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Light Energy and Pigments*# - Biology

Light Energy and Pigments*# - Biology



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The sun emits an enormous amount of electromagnetic radiation (solar energy) that spans a broad swath of the electromagnetic spectrum, the range of all possible radiation frequencies. In Bis2a we are largely concerned with the latter and we discuss some very basic concepts related light and its interaction with biology below.

Light Energy

First, however we need to refresh a couple of key properties of light.

  1. Light in a vacuum travels at the constant speed of 299,792,458 m/s. We often abbreviate the speed of light with the variable "c".
  2. Light has properties of waves. A specific "color" of light has a characteristic wavelength.

Figure 1: The distance between peaks in a wave is referred to as the wavelength and is abbreviated with the greek letter lambda (Ⲗ).
Attribution: Marc T. Facciotti (original work)

Figure 2: The inverse proportionality of frequency and wavelength. Wave 1 has a wavelength that is 2X that of wave 2 (Ⲗ1 > Ⲗ2). If the two waves are traveling at the same speed (c) - imagine that both of the the whole line that are drawn are dragged past the fixed vertical line at the same speed - then the number of times a wave peak passes a fixed point is greater for wave 2 than wave 1 (f2 > f1).
Attribution: Marc T. Facciotti (original work)

4. Finally, each frequency (or wavelength) of light is associated with a specific energy. We'll call energy "E". The relationship between frequency and energy is:

[E = h ,f]

where (h) is a constant called the Planck constant (~6.626x10-34 Joule•second when frequency is expressed in cycles per second). Given the relationship between frequency and and wavelength you can also write E = h*c/Ⲗ. Therefore, the larger the frequency (or shorter the wavelength) the more energy is associated with a specific "color". Wave 2 in the figure above has greater energy than wave 1.

Figure 3: The sun emits energy in the form of electromagnetic radiation. All electromagnetic radiation, including visible light, is characterized by its wavelength. The longer the wavelength, the less energy it carries. The shorter the wavelength the more energy is associated with that band of the electromagnetic spectrum.

The Light we See

The visible light seen by humans as white light is composed of a rainbow of colors, each with a characteristic wavelength. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. In the visible spectrum, violet and blue light have shorter (higher energy) wavelengths while the orange and red light have longer (lower energy) wavelengths.

Figure 4: The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA)

Absorption by Pigments

The interaction between light and biological systems occurs by several different mechanisms, some of which you may learn about in upper-division courses in cellular physiology or biophysical chemistry. In Bis2a we are mostly concerned about the interaction of light and biological pigments. These interactions can initiate a variety of light dependent biological process that can be grossly grouped into two functional categories: cellular signaling and energy harvesting. Signaling interactions are largely responsible for perceiving changes in the environment (in this case changes in light). An example of a signaling interaction might be the interaction between light and the pigments expressed in an eye. By contrast, light/pigment interactions that are involved in energy harvesting are used for - not surprisingly - capturing the energy in the light and transferring it to the cell to fuel biological processes. Photosynthesis, which we will learn more about soon, is one example of an energy harvesting interaction.

At the center of the biological interactions with light are groups of molecules we call organic pigments. Whether in the human retina, chloroplast thylakoid, or microbial membrane, organic pigments often have specific ranges of energy or wavelengths that they can absorb. The sensitivity of these molecules for different wavelengths of light is due to their unique chemical makeups and structures. A range of the electromagnetic spectrum is given a couple of special names because of the sensitivity of some key biological pigments: The retinal pigment in our eyes, when coupled with an opsin sensor protein, “sees” (absorbs) light predominantly between the wavelengths between of 700 nm and 400 nm. Because this range defines the physical limits of the electromagnetic spectrum that we can actually see with our eyes, we refer to this wavelength range as the "visible range". For similar reasons - plants pigment molecules tend to absorb wavelengths of light mostly between 700 nm and 400 nm - plant physiologists refer to this range of wavelengths as "photosynthetically active radiation".

