Classification

Characteristics of Living Organisms

Every living organism has certain traits that it needs to be demonstrating for it to classify as “living”. There are 7 characteristics that we need to go through. If even a single one of these characteristics is not present in an organism, then it not alive.

Movement – An action by an organism or part of an organism causing a change of position or place
Respiration – The chemical reactions in cells that break down nutrient molecules and release energy for metabolism
Sensitivity – The ability to detect or sense stimuli in the internal or external environment and to make appropriate responses
Growth – Permanent increase in size and dry mass by an increase in cell number or cell size or both
Reproduction – The processes that make more of the same kind of organism
Excretion – Removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances exceeding requirements
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

Concept and Use of Classification System

Organisms can be classified by features that they share. Classification systems aim to, therefore, classify groups of organisms systematically, to reflect their evolutionary relationships.

Before the advance of technology and science, classification was traditionally based on morphology (overall form and shape of bodies) and anatomy (detailed body structure, usually determined by dissection). This means that if certain organisms “looked” similar and shared similar features, then they would be classified under the same umbrella.

But it is important to understand that now, we can more accurately classify organisms by analysing their DNA (rather than look at appearance alone). Organisms that share similar DNA base sequences and protein amino acid sequences are more likely to be closely related.

Organisms belong to the same species if they can breed together successfully and the offspring can also breed. Every organism has a scientific name. The binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts (Genus & Species). The Genus is a generic term used in the classification of living organisms or binomial nomenclature and the Species is usually determined as a group of organisms that can reproduce to produce fertile offspring.

Example: Homo sapiens

NOTE: It is important to understand here that Homo represents the genus and is always starts with a capital letter and is written in italics, while sapiens is the species and is all lower case and in italics. Underline in IGCSE as you cannot write in italics.

Features of Organisms

All organisms are made of cells. Although the cellular structure may be different depending on the type of organism, certain things are universally shared across every single organism:

Cytoplasm
Cell membrane
DNA (that makes up genetic material)
Ribosomes
Enzymes

Now, we can organize every single organism into 5 main categories called “kingdoms”. Here are the 5 kingdoms that you need to know:

Animal (e.g. Lion)
Plant (e.g. Oak Tree)
Fungus (e.g. Yeast)
Prokaryote (e.g. Bacteria)
Protoctista (e.g. Marimo)

Now that we understand the features of the 5 kingdoms, we can branch further. The syllabus wants you to understand how to classify different “types” of animals and plants. In the animal kingdom, animals can further be classed as vertebrates (with backbone) or invertebrates (no backbone)

In the plant kingdom, plants can be classed into either flowering plants or ferns. Flowering plants are further classified into Monocotyledons and Dicotyledons.

Viruses

Viruses cause common diseases such as colds and influenza, and also more serious ones such as AIDS. Viruses are not normally considered to be alive, because they cannot do anything other than just exist until they get inside a living cell. They, then, take over the cell’s machinery to make multiple copies of themselves. 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. On their own, viruses cannot move, feed, excrete, show sensitivity, grow or reproduce. The diagram below shows one kind of virus. It is not made of a cell – it is simply a piece of RNA (a chemical similar to DNA) surrounded by some protein molecules. It is hugely magnified in this diagram. The scale bar represents a length of 10 nanometres. One nanometre is 1 × 10−9 mm. In other words, you could line up more than 15,000 of these viruses between two of the millimetre marks on your ruler.

Calculating Magnification

Magnification = Size of Drawing ÷ Actual Size

Dichotomous Keys A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, and fish. Keys consist of a series of choices that lead the user to the correct name of a given item.

For example, use the key to identify the insect.

Wings present ………………………………………….….… Go to 2
Wings absent …………………………………………………. Go to 3
One pair of wings are visible …………………………… A
Two pairs of wings are visible …………………………. B
Three pairs of legs …………………………………………. C
Two pairs of legs …………………………………………… D

Since this insect has two pairs of wings visible, the answer is B