ATP Synthase-- Movie Narrative (Advanced Look)

Concentration gradients are a key component of the biological world. The potential energy from these gradients is often used to perform biological work.

Here we will focus on hydrogen ion concentration gradients. Hydrogen ions, are also known as protons.

A gradient exists when there is a higher concentration of a molecule in one compartment compared to a neighboring compartment.

This animation will demonstrate how the potential energy that results from a hydrogen ion gradient uses ADP and inorganic phosphate, also known as Pi, to synthesize ATP.

This process involves an enzyme complex called ATP synthase.

Gradients and the potential energy they create are key aspects of the biological world. A good example of the use of a gradient occurs in the mitochondria when ATP is synthesized. ATP is synthesized by ATP synthase, a large complex of membrane-bound protein.

Here we see ATP synthase, along with other membrane-bound proteins.

Notice the large difference in the number of hydrogen ions on the two sides of the membrane. This difference is a hydrogen ion, or proton, concentration gradient.

The energy associated with this gradient is used to synthesize ATP from ADP and Pi. This occurs at the ATP synthase complex.

One hydrogen ion enters the ATP synthase complex from the intermembrane space and a second hydrogen ion leaves it on the matrix space. The upper part of the ATP synthase complex rotates when a new hydrogen ion enters.

Once three protons have entered the matrix space, there is enough energy in the ATP synthase complex to synthesize one ATP. In this way, the energy in the hydrogen ion gradient is used to make ATP.

Now let's watch the process again...

Notice how the proton enters the ATP synthase and exits into the matrix space. Once three more hydrogen ions have crossed the membrane, another molecule of ATP will be made. In this example, the hydrogen ion gradient is large enough to produce six ATP molecules.

Please watch as the remaining ATP molecules are synthesized...

The process has now completed, and the result is an equal number of protons on each side of the inner membrane. Without a gradient, there is no more energy available to make ATP.

In biological systems, however, a gradient is always maintained. The mitochondrial hydrogen ion gradient is generated as electrons pass through three membrane complexes. That process can be seen in the mitochondrial electron transport chain animation.


Electron Transport Chain -- Movie Narrative (Advanced Look)

The electron transport chain is a series of protein complexes embedded in the mitochondrial membrane. Electrons captured from donor molecules are transferred through these complexes.

Coupled with this transfer is the pumping of hydrogen ions. This pumping generates the gradient used by the ATP synthase complex to synthesize ATP.

The following complexes are found in the electron transport chain: NADH dehydrogenase, cytochrome b-c1, cytochrome oxidase, and the complex that makes ATP, ATP synthase.

In addition to these complexes, two mobile carriers are also involved: ubiquinone, and cytochrome c.

Other key components in this process are NADH and the electrons from it, hydrogen ions, molecular oxygen, water, and ADP and Pi, which combine to form ATP.

At the start of the electron transport chain, two electrons are passed from NADH into the NADH dehydrogenase complex. Coupled with this transfer is the pumping of one hydrogen ion for each electron

Next, the two electrons are transfered to ubiquinone. Ubiquinone is called a mobile transfer molecule because it moves the electrons to the cytochrome b-c1 complex.

Each electron is then passed from the cytchrome b-c1 complex to cytochrome c. Cytochrome c accepts each electron one at a time. One hydrogen ion is pumped through the complex as each electron is transfered.

The next major step occurs in the cytochrome oxidase complex. This step requires four electrons. These four electrons interact with a molecular oxygen molecule and eight hydrogen ions. The four electrons, four of the hydrogen ions, and the molecular oxygen, are used to form two water molecules. The other four hydrogen ions are pumped across the membrane.

This series of hydrogen pumping steps creates a gradient. The potential energy in this gradient is used by ATP synthase to ATP from ADP and inorganic phosphate. The ATP synthesis steps you see here are discussed in greater detail in the ATP sythase gradients animation.

This animation illustrates two full cycles of electron donation. In biological systems, however, many electron transport cycles occur simultaneously--helping to ensure that the proton gradient is always maintained.


Protein Transport (Mitochondrial) -- Movie Narrative (Advanced Look)

Most organelle proteins are synthesized in the cytoplasm from nuclear encoded mRNAs. These proteins must be imported into the organelle. Special sequences, called signal sequences, target the protein to its proper organelle. Organelles contain protein translocator complexes that are required for this transport.

