Plastids

Plastids

The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a major organelle found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. They often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's color. They possess a double-stranded DNA molecule, which is circular, like that of prokaryotes.

Plastids in plants

Those plastids which contain pigments can carry out photosynthesis. Plastids can also store products like starch and can synthesise fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis of other molecules. For example, the components of the plant cuticle and its epicuticular wax, are synthesized by the epidermal cells from palmitic acid, which is synthesized in the chloroplasts of the mesophyll tissue.[1] All plastids are derived from proplastids which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide by binary fission, but more mature chloroplasts also have this capacity.

 Plant cells with visible chloroplasts

 

In plants, plastids may differentiate into several forms, depending upon which function they play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following variants.

 

• Chloroplasts green plastids: for photosynthesis; see also etioplasts, the predecessors of chloroplasts
• Chromoplasts coloured plastids: for pigment synthesis and storage
• Gerontoplasts: control the dismantling of the photosynthetic apparatus during senescence
• Leucoplasts colourless plastids: for monoterpene synthesis; leucoplasts sometimes differentiate into more specialized plastids:
• Amyloplasts: for starch storage and detecting gravity
• Elaioplasts: for storing fat
• Proteinoplasts: for storing and modifying protein
• Tannosomes: for synthesizing and producing tannins and polyphenols

Depending on their morphology and function, plastids have the ability to differentiate, or redifferentiate, between these and other forms.

Each plastid creates multiple copies of a circular 75–250 kilobase plastome. The number of genome copies per plastid is variable, ranging from more than 1000 in rapidly dividing cells, which, in general, contain few plastids, to 100 or fewer in mature cells, where plastid divisions have given rise to a large number of plastids. The plastome contains about 100 genes encoding ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as proteins involved in photosynthesis and plastid gene transcription and translation. However, these proteins only represent a small fraction of the total protein set-up necessary to build and maintain the structure and function of a particular type of plastid. Plant nuclear genes encode the vast majority of plastid proteins, and the expression of plastid genes and nuclear genes is tightly co-regulated to coordinate proper development of plastids in relation to cell differentiation.

Plastid DNA exists as large protein-DNA complexes associated with the inner envelope membrane and called 'plastid nucleoids'. Each nucleoid particle may contain more than 10 copies of the plastid DNA. The proplastid contains a single nucleoid located in the centre of the plastid. The developing plastid has many nucleoids, localized at the periphery of the plastid, bound to the inner envelope membrane. During the development of proplastids to chloroplasts, and when plastids convert from one type to another, nucleoids change in morphology, size and location within the organelle. The remodelling of nucleoids is believed to occur by modifications to the composition and abundance of nucleoid proteins.

Many plastids, particularly those responsible for photosynthesis, possess numerous internal membrane layers.

In plant cells, long thin protuberances called stromules sometimes form and extend from the main plastid body into the cytosol and interconnect several plastids. Proteins, and presumably smaller molecules, can move within stromules. Most cultured cells that are relatively large compared to other plant cells have very long and abundant stromules that extend to the cell periphery.

Plastids in algae

In algae, the term leucoplast is used for all unpigmented plastids and their function differs from the leucoplasts of plants. Etioplasts, amyloplasts and chromoplasts are plant-specific and do not occur in algae.^{[citation needed]} Plastids in algae and hornworts may also differ from plant plastids in that they contain pyrenoids.

Glaucocystophytic algae contain muroplasts, which are similar to chloroplasts except that they have a cell wall that is similar to that of prokaryotes. Rhodophytic algae contain rhodoplasts, which are red chloroplasts that allow the algae to photosynthesise to a depth of up to 268 m.^[2]

Inheritance of plastids

Most plants inherit the plastids from only one parent. In general, angiosperms inherit plastids from the female gamete, whereas many gymnosperms inherit plastids from the male pollen. Algae also inherit plastids from only one parent. The plastid DNA of the other parent is, thus, completely lost.

In normal intraspecific crossings (resulting in normal hybrids of one species), the inheritance of plastid DNA appears to be quite strictly 100% uniparental. In interspecific hybridisations, however, the inheritance of plastids appears to be more erratic. Although plastids inherit mainly maternally in interspecific hybridisations, there are many reports of hybrids of flowering plants that contain plastids of the father. Approximately 20% of angiosperms, including alfalfa (Medicago sativa), normally show biparental inheritance of plastids.^[3]

 Origin of plastids

Plastids are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 billion years ago and enabled eukaryotes to carry out oxygenic photosynthesis.^[4] Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and muroplasts in the glaucophytes. The plastids differ by their pigmentation, but also in ultrastructure. The chloroplasts, e.g., have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead contain stroma and grana thylakoids, structures found only in plants and in closely related green algae. The glaucocystophycean plastid — in contrast to the chloroplasts and the rhodoplasts — is still surrounded by the remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes.

