How Organisms are Classified

• There are millions of species of organisms on Earth
• A species is defined as
  • A group of organisms that can reproduce to produce fertile offspring
• These species can be classified into groups by the features that they share e.g. all mammals have bodies covered in hair, feed young from mammary glands and have external ears

The Binomial System

• Organisms were first classified by a Swedish naturalist called Linnaeus in a way that allows the subdivision of living organisms into smaller and more specialised groups
• The species in these groups have more and more features in common the more subdivided they get
• He named organisms in Latin using the binomial system where the scientific name of an organism is made up of two parts starting with the genus (always given a capital letter) and followed by the species (starting with a lowercase letter)
• When typed binomial names are always in italics (which indicates they are Latin) e.g. Homo sapiens
• The sequence of classification is: Kingdom, Phylum, Class, Order, Family, Genus, Species

Dichotomous keys

A dichotomous key is a tool that can be used to identify organisms or objects in the natural world, such as plants, animals, or rocks. The key consists of a series of paired statements or clues about features or characteristics, providing a stepwise guide toward identifying each entity.

Dichotomous keys are useful for biological classification as well as identification.

The word dichotomous is derived from the Greek words meaning “two parts.” Each step in the key provides two contrasting choices about a characteristic. Each alternative choice then leads to another pair of clues and then another, until the items are identified

. A group of leaves, for example, might first be separated on the basis of vein patterns; each group could then be further subdivided on the basis of other traits such as shape and stem arrangement.

Dichotomous keys can be used to identify nonliving things as well as living things. Such keys are helpful in identifying rocks and minerals.

The number of steps in a key depends on how many entities are being identified. There is generally one less step than the total number of entities to be identified; thus a key used to identify four unknown entities would have three steps, whereas identifying eight entities would require seven pairs of clues. The clues may be presented as questions or statements.

The most important elements of a good key are that each set of clues offers two contrasting choices and that the clues gradually narrow down the list of possible objects or organisms so that identification is possible. The following key could be used to identify four familiar insects.

1a. Wings are covered by an exoskeleton — go to step 2.
1b. Wings are not covered by an exoskeleton — go to step 3.

2a. Body has a round shape — ladybug.
2b. Body has an elongated shape — grasshopper.

3a. Wings point outward from the body — dragonfly.
3b. Wings point toward the rear of the body — housefly.

Five Kingdom Classification

The five-kingdom classification that we see today was not the initial result of the classification of living organisms

The living organisms are divided into five different kingdoms – Protista, Fungi, Plantae, Animalia, and Monera

Kingdom Monera

Bacteria are categorized underneath the Kingdom Monera.

Features of Monerans

They possess the following important features:

• Bacteria occur everywhere and they are microscopic in nature.
• They possess a cell wall and are prokaryotic.
• The cell wall is formed of amino acids and polysaccharides.
• Bacteria can be heterotrophic and autotrophic.
• The heterotrophic bacteria can be parasitic or saprophytic. The autotrophic bacteria can be chemosynthetic or photosynthetic.

Types of Monerans

Bacteria can be classified into four types based on their shape:

• Coccus (pl.: cocci) – These bacteria are spherical in shape
• Bacillus (pl.: bacilli) – These bacteria are rod-shaped
• Vibrium (pl.: vibrio) – These bacteria are comma-shaped bacteria
• Spirillum (pl.: spirilla) – These bacteria are spiral-shaped bacteria

Monera has since been divided into Archaebacteria and Eubacteria.

Kingdom Protista

Features of Protista

Protista has the following important features:

• They are unicellular and eukaryotic organisms.
• Some of them have cilia or flagella for mobility.
• Sexual reproduction is by a process of cell fusion and zygote formation.

Sub-groups of Protista

Kingdom Protista is categorized into subsequent groups:

• Chrysophytes: The golden algae (desmids) and diatoms fall under this group. They are found in marine and freshwater habitats.
• Dinoflagellates: They are usually photosynthetic and marine. The colour they appear is dependent on the key pigments in their cells; they appear red, blue, brown,  green or yellow.
• Euglenoids: Most of them live in freshwater habitation in motionless water. The cell wall is absent in them, instead, there is a protein-rich layer called a pellicle.
• Slime Moulds: These are saprophytic. The body moves along putrefying leaves and twigs and nourishes itself on organic material. Under favourable surroundings, they form an accumulation and were called Plasmodial slime moulds.
• Protozoans: They are heterotrophs and survive either as parasites or predators.

Kingdom Fungi

The kingdom fungi include moulds, mushroom, yeast etc. They show a variety of applications in domestic as well as commercial purposes.

