The cell wall possesses catalytic activity
But the cell wall is even more than just polysaccharide relatives. A critical component
of cell walls is protein that provides catalytic activity for the cell wall region.
Enzymes that polymerize wall monomers, enzymes that cross-link polymers, enzymes that
cleave polymers, these and others permit the cell wall region to be a dynamically-sculpted
element for a living cell.
The cell wall provides for turgor pressure
Far from being a barrier, the wall is partially for the structural support of a multicellular,
multidimensional plant body, but has a critical function in providing a means to survive in
a dilute solution of solutes. Soil water is a hypotonic medium and the wall provides a means
to avoid cell expansion that would otherwise exceed the bursting strength of the cell membrane.
It permits the development of turgor pressure which can be a structural and functional factor
in support and movement.
The cell membrane is more than phospholipids
The cell membrane, generally just inside of the cell wall and tightly apressed against it
because of turgor pressure, is the exchange regulator for the cell and its environment. Indeed
the oligosaccharide subunits and wall-sculpting enzymes are passed from the interior of the
cell through this membrane: by exocytosis. Water, minerals, sometimes organic particles, pass
from the environment through this membrane to the cell's interior: endocytosis. While
"goodies" are allowed to pass through the membrane, other substances are kept outside.
The cell membrane is shown below.
The phosopholipids provide barrier functions
The main barrier function is provided by the phospholipid bilayer. This bilayer is composed
of amphipathic phospholipid molecules. Two C14-C24 fatty acids, one saturated (no double
bonds) and the other at least mono-unsaturated (one double bond), comprise the hydrophobic
"tails" that make passage of charged and polar molecules into and out of the cell all but
impossible. These are linked by a glycerol (3C-sugar) to a very polar and hydrophilic
phosphate-small-organic-group "head." Thus polar solutes might pass into the "heads" part of
the membrane, but cannot pass through the non-polar "tails." Similarly non-polar molecules
would pass easily through the "tails" part of the membrane, but cannot approach that because
of the polar "heads" layer.
Phospholipids spontaneously form bilayers with the polar "heads" facing the aqueous
extra-cellular fluids and the aqueous cytosol. Between these layers of "heads" the hydrophobic
"tails" inter-mingle. The double-layer of "tails" thus provides a the barrier functions for
many biologically-interesting ions and molecules.
An individual phospholipid in the bilayer is free to move about in the plane of the bilayer.
Contributing to this fluidity of movement, or stabilizing this fluidity depending on
temperature are various steroids that are often a part of the membrane. In plants the
steroid components include wide range of distinct molecules.
The cell membrane proteins provide catalytic functions
But a cell membrane that is a great barrier is not good for the cell per se. In
fact a barrier is only as good as its doors and windows. The mark of a good barrier
is that it allows for exchange of positive elements while providing only exit and never
entrance of negative elements. This lesson, hopefully recalling the Iron Curtain, is
pointing to the proteins of a cell membrane.
The phospholipid bilayer is traversed by integral proteins. These proteins have two
hydrophilic zones separated by a hydrophobic zone of suitable dimensions to become
trans-membrane transport proteins. These may permit facilitated diffusion of suitably
small charged molecules or elements, or perhaps even active transport of essential
components. Active transport of course involves the conversion of ATP into ADP and
Pi or AMP, with the released energy from the cleavage of the phosphate bond driving
the movement of the component through the membrane.
The membrane is also a site of catalytic activity. In addition to integral proteins,
there may also be peripheral proteins attached to one side or the other of the bilayer.
The attachments may involve simple hydrophobic and hydrophilic domains of the protein,
but might also involve linkage of the protein to fatty acids or other hydrophobic
attachments that anchor them within the bilayer and leaving the protein facing the
aqueous environment. These proteins likely serve various catalytic or electronic
functions. Depending on which face of the bilayer we are examining, we might find
ATP synthases or electron transfer proteins here. We might also find enzymes,
such as succinate dehydrogenase, that catalyze steps associated with pathways in
one compartment or another adjacent to that surface of the bilayer. The many
proteins in a membrane may constitute 40% of that membrane! To simply call a
membrane a "phospholipid barrier" would be a gross over-simplification.
The cytosol is more than just water
Inside the cell wall and membrane we have the fluid compartment of the cell. Indeed
this compartment is mostly water...and water has some very important roles to play
in the life of a cell...but this fluid is much more than just water!
