Chapter 2 Classification-revision notes
How Organisms are Classified
There are millions of species of organisms on Earth
A species is defined as
o 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 specialized 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 lower-case 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
Classification System Diagram
Linnaeus’s system of classification
Exam Tip
The order of classification can be remembered by using a mnemonic like:
KING PHILIP CAME OVER FOR GRAN’S SPAGHETTI
Dichotomous Keys
Keys are used to identify organisms based on a series of questions about their features
Dichotomous means ‘branching into two’ and it leads the user through to the name of the
organism by giving two descriptions at a time and asking them to choose
Each choice leads the user onto another two descriptions
In order to successfully navigate a key, you need to pick a single organism to start with
and follow the statements from the beginning until you find the name
You then pick another organism and start at the beginning of the key again, repeating until all
organisms are named
Dichotomous key Diagram
Example of a dichotomous key #1
Key
1a Body is completely or partially covered in a shell Go to 2
1b Body is not completely or partially covered in a shell Lomax flavus
2a Shell is attached to rocks by thin threads Mytilus edulis
2b Shell is not attached to rocks by thin threads Go to 3
3a Shell is a spire that comes to a point Buccino undated
3b Shell is a spire that does not comes to a point Go to 4
4a Animal has tentacles Nautilus pompilids
Planorbids
4b Animal has two tentacles
planorbids
Example of a dichotomous key #2
The Five Kingdoms
The first division of living things in the classification system is to put them into one of five
kingdoms. They are:
o Animals
o Plants
o Fungi
o Protoctists
o Prokaryotes
Main features of all animals:
o they are multicellular
o their cells contain a nucleus but no cell walls or chloroplasts
o they feed on organic substances made by other living things
A typical animal cell
Main features of all plants:
o they are multicellular
o their cells contain a nucleus, chloroplasts and cellulose cell walls
o they all feed by photosynthesis
A typical plant cell
Main features of all fungi (e.g. moulds, mushrooms, yeast)
o usually multicellular
o cells have nuclei and cell walls not made from cellulose
o do not photosynthesize but feed by saprophytic (on dead or decaying material) or
parasitic (on live material) nutrition
A typical fungal cell
Main features of all protoctists (e.g. Amoeba, Paramecium, Plasmodium)
o most are unicellular but some are multicellular
o all have a nucleus, some may have cell walls and chloroplasts
o meaning some protoctists photosynthesise and some feed on organic
substances made by other living things
Two examples of protoctist cells
Main features of all prokaryotes (bacteria, blue-green algae)
o often unicellular
o cells have cell walls (not made of cellulose) and cytoplasm but no nucleus or
mitochondria
A typical bacterial cell
The Animal Kingdom
Several main features are used to place organisms into groups within the animal kingdom
Vertebrates
All vertebrates have a backbone
o There are 5 classes of vertebrates
Class Main Features Examples
Mammals Fur/hair on skin Human, dog, mouse, horse
Gives birth to live young
Have placenta
Produces milk to feed young
External ears visible
Endothermic
Feathers cover skin
Have two legs and two wings, instead
of forelimbs
Birds Eagle, hen, duck, pigeon
Lay eggs with hard shell on land
Have a beak
Endothermic
Dry, fixed scales on skin
Reptiles Snake, lizard, turtle
Lay eggs with rubbery shell on land
Smooth, moist skin
Lays eggs without shells in water
Amphibian Frog, toad, newt
Adults live on land (no gills), larvae live
in water (have gills)
Loose, wet scales on skin
Fish Lays eggs without shells in water Flounder, tuna, clown fish
Gills to breathe
Vertebrate classification
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 phylum Arthropods
They are classified further into the following classes:
Invertebrate Table
Class Main Features Example
Body consists of many segments
Each segment contains at least one pair of
Myriapods Centipede
jointed legs
One pair of antennae
Three part body: head, thorax, abdomen
Three pairs of jointed legs
Insects Two pairs of wings (these may be non- Butterfly, ant, wasp
functional and/or underdeveloped)
One pair of antennae
Two body parts: Cephalothorax and
abdomen
Arachnids Spider, scorpion, tick
Four pairs of jointed legs
No antennae
More than four pairs of jointed legs
Chalky exoskeleton formed from calcium
Crustaceans Crab, lobster
Breathe through gills
Two pairs of antennae
Arthropod classification
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
Ferns reproduce by spores found in the underside of their 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
Wheat plants are monocotyledons
Sunflowers are 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 leaf)
Comparing monocots and dicots