Key players in this process are protein, a signal sequence, chaperonins, ATP, protein translocator complexes, and signal peptidase.

Proteins destined for transport into an organelle, such as a mitochondria or chloroplast, contain a signal sequence. This sequence acts as a targetting mechanism to ensure the protein is delivered to the proper organelle.

In addition, chaperonin proteins aid in the import process. They become associated with a protein while it is still in the cytoplasm. This association require energy from ATP. Chaperonins aid in unfolding the protein, so that it can travel through the organelle membrane. Here we see two chaperonins bound to the protein that will enter the mitochondria.

Protein translocator complexes are embedded in the mitochondrial membrane. These are multi-protein complexes required for protein import. The protein being transfered first attaches to the complex on the cytosolic side. The protein then moves into the mitochondria.

As it enters the organelle it is again bound by a chaperonin to prevent premature folding. Once the protein has fully entered the mitochondria, the first chaperonin is released and another class of chaperonins bind.

Then a complex called the signal peptidase removes the signal sequence.

Lastly, the protein folded into its final shape, and is ready to preform its proper function in the organelle.


Transcription -- Movie Narrative (Advanced Look)

Transcription is the process of making RNA from a DNA template. Several key factors are involved in this process. Including, DNA, transcription factors, RNA polymerase, and ATP.

Transcription begins with a strand of DNA. It is divided into several important regions. The largest of these is the transcription unit. This portion of the DNA will be used to produce RNA. Upstream of the transcription unit is the TATA box. An enhancer region may also be involved.

Several complexes, known as transcription factors, are required for successful transcription. The first is TFIID, the largest of the general factors. A component of this factor, TBP, binds to the DNA using the TATA box to position TFIID near the transcription initiation site. Other transcription factors, including TFIIA and TFIIB, then attach.

These complexes prepare the DNA for the successful binding of RNA polymerase. One RNA polymerase is bound, other transcription factors complete the mature transcription complex.

Now, energy must be added to the system for transcription to begin. This energy is provided by the reduction of ATP into ADP and Pi.

RNA polymerase then synthesizes an RNA template from the strand of DNA. Most factors are released after transcription begins. When the end of the transcription unit is reached, the RNA polymerase dissociates, and the newly formed strand of RNA is released.


Photosystem II -- Movie Narrative (Advanced Look)

Photosynthesis is vital to life on earth. Photosynthesis occurs in all green plants and some algae. One of the first stages of photosynthesis involves Photosystem II. The oxygen we breathe is a product of the Photosystem II reaction. This animation will describe the processes that take place within this important complex.

Photosystem II involves several key components, including: photons, light harvesting chlorophyll-binding proteins, a pair of chlorophyll molecules known as the P680 reaction center, pheophytin molecules, and plastoquinones--along with water and oxygen.

The reaction center of Photosystem II consists of multiple proteins and pigment molecules. At the heart of the reaction center is a special pair of chlorophyll molecules, p680 , which donate an electron to the electron transport system. In Photosystem II, the electron is then passed to a pheophytin molecule. The electron is then passed to plastoquinone Qa and then to plastoquinone Qb. These plastoquinone molecules are embedded in the D2 and D1 proteins.

Once Qb has accepted two electrons, it then acts as a mobile carrier to the next component of the photosynthetic electron transport system. Electrons from water are then transferred to the P680 molecules that have lost their electrons in the process. Additional proteins are involved in splitting the electrons from water. Still other proteins are necessary to build the complete photosystem reaction center.

Surrounding the reaction center are light-harvesting chlorophyll-binding proteins. These proteins provide a way to harness the unique energy contained in light. When one of the many photons of light flooding a leaf hits a chlorophyll molecule surrounding the reaction center, it creates resonance energy. You can think of this as vibrational energy. Here you can see the chlorophyll vibrating. That resonance, or vibrational, energy is then passed to a neighboring chlorophyll molecule. It is then passed through several chlorophyll molecules until it reaches the P680 reaction center . It is that energy that results in the loss of an electron from the P680 molecule.

Now we can see how these processes work as a whole. First a photon of light activates a chlorophyll molecule. The resonance, or vibrational, energy is transferred to the P680 molecules.

An electron is lost from P680. It is then donated to Qa, then to Qb. The P680 molecules are reduced by the addition of an electron generated by the splitting of water molecules at the oxygen-evolving complex.

