Biology Lecture Notes: Chapter 7
A Tour of the Cell
Section A – How We Study Cells
Topics:
1.Microscopy
2.Difference between prokaryotic and Eukaryotic
3.The two organelles; nucleus and ribosomes
Microscope provide windows to the world of the cell.
Organelle is a subcellular structure that has one or more specific jobs to perform in the cell,
much like an organ does in the body. Among the more important cell organelles are the nuclei,
which store genetic information; mitochondria, which produce chemical energy; and
ribosomes, which assemble proteins.
The discovery and early study of cells progressed with the invention and improvement of
microscopes in 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).
Microscope vary in magnification and resolving power.
Magnification is the ratio of an object’s image in real size. (How big the image you will
produce/ objective)
Resolving Power (quality of the image) is a measure of image clarity. (It is the minimum
distance two points can be separated by and still be viewed as two separate points.)
(Resolution is limited by the shortest wavelength of the source, in the case of light.)
The minimum resolution of a light microscope is about 2 microns, the size of a small
bacterium.
Light microscope can magnify effectively to about 1,000 times the actual size of the
specimen. (At lighter magnification the image blurs).
Techniques developed in the 20th century have enhanced contrast and enabled particular
cell components to be labelled so that they stand out.
While a light microscope can resolve individual cells, it cannot resolve much of internal
anatomy, specially the organelles.
To resolved smaller structures, we use electron microscope (EM), which focuses on beam of
electronic through to the specimen or into its surface. (Because resolution is inversely
related to wavelength used, electron microscopes with shorter wavelengths than visible
light have finer resolution.) (Theoretically, the resolution of a modern EM could reach 0.1
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.
Scanning Electron Microscopes (SEMs) are used for studying surface structures/external.
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 coupled with biochemistry, the study of molecules and chemical processes in
metabolism, developed into modern cell biology.
Types of Microscopy
1. Brightfield (unstained specimen)
2. Brightfield (stained specimen)
3. Fluorescence
4. Phase-contrast
5. Differential-interference-contrast (Normaski).
6. Confocal
Cell biologists can isolate organelles to study their function.
The goal of cell fractionation is to separate the major organelles of the cells so that their
individual function can be studied.
Cell refraction prepares quantities specific ell components.
His enables the function of these organelles to be isolated especially by the reaction 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 cells
1. Prokaryotic and eukaryotic cells differ in size and complexity
“PRO” = BEFORE
“KARYO” = MEANS NUCLEUS
“EU” = TRUE
SIMILARITIES
All cells are surrounded by a plasma membrane.
The semifluid substance within the membrane is the cytosol, containing the organelles.
All cells contain chromosomes which have genes in the form of DNA.
All cells also have ribosomes, tiny organelles that make proteins using the instruction
contained in genes.
DIFFERENCES
In eukaryote cells, the chromosomes are contained within a membranous nuclear
envelope.
The region between the nucleus and the plasma membrane is the cytoplasm. (All the
materials within the plasma membrane of a prokaryotic cell is cytoplasm).
within the cytoplasm of a eukaryotic cell is a variety of membrane bounded organelles of
specialized form and function. (this 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 that its surface area. (Smaller
objects have greater ration of surface area to volume.)
The plasma membrane functions as a selective barrier that allows passage of
oxygen, nutrients, and waste for the whole volume of the cell.
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 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.
2. Internal membranes compartmentalize the function of eukaryotic cell
A eukaryotic cell has extensive and elaborate internal membranes, which partition cell
into compartments.
These membranes also participate in metabolism as many enzymes are built into
membranes.
The barriers created by membranes provide different local environment that facilitates
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.)
Section C: The Nucleus and Ribosomes
1. The nucleus contains a eukaryotic cell’s genetic library
The nucleus contains most of the genes in a eukaryotic cell. (some genes are located in
mitochondria and chloroplasts.)
The nucleus averages about 5 microns in diameter.
The nucleus separated from the cytoplasm by a double membrane. (these are separated by
20-40 nm).
Where the double membranes are fused, a pore allows large macromolecules and particles
to pass through.
DNA in the mitochondria is from mother only and is different from the DNA of the nucleus.
The nuclear side of the envelope is lined by a nuclear lamina, a network of intermediate
filaments that maintain the shape of the nucleus.
Within the nucleus, the DNA and associated proteins are organized into fibrous material,
chromatin.
In normal cell they appear as a diffuse mass,
However, when the cell prepares to divide, the chromatin fibers coil up to be seen as
separate structures, chromosomes.
Each eukaryotic species has a characteristic number of chromosomes. ( a typical human cell
has 46 chromosomes, but sex cells have only 23 chromosomes.)
In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the
nucleolus.
In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled within proteins from
the cytoplasm to form ribosomal subunits.
The subunits pass from the nuclear pores to cytoplasm whereas they combine to form
ribosomes.
The nucleus directs proteins synthesis by synthesizing messenger RNA (mRNA). (The mRNA)
travels to cytoplasm and combines with ribosomes to translate its genetic message into the
primary structure of a specific polypeptide.
2. Ribosomes build a cell’s protein
Ribosomes contain rRNA and protein
A ribosome is composed of two subunits that combine to carry out protein synthesis.
Cell types that synthesize large quantities of proteins (e.g., pancreas) have large numbers of
ribosomes and prominent nuclei.
Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that
function within the cytosol.
Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic
reticulum. (these synthesize proteins that are either included into membranes of for export
from the cell).
Ribosomes can shift between roles depending on the polypeptides they are synthesizing.
Section D: The Endomembrane System
1. The endoplasmic reticulum manufactures membranes and performs many other
biosynthesis functions.
2. The Golgi apparatus finishes, sorts, and ships cell products
3. Lysosomes are digestive compartments
4. Vacuoles have diverse functions in cell maintenance.
