1. What Is a Cell?
A cell is the smallest unit of life that can carry out all the processes needed to stay alive — things like getting energy, growing, and reproducing. Every living thing you can think of, from a blade of grass to a blue whale, is made up of cells.
The Cell Theory (3 Key Ideas)
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells come from existing cells (through a process called cell division).
Animal Cells vs Plant Cells
Both animal and plant cells share some structures, but plant cells have a few extras that animal cells don't.
2. Unicellular Organisms
What Does "Unicellular" Mean?
Unicellular organisms (also called single-celled organisms) are living things made up of just one cell. That single cell has to do everything (move, feed, breathe, get rid of waste, and reproduce) without help from other specialised cells.
This is different from multicellular organisms (like humans, dogs, and trees), which are made of millions or trillions of cells working together, each with a specific job.
Examples of Unicellular Organisms
Image of Some Unicellular Organisms
How Do Unicellular Organisms Survive With Just One Cell?
Even though they're made of only one cell, unicellular organisms are fully independent living things. Their single cell is specially adapted to:
- Take in nutrients directly from their environment
- Exchange gases (like oxygen and carbon dioxide) across their cell membrane
- Reproduce quickly, often by splitting in two (binary fission)
- Respond to their surroundings, such as moving toward food or away from danger
Unicellular vs Multicellular: Quick Comparison
3. Levels of Organisation in Living Things
In multicellular organisms, cells don't work alone — they're organised into increasingly complex structures. This is known as the levels of organisation.
The Five Levels (From Simplest to Most Complex)
Cell → Tissue → Organ → Organ System → Organism
1. Cell
The basic building block of life. Example: a muscle cell.
2. Tissue
A group of similar cells working together to perform a specific function. Example: muscle tissue is made of many muscle cells grouped together to allow movement.
3. Organ
A structure made of different tissues working together to perform a particular job. Example: the heart is an organ made of muscle tissue, nervous tissue, and connective tissue, all working together to pump blood.
4. Organ System
A group of organs working together to carry out a major body function. Example: the circulatory system includes the heart, blood vessels, and blood, all working together to transport substances around the body.
5. Organism
The whole living thing, made up of all its organ systems working together. Example: a human being, complete with a circulatory system, digestive system, nervous system, and more.