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Specialist plant structures, what they do and how they work...

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Something to get your teeth in to if you're missing your Monday fix of Plant Science! Use the presentation and resources below to help you. Enjoy! And a Happy New Year!

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Lets look at some of the cornerstones of plant growth and development. Light, air, water, temperature and investigate the nature of cell growth.

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Effect of Temperature on Plant Growth and Photosynthesis

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Effect of Temperature on Plant Growth and Photosynthesis

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Worth reading again! We visiting this page in photosynthesis, but please return hear to see how temperature effects photosynthesis, doubling the rate of the light-independent reactions every 10 degrees!

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Rate of photosynthesis: Limiting Factors

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Rate of photosynthesis: Limiting Factors

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The Royal Society has captured so much with so few words. Read this and understand plant growth forever!

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This short animation might help to explain the process of plant growth and then cell differentiation to complete different functions within the plant, e.g. xylem, phloem, stem and roots.

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View the animation full screen
This animation shows the processes of growth in plants at the cellular level. It demonstrates how growth occurs at specific regions within the plant, how cells divide by mitosis, and how cells become specialised into different tissues (differentiation). The final scene demonstrates how chromosomes behave during mitosis.

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Animation - Growth, mitosis and differentiation in plants

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View the animation full screen This animation shows the processes of growth in plants at the cellular level. It demonstrates how growth occurs at specific regions within the plant, how cells divide by mitosis, and how cells become specialised into different tissues (differentiation). The final scene demonstrates how chromosomes behave during mitosis.

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Want to see Chromosomes dividing for real, with your own eyes?

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Root Tip Mitosis practical - student notes

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An improved and simplified method for showing root-tip mitosis, using toluidine blue stain.

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A-level core practicals: Root tip mitosis

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Call in here for more details about how plants retain the ability to create new plant tissues and how this can be used.

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IB Biology notes on 9.3 Growth in plants

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Can you get through most of this and complete the quiz attached to it?

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Energy in Living Systems

by Nathan H Lents, Ph.D., John Nishan

Table of contents

Terms you should know

harvest : to gather, to accumulate, to catch, to capture, to extract

pigment : a molecule that gives color and is involved in vital functions within an organism

spectrum (plural: spectra) : a continuing range such as of color or frequency; a series of colors arranged by wavelength as in a rainbow

Before scientists understood the process of photosynthesis, they were at a loss to explain how plants could grow and increase their mass so dramatically from what appeared to be a steady diet of water. A 17th century Flemish chemist named Jean Baptista van Helmont thought plants "extracted" the bulk of their food from soil (Van Helmont, 1841). Other scientists assumed plants gained their weight and size from carbon dioxide, while others assumed that water alone gave plants their heft.

None of these explanations, however, held up when tested experimentally. In test after test, mass lost by soil, water, and even carbon dioxide didn't measure up to the mass gained by a growing plant. It wasn't until Joseph Priestley's experiments a century later that scientists began to suspect sunlight as the major contributor to a plant's growth.

Early experiments towards the discovery of photosynthesis

Priestley, partially credited with the discovery of elemental oxygen, found that when he placed fresh sprigs of mint leaves inside a sealed glass container, a candle would burn longer than if the leaves were not there (Figure 1). He also found that a previously extinguished candle would reignite inside a sealed jar - sometimes days after it had ceased to burn - if mint leaves were present. This caused him to suspect that the leaves were somehow "refreshing" the air inside the container.

Several years later, a Dutch scientist named Jan Ingenhousz, having heard of Priestley's experiments, began to conduct experiments of his own. He submerged willow plants in water and saw that bubbles formed on the surface of the leaves. The bubbles, however, formed only when the experiment was conducted in the presence of sunlight. Ingenhousz later determined the gas bubbles were oxygen, but never fully understood the significance of what he had observed regarding the sunlight.

Putting it all together: Reactants and products of photosynthesis

Collectively, these chemists established the products and reactants of photosynthesis - water, oxygen, carbon dioxide, and light. But it took the musings of a German physicist named Julius Von Mayer to put the pieces together. Von Mayer, the first to propose that "energy is neither created nor destroyed," was also first to suggest that plants derive their energy for growth from sunlight.

CO ₂ + H ₂O + light energy → O ₂ + organic matter + chemical energy

Work by other scientists helped to establish the chemical formula of the organic products of photosynthesis, which is usually simplified as a glucose molecule: C ₆H ₁₂O ₆. The properly balanced general formula for photosynthesis thus becomes:

6CO ₂ + 6H ₂O + light energy → C ₆H ₁₂O ₆ + 6O ₂

The energy of photosynthesis comes from light

The principal product of photosynthesis (sugar) is a high-energy molecule, but the reactants (carbon dioxide and water), are low-energy molecules, so the process of photosynthesis needs an energy source to drive it. Molecules called pigments absorb energy from light. The main pigment in photosynthesis is called chlorophyll. Chlorophyll exists in several different forms in different organisms. Chlorophyll a is the main photosynthetic pigment found in land plants and algae. It absorbs light in the blue/violet range of the light spectrum (wavelengths of 400-450nm) as you can see in Figure 2. It also absorbs light in the red range of the spectrum (wavelengths of 650-700nm) to a lesser degree. Green light is almost completely reflected by chlorophyll, giving plants their greenish hue.

