CHAPTER 7
A TOUR OF THE CELL
Section A: How We Study Cells
1. Microscopes provide windows to the
world of the cell
2. Cell biologists can isolate organelles to study
their function
Objectives
• Distinguish between prokaryotic and
eukaryotic cells
• Explain why there are upper and lower
limits to cell size
• Explain the function of
compartmentalization in eukaryotic cells
• Describe the structure and function of the
nucleus
• Describe the structure and function of the
eukaryotic ribosome
• List the components of the endomembrane
system, describe their structures and functions
and summarize the relationships among them
• Describe the types of vacuoles and how their
functions differ
• Explain the role of peroxisomes in eukaryotic
cells
• Describe the structure of a typical
mitochondrion, detail its function and explain
how compartmentaliztion in the mitochondrion is
important to this function
• Explain the structure and functioning of the
chloroplast
• Describe the functions of the cytoskeleton
and distinguish among microtubules,
microfilaments and intermediate filaments
• Describe the structure of flagella and cilia
and briefly summarize the relationship
between
this structure and their functioning
1. Microscopes provide windows
to the world of the cell
• The discovery and early study of cells
progressed with the invention and improvement
of microscopes in the 17th century.
• In a light microscope (LMs) 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 a video screen.
– A light microscope can be used to resolve individual
cells
• 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.
– It is the minimum distance two points can be
separated and still viewed as two separate
points.
– Resolution is limited by the shortest
wavelength of the source, in this case light.
• The minimum resolution
of a light microscope is
about 2 microns, the
size of a small
bacterium
• Light microscopes can
magnify effectively to
about 1,000 times the
size of the actual
specimen.
– At higher
magnifications, the
image blurs.
Fig. 7.1
• A light microscope can resolve individual
cells but 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.
– the practical limit of a modern EM is about
about 2 nm (the size of a single rhinovirus).
• Transmission electron microscopes
(TEM) are used mainly to study the internal
ultrastructure of cells.
– A TEM aims an electron beam through a thin
section of the specimen.
– The image is focused
and magnified by
electromagnets.
– To enhance contrast,
the thin sections are
stained with atoms
of heavy metals.
Fig. 7.2a
• Scanning electron microscopes (SEM)
are useful for studying surface structures.
– The sample surface is covered with a thin film
of gold.
– The beam excites electrons on the surface.
– These secondary electrons are collected and
focused on a screen.
• The SEM has great
depth of field,
resulting in an
image that seems
three-dimensional.
Fig. 7.2b
• Electron microscopes reveal organelles,
but they can only be used on dead cells
and they may introduce some artifacts.
• Light microscopes do not have as high a
resolution, but they can be used to study
live cells.
• Microscopes are a major tool in cytology,
the study of cell structures.
• Cytology + biochemistry = modern cell
biology.
Cell Theory
1. All known living things are made up of cells.
2. The cell is structural & functional unit of all living things.
3. All cells come from pre-existing cells by division.
(Spontaneous Generation does not occur).
1838: Schleiden and Schwann proposed cell theory
These first three are the very basic foundations of Cell
Theory. All six
of these components make up modern Cell Theory
4. Cells contains hereditary information which is passed
from cell to cell during cell division.
5. All cells are basically the same in chemical composition.
6. All energy flow (metabolism & biochemistry) of life
occurs within cells.
2. Cell biologists can isolate organelles
to study their functions
• The goal of cell fractionation is to separate the
major organelles of the cells so that their
individual functions can be studied.
• Uses an ultracentrifuge, a machine that can
spin at up to 130,000 revolutions per minute and
apply forces more than 1 million times gravity
(1,000,000 g).
• Fractionation begins with homogenization,
gently disrupting the cell.
• Then, the mixture is spun in a centrifuge to
separate heavier pieces into the pellet while
lighter particles remain in the solution.
– As the process is repeated at higher speeds and
longer durations, smaller and smaller organelles can
be collected in subsequent pellets.
• Cell fractionation prepares quantities of specific
cell components.
• The functions of these organelles to be isolated,
especially by the reactions or processes
catalyzed by their proteins.
– For example, one cellular fraction is enriched in
enzymes that function in cellular respiration.
– Electron microscopy reveals that this fraction is rich in
the organelles called mitochondria.
• Cytology and biochemistry complement each
other in connecting cellular structure and
function.
Section B: A Panoramic View of the Cell
1.Prokaryotic and eukaryotic cells differ
in size and complexity
2.Internal membranes compartmentalize
the functions of a eukaryotic cell
1. Prokaryotic and eukaryotic cells
differ in size and complexity
• All cells are surrounded by a plasma
membrane. (What is this made of?)
• The “liquid” inside the membrane is the
cytosol, which contains the organelles.
• All cells contain chromosomes which
have genes in the form of DNA.
• All cells also have ribosomes, organelles
that make proteins using the instructions
contained in genes.
• A major difference between prokaryotic
and eukaryotic cells is the location of
chromosomes.
• In an 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.
Fig. 7.4 The prokaryotic cell is much simpler in structure, lacking a nucleus and the other
membrane-enclosed organelles of the eukaryotic cell.
CD-Rom Activity 7.1
• This activity will help you to review and
gain an understanding of the structures
and functions of prokaryotic cells.
• In eukaryote cells, the chromosomes are
contained within a membranous nuclear
envelope.
• The region between the nucleus and the plasma
membrane is the cytoplasm. (Sarah, this is for you.)
– All the material within the plasma membrane of a
prokaryotic cell is cytoplasm. (This includes the
organelles.)
• Within the cytoplasm of a eukaryotic cell is a
variety of membrane-bounded organelles of
specialized form and function.
– These membrane-bounded 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.
– At the lower limit, the smallest bacteria,
mycoplasmas, are between 0.1 to 1.0 micron.
– Most bacteria are 1-10 microns in diameter.
– 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.
– Smaller objects have a greater
ratio of surface area to volume.
Fig. 7.5
• The plasma membrane functions as a
selective barrier that allows passage of
oxygen, nutrients, and wastes for the whole
volume of the cell.
Fig. 7.6
• The volume of cytoplasm determines the
need for this exchange.
• Rates of chemical exchange may be
inadequate to maintain a cell with a very
large cytoplasm.
• The need for a large surface 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.
2. 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 in
metabolism as many enzymes are built
into membranes.
• The barriers created by membranes
provide different local environments that
facilitate specific metabolic functions.
• The general structure of a biological
membrane is a double layer of
phospholipids with other lipids and
diverse proteins.
• Each type of membrane has a unique
combination of lipids and proteins for
its specific functions.
– For example, those in the membranes of
mitochondria function in cellular respiration.
Fig. 7.7
CD-Rom Activity 7.2
• This activity will help you to review and
gain an understanding of the structures
and functions of animal cells.
Fig. 7.8
CD-Rom Activity 7.3
• This activity will help you to review and
gain an understanding of the structures
and functions of plant cells.