BIOL 600 Lecture Notes - Lecture 5: Cell Membrane, Cell Fractionation, Cellular Respiration
26 May 2018
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AP Bio Chapter 6 A Tour of the Cell
Lecture Outline
Overview: The Importance of Cells
All organisms are made of cells.
o Many organisms are single-celled.
o Even in multicellular organisms, the cell is the basic unit of structure and
function.
The cell is the simplest collection of matter that can live.
All cells are related by their descent from earlier cells.
Concept 6.1 To study cells, biologists use microscopes and the tools of biochemistry
The discovery and early study of cells progressed with the invention of microscopes in
1590 and their improvement in the 17th century.
In a light microscope (LM), visible light passes through the specimen and then through
glass lenses.
o The lenses refract light such that the image is magnified into the eye or onto a
video screen.
Microscopes vary in magnification and resolving power.
o Magnification is the ratio of an object’s image to its real size.
o Resolving power is a measure of image clarity.
It is the minimum distance two points can be separated and still be
distinguished as two separate points.
Resolution is limited by the shortest wavelength of the radiation used for
imaging.
The minimum resolution of a light microscope is about 200 nanometers (nm), the size of
a small bacterium.
Light microscopes can magnify effectively to about 1,000 times the size of the actual
specimen.
o At higher magnifications, the image blurs.
Techniques developed in the 20th century have enhanced contrast and enabled particular
cell components to be stained or labeled so they stand out.
While a light microscope can resolve individual cells, it cannot resolve much of the
internal anatomy, especially the organelles.
To resolve smaller structures, we use an electron microscope (EM), which focuses a
beam of electrons through the specimen or onto its surface.
o Because resolution is inversely related to wavelength used, electron microscopes
(whose electron beams have shorter wavelengths than visible light) have finer
resolution.
o Theoretically, the resolution of a modern EM could reach 0.002 nanometer (nm),
but the practical limit is closer to about 2 nm.
Transmission electron microscopes (TEMs) are used mainly to study the internal
ultrastructure of cells.
o A TEM aims an electron beam through a thin section of the specimen.
o The image is focused and magnified by electromagnets.
o To enhance contrast, the thin sections are stained with atoms of heavy metals.
Scanning electron microscopes (SEMs) are useful for studying surface structures.
o The sample surface is covered with a thin film of gold.
o The beam excites electrons on the surface of the sample.
o These secondary electrons are collected and focused on a screen.
o The result is an image of the topography of the specimen.
o The SEM has great depth of field, resulting in an image that seems three-
dimensional.
Electron microscopes reveal organelles that are impossible to resolve with the light
microscope.
o However, electron microscopes can only be used on dead cells.
Light microscopes do not have as high a resolution, but they can be used to study live
cells.
Microscopes are major tools in cytology, the study of cell structures.
Cytology combined with biochemistry, the study of molecules and chemical processes in
metabolism, to produce modern cell biology.
Cell biologists can isolate organelles to study their functions.
The goal of cell fractionation is to separate the major organelles of the cells so their
individual functions can be studied.
This process is driven by an ultracentrifuge, a machine that can spin at up to 130,000
revolutions per minute and apply forces of more than 1 million times gravity (1,000,000
g).
Fractionation begins with homogenization, gently disrupting the cell.
The homogenate is spun in a centrifuge to separate heavier pieces into the pellet while
lighter particles remain in the supernatant.
o As the process is repeated at higher speeds and for longer durations, smaller and
smaller organelles can be collected in subsequent pellets.
Cell fractionation prepares isolates of specific cell components.
This enables the functions of these organelles to be determined, especially by the
reactions or processes catalyzed by their proteins.
o For example, one cellular fraction was enriched in enzymes that function in
cellular respiration.
o Electron microscopy revealed that this fraction is rich in mitochondria.
o This evidence helped cell biologists determine that mitochondria are the site of
cellular respiration.
Cytology and biochemistry complement each other in correlating cellular structure and
function.
Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their
functions
Prokaryotic and eukaryotic cells differ in size and complexity.
All cells are surrounded by a plasma membrane.
The semifluid substance within the membrane is the cytosol, containing the organelles.
All cells contain chromosomes that have genes in the form of DNA.
All cells also have ribosomes, tiny organelles that make proteins using the instructions
contained in genes.
A major difference between prokaryotic and eukaryotic cells is the location of
chromosomes.
In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the
nucleus.
In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane
separating it from the rest of the cell.
In eukaryote cells, the chromosomes are contained within a membranous nuclear
envelope.
The region between the nucleus and the plasma membrane is the cytoplasm.
o All the material within the plasma membrane of a prokaryotic cell is cytoplasm.
Within the cytoplasm of a eukaryotic cell are a variety of membrane-bound organelles of
specialized form and function.
o These membrane-bound organelles are absent in prokaryotes.
Eukaryotic cells are generally much bigger than prokaryotic cells.
The logistics of carrying out metabolism set limits on cell size.
o At the lower limit, the smallest bacteria, mycoplasmas, are between 0.1 to 1.0
micron.
o Most bacteria are 1–10 microns in diameter.
o Eukaryotic cells are typically 10–100 microns in diameter.
Metabolic requirements also set an upper limit to the size of a single cell.
As a cell increases in size, its volume increases faster than its surface area.
o Smaller objects have a greater ratio of surface area to volume.
The plasma membrane functions as a selective barrier that allows the passage of oxygen,
nutrients, and wastes for the whole volume of the cell.
The volume of cytoplasm determines the need for this exchange.
Rates of chemical exchange across the plasma membrane may be inadequate to maintain
a cell with a very large cytoplasm.
The need for a surface sufficiently large to accommodate the volume explains the
microscopic size of most cells.
Larger organisms do not generally have larger cells than smaller organisms—simply
more cells.
Cells that exchange a lot of material with their surroundings, such as intestinal cells, may
have long, thin projections from the cell surface called microvilli. Microvilli increase
surface area without significantly increasing cell volume.
Internal membranes compartmentalize the functions of a eukaryotic cell.
A eukaryotic cell has extensive and elaborate internal membranes, which partition the
cell into compartments.
These membranes also participate directly in metabolism, as many enzymes are built into
membranes.
Document Summary
Ap bio chapter 6 a tour of the cell. All organisms are made of cells: many organisms are single-celled, even in multicellular organisms, the cell is the basic unit of structure and function. The cell is the simplest collection of matter that can live. All cells are related by their descent from earlier cells. Concept 6. 1 to study cells, biologists use microscopes and the tools of biochemistry. The discovery and early study of cells progressed with the invention of microscopes in. 1590 and their improvement in the 17th century. In a light microscope (lm), visible light passes through the specimen and then through glass lenses: the lenses refract light such that the image is magnified into the eye or onto a video screen. Microscopes vary in magnification and resolving power: magnification is the ratio of an object"s image to its real size, resolving power is a measure of image clarity.