Features of Kingdom Fungi

• The fungi are filamentous, excluding yeast (single-celled).
• Their figure comprises slender, long thread-like constructions called hyphae. The web of hyphae is called mycelium.
• Some of the hyphae are unbroken tubes which are jam-packed with multinucleated cytoplasm. Such hyphae are labelled Coenocytic hyphae.
• The other type of hyphae has cross-walls or septae.
• The cell wall of fungi is composed of polysaccharides and chitin.
• Most of the fungi are saprophytes and are heterotrophic.

• Some of the fungi also survive as symbionts. Some are parasites. Some of the symbiont fungi live in association with algae, like lichens. Some symbiont fungi live in association with roots of higher plants, as mycorrhiza.

Kingdom Plantae

Features of Kingdom Plantae

• The kingdom Plantae is filled with all eukaryotes which have chloroplast.
• Most of them are autotrophic in nature, but some are heterotrophic as well.
• The Cell wall mainly comprises cellulose.
• Plants have two distinct phases in their lifecycle. These phases alternate with each other. The diploid saprophytic and the haploid gametophytic phase. The lengths of the diploid and haploid phases vary among dissimilar groups of plants. Alternation of Generation is what this phenomenon is called.

Kingdom Animalia

Features of Kingdom Animalia

• All multicellular eukaryotes which are heterotrophs and lack cell wall are set aside under this kingdom.
• The animals are directly or indirectly dependent on plants for food. Their mode of nutrition is holozoic. Holozoic nutrition encompasses ingestion of food and then the use of an internal cavity for digestion of food.
• Many of the animals are adept for locomotion.
• They reproduce by sexual mode of rep

The five-kingdom classification of living organisms took a lot into consideration and is till now the most efficient system.

The Animal Kingdom

• Several main features are used to place organisms into groups within the animal kingdom

Vertebrates

• All vertebrates have a backbone
  • There are 5 classes of vertebrates

Class	Main Features	Examples
Mammals	Fur/hair on skin Gives birth to live young Have placenta Produces milk to feed young External ears visible Endothermic	Human, dog, mouse, horse
Birds	Feathers cover skin Have two legs and two wings, instead of forelimbs Lay eggs with hard shell on land Have a beak Endothermic	Eagle, hen, duck, pigeon
Reptiles	Dry, fixed scales on skin Lay eggs with rubbery shell on land	Snake, lizard, turtle
Amphibian	Smooth, moist skin Lays eggs without shells in water Adults live on land (no gills), larvae live in water (have gills)	Frog, toad, newt
Fish	Loose, wet scales on skin Lays eggs without shells in water Gills to breathe	Flounder, tuna, clown fish

 

Invertebrates

• Invertebrates do not possess a backbone
• One of the morphological characteristics used to classify invertebrates is whether they have legs or not
• All invertebrates with jointed legs are part of the arthropod phylum
• They are classified further into the following groups:

Invertebrate Table

Group	Main Features	Example
Myriapods	Body consists of many segments Each segment contains at least one pair of jointed legs One pair of antennae	Centipede
Insects	Three part body: head, thorax, abdomen Three pairs of jointed legs Two pairs of wings (these may be non-functional and/or underdeveloped) One pair of antennae	Butterfly, ant, wasp
Arachnids	Two body parts: Cephalothorax and abdomen Four pairs of jointed legs No antennae	Spider, scorpion, tick
Crustaceans	More than four pairs of jointed legs Chalky exoskeleton formed from calcium Breathe through gills Two pairs of antennae	Crab, lobster

The Plant Kingdom

• At least some parts of any plant are green, caused by the presence of the pigment chlorophyll which absorbs energy from sunlight for the process of photosynthesis
• The plant kingdom includes organisms such as ferns and flowering plants

 Ferns

• Have leaves called fronds
• Do not produce flowers but instead reproduce by spores produced on the underside of fronds

Flowering plants

• Reproduce sexually by means of flowers and seeds
• Seeds are produced inside the ovary found at the base of the flower
• Can be divided into two groups – monocotyledons and dicotyledons

How do you distinguish between monocotyledons and dicotyledons?

1) Flowers

• Flowers from monocotyledons contain petals in multiples of 3
• Flowers from dicotyledons contain petals in multiples of 4 or 5

2) Leaves

• Leaves from monocotyledons have parallel leaf veins
• Leaves from dicotyledons have reticulated leaf veins (meaning that they are all interconnected and form a web-like network throughout the viruses

What is a virus?

Viruses are small germs (pathogens) that can infect you and make you sick. They can infect humans, plants, animals, bacteria and fungi. Each one infects only specific types of hosts.

 

Viral infections in humans can cause no symptoms or make you extremely ill. Types of diseases they can cause include:

 

Respiratory illnesses.

Diarrhea and vomiting.

Sexually transmitted infections (STIs).

Skin conditions.

Virus structure is simply

• Genetic material (RNA or DNA)
• A protein coat

As a result, they can only replicate inside living cells

 

Shapes of viruses