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Blister pack
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Risperdal tablets in a blister pack
Blister pack[1] is a term for several types of pre-formed plastic packaging used for small consumer goods, foods, and for pharmaceuticals.
The primary component of a blister pack is a cavity or pocket made from a formable web, usually a thermoformed plastic. This usually has a backing of paperboard or a lidding seal of aluminum foil or plastic. A blister that folds onto itself is often called a clamshell.
Blister packs are useful for protecting products against external factors, such as humidity and contamination for extended periods of time. Opaque blisters also protect light-sensitive products against UV rays.
Contents
[hide]
1 Uses
1.1 Unit dose packaging of pharmaceuticals
1.2 Consumer goods
1.3 Clamshell
1.4 Medical blister trays
2 Production
2.1 Thermoforming
2.2 Cold forming
3 Materials
3.1 PVC
3.2 PVDC
3.3 PCTFE
3.4 COC
3.5 Cold form foil
3.6 Lidding foils
4 Benefits
4.1 Cost savings
4.2 Advertising space
4.3 Clear product display
4.4 Theft deterrent packaging
4.5 Tamper evident
5 See also
6 References
7 Further reading
Uses[edit]
Blister packs are used to package products such as toys, hardware, medication, etc.
Many blister packaging machines use heat and pressure via a die to form the cavity or pocket from a roll or sheet of plastic. In recent years, improvements in cold forming, specifically allowing steeper depth/angles during forming, which minimizes the amount of material used for each cavity—have helped this technology increase. The main advantages of the plastic-based blister pack are its more compact size compared to cold formed aluminum and its transparency to see the product.
Unit dose packaging of pharmaceuticals[edit]
Tablets in a blister
Blister packs are commonly used as unit-dose packaging for pharmaceutical tablets, capsules or lozenges. Blister packs can provide barrier protection for shelf life requirements, and a degree of tamper resistance. In the US, blister packs are mainly used for packing physician samples of drug products or for over-the-counter (OTC) products in the pharmacy. In other parts of the world, blister packs are the main packaging type since pharmacy dispensing and re-packaging are not common. A series of blister cavities is sometimes called a blister card or blister strip as well as blister pack. The difference between a strip pack and blister pack is that a strip pack does not have thermo-formed or cold formed cavities; the strip pack is formed around the tablet at a time when it is dropped to the sealing area between sealing moulds. In some parts of the world the pharmaceutical blister pack is known as a push-through pack (PTP), an accurate description of two key properties (i) the lidding foil is brittle, making it possible to press the product out while breaking the lidding foil and (ii) a semi-rigid formed cavity being sufficiently collapsible to be able to dispense the tablet or capsule by means of pressing it out with the thumb. Breaking the lidding foil with a fingernail for the appropriate tablet will make the pressing out easier.
The main advantages of unit-dose blister packs over other methods of packing pharmaceutical products are the assurance of product/packaging integrity (including shelf-life) of each individual dose and the ability to create a compliance pack or calendar pack by printing the days of the week above each dose.
Blister packs are created by means of a form-fill-seal process at the pharmaceutical company or designated contract packer. A form-fill-seal process means that the blister pack is created from rolls of flat sheet or film, filled with the pharmaceutical product and closed (sealed) on the same equipment. Such equipment is called a blisterline. There are two types of blister machine' design: rotary and flat-plate, depending on the mechanism for sealing the lidding foil.[2][3]
Consumer goods[edit]
A typical blister-packaged consumer good
Other types of blister packs consist of carded packaging where goods such as toys, hardware, and electrical items are contained between a specially made paperboard card and clear pre-formed plastic such as PVC. The consumer can visually examine the product through the transparent plastic. The plastic shell is vacuum-formed around a mold so it can contain the item snugly. The card is colored and designed depending on the item inside, and the PVC is affixed to the card using heat and pressure to activate an adhesive (heat seal coating) on the blister card. The adhesive is strong enough so that the pack may hang on a peg, but weak enough so that the package can be easily opened (in theory). Sometimes, with large items, the card (cold seal card) has a perforated window for access.
