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4.9: Photosynthesis - Dicovering the Secrets - Biology

4.9: Photosynthesis - Dicovering the Secrets - Biology


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This chapter talks about various scientists and their path towards discovering photosynthesis.

Van Helmont

Perhaps the first experiment designed to explore the nature of photosynthesis was that reported by the Dutch physician van Helmont in 1648. Some years earlier, van Helmont had placed in a large pot exactly 200 pounds (91 kg) of soil that had been thoroughly dried in an oven. Then he moistened the soil with rain water and planted a 5-pound (2.3 kg) willow shoot in it. He then placed the pot in the ground and covered its rim with a perforated iron plate. The perforations allowed water and air to reach the soil but lessened the chance that dirt or other debris would be blown into the pot from the outside.

For five years, van Helmont kept his plant watered with rain water or distilled water. At the end of that time, he carefully removed the young tree and found that it had gained 164 pound, 3 ounces (74.5 kg). (This figure did not include the weight of the leaves that had been shed during the previous four autumns.) He then redried the soil and found that it weighed only 2 ounces (57 g) less that the original 200 pounds (91 kg). Faced with these experimental facts, van Helmont theorized that the increase in weight of the willow arose from the water alone. He did not consider the possibility that gases in the air might be involved.

Joseph Priestley

The first evidence that gases participate in photosynthesis was reported by Joseph Priestley in 1772. He knew that if a burning candle is placed in a sealed chamber, the candle soon goes out. If a mouse is then placed in the chamber, it soon suffocates because the process of combustion has used up all the oxygen in the air — the gas on which animal respiration depends. However, Priestley discovered that if a plant is placed in an atmosphere lacking oxygen, it soon replenishes the oxygen, and a mouse can survive in the resulting mixture. Priestley thought (erroneously) that it was simply the growth of the plant that accounted for this.

Ingen-Housz

It was another Dutch physician, Ingen-Housz, who discovered in 1778 that the effect observed by Priestley occurred only when the plant was illuminated. A plant kept in the dark in a sealed chamber consumes oxygen just as a mouse (or candle) does.

Ingen-Housz also demonstrated that only green parts of plants liberated oxygen during photosynthesis. Nongreen plant structure, such as woody stems, roots, flowers, and fruits actually consume oxygen in the process of respiration. We now know that this is because photosynthesis can go on only in the presence of the green pigment chlorophyll.

Jean Senebier

The growth of plants is accompanied by an increase in their carbon content. A Swiss minister, Jean Senebier, discovered that the source of this carbon is carbon dioxide and that the release of oxygen during photosynthesis accompanies the uptake of carbon dioxide. Senebier concluded (erroneously as it turned out) that in photosynthesis carbon dioxide is decomposed, with the carbon becoming incorporated in the organic matter of the plant and the oxygen being released.

CO2 + H2O → (CH2O) + O2

(The parentheses around the CH2O signify that no specific molecule is being indicated but, instead, the ratio of atoms in some carbohydrate, e.g., glucose, C6H12O6.) The equation also indicates that the ratio of carbon dioxide consumed to oxygen release is 1:1, a finding that was carefully demonstrated in the years following Senebier's work. Using glucose as the carbohydrate product, we can write the equation for photosynthesis as

6CO2 + 6H2O → C6H12O6 + 6O2

F. F. Blackman

The above equation shows the relationship between the substances used in and produced by the process. It tells us nothing about the intermediate steps. That photosynthesis does involve at least two quite distinct processes became apparent from the experiments of the British plant physiologist F. Blackman. His results can easily be duplicated by using the setup in Figure 4.9.1. The green water plant Elodea (available wherever aquarium supplies are sold) is the test organism. When a sprig is placed upside down in a dilute solution of NaHCO3 (which serves as a source of CO2) and illuminated with a flood lamp, oxygen bubbles are soon given off from the cut portion of the stem. One then counts the number of bubbles given off in a fixed interval of time at each of several light intensities. Plotting these data produces a graph like the one in Figure 4.9.2.

Since the rate of photosynthesis does not continue to increase indefinitely with increased illumination, Blackman concluded that at least two distinct processes are involved: one, a reaction that requires light and the other, a reaction that does not. This latter is called a "dark" reaction although it can go on in the light. Blackman theorized that at moderate light intensities, the "light" reaction limits or "paces" the entire process. In other words, at these intensities the dark reaction is capable of handling all the intermediate substances produced by the light reaction. With increasing light intensities, however, a point is eventually reached when the dark reaction is working at maximum capacity. Any further illumination is ineffective, and the process reaches a steady rate.

This interpretation is strengthened by repeating the experiment as a somewhat higher temperature. Most chemical reactions proceed more rapidly at higher temperatures (up to a point). At 35°C, the rate of photosynthesis does not level off until greater light intensities are present. This suggest that the dark reaction is now working faster. The fact that at low light intensities the rate of photosynthesis is no greater at 35°C than at 20°C also supports the idea that it is a light reaction that is limiting the process in this range. Light reactions depend, not on temperature, but simply on the intensity of illumination.

The increased rate of photosynthesis with increased temperature does not occur if the supply of CO2 is limited. As the figure shows, the overall rate of photosynthesis reaches a steady value at lower light intensities if the amount of CO2 available is limited. Thus CO2 concentration must be added as a third factor regulating the rate at which photosynthesis occurs. As a practical matter, however, the concentration available to terrestrial plants is simply that found in the atmosphere: 0.035%.

