Science, Technology, and Society LET REVIEWER
General Education
Lesson 1: Hierarchy of Life
Before we delve into the hierarchy of life, we need to understand what biology is, since it falls under biology.
What is Biology?
The word "biology" comes from two Greek words: "bios" meaning life and "logos" meaning study. So biology literally means the "study of life." Biology is the science that studies everything that is alive, including plants, animals, fungi, bacteria, and all other living organisms. It looks at how living things are structured, how they work, how they interact with each other, and how they change over time.
- Biology is the study of all living things and how they work
The Hierarchy of Life
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| Image from Dreamstime |
All living things are organized from the smallest parts to the whole organism in a step-by-step way. At the very base are atoms, which combine to form molecules. These molecules then build structures called organelles, which are the working parts inside a cell. Cells are the basic units of life, and when many of them come together, they form tissues. Tissues then combine to create organs, and organs work together in systems. Finally, all these systems function as one complete organism. This arrangement shows how life is built from tiny building blocks into a fully functioning living being.
Remember this mnemonic: AMOCTOSO
Atom, Molecule, Organelle, Cell, Tissue, Organ, System, Organism
The Eight Levels of Organization
Atom
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| Image from Primary Science Online |
An atom is the smallest unit of matter. Atoms are the basic building blocks of everything in the universe. Examples include carbon atoms, oxygen atoms, and nitrogen atoms. Everything, both living and non-living, is made of atoms.
- Atoms are tiny pieces that make up everything.
Molecule
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| Image from Energy Encyclopedia |
A molecule is when two or more atoms join together. Water is a molecule made of two hydrogen atoms and one oxygen atom. Glucose (sugar) is a molecule made of carbon, hydrogen, and oxygen atoms.
- Molecules are when atoms stick together.
Organelle
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| Image from Biology Online |
Organelles are tiny structures inside cells that have special jobs. Think of organelles like the organs of the cell. The nucleus is an organelle that holds the cell's DNA. The mitochondria is an organelle that produces energy. The ribosome is an organelle that makes proteins.
- Organelles are like tiny organs inside cells that do different jobs.
Cell
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| Image from Biology Online |
A cell is the basic unit of life. All living things are made of one or more cells. Some organisms are just one cell, like bacteria. Other organisms, like humans, are made of trillions of cells working together.
- Cells are the smallest living units. They are the building blocks of all life.
Tissue
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| Image from Human Biology |
Tissue is a group of cells that work together and do the same job. For example, muscle tissue is made of muscle cells that work together to create movement. Nerve tissue is made of nerve cells that work together to send signals.
- Tissue is when many cells of the same type work together.
Organ
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| Image from Adobe Stock |
An organ is a group of different tissues that work together to do a specific job. The heart is an organ made of muscle tissue, nerve tissue, and blood vessel tissue, all working together to pump blood. The brain is an organ made of nerve tissue that controls thinking and movement.
- An organ is when different tissues work together to do an important job.
System
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| Image from Biology Online |
A system is a group of organs that work together to do a major job in the body. The circulatory system includes the heart, blood vessels, and blood, all working together to move blood throughout the body. The nervous system includes the brain, spinal cord, and nerves, all working together to control the body.
- A system is when many organs work together to do a big job for the body.
Organism
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| Image from Microbe Notes |
An organism is a complete living thing made of all the systems working together. A human is an organism. A dog is an organism. A tree is an organism. Each organism is a complete, independent living thing.
- An organism is a whole living thing made of all the parts working together.
How the Hierarchy Works Together
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| Image from Easy Peasy |
The hierarchy shows how life is organized from smallest to largest:
Atoms → Molecules → Organelles → Cells → Tissues → Organs → Systems → Organism
Example: A Human's Heart
Atoms (carbon, oxygen, hydrogen) combine to make molecules (proteins, glucose). Molecules combine to make organelles inside cells. Organelles work together to make heart muscle cells. Heart muscle cells combine to make heart muscle tissue. Heart muscle tissue combines with other tissues (nerve tissue, blood vessel tissue) to make the heart organ. The heart organ works with blood vessels to make the circulatory system. The circulatory system works with all other systems to keep the whole organism (human) alive.
We stop at the level of the organism. Why? Because these are the topics that most often appear in the LET examination. Although there are additional levels: Population, Species, Community, Ecosystem, and Biosphere. They are not the main focus of GenEd Science in the LET. However, even if there is only a small chance that they might appear, it is still good to be familiar with them.
Here is a video I recommend watching. It’s complete and will help deepen your understanding.
Hierarchy of Life Quiz: click here
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