Clamshell[edit]
Main article: Clamshell (container)
A hinged blister is known as a clamshell, used for a variety of products. It can be used as a security package to deter package pilferage for small high-value items, such as consumer electronics. It consists of one sheet folded over onto itself and sometimes fused at the edges. They can be securely heat sealed, making them difficult to open by hand to deter tampering. A pair of scissors or a sharp knife is often required to open them (although these are often sold in similar packages).[4] Care must be used to safely open some of these packages, as opening it without care can result in injury; 6,000 Americans are sent to the emergency room each year by injuries suffered in opening such packages.[5][6] Wrap rage is sometimes the result.
Medical blister trays[edit]
Medical blister trays differ from pharmaceutical blister packs in that these are not push-through packs. The thermoformed base web is made of a thicker plastic sheet, generally between 500 and 1000 µ and can not be collapsed, thus forming a solid tray. The lidding film provides a peel-open feature and is generally porous to allow sterilization (such as the Dupont medical Tyvek material). Such medical blister packs are used for sterile medical devices, used in hospitals.
Production[edit]
Thermoforming[edit]
In the case of thermoforming, a plastic film or sheet is unwound from the reel and guided though a pre-heating station on the blister line. The temperature of the pre-heating plates (upper and lower plates) is such that the plastic will soften and become pliable. The warm plastic will then arrive in a forming station where a large pressure (4 to 8 bar) will form the blister cavity into a negative mold. The mold is cooled such that the plastic becomes rigid again and maintains its shape when removed from the mold. In case of difficult shapes, the warm film will be physically pushed down partially into the cavity by a "plug-assist" feature. Plug-assist results in a blister cavity with more uniform wall distribution and is typically used when the cavity size and shape is larger than a small tablets and capsules.
Cold forming[edit]
In the case of cold forming, an aluminum-based laminate film is simply pressed into a mold by means of a stamp. The aluminum will be elongated and maintain the formed shape. In the industry these blisters are called cold form foil (CFF) blisters. The principal advantage of cold form foil blisters is that the use of aluminum offers a near complete barrier for water and oxygen, allowing an extended product expiry date. The principal disadvantages of cold form foil blisters are: the slower speed of production compared to thermoforming; the lack of transparency of the package (a therapy compliance disadvantage); and the larger size of the blister card (aluminum can not be formed with near 90 degree angles).[citation needed]
Materials[edit]
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PVC[edit]
The most basic material for the forming web is polyvinyl chloride (PVC). The principal advantages of PVC are the low cost and the ease of thermoforming. The main disadvantages are the poor barrier against moisture ingress and oxygen ingress. In the case of blister packaging the PVC sheet does not contain any plasticizer and is sometimes referred to as Rigid PVC or RPVC. In the absence of plasticizers, PVC blisters offer structural rigidity and physical protection for the pharmaceutical dosage form. On the other hand, the blister cavity must remain accessible by the push-through effect and the formed web may not be too hard to collapse when pressed upon; for this reason the PVC sheet thickness is typically chosen between 200µ to 300µ depending on the cavity size and shape. Most PVC sheets for pharmaceutical blisters are 250µ or 0.250 mm in thickness. Typical values for the Water Vapor Transmission Rate (WVTR or MVTR) of a 250µ PVC film are around 3.0 g/m2/day measured at 38 °C/90% RH and the Oxygen Transmission Rate (OTR) is around 20 mL/m2/day. In order to overcome the lack of barrier properties of PVC film, it can be coated with PVDC or laminated to PCTFE or COC to increase the protective properties. Multi-layer blister films based on PVC are often used for pharmaceutical blister packaging, whereby the PVC serves as the thermoformable backbone of the structure. Also, the PVC layer can be colored with pigments and/or UV filters. The European Pharmacopoeia (Ph Eur) references the requirements for PVC blister packs for pharmaceutical primary packaging in the monograph EP 3.1.11 "MATERIALS BASED ON NON-PLASTICISED POLY(VINYL CHLORIDE) FOR CONTAINERS FOR DRY DOSAGE FORMS FOR ORAL ADMINISTRATION". In order to be suitable for pharmaceutical blister packs, the PVC formulation also needs to comply with the US Pharmacopoeia <661>; EU food legislation; US 21.CFR and Japanese food contact requirements.
