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How are electrons entering the electron transport chain in cellular respiration energised (excited)?


In photosynthesis, electrons are excited by light energy from the sun (photoactivated).

How do they become excited in human cellular respiration?

I believe it has something to do with NADH and FADH2.


It may be useful to start with a quotation from the Introduction to Ch. 9 of Berg et al. Biochemistry (6e) that addresses the question, more or less as asked. I can't find a link to the pages in the 5e online so I will transcribe it from my text:

“A principal difference between oxidative phosphorylation and photophosphorylation is the source of the high energy electrons. In oxidative phosphorylation these electrons come from the oxidation of carbon fuels to carbon dioxide. In photosynthesis these electrons are excited to a higher energy level by the energy from photons.”

It is important to think about one of the ways in which the excited electrons are used in photosynthesis. To quote from earlier in the Introduction:

“In essence, light is used to create reducing potential.”

What this means chemically is that in photosynthesis the electrons have sufficient energy to reduce NAD+ to NADPH. In the biosynthetic (dark) reactions of photosynthesis the NADPH can be used to reduce carbon dioxide to sugars from which glucose can be produced.

So glucose is a reduced compound and can be thought of as 'embodying' the excited electrons, as its oxidation to carbon dioxide generates NADH (from NAD+). Hence:

There is no need for any additional excitation of electrons - that was done originally in photosynthesis.


How are electrons entering the electron transport chain in cellular respiration energised (excited)? - Biology

Let’s imagine that you are a cell. You’ve just been given a big, juicy glucose molecule, and you’d like to convert some of the energy in this glucose molecule into a more usable form, one that you can use to power your metabolic reactions. How can you go about this? What’s the best way for you to squeeze as much energy as possible out of that glucose molecule, and to capture this energy in a handy form?

Fortunately for us, our cells—and those of other living organisms—are excellent at harvesting energy from glucose and other organic molecules, such as fats and amino acids). Here, we’ll go through a quick overview of how cells break down fuels, then look at the electron transfer reactions (redox reactions) that are key to this process.


Biology 190 Chapter 10 Answers

CO2 and ATP.
H2O and NADPH.
sugar and O2.
ATP and NADPH.
light energy.

Which of the following sequences correctly represents the flow of electrons during photosynthesis?

NADPH → electron transport chain → O2
NADPH → O2 → CO2
H2O → NADPH → Calvin cycle
NADPH → chlorophyll → Calvin cycle
H2O → photosystem I → photosystem II

How is photosynthesis similar in C4 plants and CAM plants?

Both types of plants make most of their sugar in the dark.
In both cases, only photosystem I is used.
In both cases, thylakoids are not involved in photosynthesis.
Both types of plants make sugar without the Calvin cycle.
In both cases, rubisco is not used to fix carbon initially.

Which of the following statements is a correct distinction between autotrophs and heterotrophs?

Only heterotrophs require oxygen.
Cellular respiration is unique to heterotrophs.
Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic.
Only heterotrophs require chemical compounds from the environment.
Only heterotrophs have mitochondria.

Which of the following does not occur during the Calvin cycle?

consumption of ATP
carbon fixation
oxidation of NADPH
regeneration of the CO2 acceptor
release of oxygen

In mechanism, photophosphorylation is most similar to

the Calvin cycle.
substrate-level phosphorylation in glycolysis.
carbon fixation.
reduction of NADP+.
oxidative phosphorylation in cellular respiration.

Which process is most directly driven by light energy?

ATP synthesis
carbon fixation in the stroma
reduction of NADP+ molecules
removal of electrons from chlorophyll molecules
creation of a pH gradient by pumping protons across the thylakoid membrane

In photosynthesis, plants use carbon from __________ to make sugar and other organic molecules. (eText Overview)

water
carbon dioxide
chlorophyll
the sun
soil

Which of the following groups of organisms contains only heterotrophs? (eText Overview)

bacteria
protists
fungi
All of the listed responses are correct.
None of the listed responses are correct.

