Cell: The Building Block of Life · Lesson 4 of 9
The Cell Interior — A Coordinated Working System
“Three cell designs enter the microscope, and their internal layouts tell the whole story.”
• Identify the basic internal components shared by cells. • Compare bacterial, plant and animal cells. • Distinguish prokaryotic and eukaryotic cells. • Explain the meaning of nucleoid and membrane-bound organelle. • Interpret structural information using comparison tables and labelled diagrams. • Describe the supporting roles of the cytoskeleton and cell inclusions.
A city cannot function through its boundary wall alone. It also needs working spaces, stored materials, transport routes and a system for coordinating activities. A cell has a similar challenge. Within its membrane lies cytoplasm containing structures that carry out different tasks at the same time.
The Cell Interior
Most cells have three basic components: a selectively permeable plasma membrane, a semi-fluid jelly-like cytoplasm and genetic material. In eukaryotic cells, the genetic material is enclosed in a prominent nucleus, and the cytoplasm contains several specialised membrane-bound organelles.
| Basic component | Description | Central role |
|---|---|---|
| Cell membrane | Thin, selectively permeable boundary | Controls exchange with the surroundings |
| Cytoplasm | Semi-fluid material inside the membrane | Holds cell components and is the site of many activities |
| Genetic material | DNA carrying inherited information | Provides instructions for cell structure and activities |
A specialised structure within a cell that performs a particular function. Some organelles are surrounded by their own membranes.
Activity: Let us study
Study labelled diagrams of a bacterial cell, a plant cell and an animal cell. Do not identify a cell from one feature alone. First locate the boundary, then the genetic material, and finally check for membrane-bound organelles and structures characteristic of plant cells.
| Cell structure | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|
| Cell membrane | Present | Present | Present |
| Cell wall | Present | Present | Absent |
| Cytoplasm | Present | Present | Present |
| Well-defined nucleus | Absent | Present | Present |
| Nucleoid | Present | Absent | Absent |
| Membrane-bound organelles | Absent | Present | Present |
| Chloroplasts | Absent | Present in photosynthetic cells | Absent |
| Large central vacuole | Absent | Usually present in a mature cell | Absent |
A cell without a membrane-bound nucleus or membrane-bound organelles. Its genetic material occupies a region called the nucleoid.
A bacterial cell is prokaryotic. Most of its cellular activities occur directly in the cytoplasm because it does not contain separate membrane-bound compartments. Prokaryotic cells are usually unicellular and commonly have diameters in the range of 1–10 µm.
The region of a prokaryotic cell containing its genetic material. It is not surrounded by a nuclear membrane.
A cell with genetic material enclosed in a membrane-bound nucleus and with membrane-bound organelles in its cytoplasm.
Plant and animal cells are eukaryotic. Their internal compartments allow different processes to occur in specialised locations at the same time. Eukaryotic cells are generally larger and more complex than prokaryotic cells, with typical diameters in the range of 10–100 µm. An organism made of eukaryotic cells may be unicellular or multicellular.
| Characteristic | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Primitive nucleus or nucleoid | Present | Absent |
| Membrane-bound nucleus | Absent | Present |
| Typical diameter | 1–10 µm | 10–100 µm |
| Organisation of organism | Usually unicellular | May be unicellular or multicellular |
| Membrane-bound organelles | Absent | Present |
Problem
An unknown cell has a cell membrane, cytoplasm, ribosomes and circular genetic material in an unenclosed region. It has no mitochondria. Classify it.
- 1.The genetic material is not enclosed by a nuclear membrane, so there is no well-defined nucleus.
- 2.Mitochondria and other membrane-bound organelles are absent.
- 3.The unenclosed genetic region is a nucleoid.
- 4.These features identify a prokaryotic cell.
- 5.The cell is therefore likely to be bacterial rather than plant or animal.
Problem
A cell contains a nucleus, mitochondria, a cell membrane and cytoplasm. A rigid outer layer and a large central vacuole are also visible. Identify the cell type.
- 1.A nucleus and mitochondria establish that the cell is eukaryotic.
- 2.The rigid outer layer is a cell wall.
- 3.The large central vacuole is characteristic of a mature plant cell.
- 4.The cell is therefore a plant cell.
- 5.Chloroplasts need not be visible in every plant cell, because some plant parts do not perform photosynthesis.
Do not say that every structure in a plant cell is absent from an animal cell. Plant and animal cells both contain a cell membrane, cytoplasm, nucleus and several of the same organelles. The cell wall, plastids and usually a large central vacuole provide the clearest differences in the diagrams used here.
Ready to Go Beyond
Eukaryotic cells also contain a network of fine fibres called the cytoskeleton. It provides structural support, helps maintain cell shape and supports movement and internal transport. The cytoplasm may additionally contain stored or deposited materials, such as starch in plant cells or crystals of calcium oxalate or silica in some plant cells. These materials are called cell inclusions.
| Term | What it is | Role or example |
|---|---|---|
| Cytoskeleton | Network of fine fibres | Support, cell shape, movement and internal transport |
| Cell inclusion | Stored or deposited material in cytoplasm | Starch or certain mineral crystals |
Quiz
Which description best matches Organelle?
Which description best matches Prokaryotic cell?
Which term matches this description: A specialised structure within a cell that performs a particular function.
Which term matches this description: A cell without a membrane-bound nucleus or membrane-bound organelles.
Which statement is a key takeaway from this lesson?
Practice Problems
- List the structures shared by bacterial, plant and animal cells in the comparison diagram.
- Why is a bacterial cell described as prokaryotic?
- A cell is 60 µm wide and contains a membrane-bound nucleus. Which broad cell category fits it best, and why?
- Explain why plant and animal cells are placed in the same broad cell category despite their differences.
- Distinguish an organelle from a cell inclusion.
Key Takeaways
• A cell interior contains cytoplasm, genetic material and specialised structures. • Bacterial cells are prokaryotic and contain a nucleoid rather than a membrane-bound nucleus. • Plant and animal cells are eukaryotic and contain membrane-bound organelles. • Plant, animal and bacterial cells share a cell membrane, cytoplasm, ribosomes and genetic material. • Structural comparisons should use several features rather than one feature alone.