PVDC[edit]
Main article: Polyvinylidene chloride
Polyvinylidene chloride or PVDC can be coated onto a PVC film to obtain very high moisture and oxygen barrier properties depending on the coating weight. PVDC coated blister films are the most common and prevailing barrier films used for pharmaceutical blister packs. PVDC coatings are also the most economical method for adding water barrier and oxygen barrier properties to a PVC film. PVDC blister films are available in 2 or 3 layer specifications referred to as duplex or triplex. Since the PVDC is applied by a coating process, the coating weight is expressed in grams per square meter (gsm). Duplex structures are typically PVC/PVDC films, ranging from 250µPVC/40gsmPVDC to 250µPVC/120gsmPVDC with WVTR from 0,65 to 0,25 g/m2/d and OTR from 1 to 0,1 cc/m2/d. For very deep draw thermoformed cavities, the triplex specifications are used : PVC/PE/PVDC, where the PE layer assists when forming deeper cavities. The PE (polyethylene) forms a soft intermediate layer between the rigid PVC and PVDC layers. Triplex specifications exists in similar coating weights as duplex specifications: 250µPVC/25µPE/40gsmPVDC up to 250µPVC/25µPE/120gsmPVDC. In order to obtain high barrier properties, PVDC is always applied using an emulsion coating process using a PVDC resin dispersed in water. The film producer applies the coating in several steps, drying-off the water between each coating station.
PVDC grades are available in 2 types of polymer: (I) the historic grades offering medium to high barrier properties and (II) a super barrier coating grade offering the highest barrier. The super barrier coating grade has over two times the barrier to moisture and oxygen per gram coating weight compared to the historic grades. The most common structures using the super barrier PVDC are triplex configurations 250µPVC/25µPE/120gsmPVDC up to 250µPVC/25µPE/180gsmPVDC, with WVTR of 0,11 down to 0,06 g/m2/day and available from various suppliers.
PCTFE[edit]
Main article: Polychlorotrifluoroethylene
Polychlorotrifluoro ethylene or PCTFE can be laminated to PVC to obtain very high moisture barrier. Typical constructions used for pharmaceutical products are 250µ PVC film laminated to 15µ-150µ PCTFE film. Duplex structures are PVC/PCTFE and triplex laminates are PVC/PE/PCTFE. Deeper cavities can be formed by using the triplex structures with PE. Typical WVTR values are 0.06–0.40 g/m2/day.PCTFE films have the lowest water vapor permeation compared to all other plastic films used in blister packaging and have thermoforming properties similar to plain PVC though it is also the most expensive.Despite narrow thermoforming temperatures and required cooling steps PP is increasingly popular. This popularity is due in part to it not suffering the environmental liability that PVC suffers in discharging hydrochloric acid during incineration. Unplasticised PVC has good thermoforming properties but may not provide good moisture protection for some products. After the forming process a 250 µm film will have a final thickness of 50 to 100 µm in some deep drawn pockets.The reduction in thickness will result in an increase in WVT.[7]
COC[edit]
Cyclic olefin copolymers (COC) or polymers (COP) can provide moisture barrier to blister packs, typically in multilayered combinations with polypropylene (PP), polyethylene (PE), or glycol-modified polyethylene terephthalate (PETg). Cyclic olefin resins are generally amorphous and are noted for good thermoforming characteristics even in deep cavities, leading some to use COC in blister packaging as a thermoforming enhancer, particularly in combination with semicrystalline resins such as PP or PE. Films can be manufactured via coextrusion or lamination. WVTR values of commercial cyclic olefin-based pharmaceutical blister films typically range from 0.20 to 0.35 g/m2/day at 38 °C/90% RH. Unlike PVC and other common pharmaceutical barrier resins, cyclic olefin resins do not contain chlorine or other halogens in their molecular structure, being composed solely of carbon and hydrogen. Cyclic olefin resins are available which comply with pharmaceutical packaging guidelines in the US, Europe, and Japan.
Cold form foil[edit]
Cold form foil film is made of a 3-layer laminate: PVC/Aluminum/Polyamide. The PVC side is on the inside in contact with the product.
Cold Form Foil blister pack
Lidding foils[edit]
Pharmaceutical blister packs are mostly closed by a push-through or peel-open lidding foil. The most common lidding foil with push-through features is 20µ hard temper aluminum, which can be supplied pinhole-free from the producers. The lidding foil is coated with a heat-seal lacquer on the inside and a print primer on the outside.
Benefits[edit]
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Cost savings[edit]
Blister packaging is a cost effective way of showcasing a product. Due to the nature of the material and design, it makes it more cost effective than other types of packages that are on the market. There are several different types of blister packaging – Face Seal, Trap, Mock, Slide and Interactive. Each one has its unique qualities and price points, from entry level to high end.
