In order to identify living organisms, scientists have listed 7 characteristics which all Living Organisms have:

Characteristic Definition
M Movement An action by an organism or part of an organism causing a change of position or place
R Respiration The chemical reactions in cells that break down nutrient molecules and release energy for metabolism
S Sensetivity The ability to detect or sense stimuli in the internal or external environment and to make appropriate responses
G Growth A permanent increase in size and dry mass by an increase in cell number or cell size or both
R Reproduction The processes that make more of the same kind of organism
E Excretion Removal of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements
N Nutrition Taking in of materials for energy, growth and development; plants require light, carbon dioxide, water and ions; animals need organic compounds and ions and usually need water

The Binomial System:

The Binomial System of scientifically naming organisms was developed by Carl Linnaeus of Sweden. It consists of the organism’s Genus and Species name, and thus it is called as Binomial. It consists of 7 levels:

  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

Now, let’s take up a simple example: Humans

  • Kingdom- Animal
  • Phylum- Vertebrates
  • Class- Mammalia
  • Order- Primate
  • Family- Hominidae
  • Genus- Homo
  • Species- Sapiens

Rules for writing scientific names:

  • The first letter of the genus is always CAPITALISED
  • The first letter of the species is NEVER capitalised
  • Scientific names of organisms are always italicised or underlined

Here is an example for the scientific name for Blue Whales.

Scientific name Correct or Incorrect?
balaenoptera musculus Incorrect because first letter of Genus is not Capitalised
Balaenoptera Musculus Incorrect because first letter of Species is Capitalised
Balaenoptera musculus Correct!

The classification of living organisms

Diagram explaining how living organisms are classified

Eukaryotes

As seen in the diagram above, Eukaryotes consist of Protoctists, Fungi, Plant, and Animal kingdoms.

Protoctists

are organisms with a nucleus, and many flexible organelles amongst their species (for example, some have chloroplasts and cell walls like plants and some like animal cells without these distinguishing characteristics). Their main characteristics include:

  • Unicellular or multi-cellular bodies
  • Cells with or without cell wall and chloroplasts
  • Some species are autotrophic, rest are heterotrophic
  • All species have cells with nucleus

Examples of Protoctists:

  • Paramecium
  • Chlamydomonas
  • Seaweeds

Fungi

Photo by Andrew Ridley

Fungi are organisms which do not have chlorophyll, thus are heterotrophic and feed on dead organic matter parasitically. The most common known is the edible mushroom; others include fungi  causing diseases like athlete’s foot, ringworm, panama disease etc.Their characteristics include:

  • Multicellular bodies (very few are unicellular)
  • Have nuclei
  • Reproduce by spore production
  • Are heterotrophic
  • Don’t have chloroplasts
  • Feed by parasitic or saprophytic means on organic dead matter

Examples include:

  • Saccharomyces cerevisiae
  • Penicillium
  • Mushroom

Animals

Within the Animal Kingdom, there are several phylums such as Arthropods,

Phylum Arthropoda

  • Hard exoskeleton
  • Segmented bodies
  • jointed appendages
  • exoskeleton composed of protein and chitin
  • Open circulatory systems in which heart pumps hemolymph through short arteries into open spaces (sinuses)
  • Aquatic members have gills for gas exchange
  • terrestrial members have tracheal system of branched tubes leading from their surface throughout body

Class Crustaceans

Found these two on a rock in the Galapagos Islands. Taken from about a foot away.
Photo by Jim Strasma 
  • More than 4 pairs of jointed legs
  • Breathe through gills
  • Antennae present
  • Mostly marine

Examples of crustaceans: Crabs, lobsters etc.

Class Arachnids

Photo by Joao Alexandre Paulo 
  • With 4 pairs of jointed legs
  • Breathe through gills and book lungs
  • Mostly terrestrial

Examples of arachnids: Scorpion, spider etc.

Class Insects

A photo of a bee.
Photo by Christian Cagni
  • Have 3 pairs of jointed legs
  • Have 2 pairs of wings
  • Breathe through trachea
  • Antennae present
  • Mostly terrestrial

Examples of insects: Locust, Moth, House Fly, Grasshopper etc.

Class Myriapods

Otmoor
Photo by James Wainscoat
  • Body consists of many segments
  • Each segment has jointed legs
  • They can be both herbivores and carnivores
  • Terrestrial

Examples of myriapods: Centipede, Millipede etc.

