1.04 Units of Measurement and Organelle Sizes

Importance of Small Units in Microscopy

• In microscopy, measuring cellular structures requires extremely small units.
• The International System of Units (SI units) is used worldwide, with the basic length unit being the metre (m).

Metric Prefixes and Conversions

• SI units use standard prefixes to denote multiples or fractions of a base unit.
• Example: kilo- (1000 times), so 1 kilometre = 1000 metres.

Units of Length Relevant to Cell Studies

• Millimetre (mm): One thousandth of a metre (10⁻³ m).
• Micrometre (μm): One millionth of a metre (10⁻⁶ m); a thousandth of a millimetre.
• Nanometre (nm): One billionth of a metre (10⁻⁹ m); a thousandth of a micrometre.

Organelle Sizes and Visibility Under a Light Microscope

Cell Wall

• Size: 1 – 10 μm (varies with cell type)
• Present In: Plant Cells, Fungi, Some Protists
• Visibility: Yes. Appears as a rigid, outer layer surrounding the cell membrane in plant cells. Typically stained (e.g., with iodine) to enhance visibility under a bright-field microscope.

Vacuole

• Size: 5 – 30 μm
• Present In: Primarily Plant Cells; small in Animal Cells
• Visibility: Yes. In plant cells, a large central vacuole is often visible as a clear or lightly stained area occupying a significant portion of the cell. In animal cells, smaller vacuoles are less prominent and may require specific staining.

Nucleus

• Size: 6 – 10 μm
• Present In: Both Plant and Animal Cells
• Visibility: Yes. The nucleus is usually the most prominent organelle, visible as a large, round or oval structure with a darker region (nucleolus) when stained with dyes like hematoxylin.

Chloroplast

• Size: 4 – 10 μm
• Present In: Plant Cells
• Visibility: Yes. Chloroplasts are visible as green, lens-shaped structures due to the presence of chlorophyll. They can be seen in high concentrations in leaf cells under a light microscope, especially when stained with specific dyes.

Flagella/Cilia

• Size: 5 – 20 μm (Flagella), ~0.2 μm diameter
• Present In: Both Plant and Animal Cells (Flagella in some plant cells like sperm cells; Cilia primarily in animal cells)
• Visibility: Yes (Flagella). Flagella appear as long, whip-like appendages moving in a wave-like manner. Cilia are shorter, more numerous, and beat in coordinated patterns, primarily observed in animal cells like epithelial cells lining the respiratory tract.

Golgi Apparatus

• Size: 1 – 2 μm
• Present In: Both Plant and Animal Cells
• Visibility: Sometimes. When stained, the Golgi apparatus may appear as a series of stacked, flattened sacs (cisternae) near the nucleus, but it can be challenging to distinguish without specific markers.

Endoplasmic Reticulum (ER)

• Size: 0.1 – 1 μm (individual tubules and sheets)
• Present In: Both Plant and Animal Cells
• Visibility: Sometimes. Rough ER can appear as a network of interconnected tubules with ribosomes (giving a “rough” appearance), while Smooth ER lacks ribosomes and appears smoother. Visibility depends on the cell type and staining methods.

Mitochondria

• Size: 0.5 – 1 μm
• Present In: Both Plant and Animal Cells
• Visibility: Rarely. Typically not visible under standard light microscopes due to their small size and lack of distinctive staining. Electron microscopy is required for clear visualization.

Centrosome/Centriole

• Size: 0.5 – 1 μm
• Present In: Animal Cells
• Visibility: Rarely. Difficult to visualize under light microscopy. May appear as small, dot-like structures during specific cell cycle stages if stained with appropriate markers.

Lysosome

• Size: 0.1 – 1.2 μm
• Present In: Animal Cells
• Visibility: No. Generally not visible under light microscopes. Function in breaking down waste materials and cellular debris.

Ribosome

• Size: 0.025 μm (25 nm)
• Present In: Both Plant and Animal Cells
• Visibility: No. Non-membrane-bound organelles responsible for protein synthesis. Visible only with electron microscopy or specialized fluorescence techniques.

Cytoplasm

• Size: Variable (entire cell excluding nucleus and organelles)
• Present In: Both Plant and Animal Cells
• Visibility: Yes. The cytoplasm appears as a gel-like substance filling the cell, providing a medium for organelles to reside and facilitating cellular processes. Organelles are suspended within the cytoplasm and can be seen as distinct structures when stained.

Cytoskeleton

• Size: Variable (fibers range from nanometers to micrometers)
• Present In: Both Plant and Animal Cells
• Visibility: No under standard light microscopy. Composed of microtubules, actin filaments, and intermediate filaments, which provide structural support and facilitate movement within the cell. Visible with fluorescence microscopy using specific dyes.

Microvilli

• Size: 0.1 – 1 μm in diameter; up to 1-2 μm in length
• Present In: Animal Cells (especially epithelial cells)
• Visibility: No under standard light microscopy. These finger-like projections increase surface area for absorption and secretion, particularly in intestinal and kidney cells. Visible with electron microscopy.

Plasmodesmata

• Size: <1 μm
• Present In: Plant Cells
• Visibility: No under light microscopy. Microscopic channels that traverse the cell walls of plant cells, facilitating transport and communication between them. Visible with electron microscopy.

Table of Relevant Units:

Note: μ\muμ represents the Greek letter “mu”; 1 micrometre = 1/1000 of a millimetre, and 1 nanometre = 1/1000 of a micrometre.

Conversion Examples