Characteristics and classification — Cambridge IGCSE Biology
The seven characteristics of living organisms, species, binomial names, dichotomous keys and how classification reflects evolution.
Revision notes
4 short notes, in the order of the specification. Each one in short:
The seven characteristics of living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition, remembered as MRS GREN. Every living organism shows all seven, while a non-living thing may show some but never all. Growth is a permanent increase in size and dry mass, and excretion is the removal of metabolic waste, not the egestion of faeces.
A species is a group of organisms that can reproduce to produce fertile offspring. The binomial system is an internationally agreed system for naming species: the first part of the name is the genus and the second is the species, as in Homo sapiens. The genus starts with a capital letter, and the name is printed in italics.
A dichotomous key is a series of paired statements that identifies an organism. At each step you choose between two contrasting descriptions, such as 'has fur' and 'does not have fur', and each choice sends you to another step or to the organism's name. Good keys use features that can be observed and do not change, not size.
Classification, evolution and DNA
SupplementClassification systems aim to reflect evolutionary relationships, grouping organisms that are believed to have evolved from a common ancestor. The sequences of bases in DNA are used for classification: organisms that share a more recent ancestor have more similar base sequences. Organisms that separated long ago have gathered more differences, because mutations change the sequence over time.
Exam questions
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Quick check questions
Define a species.
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A group of organisms that can reproduce to produce fertile offspring.
Which two parts make up a binomial name?
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The genus and the species.
What is excretion?
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The removal of the waste products of metabolism and substances in excess of requirements.
What does 'dichotomous' mean?
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Divides into two: each step of the key offers a choice between two statements.
Supplement How does DNA show that two species are closely related?
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Their DNA base sequences are more similar, because they share a more recent common ancestor.
Frequently asked questions
What are the seven characteristics of living organisms?
The seven characteristics of living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition, often remembered as MRS GREN. Every living organism shows all seven. A car or a flame may seem to show some of them, such as movement, but a non-living thing never shows all seven.
What is the difference between excretion and egestion?
Excretion is the removal of the waste products of metabolism and substances in excess of requirements, such as carbon dioxide and urea. Egestion is the removal of faeces. Faeces are undigested food that never entered the body's cells, so they are not metabolic waste, and removing them is not excretion.
Why are horses and donkeys different species?
Horses and donkeys are different species because their offspring, the mule, is sterile. A species is a group of organisms that can reproduce to produce fertile offspring. Horses and donkeys look similar and can mate, but the mule cannot produce offspring of its own, so the definition is not met.
How do you write a binomial name correctly?
A binomial name has two parts: the genus first, with a capital letter, then the species, with a lower-case letter, as in Homo sapiens. In print the name is in italics; when handwritten, underline each word separately. The system is agreed internationally, so scientists everywhere use the same name for the same organism.
How is DNA used to classify organisms?
Supplement Scientists compare the base sequence of the same gene in different organisms. The more similar the DNA base sequences, the more recent the common ancestor the organisms share, so the more closely related they are. Organisms that separated long ago have gathered more differences, because mutations change the sequence over time.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026