Plants do not make equal use of all the wavelengths present in the full range of visible light - a fact first demonstrated by German plant physiologist T. W. Engelmann in 1882. He used a simple experiment to demonstrate that the blue and red wavelengths of light, in particular, were the biggest drivers of photosynthesis.

The action spectrum of photosynthesis

Engelmann split white light into its spectral components using a prism and shone it on a dish of liquid solution containing a photosynthetic green algae called Chladophora. He then released bacteria into the solution. The bacteria, which need oxygen to survive, migrated toward those areas in the dish where blue and red wavelength light was shining. Why? Because where the red and blue range of light was shining, the photosynthetic algae produced more oxygen due to increased photosynthetic activity. With this demonstration, Engelmann had established the first action spectrum of photosynthesis.

Chlorophyll a does not perfectly overlap with the action spectrum of photosynthesis identified by Engelmann (see Table 1). This led scientists to suspect that there are additional pigments in plants that absorb light at different wavelengths. Land plants have pigments such as chlorophyll b and carotene, while other photosynthetic organisms, like protists, have chlorophyll c and chlorophyll a.

Pigment

Peak absorbance

Reflects

Chlorophyll

Chlorophyll a

400-450nm

green

Chlorophyll b

450-500nm

yellow

Carotenoids (α and β forms)

425-475nm

Phycobilins-in red algae & cyanobacteria

Wavelengths not absorbed by chlorophyll a

Red, orange blue

Table 1: Three basic classes of photosynthetic pigments give plants and other photosynthetic organisms their color.

Plant pigments are classified as either chlorophylls or carotenoids. Chlorophylls reflect green light while carotenoids reflect light in the red, orange, and yellow range. Carotenoids give carrots their color. They are considered an accessory pigment because they cannot transfer sunlight energy directly to the photosynthetic pathway. Carotenoids pass their absorbed energy to chlorophyll, which in turn transfers energy to the photosynthetic pathway.

Phase One: The light-dependent reactions

Photosynthesis occurs in two phases: the light-dependent reactions and the Calvin-Benson Cycle (see the Photosynthesis I video below). The light-dependent reaction is the first phase, when pigments like chlorophyll harvest light energy. The Calvin-Benson Cycle uses that energy to synthesize high-energy sugar molecules from carbon dioxide. In plants and algae, the light reactions occur within the thylakoid membranes of chloroplasts. The animation below provides an overview of photosynthesis.

Like the light-harvesting complexes, the reaction centers are also made of proteins, cofactors, and pigments, but there are two types of reaction centers: photosystem I and photosystem II. Photosystem I, so named because it was discovered first, is also referred to as P700 because the special chlorophyll a pigment molecules that form it best absorb light of wavelength 700nm. Photosystem II is also referred to as P680, because the chlorophyll molecules that form it best absorb light in the 680nm wavelength. In both cases, after either P700 or P680 become excited, either by a photon or another excited pigment molecule, one of its electrons moves to a higher energy state. The difference between these two photosystems lies in what happens next with this harnessed energy. View a video of photosystems I and II below.

Photosystem II

Even though it was discovered and named second, photosystem II is actually where the story begins. When a photon of light strikes the reaction center of photosystem II, it excites an electron that leaves and begins its journey through a series of high-energy electron acceptors and donors collectively known as the electron transport chain (ETC) as shown in Figure 5. (This particular ETC is called the cytochrome ETC, after one of the members of the chain that was discovered first.)

At the same time, two water molecules bind to a water-splitting enzyme at the reaction center of photosystem II, as seen in Figure 6. When the water molecules split, ionized hydrogen atoms (H⁺) enter the thylakoid space. An enzyme called cytochrome b6f, the next stop in the chain after photosystem II, generates more ions for the proton pump and sends the excited electrons along toward photosystem I. As the hydrogen ions accumulate within the thylakoid space, they create the H⁺ gradient that drives ATP synthesis. ATP will be used for sugar synthesis later, in the Calvin-Benson Cycle.

Figure 6: Formation of O₂ by photosystem II.

Oxygen atoms from the split water molecules also accumulate within the thylakoid space. Lone oxygen atoms are very reactive and rapidly combine to form molecular oxygen (O ₂) that is released as a waste product of photosynthesis. Yes, each molecule of oxygen that we breathe was formed in a chloroplast somewhere as an accidental by-product of the splitting of water. Electrons are at a much lower energy state at the end of the ETC than they were at the beginning of the process. They get a badly needed boost at the reaction centers in photosystem I.

Photosystem I

Photosystem I also consists of light-harvesting complexes with lots of pigment molecules for capturing light energy. Light energy harvested from photons and intermediate-energy electrons from photosystem II flow to a special chlorophyll a molecule structure called P700 in photosystem I. Electrons jump up to a high-energy state when a photon arrives at P700, either directly from sunlight, or through a collision with an already excited pigment.

Once re-excited to a high energy level, the electrons don't stay for long. Excited electrons leave photosystem I and flow through another ETC, but this one, called the Ferredoxin ETC, is much shorter and does not drive ATP synthesis. The Ferredoxin ETC passes the excited electrons to the high-energy electron acceptor NADP⁺, which then combines with a proton (H⁺) from the surrounding solution and forms NADPH. NADPH then delivers high-energy electrons to the Calvin Cycle for long-term energy storage in the form of sugar (Figure 7).