How does carbon dioxide enter the leaf? (eText Concept 10.1)

through the chloroplasts
through the roots
through the thylakoids
through the stomata
through the vascular system

In a rosebush, chlorophyll is located in __________. (eText Concept 10.1)

chloroplasts, which are in mesophyll cells in the thylakoids of a leaf
mesophyll cells, found within the thylakoids of a leaf’s chloroplasts
thylakoids, which are in mesophyll cells in the chloroplasts of a leaf
chloroplasts, which are in thylakoids in the mesophyll cells of a leaf
thylakoids, which are in chloroplasts in the mesophyll cells of a leaf

Chlorophyll molecules are in which part of the chloroplast? (eText Concept 10.1)

thylakoid membranes
stroma
stomata
plasma membrane
thylakoid lumen

The source of the oxygen produced by photosynthesis has been identified through experiments using radioactive tracers. The oxygen comes from __________. (eText Concept 10.1)

carbon dioxide
glucose
radioisotopes
water
light

In photosynthesis, what is the fate of the oxygen atoms present in CO2? They end up __________. (eText Concept 10.1)

as molecular oxygen
in sugar molecules
in water
as molecular oxygen and in sugar molecules
in sugar molecules and in water

Open Hint for Question 8 in a new window Molecular oxygen is produced during __________. (eText Concept 10.1)

glycolysis
linear electron flow during the light reactions
the Calvin cycle
cyclic electron flow during the light reactions
re-energization of electrons by PSI

Open Hint for Question 9 in a new window The reactions of the Calvin cycle are NOT directly dependent on light, but they usually do NOT occur at night. Why? (eText Concept 10.1)

It is often too cold at night for these reactions to take place.
Carbon dioxide concentrations decrease at night.
The Calvin cycle requires products only produced when the photosystems are illuminated.
Plants usually open their stomata at night.
At night, no water is available for the Calvin cycle.

The Calvin cycle occurs in the __________. (eText Concept 10.1)

thylakoid membrane
thylakoid lumen
stroma
stomata
matrix

What is the role of NADP+ in photosynthesis? (eText Concept 10.1)

It helps produce ATP from the light reactions.
It absorbs light energy.
It forms part of photosystem II.
It is the primary electron acceptor.
It forms NADPH to be used in the Calvin cycle.

A photon of which of these colors would carry the most energy? (eText Concept 10.2)

green
yellow
blue
orange
red

What is the range of wavelengths of light that are absorbed by the pigments in the thylakoid membranes? (eText Concept 10.2)

green, which is why plants are green
blue-violet and red-orange
the entire spectrum of white light
the infrared
the range absorbed by carotenoids

The most important role of pigments in photosynthesis is to __________. (eText Concept 10.2)

capture light energy
screen out harmful ultraviolet rays
store energy
catalyze the hydrolysis of water
catalyze the synthesis of ATP

Based on the work of Engelmann, a plot of photosynthetic activity versus wavelength of light is referred to as __________. (eText Concept 10.2)

an effective spectrum
an absorption spectrum
an electromagnetic spectrum
a visible light spectrum
an action spectrum

When chloroplast pigments absorb light, __________. (eText Concept 10.2)

the pigments become reduced
the pigments lose potential energy
the pigments’ electrons become excited
the Calvin cycle is triggered
the pignments’ photons become excited

What structure is formed by the reaction center, light-harvesting complexes, and primary electron acceptors that cluster, and is located in the thylakoid membrane? (eText Concept 10.2)

the fluorescence center
the photosystem
the electron transport chain
NADP+ reductase
ATP synthase

Where do the electrons entering photosystem II come from? (eText Concept 10.2)

chlorophyll molecules in the antenna complex
ATP
the electron transport chain
light
water

During photosynthesis in chloroplasts, O2 is produced from __________ via a series of reactions associated with __________. (eText Concept 10.2)

CO2 … photosystem II
H2O … photosystem II
CO2 … the Calvin cycle
H2O … photosystem I
CO2 … both photosystem I and the Calvin cycle

Which of the following is cycled in the cyclic variation of the light reactions? (eText Concept 10.2)

electrons
ATP
NADPH
ribulose bisphosphate
protons


Pyruvate Oxidation

If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A (CoA). The resulting compound is called acetyl CoA. CoA is made from vitamin B5, pantothenic acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.

Breakdown of Pyruvate

In order for pyruvate (which is the product of glycolysis) to enter the Citric Acid Cycle (the next pathway in cellular respiration), it must undergo several changes. The conversion is a three-step process (Figure 5).

Figure 5. Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA. In the process, carbon dioxide is released and one molecule of NADH is formed.

Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme (pyruvate dehydrogenase). This is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice (remember: there are two pyruvate molecules produced at the end of glycolysis) for every molecule of glucose metabolized thus, two of the six carbons will have been removed at the end of both steps.

Step 2. NAD + is reduced to NADH. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD + , forming NADH. The high-energy electrons from NADH will be used later to generate ATP.