Since Qb needs two electrons to become mobile, a second photon of light is required. The resonance energy is again transferred to the reaction center. An electron is lost from P680 and transferred via Qa to the Qb, which already contains one electron. The fully reduced Qb is then transferred to the cytochrome b6f complex.

The P680 molecules are again reduced by the oxygen-evolving complex. Here you can see water being split at the oxygen-evolving complex. Two water molecules must be split to provide electrons to reduce p680. The oxygen we breathe is a product of this water-splitting process.


Protein Trafficking (Golgi) -- Movie Narrative

It is important that translated proteins are delivered to their specific cellular location. To accomplish this, the protein is transferred through a series of membrane structures. A principle membrane component is the Golgi apparatus.

The Golgi apparatus is the sorting organelle of the cell. Proteins from the rough endoplasmic reticulum are sent to the Golgi. As the proteins move through the Golgi apparatus, they are modified and packaged into vesicles. Because the Golgi apparatus receives proteins from one location and targets them for delivery to a second location, it is sometimes considered the “Post Office” of the cell.

The Golgi apparatus consists of general components: the cis cisterna nearest the endoplasmic reticulum or ER, the medial and trans cisternae, and the trans Golgi network.

Other key players in this process are the proteins being transported and the enzymes that modify them.

Translated proteins are encapsulated in vesicles in the ER. A group of these vesicles fuse, and these fused vesicles form the cis-cisterna. As the protein moves through the stack, it is modified by RESIDENT Golgi enzymes at specific locations in the apparatus. These modifications are important because they provide the signal that determines the final destination of the protein.

So how does the protein move through the Golgi? Movement occurs in waves. First the cis-cisterna becomes part of the medial Golgi cisternae. Behind it, a new cis-cisterna is formed by the fusion of vesicles from the endoplasmic reticulum. Meanwhile, one of the medial cisternae migrates and becomes the new trans-cisterna. Collectively, this process is known as the cis-maturation model.

Proteins are sorted within the trans Golgi network. Proteins with the same target sequence and are destined for delivery to the same location.

The trans Golgi network then buds off into vesicles. These vesicles then migrate to their target location. These locations include internal organelles such the lysosome, the digestive organelle of the cell. The vesicles can also be targeted to the cell membrane where the targeted protein can be released from the cell for delivery elsewhere in the organism.


Constitutive Secretion (Golgi) -- Movie Narrative

Different cells secrete different types of proteins. Some cells, such as white blood cells, only secrete only one type of protein and are known as unpolarized cells. Other cells, called polarized cells, secrete several classes of proteins that are each destined for delivery to a different location.

As seen in the Protein Modification animation, some of these proteins can be destined for the endosome. They are targeted to the endosome by a specific mannose-phosphate signal. Other proteins are destined for delivery outside of the cell. These proteins are packaged into secretory vesicles and delivered to the cell membrane.

Some secretory vesicles congregate in the cell awaiting an external signal. Once that signal is received, the vesicles rapidly fuse with the membrane, and a large quantity of cargo proteins are released simultaneously. This is called regulated secretion. During any secretion process, the vesicle fuses with the cell membrane by a process called exocytosis.

Alternatively, secretory vesicles are delivered to the cell membrane after they are formed, and the protein product is immediately released. This is called constitutitive secretion . The constitutive pathway is required to maintain the cell membrane and exists in all eucaryotic cells. This animation focuses on constitutive secretion.

Protein cargo originally from the endoplasmic reticulum migrates through the Golgi apparatus. These proteins leave the Trans Golgi Network in a secretory vesicle. Some of these proteins are not modified as they move through the Golgi apparatus.

Unlike modified proteins, these proteins do not contain any signal that would direct their transport to a specific intracellular location such as the ER, endosome, or a previous Golgi cisternae. These proteins enter the default secretion pathway and are immediately secreted from the cell. Because there is no control over their secretion, this process is called constitutive secretion.

Once the secretory vesicle reaches the cell membrane, it fuses immediately with the surface membrane and releases its cargo protein into the extracellular space. The secretory vesicle itself contributes new lipids and its membrane to the plasma membrane of the cell.

