The Cell Theory
The Discovery of Cells
1. Robert Hooke (1665), English microscopist described
chambers in cork; called them cells (cellulae) since they
reminded him of cells occupied by monks living in a monastery
Matthias Schleiden, botanist (1838) - all plant tissues are
composed of cells; plant embryos arise from single cell
Theodor Schwann, zoologist (1839) - same conclusion about
animals; plants & animals similar
He was looking at empty cell walls, the remains of dead
cells; no internal structure
Schwann then proposed first two parts of The Cell Theory
2. Anton van Leeuwenhoek (1665-1675), Dutch, was first to
describe living single cells; results were checked and confirmed
by Hooke
2. The cell is the structural unit of life for all organisms.
He saw “animalcules” in pond water using the single lens
microscopes of remarkable quality that he made, and also
described bacteria from tooth scrapings
Basic Properties of Cells
I. Life – most basic property of cells; cells are the smallest units to
exhibit this property; plant or animal cells can be removed from
organism & cultured in laboratory
They can grow and reproduce for a long time in culture, unlike
their parts which soon deteriorate. In 1951 the first human cell
culture (HeLa cells) was established; these cells are still grown in
laboratories today. Cultured cells are simpler to study than cells in
body, serve as model systems
II. Cells are highly complex and organized
Cellular organelles have consistent macromolecular composition
arranged in a predictable pattern
Cell structure is similar, organism to organism despite differences
in higher anatomical features
VI. Cells carry out many chemical reactions (metabolism); to do
this, cells use enzymes that allow chemical reactions to take place
under biological conditions.
VII. Cells engage in numerous mechanical activities based on
dynamic, mechanical changes in cell. Includes extracellular and
intracellular movement.
VIII. Cells able to respond to stimuli - have receptors that sense
environment & initiate responses (move away from object in path
or toward nutrient source). Receptors bind to hormones, growth
factors, extracellular materials, surfaces of other cells
IX. Cells are capable of self-regulation
Importance of regulatory mechanisms most evident when they
break down. Failure of cell to correct error in DNA replication ->
may lead to debilitating mutation
1. All organisms are composed of one or more cells.
Rudolf Virchow, German pathologist (1855) - added third
part of The Cell Theory derived from his cell division
observations.
3. Cells can arise only by division from a preexisting cell.
III. Cells possess a genetic program (DNA) & the means to use it
(a blueprint); encoded in collection of genes
A. Blueprint for constructing cellular structures
B. Directions for running cell activities
C. Program for making more cells
IV. Cells are capable of producing more of themselves - mitosis
and meiosis
Before division, genetic material is faithfully copied; each
daughter cell gets complete & equal share of genetic information
(usually)
V. Cells acquire & utilize energy to develop & maintain
complexity - photosynthesis & respiration
Two Fundamentally Different Classes of Cells:
Prokaryotes and Eukaryotes
Two cell types were distinguished by size & types of internal
structures (organelles)
Prokaryotes (pro - before; karyon - nucleus) – all
bacteria, cyanobacteria (blue-green algae); structurally simpler
Prokaryotes now living are very similar to those
fossilized in >3.5 billion year old rocks (Australia, S. Africa);
sole life on planet for nearly 2 billion years before first eukaryote
Eukaryotes (eu - true) - structurally more complex;
protists, fungi, plants, animals
Breakdown in growth control -> may lead to cancer cell & death
1
Similarities between prokaryotes and eukaryotes
•
A. share an identical genetic language
B. share a common set of metabolic pathways
C. share common structural features - cell membrane, cell walls
(same function, different structure)
Differences between prokaryotes and eukaryotes 1. Eukaryote cells are internally more complex, with a
membrane-bound nucleus with complex nuclear envelope & other
organelles
Prokaryotes have a nucleoid region, no true membrane bound
nucleus, and no membrane-bound organelles
2. Eukaryotic chromosomes are numerous; contain linear DNA
tightly associated with protein
Prokaryotes have single, circular chromosome with DNA that
is nearly naked
Figure 1.9
The Sizes of Cells and Their Components
Units of measurement most often used to describe cell structures
Micrometers (µm; 10-6 m), nanometers (nm; 10-9 m)
BÅngstroms (Å; 10-10 m) – often used by molecular biologists for
atomic dimensions; ~1 Å = diameter of H atom
Examples
Globular protein (myoglobin) - ~ 4.5 nm x 3.5 nm x 2.5 nm
Elongated proteins (collagen, myosin) - over 100 nm in length
DNA - ~2 nm in width
Large molecular complexes (ribosomes, microtubules,
microfilaments); 5 - 25 nm dia.
Nuclei - about 10 µm diameter;
Mitochondria - about 2 µm in length
Bacteria - 1 to 5 µm in length
Eukaryotic cells - 10 to 30 µm in length
Figure 1.21
Viruses and prions: cells – no, alive - ?
