Eukaryotic Cells Under the Microscope

Light (Optical) Microscopy

• Mechanism: Uses visible light and glass lenses to focus an image. It is suitable for viewing living specimens in color and is relatively inexpensive.
• Capabilities: Can resolve whole cells, nuclei, and larger organelles like mitochondria and chloroplasts, but cannot see internal organelle details like ribosomes.

Electron Microscopy (EM)

• Transmission Electron Microscope (TEM): Electrons pass through a thin specimen. Denser parts absorb more electrons and appear darker. It produces high-resolution 2D images of internal cell ultrastructure.
• Scanning Electron Microscope (SEM): Electrons bounce off the surface of a specimen coated in metal. It produces 3D images of the surface topography but has a lower resolution than TEM.

• The Transparency Problem: Most biological tissues are naturally transparent to light and electrons, making them difficult to see without contrast enhancement.

• Light Microscopy Stains: Colored dyes are used to bind to specific structures. For example, Toluidine blue turns cells blue, while Phloroglucinol stains lignin red/pink. Differential staining involves using multiple dyes to distinguish different tissues.

• Electron Microscopy Stains: Specimens are treated with heavy metal compounds (e.g., Osmium tetroxide). These metals absorb or scatter electrons, creating the contrast needed to see membranes and organelles in grayscale.

• Mitochondria: Identified by their double membrane and inner folds called cristae. They are just visible under high-power light microscopes but require EM for internal detail.

• Chloroplasts: Large green organelles in light microscopy; EM reveals internal stacks of membranes called grana.

• Identify the Microscope: If the image is in color or shows living movement, it is a light microscope. If it is high-resolution grayscale showing internal organelle membranes, it is a TEM. If it shows a 3D surface, it is an SEM.

• Unit Consistency: Always convert all measurements to the same units (usually micrometers $\mu m$) before using the magnification formula. Remember: $1\ mm = 1000\ \mu m$ and $1\ \mu m = 1000\ nm$.

• Biological Drawings: When asked to draw from a microscope, use clear, single lines with no shading. Ensure proportions are accurate and label lines are straight, drawn with a ruler, and do not have arrowheads.