Two Key Types of Pigments We Discuss in Bis2a

Chlorophylls (including bacteriochlorophylls) are part of a large family of pigment molecules. There are five major chlorophyll pigments named: a, b, c, d, and f. Chlorophyll a is related to a class of more ancient molecules found in bacteria called bacteriochlorophylls. Chlorophylls are structurally characterized by ring-like porphyrin group that coordinates a metal ion. This ring structure is chemically related to the structure of heme compounds that also coordinate a metal and are involved in oxygen binding and/or transport in many organisms. Different chlorophylls are distinguished from one another by different "decorations"/chemical groups on the porphyrin ring.

Figure 5: The structure of heme and chlorophyll a molecules. The common porphyrin ring is colored in red. Attribution: Marc T. Facciotti (original work)

Carotenoids are the red/orange/yellow pigments found in nature. They are found in fruit — such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene) — which are used as biological "advertisements" to attract seed dispersers (animals or insects that may carry seeds elsewhere). In photosynthesis, carotenoids function as photosynthetic pigments. In addition, when a leaf is exposed to full sun, that surface is required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids help absorb excess energy in light and safely dissipate that energy as heat.

Flavonoids ENTER DESCRIPTION HERE

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light. This characteristic is known as the pigment's absorption spectrum. The graph in the figure below shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. These differences in absorbance are due to differences in chemical structure (some are highlighted in the figure). Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

Figure 6: (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. Note how the small amount of difference in chemical composition between different chlorophylls leads to different absorption spectra. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum.

Importance of having multiple different pigments

Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and available wavelengths decrease and change, respectively, with depth. Other organisms grow in competition for light. Plants on the rainforest floor for instance must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation. To account for these variable light conditions, many photosynthetic organisms have a mixture of pigments whose expression can be tuned to improve the organism's ability to absorb energy from a wider range of wavelengths than would be possible with one pigment alone.


What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength , the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure).

The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure). The difference between wavelengths relates to the amount of energy carried by them.

The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.


Introduction and Goals

The majority of life on Earth could not exist without photosynthesis. Recall from the tutorial on Thermodynamics that during photosynthesis light energy is converted to chemical energy. Specifically, it is the process whereby plants, protists, and some bacteria use light, water, and carbon dioxide to make sugars. Photosynthesis is not the exact opposite of cellular respiration, but rather a separate process that just so happens to contain many similar features (e.g., the electron transport chain).


Photosynthesis can be divided into two parts: the light reactions and the light-independent reactions (also referred to as the "dark" reactions). This tutorial will cover the light reactions. This is when light energy is transformed into energy that can be used by cells. The next tutorial will cover the light-independent reactions, which is when sugar is actually made.
By the end of this tutorial you should have an understanding of:

  • Plants' requirements for photosynthesis and respiration
  • The light reactions of photosynthesis
  • Characteristics of light
  • Channeling of light energy
  • Electron transport chains and the Z-scheme
  • Generation of ATP.

Photosynthesis Occurs on Thylakoid Membranes

Chloroplasts are bounded by two membranes, which do not contain chlorophyll and do not participate directly in photosynthesis (Figure 16-34). Of these two membranes, the outer one, like the outer mitochondrial membrane, is permeable to metabolites of small molecular weight it contains proteins that form very large aqueous channels. The inner membrane, conversely, is the permeability barrier of the chloroplast it contains transporters that regulate the movement of metabolites into and out of the organelle.