Saying "just water" is really an injustice to this critical molecule for life! We
will get into that later in the course in some detail. Let's remember it as the
medium for the creation of life, as a UV screen until the ozone layer helped out,
and as more than 90% of the weight of metabolically active cells.
This region was called cytoplasm (literally cell fluid) back in the days when
light microscopy was a cutting-edge technology. When electron microscopy was
developed, we learned that this cytoplasm was not just fluid...it contained
previously-invisible structures that are not simply fluid. So we use cytoplasm
generally to mean the fluid and all of these contained organelles together. A
newer word, cytosol (literally cell solvent), is used to mean only the fluid
in which the organelles are suspended.
The cytosol hosts tremendous catalytic activity
The cytosol contains dissolved minerals, gases, and organic molecules. These
provide for the many catalytic functions of the cytosol. Dissolved proteins,
often called soluble enzymes, are responsible for a range of biochemical sytheses
and degradations that characterize a living cell. The cytosol is host to a range
of entire pathways of regulated biochemistry! Examples include glycolysis,
fermentation, and (from a certain perspective) translation!
In addition to these functions, the cytosol serves as a hydraulic fluid for
organismal support, as a medium for diffusion, and as a compartment for osmosis.
It provides thermal buffering capacity and as a transparent medium for light
penetration. The differential solubility of oxygen gas and carbon-dioxide gas
provides for a medium in which photosynthesis can occur. Dissolved pigments
may provide protection in times of excessive light irradiation.
The nucleus is more than just a DNA container
The nucleus is an organelle of the cell that is not considered part of the term
cytoplasm. It was obvious in the early days of microscopy. Various staining
procedures revealed that it contained nucleic acids (DNA and RNA) well before
the hereditary roles of these molecules was known. But early tests also
indicated the presence of proteins.
The nuclear envelope is composed of two bilayer membranes
The outer membrane and inner membrane of the nuclear envelope are separated
by a perinuclear space. The bilayer facing the cytosol is perhaps very much
like that of the endoplasmic reticulum and often hosts polyribosomes. The
bilayer facing the nucleoplasm is coated with the nuclear lamina...a layer of
intermediate filaments. During mitosis the envelope is dispersed as small
vesicles that coalesce after mitoses.
The nuclear envelope has "pores" for the passage of large materials
The nuclear pore complex is a formation of structural and functional proteins
that permit the movement of molecules, macromolecules, and even the subunits
of ribosomes through the envelope. The pore complex is depicted below. A
key element of this complex is the transporter protein, which can be
regulated to permit or prohibit movement through the envelope.
DNA in the nucleus is found in chromosomes
The hereditary molecule, DNA, is amazingly long. Each molecule, called a
chromosome, is composed of many shorter lengths of DNA, called genes, that
are linked end to end. The sequence of nucleotides in each gene constitute
the instructions for the synthesis of a single protein. For various portions
at one time or another, and for the whole chromosome at the start of mitosis,
the chromosome is condensed in a process shown below.
Histone proteins are a primitive feature of most eukaryotic organisms. The
amino acid sequence of these proteins is exceedingly conservative among plants,
animals, and fungi. The DNA molecule is wrapped around histones in the form of
nucleosomes. These are cross-linked by other histones, and coiled and looped in
repeatable ways to ultimately produce the familiar X-shaped chromosomes we
observe during mitosis. Because the set of n chromosomes, as a group, represent
one entire genome, and two such sets are present in most plant nuclei, it is
no surprise that most of the DNA in any particular cell is not being expressed
(used) at any point in time. When one considers that each cell contains two
complete sets of instructions for a whole organism, it is clear that most of
the DNA must not be "active."
The nucleus often demonstrates a nucleolus
Early light microscope preparations were stained and even in interphase, when
the chromosomes are NOT condensed, one or more intensely-staining zones were
observed in the nucleus. These were called nucleoli. RNA-specific stains and
probes reveal that the nucleolus is a region with much RNA compared to the
rest of the nucleus. The nucleoli are considered to represent the location
of genes coding for ribosomal and transfer RNAs. In metabolically active
interphase cells, many ribosomes and transfer RNAs are needed to maintain
the protein pools needed for active metabolism. These genes, then, are being
transcribed at a very high rate and the RNA products are locally abundant,
explaining the intense local staining.