Van Niel

It was the American microbiologist Van Niel who first glimpsed the role that light plays in photosynthesis. He studied photosynthesis in purple sulfur bacteria. These microorganisms synthesize glucose from CO2 as do green plants, and they need light to do so. Water, however, is not the starting material. Instead they use hydrogen sulfide (H2S). Furthermore, no oxygen is liberated during this photosynthesis but rather elemental sulfur. Van Niel reasoned that the action of light caused a decomposition of H2S into hydrogen and sulfur atoms. Then, in a series of dark reactions, the hydrogen atoms were used to reduce CO2 to carbohydrate:

[ce{CO2 + 2H2S → (CH2O) + H2O + 2S}]

Van Niel envisioned a parallel to the process of photosynthesis as it occurs in green plants. There the energy of light causes water to break up into hydrogen and oxygen. The hydrogen atoms are then used to reduce CO2 in a series of dark reactions:

[ce{CO2 + 2H2O → (CH2O) + H2O + O2}]

If this theory is correct, then it follows that all of the oxygen released during photosynthesis comes from water just as all the sulfur produced by the purple sulfur bacteria comes from H2S. This conclusion directly contradicts Senebier's theory that the oxygen liberated in photosynthesis comes from the carbon dioxide. If Van Niel's theory is correct, then the equation for photosynthesis would have to be rewritten:

[ce{6CO2 + 12H2O → C6H12O6 + 6 H2O + 6O2}]

In science, a theory should be testable. By deduction, one can make a prediction of how a particular experiment will come out if the theory is sound. In this case, the crucial experiments needed to test the two theories had to await the time when the growth of atomic research made it possible to produce isotopes other than those found naturally or in greater concentrations than are found naturally.

Samuel Ruben

In air, water and other natural materials containing oxygen, 99.76% of the oxygen atoms are 16O and only 0.20% of them are the heavier isotope 18O. In 1941, Samuel Ruben and his coworkers at the University of California were able to prepare specially "labeled" water in which the 0.85% of the molecules contained 18O atoms. When this water was supplied to a suspension of photosynthesizing algae, the proportion of 18O in the oxygen gas that was evolved was 0.85%, the same as that of the water supplied, and not simply the 0.20% found in all natural samples of oxygen (and its compounds like CO2).

% 18O FOUND IN
EXPERIMENTH2OCO2O2
1.START0.850.20
FINISH0.850.61*0.86
2.START0.200.68
FINISH0.200.570.20

* A non-biochemical exchange of oxygen atoms between the water and the bicarbonate ions used as a source of CO2 explains the uptake of the isotope by CO2 in the first experiment.

These results clearly demonstrated that Senebier's interpretation was in error. If all the oxygen liberated during photosynthesis comes from the carbon dioxide, we would expect the oxygen evolved in Ruben's experiment to contain simply the 0.20% found naturally. If, on the other hand, both the carbon dioxide and the water contribute to the oxygen released, we would expect its isotopic composition to have been some intermediate figure. In fact, the isotopic composition of the evolved oxygen was the same as that of the water used.

Ruben and his colleagues also prepared a source of carbon dioxide that was enriched in 18O atoms. When algae carried out photosynthesis using this material and natural water, the oxygen that was given off was not enriched in 18O. It contained simply the 0.20% 18O found in the natural water used. The heavy atoms presumably became incorporated in the other two products (carbohydrate and by-product water).

These experiments lent great support to Van Niel's idea that one function of light in photosynthesis was the separation of the hydrogen and oxygen atoms of water molecules. But there remained to work out just how the hydrogen atoms were made available to the dark reactions.


Surprising Research Reveals Photosynthesis Could Be As Old as Life Itself

The finding also challenges expectations for how life might have evolved on other planets. The evolution of photosynthesis that produces oxygen is thought to be the key factor in the eventual emergence of complex life. This was thought to take several billion years to evolve, but if in fact the earliest life could do it, then other planets may have evolved complex life much earlier than previously thought.

“Now, we know that Photosystem II shows patterns of evolution that are usually only attributed to the oldest known enzymes, which were crucial for life itself to evolve.” — Dr. Tanai Cardona

The research team, led by scientists from Imperial College London, traced the evolution of key proteins needed for photosynthesis back to possibly the origin of bacterial life on Earth. Their results are published and freely accessible in BBA – Bioenergetics.

Lead researcher Dr. Tanai Cardona, from the Department of Life Sciences at Imperial, said: “We had previously shown that the biological system for performing oxygen-production, known as Photosystem II, was extremely old, but until now we hadn’t been able to place it on the timeline of life’s history.

“Now, we know that Photosystem II shows patterns of evolution that are usually only attributed to the oldest known enzymes, which were crucial for life itself to evolve.”

Early oxygen production

Photosynthesis, which converts sunlight into energy, can come in two forms: one that produces oxygen, and one that doesn’t. The oxygen-producing form is usually assumed to have evolved later, particularly with the emergence of cyanobacteria, or blue-green algae, around 2.5 billion years ago.

While some research has suggested pockets of oxygen-producing (oxygenic) photosynthesis may have been around before this, it was still considered to be an innovation that took at least a couple of billion years to evolve on Earth.

