Branches of biology and levels of organisation

Biology · Introduction. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Biology is the study of living things. It is divided into branches, and each branch looks at life from one side: morphology studies form and structure, anatomy the internal structure, histology the tissues under a microscope, physiology the functions of the parts, and so on.

Life is also studied at levels of organisation. Atoms form molecules, molecules build organelles and cells, cells form tissues, organs and systems, and these make one individual. Individuals of one species form a population, and the populations of different species living together form a community.

Biology also serves mankind, for example through hydroponics, biological control, cloning and conservation.

Rules to remember

Order of levels: subatomic and atomic → molecular → organelle and cell → tissue → organ and system → individual → population → community → biosphere.
Six bioelements make up about 99% of the mass of protoplasm: O 65%, C 18%, H 10%, N 3%, Ca 2%, P 1%.
Morphology = form and structure; anatomy = internal structure; histology = microscopic study of tissues; physiology = functions of the parts.
Embryology = development from the fertilised egg; taxonomy = naming and classification; palaeontology = fossils; genetics = heredity; ecology = organisms and their environment.
Population = members of one species living in one place at one time; community = all the populations of different species living in one place.
Biosphere = the part of the earth where life exists.
Hydroponics = growing plants in an aerated nutrient solution without soil; biological control = using a living organism to control a pest; cloning = making genetically identical copies.

Common mistake

Students mix up population and community. A population has one species only; as soon as the question lists several species living together, the answer is community (an ecosystem also includes the non-living surroundings).

Worked example

A scientist counts all the house sparrows living in one park in Lahore this year. Which level of organisation is she studying?

Only one species is counted: the house sparrow.
The birds live in the same place at the same time.
One species, one place, one time is the definition of a population; a community would need all the species of the park.

Answer: A population.

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