Cell Structures: Functions and Optical Visibility (Onion Cells vs. Yeast Cells)

This page summarizes all major cell structures, what they do, and whether they can be observed under a standard optical microscope.
Comparisons are made using:

• Onion epidermal cells (large plant cells, easy for beginners)
• Yeast cells (Saccharomyces cerevisiae, very small single-celled fungi)

1. Cell Membrane (Plasma Membrane)

Function:
A lipid bilayer that controls what enters and leaves the cell; maintains homeostasis.

Optical visibility:
- Onion: Barely visible. You usually see the cell wall, not the membrane itself.
- Yeast: Not visible. Too small and closely apposed to the cell wall.

2. Cell Wall (plants, fungi, bacteria — but different compositions)

Function:
Rigid outer layer providing structure and protection.
- Plants → cellulose
- Yeast → glucans + mannoproteins

Optical visibility:
- Onion: Yes, very visible. Thick outline around each cell.
- Yeast: Yes, faintly visible as the border of each yeast cell.

3. Nucleus

Function:
Contains the genetic material (DNA). Coordinates gene expression and cell division.

Optical visibility:
- Onion: Yes, with staining (e.g., iodine, methylene blue). Appears as a large round region.
- Yeast: No under standard light microscopy; the cell is too small. Requires fluorescence or high-resolution techniques.

4. Nucleolus

Function:
Produces ribosomal RNA (rRNA). Dense region inside the nucleus.

Optical visibility:
- Onion: Sometimes, if strongly stained.
- Yeast: Not visible with light microscopy.

5. Cytoplasm

Function:
Gel-like interior containing water, ions, proteins, and organelles.

Optical visibility:
- Onion: Indirectly visible as the uniform interior of the cell.
- Yeast: Visible only as a general “filled” appearance; no structures discernible.

6. Vacuole (in plant/yeast cells)

Function:
Large storage compartment for water, nutrients, pigments, and waste. Maintains cell pressure (turgor).

Optical visibility:
- Onion: Yes. The vacuole is huge in plant cells, often occupying most of the cell.
- Yeast: Sometimes visible as a clear bubble inside the cell, but very small.

7. Mitochondria

Function:
Powerhouses of the cell; produce ATP through cellular respiration.

Optical visibility:
- Onion: No. Typically below 1 µm, below light microscope resolution.
- Yeast: No. Too small; require fluorescence dyes or TEM.

8. Endoplasmic Reticulum (ER)

Smooth ER & Rough ER

Function:
- Rough ER → protein synthesis
- Smooth ER → lipid synthesis and detoxification

Optical visibility:
- Onion: Not visible under standard light microscopy.
- Yeast: Not visible.

9. Golgi Apparatus

Function:
Processes, modifies, and ships proteins and lipids.

Optical visibility:
- Onion: Not visible with optical microscopy.
- Yeast: Not visible.

10. Ribosomes

Function:
Molecular machines that build proteins (translation).

Optical visibility:
- Onion: Not visible (20–30 nm).
- Yeast: Not visible.

Light microscopy cannot resolve objects this small.

11. Chloroplasts (in plant cells only)

Function:
Perform photosynthesis.

Optical visibility:
- Onion: No chloroplasts in onion epidermal cells (only in green tissues).
- Yeast: Not applicable.

(If you observe a green leaf instead, chloroplasts are extremely visible — bright green dots.)

12. Lysosomes / Peroxisomes

Function:
Waste digestion (lysosomes) and detoxification (peroxisomes).

Optical visibility:
- Onion: Not visible under standard optical microscopy.
- Yeast: Not visible.

13. Cytoskeleton (microtubules, microfilaments, intermediate filaments)

Function:
Maintains cell shape, enables movement, organizes organelles.

Optical visibility:
- Onion: Not visible with light microscopy.
- Yeast: Not visible.

Requires fluorescence labeling.

Summary Table