The new research finds that enzymes capable of performing the key process in oxygenic photosynthesis – splitting water into hydrogen and oxygen – could actually have been present in some of the earliest bacteria. The earliest evidence for life on Earth is over 3.4 billion years old and some studies have suggested that the earliest life could well be older than 4.0 billion years old.

Colonies of cyanobacteria under the microscope.

Like the evolution of the eye, the first version of oxygenic photosynthesis may have been very simple and inefficient as the earliest eyes sensed only light, the earliest photosynthesis may have been very inefficient and slow.

On Earth, it took more than a billion years for bacteria to perfect the process leading to the evolution of cyanobacteria, and two billion years more for animals and plants to conquer the land. However, that oxygen production was present at all so early on means in other environments, such as on other planets, the transition to complex life could have taken much less time.

Measuring molecular clocks

The team made their discovery by tracing the ‘molecular clock’ of key photosynthesis proteins responsible for splitting water. This method estimates the rate of evolution of proteins by looking at the time between known evolutionary moments, such as the emergence of different groups of cyanobacteria or land plants, which carry a version of these proteins today. The calculated rate of evolution is then extended back in time, to see when the proteins first evolved.

“We could develop photosystems that could carry out complex new green and sustainable chemical reactions entirely powered by light.” — Dr. Tanai Cardona

They compared the evolution rate of these photosynthesis proteins to that of other key proteins in the evolution of life, including those that form energy storage molecules in the body and those that translate DNA sequences into RNA, which is thought to have originated before the ancestor of all cellular life on Earth. They also compared the rate to events known to have occurred more recently, when life was already varied and cyanobacteria had appeared.

The photosynthesis proteins showed nearly identical patterns of evolution to the oldest enzymes, stretching far back in time, suggesting they evolved in a similar way.

First author of the study Thomas Oliver, from the Department of Life Sciences at Imperial, said: “We used a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins.

“These sequences give us information about how the ancestral Photosystem II would have worked and we were able to show that many of the key components required for oxygen evolution in Photosystem II can be traced to the earliest stages in the evolution of the enzyme.”

Directing evolution

Knowing how these key photosynthesis proteins evolve is not only relevant for the search for life on other planets, but could also help researchers find strategies to use photosynthesis in new ways through synthetic biology.

Dr. Cardona, who is leading such a project as part of his UKRI Future Leaders Fellowship, said: “Now we have a good sense of how photosynthesis proteins evolve, adapting to a changing world, we can use ‘directed evolution’ to learn how to change them to produce new kinds of chemistry.

“We could develop photosystems that could carry out complex new green and sustainable chemical reactions entirely powered by light.”


Joseph Priestley

The first evidence that gases participate in photosynthesis was reported by Joseph Priestley in 1772. He knew that if a burning candle is placed in a sealed chamber, the candle soon goes out. If a mouse is then placed in the chamber, it soon suffocates because the process of combustion has used up all the oxygen in the air — the gas on which animal respiration depends. However, Priestley discovered that if a plant is placed in an atmosphere lacking oxygen, it soon replenishes the oxygen, and a mouse can survive in the resulting mixture. Priestley thought (erroneously) that it was simply the growth of the plant that accounted for this.


Incremental discovery may one day lead to photosynthetic breakthrough

Photosynthesis is one of the most complicated and important processes -- responsible for kick-starting Earth's food chain. While we have modeled its more-than-100 major steps, scientists are still discovering the purpose of proteins that can be engineered to increase yield, as scientists recently proved in Science. Now researchers have uncovered secrets about another protein, CP12 -- the full understanding of which may provide an additional route to boost yields in the future.

There are three forms of the protein CP12 that regulate the enzymes GAPDH and PRK. Think of the enzymes as the workhorses and CP12 as the groom holding the reins. CP12 tells them to get to work when there's light and reins them in when it's dark.

"CP12 is an important component because it helps plants respond to changing light levels, for example when the plant is shaded by a leaf or cloud," said first author Patricia Lopez, a postdoctoral researcher for Realizing Increased Photosynthetic Efficiency (RIPE) who led this research. "CP12 stops the activity of the enzymes within seconds but without CP12, it will take several minutes to slow the activity, costing the plant precious energy."

Published in the Journal of Experimental Botany, Lopez and co-authors found not all CP12 enzymes are created equal. Turns out that CP12-3 is not part of this process -- whereas CP12-1 and CP12-2 are in charge and can cover for each other. Get rid of all three, and the plant can't photosynthesize efficiently, resulting in a drastically smaller plant with fewer, smaller seeds.

In fact, without CP12 to hold the reins, PRK also disappears. "PRK is a vital workhorse that provides the raw materials for the enzyme Rubisco to turn into carbohydrates -- the sugars the plant uses to grow bigger and produce more yield," said lead author Christine Raines, a professor of plant molecular physiology at the University of Essex.

Agriculture is approaching the limits of the yield traits that drove the remarkable yield increases over the past century, said RIPE Associate Director Don Ort, USDA/ARS scientist and the Robert Emerson Professor of Plant Biology at the Carl R. Woese Institute for Genomic Biology. "Improving photosynthesis has the promise of being the next frontier to dramatic boost crop yields, and for the first time there is both a molecular understanding of photosynthesis and powerful technological tools to make engineering photosynthesis a realistic and attainable goal."