Complex plastids start by secondary endosymbiosis, when a eukaryote engulfs a red or green alga and retains the algal plastid, which is typically surrounded by more than two membranes. In some cases these plastids may be reduced in their metabolic and/or photosynthetic capacity. Algae with complex plastids derived by secondary endosymbiosis of a red alga include the heterokonts, haptophytes, cryptomonads, and most dinoflagellates (= rhodoplasts). Those that endosymbiosed a green alga include the euglenids and chlorarachniophytes (= chloroplasts). The Apicomplexa, a phylum of obligate parasitic protozoa including the causative agents of malaria (Plasmodium spp.), toxoplasmosis (Toxoplasma gondii), and many other human or animal diseases also harbor a complex plastid (although this organelle has been lost in some apicomplexans, such as Cryptosporidium parvum, which causes cryptosporidiosis). The 'apicoplast' is no longer capable of photosynthesis, but is an essential organelle, and a promising target for antiparasitic drug development.

Some dinoflagellates and sea slugs, in particular of the genus Elysia, take up algae as food and keep the plastid of the digested alga to profit from the photosynthesis; after a while, the plastids are also digested. This process is known as kleptoplasty, from the Greek, kleptes, thief.

From :

http://en.wikipedia.org/wiki/Plastids

https://www.youtube.com/watch?v=cldx7bObtto

Mitochondria

Mitochondria are unusual organelles. They act as the power plants of the cell, are surrounded by two membranes, and have their own genome. They also divide independently of the cell in which they reside, meaning mitochondrial replication is not coupled to cell division. Some of these features are holdovers from the ancient ancestors of mitochondria, which were likely free-living prokaryotes.

What Is the Origin of Mitochondria?

Mitochondria are thought to have originated from an ancient symbiosis that resulted when a nucleated cell engulfed an aerobic prokaryote. The engulfed cell came to rely on the protective environment of the host cell, and, conversely, the host cell came to rely on the engulfed prokaryote for energy production. Over time, the descendants of the engulfed prokaryote developed into mitochondria, and the work of these organelles — using oxygen to create energy — became critical to eukaryotic evolution (Figure 1).

Figure 1: A mitochondrion

© 2010 Nature Education All rights reserved.

Modern mitochondria have striking similarities to some modern prokaryotes, even though they have diverged significantly since the ancient symbiotic event. For example, the inner mitochondrial membrane contains electron transport proteins like the plasma membrane of prokaryotes, and mitochondria also have their own prokaryote-like circular genome. One difference is that these organelles are thought to have lost most of the genes once carried by their prokaryotic ancestor. Although present-day mitochondria do synthesize a few of their own proteins, the vast majority of the proteins they require are now encoded in the nuclear genome.

What Is the Purpose of a Mitochondrial Membranes?

As previously mentioned, mitochondria contain two major membranes. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein. In contrast, the inner membrane has much more restricted permeability, much like the plasma membrane of a cell. The inner membrane is also loaded with proteins involved in electron transport and ATP synthesis. This membrane surrounds the mitochondrial matrix, where the citric acid cycle produces the electrons that travel from one protein complex to the next in the inner membrane. At the end of this electron transport chain, the final electron acceptor is oxygen, and this ultimately forms water (H20). At the same time, the electron transport chain produces ATP. (This is why the the process is called oxidative phosphorylation.)
During electron transport, the participating protein complexes push protons from the matrix out to the intermembrane space. This creates a concentration gradient of protons that another protein complex, called ATP synthase, uses to power synthesis of the energy carrier molecule ATP (Figure 2).

Figure 2: The electrochemical proton gradient and ATP synthase

At the inner mitochondrial membrane, a high energy electron is passed along an electron transport chain. The energy released pumps hydrogen out of the matrix space. The gradient created by this drives hydrogen back through the membrane, through ATP synthase. As this happens, the enzymatic activity of ATP synthase synthesiszes ATP from ADP.

© 2010 Nature Education All rights reserved.

Figure Detail

Is the Mitochondrial Genome Still Functional?

Mitochondrial genomes are very small and show a great deal of variation as a result of divergent evolution. Mitochondrial genes that have been conserved across evolution include rRNA genes, tRNA genes, and a small number of genes that encode proteins involved in electron transport and ATP synthesis. The mitochondrial genome retains similarity to its prokaryotic ancestor, as does some of the machinery mitochondria use to synthesize proteins. In fact, mitochondrial rRNAs more closely resemble bacterial rRNAs than the eukaryotic rRNAs found in cell cytoplasm. In addition, some of the codons that mitochondria use to specify amino acids differ from the standard eukaryotic codons.

Still, the vast majority of mitochondrial proteins are synthesized from nuclear genes and transported into the mitochondria. These include the enzymes required for the citric acid cycle, the proteins involved in DNA replication and transcription, and ribosomal proteins. The protein complexes of the respiratory chain are a mixture of proteins encoded by mitochondrial genes and proteins encoded by nuclear genes. Proteins in both the outer and inner mitochondrial membranes help transport newly synthesized, unfolded proteins from the cytoplasm into the matrix, where folding ensues (Figure 3).