Figure 16-34

The structure of a leaf and chloroplast. The chloroplast is bounded by a double membrane: the outer membrane contains proteins that render it permeable to small molecules (MW <�) the inner membrane forms the permeability barrier (more. )

Unlike mitochondria, chloroplasts contain a third membrane — the thylakoid membrane — that is the site of photosynthesis. In each chloroplast, the thylakoid membrane is believed to constitute a single, interconnected sheet that forms numerous small flattened vesicles, the thylakoids, which commonly are arranged in stacks termed grana (see Figure 16-34). The spaces within all the thylakoids constitute a single continuous compartment, the thylakoid lumen. The thylakoid membrane contains a number of integral membrane proteins to which are bound several important prosthetic groups and light-absorbing pigments, most notably chlorophyll. Carbohydrate synthesis occurs in the stroma, the soluble phase between the thylakoid membrane and the inner membrane. In photosynthetic bacteria extensive invaginations of the plasma membrane form a set of internal membranes, also termed thylakoid membranes, or simply thylakoids, where photosynthesis occurs.


Understanding Pigments

Different kinds of pigments exist, and each absorbs only certain wavelengths (colors) of visible light. Pigments reflect the color of the wavelengths that they cannot absorb.

All photosynthetic organisms contain a pigment called chlorophyll a, which humans see as the common green color associated with plants. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not from green. Because green is reflected, chlorophyll appears green.

Other pigment types include chlorophyll b (which absorbs blue and red-orange light) and the carotenoids. Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is its absorption spectrum.

Many photosynthetic organisms have a mixture of pigments between them, the organism can absorb energy from a wider range of visible-light wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity decreases with depth, and certain wavelengths are absorbed by the water. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees block most of the sunlight (Figure 5.11).

Figure 5.11 Plants that commonly grow in the shade benefit from having a variety of light-absorbing pigments. Each pigment can absorb different wavelengths of light, which allows the plant to absorb any light that passes through the taller trees. (credit: Jason Hollinger)


Biology 171

By the end of this section, you will be able to do the following:

  • Explain how plants absorb energy from sunlight
  • Describe short and long wavelengths of light
  • Describe how and where photosynthesis takes place within a plant

How can light energy be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build basic carbohydrate molecules ((Figure)). However, autotrophs only use a few specific wavelengths of sunlight.


What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy in a spectrum from very short gamma rays to very long radio waves). Humans can see only a tiny fraction of this energy, which we refer to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength (shorter wavelengths are more powerful than longer wavelengths)—the distance between consecutive crest points of a wave. Therefore, a single wave is measured from two consecutive points, such as from crest to crest or from trough to trough ((Figure)).


Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation ((Figure)). The difference between wavelengths relates to the amount of energy carried by them.


Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less energy it carries. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum ((Figure)) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms.

Absorption of Light

Light energy initiates the process of photosynthesis when pigments absorb specific wavelengths of visible light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, in a process called bleaching. Our retinal pigments can only “see” (absorb) wavelengths between 700 nm and 400 nm of light, a spectrum that is therefore called visible light. For the same reasons, plants, pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm plant physiologists refer to this range for plants as photosynthetically active radiation.

The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy ((Figure)).


Understanding Pigments

Different kinds of pigments exist, and each absorbs only specific wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear a mixture of the reflected or transmitted light colors.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light: This is termed the absorption spectrum . The graph in (Figure) shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.


Many photosynthetic organisms have a mixture of pigments, and by using these pigments, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation ((Figure)).


When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in (Figure). Protein complexes and pigment molecules work together to produce NADPH and ATP. The numbering of the photosystems is derived from the order in which they were discovered, not in the order of the transfer of electrons.


The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem , two types of which are found embedded in the thylakoid membrane: photosystem II (PSII) and photosystem I (PSI) ((Figure)). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex , which passes energy from sunlight to the reaction center it consists of multiple antenna proteins that contain a mixture of 300 to 400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.


What is the initial source of electrons for the chloroplast electron transport chain?

The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation they can actually give up an electron in a process called a photoact . It is at this step in the reaction center during photosynthesis that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate. PSII and PSI are two major components of the photosynthetic electron transport chain , which also includes the cytochrome complex. The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII (called P680 ) delivers its high-energy electrons, one at the time, to the primary electron acceptor , and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water thus, water is “split” during this stage of photosynthesis, and PSII is re-reduced after every photoact. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. However, splitting two molecules is required to form one molecule of diatomic O2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

As electrons move through the proteins that reside between PSII and PSI, they lose energy. This energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700 ). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine-tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Generating an Energy Carrier: ATP

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP ((Figure)). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure of the thylakoid.