Why are most cells so small?
Most eukaryotic cells have a single nucleus with only
two copies of most genes
Thus, cells can only produce a limited number of
mRNAs in a given amount of time
The larger a cell's volume, the longer it takes to make
the number of mRNAs the cell needs
As a cell increases in size, the surface area/volume
ratio decreases. Plasma membrane area is limited, also
diffusion is a slow process to move things inside cells
Viruses first found as pathogens smaller and, presumably,
simpler than smallest bacteria
In 1935) - tobacco mosaic virus (TMV), a rod-shaped particle
was crystallized & found to be infective.
It is a single RNA molecule surrounded by helical shell
of protein subunits
Common virus properties – often not considered living
since need host to reproduce, metabolize, etc. (obligatory
intracellular parasites). Alone, they are unable to reproduce,
metabolize or carry on other life-associated activities. Thus, they
are not considered to be organisms
Outside of living cell, they exist as particle or virion,
essentially a macromolecular package.
2
Viruses
Have genetic material (single/double stranded DNA or
RNA); 3 or 4 genes up to several 100. The genetic material
surrounded by protein capsule (capsid) usually made up of a
specific number of subunits; efficient (need only a few genes
to make capsid)
Bacteriophages are viruses that infect bacteria.
Viruses have virtues - research tool to study host DNA
replication/gene expression, insect-killing viruses (pest
control), used to introduce foreign genes into human cells as
treatment (gene therapy)
Prions
Infectious agents lacking DNA or RNA
Misfolded proteins
Associated with spongiform encephalopathy (ex: BSE, CJD)
Figure 1.23
Chemical Synthesis of Poliovirus cDNA: Generation of Infectious Virus in the
Absence of Natural Template
Jeronimo Cello, Aniko V. Paul, Eckard Wimmer*
Full-length poliovirus complementary DNA (cDNA) was synthesized by assembling
oligonucleotides of plus and minus strand polarity. The synthetic poliovirus cDNA
was transcribed by RNA polymerase into viral RNA, which translated and replicated
in a cell-free extract, resulting in the de novo synthesis of infectious poliovirus. The
synthetic virus had biochemical and pathogenic characteristics of poliovirus, that is, it
could infect and kill human cells. Results show that it is possible to synthesize an
infectious agent by in vitro chemical-biochemical means solely by following
instructions from a written sequence.
Science, Vol. 297, Issue 5583, 1016-1018, August 9, 2002
According to plan. Poliovirus reconstructed
from its genetic sequence is indistinguishable
from the original, shown here.
CREDIT: GELDERBLOM/EYE OF
SCIENCE/PHOTO RESEARCHERS INC.
Types of Eukaryotic Cells
Unicellularity vs. multicellularity – the most complex
eukaryotic cells are among single-celled protists
A. Protists - must do everything an organism must do to
survive; one evolutionary pathway
B. Multicellular organisms exhibit differentiation - different
activities conducted by different types of specialized cells
C. Differentiation – process by which a relatively
unspecialized cell becomes highly specialized activities
Differentiation of each eukaryotic cell depends primarily on
signals received from environment
Organelles
Membrane bound, keeps their internal contents separated from
the cytoplasm. Specialized structures for specialized functions.
Nucleus – large, contained by 2 layers of phospholipid bilayer
membranes called the nuclear envelope. Contains nuclear
pores that pass through both membranes. Pores are large (7090 nm diameter) and allow traffic into and out of the nucleus.
A mass of proteins called the annulus regulates movement of
large items like ribonucleoprotein particles through the pores.
Chromatin fibers 10-30 nm take up most of the space inside
the nucleus, composed of DNA, histones and nonhistone
[chromosomal] proteins.
Chromosomes – 46 in humans, DNA = 1 meter in length!
Liver cell nucleus – nucleolus and chromatin within,
glycogen, mitochondria and ER present in surrounding
cytoplasm
3
Nucleoli – small fibers and granuals, not separated off
from nucleus by membranes. Assembly of ribosomal
subunits occurs here. rRNA is synthesized from rRNA
genes in the nucleolus. Ribosomal proteins [many!] are
synthesized in the cytoplasm and transported in through
pores for ribosome assembly in the nucleolus.
Genes in DNA encode mRNA, rRNA, tRNA, others.
These RNAs are transcribed from the DNA of genes.
All DNA and chromosomal proteins are duplicated before
cell division. Copying of DNA is called replication.
Both transcription and replication occur in the nucleus.
So how does chromatin get assembled and how do
mRNAs get out into the cytoplasm to be translated into
proteins on ribosomes?