Discovering the secret code behind photosynthesis

(PhysOrg.com) -- Scientists from Queen Mary, University of London have discovered that an ancient system of communication found in primitive bacteria, may also explain how plants and algae control the process of photosynthesis.

Two-component signal transduction systems (TCSTs) have long been recognised as the main way in which bacteria coordinate their responses to changes in their environment. But recent research has shown that these 'bacterial' two-component systems have also survived in plants and algae, as a way of sending signals within their cells.

These systems, which are thought to have evolved from ancient cyanobacteria, are found in chloroplasts - the part of a cell of a plant which conducts photosynthesis, converting light to chemical energy.

Writing in the Royal Society journal Proceedings of the Royal Society:B , Dr Sujith Puthiyaveetil and Professor John F Allen from Queen Mary's School of Biological and Chemical Sciences report that these two-component systems have played a fundamental role in linking the process of photosynthesis with gene expression, thereby determining the way that all plants adapt to changing environments.

Dr Puthiyaveetil explains: "We already know that two-component systems act as a type of on/off switch for genes in bacteria. But the survival of these bacterial-type on/off switches in chloroplasts suggests a new model for gene regulation in plants."

Professor Allen adds: "To many, it will be shock to learn that some messages are sent within plant cells - and, probably, animal cells - using the same telegraph system as the one found in 'primitive' bacteria. It would be like discovering Morse code in your computer network, or a wax cylinder at the heart of your new, shiny digital HiFi. To us, however, the discovery is exciting evidence for an unorthodox theory of cell evolution first published sixteen years ago in the Journal of Theoretical Biology ."

More information: 'Chloroplast two-component systems: evolution of the link between photosynthesis and gene expression‘, will be published in the online edition of Proceedings of the Royal Society:B on 25 February 2009.


10.1 Using Microbiology to Discover the Secrets of Life

Alex is a 22-year-old college student who vacationed in Puerta Vallarta, Mexico, for spring break. Unfortunately, two days after flying home to Ohio, he began to experience abdominal cramping and extensive watery diarrhea. Because of his discomfort, he sought medical attention at a large Cincinnati hospital nearby.

Jump to the next Clinical Focus box.

Through the early 20th century, DNA was not yet recognized as the genetic material responsible for heredity , the passage of traits from one generation to the next. In fact, much of the research was dismissed until the mid-20th century. The scientific community believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring this hypothetical process appeared to be correct because of what we know now as continuous variation, which results from the action of many genes to determine a particular characteristic, like human height. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case.

Two separate lines of research, begun in the mid to late 1800s, ultimately led to the discovery and characterization of DNA and the foundations of genetics, the science of heredity. These lines of research began to converge in the 1920s, and research using microbial systems ultimately resulted in significant contributions to elucidating the molecular basis of genetics .

Discovery and Characterization of DNA

Modern understanding of DNA has evolved from the discovery of nucleic acid to the development of the double-helix model. In the 1860s, Friedrich Miescher (1844–1895), a physician by profession, was the first person to isolate phosphorus-rich chemicals from leukocytes (white blood cells) from the pus on used bandages from a local surgical clinic. He named these chemicals (which would eventually be known as RNA and DNA) “ nuclein ” because they were isolated from the nuclei of the cells. His student Richard Altmann (1852–1900) subsequently termed it “ nucleic acid ” 20 years later when he discovered the acidic nature of nuclein. In the last two decades of the 19th century, German biochemist Albrecht Kossel (1853–1927) isolated and characterized the five different nucleotide bases composing nucleic acid. These are adenine, guanine, cytosine, thymine (in DNA), and uracil (in RNA). Kossell received the Nobel Prize in Physiology or Medicine in 1910 for his work on nucleic acids and for his considerable work on proteins, including the discovery of histidine .

Foundations of Genetics

Despite the discovery of DNA in the late 1800s, scientists did not make the association with heredity for many more decades. To make this connection, scientists, including a number of microbiologists, performed many experiments on plants, animals, and bacteria.

Mendel’s Pea Plants

While Miescher was isolating and discovering DNA in the 1860s, Austrian monk and botanist Johann Gregor Mendel (1822–1884) was experimenting with garden peas, demonstrating and documenting basic patterns of inheritance, now known as Mendel’s laws.

In 1856, Mendel began his decade-long research into inheritance patterns. He used the diploid garden pea, Pisum sativum, as his primary model system because it naturally self-fertilizes and is highly inbred, producing “true-breeding” pea plant lines—plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if he used plants that were not true-breeding. Mendel performed hybridizations, which involve mating two true-breeding individuals (P generation) that have different traits, and examined the characteristics of their offspring (first filial generation, F1) as well as the offspring of self-fertilization of the F1 generation (second filial generation, F2) (Figure 10.2).

In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits. In 1866, he published his work, “Experiments in Plant Hybridization,” 1 in the Proceedings of the Natural History Society of Brünn. Mendel’s work went virtually unnoticed by the scientific community, which believed, incorrectly, in the theory of blending of traits in continuous variation.

He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

The Chromosomal Theory of Inheritance

Mendel carried out his experiments long before chromosomes were visualized under a microscope. However, with the improvement of microscopic techniques during the late 1800s, cell biologists could stain and visualize subcellular structures with dyes and observe their actions during meiosis . They were able to observe chromosomes replicating, condensing from an amorphous nuclear mass into distinct X-shaped bodies and migrating to separate cellular poles. The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel’s publications and re-evaluate his model in terms of the behavior of chromosomes during mitosis and meiosis.