View Photosynthesis: Light Reactions (Flash animation) to learn more about the process of photosynthesis within a leaf.

Section Summary

The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a and then to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of hydrogen ions. The hydrogen ions flow through ATP synthase during chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing carrier for the light-independent reactions.

Art Connections

(Figure) What is the source of electrons for the chloroplast electron transport chain?


Pigments In Plants

“Substances having ability to absorb light of certain wavelength and produce color”.

Importance:

The light is absorbed by pigments contained within the chloroplasts of plant cells energizes electrons, raising them to a higher energy level. It is the energy that is used to produce ATP and to reduce NADP to NADPH. The release of energy from ATP and the oxidation of NADPH are then used to incorporate CO2 into organic molecules.

Types:

There are following pigments which help out in the process of photosynthesis and color production:

  • Chlorophyll
  • Carotenoids
  • Phycobilins
Chlorophyll:

Chlorophyll is any of several related green pigments found in cyanobacteria and the chloroplasts of algae and plants. Its name is derived from the Greek words chloros (“green”) and phyllon (“leaf”).”

Physical properties:
  • Itis the primary pigment in plant.it is a chlorine that absorbs yellow and blue wavelengths of light while reflecting green
  • Chlorophyll is found in the thylakoid sacs of the chloroplast. The chloroplast is a specialized part of the cell that functions as an organelle.
  • Once the appropriate wavelengths of light are absorbed by the chlorophyll into the thylakoid sacs, the important process of photosynthesis is able to begin.
  • They are fat soluble.
  • Classification and distribution of Chlorophyll:
  • Chl a found in all green plants including algae.
  • Chl b found in higher plants and some algae e.gchlorophyceae,euglenophyceae and charophyceae.
  • Chl c found in lower vascular plants e.g bryophytes and in bacillariophyta,phaephyta.
  • Chl d found in red algae.
  • Chl e is rare found in some golden algae

Figure: Chlorophyll

Molecular Nature:

There are several types of chlorophyll, but all shares the chlorin magnesium ligand which forms the right side of the diagram.The molecule is consisting of 4_porphyrin ring as a head and phytole as tail.

This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll “captures” the energy of sunlight.

Chemical Nature:

Chemically chlorophyll consist of C,H,O with Mg as central atom.

Chlorophyll a: C55H72O5N4Mg (CH3), Special group is methyl

Chlorophyll b: C55H70O6N4Mg (CHO), special group is aldehyde

Biosynthesis of chlorophyll:

Succinyl-coA and glycine react to form an intermediate Product δ-Aminolevulinic acid 2 molecules of δ-Aminolevulinic acid react with each other to give porphobilinogen 4 molecules of porphobilinogen fuse to give protochlorophyll.Then protochlorophyll react with 2H to synthesis chlorophyll in the presence of sunlight.

2 molecules of δ -Aminolivolinic acid ––—————-–>Porphobilinogen

4 molecules of porphobilinogen ––—————-–>protochlorophyll

Protochlorophyll + 2 H ———––(Sunlight)––—->Chlorophyll

Carotenoids:

“Carotenoids are fat-soluble pigments and open chain conjugate molecules found throughout nature”

Occurrence:

They are found principally in plants, algae, and photosynthetic bacteria, where they play a critical role in the photosynthetic process. They also occur in some non photosynthetic bacteria, yeasts and molds, where they may carry out a protective function against damage by light and oxygen.

Carotenoids are responsible for many of the red, orange, and yellow hues of plant leaves, fruits, and flowers, as well as the colors of some birds, insects, fish, and crustaceans.

Classification:

Carotenoids are classified according to the structure as follows:

1.The hydrocarbon carotenoids are known as carotenes example alpha-carotene, Βeta-carotene etc.