Certain white blood cells constitutively secrete specific Interleukins, which are signaling molecules, for the purpose of intercellular communication and play an important role in the function of the immune system. Cells such as Fibroblasts constitutively secrete proteins like Collagen and Proteoglycans into the extracellular matrix and play an important role in maintaining the structural integrity of connective tissues.


Through the Virtual Cell - Narration

Welcome to the NDSU Virtual Cell. It's time to climb into one of our cell submarines and take a virtual tour of our cellular landscape.

Out on the horizon, you should see a large blue object. That is the nucleus of the cell. It will be our first stop today. The nucleus is uniquely recognizable by the system of pores embedded within its outer membrane. Biological materials move in and out through the pores. They are the communication channel between the internal world of the nucleus and the cellular cytoplasm.

The nucleus contains the vast majority of the DNA in the cell. The DNA contains all of the genetic information necessary to carry out all of the functions of the cell, as well as the tissues and organs in which the cell can be found. That information is mobilized first by the process of transcription. Let's pop inside and take a short look at the process. During this process, the DNA is used as a template to make RNA. Here, you can see that process in action. The final product of transcription is then spliced and modified into one of three RNA molecules: messenger RNA (or mRNA), transfer (or tRNA), or ribosomal (or rRNA). These are all important components of the process call translation that is used to make proteins.

If we follow the final RNA products out of the nucleus we can see them in action during the process of translation. Here you can see a particulate organelle called the ribosome. It is partially composed of the rRNA we just spoke of. Attached to it is the mRNA. As translation starts, a tRNA molecule binds to it and delivers the correct amino acid. As the protein grows, additional amino acids are brought into place by the correct tRNA molecules. The correct tRNA is determined by triplet codes found in the mRNA.

Proteins are the product of translation. Although some of these proteins stay within the cytoplasm, others are trafficked to different locations within the cell and some are exported from the cell. Most of this trafficking involves the endoplasmic reticulum (or ER) and the golgi apparatus.

We are now looking at the ER, or endoplasmic reticulum. Some of the cell's ribosomes are attached to the ER and the proteins they manufacture are inserted directly into the space inside the ER. Those proteins can be packaged into vesicles which depart the ER and migrate to the golgi apparatus.

Here, the vesicles merge to form a golgi cisterna with the proteins located inside the cisterna's membrane. As the cisterna matures, enzymes inside can modify the protein. This modification creates a molecular tag that is used to target the protein to a specific cellular location. Eventually, the cisterna will mature into the trans golgi network. From here vesicles can deliver proteins to cellular locations such as endsome or the cell membrane where the proteins can be embedded or exported from the cell.

Once proteins have been translated, they can also be delivered to other organelles in the cell, such as the mitochondria and the chloroplast. Delivery here is by a different process. Some proteins are produced with transit peptides. These are specialized sequences on the end of the protein recognized by pores in the mitochondrial membrane. With the help of additional specialized proteins, the protein is delivered into the organelle. Transport to the chloroplast involves a similar process.

Mitochondrias and chloroplasts are the sites for unique cellular processes. We'll first look at the mitochondria. This organelle produces ATP, an energy molecule that is used by many other cellular processes. ATP is produced by a complex in the mitochondrial membrane called ATP synthase. The energy to produce ATP is provided by a gradient of protons (or hydrogen ions) found on the two sides of the membrane. Protons flow from the area of high proton concentration through the ATP synthase, to the area of low concentration. As they flow through ATP synthase, ATP is produced.

This gradient is produced by the action of the electron transport system. As electrons are passed from one carrier to another in the system, protons are pumped across the membrane. This creates the gradient required for ATP production. Here we see can see the electrons moving between the carriers.

Finally, we see an organelle only found within plant cells. This is the chloroplast. Here light energy from the sun is converted into chemical energy in the form of ATP. As with the electron transport system, electrons are passed from one carrier to another and protons are passed across the membrane. The gradient this creates produces ATP by a similar ATP synthase complex. It begins with a photon of light that produces the energy necessary to release an electron down the carrier chain. Here you can see the electron moving between the carriers along with ATP being produced by ATP synthase.

And that brings us to the end of our fly through. There are many processes going on within our virtual cell, and this trip featured only a few of the major functions. We are always working to expand our collection, and we hope you will return to explore our newest additions.


Insulin Signaling -- Movie Narrative

A biological individual consists of multiple organs with specialized functions. For the organism to function properly in its environment, these organs must communicate. This communication often involves a signal sent from one location to another that instructs the second organ about the status of some cellular feature. Glucose is a good example.