Phase Two: The Calvin-Benson Cycle

After the energy of light is harvested as high-energy electrons held by NADPH, these electrons are then used to synthesize high-energy sugar molecules from the low-energy starting material of carbon dioxide. The Calvin-Benson Cycle used to be called "the dark reactions" because light is not directly involved. However, this name is misleading because the products of the light reactions are required to drive the Calvin Cycle. Thus, light is required, just not directly.

So far, we've seen how the flow of electrons in the light reactions goes like this (note: PSI and PSII stand for photosystem I and II):

This linear path is called noncyclic electron transport. However, not all electrons flow in this linear path. Some electrons double back and return to PSII after the PSI. This is called cyclic electron flow.

Why would some electrons take the redundant path of twice being energized by PSII and twice flowing through the cytochrome ETC? The answer is found when thinking about what the ETC produces - ATP. The simple noncyclic flow of electrons produces ATP and NADPH in roughly equal amounts. However, the Calvin Cycle needs more ATP than NADPH. Thus, the extra trip through the ETC that occurs in cyclic electron flow provides a little "boost" of ATP so that the Calvin Cycle has what it needs to synthesize sugars.

In roughly 300 years, our understanding of photosynthesis has progressed from mere identification of all the basic products and reactants of photosynthesis to a detailed picture of the molecular processes involved. We have summarized in this module how electrons are harvested, energized, and stored in the covalent bonds of NADPH, a process called light reactions. In the next module, we explore the Calvin-Benson Cycle where high-energy electrons from NADPH drive synthesis of carbohydrates - the sugars that provide sustenance to nearly every living thing on the Earth.

Summary

Through photosynthesis, plants harvest energy from the sun to produce oxygen and sugar, the basic energy source for all living things. This module introduces photosynthesis, beginning with experiments leading to its discovery. The stages of photosynthesis are explained. Topics include the role of chlorophyll, the action spectrum of photosynthesis, the wavelengths of light that drive photosynthesis, light-harvesting complexes, and the electron transport chain.

Key Concepts

Photosynthesis is a process by which an organism converts light energy from the sun into chemical energy for its sustenance.

Photosynthesis occurs in plants, algae, and some species of bacteria.

In plants, chloroplasts contain chlorophyll that absorbs light in the red and blue-violet regions of the spectrum.

Photosynthesis occurs in two stages: the light-dependent stage that occurs in the thylakoid membrane of the chloroplast and harvests solar energy, and the light-independent stage that takes that energy and makes sugar from carbon dioxide.

NGSS

HS-C5.2, HS-C5.4, HS-LS1.C1

References

Van Helmont, J.B. Ortus medicinae: id est initia phisicae inaudita: progressus medicinae novus in morborum vltionem ad vitam longam, (sumptibus Joan. Ant. Huguetan [et] Guillielmi Barbier, 1952).

Liebig, J. Annalen der Chemie und Pharmacie, (CF Winter'sche, 1841).

Nathan H Lents, Ph.D., John Nishan "Photosynthesis I" Visionlearning Vol. BIO-3 (6), 2014.

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Photosynthesis I | Biology | Visionlearning

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Energy in Living Systems by Nathan H Lents, Ph.D., John Nishan Table of contents Terms you should know harvest : to gather, to accumulate, to catch, to capture, to extract pigment : a molecule that gives color and is involved in vital functions within an organism spectrum (plural: spectra) :

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Watch out for a couple of video links labelled L3-4, just there if you really want to get in to this, degree ready stuff. Enjoy

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Amazing animation! This team have worked for hours on this, but this is just the tip of the iceberg. The website is stacked with knowledge and understanding of the living world, enjoy!

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This animation is a model, and has its strengths and limitations. In order to model something well, visual artists have to make decisions about what to represent and how best to do so. What's present in this model, and what's intentionally missing or altered?

To animate a scientifically accurate leaf, artists studied the texture of a redwood leaf specimen on a glass slide at high resolution. They even counted the stomata, and used that exact count for this film!

Because light is required for photosynthesis, the plant cells and their associated organelles must be translucent to allow light to pervade the space. That's why you'll see translucent organelles in this visualization. Animators, however, "lit up" the color to make the interior of the cell appear bright.

To highlight this photosystem well, animators simplified the process by emphasizing the role played by one plant pigment: chlorophyll, which functions to collect light from the sun to fuel photosynthesis. Because chlorophyll absorbs light in the blue and red portions of the visible light spectrum, reflecting the green wavelengths, most of the plant's organelles appear green, since the animation is designed for humans vision. Many other plants pigments-red, yellow, orange, even blue!-exist in nature and have particular roles in photosynthesis or plant structure. They are not evident here.

During the last scene, we see the light-dependent reactions of photosynthesis at the thylakoid membrane assuming a continuous source of light energy from the leaf's external environment. To accurately visualize movement at the molecular level, animators had to slow down time. The molecules shown are moving at least a million times slower than they would in real life! What does this tell you about the nature of energy and matter?

Look closely to see bright pulses traveling through the lipid layer of the thlyakoid's surface: this represents electrons passing energy along in the classic bucket brigade!