Step 3. An acetyl group is transferred to conenzyme A, resulting in acetyl CoA. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.

Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.

Acetyl CoA to CO2

In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names, but we will primarily call it the Citric Acid Cycle.

In Summary: Pyruvate Oxidation

In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked up by NAD + , and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.


Cellular Respiration

Cellular respiration in the presence of oxygen (aerobic respiration) is the process by which energy-rich organic substrates are broken down into carbon dioxide and water, with the release of a considerable amount of energy in the form of adenosine triphosphate (ATP). Anaerobic respiration breaks down glucose in the absence of oxygen, and produces pyruvate, which is then reduced to lactate or to ethanol and CO2. Anaerobic respiration releases only a small amount of energy (in the form of ATP) from the glucose molecule.

Respiration occurs in three stages. The first stage is glycolysis, which is a series of enzyme-controlled reactions that degrades glucose (a 6-carbon molecule) to pyruvate (a 3-carbon molecule) which is further oxidized to acetylcoenzyme A (acetyl CoA). Amino acids and fatty acids may also be oxidized to acetyl CoA as well as glucose.

In the second stage, acetyl CoA enters the citric acid (Krebs) cycle, where it is degraded to yield energy-rich hydrogen atoms which reduce the oxidized form of the coenzyme nicotinamide adenine dinucleotide (NAD + ) to NADH, and reduce the coenzyme flavin adenine dinucleotide (FAD) to FADH2. (Reduction is the addition of electrons to a molecule, or the gain of hydrogen atoms, while oxidation is the loss of electrons or the addition of oxygen to a molecule.) Also in the second stage of cellular respiration, the carbon atoms of the intermediate metabolic products in the Krebs cycle are converted to carbon dioxide.

The third stage of cellular respiration occurs when the energy-rich hydrogen atoms are separated into protons [H + ] and energy-rich electrons in the electron transport chain. At the beginning of the electron transport chain, the energy-rich hydrogen on NADH is removed from NADH, producing the oxidized coenzyme, NAD + and a proton (H+) and two electrons (e-). The electrons are transferred along a chain of more than 15 different electron carrier molecules (known as the electron transport chain). These proteins are grouped into three large respiratory enzyme complexes, each of which contains proteins that span the mitochondrial membrane, securing the complexes into the inner membrane. Furthermore, each complex in the chain has a greater affinity for electrons than the complex before it. This increasing affinity drives the electrons down the chain until they are transferred all the way to the end where they meet the oxygen molecule, which has the greatest affinity of all for the electrons. The oxygen thus becomes reduced to H2O in the presence of hydrogen ions (protons), which were originally obtained from nutrient molecules through the process of oxidation.

During electron transport, much of the energy represented by the electrons is conserved during a process called oxidative phosphorylation. This process uses the energy of the electrons to phosphorylate (add a phosphate group) adenosine diphosphate (ADP), to form the energy-rich molecule ATP.

Oxidative phosphorylation is driven by the energy released by the electrons as they pass from the hydrogens of the coenzymes down the respiratory chain in the inner membrane of the mitochondrion. This energy is used to pump protons (H + ) across the inner membrane from the matrix to the intermediate space. This sets up a concentration gradient along which substances flow from high to low concentration, while a simultaneous current of OH - flows across the membrane in the opposite direction. The simultaneous opposite flow of positive and negative ions across the mitochondrial membrane sets up an electrochemical proton gradient. The flow of protons down this gradient drives a membrane-bound enzyme, ATP synthetase, which catalyzes the phosphorylation of ADP to ATP.

This highly efficient, energy conserving series of reactions would not be possible in eukaryotic cells without the organelles called mitochondria. Mitochondria are the "powerhouses" of the eukaryotic cells, and are bounded by two membranes, which create two separate compartments: an internal space and a narrow intermembrane space. The enzymes of the matrix include those that catalyze the conversion of pyruvate and fatty acids to acetyl CoA, as well as the enzymes of the Krebs cycle. The enzymes of the respiratory chain are embedded in the inner mitochondrial membrane, which is the site of oxidative phosphorylation and the production of ATP.

In the absence of mitochondria, animal cells would be limited to glycolysis for their energy needs, which releases only a small fraction of the energy potentially available from the glucose.

The reactions of glycolysis require the input of two ATP molecules and produce four ATP molecules for a net gain of only two molecules per molecule of glucose. These ATP molecules are formed when phosphate groups are removed from phosphorylated intermediate products of glycolysis and transferred to ADP, a process called substrate level phosphorylation (synthesis of ATP by direct transfer of a high-energy phosphate group from a molecule in a metabolic pathway to ADP).