In 1902, Theodor Boveri (1862–1915) observed that in sea urchins, nuclear components (chromosomes) determined proper embryonic development. That same year, Walter Sutton (1877–1916) observed the separation of chromosomes into daughter cells during meiosis. Together, these observations led to the development of the Chromosomal Theory of Inheritance , which identified chromosomes as the genetic material responsible for Mendelian inheritance.

Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel’s observations, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes. Thomas Hunt Morgan (1866–1945) and his colleagues spent several years carrying out crosses with the fruit fly, Drosophila melanogaster. They performed meticulous microscopic observations of fly chromosomes and correlated these observations with resulting fly characteristics. Their work provided the first experimental evidence to support the Chromosomal Theory of Inheritance in the early 1900s. In 1915, Morgan and his “Fly Room” colleagues published The Mechanism of Mendelian Heredity, which identified chromosomes as the cellular structures responsible for heredity. For his many significant contributions to genetics, Morgan received the Nobel Prize in Physiology or Medicine in 1933.

In the late 1920s, Barbara McClintock (1902–1992) developed chromosomal staining techniques to visualize and differentiate between the different chromosomes of maize (corn). In the 1940s and 1950s, she identified a breakage event on chromosome 9, which she named the dissociation locus (Ds). Ds could change position within the chromosome. She also identified an activator locus (Ac). Ds chromosome breakage could be activated by an Ac element (transposase enzyme). At first, McClintock’s finding of these jumping genes , which we now call transposons , was not accepted by the scientific community. It wasn’t until the 1960s and later that transposons were discovered in bacteriophages, bacteria, and Drosophila. Today, we know that transposons are mobile segments of DNA that can move within the genome of an organism. They can regulate gene expression, protein expression, and virulence (ability to cause disease).

Microbes and Viruses in Genetic Research

Microbiologists have also played a crucial part in our understanding of genetics. Experimental organisms such as Mendel ’s garden peas, Morgan’s fruit flies, and McClintock ’s corn had already been used successfully to pave the way for an understanding of genetics. However, microbes and viruses were (and still are) excellent model systems for the study of genetics because, unlike peas, fruit flies, and corn, they are propagated more easily in the laboratory, growing to high population densities in a small amount of space and in a short time. In addition, because of their structural simplicity, microbes and viruses are more readily manipulated genetically.

Fortunately, despite significant differences in size, structure, reproduction strategies, and other biological characteristics, there is biochemical unity among all organisms they have in common the same underlying molecules responsible for heredity and the use of genetic material to give cells their varying characteristics. In the words of French scientist Jacques Monod , “What is true for E. coli is also true for the elephant,” meaning that the biochemistry of life has been maintained throughout evolution and is shared in all forms of life, from simple unicellular organisms to large, complex organisms. This biochemical continuity makes microbes excellent models to use for genetic studies.

In a clever set of experiments in the 1930s and 1940s, German scientist Joachim Hämmerling (1901–1980), using the single-celled alga Acetabularia as a microbial model, established that the genetic information in a eukaryotic cell is housed within the nucleus . Acetabularia spp. are unusually large algal cells that grow asymmetrically, forming a “foot” containing the nucleus, which is used for substrate attachment a stalk and an umbrella-like cap—structures that can all be easily seen with the naked eye. In an early set of experiments, Hämmerling removed either the cap or the foot of the cells and observed whether new caps or feet were regenerated (Figure 10.3). He found that when the foot of these cells was removed, new feet did not grow however, when caps were removed from the cells, new caps were regenerated. This suggested that the hereditary information was located in the nucleus-containing foot of each cell.

In another set of experiments, Hämmerling used two species of Acetabularia that have different cap morphologies, A. crenulata and A. mediterranea (Figure 10.4). He cut the caps from both types of cells and then grafted the stalk from an A. crenulata onto an A. mediterranea foot, and vice versa. Over time, he observed that the grafted cell with the A. crenulata foot and A. mediterranea stalk developed a cap with the A. crenulata morphology. Conversely, the grafted cell with the A. mediterranea foot and A. crenulata stalk developed a cap with the A. mediterranea morphology. He microscopically confirmed the presence of nuclei in the feet of these cells and attributed the development of these cap morphologies to the nucleus of each grafted cell. Thus, he showed experimentally that the nucleus was the location of genetic material that dictated a cell’s properties.

Another microbial model, the red bread mold Neurospora crassa , was used by George Beadle and Edward Tatum to demonstrate the relationship between genes and the proteins they encode. Beadle had worked with fruit flies in Morgan ’s laboratory but found them too complex to perform certain types of experiments. N. crassa, on the other hand, is a simpler organism and has the ability to grow on a minimal medium because it contains enzymatic pathways that allow it to use the medium to produce its own vitamins and amino acids.

