Cell Study Methods and Cell Types

Microscopy and Cell Study

Principles of Microscopy

  • Light Microscope (LM): Uses visible light, magnifies via glass lenses bending light. Effective magnification up to ≈1000×\approx 1000 \times.
  • Three Parameters:
    • Magnification: Ratio of image size to real size.
    • Resolution: Clarity of image; minimum distance to distinguish two points (≈0.2μm\approx 0.2 \mu m or 200nm200 nm for LM). Inversely related to wavelength.
    • Contrast: Difference in brightness, enhanced by staining.
  • Electron Microscope (EM): Uses electron beam for imaging. Introduced 1950s. Higher resolution than LM (≈2nm\approx 2 nm in practice). Specimen preparation often kills cells.
    • Scanning Electron Microscope (SEM): Studies specimen surface topography, produces 3-D image.
    • Transmission Electron Microscope (TEM): Studies internal cell structures, beams electrons through thin sections.
    • Cryo-electron microscopy (cryo-EM): Freezes specimens at ultra-low temperatures ( < -160^
      \circ C ) to preserve structure without chemical fixatives, allowing visualization in cellular environment.
  • Advanced Light Microscopy:
    • Fluorescent markers, confocal, and deconvolution microscopy for sharper 3D images.
    • Super-resolution microscopy: Breaks resolution limit, distinguishing structures as small as 10−20nm10-20 nm.

Cell Fractionation

  • Technique: Separates cell components by size and density using a centrifuge.
  • Process: Cells homogenized, then undergo differential centrifugation at increasing speeds, forming successive pellets of different organelles (e.g., nuclei, mitochondria, ribosomes).
  • Purpose: Enables the study of isolated organelle functions and correlation of cell structure with biochemical processes.

Prokaryotic vs. Eukaryotic Cells

Basic Cell Features

  • All cells possess: plasma membrane, cytosol, chromosomes (DNA), and ribosomes.

Key Differences

  • DNA Location:
    • Eukaryotic: DNA in a membrane-bound nucleus. Cytoplasm is region between nucleus and plasma membrane.
    • Prokaryotic: DNA in non-membrane-enclosed nucleoid region.
  • Organelles:
    • Eukaryotic: Contains membrane-bound organelles.
    • Prokaryotic: Lacks membrane-bound organelles.
  • Size:
    • Eukaryotic: Typically 10−100μm10-100 \mu m in diameter.
    • Prokaryotic: Typically 1−5μm1-5 \mu m in diameter.
  • Surface Area to Volume Ratio:
    • Metabolic demands limit cell size.
    • Smaller cells have a higher surface area/volume\text{surface area} / \text{volume} ratio, which is crucial for efficient nutrient, oxygen, and waste exchange.
    • Surface area is proportional to (linear dimension)2^2; volume is proportional to (linear dimension)3^3.