Figure 3: Protein import into a mitochondrion

A signal sequence at the tip of a protein (blue) recognizes a receptor protein (pink) on the outer mitochondrial membrane and sticks to it. This causes diffusion of the tethered protein and its receptor through the membrane to a contact site, where translocator proteins line up (green). When at this contact site, the receptor protein hands off the tethered protein to the translocator protein, which then channels the unfolded protein past both the inner and outer mitochondrial membranes.

© 2010 Nature Education All rights reserved.

Figure Detail

How Many Mitochondria Do Cells Have?

Mitochondria cannot be made "from scratch" because they need both mitochondrial and nuclear gene products. These organelles replicate by dividing in two, using a process similar to the simple, asexual form of cell division employed by bacteria. Video microscopy shows that mitochondria are incredibly dynamic. They are constantly dividing, fusing, and changing shape. Indeed, a single mitochondrion may contain multiple copies of its genome at any given time.

Logically, mitochondria multiply when a the energy needs of a cell increase. Therefore, power-hungry cells have more mitochondria than cells with lower energy needs. For example, repeatedly stimulating a muscle cell will spur the production of more mitochondria in that cell, to keep up with energy demand.

Conclusion

Mitochondria, the so-called "powerhouses" of cells, are unusual organelles in that they are surrounded by a double membrane and retain their own small genome. They also divide independently of the cell cycle by simple fission. Mitochondrial division is stimulated by energy demand, so cells with an increased need for energy contain greater numbers of these organelles than cells with lower energy needs.

From : http://www.nature.com/scitable/topicpage/mitochondria-14053590
From : https://www.youtube.com/watch?v=4J1GgSg0u1s

Vacuoles

Vacuoles

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal^[1] and bacterial cells.^[2] Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these.^[3] The organelle has no basic shape or size; its structure varies according to the needs of the cell.
The function and significance of vacuoles varies greatly according to the type of cell in which they are present, having much greater prominence in the cells of plants, fungi and certain protists than those of animals and bacteria. In general, the functions of the vacuole include:

 

Plant Cell 

Animal Cell

• Isolating materials that might be harmful or a threat to the cell
• Containing waste products
• Containing water in plant cells
• Maintaining internal hydrostatic pressure or turgor within the cell
• Maintaining an acidic internal pH
• Containing small molecules
• Exporting unwanted substances from the cell
• Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole
• In seeds, stored proteins needed for germination are kept in 'protein bodies', which are modified vacuoles.^[4] 

Vacuoles also play a major role in autophagy, maintaining a balance between biogenesis (production) and degradation (or turnover), of many substances and cell structures in certain organisms. They also aid in the lysis and recycling of misfolded proteins that have begun to build up within the cell. Thomas Boller ^[5] and others proposed that the vacuole participates in the destruction of invading bacteria and Robert B Mellor proposed organ-specific forms have a role in 'housing' symbiotic bacteria. In protists, vacuoles have the additional function of storing food which has been absorbed by the organism and assisting in the digestive and waste management process for the cell.^[6]
The vacuole probably evolved several times independently, even within the Viridiplantae.^[7]

Bacteria

Large vacuoles are found in three genera of filamentous sulfur bacteria, the Thioploca, Beggiatoa and Thiomargarita. The cytosol is extremely reduced in these genera and the vacuole can occupy between 40–98% of the cell.^[8] The vacuole contains high concentrations of nitrate ions and is therefore thought to be a storage organelle.^[2]
Gas vacuoles, which are freely permeable to gas,^[9] are present in some species of Cyanobacteria. They allow the bacteria to control their buoyancy.

Plants

Most mature plant cells have one large vacuole that typically occupies more than 30% of the cell's volume, and that can occupy as much as 80% of the volume for certain cell types and conditions.^[10] Strands of cytoplasm often run through the vacuole.
A vacuole is surrounded by a membrane called the tonoplast (word origin: Gk tón(os) + -o-, meaning “stretching”, “tension”, “tone” + comb. form repr. Gk plastós formed, molded). Also called the vacuolar membrane, the tonoplast is the cytoplasmic membrane surrounding a vacuole, separating the vacuolar contents from the cell's cytoplasm. As a membrane, it is mainly involved in regulating the movements of ions around the cell, and isolating materials that might be harmful or a threat to the cell.^[11]
Transport of protons from the cytosol to the vacuole stabilizes cytoplasmic pH, while making the vacuolar interior more acidic creating a proton motive force which the cell can use to transport nutrients into or out of the vacuole. The low pH of the vacuole also allows degradative enzymes to act. Although single large vacuoles are most common, the size and number of vacuoles may vary in different tissues and stages of development. For example, developing cells in the meristems contain small provacuoles and cells of the vascular cambium have many small vacuoles in the winter and one large one in the summer.
Aside from storage, the main role of the central vacuole is to maintain turgor pressure against the cell wall. Proteins found in the tonoplast (aquaporins) control the flow of water into and out of the vacuole through active transport, pumping potassium (K⁺) ions into and out of the vacuolar interior. Due to osmosis, water will diffuse into the vacuole, placing pressure on the cell wall. If water loss leads to a significant decline in turgor pressure, the cell will plasmolyze. Turgor pressure exerted by vacuoles is also required for cellular elongation: as the cell wall is partially degraded by the action of expansins, the less rigid wall is expanded by the pressure coming from within the vacuole. Turgor pressure exerted by the vacuole is also essential in supporting plants in an upright position. Another function of a central vacuole is that it pushes all contents of the cell's cytoplasm against the cellular membrane, and thus keeps the chloroplasts closer to light.^[12] Most plants store chemicals in the vacuole that react with chemicals in the cytosol. If the cell is broken, for example by a herbivore, then the two chemicals can react forming toxic chemicals. In garlic, alliin and the enzyme alliinase are normally separated but form allicin if the vacuole is broken. A similar reaction is responsible for the production of syn-propanethial-S-oxide when onions are cut.^{[citation needed]}