Beadle and Tatum irradiated the mold with X-rays to induce changes to a sequence of nucleic acids, called mutations . They mated the irradiated mold spores and attempted to grow them on both a complete medium and a minimal medium. They looked for mutants that grew on a complete medium, supplemented with vitamins and amino acids, but did not grow on the minimal medium lacking these supplements. Such molds theoretically contained mutations in the genes that encoded biosynthetic pathways. Upon finding such mutants, they systematically tested each to determine which vitamin or amino acid it was unable to produce (Figure 10.5) and published this work in 1941. 2

Subsequent work by Beadle, Tatum, and colleagues showed that they could isolate different classes of mutants that required a particular supplement, like the amino acid arginine (Figure 10.6). With some knowledge of the arginine biosynthesis pathway, they identified three classes of arginine mutants by supplementing the minimal medium with intermediates (citrulline or ornithine) in the pathway. The three mutants differed in their abilities to grow in each of the media, which led the group of scientists to propose, in 1945, that each type of mutant had a defect in a different gene in the arginine biosynthesis pathway. This led to the so-called one gene–one enzyme hypothesis , which suggested that each gene encodes one enzyme.

Subsequent knowledge about the processes of transcription and translation led scientists to revise this to the “one gene–one polypeptide” hypothesis. Although there are some genes that do not encode polypeptides (but rather encode for transfer RNAs [tRNAs] or ribosomal RNAs [rRNAs], which we will discuss later), the one gene–one enzyme hypothesis is true in many cases, especially in microbes. Beadle and Tatum’s discovery of the link between genes and corresponding characteristics earned them the 1958 Nobel Prize in Physiology and Medicine and has since become the basis for modern molecular genetics.

Link to Learning

To learn more about the experiments of Beadle and Tatum, visit this website from the DNA Learning Center.

Check Your Understanding

  • What organism did Morgan and his colleagues use to develop the Chromosomal Theory of Inheritance? What traits did they track?
  • What did Hämmerling prove with his experiments on Acetabularia?

DNA as the Molecule Responsible for Heredity

By the beginning of the 20th century, a great deal of work had already been done on characterizing DNA and establishing the foundations of genetics, including attributing heredity to chromosomes found within the nucleus. Despite all of this research, it was not until well into the 20th century that these lines of research converged and scientists began to consider that DNA could be the genetic material that offspring inherited from their parents. DNA, containing only four different nucleotides , was thought to be structurally too simple to encode such complex genetic information. Instead, protein was thought to have the complexity required to serve as cellular genetic information because it is composed of 20 different amino acids that could be combined in a huge variety of combinations. Microbiologists played a pivotal role in the research that determined that DNA is the molecule responsible for heredity .

Griffith’s Transformation Experiments

British bacteriologist Frederick Griffith (1879–1941) was perhaps the first person to show that hereditary information could be transferred from one cell to another “horizontally” (between members of the same generation), rather than “vertically” (from parent to offspring). In 1928, he reported the first demonstration of bacterial transformation , a process in which external DNA is taken up by a cell, thereby changing its characteristics. 3 He was working with two strains of Streptococcus pneumoniae , a bacterium that causes pneumonia: a rough (R) strain and a smooth (S) strain. The R strain is nonpathogenic and lacks a capsule on its outer surface as a result, colonies from the R strain appear rough when grown on plates. The S strain is pathogenic and has a capsule outside its cell wall, allowing it to escape phagocytosis by the host immune system. The capsules cause colonies from the S strain to appear smooth when grown on plates.

In a series of experiments, Griffith analyzed the effects of live R, live S, and heat-killed S strains of S. pneumoniae on live mice (Figure 10.7). When mice were injected with the live S strain, the mice died. When he injected the mice with the live R strain or the heat-killed S strain, the mice survived. But when he injected the mice with a mixture of live R strain and heat-killed S strain, the mice died. Upon isolating the live bacteria from the dead mouse, he only recovered the S strain of bacteria. When he then injected this isolated S strain into fresh mice, the mice died. Griffith concluded that something had passed from the heat-killed S strain into the live R strain and “transformed” it into the pathogenic S strain he called this the “transforming principle.” These experiments are now famously known as Griffith’s transformation experiments .

In 1944, Oswald Avery , Colin MacLeod , and Maclyn McCarty were interested in exploring Griffith’s transforming principle further. They isolated the S strain from infected dead mice, heat-killed it, and inactivated various components of the S extract, conducting a systematic elimination study (Figure 10.8). They used enzymes that specifically degraded proteins, RNA, and DNA and mixed the S extract with each of these individual enzymes. Then, they tested each extract/enzyme combination’s resulting ability to transform the R strain, as observed by the diffuse growth of the S strain in culture media and confirmed visually by growth on plates. They found that when DNA was degraded, the resulting mixture was no longer able to transform the R strain bacteria, whereas no other enzymatic treatment was able to prevent transformation. This led them to conclude that DNA was the transforming principle. Despite their results, many scientists did not accept their conclusion, instead believing that there were protein contaminants within their extracts.

Check Your Understanding

  • How did Avery, MacLeod, and McCarty’s experiments show that DNA was the transforming principle first described by Griffith?

Hershey and Chase’s Proof of DNA as Genetic Material

Alfred Hershey and Martha Chase performed their own experiments in 1952 and were able to provide confirmatory evidence that DNA , not protein, was the genetic material (Figure 10.9). 4 Hershey and Chase were studying a bacteriophage , a virus that infects bacteria. Viruses typically have a simple structure: a protein coat, called the capsid, and a nucleic acid core that contains the genetic material, either DNA or RNA (see Viruses). The particular bacteriophage they were studying was the T2 bacteriophage, which infects E. coli cells. As we now know today, T2 attaches to the surface of the bacterial cell and then it injects its nucleic acids inside the cell. The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell bursts, releasing a large number of bacteriophages.