In order to see parts of the system well, the animators removed other elements of this visual molecular model, including water, oxygen, various proteins, ADP, photons, and electrons. Instead of seeing all reactants and products of the light-dependent reactions of photosynthesis, the animation focuses on the cellular machinery that allows for the creation of ATP.

You won't spot glucose or carbon dioxide, either. These molecules play a role in the Calvin Cycle, which takes place in the fluid-filled space outside of the thylakoids.

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Take a journey inside a leaf of a redwood tree! Enter the stoma and view the inside of a plant cell, translucent enough to capture light from the sun. Fly by familiar structures like the nucleus and mitochondria, and settle into the chloroplast to watch photosynthesis at work.

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Although a commercial site, I like the language, Lopez explains respiration clearly without over-simplifying it.

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The process of respiration in plants involves using the sugars produced during photosynthesis plus oxygen to produce energy for plant growth. In many ways, respiration is the opposite of photosynthesis. In the natural environment, plants produce their own food to survive.

They use the carbon dioxide (CO ₂) from the environment to produce sugars and oxygen (O ₂), which can later be utilized as a source of energy. While photosynthesis takes place in the leaves and stems only, respiration occurs in the leaves, stems and roots of the plant. The process of respiration is represented as follows:

As with photosynthesis, plants get oxygen from the air through the stomata. Respiration takes place in the mitochondria of the cell in the presence of oxygen, which is called "aerobic respiration". In plants, there are two types of respiration: dark respiration and photo respiration. The first kind occurs in the presence or absence of light, while the second occurs exclusively in the presence of light.

Role of air temperature: Plant respiration occurs 24 hours per day, but night respiration is more evident since the photosynthesis process ceases. During the night, it is very important that the temperature is cooler than during the day because plants can experience stress. Imagine a runner in a marathon. The runner respires at higher rates than a person standing still; therefore, a runner's rate of respiration is higher and the temperature of the body increases. The same principle applies to plants, as temperature at night increases, the respiration rate increases and consequently temperature increases. This action could result in flower damage and poor plant growth.

Roots need oxygen: As mentioned above, roots respire too! One of the functions of the substrate is to serve as a site for air exchange between the root zone and atmosphere. In other words, roots breathe oxygen like we do. Different plants have different oxygen requirements for their root systems. For example, the root system of a poinsettia requires a lot of oxygen, so it is best to use a substrate with high air porosity, while hostas can live well in a substrate with a high water holding capacity. A defense mechanism for plants under waterlogged or excessively dry conditions is to grow aerial roots from the stem just above the root crown; however, the ambient relative humidity must be high to sustain root growth outside the substrate.
"Figure 1. Plant roots not only absorb water and nutrients for plant growth, but also respire.
It is important that the growing medium has enough oxygen for plant roots to function properly.
Source: Premier Tech Horticulture."

Ideal root zone conditions: The key to ideal plant growth is to maintain the optimum root zone environment without sacrificing finances. Did you know that roots can take oxygen from the water for respiration, although not as much as from the air? Therefore, it is important to water plants until you obtain some leachate (15-30% by volume is recommended) as this will flush out old stagnant air and exchange it for fresh oxygen. Another factor to consider is the substrate's temperature. As temperature in the root zone increases, the oxygen concentration of the water decreases.

Importance of air in organic substrates: Root respiration is more important to consider in organic production because the root zone is full of natural microorganisms responsible for converting organic nutrients into usable ions. These microorganisms require oxygen since they work and respire too, so the substrate must maintain enough oxygen for both the roots and microorganisms. Therefore, it is a good idea to select a high porosity growing medium and to use containers that are deeper, because they will drain well after watering, leaving behind a good reservoir of air.

More in-depth information about plant respiration can be found in scientific literature.

PRO-MIX® is a registered trademark of PREMIER HORTICULTURE Ltd.

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The process of respiration in plants involves using the sugars produced during photosynthesis plus oxygen to produce energy for plant growth. In many ways, respiration is the opposite of photosynthesis. In the natural environment, plants produce their own food to survive.

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Priestly discovered that plants could allow a flame to continue burning

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By 1774 Priestly discovered that plants could allow a candle to continue burning. He had discovered Oxygen, although he didn't know it at the time!

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We now know every step of every atom in the photosynthetic pathway. The good news is that you don't need to know that, but, some of you will be desperate to know! This page is for you, c/o the Khan Academy

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The Calvin cycle (article) | Photosynthesis | Khan Academy

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How the products of the light reactions, ATP and NADPH, are used to fix carbon into sugars in the second stage of photosynthesis.

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Yet another site on photosynthesis! If you're starting to feel a little overwhelmed, this site is really well written in plain English, but it might allow you to reach even further in to the detail. See if you can get as far as "Messages to remember". Let me know how you get on?

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What is the most essential for life on Earth? Water, of course; but probably light too... and photosynthesis that enhances it for the benefit of living organisms. Yet photosynthesis uses only a very small part (5 to 6% in the best conditions, less than 1% on average) of the solar energy arriving on Earth. This energy allows the annual fixation of 115 to 120 billion tons of carbon from the atmospheric CO₂ into the biomass (see The path of carbon in photosynthesis). Over geological times, this has led to the formation of fossil fuels (coal, oil, gas) which provide 80% of the energy used by our societies (see Oil: Evidence of its biological origin). Photosynthesis is also responsible for producing the oxygen we breathe. But then how do photosynthetic organisms manage to collect solar light and how do they recover the energy it contains? How do these solar collectors that are chloroplasts work?