Protein stains also highlight the nucleolus. Proteins are translated in the
cytosol and those destined for the nucleus pass through the pore complex
into the nucleoplasm. Some of these are enzymes are involved in replication
and transcription, including DNA and RNA polymerase. Other proteins arriving
are the components that must join ribosomal RNA to form ribosome subunits.
The two ribosome subunits, the large and small subunits, are assembled in
the nucleolus region explaining the protein stain results. The ribosome
subunits are transported out of the nucleoplasm through the pore complex
separately. They join only after arrival in the cytosol and in the presence
of messenger RNA and transfer RNAs.
The nucleus is the site of mRNA transcription
In addition to the production of rRNA and tRNA, the nucleus is responsible
for producing messenger RNA. This process is called transcription. Regions
of the "active" genes in the genome possess nucleotide sequences which are
called the promoter. This region is recognized by RNA polymerase (a protein)
as a site for binding to the DNA. The RNA polymerase slides down the length
of the gene and catalyzes the formation of a single-stranded RNA transcript.
If this gene codes for a protein, the RNA transcript is called messenger RNA.
The processes of transcription and translation are depicted below.
The messenger RNA will be modified (introns removed, etc.) and shipped out of
the nucleus through the pore complex. In the cytosol, it will join to the small
and large subunits of a ribosome and through the interaction of the ribosome
with transfer RNAs, a protein will be synthesized.
The ribosome is the site of protein translation
The ribosome subunits attach to the mRNA and together slide along the RNA. As
the subunits pass along the RNA, transfer RNAs deliver amino acids as coded by
the sequence of nucleotides in the RNA. The ribosome proteins and ribozymes catalyze the
formation of the peptide bond between the amino acids to provide the primary
structure of the developing polypeptide.
Ribosomes can be found freely in the cytosol and produce "soluble" proteins
that are used locally in the cytosol. Other ribosomes are associated with
the endoplasmic reticulum. These are attahed to mRNAs that code for a hydrophobic
signal sequence of 18-30 amino acids in the developing polypeptide. These
sequences bind to a signal recognition particle which facilitates the attachment
of the ribosome to the bilayer of the endoplasmic reticulum and the penetration
of the developing polypeptide into the lumen of the ER. Once inside the lumen,
the polypeptide is processed in various ways to attach a particular
oligosaccharide to the polypeptide. This "label" facilitates the sorting,
packaging, and transport of the polypeptides to ensure their delivery to
the correct intracellular or extracellular location.
The difference between rough ER and smooth ER is both structural
and functional. Areas where the ER is associated with ribosomes (rough ER)
the main functions are translation of export proteins. Areas where the ER
is not associated with ribosomes (smooth ER) carry out synthesis of lipids
and other membrane components. ER located near the nucleus is often rough
ER; areas of the cell remote from the nucleus are more likely to be
smooth ER.
The endoplasmic reticulum processes and transports materials across the cell
The endoplasmic reticulum is a network of connected membrane sacs and
tubules. Materials crossing into the lumen from the cytosol are transported
from one region of the cell to another. Precisely how the ER accomplishes
the movements is apparently unknown.
Ultimately proteins that require packaging and export arrive through the
network near an ending close to a Golgi apparatus. Here the ER produces
a vesicle that contains the protein and carries it across the cytosol
toward the Golgi.
The Golgi (dictyosome) has two faces
The Golgi apparatus is very much like a specialized stack of ER; it is
depicted. The layers of the stack which are closest
to the ER are called the cis face; the layers which are closer to
the cell membrane are called the trans face. Each layer in the stack
is called a cisternum.
The cisternae of the cis face of the Golgi receive the vesicles from the
ER with their contents. Within the cisternae, the oligosaccharides at the
end of the protein are modified again prior to export. Cisternae on the
trans face either produce secretory vesicles or disintegrate into
large populations of export vesicles. In the latter case, new cisternae are
produced on the cis face to replace those breaking up on the trans face.
Which of these two mechanisms, or some combination, is reality has not
been determined precisely.
The secretion vesicles can migrate to the cell membrane and participate in exocytosis.
One type of vesicle that is critical for plants is a secretory vesicle
that releases cell wall oligosaccharides and wall-enzymes by exocytosis
into the cell wall environment. Another vesicle, found in storage cells, are
coated with clathrin and contain proteins and other materials for digestion.
These transport materials to special vacuoles for intracellular digestion.
There are also means for lysosomes to participate in this process.
We will continue with Basic Cytology in the next lecture.
