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Biology · Chapter 1: Introduction to Biology
Levels of Organisation
Life is organised in levels: atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystems and the biosphere.
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01Concept
Life is organised in levels, from the tiniest particles to the whole Earth. Each level is made of the level below it, and each new level has features that the smaller parts do not have on their own.
1. Subatomic and atomic level
All matter, living or non-living, is made of atoms, which are made of even smaller subatomic particles: protons, neutrons and electrons. The elements found in living things are called bioelements. Of the 92 elements that occur naturally, about 16 are bioelements.
Just six elements make up about 99% of the mass of the human body: oxygen (about 65%), carbon (about 18%), hydrogen (about 10%), nitrogen (about 3%), calcium (about 1.5%) and phosphorus (about 1%).
2. Molecular level
Atoms join to form molecules. The molecules found in living things are called biomolecules. Small biomolecules, such as glucose, amino acids and water, are micromolecules. Large ones, such as starch, proteins and DNA, are macromolecules.
3. Organelle and cell level
Biomolecules build organelles: tiny parts of a cell, each with its own job. The nucleus controls the cell; mitochondria release energy. Organelles together form a cell. The cell is the smallest unit that is alive.
4. Tissue level
A tissue is a group of similar cells that work together to do the same job. Muscle tissue contracts to cause movement. In plants, xylem tissue carries water.
5. Organ and organ system level
An organ is made of different tissues working together for a particular job. The heart, stomach, brain and a leaf are organs.
An organ system is a group of organs that work together. The digestive system includes the mouth, stomach, intestines and liver. Plants have two main systems: the root system and the shoot system.
6. Organism (individual) level
An organism is a complete living thing that can carry out all life processes on its own. A human, a mango tree, a frog.
7. Population level
A population is a group of organisms of the same species living in the same place at the same time. All the people living in Karachi; all the mango trees in an orchard.
8. Community level
A community is all the populations of different species living together in an area and affecting each other. The fish, frogs, water plants and insects in a pond.
9. Ecosystem level
An ecosystem is a community together with its non-living surroundings, such as water, soil, air and sunlight. A pond with its water, mud and sunlight.
10. Biosphere
The biosphere is the part of the Earth where life exists: the land, water and lower atmosphere together. It is the highest level of organisation.
Unicellular, colonial and multicellular organisation
| Type | Meaning | Example |
|---|---|---|
| Unicellular | The whole organism is a single cell that does every life process | Amoeba, Paramecium, bacteria |
| Colonial | Many similar cells live together, but each cell still lives fairly independently | Volvox, a green alga |
| Multicellular | Many cells, divided into tissues and organs that do different jobs | Frog, mustard plant, human |
02Real World Examples
Medicine works at many levels
A blood test looks at molecules and cells, a heart transplant replaces an organ, and a polio vaccination campaign protects a whole population.
Saving wildlife
To protect the Indus river dolphin, scientists count its population and also protect its ecosystem: the river's water, fish and banks.
Farming
Farmers care for single organisms (a sick cow) and also whole populations (a field of wheat).
Your own body
Right now, atoms form molecules in your cells, cells form muscle tissue, and tissues form your heart, which is part of your circulatory system.
03Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What are bioelements? Name the six most common bioelements in the human body.Show answer
Answer
Bioelements are the elements that are found in living things. About 16 of the 92 naturally occurring elements are bioelements.
The six most common are oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus, which together make up about 99% of body mass.
Q2.Define tissue and organ, giving one example of each.Show answer
Answer
- Tissue: a group of similar cells doing the same job. Muscle tissue.
- Organ: different tissues working together for a particular job. The heart or a leaf.
Q3.Differentiate between a population and a community.Show answer
Answer
| Population | Community |
|---|---|
| Organisms of the same species | Populations of different species |
| Living in one area at the same time | Living together and interacting in one area |
| All the frogs in a pond | Frogs, fish, insects and plants in a pond |
Q4.What is colonial organisation? Give an example.Show answer
Answer
In colonial organisation, many similar cells live together in a group, but each cell still carries out its own life processes fairly independently. Example: Volvox, a green alga.
Q5.What is the biosphere?Show answer
Answer
The biosphere is the part of the Earth where life exists: the land, water and lower atmosphere. It is the highest level of organisation.
Long questions
Q6.Describe the levels of organisation of life, from atoms to the biosphere.Show answer
Answer
- Subatomic and atomic level: all matter is made of atoms. About 16 elements are bioelements; O, C, H, N, Ca and P make up about 99% of body mass.
- Molecular level: atoms form biomolecules: small ones (glucose, amino acids) and large ones (proteins, DNA).
- Organelle and cell level: biomolecules form organelles (nucleus, mitochondria), which form cells, the smallest living units.
- Tissue level: similar cells doing the same job. Muscle tissue, xylem.
- Organ and organ system level: tissues form organs (heart, leaf); organs form systems (digestive system).
- Organism level: a complete living thing. A human, a mango tree.
- Population: one species in one area. All mango trees in an orchard.
- Community: all the populations living together in an area. Life in a pond.
- Ecosystem: a community plus its non-living surroundings.
- Biosphere: all parts of the Earth where life exists.
Each level is built from the level below it.