Phylum Annelida

Photo by sippakorn yamkasikorn
  • They are worms
  • Have bodies made up of ring like segments
  • Live in water and moist soil

Example of Annelids: Earthworm

Phylum Mollusca

Jellyfish Exotica
Photo by Ganapathy Kumar
  • Soft bodied animals
  • Have unsegmented bodies
  • With or without shell

Examples of Molluscs: Octopus, Jellyfish, Squid etc.

Phylum Nematodes

  • They are worms
  • Bodies are not divided into segments
  • Usually white, long and thin bodied
  • feed by parasitic means

Examples of Nematodes: Hookworm, Roundworm etc.

Phylum Vertebrates

  • Internal skeleton with spine
  • Their nervous system has encephalon (brain) and a spinal cord.
  • The encephalon is placed inside the skull and spinal cord is placed inside the spine.

Class Fish

Photo by David Dvořáček
  • Are cold blooded
  • Have streamlined bodies
  • Aquatic
  • Have scales on their bodies
  • May be Herbivores or Carnivores
  • Lay eggs in water
  • Have fins
  • Breathe through gills

Example of Fish: Mackerel, Pomfret, Tuna, Salmon etc.

Class Amphibians

Frog in the Water
Photo by henry fournier
  • Give birth to offspring by laying eggs
  • Have 4 limbs
  • Their habitat is both terrestrial and aquatic
  • Have moist skin
  • Breathe through gills when young; when mature, breathe through lungs
  • Adult often lives on land

Example of Amphibian: Frog, Salamander etc.

Class Reptiles

Tree Agama, Manzini Swaziland
Photo by Peter Scholten
  • Have scales on body
  • Are cold blooded
  • Terrestrial
  • Lay eggs to give birth to offsprings
  • Egg shells are rubbery

Examples of Reptiles: Lizard, Snake etc.

Class Birds

22 Species of Birds https://ark.digitalcommonwealth.org/ark:/50959/6d56zx52j

Please visit Digital Commonwealth to view more images: https://www.digitalcommonwealth.org.
Photo by Boston Public Library
  • Don’t have teeth, instead have a beak
  • Have hollow bones
  • Are warm blooded
  • Lay hard shelled eggs
  • Forelimbs replaced by wings
  • Breathe through lungs

Examples of Birds: Flamingo, Eagle, Hawk, Sparrow etc.

Class Mammals

[Portrait of President-elect Donald Trump]. Digital photograph, 2016. Library of Congress Prints & Photographs Division.

https://www.loc.gov/item/2017645723/
Photo by Library of Congress
  • Warm blooded
  • Can maintain a constant body temperature
  • Have different types of teeth
  • Have skin covered by Hair
  • Give birth to live young offsprings
  • Have sudoriferous (sweat) glands
  • Females have mammary (milk secreting) glands that produce milk to feed young ones.

Example: Human being

The Plant Kingdom

Phylum Angiosperm:

  • Have root, stem and leaves
  • Have xylem and phloem
  • Reproduce by seed production
  • Seeds are produced inside the ovary of the flower
  • Phloem: Transports sugar
  • Xylem: Transports water and minerals
Plant Part Monocot Dicot
Root System Adventitious root only Adventitious root, Taproot or both
Cotyledons One Two
Stem Vascular bundles scattered Vascular bundles arranged in a ring
Leaves Exhibit parallel venation Exhibit reticulate venation
Flower Either trimerous, tetramerous, seldom pentamerous Either pentamerous, tetramerous, seldom trimerous

Viruses

This illustration, created at the Centers for Disease Control and Prevention (CDC), reveals ultrastructural morphology exhibited by coronaviruses. Note the spikes that adorn the outer surface of the virus, which impart the look of a corona surrounding the virion, when viewed electron microscopically. A novel coronavirus, named Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2), was identified as the cause of an outbreak of respiratory illness first detected in Wuhan, China in 2019. The illness caused by this virus has been named coronavirus disease 2019 (COVID-19).
Photo by CDC
  • Entirely microscopic
  • Consisting of a single nucleic acid surrounded by a protein coat
  • Capable of replication only within living cells of bacteria, animals or plants.

Example of Viruses: Coronavirus, Human Immunodeficiency Virus, Tuberculosis, etc.

Prokaryotes

This illustration depicted a three-dimensional (3D), computer-generated image, of a group of Gram-positive, Corynebacterium diphtheriae, bacteria. The artistic recreation was based upon scanning electron microscopic (SEM) imagery.
Photo by CDC

Bacteria are prokaryotic and unicellular. they have cell walls and circular DNA called plasmids. They are Heterotrophs or Autotrophs.

Example: L.bulgaricus

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