Cell cytoplasmic membrane
Copyright Dennis Kunkel Microscopy, Inc.
www.DennisKunkel.com
Cytoplasm and organelles
Contains ribosomes with different proteins and rRNAs in
prokaryotes than eukaryotes.
Free ribosomes make proteins that go into solution in the
cytoplasm or form cytoplasmic structures.
Ribosomes on rough endoplasmic reticulum (ER) make
proteins that become parts of membranes or are packaged into
vesicles for storage or export. Chemical modifications of proteins
occurs inside the ER.
Processing and sorting of proteins takes place in the Golgi
complex, also some modification of proteins made in rough ER.
Protein sorting occurs here.
Smooth endoplasmic reticulum does not have ribosomes
attached. Breakdown of fats, lipid synthesis, toxin breakdown
Rough endoplasmic reticulum with ribosomes
Copyright Dennis Kunkel Microscopy, Inc.
www.DennisKunkel.com
Rough endoplasmic reticulum with numerous
ribosomes on cisternae surface (from a neuron)
Ribosomes and polyribosomes (liver cell)
4
Golgi apparatus, stacks of cisternae, and vesicles near the
nucleus of a nerve cells. The nuclear membrane has pores
in it.
Liver cell – with glycogen, mitochondria, nucleus, ER
Lysosomes - contains enzymes that break down biological
molecules. If they rupture inside the cell, cell dies. Sometimes
desirable – loss of webbing between fingers and toes. Apoptosis –
programmed cell death.
Mitochondria – reactions occurring here provide chemical energy to
cells. Change be different shapes, or change shape. Outer and inner
membrane systems. Cristae are folds of the inner layer, where
respiratory enzyme complexes are found. The soluble interior of the
mitochondrion is called the matrix. Mitochondria also have DNA,
ribosomes and make some proteins.
Microbodies (peroxisomes) – make hydrogen peroxide. In plants
some microbodies (glyoxisomes) convert fat into sugar.
Cytoplasmic filament systems for cell movement. Convert chemical
energy to mechanical energy
Microtubules – 25 nm diameter, tubulin, mitosis, other
Microfilaments – 5-7 nm, actin, cell motility, muscle, other
Intermediate filaments – 10 nm diameter
Mitochondrion from a heart muscle cell showing
numerous cristae.
Plant only structures:
Central vacuole – large membrane bound sacs, can occupy
90% of the interior of a mature plant cell. Support via osmotic
pressure. Contain salts, sugars, enzymes, waste products, etc.
Cellulose containing cell walls – outside the plasma
membrane. Provide support, tensile strength. Cytoplasmic channels
through the cell walls – plasmodesmata.
Chloroplasts – outer and inner membrane systems, inner is
folded and specialized for large surface area. Stacks of these
photosynthetic membranes – thylakoids. Here, light energy is
captures and converted into chemical energy. Interior fluid portion –
stroma. Chloroplasts also have DNA, ribosomes and more.
Cellular organelles in a liver cell
5
Chloroplasts and other organelles near the plasma membrane of a plant
cell. Note also the cell wall and central vacuoles. A second cell in
present. Its chloroplasts are not included in the image.
Image credits – Molecular Biology of the Cell, 4th Edition, Alberts et al. Cell Biology
Interactive 2002
Figure 1.19
Red blood cell in a tiny capillary. Note the endothelial
cell (one containing a large nucleus) that forms the
capillary.
Parenchyma cell from a plant (arum or voodoo lily,
Sauromatum guttum). Note the cell wall, nucleus with
nucleolus, amyloplast with starch grains and mitochondria.
Nucleus from a neuroglial cell (insect nervous system).
Note the nucleolus and condensed DNA in the nucleus.
Mitochondria and ER are in the surrounding cytoplasm.
Small intestine villus. Columnar epithelial cells are in the periphery
with microvilli at tips. Goblet cells are in the infoldings of the
columnar epithelial cells.The central region of the villus is the lamina
propia mucosa. Various connective cells with capillaries are nearby.
6
Stem Cells: A Primer
NIH 2000
Definitions DNA - abbreviation for deoxyribonucleic acid which makes up genes.
Gene - a functional unit of heredity which is a segment of DNA located in a specific site on a
chromosome. A gene directs the formation of an enzyme or other protein.
Somatic cell - cell of the body other than egg or sperm.
Somatic cell nuclear transfer - the transfer of a cell nucleus from a somatic cell into an egg
from which the nucleus has been removed.
Stem cells - cells that have the ability to divide for indefinite periods in culture and to give
rise to specialized cells.
Pluripotent -capable of giving rise to most tissues of an organism.
Totipotent - having unlimited capability. Totipotent cells have the capacity to specialize into
extraembryonic membranes and tissues, the embryo, and all postembryonic tissues and organs
EP Figure 1
7