The Invisible Living World: Beyond Our Naked Eye · Lesson 8 of 9
Why the Cell Is the Basic Unit of Life
“Bring plants, animals and microbes together through the idea of the cell.”
• Explain why one cell can be an organism while many cells can cooperate as one organism. • Compare yeast and mould as one-celled and many-celled fungi. • Compare bacterial, fungal, plant and animal cell features. • Explain what the nucleoid shows about a bacterial cell.
One cell can be a whole organism
An Amoeba or a bacterium can be an entire organism made of one cell. That cell must carry out all the activities needed to stay alive. Plants and animals usually have many cells, with specialised cells cooperating to keep the organism alive. In either case the cell is the smallest living building unit used to understand the body. An organism is not defined by how large it looks to us.
Made of a single cell that carries out the organism’s essential life processes.
Made of many cells, which may perform different functions and work together.
Problem
Both are fungi. Why can one be called unicellular and the other multicellular?
- 1.Count the cells, rather than choosing by the group name alone.
- 2.A yeast organism is one cell in the chapter example; filamentous mould is made of many cells.
Cell coverings and inner regions
The cells of plants, animals, fungi and bacteria have a cell membrane. Plant, fungal and bacterial cells also have a wall outside that membrane, while the typical animal cell in this chapter does not. A fungal cell has no chloroplast and cannot make its own food by photosynthesis. Bacteria lack a well-defined nucleus surrounded by a nuclear membrane. Their genetic material occupies a region called the nucleoid. Yeast, other fungi, protozoa, algae, plants and animals are contrasted with bacteria on this feature.
The region containing a bacterium’s genetic material, not enclosed by a nuclear membrane.
| Feature | Plant cell | Animal cell | Fungal cell | Bacterial cell |
|---|---|---|---|---|
| Cell membrane | Yes | Yes | Yes | Yes |
| Cell wall | Yes | No | Yes | Yes |
| Well-defined nucleus | Yes | Yes | Yes | No; nucleoid region |
| Chloroplasts | In many green cells | No | No | No |
The table is a teaching comparison of the cells discussed here. A plant’s non-green cells may not have chloroplasts. The earlier onion-peel slide illustrated this caution. A microscope’s resolving power also matters: an ordinary school microscope shows only some structures; a powerful electron microscope can reveal much finer detail.
Problem
A cell has a membrane, wall and nucleoid but no defined nucleus. Which group best matches?
- 1.A wall alone cannot separate plant, fungal and bacterial cells.
- 2.The nucleoid without a membrane-bound nucleus points to a bacterium.
Problem
A student says every cell with a wall must be a plant cell.
- 1.The table shows that fungi and bacteria can also have walls.
- 2.A cell wall is useful evidence, but other structures are needed to classify the cell.
A bacterium’s nucleoid is not surrounded by a nuclear membrane. Calling it a conventional, well-defined nucleus erases the distinction the chapter is teaching.
What does “basic unit” mean?
The idea unifies the chapter. A microscope makes cells observable; cells have working parts; their shapes suit tasks; they can cooperate as tissues and organs; and tiny microbes can themselves be single cells or groups of cells. Viruses were described separately because they are acellular and multiply inside host cells. These relationships explain why the cell is a powerful starting point for studying living organisms.
Check your understanding
Compare cells by several features together, and always separate the number of cells in an organism from the structures inside one cell.
Quiz
Which example is a unicellular fungus?
What distinguishes the bacterial cell described from a typical cheek cell?
Which listed cell types have walls?
Why are viruses treated separately from cells?
What does multicellular mean?
Practice Problems
- Explain how one bacterium can be an organism, while one human muscle cell is part of an organism.
- Compare yeast and bread mould using cell number and their shared fungal group.
- Place membrane, cytoplasm, nucleus, wall, chloroplast and nucleoid in an appropriate plant–animal–bacterial comparison.
- Why is a cell wall alone insufficient to identify a plant cell?
- Explain the difference between a bacterial nucleoid and a well-defined nucleus.
- Use three earlier lessons to defend the statement that the cell is the basic unit of life.
Key Takeaways
• All living organisms studied here are made of one or more cells; viruses are acellular. • A unicellular organism performs life functions in one cell, while multicellular organisms use cooperating cells. • Yeast is a unicellular fungus; mould is multicellular. • Plant, fungal and bacterial cells have walls, but a typical animal cell does not. • Bacteria have a nucleoid rather than a well-defined membrane-bound nucleus.