Fungi

Vacuoles in fungal cells perform similar functions to those in plants and there can be more than one vacuole per cell. In yeast cells the vacuole is a dynamic structure that can rapidly modify its morphology. They are involved in many processes including the homeostasis of cell pH and the concentration of ions, osmoregulation, storing amino acids and polyphosphate and degradative processes. Toxic ions, such as strontium (Sr2+), cobalt(II) (Co2+), and lead(II) (Pb2+) are transported into the vacuole to isolate them from the rest of the cell.^[13]

Animals

In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis.
Animal vacuoles are smaller than their plant counterparts but also usually greater in number.^[14] There are also animal cells that do not have any vacuoles.^[15]
Exocytosis is the extrusion process of proteins and lipids from the cell. These materials are absorbed into secretory granules within the Golgi apparatus before being transported to the cell membrane and secreted into the extracellular environment. In this capacity, vacuoles are simply storage vesicles which allow for the containment, transport and disposal of selected proteins and lipids to the extracellular environment of the cell.
Endocytosis is the reverse of exocytosis and can occur in a variety of forms. Phagocytosis ("cell eating") is the process by which bacteria, dead tissue, or other bits of material visible under the microscope are engulfed by cells. The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. Pinocytosis ("cell drinking") is essentially the same process, the difference being that the substances ingested are in solution and not visible under the microscope.^[16] Phagocytosis and pinocytosis are both undertaken in association with lysosomes which complete the breakdown of the material which has been engulfed.^[17]
Salmonella is able to survive and reproduce in the vacuoles of several mammal species after being engulfed.<sup>[18]

From: http://en.wikipedia.org/wiki/Vacuole</sup>

 

 

Plasma Membrane

Plasma Membrane

All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents and serves as a semi-porous barrier to the outside environment. The membrane acts as a boundary, holding the cell constituents together and keeping other substances from entering. The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

According to the accepted current theory, known as the fluid mosaic model, the plasma membrane is composed of a double layer (bilayer) of lipids, oily substances found in all cells (see Figure 1). Most of the lipids in the bilayer can be more precisely described as phospholipids, that is, lipids that feature a phosphate group at one end of each molecule. Phospholipids are characteristically hydrophilic ("water-loving") at their phosphate ends and hydrophobic ("water-fearing") along their lipid tail regions. In each layer of a plasma membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate groups are aligned so they face outwards, either toward the aqueous cytosol of the cell or the outside environment. Phospholipids tend to spontaneously aggregate by this mechanism whenever they are exposed to water.
Within the phospholipid bilayer of the plasma membrane, many diverse proteins are embedded, while other proteins simply adhere to the surfaces of the bilayer. Some of these proteins, primarily those that are at least partially exposed on the external side of the membrane, have carbohydrates attached to their outer surfaces and are, therefore, referred to as glycoproteins. The positioning of proteins along the plasma membrane is related in part to the organization of the filaments that comprise the cytoskeleton, which help anchor them in place. The arrangement of proteins also involves the hydrophobic and hydrophilic regions found on the surfaces of the proteins: hydrophobic regions associate with the hydrophobic interior of the plasma membrane and hydrophilic regions extend past the surface of the membrane into either the inside of the cell or the outer environment.
Plasma membrane proteins function in several different ways. Many of the proteins play a role in the selective transport of certain substances across the phospholipid bilayer, either acting as channels or active transport molecules. Others function as receptors, which bind information-providing molecules, such as hormones, and transmit corresponding signals based on the obtained information to the interior of the cell. Membrane proteins may also exhibit enzymatic activity, catalyzing various reactions related to the plasma membrane.
Since the 1970s, the plasma membrane has been frequently described as a fluid mosaic, which is reflective of the discovery that oftentimes the lipid molecules in the bilayer can move about in the plane of the membrane. However, depending upon a number of factors, including the exact composition of the bilayer and temperature, plasma membranes can undergo phase transitions which render their molecules less dynamic and produce a more gel-like or nearly solid state. Cells are able to regulate the fluidity of their plasma membranes to meet their particular needs by synthesizing more of certain types of molecules, such as those with specific kinds of bonds that keep them fluid at lower temperatures. The presence of cholesterol and glycolipids, which are found in most cell membranes, can also affect molecular dynamics and inhibit phase transitions.
In prokaryotes and plants, the plasma membrane is an inner layer of protection since a rigid cell wall forms the outside boundary for their cells. The cell wall has pores that allow materials to enter and leave the cell, but they are not very selective about what passes through. The plasma membrane, which lines the cell wall, provides the final filter between the cell interior and the environment.
Eukaryotic animal cells are generally thought to have descended from prokaryotes that lost their cell walls. With only the flexible plasma membrane left to enclose them, these primordial creatures would have been able to expand in size and complexity. Eukaryotic cells are generally ten times larger than prokaryotic cells and have membranes enclosing interior components, the organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials, allowing the cell to segregate its chemical functions into discrete internal compartments.