Advertising space[edit]
There is a wide range of colors and finishes that blister packaging can be constructed to feature. Manufacturers can use that to help make their products stand out on shelves. Blister packaging allows either the manufacturer or retailer to include promotional materials or advertisements to help build a brand and increase customer loyalty.
Clear product display[edit]
Blister packaging helps shoppers see product, further aiding in their purchase decision. More than half of shoppers believe it is important to see a product through its packaging.[citation needed]
Theft deterrent packaging[edit]
To prevent retail theft, packages are specifically designed so that the customer cannot “touch and feel” the product. Clear, protective, and durable thermoformed plastic packaging enables the customer to fully view the product while maintaining the security of the product. Blister packaging can easily be sealed using heat sealing machinery.
Tamper evident[edit]
A tamper-evident package, according to the regulations of the Food and Drug Administration (21 CFR § 211.132), “is one having one or more indicators or barriers to entry which, if breached or missing, can reasonably be expected to provide visible evidence to consumers that tampering has occurred.” In addition, the indicator or barrier must be “distinctive by design,” which means the tamper-evident feature is designed from material not readily available to the public. Therefore, it can’t be easily duplicated. The labeling must also include a description of the safety feature. For blister packaging, each tablet or capsule is individually sealed, so any form of tampering is immediately visible. The product label needs to include a statement similar to the following: “Do not use if blister is cut or broken.”
See also[edit]
Heat sealer
Pharmacy automation
Skin pack
Ultrasonic welding
Wrap rage
References[edit]
Jump up ^ Soroka, W. Illustrated Glossary of Packaging Terminology (Second ed.). Institute of Packaging Professionals.
Jump up ^ Pilchik, R (Nov 2000), "Pharmaceutical Blister Packaging, Part 1, Rationale and Materials" (PDF), Pharmaceutical Technology: 68–77, retrieved 26 June 2017
Jump up ^ Pilchik, R (Dec 2000), "Pharmaceutical Blister Packaging, Part 2, Machinery and Assembly" (PDF), Pharmaceutical Technology: 56–60, retrieved 26 June 2017
Jump up ^ Clifford, Stephanie (2 June 2011). "Clamshell packaging is being tossed". The Bulletin. Bend, OR. New York Times News Service. Retrieved 6 June 2011.
Jump up ^ Stone, Brad (2008-11-14). "Packages You Won't Need a Saw to Open". Nytimes.com. Retrieved 2012-11-21.
Jump up ^ Elliot, V S (22 December 2008). "Ho ho woes: Wrap rage results in lacerations and bad tempers". American Medical News. Retrieved 4 January 2010.
Jump up ^ "How To Minimize Blister Packaging Water Vapor Transmission Rates". PharmaCMC. Retrieved 27 September 2016.
Further reading[edit]
Yam, K. L., Encyclopedia of Packaging Technology, John Wiley & Sons, 2009, ISBN 978-0-470-08704-6
Lockhart, H., and Paine, F.A., Packaging of Pharmaceuticals and Healthcare Products, 2006, Blackie, ISBN 0-7514-0167-6
Wikimedia Commons has media related to Blister packs.
Wikimedia Commons has media related to Blister packs (pharmaceutical products).
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Light-dependent reactions
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Light-dependent reactions of photosynthesis at the thylakoid membrane
In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.
The four photosystems absorb light energy through pigments—primarily the chlorophylls, which are responsible for the green color of leaves. The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI, the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis various electron donors are used.
Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.
The net-reaction of all light-dependent reactions in oxygenic photosynthesis is:
2H
2O + 2NADP+
+ 3ADP + 3Pi → O
2 + 2NADPH + 3ATP
The two photosystems are protein complexes that absorb photons and are able to use this energy to create an electron transport chain. Photosystem I and II are very similar in structure and function. They use special proteins, called light-harvesting complexes, to absorb the photons with very high effectiveness. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and is unique because it transforms light energy into chemical forms.