In contrast, mitochondria supplied with oxygen produce about 36 molecules of ATP for each molecule of glucose oxidized. Procaryotic cells, such a bacteria, lack mitochondria as well as nuclear membranes. Fatty acids and amino acids when transported into the mitochondria are degraded into the two-carbon acetyl group on acetyl CoA, which then enters the Krebs cycle. In animals, the body stores fattyacids in the form of fats, and glucose in the form of glycogen in order to ensure a steady supply of these nutrients for respiration.

While the Krebs cycle is an integral part of aerobic metabolism, the production of NADH and FADH 2 is not dependent on oxygen. Rather, oxygen is used at the end of the electron transport chain to combine with electrons removed from NADH and FADH2 and with hydrogen ions in the cytosol to produce water.

Although the production of water is necessary to keep the process of electron transport chain in motion, the energy used to make ATP is derived from a different process called chemiosmosis.

Chemiosmosis is a mechanism that uses the proton gradient across the membrane to generate ATP and is initiated by the activity of the electron transport chain. Chemiosmosis represents a link between the chemical and osmotic processes in the mitochondrion that occur during respiration.

The electrons that are transported down the respiratory chain on the mitochondrion's inner membrane release energy that is used to pump protons (H + ) across the inner membrane from the mitochondrial matrix into the intermembrane space. The resulting gradient of protons across the mitochondrial inner membrane creates a backflow of protons back across the membrane. This flow of electrons across the membrane, like a waterfall used to power an electric turbine, drives a membrane-bound enzyme, ATP synthetase. This enzyme catalyzes the phosphorylation of ADP to ATP, which completes the part of cellular respiration called oxidative phosphorylation. The protons, in turn, neutralize the negative charges created by the addition of electrons to oxygen molecules, with the resultant production of water.

Cellular respiration produces three molecules of ATP per pair of electrons in NADH, while the pair of electrons in FADH2 generate two molecules of ATP. This means that 12 molecules of ATP are formed for each acetyl CoA molecule that enters the Krebs cycle and since two acetyl CoA molecules are formed from each molecule of glucose, a total of 24 molecules of ATP are produced from each molecule of this sugar. When added to the energy conserved from the reactions occurring before acetyl CoA is formed, the complete oxidation of a glucose molecule gives a net yield of about 36 ATP molecules. When fats are burned, instead of glucose, the total yield from one molecule of palmitate, a 16-carbon fatty, is 129 ATP.


Aerobic Respiration

Cellular Respiration. Left side is glycolysis (anaerobic). The Right side is what occurs in the presence of oxygen in eukaryotes. The aerobic reactions occur inside the mitochondria after being fed Acetyl-CoA molecules from the cytoplasmic preparatory reaction. Credit: RegisFrey (CC-BY-SA 3.0)

Closeup of the Electron Transport Chain (ETC) that takes place on the inner membrane of mitochondria. This is where oxygen is utilized as the final electron acceptor. Reduction of 1/2 O2 results in the generation of a water molecule ( chemiosmosis ). Credit: Jeremy Seto (CC-BY-NC-SA 3.0)


Anaerobic Respiration

  • In anaerobic respiration Nitrate, Sulfate, Co2 functions as a final electron acceptor in ATC.
  • Anaerobic respiration produces less energy or ATP as compared to aerobic respiration.
  • The aerobic respiration is performed by few bacteria, archaea, and some eukaryotic microbes.
  • Paracoccus denitrificans performs both anaerobic respiration in presence of O2 and anaerobic respiration in absence of O2.
  • The anaerobic respiration carried out different enzymes such as nitrate reductase (Nar), nitrite reductase (Nir), nitric oxide reductase (Nor), and nitrous oxide reductase (Nos) for the formation of gaseous Nitrogen from Nitrate.
  • Nitrite is reduced to nitric oxide (NO) by the periplasmic enzyme nitrite reductase. Nitric oxide reductase catalyzes the formation of nitrous oxide (N20) from NO. It is part of the membrane-bound cytochrome b complex. Finally, the periplasmic enzyme nitrous oxide reductase catalyzes the formation of N2 from N20.

Why is the energy yield or ATP yield in Anaerobic respiration is Low?