  • Generala formula: C40H56
  • Show photosystem 1
  • Colour: orange

Figure: Beta-carotene

2. The oxygenated carotenoids which are derivatives of these hydrocarbons known as xanthophylls examples of these compounds are zeaxanthin and luteinetc.

  • Show photosystem 2
  • formula: C40H56O2
  • Color: yellow

Figure:Zeaxanthin

Function:

Carotenoids are essential for plant life, providing important photoprotective functions during photosynthesis light harvesting and prevention of photo-oxidative damage, and serving as precursors for the biosynthesis of the phytohormoneabscisic acid (ABA). They have a role in attraction of pollinators.

PHYCOBILINS:

“A group of red or blue photosynthetic water soluble pigments”They are open-chain tetrapyrroles. The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a.

Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there.There are three types of phycobilins:

  • Phycoerythrin:Phycoerythrin absorbs green, yellow and blue light and transmit red light.It is found in red algae.
  • Phycocyanin:Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliproteinfamily.Phycocyanins are found in Cyanobacteria.
  • Allophycocyanin:Allophycocyanin (APC) is an intensely bright phycobiliprotein isolated from red algae that exhibits far-red fluorescence with high quantum yields. It is excited by laser lines at 594 and 633 nm, with an absorbance maximum at 650 nm and a fluorescence emission peak at 660 nm.
Location of phycobilins:

They are found outside the thylakoid membrane in small bodies called phycobilisomes.


Functions of Chlorophyll

Chlorophyll in the Biosynthesis of Sugars

Plants use both forms of chlorophyll to collect the energy from light. Chlorophyll is concentrated in the thylakoid membranes of chloroplasts. Chloroplasts are the organelles in which photosynthesis takes place. The thylakoids are small sacs of membrane, stacked on top of each other. Embedded in these membranes are a variety of proteins that surround chlorophyll. These proteins work together to transfer the energy from light, through chlorophyll, and into the bonds of ATP – the energy transferring molecule of cells. ATP can then be used in the Calvin cycle, or dark cycle, to create sugars.

The series of proteins that transfer energy from light and channel it into the synthesis of sugars are known as photosystems. The entire process, both light and dark cycles together, is known as photosynthesis, and occurs in plants, algae, and some bacteria. These organisms take in carbon dioxide (CO2), water (H2O) and sunlight to produce glucose. They can use this glucose in the process of cellular respiration to create ATP, or they can combine the glucose into more complex molecules to be stored.

Chlorophyll in the production of oxygen

A by-product of photosynthesis is oxygen. Plants can use this oxygen in cellular respiration, but they also release excess oxygen into the air. This oxygen allows many non-plants to undergo respiration as well, thereby supporting life on Earth. The oxygen is produced in the first part of the light cycle of photosynthesis. Plants split water molecules to produce electrons, hydrogen ions, and diatomic oxygen (O2). The electrons supply the electron transport chain that drives ATP production. The oxygen is released into the air. In this way, all the oxygen we breathe is produced.


Light Energy and Pigments*# - Biology

Department of Biology
Dickinson College, Carlisle, PA 17013
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The functioning of the Earth's ecosystem is dependent upon a continuous input of light energy in the form of photons coming from the sun. The process by which light energy is captured and incorporated into molecules that can be used by living organisms is called photosynthesis. It takes place in green plants, algae, and some kinds of bacteria. Organisms can be grouped into two major categories: autotrophs, which carry on photosynthesis and manufacture their own food, and heterotrophs, which do not perform photosynthesis themselves but must consume autotrophs or other organisms that ate autotrophs. In other words, photosynthetic organisms are at the base of the food chain, and with only a very few exceptions all organisms on Earth depend upon photosynthesis as the source of their food and energy.