Hershey and Chase labeled the protein coat in one batch of phage using radioactive sulfur, 35 S, because sulfur is found in the amino acids methionine and cysteine but not in nucleic acids. They labeled the DNA in another batch using radioactive phosphorus, 32 P, because phosphorus is found in DNA and RNA but not typically in protein.

Each batch of phage was allowed to infect the cells separately. After infection, Hershey and Chase put each phage bacterial suspension in a blender, which detached the phage coats from the host cell, and spun down the resulting suspension in a centrifuge. The heavier bacterial cells settled down and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube with the protein labeled, the radioactivity remained only in the supernatant. In the tube with the DNA labeled, the radioactivity was detected only in the bacterial cells. Hershey and Chase concluded that it was the phage DNA that was injected into the cell that carried the information to produce more phage particles, thus proving that DNA, not proteins, was the source of the genetic material. As a result of their work, the scientific community more broadly accepted DNA as the molecule responsible for heredity.

By the time Hershey and Chase published their experiment in the early 1950s, microbiologists and other scientists had been researching heredity for over 80 years. Building on one another’s research during that time culminated in the general agreement that DNA was the genetic material responsible for heredity (Figure 10.10). This knowledge set the stage for the age of molecular biology to come and the significant advancements in biotechnology and systems biology that we are experiencing today.

Link to Learning

To learn more about the experiments involved in the history of genetics and the discovery of DNA as the genetic material of cells, visit this website from the DNA Learning Center.


Pseudocyclic Photophosphorylation

Another way to make up the deficit is by a process called pseudocyclic photophosphorylation in which some of the electrons passing to ferredoxin then reduce molecular oxygen back to H2O instead of reducing NADP + to NADPH.

At first glance, this might seem a fruitless undoing of all the hard work of photosynthesis. But look again. Although the electrons cycle from water to ferredoxin and back again, part of their pathway is through the chemiosmosis-generating stem of cytochrome b6/f.

Here, then, is another way that simply by turning on a light, enough energy is imparted to electrons that they can bring about the synthesis of ATP.


The Steps

  • CO2 combines with the phosphorylated 5-carbon sugar ribulose bisphosphate.
  • This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO)(an enzyme which can fairly claim to be the most abundant enzyme on earth).
  • The resulting 6-carbon compound breaks down into two molecules of 3-phosphoglyceric acid (PGA).
  • The PGA molecules are further phosphorylated (by ATP) and are reduced (by NADPH) to form phosphoglyceraldehyde (PGAL).
  • Phosphoglyceraldehyde serves as the starting material for the synthesis of glucose and fructose.
  • Glucose and fructose make the disaccharidesucrose, which travels in solution to other parts of the plant (e.g., fruit, roots).
  • Glucose is also the monomer used in the synthesis of the polysaccharidesstarch and cellulose.

The graphic shows the steps in the fixation of carbon dioxide during photosynthesis. All of these reactions occur in the stroma of the chloroplast.

These steps were worked out by Melvin Calvin and his colleagues at the University of California and, for this reason, are named the Calvin cycle.


Scientists Discover a New Type of Photosynthesis

The discovery changes our understanding of the basic mechanism of photosynthesis and should rewrite the textbooks. It will also tailor the way we hunt for alien life and provide insights into how we could engineer more efficient crops that take advantage of longer wavelengths of light.

The discovery, published today in Science, was led by Imperial College London, supported by the BBSRC, and involved groups from the ANU in Canberra, the CNRS in Paris and Saclay and the CNR in Milan.

The vast majority of life on Earth uses visible red light in the process of photosynthesis, but the new type uses near-infrared light instead. It was detected in a wide range of cyanobacteria (blue-green algae) when they grow in near-infrared light, found in shaded conditions like bacterial mats in Yellowstone and in beach rock in Australia.

As scientists have now discovered, it also occurs in a cupboard fitted with infrared LEDs in Imperial College London.

Photosynthesis beyond the red limit

The standard, near-universal type of photosynthesis uses the green pigment, chlorophyll-a, both to collect light and use its energy to make useful biochemicals and oxygen. The way chlorophyll-a absorbs light means only the energy from red light can be used for photosynthesis.

Since chlorophyll-a is present in all plants, algae and cyanobacteria that we know of, it was considered that the energy of red light set the ‘red limit’ for photosynthesis that is, the minimum amount of energy needed to do the demanding chemistry that produces oxygen. The red limit is used in astrobiology to judge whether complex life could have evolved on planets in other solar systems.

Cross-section of beach rock (Heron Island, Australia) showing chlorophyll-f containing cyanobacteria (green band) growing deep into the rock, several millimetres below the surface. Dennis Nuernberg

However, when some cyanobacteria are grown under near-infrared light, the standard chlorophyll-a-containing systems shut down and different systems containing a different kind of chlorophyll, chlorophyll-f, takes over.

Until now, it was thought that chlorophyll-f just harvested the light. The new research shows that instead chlorophyll-f plays the key role in photosynthesis under shaded conditions, using lower-energy infrared light to do the complex chemistry. This is photosynthesis ‘beyond the red limit’.