The final electron acceptor of anaerobic respiration is nitrate, which has a low positive reduction potential as compared to oxygen(electron acceptor of aerobic respiration). The difference in standard reduction potential of NADH and Nitrate is lower than the difference of reduction potential between the NADH and O2.

That’s why the energy yield of anaerobic respiration is low, because the energy yield is directly related to the magnitude of reduction potential difference.


6.1: Photosynthesis

  • Contributed by E. V. Wong
  • Axolotl Academica Publishing (Biology) at Axolotl Academica Publishing

In one way or another, the energy of sugar and fat fuel molecules is derived from photosynthesis - the conversion of solar light energy into chemical bond energy, whether directly in photosynthetic plant cells and certain photosynthetic bacteria, or indirectly by the ingestion of those plants and bacteria. Photosynthesis is a simple idea: atmospheric carbon dioxide molecules are joined with water molecules to form sugars and oxygen:

The production of usable energy from sunlight and the fixation of atmospheric carbon dioxide are two separate sets of reactions. In plants, photosynthesis takes place only in cells containing chloroplasts. Chloroplasts are organelles with an evolutionary origin suspected to be similar to that of mitochondria, and like mitochondria, chloroplasts generate ATP and use a nicotinamide-based high-energy electron carrier. There are further similarities: they both have highly folded inner membranes, though in chloroplasts, there are three membranes in all, while mitochondria only have two. Finally, an electron transport chain is embedded in the thylakoid membrane of chloroplasts, functioning very similarly to electron transport in the mitochondria. In addition to the electron transport components and ATP synthase (structurally and functionally almost identical to mitochondrial ATP synthase), the thylakoid membrane is also rich in a set of molecules that are not found in the inner mitochondrial membrane: light-absorbing pigment molecules.

In plants, these pigment molecules fall into two classes: the chlorophylls and the carotenoids (Figure (PageIndex<1>)) but only the chlorophylls can mediate photosynthesis. Photosynthetic bacteria do not contain chlorophyll, but do have carotenoid pigments that can carry out photosynthesis. Both are hydrophobic hydrocarbons that are held in place within the plane of the membrane by transmembrane proteins. Chlorophylls are easily recognizable by the very large Mg 2+ -containing porphyrin ring, while the carotenoids are long hydrocarbon chains that may or may not have small ring structures on the ends (e.g. &beta-carotene). While there is variation in the chlorophyll family, they all impart a green color to the leaf. Carotenoids, on the other hand have a much wider range of colors from yellows to reds. Both chlorophylls and carotenoids are able to absorb light energy of a particular wavelength/energy range and enter an unstable excited state. When the molecule returns to its ground state, the energy would be emitted as heat or light in an isolated situation. However within the context of the pigment arrays (antenna complex) in a living cell, most of the energy is shuttled to another pigment molecule of lower energy by resonance transfer. As described below, only one pair of chlorophyll molecules in an antenna complex will actually eject an electron as it drops from an excited state back to ground state. It is the transfer of that high-energy electron that powers photosynthesis.

Figure (PageIndex<1>). Chlorophyll (top) and &beta-carotene (bottom)

Chlorophyll molecules are made up of a phytol hydrocarbon tail that anchors the molecule within a membrane, and an electron- carrying porphyrin ring containing a magnesium cation. Note that the phytol tail is not drawn to scale with the porphyrin ring in Figure (PageIndex<1>). Among different types of chlorophyll, the chemical groups attached to the ring may vary, and this variation is responsible for differences in the absorption spectrum from one type of chlorophyll to the next. For example, chlorophyll a has absorption peaks at approximately 430 and 662 nm, whereas chlorophyll b has peaks at 453 and 642 nm. The difference between the two is small: at C7, there is a &mdashCH3 group on chlorophyll a, but a &mdashCHO group on chlorophyll b. Presently, there are five known chlorophylls: chlorophyll a is found in all photosynthetic organisms, chlorophyll b is only found in plants, chlorophylls c1 and c2 are found in photosynthetic algae, and chlorophyll d is found in cyanobacteria.

Carotenoids have two functions. As noted in the primary text at left, they can participate in energy transfer in toward the reaction center chlorophylls. They are also a protectant molecule, preventing reaction center auto-oxidation. Carotenoids can be highly efficient free radical scavengers due to the conjugation of alternating single-double carbon bond structures.

