Chapter 6 A Tour of the Cell

All organisms are composed of cells

What is a living cell . . . . . . self-contained, self assembling, self adjusting, self perpetuating, isothermal mix of biomolecules, held in a 3-D conformation by weak non-covalent forces, which extract raw materials (precursors) and free energy from the surrounding environment and which shows great efficiency and economy of metabolic regulation and that maintains a dynamic steady state far from equilibrium and which can self-replicate using the informational molecule DNA.

Cell A collection of living matter enclosed by a barrier that separates the cell from its surroundings; Basic Unit of all forms of life; Smallest unit that displays the properties of life; Composed of cytoplasm and surrounded by a plasma membrane.

CELL THEORY (1838) All life forms are made from one or more cells. Theodor Schwann Matthias Schleiden All life forms are made from one or more cells. The cell is the smallest form of life. Cells only arise from pre-existing cells.

CELL THEORY Rudolph Virchow (1855) Summarized the cell theory with the Latin phrase omnis cellula a cellula (all cells arise from cells) in 1855. Cells are capable of self-reproduction; (e.g. only from preexisting cells.

Plasma Membrane The interface boundary for intra and extracellular exchange of materials.

Cell Size 1 mm (frog’s egg) - 1 m (1/1000th mm.) Determined by membrane transfer characteristics. As a cell get larger in volume, relative surface area (and relative capability for material exchange) decreases. Cells needing higher exchange rates have evolved to develop modifications such as folding, microvilli, multiple nucleii, long skinny shape, etc.

Surface-area-to volume ratio Take Home Lesson Surface-area-to volume ratio Limits how large an actively metabolizing cells can become. Requires that cells be small.

Reasons for Small Cell Size As linear dimensions of a cell increase, the volume increases twice as fast as the surface area. Nutrients and wastes must enter and leave the cell through the plasma membrane. The nucleus can only control a certain amount of cytoplasm. Materials must be able to move quickly and efficiently through the cytoplasm.

Microscopy Magnification Making something larger Function of wavelengths; shorter wavelengths of light or electrons allow greater magnification. Resolution Minimum distance between two objects before they are seen as one larger object.

Types of Microscopy Bright-Field (Compound) Light Microscope Use light rays focused by glass lenses.

Types of Microscopy Dark-field Uses angular light reflected off specimen.

Types of Microscopy Phase-contrast Differential-interference contrast Light is out of phase and gives different contrast when brought back together. Differential-interference contrast Light is out of phase and combined with other light.

Types of Microscopy Fluorescence Confocal Uses fluorescent antibodies to reveal protein in cells. Confocal Uses laser beam to focus on shallow plane; forms series of optical sections.

Transmission Electron Microscope (TEM) Electrons pass through specimens; focused by magnets. Used when internal detail is critical

Scanning Electron Microscope Uses electrons scanned across metal-coated specimen. Used when external detail is critical.

SEM vs. TEM

Let’s view some examples. Microscopy Let’s view some examples.

Use of the light microscope. Magnification and Field of View.

Review List the three principals of the cell theory. Define the term “cell” Explain the use of and differences between the compound light, transmission electron and scanning electron microscopes with respect to their ability to increase contrast. Define resolution and magnification. Why is the size of a cell dependent on the cell-volume-to-cell-surface ratio.