1. Autotrophy and photosynthesis

" On August 16, 1771, I put a spring of mint into a transparent closed space with a candle that burned in the air until it soon went out. After 27 days, I relit the extinguished candle again by focusing sunlight beams with a mirror onto the candle wick and it burned perfectly well in the air that previously would not support it." This is how Joseph Priestley (see Focus Some pioneers of photosynthesis) reported the experiment that allowed him to discover oxygen and to reveal a fundamental aspect of green plants metabolism*: namely photosynthesis.

1.1. What is photosynthesis?

This metabolic process confers autotrophy* to photosynthetic organisms (plants, algae, cyanobacteria). Thanks to light, these organisms oxidize water, release oxygen ( dioxygen) and fix carbon dioxide by synthesizing their organic matter ( biomass) from mineral substances taken from their surrounding environment (air, soil, aquatic environment...). The following equation summarizes this process of photosynthesis (Figure 1) :

Prokaryotic organisms, ancestors of today's cyanobacteria (which appeared in the primordial Precambrian ocean more than three billion years ago) were among the first organisms capable of photosynthesis (See The Biosphere, a major geological player). Using solar energy, they produced oxygen that slowly accumulated in the environment, causing a real "revolution in evolution". The oxygen enrichment of the original atmosphere led to the creation of the ozone layer, which protects the Earth from solar ultraviolet radiation, causing changes in climate and in the composition of the Earth's crust. These changes have enabled the colonization of continents by new forms of bacterial, animal and plant life. [1]

1.2. Chloroplasts, site of photosynthesis

Photosynthesis takes place in higher plants leaves, in algae thallus* and in unicellular micro-organisms (algae, cyanobacteria, ...). The -generally flattened- shape of the leaf of a green plant, its orientation -facing the light- and its thinness make it an efficient receiver for solar radiation (Figure 2).

Leaf cells contain a large number of chloroplasts in their cytoplasm (Figure 3). These organelles, highly differentiated and containing all of the leaf chlorophyll, are specialized in the accomplishment of photosynthesis. [2] In one gram of spinach leaf, there are about 500 million chloroplasts. On average, almost 60% of the total mass of leaf proteins is located in the chloroplasts.

Video " Chloroplast movement in a remote cell " :

Observed under an electron microscope (Figure 4), a chloroplast appears as an ovoid disc, 7 to 8 microns long by 2 to 3 microns in diameter. It consists of three parts:

the envelope, a double membrane system (made of an outer and an inner membranes, separated by an intermembrane space), delimits the chloroplast ;

thylacoids, a membrane network in the form of flattened bags; often stacked in grana linked together by intergrana lamellae. The inner space bounded by the thylakoid membranes is called the lumen.

the stroma is the space limited by the envelope and in which thylacoids are bathing. Under electron microscopy, it has a granular appearance. It contains many enzymes, DNA, protein-synthesizing machinery (ribosomes) and a few lipid droplets.

Photosynthesis reactions are coordinated in these various compartments:

the absorption of light and the release of oxygen take place within the thylacoids during primary reactions (see Focus Z as [photosynthesis);

carbon dioxide fixation and synthesis of carbon molecules then take place within the stroma (see The path of carbon in photosynthesis);

the transport of molecules between the chloroplast and its cellular environment involves the chloroplast envelope (see Focus Sucrose or starch?). It feeds biomass synthesis.

An overview of the chloroplast compartments, their various constituents and functions is available interactively on the "SUN Chloroplast E-book" website: http://www.markhoelzer.com/SUN-chlorophyllEbookWorking/chloroplast.html

2. The leaf is a solar collector

2.1. Why are leaves green?

Most leaves are green, with a few exceptions with white or coloured parts. Why are they green? The answer to this question seems simple: because they contain chlorophyll (see Focus The colour of leaves).

But things are a little more complex because they involve the special characteristics of our vision and the coordination by our brain. We only see the wavelengths of the electromagnetic spectrum that activate receptors in the cells of our retina. These receivers are sensitive to 3 colours (blue, green and red) and detect the light reflected to the eye by objects in our environment. They allow us to see an infinite variety of shades in the color spectrum of a rainbow (violet, indigo, blue, green, yellow, orange, red) [3]. However, people with various forms of colour blindness do not distinguish all of these colours, the most common confusion being between green and red. Each animal species has its own specific vision (for example, bees see in ultraviolet light) and very few animals see green leaves (see Light, Vision and Biological Clocks and The Colours of the Sky).

For photosynthesis, the quality of light is more important than the quantity. A molecule such as chlorophyll a (the most abundant form of chlorophyll) effectively absorbs light in violet, red, orange and some in yellow. Chlorophyll b is more effective in the blue part of the spectrum. On the other hand, they are very inefficient at absorbing green light, which is then reflected by the leaf. That is why the leaves appear green to us (Figure 5). If chlorophyll was optimally absorbed in all regions of the spectrum, plants would look black to us, even in daylight!

2.2. How much light do you need?

In nature, leaves absorb less than 1% of the sunlight they receive, so this is generally not a limiting factor. [4] However, very strong light can lead to excess energy in the leaves, which leads to photoinhibition: oxidative stresses* can then damage the light-capturing structures (see How do plants cope with alpine stresses?).