Photosynthesis can be divided into two mechanisms: the light reactions, which use light energy to excite the electrons of certain chlorophylls, and participate in the electron transport chain to generate ATP and NADPH, and the dark reactions, which use that ATP and NADPH to fix carbon from CO2 into organic molecules (carbohydrates). As the name implies, the light reactions require light energy to excite the chlorophyll and begin electron transport. Dark reactions, however, do not require darkness. They are technically light-independent, but in some plants, the dark reactions run better in the light for reasons to be discussed.

The light reactions are intimately tied to the anatomy of the thylakoid membrane specifically, the arrangement of light-absorbing pigment molecules in antenna complexes, also called light-harvesting complexes (sometimes abbreviated LHC, not to be confused with the Large Hadron Collider). These pigments are held by proteins in ordered three-dimensional groups so that the pigments that absorb the highest energy light are toward the periphery, and the lowest-energy-absorbing chlorophylls are in the center (Figure (PageIndex<2>)). Sunlight is composed of a broad range of wavelengths, some of which are transiently absorbed by the pigments. After a pigment molecule absorbs a photon, the energy is released and passed on to a pigment tuned to a slightly lower energy level (longer wavelength), and from there to an even lower-energy pigment, and so on until it reaches the reaction center chlorophylls. In this way, energy from a wide range of light wavelengths/energies can all contribute to the ATP and NADPH production by the light reactions. The antenna complex is crucial because it allows the use of a greater portion of the solar light spectrum. And, as a tightly-packed three-dimensional array, photons that pass by one pigment molecule may well hit another one on its way through the array. All of these characteristics combine to increase the efficiency of light use for photosynthesis. The reaction center chlorophylls (P680 for photosystem II, P700 for photosystem I) are the only chlorophylls that actually send excited electrons into the electron transport chain. The other chlorophylls and pigments only act to transfer the energy to the reaction center.

Figure (PageIndex<2>). Pigment molecules are arranged in an antenna complex in the thylakoid membrane.

When excited, the reaction center chlorophyll of photosystem II (Figure (PageIndex<3>)) begins the process of electron transport. This chlorophyll is part of a protein complex that also includes a Mn-based oxygen-evolving complex (OEC), pheophytin, and a docking site for plastoquinone. Although the chlorophyll electron is the one excited by the solar energy, the origin of the electrons to keep the chlorophyll replenished comes from the splitting (oxidation) of water to O2 and 4 H + .

The OEC, or oxygen-evolving complex (also WOC, water oxidizing complex) is a metalloenzyme with a Mn4OXCa catalytic cluster, where X is the number of m-oxo-bridges connecting the metal atoms, with surrounding amino acids, especially crucial tyrosines, also playing a role in the coordination sphere of the active site. The overall complex undergoes a series of 4 oxidation state changes as the P680 chlorophylls are excited by the light energy and transfer electrons, but at present it is not known what the exact oxidation state of any given Mn atom is through this series of state changes. The crucial reaction is the formation of the O-O bond to form O2. There are two proposed models for this mechanism. One is that the O-O bond is formed when the OEC has reached its fully oxidized state, and an oxygen in a m-oxo-bridge radical state interacts with a water molecule. The other proposed mechanism is that the O-O bond forms earlier as a complexed peroxide held by the OEC center.

Figure (PageIndex<3>). Photosystem II (which feeds electrons into photosystem I).

The question of how a cell could generate the energy needed to split water was long a thorny issue because water is an exceptionally stable molecule. The current model suggests that the energy comes from an extremely strong oxidizer in the form of P680 + . After P680 is energized by light, an excited electron has enough energy to break away from the chlorophyll and jumps to pheophytin. Pheophytin becomes Pheo- temporarily, and the charge separation in the complex between P680 + and Pheo - helps to enhance the oxidative power of P680 + . That extraordinarily strong attraction for electrons is what allows the P680 chlorophyll to tear them away from H2O and split the water. In fact, P680 + is one of the strongest biological oxidizers known. Since four electrons must be taken to fully oxidize two water molecules and generate molecular oxygen, four photoexcitation events are needed. While the exact mechanism is still to be elucidated, it appears that the OEC helps to stabilize the water molecule during this process as well as holding onto each electron as it comes off.

Figure (PageIndex<4>). Change in electron energy moving through photosystems II and I. Light energy is needed in both photosystems to boost the electron energy high enough to move to the next electron carrier.

