Introduction to Cells and Microscopy

The hierarchy of biological organization ranges from molecules to organelles, cells, tissues, organs, and organ systems.
Cells represent the fundamental unit of structure and function in all organisms.
Cell biology explores the interaction of molecules, structure, function, and processes.
All cells are enclosed by a membrane and utilize $DNA$ as genetic information.
Reproduction, growth, and repair in organisms are based on cell division; all cells arise from existing cells.

Robert Hooke (1665): First used the term "cells" after observing the cell walls of dead cork tissue through a microscope, noting their resemblance to monastery rooms.
van Leeuwenhoek (late 1600s): First to observe living cells, which he called "animalcules."
Lorenz Oken (1805): Stated all living things consist of vesicles or cells.
Matthias Schleiden and Theodor Schwann (1839): Established cells as the basic unit of structure and function.
Rudolf Virchow (1859): Added that all cells originate from other cells.
Modern Cell Theory Summary:
- All organisms are composed of one or more cells.
- The cell is the smallest living thing and the basic unit of organization.
- Cells arise only from the division of previously existing cells.

́Angstrm ( $\text{}$ ): $10^{-10} m$ or $0.1 nm$ . Named after Anders Jonas ́Angstrm.
Micrometer (́́̂m): $10^{-6} m$ .
Nanometer ( $nm$ ): $10^{-9} m$ .

Metabolic logistics limit cell size.
The surface area-to-volume ratio ( $SA:V$ ) is critical; as a cell grows, volume increases by a factor of $n^{3}$ while surface area increases only by $n^{2}$ .
Smaller cells maintain a higher $SA:V$ ratio, facilitating efficient exchange of oxygen, nutrients, and waste.

Magnification: Ratio of image size to real size; effective up to $1,000 \times$ (limit of usefulness is approx. $1,400 \times$ ).
Resolution: Measure of clarity. It is limited by the wavelength (́̃) of light; points closer than 1/2 ́̃ cannot be resolved.
Contrast: Visible differences in sample parts.
LM Techniques:
- Brightfield (Unstained/Stained): Passes light directly through; staining improves contrast but often kills cells.
- Phase-Contrast: Enhances density variations; useful for living, unpigmented cells.
- Differential-Interference-Contrast (Nomarski): Uses density differences for a $3́D$ appearance.
- Fluorescence: Labels specific molecules with fluorescent dyes that emit visible light under $UV$ .
- Confocal: Uses a pinhole to eliminate out-of-focus light, providing sharp "optical sections" for $3́D$ reconstruction.

Increases resolving power by over $10,000 \times$ compared to LM using electron beams.
Scanning Electron Microscope (SEM): Coats surfaces with gold to provide $3́D́looking$ images of the specimen exterior.
Transmission Electron Microscope (TEM): Uses heavy metal stains and thin sections to study internal ultrastructure.
Limitations: Specimen preparation (vacuum, fixing, staining) can introduce artifacts and kills the specimen.

A process used to separate and purify organelles based on size and density while preserving function.
Homogenization: Breaking up cells to create a homogenate.
Differential Centrifugation: Spinning the homogenate at increasing speeds (expressed in $g$ force):
- Low speed ( $1,000 g$ ): Pellets nuclei and debris.
- Medium speed ( $20,000 g$ ): Pellets mitochondria and chloroplasts.
- High speed ( $80,000 g$ ): Pellets microsomes (membrane fragments).
- Very high speed ( $150,000 g$ ): Pellets ribosomes.

Question: Why does cork float?
Discussion Points/Tasks:
- How many $nm$ are in $1 mm$ ?
- How many $nm$ are in 1 ́́̂m?
- How does the molecular makeup of organelles influence their structure and function?