Light is the driving force of photosynthesis. Experimentally, we observe that photosynthesis increases steadily with light intensity. However, there is saturation at high intensities: some reactions become limiting (for example because of CO ₂ concentration or temperature). These observations revealed the existence of two types of reactions: those that required light (the so-called "light" or photochemical reactions) and those that could take place in the absence of light (improperly called "dark" reactions, but rather biochemical reactions).

In 1932, Emerson [5] and Arnold illuminated chlorella - a photosynthetic unicellular green algae - with intense flashes of light lasting a few milliseconds, and demonstrated that the light provided by the flashes allowed the emission of a single oxygen molecule for every 2,500 chlorophyll molecules (Figure 6). It takes about 9 to 10 photons to allow the production of this oxygen molecule. This corresponds to a quantum efficiency - i.e. the ratio between the number of oxygen molecules emitted and the number of photons absorbed - of about 0.1. This experiment led to the concept of a photosynthetic unit which will be demonstrated later with the characterization of photosystems.

2.3. Are all wavelengths the same?

Do all wavelengths have the same importance for photosynthesis? In the 19 th century, Engelmann [6], showed the importance of the light color in an experiment with filamentous algae (spirogyre type) illuminated by light spots and then with a prism where aerobic bacteria* serve as an indicator of oxygen production. Bacteria density was highest in the areas illuminated by the blue and red lights.

Measurement of photosynthetic activity as a function of different wavelengths shows that photosynthesis is active over the entire visible light range, even in the 500-600 nm range where chlorophylls are not very efficient (see Figure 5). This is due to the so-called accessory pigments, which are also capable of absorbing light energy. This is particularly the case with carotenoids, which absorb light in the violet-to-red range of the spectrum. All the pigments therefore absorb energy over practically the entire wavelength range and release it for photosynthesis.

However, by illuminating chloroplasts with monochromatic light, Emerson and Lewis showed a sharp drop in quantum efficiency above 680 nm (Figure 7) [7], whereas chlorophylls are able to absorb in this region of the spectrum. This effect, called " red drop", shows that wavelengths above 680 nm are not capable - on their own - of allowing photosynthesis (measured here by the release of oxygen). On the other hand, this drop in red is suppressed by adding radiation of shorter wavelength, e.g. 600 nm, to the dark red light. This experiment suggests the existence of two distinct pigment systems, later described as the two photosystems (photosystem I, or PSI, and photosystem II, or PSII):

one that does not absorb light beyond 680 nm and is associated with the release of oxygen.

the other which absorbs beyond 680 nm and does not allow the release of oxygen.

This synergistic effect suggests that two distinct systems cooperate under normal illumination conditions to carry out reactions leading to the emission of oxygen.

2.4. How are pigments organized in the leaf?

Leaf chlorophyll is localized within the thylacoid membrane (see Figure 5), but is not uniformly distributed there. It is associated with proteins in photosystems each organized around a reaction centre and an array of light collecting antennas (Figure 8). These photosystems are embedded in the membrane lipids that make up the thylacoid membrane.

The antenna combines proteins and a large number of photoreceptor pigments: chlorophylls (about 300 chlorophyll molecules in each photosystem), but also, depending on the organism, different pigments such as carotenoids. Like chlorophylls, carotenoids participate in the process of capturing light energy within the collecting antennae and can therefore transfer their energy to the chlorophyll. Photosynthesis is first of all a membrane phenomenon!

3. From photons to electrons: how light becomes electricity

3.1. Chlorophyll and light energy recovery

The various pigments of the antenna (chlorophylls and carotenoids) absorb light mainly in the visible part of the solar radiation (see Figure 5) and the molecules switch to an excited state during the capture of a photon (Figure 9B). 8] Once excited, the chlorophyll returns to its ground state, which is more thermodynamically stable (Figure 8C), according to three processes:

by emitting light and heat: this is called fluorescence (Figure 9, C1) [9]. This mechanism is functional under excess light conditions, the excess light energy is then dissipated as heat;

by transferring its energy to an adjacent molecule: it is called resonance. This great proximity between pigment molecules explains the extreme speed of the reaction: it takes place in less than one picosecond (i.e. less than one millionth of a millionth of a second or 10⁻¹² s). This transfer of excitation energy takes place with virtually no loss of energy. This is how the pigments of the collecting antenna (chlorophylls and carotenoids) transfer the energy brought by the light from molecule to molecule (Figure 9, C2) to the special chlorophyll pair at the reaction centre.

by losing an electron: that's what happens within the core of the reaction centre.

The core of the reaction centre is formed by a pair of so-called special chlorophyll molecules (Figure 7). This chlorophyll serves as an energy trap: it receives, in the form of electronic excitation, the energy of the solar photons captured by all the pigments of the antenna. All the light energy picked up by the antenna is therefore concentrated on this special pair.

Thus excited, the "special" pair of chlorophylls transfers an electron to an acceptor - called primary - (see Figure 7) which will then be reduced. It is the " charge separation" where an electron of the chlorophyll passes from the inner side of the thylacoids to the stromatic side. The reaction centre can therefore be considered as a molecular photopile. Its positive pole is formed by the special oxidized pair with a positive charge. Its negative pole consists of a reduced molecule with a negative charge: the primary acceptor.