Glucose is a critical product of digestion. It is an essential energy source for cellular metabolism. This energy is produced when glucose is used as a substrate for glycolysis and then the Krebs or Citric Acid Cycle.

Following the digestion of food, higher levels of glucose circulate through the blood stream where it enters different cell types. In muscle cells glucose is readily used to produce energy and is also stored as glycogen, a secondary short term energy source. In fat cells, glucose is used for Triglyceride production, and acts as an important energy reserve molecule.

Here we will illustrate the signaling pathway that occurs when glucose is at high levels. This pathway involves multiple proteins and signaling events. This is termed cytoplasmic signaling. Different types of cells perform similar signaling steps in response to changes in their environment.

In the Protein Recycling Animation we see a group of storage vesicles enriched with GLUT4 proteins continuously recycling from the Cell Membrane to an inactive location in the cytosol. GLUT4 is a protein that facilitates the movement of glucose into the cell.

When high levels of glucose are detected by beta cells in the pancreas, insulin is released by the cells. The insulin circulates through the blood stream until it binds to an insulin receptor embedded in the cell membrane of a muscle, fat, or brain cell. Once the insulin binds to the receptor, phosphate groups are added to the intracellular domain of the receptor. Since the receptor itself adds the phosphate groups, the process is called autophosphorylation.

This phosphorylation event sets off a cascade of molecular events. The activated receptor protein then adds a phosphate group to another closely associated protein. This effectively passes the signal from the receptor to the next step in the signal pathway.

Proteins that add phosphate groups to another protein are called kinases. Kinases are often components of signal pathways, and phosphorylation is an important component in the transmission of a signal from one compartment to another. In this system, the signal corresponds to the level of blood glucose and is transmitted from outside to inside the cell.

Next we see a large pool of molecules that are embedded in the membrane also being phosphorylated. Other proteins are then in turn phosphorylated, further transmitting the first extracellular signal that was originally sent from outside the cell membrane.

So how does this affect the uptake of glucose? As we mentioned before, Glut4 is a glucose transporter, and Glut4 Storage Vesicles are held in a recycling state near the cell membrane. The vesicles are held mostly in this region because the RAB proteins that interact with the motor proteins necessary to move the vesicles to the membrane are in an inactive state.

The final step in the signal pathway involves the phosphorylation of a protein that prevents the RAB proteins from interacting with the vesicles.

When the RAB proteins are no longer inhibited, the storage vesicles can freely merge with the membrane. Once the vesicles have merged many Glut4 proteins are embedded in the membrane and large quantities of glucose can move into the cell. It is the signaling pathway that insures only the correct molecules will be allowed to enter the target cell.

mRNA Processing -- Movie Narrative (Advanced Look)

Before mRNA can be used by ribosomes as a template for building proteins, it must first be processed. Key steps are the addition of a methylated cap and a polyadenylated tail. Involved in processing are: RNA polymerase, cleavage factors, and poly A polymerase.

Processing of mRNA, begins with transcription. Soon afer RNA polymerase begins transcription, a methylated cap is added to the 5' end. Transcription then continues to completion.

Following completion, RNA polymerase releases the capped strand of pre-mRNA. Specific nucleotide sequences in the mRNA are bound by cleavage factors. The 3' end of the mRNA is next moved into the correct configuration for cleavage. Stabilizing factors are then added to the complex.

Poly A polymerase now binds to the mRNA and cleaves the 3' end. The complex begins to dissociate, and the cleaved 3' end quickly degrades. Poly A polymerase now synthesizes the polyadenylated tail by adding adenine residues to the cleavage site. Additional proteins then bind to the tail, increasing the rate at which it grows.

When the tail reaches its full length, the poly A polymerase is signaled to stop adding residues, and the polyadenylation process is completed. The processed mRNA is now ready to undergo splicing in preparation for translation.


mRNA Splicing -- Movie Narrative (Advanced Look)

The following animation will describe the process of RNA splicing--an important step in creating the mRNA that is involved in protein synthesis, via the process of translation. Key factors in this process include: RNA, possessing introns and exons, and the spliceosome.

