The Cell Theory The Discovery of Cells Basic Properties of Cells Two Fundamentally Different Classes of Cells: Prokaryotes and Eukaryotes

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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

He was looking at empty cell walls, the remains of dead cells; no internal structure

2. Anton van Leeuwenhoek (1665-1675), Dutch, was first to describe living single cells; results were checked and confirmed by Hooke

He saw “animalcules” in pond water using the single lens microscopes of remarkable quality that he made, and also described bacteria from tooth scrapings

The Cell Theory

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

Schwann then proposed first two parts of The Cell Theory 1. All organisms are composed of one or more cells. 2. The cell is the structural unit of life for all organisms.

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.

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

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

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

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


Figure 1.9

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

The Sizes of Cells and Their Components

Units of measurement most often used to describe cell structures Micrometers (µm; 10-6m), nanometers (nm; 10-9m) BÅngstroms (Å; 10-10m) – often used by molecular biologists for atomic dimensions; ~1 Å = diameter of H atom


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

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 and prions: cells – no, alive - ?

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.



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)


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.


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


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 poresthat pass through both membranes. Pores are large (70-90 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 fibers10-30 nm take up most of the space inside the nucleus, composed of DNA, histones and nonhistone [chromosomal] proteins.


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. Genesin 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 onribosomes?

Cell cytoplasmic membrane

Copyright Dennis Kunkel Microscopy, Inc.

Cytoplasm and organelles

Contains ribosomeswith 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 numerous

ribosomes on cisternae surface (from a neuron)

Rough endoplasmic reticulum with ribosomes

Copyright Dennis Kunkel Microscopy, Inc.


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.

Cellular organelles in a liver cell

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.


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, 4thEdition, Alberts et al. Cell Biology

Interactive 2002

Parenchyma cell from a plant (arum or voodoo lily,

Sauromatum guttum

). Note the cell wall, nucleus with

nucleolus, amyloplast with starch grains and mitochondria


Figure 1.19

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.

Red blood cell in a tiny capillary. Note the endothelial

cell (one containing a large nucleus) that forms the


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.


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




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