Reece et al. 9 Edition
ATour of the Cell
• Cells are the fundamental unit of life (i.e., in the hierarchy of biological organization, cells
are the simplest collection of matter that can be alive)
• Cells are the basic unit an organisms basic unit of structure and function
• All cells arise from preexisting cells
• Cells show an incredible diversity in structure and function and yet share many common
I. Tools to Study Cells
The invention of the light microscope led to the discovery of cells.
Three important concepts in microscopy are magnification, resolution and contrast
Magnification = ratio of size of object image to real size of object
• How much larger an object is made to appear compared to its real size
• Resolution is a measure of clarity of the image; it is the minimum distance two points can be
separated by and still be distinguished as two separate points
• Resolving power is inversely related to wavelength of electromagnetic radiation used by the
• Accentuate different parts of the specimen
Types of Microscopes
a. Light microscope
• Visible light is focused on the specimen with a condenser.
• Light passing through the specimen is refracted through an objective lens and an ocular lens
producing an inverted magnified image.
• 1000 x magnification and 0.2 µm resolution (limited by wavelength of visible light) for
conventional light microscopy such as brightfield microscopy.
• Above 1000 x magnification objects become increasingly blurry – This is often referred to as
“empty magnification” because no useful information is gained from increasing
magnification beyond 1000 x.
• Contrast my be created due to natural pigmentation of specimens, staining or labeling cell
components, or specialized microscopes with light modifying condensers.
b. Electron microscope
• These instruments use electromagnets as lens to focus a beam of electrons on specimen. Reece et al. 9 Edition
• Electron microscopes have a much higher resolving power. Theoretically it is around 2 pm
for transmission electron microscopy due to the much shorter wavelength of the electron
beam. However, the practical limit for biological systems is 2 nm.
• The development of electron microscopes allowed researchers to study internal cell
ultrastructure (e.g., intracellular organelles) using transmission electron microscopy.
i) Transmission electron microscopy (TEM)
• Electrons transmitted through the specimen
• Specimens are stained with heavy metals resulting in
regions with different electron densities. When exposed to a beam of electrons,
regions with high electron densities allow fewer electrons to pass through. Displayed
images represent a pattern of transmitted electrons
• TEM is used to study internal cellular ultrastructure
ii) Scanning electron microscopy (SEM)
• Specimens are coated with a think film of gold.A
scanning beam of electrons excites secondary electrons on gold specimen surface.
These are collected and focused onto a viewing screen resulting in a 3-D image with
great depth of field.
• SEM used for studying the surface of a specimen.
What are the advantages and disadvantages of the different types of microscopy?
2. Cell Fractionation and BiochemicalAnalysis
II. Cell Size (Figure 6.2)
• Cells come in a variety of sizes & shapes
• Size limit is set by the logistics required to carry out metabolism
• The smallest cells are nanobacteria and mycoplasmas (diameter of 0.1 to 1.0 µm).
• Most bacterial cells are 10 times larger than mycoplasmas (1 to 5 µm in diameter)
• Eukaryotic cells are typically 10 times larger than bacteria (10 - 100 µm in diameter). Reece et al. 9 Edition
What factors constrain the lower size limit of cells?
What factors constrain the upper size limit of cells?
Hint: Fig. 6.7
Size does not always correlate with class of cells (i.e., prokaryotic vs eukaryotic cells)
• Generally prokaryotic cells are smaller than eukaryotic cells. But there are exceptions.
e.g., Epulopiscium fishelsoni
• Generally cells are microscopic. But there are exceptions to this as well
• Loligo -Atlantic squid has neurons with axon
diameters as large as 1.0 mm.
• Human motor neurons
• Ostrich egg
Cell shapes vary in shape from sphere and cylinders to very irregular nerve cells.
III. Cell Types
You should be able to compare and contrast prokaryotic and eukaryotic cells.
• Cells are the basic unit of life.
• Cells are highly organized structures
• Cells are separated from their external environment by a plasma membrane.