Here we see an RNA molecule with a single intron. Several signals exist within the intron that are used in the splicing process. From the 5' end of the intron, these are, GU, the A branch site, a pyrimidine-rich region, and the 3' AG. The AG and GU sequences define the beginning and end of the intron.

Splicing is mediated by the spliceosome, which consists of several protein-RNA complexes. The first step involves two complexes that bind near the GU sequence. The RNA in then looped, and three other protein-RNA complexes bind. This final complex then undergoes a conformation change.

Introns are non-coding RNA sequences that must be removed before translation. The process of removing the intron is called splicing. The intron is then cleaved at the 5' GU sequence and forms a lariat at the A branch site. The 3' end of the intron is next cleaved at the AG sequence, and the two exons are ligated together.

As the spliced mRNA is released from the spliceosome, the intron debranches, and is then degraded.

Translation -- Movie Narrative (Advanced Look)

Translation is the synthesis of a protein from an mRNA template. This process involves several key moelcules including mRNA, the small and large subunits of the ribsome, tRNA, and finally, the release factor. The process is broken into three stages: initiation, elongation, and termination. Let's see the process in action...

Eukaryotic mRNA, the substrate for translation, has a unique 3'-end called the poly-A tail. mRNA also contains codons that will encode for specific amino acids.

A methylated cap is found at the 5'-end. Translation initiation begins when the small subunit of the ribosome attaches to the cap and moves to the translation initiation site.

tRNA is another key molecule. It contains an anticodon that is complementary to the mRNA codon to which it binds. The first codon is typically AUG. Attached to the end of tRNA is the corresponding amino acid. Methionine corresponds to the AUG codon.

The large subunit now binds to create the peptidyl (or P) site and the aminoacyl (or A) site. The first tRNA occupies the Psite. The second tRNA enters the A-site and is complementary to the second codon.

The methionine is transferred to the A-site amino acid, the first tRNA exits, the ribosome moves along the mRNA, and the next tRNA enters. These are the basic steps of elongation.

As elongation continues, the growing peptide is continually transferred to the A-site tRNA, the ribosome moves along the mRNA, and new tRNAs enter.

Whena stop codon is encountered in the A-site, a release factor enters the A-site and translation is terminated. When termination is reached, the ribosome dissociates, and the newly formed protein is released.


The Lac Operon -- Movie Narrative (Advanced Look)

The E. coli lac operon is an example of an inducible set of genes. These genes are responsible for the breakdown of lactose into sugars used for cellular metabolism.

This inducible system also involves bacterial DNA, a repressor, mRNA, and the sugar molecule lactose. This animation will only focus on two of the three proteins encoded by the lac operon, ß-galactosidase and permease.

Gene expression can be induced (or turned on) when a specific inducer molecule appears in a cell.

For inducible systems, a repressor molecule prevents gene expression by binding to the upstream controlling region.

Lactose is the lac operon inducer molecule. After first appearing in the cellular enviroment, lactose passively enters the E. coli cell and binds to the repressor molecule. This binding releases the repressor from the controlling region.

At this point, RNA polymerase can begin transcription of the operon. Here we show two of the three lac operon genes being transcribed into mRNA.

Ribosomes then bind to the mRNA, and the two proteins are translated.

The first protein is ß-galactosidase which breaks down lactose into two simple sugars.

The second protein is permease, a membrane bound protein.

When embedded in the cell membrane, permease functions to provide a direct route for the lactose outside the cell to be imported into the cell. This import occurs at a much greater rate than the passive transfer we first observed. Because translation continues inside the cell, other permease proteins become embedded in the membrane. This further increases the rate at which lactose enters the cell.

ß-galactosidase breaks the cellular lactose into the simple sugars glucose and galactose. Once its concentration is greatly reduced, the lactose bound to the repressor are released.

At this point, the repressor again binds to the controlling region and gene expression is halted.

For all inducible systems, like the lac operon, it is the interaction of the repressor and inducer molecules that mediate gene expression.


Photosynthesis (The Light Reactions)-- Movie Narrative (Advanced Look)

The process of Photosynthesis produces ATP from ADP and Pi by using the energy from light to excite electrons that are passed along an electron transport chain. Coupled with the transfer of electrons is the pumping of hydrogen ions and the splitting of water molecules.

