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Chapter 1

BIO206H5 Chapter Notes - Chapter 1: Transmission Electron Microscopy, Scanning Electron Microscope, Electron Microscope

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George S Espie

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Chapter 1: Cells - The Fundamental Unit of Life
Unity and Diversity of Cells
All living things are built from cells
Cells: small, membrane enclosed units that are filled with concentrated aqueous solutions or chemicals and given the ability to create
copies of themselves by growing and then dividing into two
Cells Vary Enormously In Appearance and Function
Bacterial cells are measured in micrometers (25x smaller than the width of a human hair)
Frog eggs have a diameter of 1mm
Thus, cells vary in size
Cells also vary in shape
They are also different in their chemical requirements. Some require oxygen to live, for others oxygen is deadly. Some cells consume
more than air, sunlight, and water as their raw material, others need complex mixtures of molecules that are produced by other cells
A difference in size, shape, and chemical requirements reflect a difference in cell function
Some modifications specialize a cell so much that they spoil their chances of leaving a descendent
This specialization is senseless for cells that live a solitary life. However, for cells that live a multicellular life, there is a division of labour
among the cells. This allows some cells to specialize to extreme degrees for a particular task
Living Cells all have a Similar Basic Chemistry
Even though the cells of all living things are different when viewed from the outside, they are fundamentally similar from the inside
They are composed of the same sorts of molecules, which participates in the same type of chemical reactions
In all organisms, genetic information is carried in DNA molecules in the form of genes
This information is written in the same genetic code, constructed out of the same building blocks, interpreted by the same chemical
machinery, and replicated in the same way when reproducing
In all cells, the long DNA polymer chains are made up of the same 4 monomers, called Nucleotide
In every cell, the information that is encoded in the DNA is read out (transcribed) into chemically related sets of polymers called RNA
Flow of information from DNA to RNA to Protein is called Central Dogma
Protein molecules determine the appearance and behaviour of a cell. Proteins serve as structural support, chemical catalysts, molecular
motors, etc
Proteins are built from amino acids. All organisms use the same 20 amino acids to make their protein
Amino acids are linked in different sequences. This is what gives protein molecules a different, 3D shape
Virus: compact packages of genetic information (DNA or RNA), that is encased in protein but they don't have the ability to reproduce
themselves by their own effort
Viruses get themselves copied by parasitizing the reproductive machinery of other cells that they invade
All Present Day Cells have Apparently Evolved from the Same Ancestral Cell
Cells reproduce by replicating its DNA and then dividing into two. This way, the cells can pass on a copy of the genetic instructions that
is encoded in DNA to each of the daughter cells
If a mutation occurs during replication, it can change the daughter cells for the worse (not able to survive and reproduce) or for the
better (better able to survive and reproduce), or in a neutral way (genetically different but equally viable)
These principles of genetic change and selection that are applied repeatedly over many cell generations is called Evolution
Evolution: process by which living species become gradually modified and adapted to their environment in more sophisticated ways
All present day cells are similar in their fundamentals because they have all inherited their genetic instructions from the same common
Genes Provide the Instructions for Cell Form, Function, and Complex Behaviour
A cell's genome provides a genetic program that instructs the cell on how to behave
Genome: entire sequence of nucleotides in an organisms DNA
In plant and animal embryos, the genome directs the growth and development of an adult organism with hundreds of different cell
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Different cells express different genes. They use their genes to produce some proteins and not others. This depends on their internal
state and on the cues that they and their ancestor cells have received from their surroundings
Each cell is capable of carrying out a variety of biological tasks, and it selectively uses the information that is encoded in its DNA to
guide its activity
Cells Under the Microscope
Cells were not visible until the 17th century. This is when the microscopes were invented
Light Microscopes: use visible light to illuminate specimens.
Light microscopes allowed biologists to see complex structures for the first time
Electron microscope: use beams of electrons instead of beams of light as the source of illumination. This extends the ability to see the
fine details of the cell, and even make some of the larger molecules visible individually
The Invention of the Light Microscope led to the Discovery of Cells
The development of the light microscope advanced after the production of glass lenses
Using an instrument with the lens, Robert Hooke examined a cork and reported to the Royal Society of London that the cork was
composed of a mass of chambers. He called the chambers Cells
Later, Hooke and Antoni van Leeuwenhoek were able to observe living cells
the development of Cell Biology happened very gradually, but its official birth was done by two people. One was a botanist, Matthew
Schleiden, in 1838, and another was a zoologist, Theodor Schwann, in 1839. In the paper, these two reported the results of a systematic
investigation of plants and animal tissues with the light microscope.
Their work, and the work of other 9 microscopists, led to the realization that all living things are formed by the growth and division of
already existing cells. (one principle in Cell Theory)
People first debated whether living organism arise spontaneously or if they are generated only from existing organisms. They came to a
conclusion in 1860s after a set of experiments performed by Louis Pasteur
To understand why present day cells and organisms behave as they do, we need to understand their history. Charles Darwin provided
the information about the history.
His theory of evolution explained how random variations and natural selection can give rise to diversity among organisms that share a
common ancestor
Light Microscopes Allow Examination of Cells and Some of Their Components
Cells can either by closely packed, or separated from one another by an extracellular matrix
Extracellular Matrix: dense material made of protein fibers that are embedded in a polysaccharide gel
Seeing internal structures of a cell is difficult because the parts are small and also because they are transparent and colorless
One way to see cells is to stain them with dyes that color particular components differently
By dying a cell, we found out that the cell has a sharply defined boundary, indicating the presence of a membrane. It also has a large
round structure called the nucleus, which is located in the center of the cell. Around the nucleus and filling the cells interior is the
Cytoplasm: contents of a cell that are contained within its plasma membrane, but in an eukaryotic cell, it is contained outside the
Fluorescence Microscope: use sophisticated methods of illumination and electronic image processing to see fluorescently labeled cell
components in finer detail
Ribosome: large macromolecule complex that is composed of 80-90 individual proteins and RNA molecules ***
The Fine Structure of a Cell is Revealed by Electron Microscopy
Electron Microscope is used for highest magnification and best resolution
When thin sections are cut and placed in the electron microscope, the jumble of cell components become sharply resolved into
Organelles: separate, recognizable substructures with specialized functions
Membrane that separates the interior of the cell from the external environment is called the plasma membrane
Membranes that surround the organelles are called internal membranes
All of these membranes are only two molecules thick
Transmission Electron Microscope is used to look at thin sections of tissues
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