Cell wall

Cell Walls - Structure & Function

I. Functions of the cell wall:   The cell wall serves a variety of purposes including:

1. maintaining/determining cell shape (analogous to an external skeleton for every cell).  Since protoplasts are invariably round, this is good evidence that the wall ultimately determines the shape of plant cells.
    
2. Support and mechanical strength (allows plants to get tall, hold out thin leaves to obtain light)
    
3. prevents the cell membrane from bursting in a hypotonic medium (i.e., resists water pressure)
    
4. controls the rate and direction of cell growth and regulates cell volume
    
5. ultimately responsible for the plant architectural design and controlling plant morphogenesis since the wall dictates that plants develop by cell addition (not cell migration)
    
6. has a metabolic role (i.e., some of the proteins in the wall are enzymes for transport, secretion)
    
7. physical barrier to: (a) pathogens; and (b) water in suberized cells.  However, remember that the wall is very porous and allows the free passage of small molecules, including proteins up to 60,000 MW. The pores are about 4 nm (Tepfert & Taylor 1987)
    
8. carbohydrate storage - the components of the wall can be reused in other metabolic processes (especially in seeds).  Thus, in one sense the wall serves as a storage repository for carbohydrates
    
9. signaling  - fragments of wall, called oligosaccharins, act as hormones. Oligosaccharins, which can result from normal development or pathogen attack, serve a variety of functions including:  (a) stimulate ethylene synthesis; (b) induce phytoalexin (defense chemicals produced in response to a fungal/bacterial infection) synthesis; (c) induce chitinase and other enzymes; (d) increase cytoplasmic calcium levels and (d) cause an "oxidative burst".  This burst produces hydrogen peroxide, superoxide and other active oxygen species that attack the pathogen directly or cause increased cross-links in the wall making the wall harder to penetrate.
   
       Let's look at how this system works.  Consider a pathogenic fungus like Phytophthora.  In contact with the host plant the fungus releases enzymes such as pectinase that break down plant wall components into oligosaccharins.  The oligosaccharins stimulate the oxidative burst and phytoalexin synthesis, both which will deter the advance of the fungus.  In addition, the oligosaccharins stimulate chitinase and glucanase production in the plant.  These are released and begin to digest the fungal wall.  Fragments of fungal wall also act as oligosaccharins in the plant to further induce phytoalexin synthesis.  Cool!
    
10. recognition responses - for example: (a) the wall of roots of legumes is important in the nitrogen-fixing bacteria colonizing the root to form nodules; and (b) pollen-style interactions are mediated by wall chemistry.
     
11. economic products - cell walls are important for products such as paper, wood, fiber, energy, shelter, and even roughage in our diet.
    