The following complexes are found in the photosynthesis electron transport chain: Photosystem II, Cytochrome b6-f, Photosystem I, Ferredoxin NADP Reductase (FNR), and the complex that makes ATP, ATP Synthase. In addition to the complexes, three mobile carriers are also involved: Plastoquinone Qb, Plastocyanin, and Ferredoxin. Other key components include: photons, chlorophyll molecules, protons, water, molecular oxygen, NADP+ and the electrons to form NADPH, and ADP and Pi, which combine to form ATP.

Photosynthesis occurs in the chloroplasts of plants and algae. The process is also found in single-cell organisms such as cyanobacteria that do not have chloroplasts. Like its mitochondrial counterpart, the chloroplast electron transport chain consists of several protein complexes and mobile electron carriers.

First, a photon of light hits a chlorophyll molecule surrounding the Photosystem II complex. This creates resonance energy that is transferred through neighboring chlorophyll molecules. When this energy reaches the reaction center embedded in photosystem II, an electron is released. The reaction center chlorophyll contains electrons that can be transferred when excited. One photon is needed to excite each of the electrons in this chlorophyll.

Once excited, two electrons are transferred to plastoquinone Qb, the first mobile carrier. In addition to the two electrons, Qb also picks up two protons from the stroma.

The two electrons lost from photosystem II are replaced by the splitting of water molecules. Water splitting also releases hydrogen ions into the lumen. This contributes to a hydrogen ion gradient similar to the one created by mitochondrial electron transport. After two water molecules have been split, one molecule of molecular oxygen is created.

Plastoquinone Qb then transfers the two electrons to the cytochrome b6-f complex. The two protons it picked up are released into the lumen. These transfers are coupled with the pumping of two more hydrogen ions into the lumen space by cytochrome b6-f.

The electrons are next transferred to plastocyanin, another mobile carrier. Next the electrons are transferred from plastocyanin to the Photosystem I complex.

It is here that photons again energize each electron and propel their transfer to ferredoxin. Ferredoxin then transfers the electrons to the ferredoxin-NADP-reductase, also known as FNR. After two electrons are transfered to FNR, NADPH is made by adding the two electrons and a hydrogen ion to NADP+.

The hydrogen ion gradient created by the electron transport chain is utilized by ATP synthase to create ATP from ADP and Pi. This is similar to the way ATP is synthesized in the mitochondria. ATP, NADPH, and molecular oxygen are the final, vital, products of photosynthesis.


Protein Modification (Golgi) -- Movie Narrative

Proteins targeted to organelles such as the endosome, cellular membranes, or for extracellular secretion, must be modified. The modification is necessary for the correct delivery of the protein to its final cellular location.

The modification occurs when specific sugar molecules are added to a core oligosaccharide that is attached to the protein. These sugar complexes are the signal often required to direct the protein to its final destination. One example of this, is mannose 6-phosphate.

These sugar side chains modifications occur within the Golgi apparatus.

We focus here on the delivery of a hydrolase enzyme to the endosome. Hydrolases are enzymes that degrade other molecules. The endosome is an organelle that contains molecules to be degraded. Other key components include the M6P receptor protein.

So let's follow a hydrolase from the endoplasmic reticulum, or ER, where it is synthesized, to the endosome.

First, the hydrolase is delivered from the ER to the Golgi Apparatus via a vesicle. While it is being transfered through the ER and cis-cisterna of the Golgi apparatus, modification of the sugar core oligosaccharide begins. The term for this process is glycosylation.

Here we show two steps involved in the production of the mannose 6-phosphate signal. In humans, defects in Golgi glycosylation can lead to specific diseases. Once the hydrolase reaches the trans-golgi cisternae the mannose 6-phosphate signal has been completed. Only proteins destined for the endosome have the mannose 6-phosphate signal.

Once modified the hydrolase is bound to the mannose-6 receptor protein through the mannose-6-phosphate molecule. The receptor has a domain that extends through the trans-golgi membrane. Through the interaction with the receptor the hydrolase is associated with the trans-golgi membrane.

Next, a vesicle containing the hydrolase buds off from the trans-Golgi and moves to the endosome. Endosomes eventually mature into Lysosomes. Other proteins have different sugar side chains and they are delivered to other cellular locations or to the cell membrane where it is embedded or secreted.

The vesicle docks and fuses with the endosome. At this point, the hydrolase is released. Soon after, the phosphate portion of the mannose 6-phosphate signal is removed. Before it can go on to degrade other molecules, the hydrolase will undergo a final modification to become an active enzyme. The M6P receptors are then recycled back to the Golgi.