Although photosynthesis is a very complex process, it can be divided into two parts. In the first part, light energy is captured and used to make high energy chemical molecules. In the second part, those high energy molecules are used to convert atmospheric carbon dioxide (CO 2 ) into carbohydrates and other energy containing compounds. These then serve as food for the photosynthetic plant itself or for an animal that eats the plant. During this process, water molecules (H 2 O) are split apart and the oxygen from them is released into the atmosphere as oxygen gas (O 2 ). This is an important byproduct of photosynthesis because most organisms have evolved ways of using O 2 to help them process their foods more efficiently. Some of the O 2 also moves into the upper atmosphere where it forms ozone (O 3 ), which is important in protecting the Earth's surface from ultraviolet radiation.

We will now examine these and other aspects of photosynthesis in more detail.


Capture of Light Energy and Formation of High Energy Molecules

This first part of photosynthesis is sometimes referred to simply as the "light reactions." In essence it is an energy conversion process, i.e. the conversion of light energy into chemical energy. This discussion will be based upon the way the process is believed to function in higher plants, although the fundamental mechanisms are generally similar in other types of organisms.


Photosynthetic Pigments and the Structure of the Leaf

The first law of photochemistry and photobiology tells us that in order for light to have an effect in a biological system it must first be absorbed. Pigments are molecules that are specialized for the absorption of light, and they therefore must be found in all photosynthetic organisms. Two major classes of photosynthetic pigments that occur in higher plants are the chlorophylls and the carotenoids.

Chlorophylls consist of a light-absorbing porphyrin ring with a magnesium atom at the center and a long phytol tail that anchors the molecule in a membrane (Figure 1). They absorb light in the blue and red parts of the spectrum, but the green wavelengths are transmitted or reflected. This is what makes leaves appear green in color. Chlorophylls a and b both occur in higher plants. They have slight differences in structure and absorbance spectra. Protochlorophyll is a biosynthetic precursor of chlorophyll.

The carotenoids and other non-chlorophyll pigments that participate in photosynthesis are referred to as accessory pigments. Their structures and properties vary among different species (Figure 2).

The photosynthetic pigments are contained in the membranes of subcellular organelles called chloroplasts (Figure 3), which reside inside leaf cells. The interior air spaces of the leaf are connected to the outside atmosphere through tiny pores called stomata, which allow for the inward movement of CO 2 and the outward movement of O 2 . The double membrane surrounding the chloroplast is permeable to small uncharged molecules such as CO 2 and O 2 , thereby allowing for exchange of these gases during photosynthesis.

The pigment-containing membranes inside the chloroplast are referred to as thylakoids, and are arranged in stacks called grana, which are strikingly similar to the arrangement of disks in the photoreceptor cells of the vertebrate retina. This arrangement of stacked membranes has apparently evolved independently several times as an extremely efficient system for the interception and capture of light energy. The chlorophyll and carotenoid molecules in the thylakoid membranes are not free, but exist in the form of pigment-protein complexes, sometimes referred to as "light-harvesting complexes." These proteins maintain the pigments in the proper orientation for efficient energy transfer, and also bring about slight changes in the wavelengths of light they can absorb.

The non-membranous region inside the chloroplast is called the stroma. It contains an aqueous solution of enzymes and various metabolites that participate in the reduction of CO 2 to carbohydrate via the Calvin Cycle, as described below.

Absorption of Photons and Transfer of Energy

When a plant is exposed to light, photons of appropriate wavelength will strike and be absorbed by the pigment-protein complexes arrayed on the thylakoid membranes. When this happens, the energy of the photon is transferred to the pigment molecule, thus causing the pigment to go into an electronically excited state. In other words, the energy originally associated with the photon is now contained within the pigment molecule. Once a pigment molecule has gone into an excited state, it is possible for the excitation energy to be transferred to an adjacent pigment molecule.

Dispersed among the pigment-protein complexes on the thylakoid membranes are special structures called reaction centers. They are fewer in number than the light-harvesting complexes, and consist of proteins, chlorophyll, and several other pigments in a highly structured arrangement. This will be described later in a detailed module. There are two types of reaction centers, called Photosystem I (PSI or P700) and Photosystem II (PSII or P680).