Very quickly, the reduced primary acceptor will yield the electron to another acceptor and so on during a series of cascade redox reactions* that will allow the production of chemical energy from light energy (Figure 10) (see Focus Z as photosynthesis).

3.2. Back to basics: where does the oxygen comes from?

For the system to remain functional, however, the special pairs of chlorophylls in the reaction centres must return to their ground state. They do so by accepting an electron from a primary donor. Each photosystem is characterized as follows:

by a pair of chlorophylls absorbing light at a given wavelength (with a maximum at 700 nm for PSI and 680 nm for PSII, hence their name, P700 and P680) ;

by primary donors and acceptors specific to each photosystem (Figure 10).

In the PSII, the photo-induced departure of an electron leaves the primary electron donor P680 in an oxidized state: it then carries a positive charge (P680⁺). For the system to work, it is essential that retrieves each individual electron. Photosynthetic organisms solved this problem very early in the history of evolution by developing an original process more than three billion years ago: they took electrons from water, according to the following reaction :

2 H ₂O (water) → O ₂ (oxygen) + 4 H⁺ (protons) + 4 e ⁻ (electrons) (equation 1)

Thus, electrons from the oxidation of water are transferred to P680⁺, gaseous oxygen is released and protons are released into the inner space of the thylacoids (Figure 11). This step returns the P680⁺ to its neutral state (P680) and allows a new photochemical cycle to occur. [10]

The mechanism responsible for this reaction was identified around 1970 by two groups of researchers: the French P. Joliot [11] and the American B. Kok [12]. By illuminating thylacoids with a series of flashes, Joliot showed that oxygen production has a periodicity of order four until the differences gradually subside (Figure 11A). Kok then offers an explanation: the water oxidation cycle. Oxygen formation requires the sequential accumulation of four positive charges on the donor side of PSII (Oxygen Emission Complex) in a cyclic mechanism where manganese plays a central role (Figure 11B). Each electron released during this process allows the special pair of chlorophylls (P680) to return to its ground state and to be available again to recover the energy of the photons captured by the antenna pigments.

So the oxygen we breathe is a by-product of this reaction. [13]

3.3. How does the current flow?

The electrons stripped from the P680 at the primary acceptor of the PSII then flow to the other photosystem (PSI) through a series of redox reactions that allow the oxidized P700⁺ to return to its initial state. Working in series, photosystems energetically couple their photochemical reactions on the electron transfer chain (see Focus Z as photosynthesis).

Electron transfers are therefore organized within the photosynthetic membrane to result in the chemical reduction of nicotinamide adenine dinucleotide phosphate (NADP⁺) by an enzyme, Ferredoxine-NADP+ oxidoreductase (or FNR) located in the stroma. This final stage of electron transfer allows the formation of a reducing power in the form of NADPH.

A true electric current therefore flows through the thylacoid membrane from the water (at the lumen) to the NADP⁺ on the stromatic side of the membrane (see Figure 10). This electron transfer can be viewed in animated form on the SUN Chloroplast E-book website: http://www.markhoelzer.com/SUN-chlorophyllEbookWorking/chloroplast.html

3.4. A proton gradient coupled to the electron transfer

Electron transfer is coupled with the establishment of a proton gradient across the thylacoid membrane, through reactions leading to acidification of the lumen relative to the stroma (see Figure 10):

Oxidation of water releases protons into the lumen of the thylacoids (see equation 1);

By transferring electrons between the two photosystems, the cytochrome b6f complex pumps protons from the stroma that accumulate in the lumen through the thylakoid membrane;

The synthesis of NADPH in the stroma consumes protons, which amplifies the difference in pH between stroma and lumen.

The chemical energy potential resulting from this difference in proton concentrations between the two sides of the photosynthetic membrane (or electrochemical proton gradient) is used by a membrane protein, ATP-synthase - a true nanomachine - to synthesize adenosine triphosphate (ATP) (see Focus on ATP synthesis).

In summary, the photon energy recovered from photosystems is converted into reducing power (NADPH) and chemical energy (ATP).

4. Coupling between photochemical and biochemical reactions

ATP and NADPH are used to fuel reactions in the next stage of photosynthesis: carbon dioxide fixation (see The path of carbon in photosynthesis). This last phase of photosynthesis is closely coupled to the primary reactions (clear phase) because it uses the ATP and NADPH generated by the primary reactions and takes place in the stroma. It allows the integration of atmospheric CO ₂ carbon into triose-phosphate, phosphorylated 3-carbon molecules, within the chloroplast stroma during a set of reactions called the Benson-Bassham-Calvin Cycle (Figure 12).

Phosphate trioses are then used in the chloroplast for the synthesis of starch, amino acids or lipids or exported out of the chloroplast and transformed into sugars (sucrose) by the enzymes of the cytoplasm: this is the origin of the constituent biomass of all living organisms (Read Focus Sucrose or starch?).

