Characteristics and Classification of Living Organisms
Characteristics and Classification
Cambridge IGCSE Biology
Characteristics of living organisms
| Characteristic | Definition |
|---|---|
| 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 stimuli in the internal or external environment and make appropriate responses |
| Growth | a permanent increase in size and dry mass |
| Reproduction | the processes that make more of the same kind of organism |
| Excretion | the removal of waste products of metabolism and substances in excess of requirements |
| Nutrition | the taking in of materials for energy, growth and development |
Classification and species
- Organisms are classified into groups by the features that they share.
- A species is a group of organisms that can reproduce to produce fertile offspring.
The binomial system
- The binomial system is an internationally agreed system: the scientific name of an organism has two parts, the genus then the species.
- The genus is the first word and starts with a capital letter; the species is the second word, in lower case.
- Copy a genus name with its spelling exactly as printed.
Dichotomous keys
- A key is based on identifiable features. Each step splits a group into two, using one visible feature.
- Write each statement as a feature that is present or absent: "has ..." against "... absent".
- Use a key from step 1, following each "go to" until a single name is reached.
| Step | Statement | Go to |
|---|---|---|
| 1a | has the feature | 2 |
| 1b | feature absent | name of organism |
| 2a | has a second feature | name of organism |
| 2b | second feature absent | name of organism |
Classification by DNA
- Classification systems aim to reflect evolutionary relationships.
- The sequences of bases in DNA are used to classify organisms: use extracted DNA, determine the base sequences and compare them with other known species.
- Groups that share a more recent common ancestor are more closely related: their base sequences in DNA are more similar, with fewer differences, than groups that share only a distant ancestor.
- DNA data is more accurate and not subjective.
- On a classification diagram, a branch point is a common ancestor: the more recent the shared branch point, the more similar the base sequences. The group whose branch splits off earliest is the least similar.
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