What you need to know
- State that cells are the fundamental units of living organisms.
- Put cell, tissue, organ, organ system and organism in the correct order.
- Distinguish unicellular from multicellular organisms.
- Compare eukaryotic and prokaryotic cells.
- Explain why viruses are not cells.
The cell idea
A cell is the smallest structural and functional unit of a living organism. It is structural because organisms are built from cells, and functional because essential life processes take place inside cells.
Living organisms are made of cells, cells are the basic units of life, and new cells arise from existing cells.
A unicellular organism, such as a bacterium or yeast cell, consists of one cell. That single cell must carry out every process needed for survival. A multicellular organism contains many cells. Its cells can specialise and cooperate, making larger and more complex organisms possible.
Unicellular
- One cell is the whole organism.
- The cell carries out all life processes.
- Examples include bacteria and yeast.
Multicellular
- Many cells work together.
- Cells can become specialised.
- Examples include animals and plants.
Levels of biological organisation
In a multicellular organism, specialised cells are organised into increasingly large structures.
For example, a muscle cell is part of muscle tissue. Several tissues form the heart. The heart is one organ in the circulatory system, which is part of the human organism.
An organ contains different tissues working together. A tissue contains a group of cells with a similar structure and function.
Eukaryotic and prokaryotic cells
Cells can be placed into two broad groups. Eukaryotic cells contain genetic material enclosed inside a nucleus. Animals, plants, fungi and protists are eukaryotes. Prokaryotic cells are smaller and simpler; their main loop of DNA is free in the cytoplasm rather than enclosed inside a nucleus. Bacteria are prokaryotes.
| Feature | Eukaryotic cell | Prokaryotic cell |
|---|---|---|
| Nucleus | Present; contains the main genetic material | Absent |
| DNA | Linear chromosomes inside the nucleus | A circular DNA loop in the cytoplasm; plasmids may also occur |
| Size | Usually larger | Usually much smaller |
| Membrane-bound organelles | May contain mitochondria and, in plants, chloroplasts | Absent |
| Examples | Animal, plant, fungal and protist cells | Bacterial cells |
Misconception AlertProkaryotic cells have no DNA.Select to reveal the correctionSelect to hide the correction
Bacterial cells contain DNA, but their main DNA loop is not enclosed inside a nucleus.
Why viruses are different
A virus is not a cell. It consists of genetic material surrounded by a protein coat, and some viruses also have a lipid envelope. Viruses contain no cytoplasm, ribosomes or organelles and cannot carry out respiration or protein synthesis independently.
A virus reproduces only after entering a suitable host cell. It uses the host cell's machinery to copy viral genetic material and make new virus particles. For this reason, viruses are generally not described as living organisms.
A living cell that a virus enters and uses to produce new virus particles.
Quick retrieval check
1. What is the smallest structural and functional unit of life?
2. Put tissue, organ, cell, organism and organ system in order.
3. Give one difference between a prokaryotic and a eukaryotic cell.
4. Why can a virus not reproduce independently?
Exam connection
A student says, “Bacteria and viruses are both simple cells because neither has a nucleus.” Evaluate the statement.
Show the mark points
- Bacteria are cells and are prokaryotic.
- A bacterial cell has cytoplasm, a cell membrane and ribosomes, but no nucleus.
- Viruses are not cells.
- Viruses lack cytoplasm and ribosomes and reproduce only inside host cells.
Command word: Evaluate means consider the evidence in the statement and reach a supported judgement. Here the statement is partly correct about nuclei but incorrect overall.