Lead researcher Professor Bill Rutherford, from the Department of Life Sciences at Imperial, said: “The new form of photosynthesis made us rethink what we thought was possible. It also changes how we understand the key events at the heart of standard photosynthesis. This is textbook changing stuff.”

Preventing damage by light

Another cyanobacterium, Acaryochloris, is already known to do photosynthesis beyond the red limit. However, because it occurs in just this one species, with a very specific habitat, it had been considered a ‘one-off’. Acaryochloris lives underneath a green sea-squirt that shades out most of the visible light leaving just the near-infrared.

The chlorophyll-f based photosynthesis reported today represents a third type of photosynthesis that is widespread. However, it is only used in special infrared-rich shaded conditions in normal light conditions, the standard red form of photosynthesis is used.

It was thought that light damage would be more severe beyond the red limit, but the new study shows that it is not a problem in stable, shaded environments.

Co-author Dr Andrea Fantuzzi, from the Department of Life Sciences at Imperial, said: “Finding a type of photosynthesis that works beyond the red limit changes our understanding of the energy requirements of photosynthesis. This provides insights into light energy use and into mechanisms that protect the systems against damage by light.”

These insights could be useful for researchers trying to engineer crops to perform more efficient photosynthesis by using a wider range of light. How these cyanobacteria protect themselves from damage caused by variations in the brightness of light could help researchers discover what is feasible to engineer into crop plants.

Textbook-changing insights

More detail could be seen in the new systems than has ever been seen before in the standard chlorophyll-a systems. The chlorophylls often termed ‘accessory’ chlorophylls were actually performing the crucial chemical step, rather than the textbook ‘special pair’ of chlorophylls in the centre of the complex.

This indicates that this pattern holds for the other types of photosynthesis, which would change the textbook view of how the dominant form of photosynthesis works.

Dr Dennis Nürnberg, the first author and initiator of the study, said: “I did not expect that my interest in cyanobacteria and their diverse lifestyles would snowball into a major change in how we understand photosynthesis. It is amazing what is still out there in nature waiting to be discovered.”

Peter Burlinson, lead for frontier bioscience at BBSRC – UKRI says, “This is an important discovery in photosynthesis, a process that plays a crucial role in the biology of the crops that feed the world. Discoveries like this push the boundaries of our understanding of life and Professor Bill Rutherford and the team at Imperial should be congratulated for revealing a new perspective on such a fundamental process.”


Unlocking secrets of photosynthesis

A researcher at the University of Essex has uncovered secrets about a protein which could help feed the growing global population.

Professor Christine Raines,, working with scientists at the Carl R Woese Institute for Genomic Biology Photosynthesis at the University of Illinois investigated the protein CP12, which could boost crop yields in the future.

Photosynthesis is one of the most complicated and important processes responsible for kick-starting Earth’s food chain. While researchers have modeled its more-than-100 major steps, scientists are still discovering the purpose of proteins that can be engineered to increase yield.

There are three forms of the protein CP12 that regulate the enzymes GAPDH and PRK. The enzymes are the workhorses while CP12 holds the reins. CP12 tells them to get to work when there’s light and reins them in when it’s dark.

“CP12 is an important component because it helps plants respond to changing light levels, for example when the plant is shaded by a leaf or cloud,” said first author Dr Patricia Lopez, a postdoctoral researcher for Realizing Increased Photosynthetic Efficiency (RIPE) who led the research. “CP12 stops the activity of the enzymes within seconds but without CP12, it will take several minutes to slow the activity, costing the plant precious energy.”

Published in the Journal of Experimental Botany, Lopez and co-authors found not all CP12 enzymes are created equal. In fact CP12-3 is not part of this process—whereas CP12-1 and CP12-2 are in charge and can cover for each other. Get rid of all three, and the plant can’t photosynthesize efficiently, resulting in a drastically smaller plant with fewer, smaller seeds.

In fact, without CP12 to hold the reins, PRK also disappears. “PRK is a vital workhorse that provides the raw materials for the enzyme Rubisco to turn into carbohydrates—the sugars the plant uses to grow bigger and produce more yield,” said lead author Professor Raines, a professor of plant molecular physiology.

Agriculture is approaching the limits of the yield traits that drove the remarkable yield increases over the past century, said RIPE Associate Director Don Ort, USDA/ARS scientist and the Robert Emerson Professor of Plant Biology at the Carl R Woese Institute for Genomic Biology. “Improving photosynthesis has the promise of being the next frontier to dramatic boost crop yields, and for the first time there is both a molecular understanding of photosynthesis and powerful technological tools to make engineering photosynthesis a realistic and attainable goal.”

The paper 'Arabidopsis CP12 mutants have reduced levels of phosphoribulokinase and impaired function of the Calvin–Benson cycle' is published by the Journal of Experimental Botany. Co-authors include Amani Omar Abuzaid and Professor Tracy Lawson.

Realising Increased Photosynthetic Efficiency (RIPE) is an international research project engineering plants to more efficiently turn the sun’s energy into food to sustainably increase worldwide food productivity. The RIPE project is supported by the Bill and Melinda Gates Foundation.

The Carl R. Woese Institute for Genomic Biology advances life sciences research through interdisciplinary collaborations within a state-of-the-art genomic research facility at the University of Illinois.


Watch the video: Φωτοσύνθεση (June 2022).


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