The excited electrons, moving from the OEC to P680 + to pheophytin, next move to the lipid-soluble carrier, plastoquinone. The similarity of the name with the mitochondrial carrier ubiquinone is not a coincidence. They function similarly, and as the plastoquinone takes on the electrons, it also takes on protons from the stromal side of the thylakoid membrane. The PQ moves within the membrane from pheophytin to cytochrome b6f. As the electrons are transferred to cytochrome b6f, the protons are then dropped off on the lumenal side of the membrane, increasing their concentration in the chloroplast lumen, and building a proton gradient to power ATP synthase. Cytochrome b6f passes the electrons on to plastocyanin, an aqueous-phase carrier, which shuttles the electrons to the P700 reaction center chlorophyll of photosystem I. However, after all the transfers, the energy level of the electrons is now fairly low (Figure (PageIndex<4>)) and unable to power the upcoming reactions. Since it is now on a reaction center chlorophyll, the obvious answer is to re-energize it with a bit of sunlight. This raises the electron energy sufficiently to reduce ferredoxin. Now things get a little complicated.

This part of photosynthesis can take one of two directions, the linear pathway, which generates both NADPH and ATP, and the cyclic pathway which mostly generates ATP. Most of the time, the linear pathway is taken, with the electrons on ferredoxin transferred via ferredoxin-NADPH reductase (FNR) onto NADPH. However, sometimes the cell requires significantly more ATP than NADPH, in which case, the electrons from ferredoxin are transferred back to plastoquinone via ferredoxin-plastoquinone reductase. This acts just as described above, and pumps more protons across the membrane to power the ATP synthase. ATP synthesis goes up and NADPH synthesis goes down.

Figure (PageIndex<5>). Photosystem I initiates electron movement through two pathways.

The ATP and NADPH generated by the chloroplast are almost exclusively used by the chloroplast itself (and not distributed to the rest of the cell) to power the dark reactions, which are energetically expensive. In fact, when the light reactions are not running due to darkness, some plant cells have mechanisms to prevent the dark reactions from using the limited resources of cellular, non-chloroplastic, respiration. The simplest method of such limitation is the pH sensitivity of rubisco (ribulose bis-phosphate carboxylase), at least in C3 plants (see below). Rubisco has a very sharp pH optimum at about pH 8.0, so while the light reactions are running and the protons are being pumped, the pH rises to about 8 and rubisco works, but in the dark, the pH drops back to its basal level close to 7.0, inhibiting rubisco activity.


Cellular Respiration…I think I’ve got it!

At my previous workplace, we typically devoted about 2 days to cellular respiration and photosynthesis in our freshman biology classes.

Yes, biologists, you read that correctly. Two days.

Clearly, these topics are much more complex than 2 days worth of material. Are they enough for 3 weeks, though, in a high school biology 1 class?

This is what I’ve been struggling with over the past week and a half. Since I’ve never taught this material in more detail than two days worth, it’s been challenging to recall some of the information I haven’t had to think about since I was in college. Even the textbook we use (Biology: Exploring Life), which I really like, seems a little too detailed over this particular unit.

I think I’ve managed, though, to get it narrowed down to where I want it. Here’s what we covered for cellular respiration (we haven’t finished photosynthesis yet, so more on that later):

  • The overall equation for cellular respiration (and recognizing that the reactants in cellular respiration are the products in photosynthesis, and vice versa)
  • Glycolysis summary
  • Krebs Cycle summary
  • Electron Transport Chain and ATP Synthase summary

Here’s how we summarized each stage:

  • takes place in cytoplasm
  • splits glucose in half, forming 2 pyruvic acid molecules—-these go into the Krebs Cycle
  • also forms 2 NADH (we had to discuss what an electron carrier was, of course)—-these go to the ETC
  • net gain of 2 ATP
  • takes place in matrix of mitochondria
  • “strips off” carbons from the pyruvic acids, which become part of CO2 molecules
  • forms many more electron carriers (NADH and FADH2)—-go to the ETC
  • gain 2 more ATP

  • found on the inner membrane of the mitochondria
  • electron carriers (NADH and FADH2) give up their electrons to the ETC, creating a H+ gradient
  • gradient powers the enzyme ATP Synthase, making 34 ATP
  • oxygen accepts the electrons and hydrogen from the electron carriers, forming H2O

We didn’t name all of the other enzymes involved, and we didn’t really mention acetyl coA (even though it was in the textbook). It’s not perfect, but I think it makes a decent mix between only 2 days of coverage versus college-level detail. We also discussed fermentation (what happens when there is little/no oxygen present), and I stressed the fact that plants, too, do cellular respiration (some students get confused and think that autotrophs do photosynthesis, making glucose, and therefore they don’t need to do respiration–they fail to see that the glucose still needs to be broken down to make ATP).