Contents
[hide]
1 The reaction center
2 Photosynthetic electron transport chains in chloroplasts
2.1 Photosystem II
2.1.1 The water-splitting complex
2.1.2 The reaction center
2.1.3 Link of water-splitting complex and chlorophyll excitation
2.1.4 Summary
2.2 Cytochrome b6
2.3 Photosystem I
3 Photosynthetic electron transport chains in bacteria
3.1 Cyanobacteria
3.2 Purple bacteria
3.3 Green sulfur bacteria
4 History
5 See also
6 References
The reaction center[edit]
Main article: Photosynthetic reaction center
The reaction center is in the thylakoid membrane. It transfers light energy to a dimer of chlorophyll pigment molecules near the periplasmic (or thylakoid lumen) side of the membrane. This dimer is called a special pair because of its fundamental role in photosynthesis. This special pair is slightly different in PSI and PSII reaction center. In PSII, it absorbs photons with a wavelength of 680 nm, and it is therefore called P680. In PSI, it absorbs photons at 700 nm, and it is called P700. In bacteria, the special pair is called P760, P840, P870, or P960. Where "P" means pigment, and the number following it is the wavelength of light absorbed.
If an electron of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer. In normal circumstances, the electron should return to the ground state, but, because the reaction center is arranged so that a suitable electron acceptor is nearby, the excited electron can move from the initial molecule to the acceptor. This process results in the formation of a positive charge on the special pair (due to the loss of an electron) and a negative charge on the acceptor and is, hence, referred to as photoinduced charge separation. In other words, electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they exist at the lowest possible energy level they can. However, if there is enough energy to move them into the next energy level, they can absorb that energy and occupy that higher energy level. The light they absorb contains the necessary amount of energy needed to push them into the next level. Any light that does not have enough or has too much energy cannot be absorbed and is reflected. The electron in the higher energy level, however, does not want to be there; the electron is unstable and must return to its normal lower energy level. To do this, it must release the energy that has put it into the higher energy state to begin with. This can happen various ways. The extra energy can be converted into molecular motion and lost as heat. Some of the extra energy can be lost as heat energy, while the rest is lost as light. (This re-emission of light energy is called fluorescence.) The energy, but not the e- itself, can be passed onto another molecule. (This is called resonance.) The energy and the e- can be transferred to another molecule. Plant pigments usually utilize the last two of these reactions to convert the sun's energy into their own.
This initial charge separation occurs in less than 10 picoseconds (10−11 seconds). In their high-energy states, the special pigment and the acceptor could undergo charge recombination; that is, the electron on the acceptor could move back to neutralize the positive charge on the special pair. Its return to the special pair would waste a valuable high-energy electron and simply convert the absorbed light energy into heat. In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition.[1][2] Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely.
Another electron acceptor is less than 10 Å away from the first acceptor, and so the electron is rapidly transferred farther away from the special pair.
An electron donor is less than 10 Å away from the special pair, and so the positive charge is neutralized by the transfer of another electron
The electron transfer back from the electron acceptor to the positively charged special pair is especially slow. The rate of an of electron transfer reaction increases with its thermodynamic favorability up to a point and then decreases. The back transfer is so favourable that it takes place in the inverted region where electron-transfer rates become slower.[1]
Thus, electron transfer proceeds efficiently from the first electron acceptor to the next, creating an electron transport chain that ends if it has reached NADPH.
Photosynthetic electron transport chains in chloroplasts[edit]
The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher-energy level. This energy is used to reduce a chain of electron acceptors that have subsequently lowered redox-potentials. This chain of electron acceptors is known as an electron transport chain. When this chain reaches PS I, an electron is again excited, creating a high redox-potential. The electron transport chain of photosynthesis is often put in a diagram called the z-scheme, because the redox diagram from P680 to P700 resembles the letter z.[3]
The final product of PSII is plastoquinol, a mobile electron carrier in the membrane. Plastoquinol transfers the electron from PSII to the proton pump, cytochrome b6f. The ultimate electron donor of PSII is water. Cytochrome b6f proceeds the electron chain to PSI through plastocyanin molecules. PSI is able to continue the electron transfer in two different ways. It can transfer the electrons either to plastoquinol again, creating a cyclic electron flow, or to an enzyme called FNR (Ferredoxin—NADP(+) reductase), creating a non-cyclic electron flow. PSI releases FNR into the stroma, where it reduces NADP+
to NADPH.
Activities of the electron transport chain, especially from cytochrome b6f, lead to pumping of protons from the stroma to the lumen. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.
