Levels of Organisation: Grade 9 Understanding for IGCSE Biology 2.1

The Easter holiday is the most important time of year for this iGCSE Biology blog.  With exams in early May, the next few weeks should be the time when students are working at their maximal rate.  I intend to add one post a day such that by the middle of April, the entire EdExcel iGCSE Biology specification has been covered on this blog.  This should then make it a useful resource for all GCSE Biology students to help them with their revision.

Today I will put up two posts that will look at two of the simplest topics in the specification:  the first one will be Levels of Organisation and then Cell Structure (2.1, 2.2, 2.3 and 2.4)


Living things (or organisms to be precise) are complex entities.  Even the simplest organism will be made up of millions of different molecules arranged in an organised and complex way.  Human beings are organisms made up of about 10 trillion cells of roughly 210 different cell types all put together in a organised and systematic way.  It makes it much easier to study such complexity if we have a system to break the complexity down into constituent parts.  This is what scientists mean by levels of organisation.

So, starting with the smallest things that might be of interest to a biologist……

220px-Levels_of_Organization.svgAll matter on earth including the matter of living things is made of atoms (e.g. a carbon atom, an oxygen atom etc.).  Atoms can combine together in a variety of ways to form molecules (a water molecule H2O, a carbon dioxide molecule CO2,)  How atoms combine to form molecules is chemistry, and the levels of organisation smaller than an atom forms part of physics, so we won’t worry too much about them….

But molecules in an organism are interesting and worth studying – you learn about carbohydrates, lipids, proteins, DNA in your iGCSE course.  These molecules can be grouped together to form structures inside cells called organelles.  If you are being really precise with your terminology, an organelle is a membrane-bound compartment inside a eukaryotic cell (remember bacterial cells have no organelles at all).  Examples of organelles are structures like the nucleus, chloroplasts, mitochondria and so on.

Cells are structures enclosed by a cell membrane that contain many different organelles.  You have probably looked at a human cheek cell using a light microscope at some point in the past.  In multicellular organisms, cells of the same type are often attached together to form a Tissue.  A tissue is a group of similar cells often attached to each other that carry out the same function in an organism.  (It is also a small disposable piece of rectangular fabric used for blowing your nose but that is something quite different….)  Tissues are grouped together to form larger structures called Organs.  For example, the lungs are an organ made up of a particular arrangement of epithelial tissues together with some blood and connective tissues.   Organs can be grouped into Organ Systems based on their function such as the Digestive System (oesophagus, tongue, stomach, pancreas, liver, intestines etc.)  An Organism such as you or I is made up of many organ systems (nervous system, cardiovascular system, digestive system, excretory system and so on….. You get the idea I’m sure!)

You can study levels of organisation bigger than the organism.  This branch of biology is called Ecology – and indeed you should know the meaning of the terms population, community and ecosystem – but perhaps that is for another post……



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