We also did a little dance with hand motions. I’ll need to get the exact words later.


Electron Transport Chain Steps Explained with Diagram

The electron transport chain is an essential metabolic pathway that produces energy by carrying out a series of redox reactions. This BiologyWise article provides a simple explanation of this pathway.

The electron transport chain is an essential metabolic pathway that produces energy by carrying out a series of redox reactions. This BiologyWise article provides a simple explanation of this pathway.

Did You Know?

One cycle of the electron transport chain yields about 30 molecules of ATP (Adenosine triphosphate) as compared to the 2 molecules produced each via glycolysis and the citric acid cycle.

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The electron transport chain is made up of a series of spatially separated enzyme complexes that transfer electrons from electron donors to electron receptors via sets of redox reactions. This is also accompanied by a transfer of protons (H + ions) across the membrane. This leads to the development of an electrochemical proton gradient across the membrane that activates the ATP synthase proton pump, thereby, driving the generation of ATP molecules (energy). The cycle ends by the absorption of electrons by oxygen molecules.

In eukaryotic organisms, the electron transport chain is found embedded in the inner membrane of the mitochondria, in bacteria it is found in the cell membrane, and in case of plant cells, it is present in the thylakoid membrane of the chloroplasts.

In chloroplasts, photons from light are used produce the proton gradient whereas, in the mitochondria and bacterial cells, the conversions occurring in the enzyme complexes, generate the proton gradient.

Overview of Electron Transport Chain

This pathway is the most efficient method of producing energy. The initial substrates for this cycle are the end products obtained from other pathways. Pyruvate, obtained from glycolysis, is taken up by the mitochondria, where it is oxidized via the Krebs/citric acid cycle. The substrates required for the pathway are NADH (nicotinamide adenine dinucleotide), succinate, and molecular oxygen.

NADH acts as the first electron donor, and gets oxidized to NAD + by enzyme complex I, accompanied by the release of a proton out of the matrix. The electron is then transported to complex II, which brings about the conversion of succinate to fumarate. Molecular oxygen (O2) acts as an electron acceptor in complex IV, and gets converted to a water molecule (H2O). Each enzyme complex carries out the transport of electrons accompanied by the release of protons in the intermembrane space.

The accumulation of protons outside the membrane gives rise to a proton gradient. This high concentration of protons initiates the process of chemiosmosis, and activates the ATP synthase complex. Chemiosmosis refers to the generation of an electrical as well as a pH potential across a membrane due to large difference in proton concentrations. The activated ATP synthase utilizes this potential, and acts as a proton pump to restore concentration balance. While pumping the proton back into the matrix, it also conducts the phosphorylation of ADP (Adenosine Diphosphate) to yield ATP molecules.

Enzyme Complexes of Electron Transport Chain

Complex I – NADH-coenzyme Q oxidoreductase
The reduced coenzyme NADH binds to this complex, and functions to reduce coenzyme Q10. This reaction donates electrons, which are then transferred through this complex using FMN (Flavin mononucleotide) and a series of Fe-S (Iron-sulpur) clusters. The transport of these electrons brings about the transfer of protons across the membrane into the intermembrane space.

Complex II – Succinate-Q oxidoreductase
This complex acts on the succinate produced by the citric acid cycle, and converts it to fumarate. This reaction is driven by the reduction and oxidation of FAD (Flavin adenine dinucleotide) along with the help of a series of Fe-S clusters. These reactions also drive the redox reactions of quinone. These sets of reactions help in transporting the electrons to the third enzyme complex.

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Complex III – Q-cytochrome c oxidoreductase
This complex oxidizes ubiquinol and also reduces two molecules of cytochrome-c. The electron is transported via these reactions onto complex IV accompanied by the release of protons.

Complex IV – ytochrome c oxidase
The received electron is received by a molecular oxygen to yield a water molecule. This conversion occurs in the presence of Copper (Cu) ions, and drives the oxidation of the reduced cytochrome-c. Protons are pumped out during the course of this reaction.

ATP Synthase
The protons produced from the initial oxidation of the NADH molecule, and their presence in the intermembrane space gives rise to a potential gradient. It is utilized by this complex to transport the protons back into the matrix. The transport itself also generates energy that is used to achieve phosphorylation of the ADP molecules to form ATP.

Any anomalies or defects in any of the components that constitute the electron transport chain leads to the development of a vast array of developmental, neurological, and physical disorders.

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