II. Wall Components - Chemistry
    The main ingredient in cell walls are polysaccharides (or complex carbohydrates or complex sugars) which are built from monosaccharides (or simple sugars).  Eleven different monosaccharides are common in these polysaccharides including glucose and galactose. Carbohydrates are good building blocks because they can produce a nearly infinite variety of structures.  There are a variety of other components in the wall including protein, and lignin. Let's look at these wall components in more detail:
A. Cellulose
   β1,4-glucan (structure provided in class). Made of as many as 25,000 individual glucose molecules. Every other molecule (called residues) is "upside down". Cellobiose (glucose-glucose disaccharide) is the basic building block. Cellulose readily forms hydrogen bonds with itself (intra-molecular H-bonds) and with other cellulose chains (inter-molecular H-bonds). A cellulose chain will form hydrogen bonds with about 36 other chains to yield a microfibril. This is somewhat analogous to the formation of a thick rope from thin fibers. Microfibrils are 5-12 nm wide and give the wall strength - they have a tensile strength equivalent to steel. Some regions of the microfibrils are highly crystalline while others are more "amorphous". 
B. Cross-linking glycans (=Hemicellulose)
    Diverse group of carbohydrates that used to be called hemicellulose.  Characterized by being soluble in strong alkali. They are linear (straight), flat, with a β-1,4 backbone and relatively short side chains. Two common types include xyloglucans and glucuronarabinoxylans. Other less common ones include glucomannans, galactoglucomannans, and galactomannans.  The main feature of this group is that they don’t aggregate with themselves - in other words, they don’t form microfibrils. However, they form hydrogen bonds with cellulose and hence the reason they are called "cross-linking glycans". There may be a fucose sugar at the end of the side chains which may help keep the molecules planar by interacting with other regions of the chain.
C. Pectic polysaccharides
    These are extracted from the wall with hot water or dilute acid or calcium chelators (like EDTA).  They are the easiest constituents to remove from the wall. They form gels (i.e., used in jelly making). They are also a diverse group of polysaccharides and are particularly rich in galacturonic acid (galacturonans = pectic acids).   They are polymers of primarily β 1,4 galacturonans (=polygalacturonans) are called homogalacturons (HGA) and are particularly common.  These are helical in shape. Divalent cations, like calcium, also form cross-linkages to join adjacent polymers creating a gel.  Pectic polysaccharides can also be cross-linked by dihydrocinnamic or diferulic acids.  The HGA's (galacturonans) are  initially secreted from the golgi as methylated polymers; the methyl groups are removed by pectin methylesterase to initiate calcium binding.

    Other pectic acids include Rhamnogalacturonan II (RGII) which features rhamnose and galacturonic acid in combination with a large diversity of other sugars in varying linkages.  Dimers of RGII can be cross-linked by boron atoms linked to apiose sugars in a side chain.

    Although most pectic polysaccharides are acidic, others are composed of neutral sugars including arabinans and galactans.  The pectic polysaccharides serve a variety of functions including determining wall porosity, providing a charged wall surface for cell-cell adhesion - or in other words gluing cells together (i.e,. middle lamella), cell-cell recognition, pathogen recognition and others.
D. Protein
    Wall proteins are typically glycoproteins (polypeptide backbone with carbohydrate side chains). The proteins are particularly rich in the amino acids hydroxyproline (hydroxyproline-rich glycoprotein, HPRG), proline (proline-rich protein, PRP), and glycine (glycine-rich protein, GRP).  These proteins form rods (HRGP, PRP) or beta-pleated sheets (GRP).  Extensin is a well-studied HRGP.   HRGP is induced by wounding and pathogen attack. The wall proteins also have a structural role since: (1) the amino acids are characteristic of other structural proteins such as collagen; and (2) to extract the protein from the wall requires destructive conditions. Protein appears to be cross-linked to pectic substances and may have sites for lignification. The proteins may serve as the scaffolding used to construct the other wall components.

    Another group of wall proteins are heavily glycosylated with arabinose and galactose. These arabinogalactan proteins, or AGP's, seem to be tissue specific and may function in cell signaling. They may be important in embryogenesis and growth and guidance of the pollen tube.
E. Lignin
    Polymer of phenolics, especially phenylpropanoids. Lignin is primarily a strengthening agent in the wall. It also resists fungal/pathogen attack.
F. Suberin, wax, cutin
    A variety of lipids are associated with the wall for strength and waterproofing.
G. Water
    The wall is largely hydrated and comprised of between 75-80% water. This is responsible for some of the wall properties. For example, hydrated walls have greater flexibility and extensibility than non-hydrated walls.
III. Morphology of the Cell Wall - there are three major regions of the wall:

1. Middle lamella - outermost layer, glue that binds adjacent cells, composed primarily of pectic polysaccharides.
   
2. Primary wall - wall deposited by cells before and during active growth. The primary wall of cultured sycamore cells is comprised of pectic polysaccharides (ca. 30%), cross-linking glycans (hemicellulose; ca 25%), cellulose (15-30%) and protein (ca. 20%) (see Darvill et al, 1980). The actual content of the wall components varies with species and age.  All plant cells have a middle lamella and primary wall.
   
3. Secondary Wall - some cells deposit additional layers inside the primary wall. This occurs after growth stops or when the cells begins to differentiate (specialize). The secondary wall is mainly for support and is comprised primarily of cellulose and lignin. Often can distinguish distinct layers, S1, S2 and S3 - which differ in the orientation, or direction, of the cellulose microfibrils.