The overall process of the photosynthetic electron transport chain in chloroplasts is:
H
2O → PS II → plastoquinone → cytb
6 → plastocyanin → PS I → NADPH
Photosystem II[edit]
Main article: Photosystem II
PS II is extremely complex, a highly organized transmembrane structure that contains a water-splitting complex, chlorophylls and carotenoid pigments, a reaction center (P680), pheophytin (a pigment similar to chlorophyll), and two quinones. It uses the energy of sunlight to transfer electrons from water to a mobile electron carrier in the membrane called plastoquinone:
H
2O → P680 → P680* → plastoquinone
Plastoquinone, in turn, transfers electrons to cytb
6, which feeds them into PS I.
The water-splitting complex[edit]
The step H
2O → P680 is performed by a poorly understood structure embedded within PS II called the water-splitting complex or the oxygen-evolving complex. It catalyzes a reaction that splits water into electrons, protons and oxygen:
2H
2O → 4H+ + 4e− + O
2
The actual steps of the above reaction are running in the following way (Dolai's diagram of S-states): (I) 2H2O(monoxide) (II) OH. H2O (hydroxide) (III) H2O2(peroxide) (IV)HO2(super oxide)(V) O2(di-oxygen).[4]
The electrons are transferred to special chlorophyll molecules (embedded in PS II) that are promoted to a higher-energy state by the energy of photons.
The reaction center[edit]
The excitation P680 → P680*of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PS II absorb the energy of photons, with maximal absorption at 680 nm. Electrons within these molecules are promoted to a higher-energy state. This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency (greater than 90%) because, in addition to direct excitation by light at 680 nm, the energy of light first harvested by antenna proteins at other wavelengths in the light-harvesting system is also transferred to these special chlorophyll molecules.
This is followed by the step P680*→ pheophytin, and then on to plastoquinone, which occurs within the reaction center of PS II. High-energy electrons are transferred to plastoquinone before it subsequently picks up two protons to become plastoquinol. Plastoquinol is then released into the membrane as a mobile electron carrier.
This is the second core process in photosynthesis. The initial stages occur within picoseconds, with an efficiency of 100%. The seemingly impossible efficiency is due to the precise positioning of molecules within the reaction center. This is a solid-state process, not a chemical reaction. It occurs within an essentially crystalline environment created by the macromolecular structure of PS II. The usual rules of chemistry (which involve random collisions and random energy distributions) do not apply in solid-state environments.
Link of water-splitting complex and chlorophyll excitation[edit]
When the chlorophyll passes the electron to pheophytin, it obtains an electron from P680*. In turn, P680* can oxidize the Z (or YZ) molecule. Once oxidized, the Z molecule can derive electrons from the oxygen-evolving complex (OEC).[5] Dolai's S-state diagrams show the reactions of water splitting in the oxygen-evolving complex.
Summary[edit]
PS II is a transmembrane structure found in all chloroplasts. It splits water into electrons, protons and molecular oxygen. The electrons are transferred to plastoquinone, which carries them to a proton pump. Molecular oxygen is released into the atmosphere.
The emergence of such an incredibly complex structure, a macromolecule that converts the energy of sunlight into potentially useful work with efficiencies that are impossible in ordinary experience, seems almost magical at first glance. Thus, it is of considerable interest that, in essence, the same structure is found in purple bacteria.
Cytochrome b
6[edit]
PS II and PS I are connected by a transmembrane proton pump, cytochrome b
6 complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1). Electrons from PS II are carried by plastoquinol to cytb
6, where they are removed in a stepwise fashion (reforming plastoquinone) and transferred to a water-soluble electron carrier called plastocyanin. This redox process is coupled to the pumping of four protons across the membrane. The resulting proton gradient (together with the proton gradient produced by the water-splitting complex in PS II) is used to make ATP via ATP synthase.
The similarity in structure and function between cytochrome b
6 (in chloroplasts) and cytochrome bc
1 (Complex III in mitochondria) is striking. Both are transmembrane structures that remove electrons from a mobile, lipid-soluble electron carrier (plastoquinone in chloroplasts; ubiquinone in mitochondria) and transfer them to a mobile, water-soluble electron carrier (plastocyanin in chloroplasts; cytochrome c in mitochondria). Both are proton pumps that produce a transmembrane proton gradient.