• All cells use DNA as their genetic information. The DNA is found in chromosomes
(although there are significant differences between prokaryotic and eukaryotic chromosomes)
• All cells have ribosomes
• All present day cells have apparently evolved from the same ancestor Reece et al. 9 Edition
Differences (Note this is not an exhaustive list but a good starting point)
Prokaryotic (before nucleus) cell Eukaryotic (true nucleus) cell
• no membrane bound nucleus - membrane bound nucleus
• no membrane bound organelles - membrane bound organelles
• simple cell structure - complex cell structure
• generally smaller than eukaryotic cells - generally larger than prokaryotic cells
• Domains: Bacteria &Archaea - Domain: Eukarya
1. Prokaryotic cells
• Generally single cell organisms (Figure 6.5)
• Life cycle can be as short as twenty minutes.
• Vegetative reproduction in which an individual produces two daughter cells by a process
called binary fission.
Prokaryotic cell structure
i) Cell or plasma membrane (discussed in more detail in Chapter 7)
• every cell is surrounded by this selective barrier
• allows passage of sufficient nutrients, oxygen, and wastes to service the cell
• consists of a lipid bilayer composed of phospholipids and other lipids
• proteins serving a variety of functions (enzymes, receptors, attachment, transport…) are
embedded or attached to the surface of the lipid bilayer
• membrane composition varies depending on the type of cell and composition is related to
ii) Nucleoid region - region where the chromosome (DNA) of a prokaryotic cell is located. This
region is NOT membrane bound.
iii) Cytoplasm – interior of the prokaryotic cell
iv) Cell wall – rigid structure outside the plasma membrane
v) Capsule - a gelatinous or slimy layer external to the cell wall composed of polypeptides or
vi) Pilus (pl. Pili) and fimbria (pl. fimbriae) - attachment structures on the cell surface Reece et al. 9 Edition
vii) Flagellum (pl - Flagella) - locomotory organelle
viii) Ribosomes - site of protein synthesis
Mesosome - at one time thought to be involved in cell division, now thought to be an artifact.
Note Prokaryotic cells lack a nucleus and membrane bound organelles
2. Eukaryotic Cells
How do eukaryotic cells differ from prokaryotic cells?
Structure of a Eukaryotic Cell
i) Cell Membrane - separates cell from the surrounding environment. Described above and will
be covered extensively in chapter 7.
• contains most of a cell's genetic material
• usually the most prominent organelle in an eukaryotic cell.
• surrounded by nuclear envelope composed of a double membrane. Pores in the nuclear
envelope regulate the movement of proteins and RNAs into and out of the nucleus. The
inside of the nuclear envelope is lined by the nuclear lamina (a network of protein filaments
that maintain the structure of the nucleus).
• Chromosomes in the nucleus are formed of chromatin (complex of DNA and protein). Reece et al. 9 Edition
• When a cell prepares to divide the chromatin condenses and forms into structures that are
visible with the light microscope. The number of chromosomes is a characteristic of a
• Nuclear pore
• Nuclear matrix
iii) Cytoplasm - cellular contents excluding the nucleus - strip away membrane, everything that
is left excluding the nucleus.
iv) Cytosol – semi-fluid (i.e., gel) substance in which the organelles are found.
• sites of protein synthesis
• found free within the cytoplasm and associated with the endoplasmic reticulum and nuclear
envelope as well as within mitochondria and chloroplasts
• number varies depending on the level of protein synthesis going on within the cell
• two types of ribosomes based on local. The structures of these ribosomes are identical.
Free ribosomes - in the cytosol - produce proteins that will function within the cytosol.
Also most of the mitochondrial and chloroplast proteins are produced by these ribosomes.
Bound ribosomes - attached to the outer surface of the nuclear envelope and
endoplasmic reticulum. Produce proteins that are to be included in membranes, packaged
into membrane bound organelles or exported from the cell (secretion).
The Endomembrane System (Fig 6.15)
• Anumber of related membranous structures including nuclear envelope, endoplasmic
reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane
• Believed to have arisen by invagination of the plasma membrane.
• Carries out a variety of functions, including synthesis of proteins, transport of protein into
membranes and organelles or out of the cell, metabolism and movement of lipoids and
detoxification of toxins Reece et al. 9 Edition
vi) Endoplasmic reticulum (“within cytoplasm network”) = ER
• extensive membrane network of tubules and sacs (cisternae; singular - cisterna) that is
continuous with the nuclear envelope
• the ER represents up to half of an eukaryotic cell’s total membrane
Two types of ER that differ in structure and function though interconnected
• lacks ribosomes
Functions (dependent upon cell type)
• synthesis of lipids (fatty acids, phospholipids and steroids)
• metabolism of carbohydrates
• detoxification of drugs and poisons