Each tissue and region is composed of various differentiated cell types which together provide for the functions achieved in the tissues. Several different cell types can be observed in plants.
There are wax-coated dermal cells, isodiametric parenchyma and elongate collenchyma ground tissue cells. Some of the latter are sclerenchyma cells. Each of these have a range of structural and functional features that distinguish them.
The hollow tracheary elements of xylem are most closely related to sclerenchyma in structure and function. The living sieve tube elements are most closely related to parenchyma in structure and function, but are clearly more derived. The sieve tube elements typically lose their membrane-bound organelles including nucleus, mitochondria, and plastids. An adjacent complete parenchyma-type cell, called the companion cell keeps the cytoplasm of the sieve tube element alive through plasmodesma connections.
To be honest, perhaps there is nothing. Some cells lack cell walls (plant sperm cells, for example) and other cells lack everything else (xylem vessel element, for example). But because of cells that are dead at functional maturity, the best answer would probably be the cell wall. In consideration of the sheer mass and volume of dead cells in the trunk of a Sequoia tree, for example, a dead cell might arguably be more "typical" than what is often depicted as a "typical" plant cell in textbooks.
Because a parenchyma cell is alive and is responsible for virtually all of the photosynthesis and respiration in a plant, most books will show the parenchyma cell as "typical." Below is a cartoon of what a parenchyma cell looks like in an electron microscope view. This cell could be, for example, a spongy mesophyll cell from a leaf.
Many people know that the cell wall is made of layers of variously arranged/aligned cellulose microfibrils. But this polymer of glucose is not the only wall element by any means! In addition to cellulose, walls have a range of various polymers of sugars and sugar-derivatives. Hemicellulose rhamnogalacturonan, and pectins are shown here.
Hemicellulose provides cross-linking of the cellulose microfibrils. Pectins are the glue that holds adjacent cells together...the middle-lamella is comprised of much pectin. In secondary xylem cells and others, another polymer class is the condensed lignins. In some kinds of wood, up to 40% of the weight of the wood can be attributed to lignins. One possible structure of lignin is shown below.
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.
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, 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 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.
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.
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 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 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 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 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.
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."
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.
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 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 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 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.