Photosystem I[edit]
The cyclic light-dependent reactions occur only when the sole photosystem being used is photosystem I. Photosystem I excites electrons which then cycle from the transport protein, ferredoxin (Fd), to the cytochrome complex, b6f, to another transport protein, plastocyanin (Pc), and back to photosystem I. A proton gradient is created across the thylakoid membrane (6) as protons (3) are transported from the chloroplast stroma (4) to the thylakoid lumen (5). Through chemiosmosis, ATP (9) is produced where ATP synthase (1) binds an inorganic phosphate group (8) to an ADP molecule (7).
Main article: Photosystem I
PS I accepts electrons from plastocyanin and transfers them either to NADPH (noncyclic electron transport) or back to cytochrome b
6 (cyclic electron transport):
                        plastocyanin → P700 → P700* → FNR → NADPH ↑ ↓ b
6 ← plastoquinone

PS I, like PS II, is a complex, highly organized transmembrane structure that contains antenna chlorophylls, a reaction center (P700), phylloquinine, and a number of iron-sulfur proteins that serve as intermediate redox carriers.
The light-harvesting system of PS I uses multiple copies of the same transmembrane proteins used by PS II. The energy of absorbed light (in the form of delocalized, high-energy electrons) is funneled into the reaction center, where it excites special chlorophyll molecules (P700, maximum light absorption at 700 nm) to a higher energy level. The process occurs with astonishingly high efficiency.
Electrons are removed from excited chlorophyll molecules and transferred through a series of intermediate carriers to ferredoxin, a water-soluble electron carrier. As in PS II, this is a solid-state process that operates with 100% efficiency.
There are two different pathways of electron transport in PS I. In noncyclic electron transport, ferredoxin carries the electron to the enzyme ferredoxin NADP+
reductase (FNR) that reduces NADP+
to NADPH. In cyclic electron transport, electrons from ferredoxin are transferred (via plastoquinone) to a proton pump, cytochrome b
6. They are then returned (via plastocyanin) to P700.
NADPH and ATP are used to synthesize organic molecules from CO
2. The ratio of NADPH to ATP production can be adjusted by adjusting the balance between cyclic and noncyclic electron transport.
It is noteworthy that PS I closely resembles photosynthetic structures found in green sulfur bacteria, just as PS II resembles structures found in purple bacteria.
Photosynthetic electron transport chains in bacteria[edit]
PS II, PS I, and cytochromeb
6 are found in chloroplasts. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP by the mechanisms described above. In essence, the same transmembrane structures are also found in cyanobacteria.
Unlike plants and algae, cyanobacteria are prokaryotes. They do not contain chloroplasts. Rather, they bear a striking resemblance to chloroplasts themselves. This suggests that organisms resembling cyanobacteria were the evolutionary precursors of chloroplasts. One imagines primitive eukaryotic cells taking up cyanobacteria as intracellular symbionts in a process known as endosymbiosis.
Cyanobacteria[edit]
Cyanobacteria contain structures similar to PS II and PS I in chloroplasts. Their light-harvesting system is different from that found in plants (they use phycobilins, rather than chlorophylls, as antenna pigments), but their electron transport chain
         H
2O → PS II → plastoquinone → b
6 → cytochrome c
6 → PS I → ferredoxin → NADPH ↑ ↓ b
6 ← plastoquinone

is, in essence, the same as the electron transport chain in chloroplasts. The mobile water-soluble electron carrier is cytochrome c
6 in cyanobacteria, plastocyanin in plants.
Cyanobacteria can also synthesize ATP by oxidative phosphorylation, in the manner of other bacteria. The electron transport chain is
           NADH dehydrogenase → plastoquinone → b
6 → cytochrome c
6 → cytochrome aa
3 → O
2

where the mobile electron carriers are plastoquinone and cytochrome c
6, while the proton pumps are NADH dehydrogenase, b
6 and cytochrome aa
3.
Cyanobacteria are the only bacteria that produce oxygen during photosynthesis. Earth's primordial atmosphere was anoxic. Organisms like cyanobacteria produced our present-day oxygen-containing atmosphere.
The other two major groups of photosynthetic bacteria, purple bacteria and green sulfur bacteria, contain only a single photosystem and do not produce oxygen.