IV. Tire analogy for the cell wall
    The wall is similar to a tire that has a series of steel belts or cords embedded in an amorphous matrix of rubber. In the plant cell wall, the "cords" are analogous to the cellulose microfibrils and they provide the structural strength of the wall. The matrix of the wall is analogous to the rubber in the tire and is comprised of non-cellulosic wall components. How are the various wall polymers arranged?  It appears that:

1. cross-linking glycans (hemicellulosic polysaccharides) are hydrogen bonded to the cellulose microfibrils
2. cross-linking glycans may also be entrapped inside cellulose microfibrils as they form
3. the different types of pectic polysaccharides are covalently bonded to one another
4. calcium bridges link pectic acids
5. connections between the protein and other wall polymers are still not clear
6. pectic polysaccharides and cross-linking glycans interact 
7. cross-linking glycans are linked by ferulic acid bridges or boron

V. Wall Formation
    The cell wall is made during cell division when the cell plate is formed between daughter cell nuclei. The cell plate forms from a series of vesicles produced by the golgi apparatus. The vesicles migrate along the cytoskeleton and move to the cell equator. The vesicles coalesce and dump their contents. The membranes of the vesicle become the new cell membrane. The golgi synthesizes the non-cellulosic polysaccharides. At first, the golgi vesicles contain mostly pectic polysaccharides that are used to build the middle lamella. As the wall is deposited, other non-cellulosic polysaccharides are made in the golgi and transported to the growing wall.
    Cellulose is made at the cell surface. The process is catalyzed by the enzyme cellulose synthase that occurs in a rosette complex in the membrane. Cellulose synthase, which is initially made in by the ribosomes (rough ER) and move from the ER → vesicles →  golgi →  vesicle →  cell membrane.  The enzyme apparently has two catalytic sites that transfer two glucoses at a time (i.e., cellobiose) from UDP-glucose to the growing cellulose chain.  Sucrose may supply the glucose that binds to the UDP.   Wall protein is presumably incorporated into the wall in a similar fashion.
    Remember that the wall is made from the outside in. Thus, as the wall gets thicker the lumen (space within the wall) gets smaller.
    Exactly how the wall components join together to form the wall once they are in place is not completely understood. Two methods seem likely: 

1. self assembly. This means that the wall components spontaneously aggregate; and
    
2. enzymatic assembly – various enzymatic reactions (XET) are designed for wall assembly. For example, one group of enzymes "stitches" xylans together in the wall to form long chains.  Oxidases may catalyze additional cross-linking between wall components and pectin methyl esterase may play an important role (see below).

VI. Strong Wall/Cell Expansion Paradox (don’t you just love a good paradox?)
    How can the wall be strong (it must withstand pressures of 100 MPa!), yet still allow for expansion? Good question, eh? The answer requires that the wall:
A. Be capable of expansion
    In other words, only cells with primary walls are capable of growth since the formation of the secondary wall precludes further expansion of the cell. The sequence of microfibril orientation changes during development. Initially the microfibrils are laid down somewhat randomly (isotropically). Such a cell can expand in any direction. As the cell matures, most microfibrils are laid down laterally, like the hoops of a barrel, which restricts lateral growth but permits growth in length.  As the cell elongates the microfibrils take on an overlapping cross-hatched pattern, similar to fiberglass. This occurs because the cell expands like a slinky - the width of the cell doesn't change by the microfibrils become aligned in the direction of growth just like the spring. This overlapping of microfibrils, which is strong and lightweight, prohibits further expansion.  
    But, what determines the orientation of the microfibrils? They are correlated with the direction of the microtubules in the cell. Evidence: treating a cell with colchicine or oryzalin (which inhibit microtubule formation) destroys the orientation of the microfibrils. The microtubules apparently direct the cellulose synthesizing enzymes to the plasma membrane.
    In addition to cellulose microfibril orientation, mature walls apparently loose their ability to expand because the wall components become resistant to loosening-activities. This would occur if there were increased cross-linking between wall components during maturation. This would result from:

1. producing wall polysaccharides in a form that makes tighter complexes with cellulose or other materials
2. increasing the lignin in the wall would increase cross-links between polymers
3. de-esterifying the pectic acids would increase calcium bridges;

B. Loosening (or Stress relaxation) the wall at the appropriate time
    Even though the microfibrils may be in the proper position to permit loosening, the wall is still rather strong. Recall that our wall model proposed strong (covalent) and weak (hydrogen bonds) links between the wall components. When the wall is loosened, weak bonds are temporarily broken to allow the wall components to slide or creep past one another. So, how is the wall temporarily loosened?

1. Protons are the primary wall loosening factor (Acid Growth Hypothesis). This idea was first proposed by David Rayle and R. Cleland in 1970. Some evidence:

• acid buffers stimulate elongation and rapid responses 5-15 min even in non-living tissues (Evans,1974);
• acid secretion is associated with sites of cell elongation (see Evans & Mulkey, 1981)
• Fusicoccin, a diterpene glycoside extracted from a fungus, stimulates proton secretion (activates a H⁺/K⁺ pump) and stimulates elongation.

2. Mechanism of proton action: Protons stimulate wall loosening by:

• disrupting acid-labile bonds such as H-bonds and calcium bridges; and
  
• enhancing the activity of enzymes that break wall cross-links including H-bonds and calcium bridges. Evidence for the enzyme involvement includes: (1) when primary walls are heated or treated with protein denaturing agents they can't be "loosened" by acid; and (2) adding proteins extracted from growing walls to heat-treated walls restores the acid response.
  
