Biologists Use Microscopes and Biochemistry to Study Cells

Invention and Early Refinements:
- Microscopes were first invented in $1590$ and underwent further refinements during the $1600\text{s}$ .
- The development of instruments that extend human senses has progressed alongside scientific advancements.
Pioneering Cell Biologists:
- Robert Hooke ( $1665$ ): First observed cell walls while examining dead cells from the bark of an oak tree under a microscope.
- Antoni van Leeuwenhoek: Crafted high-quality lenses that enabled the first visualization of living cells.
- Hooke's Visit ( $1674$ ): Hooke visited van Leeuwenhoek in $1674$ , uncovering the microscopic world of microorganisms, which van Leeuwenhoek termed "very little animalcules."

Mechanism of Light Microscopes (LM):
- Visible light is passed through a specimen and then through glass lenses.
- Glass lenses refract (bend) the light in a manner that magnifies the specimen's image as it is projected into the eye or camera.
Three Core Parameters in Microscopy:
1. Magnification:
- The ratio of an object's image size to its actual size.
- Standard light microscopes can magnify effectively up to approximately $1{,}000\times$ the actual size of the specimen.
- Magnifications beyond $1{,}000\times$ fail to reveal additional detail clearly.
1. Resolution:
- A measure of image clarity; defined as the minimum distance two points can be separated and still be distinguished as separate points.
- Analogy: An object appearing as a single star to the unaided eye can be resolved as twin stars when viewed through a telescope with higher resolving power.
- Using standard techniques, light microscopy cannot resolve detail finer than about $0.2\,\mu\text{m}$ ( $200\,\text{nm}$ ), regardless of magnification.
1. Contrast:
- The difference in brightness between light and dark areas of an image.
- Enhanced in light microscopy by staining or labeling specific cell components to make them visually distinct.

Metric Units and Conversions:
- $1\,\text{centimeter (cm)} = 10^{-2}\,\text{meter (m)} = 0.4\,\text{inch}$
- $1\,\text{millimeter (mm)} = 10^{-3}\,\text{m}$
- 1\,\text{micrometer (\mu m)} = 10^{-6}\,\text{m}
- $1\,\text{nanometer (nm)} = 10^{-9}\,\text{m}$
Size Ranges Across Observation Domains:
- Unaided Human Eye ( $\sim 100\,\mu\text{m}$ to $10\,\text{m}$ ):
- Human height: $1\text{--}2\,\text{m}$
- Length of certain nerve and muscle cells: up to $1\,\text{m}$
Cryo-Electron Microscopy (cryo-EM):
- A specialized TEM technique where specimens are frozen rapidly at extremely low temperatures.
- Prevents the need for chemical preservatives, allowing visualization of structures in their natural cellular environment.
- Complements X-ray crystallography to determine the structures of protein complexes and subcellular machinery (such as ribosomes), and can resolve individual proteins.

Integrating Disciplines:
- Cytology: The study of cell structure.
- Biochemistry: The study of chemical processes (metabolism) within cells.
- Full understanding of cell function requires combining cytological observations with biochemical analysis.
Cell Fractionation Procedure:
- A technique used to separate cell components in bulk based on size and density:
1. Cells are broken up (homogenized) and placed in a centrifuge tube.
2. Centrifugation at a given speed causes the largest cellular components to settle at the bottom as a pellet.
3. The liquid above the pellet (supernatant) is transferred to a new tube and spun at a higher speed for a longer duration.
4. Sequential repetition at increasing speeds yields pellets containing specific components:
  - Low speed: Nuclei and larger structures.
  - Medium speed: Mitochondria (and chloroplasts if from photosynthetic cells).
  - Higher speed: Membrane fragments.
  - Very high speed: Ribosomes (the smallest components).
Function Identification Example:
- Fractionation enabled isolation of specific cellular parts in bulk.
- Biochemical tests on isolated organelle fractions revealed high concentrations of enzymes involved in cellular respiration.
- Electron microscopy confirmed the abundance of organelles called mitochondria in the exact same fractions.
- Joint evidence established that mitochondria are the primary sites of cellular respiration.

Question 1: How do stains used for light microscopy compare with those used for electron microscopy?
- Response: Stains for light microscopy consist of colored dyes or fluorescent labels that absorb or emit light. Stains for electron microscopy rely on heavy metal atoms (such as gold or osmium) that attach to cellular structures and scatter electrons to create contrast.
Question 2: Which type of microscope would you use to study (a) the changes in shape of a living white blood cell and (b) the details of surface texture of a hair?
- Response:
- (a) Light microscope: Electron microscopy preparation kills cells, making light microscopy (such as phase-contrast or live-cell imaging) required to observe living cells dynamic in shape.
- (b) Scanning electron microscope (SEM): Ideal for rendering high-resolution, three-dimensional views of surface topographies.