The sugar side chain signal, added by glycosylation in the Golgi Apparatus, is a key element of the process that directs certain proteins to their proper cellular locations.


Regulated Secretion (Golgi) -- Movie Narrative

Cells produce proteins that serve specific functions. Some cells have a specialized function to release specific proteins required under certain cellular conditions. Beta cells in the pancreas secrete insulin, some nerve cells (or neurons) release neurotransmitter proteins, and specific pituitary cells release one of many peptide hormones.

All of these cells have one common feature: secretory vesicles. A secretory vesicle is a membrane bound compartment that stores large amounts of a specialized protein. Looking into a pancreatic beta cell, you will find secretory vesicles filled with insulin, while in a nerve cell the secretory vesicle may be filled with serotonin or another neurotransmitter protein.

Remember that in constitutive secretion, vesicles from the trans-Golgi network are sent to the cell surface for immediate secretion. Whereas in regulated secretion, vesicles containing the product for secretion remain near the cell surface until a specific signal arrives that triggers secretion.

Here we see a pancreatic beta cell--one of the cell types found in the pancreas. In these beta cells, the secretory vesicles are filled with insulin. As with many proteins, the insulin protein is processed in the Golgi apparatus and secreted in vesicles. Secretory vesicles are all derived from transport vesicles leaving the trans-Golgi network. In these specialized cells, vesicles containing the same cargo protein will fuse to form larger secretory vesicles.

Vesicles containing insulin build up in the cell until a glucose signal enters the cell. Glucose itself does not directly cause the release, but in the presence of glucose a complicated process is initiated, leading to the release. The key point, is that the secretory vesicles containing high concentrations of insulin move to the plasma membrane at the cell surface and release the protein to neighboring blood capillaries.

This entire process couples the creation of the secretory vesicles with regulated release and is called exocytosis. In this case, the released insulin in turn acts as a regulator that stimulates the release of glucose transporters in other cells via exocytosis. Collectively, this is a classic example of how different exocytosis events are coupled for the normal cellular and physiological function of an organism.


Protein Recycling -- Movie Narrative

Glucose is an important sugar source in humans. As the precursor for glycolysis, it is at the beginning of the chemical chain that leads to the production of ATP, the universal energy molecule.

In humans, most of the glucose is utilized in muscle and fat tissues. When we exercise, glucose is rapidly taken up to support our muscles. After eating, the circulated glucose is taken up by fat cells and used immediately or stored as glycogen for later use.

The cells in these two tissue types have a common mechanism for taking up glucose. When glucose levels rise in these tissues, the glucose transporter 4 (or GLUT4) protein is mobilized from the cytoplasm and deposited in the plasma membrane.

This animation illustrates how proteins are recycled in the cell, a basic concept critical to understanding how external signals affect protein trafficking.

As illustrated in the Protein Trafficking animation, many proteins are processed through the Golgi apparatus and the trans-Golgi network, and eventually stored in vesicles. The vesicles are membrane-bound cargo vessels that can move to different cellular locations.

Here we focus on glucose transporter 4 (again GLUT4), the main protein through which glucose moves into the cell in humans. The glucose transporter proteins are found in two locations. One portion is found in the plasma membrane. The second pool of GLUT4 proteins are found in GLUT4 storage vesicles (or GSVs). This is a pool of vesicles that cluster together.

When glucose levels are low in muscle and fat tissues, only a small portion of the cellular GLUT4 protein is found in the plasma membrane. The vast majority (about 90%) is found in GLUT4 storage vesicles (or GSVs). The membrane bound GLUT4 proteins are needed to transport even the low level of glucose into the cell. Here you can see a few glucose molecules moving into the cell.

There is a steady recycling of the plasma membrane GLUT4 protein pool. These proteins bud off and move to the GSV pool in a membrane vesicle. The membrane pool is refilled by the movement of vesicles from the GSV pool to the plasma membrane where the vesicle merges and the GLUT4 protein is embedded in the membrane.

Recycling is a common feature of protein stored in vesicles that balances the current need for a certain protein and the ability to rapidly mobilize that protein to its site of action when the proper signal is received. An additional animation will show what happens in muscle and fat cells when they are stimulated by insulin.