Lecture 2: Unity and diversity of cells, cell theory and microscopy

cell theory

� All living organisms are made of one or more cells � Cell is the basic unit of life � Cells are produced only from pre-existing cells

all cells ____ their genetic instructions from a common ____ cell

inherited, ancestral

Basic chemistry of a cell:

�DNA encodes genetic information
�Enclosed by membrane composed of a lipid-bilayer
�Essential biochemical processes

Three major divisions of the living world:

bacteria, archaea, and eukaryotes

Diversity is generated by ____ processes (gene mutation and natural selection) through billions of years


Prokaryotic Cell Structure

� Small and simple (0.2-2 �m)
� Does not have a nucleus and other organelles
� Has a single circular DNA in the cytoplasm
� unicellular

Eukaryotic cell structure

� Has a nucleus and other organelles (Mitochondrion Golgi apparatus ER)
� Has a single circular DNA in the cytoplasm
� Has multiple linear DNA's in the nucleus
� Large and complex (10 - 100 �m)
� unicellular or multicellular

Cell -> _____ -> organ -> _____ -> organism

tissue, organ system

Cells in different ____ have different ___ and _____

tissues, sizes, shapes

light microscopy

use light as a source of illumination

Living cells growing in a dish can be observed by light microscopy with various optics such as:

Bright field and phase contrast

Cells in a tissue biopsy can be observed under a light microscopy by ____ and ____ the specimen

fixing, staining


a procedure to terminate all ongoing biochemical reactions


color or fluorescence


stains nuclei purple


stains cytoplasm pink

Fluorescence microscopy

imaging cells with fluorescently labeled molecules
Yellow: actin filaments within microvilli Blue: microtubules Pink: DNA within the nuclei


proteins are fluorescently labeled with antibodies

direct labeling

primary antibody carries the die

indirect labeling

primary antibody needs secondary antibody that has the dye

Live cell imaging by tagging a protein with _____

green fluorescent protein (GFP)

Confocal microscopy

eliminates unfocused background fluorescence
a 3D image can be constructed from optical sections

Resolving power (resolution)

ability to discriminate between two separate points

Magnification power

the extent of enlargement by the lenses of the microscope

Limitation of light microscope

low resolving power (ability to discriminate between two separate points)

Transmission electron microscopy (TEM)

� samples are fixed and sectioned into very thin slices
� stained with heavy metal salts that block transmission of electrons
� significantly higher magnification and resolution (resolution limit: 0.2 nm)
� visualize fine structures inside an organelle or

Scanning electron microscopy (SEM)

� Samples are fixed and the surfaces are coated with heavy metal
� Electrons excite emission of secondary electrons from the surface of the specimen
� Produce 3D image of a surface
� Allow detailed study of surfaces
� Resolution limit: 1 nm
� 3D image of

Atomic force microscopy (AFM)

� scanning mode: 3D imaging of a surface (3D topology) by scanning the surface with the tip of a cantilever
� force mode: measuring the mechanical properties of cells and tissues (will be discussed in L4)
� Resolution limit: 0.2 nm