5. Messages to remember

With the help of light, autotrophic organisms oxidize water, release oxygen (dioxygen) and fix carbon dioxide (carbon dioxide) by synthesizing their organic matter from mineral substances taken from the surrounding environment;

The leaf is a solar collector, it contains chloroplasts and chlorophyll; the chlorophylls absorb blue and red light, but are very inefficient at absorbing green light, which is then reflected by the leaf. That's why the leaves appear green to us;

The two photosystems (photosystem I, or PSI, and photosystem II, or PSII) consist of a light collecting antenna and a reaction center that will transfer electrons in a process called charge separation ;

Working in series, the photosystems energetically couple their photochemical reactions on the electron transfer chain: the photon energy recovered in the photosystems is thus converted into reducing power (NADPH) and chemical energy (ATP).

ATP and NADPH are used to fuel the reactions in the next stage of photosynthesis: the fixation of dioxide which takes place in the stroma during the Benson-Bassham-Calvin Cycle.

Notes and References

Cover image. [Source: © Diverticimes]

[1] Rutherford A.W. & Boussac A. (2004), Photosynthesis, a green chemistry triggered by solar energy. Photosynthesis and oxygen production. KEYS CEA 49:86-92.

[2] It was in 1937 that Robin Hill (British biologist, 1899-1991) discovered that chloroplasts are "grains of chlorophyll" that ensure photosynthesis. It manages to isolate chloroplasts (actually thylacoids) and will achieve oxygen production by a suspension of illuminated chloroplasts in the presence of an artificial electron acceptor (oxidant). It's the "Hill reaction".

[3] A photon of blue light has more energy than a photon of red light (Planck's Law, see Planck's Theory). The order of the colours of the rainbow thus corresponds to a continuous spectrum of energy increasing from red to blue. The energy (e) of a photon is given by the equation e = hc/λ, where c is the speed of light, h is Planck's constant, and λ is the wavelength of light. The energy (E) of an einstein is E = Ne = Nhc/λ = 28,600/λ, when E is in kilocalories and λ is given in nanometers (nm; 1 nm = 10⁻⁹ meters). An einstein of red light with a wavelength of 680 nm has an energy of about 42 kcal. Blue light has a shorter wavelength and therefore more energy than red light. The part of the solar spectrum used by plants has an estimated average wavelength of 570 nm; therefore, the light energy used during photosynthesis is about 28,600/570, or 50 kcal per einstein.

[4] Even under the canopy, light is not really a limiting factor. The plants living there have leaf structures adapted to the light environment to balance light capture (photochemical reactions) and CO ₂ fixation (biochemical reactions). The distribution of plants in undergrowth and during cutting is linked to a phenomenon of tolerance (or not) to shade. This phenomenon (called "Shade avoidance") is related to phytochrome signalling and not at all to chlorophyll. Here again it is a question of light quality (i.e. signalling) and very little light quantity (substrate).

[5] Robert Emerson (1903-1959), American biologist, author of numerous major works for the understanding of the impact of light on photosynthesis (quantum efficiency, "Emerson" effect, etc.). His work is the first experimental demonstration of the existence of two photosystems in chloroplasts. Emerson R. & Arnold W. (1932) A separation of the reactions in photosynthesis by means of intermittent light. J Gen Physiol 15:391-420.

[6] Theodor Wilhelm Engelmann (1843-1909), a German physiologist, played a decisive role in the analysis of the mechanisms of muscle contraction (striated muscles) and photosynthesis.

[7] Govindjee (1963) Emerson enhancement effect and two light reactions in photosynthesis. In: Photosynthetic Mechanisms in Green Plants. Publication 1145, Published by National Academy of Sciences - National Research Council, pp. 318-334

[8] This excitation is due to the presence of conjugated bonds (and thus of delocalized electrons): the arrival of a photon makes a delocalized electron pass from a ground state (unexcited) to an excited state. In chlorophyll, there are two excited states: an upper state (Sa) and a lower state (Sb), depending on the energy of the excitation photon (blue or red).

[9] The emission kinetics of chlorophyll fluorescence from plants is an excellent indicator of their photosynthetic performance. His study thus makes it possible to accurately measure the impact of various stresses that disrupt the photosynthetic activity of plants.

[10] Govindjee & Coleman W. (1990) Oxygen production by plants. Special Issue For Science, January 2000

[11] Joliot, P., Barbieri, G. & Chabaud, R. (1969) A new model of photochemical centers in system-2. Photochem. Photobiol. 10, 309-329.

[12] Kok, B., Forbush, B. & McGloin, M. (1970) Cooperation of charges in photosynthetic O ₂ evolution-I. A linear four step mechanism. Photochem. Photobiol. 11, 457-475; Bessel Kok (1918-1979) is an American biophysicist of Dutch origin.

[13] The oxygen content of the atmosphere has varied enormously over geological time. Before 2.5 Ga, there was no oxygen in the atmosphere. Since 1.8 Ga, the oxygen content has been above 0.1%. There is thus a very significant rise between -2.5 and -1.8 Ga, confirmed by the general precipitation of Fe ₂O ₃ in the oceans at that time. Currently, there are 1,000,000 Gt of oxygen in the atmosphere, i.e. a content of 21% (see The Biosphere, a major geological player).

The Environmental Encyclopedia is published by the Association des Encyclopédies de l'Environnement et de l'Énergie ( www.a3e.fr), which is contractually linked to Grenoble Alpes University and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: JOYARD Jacques, MOROT-GAUDRY Jean-François (2020), Shedding light on photosynthesis, Encyclopedia of the Environment, [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/shedding-light-on-photosynthesis/.

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