they enter a living cell.
Viruses are complicated assemblies of molecules including proteins, nucleic acids, lipids, carbohydrates, etc.
When viruses encounter a cell:
- they take over the cell’s machinery.
- A series of chemical reactions occur that lead to the production of new viruses.
- These new viruses burst out of the cell and invade others, where the process is repeated.
- The host cell is usually killed when this happens.
- These steps are completely passive, i.e., they are predefined by the nature of the molecules that comprise the virus particle.
- Viruses don’t actually do anything. Without cells,viruses would not be able to multiply.
Scientists do not classify a virus as a living thing, because:
- it does not show all seven processes for life.
- when it enters a cell it changes the way the cell works so it can make copies of the virus.
Key features of a virus:
- It is very small (100 times smaller than bacteria)
- It does not have a typical cell structure.
- It contains a strand of DNA or RNA.
- It is surrounded by a protein coat called a capsid.
- The only life process they show is reproduction (inside host cell)
  
  Photo Credit: Fsu.Edu

Bacteria cells are very different from the cells of all other organisms.
They do not have a nucleus.
Some bacteria can carry out photosynthesis.
The oldest fossils belong to this kingdom, so we think they were the first kinds of organisms to evolve.
Key features of bacteria:
- They are small (1000 times smaller than a plant cell).
- They are often unicellular (single-celled).
- They have cell wall, which does not contain cellulose nor chitin.
- They have cell membrane, and cytoplasm, containing glycogen granules.
- They do not have a nucleus, only DNA in the form of a single coiled chromosome.
- Some of them have a slime capsule.
- Some of them have one or more flagella (a lash-like appendage).
  
  Bacteria Cell Diagram (Photo Credit: SpyAlly.com)

PLANTS	ANIMALS
Contain cell wall, chloroplast, vacuole, centriole	Do not contain cell wall, chloroplast, vacuole, centriole
They are autotrophs	They are heterotrophs
Store carbohydrate in form of starch	Store carbohydrate in form of glycogen
They are non-motile	They are motile
Gaseous exchange takes place through stomata	Different animals have different organs for gaseous exchange, e.g., gills, lungs, etc.
Reproduce asexually and with vegetative methods	Reproduce sexually and asexually
Response to stimuli is slow	Response to stimuli is fast