Purple bacteria[edit]
Purple bacteria contain a single photosystem that is structurally related to PS II in cyanobacteria and chloroplasts:
P870 → P870* → ubiquinone → bc
1 → cytochrome c
2 → P870
This is a cyclic process in which electrons are removed from an excited chlorophyll molecule (bacteriochlorophyll; P870), passed through an electron transport chain to a proton pump (cytochrome bc
1 complex, similar but not identical to cytochrome bc
1 in chloroplasts), and then returned to the chlorophyll molecule. The result is a proton gradient, which is used to make ATP via ATP synthase. As in cyanobacteria and chloroplasts, this is a solid-state process that depends on the precise orientation of various functional groups within a complex transmembrane macromolecular structure.
To make NADPH, purple bacteria use an external electron donor (hydrogen, hydrogen sulfide, sulfur, sulfite, or organic molecules such as succinate and lactate) to feed electrons into a reverse electron transport chain.
Green sulfur bacteria[edit]
Green sulfur bacteria contain a photosystem that is analogous to PS I in chloroplasts:
                           P840 → P840* → ferredoxin → NADH ↑ ↓ cyt c553 ← bc
1 ← menaquinone

There are two pathways of electron transfer. In cyclic electron transfer, electrons are removed from an excited chlorophyll molecule, passed through an electron transport chain to a proton pump, and then returned to the chlorophyll. The mobile electron carriers are, as usual, a lipid-soluble quinone and a water-soluble cytochrome. The resulting proton gradient is used to make ATP.
In noncyclic electron transfer, electrons are removed from an excited chlorophyll molecule and used to reduce NAD+ to NADH. The electrons removed from P840 must be replaced. This is accomplished by removing electrons from H
2S, which is oxidized to sulfur (hence the name "green sulfur bacteria").
Purple bacteria and green sulfur bacteria occupy relatively minor ecological niches in the present day biosphere. They are of interest because of their importance in precambrian ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants.
History[edit]
The first ideas about light being used in photosynthesis were proposed by Colin Flannery in 1779[6] who recognized it was sunlight falling on plants that was required, although Joseph Priestley had noted the production of oxygen without the association with light in 1772.[7] Cornelius Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce CO
2.[8] Then in 1939, Robin Hill showed that isolated chloroplasts would make oxygen, but not fix CO
2 showing the light and dark reactions occurred in different places. Although they are referred to as light and dark reactions, both of them take place only in the presence of light.[9] This led later to the discovery of photosystems I and II.
See also[edit]
Light-independent reaction
Photosynthetic reaction centre
Photosystem II
Compensation point
References[edit]
^ Jump up to: a b Vass, Imre; Cser, Krisztian (April 2009). "Janus-faced charge recombinations in photosystem II photoinhibition". Trends in Plant Science. 14 (4): 200–205. doi:10.1016/j.tplants.2009.01.009. ISSN 1360-1385. PMID 19303349. Retrieved 2012-10-30.
Jump up ^ Keren, Nir; Berg, Alexander; van Kan, Paul J. M.; Levanon, Haim; Ohad, Itzhak (1997-02-18). "Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: The role of back electron flow". Proceedings of the National Academy of Sciences. 94 (4): 1579–1584. doi:10.1073/pnas.94.4.1579. ISSN 0027-8424. PMC 19834  . PMID 11038602. Retrieved 2012-10-30.
Jump up ^ Rajni Govindjee. "The Z-Scheme Diagram of Photosynthesis". Retrieved March 2, 2006.
Jump up ^ Dolai, U. (2017). "Chemical Scheme of Water -Splitting Process during Photosynthesis by the way of Experimental Analysis ". IOSR Journal of Pharmacy and Biological Sciences. 12(6):65-67. doi: 10.9790/3008-1206026567. ISSN 2319-7676.
Jump up ^ "Photosynthesis". McGraw Hill Encyclopedia of Science and Technology. 2007. p. 472.
Jump up ^ Ingenhousz, J (1779). Experiments Upon Vegetables. London: Elmsly and Payne.
Jump up ^ Priestley, J (1772). "Observations on Different Kinds of Air". 62. London: Phil. Trans. Roy. Soc.: 147–264. doi:10.1098/rstl.1772.0021.
Jump up ^ van Niel, C. B. (1931). "On the morphology and physiology of the purple and green sulfur bacteria". Arch. Microbiol. 3: 1–114. doi:10.1007/BF00454965.
Jump up ^ Hill, R. (May 1939). "Oxygen Produced by Isolated Chloroplasts". Proceedings of the Royal Society of London. Series B, Biological Sciences. 127 (847): 192–210. doi:10.1098/rspb.1939.0017.
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