      Expansins – appear to be the primary wall-loosening enzymes. This class of proteins are activated by low pH and break the hydrogen bonds between cellulose and the cross-linking glycans. Other candidates for enzymes involved include: (1) pectin methyl esterase which would break the calcium bridges between pectins by esterifying the carboxyl groups; and (2) hydrolases – which would hydrolyze the cross-linking glycans (hemicelluloses).  For example, xyloglucan endotransglycosylase (XET) has been shown to cleave cross-linking glycans that could allow slippage of the wall components

 3. The acid effect is induced by indole-3-acetic acid (IAA, auxin), one of the major plant hormones. IAA stimulates proton excretion and cell growth/elongation.  Evidence:

• peeled coleoptiles + IAA → medium acidic; peeled coleoptiles + water → not acidic; and
• flooding auxin-treated tissue with neutral buffers prevents the growth response.

4. Mechanism of Auxin Action – How does auxin stimulate proton excretion and wall elongation?  There are two ideas:

Hypothesis 1:  Auxin activates pre-existing H⁺-ATPase pump proteins in the cell membrane. These proteins transport protons from the protoplast into the wall. Auxin probably first binds to a receptor molecule and this complex then actives the pump. This process is active - thus the pump requires ATP. Evidence: ATP stimulated acidification is observed soon after auxin treatment. Hypothesis 2: Auxin stimulates transcription and translation. Transcription/translation (protein synthesis) would be required to produce proton pump proteins (a wonderful alliteration), respiratory enzymes to provide ATP to power the process; and even enzymes for the synthesis of wall components and cell solutes (see C. & D. below). Evidence for the involvement of transcription/translation:

• Nooden (1968) found that artichoke disks increased in size when incubated with IAA but that the addition of antimycin (a protein synthesis inhibitor) prevented this response;
• soybean hypocotyls incubated with 2,4-D (an analog of IAA) produce at least 3 new polypeptides within three hours (Zurfluh & Guilfoyle, 1980);
• in vitro translation of mRNA occurs within 15 minutes of IAA treatment
• The proton effect is short-lived. Cell elongation stops 30-60 minutes after acidification. Continuous elongation requires longer term metabolic changes such as protein synthesis.

C. Wall synthesis occurs
    As the cell grows, wall synthesis needs to occur. Think about the color of a balloon as it is blown up - it gets lighter in color as the balloon gets larger because the thickness of the balloon decreases as it expands and stretches. Using this logic, we expect that plant cells should become thinner as they expand. Right? Wrong - cell walls remain a relatively uniform thickness throughout cell growth. Thus, we can conclude that new wall material must be made during cell elongation.
D. Enhanced solute synthesis
    The solute concentration of the cell remains constant during cell enlargement. This suggests that solutes are being synthesized since the volume of the cell is increasing. Maintaining a high solute concentration is necessary to allow for water uptake.
E.  Lock wall in place after expansion is complete
    Once wall elongation is completed, the cell needs to "lock it" in place.  This likely happens as the temporary bonds that were broken reform, and due to increased interactions (including enzymatic) between wall molecules.
F.  Water Uptake/Pressure

Web Sites:

• Carpita, N, McCann (2000) Cell Walls. Chapter 2. In Biochemistry & Molecular Biology of Plants.  Buchanan, BB, Gruissem W, Jones, RL. eds. American Society of Plant Biology, Beltsville, MD.  This is a great article!

References

• Albersheim, P. 1975. The Walls of Growing Plant Cells. Sci. Amer. 232: 81-95.
• Albersheim, P. 1985. Oligosaccharins. Sci. Amer. 253: 58.
• Brett, C. T. and J.R.Hillman. 1985. Biochemistry of Plant Cell Walls. Cambridge University Press, NY.
• Brett, C. & K. Waldron. 1996. Physiology and Biochemistry of Plant Cell Walls. 2nd edn. Chapman & Hall, NY.
• Cosgrove, D. 1986. Cell growth. ARPP 37: 377
• Delmer, D. 1987. Cellulose biosynthesis. ARPP 38: 259
• Fry, S.C. 1989. Dissecting the complexity of the plant cell wall. Plants Today 2: 126-132.
• Mulkey, T.J., K.M. Kuzmanoff and M.L. Evans. 1981. The agar-dye method for visualizing acid efflux paterns during tropistic curvatures. What's New in Plant Physiol. 12:9-12.
• Preston, R.D. 1979. Polysaccharide conformation and cell wall formation. ARPP 30:50.
• Taiz, L. 1984. Plant cell expansion. ARPP 35: 585-647
• Ruszkowski, Martha - check this link for the Ukrainian translation by M Ruszkowski.

From: http://employees.csbsju.edu/ssaupe/biol327/lecture/cell-wall.htm