The array of pigment-protein complexes on the thylakoid membranes functions as an antenna for gathering light energy. Excitation energy is transferred from one chlorophyll to another until the energy reaches a reaction center (Figure 4). This causes one of the chlorophyll molecules within the reaction center to go into an excited state.

Formation of High Energy Chemical Compounds

Two kinds of energy-containing molecules are formed in the light reactions of photosynthesis: adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), which serves as a source of reducing power.

The PSI and PSII reaction centers are interconnected by a series of compounds that function as electron carriers (Figure 5). An excited electron in a PSII reaction center enters this chain of carriers and moves from one to the next until it reaches a PSI reaction center. This transport of an electron between the two types of reaction centers results in the pumping of hydrogen ions (H + ) across the thylakoid membrane, thus forming a gradient with a high H + concentration inside the thylakoid compartments and a relatively low concentration on the stroma side. The potential energy associated with this gradient is then used to form ATP by a mechanism similar to that by which ATP is generated in mitochondria.

The next step in the process is that an excited electron in PSI is transferred to a molecule of NADP, along with an H + , thereby reducing it to NADPH. Notice that the electrons leaving PSI are being replaced by ones that came through the electron carriers from PSII.

Finally, in order for this process to continue, the electrons that were removed from PSII have to be replaced. This is achieved by the splitting of an H 2 O molecule, which yields electrons, H + , and oxygen. The oxygen from two water molecules forms O 2 , which then passes through the stomata into the atmosphere.

In summary, what has been achieved is the conversion of photon energy into chemical energy in the form of ATP and NADPH, with the concomitant formation of O 2 .


Reduction of Atmospheric CO 2 to Carbohydrate

The second major phase of photosynthesis involves the conversion of CO 2 from the atmosphere into carbohydrates and other biological molecules. This set of reactions is sometimes referred to as the "dark reactions" of photosynthesis because light is not directly involved. However, this terminology is somewhat misleading because light is required for the formation of the ATP and NADPH needed for energy and reducing power. If a photosynthesizing plant was suddenly put in the dark, the so-called dark reactions would only continue until the supply of ATP and NADPH was depleted.

The conversion of CO 2 into carbohydrate takes place in the chloroplast stroma and follows a complex metabolic pathway called the Calvin Cycle or the Reductive Pentose Phosphate Pathway, each step of which is catalyzed by a specific enzyme. The details of this pathway will be dealt with in another module, but its overall form is shown in Figure 6. Carbon dioxide first combines with a five carbon compound called ribulose-1,5 -bisphosphate (RUBP), which then immediately splits into two three carbon compounds. Using energy and reducing power from ATP and NADPH, these are converted into 3-phosphoglyceraldehyde (3PGAL), which can be used by the plant to manufacture carbohydrates and various other biological molecules and to regenerate RUBP.

In order for the Calvin Cycle to continue, there must be a continuous supply of RUBP. Thus not all of the 3PGAL produced can go into the synthesis of carbohydrates - a significant portion (about 5 molecules out of 6) must be used to make fresh RUBP. This also requires ATP coming from the light reactions, and is what gives the pathway its cyclic nature.

In summary, ATP and NADPH formed in the light reactions of photosynthesis are used to convert atmospheric CO 2 into carbohydrates, which represent a stable, long term form of energy storage. Overall, energy that originated as photons from the sun is now contained in carbohydrates and other molecules that serve as food for the plant or for an animal that eats the plant.


Environmental Aspects of Photosynthesis

Because of the fundamental biological significance of photosynthesis and its importance in agricultural productivity, a considerable amount of research has been directed toward understanding the effects that environmental factors such as light, temperature, rainfall, salinity, and disease have on the process. In recent years, the effects of elevated atmospheric CO 2 and of increased ultraviolet radiation exposure due to depletion of the Earth's ozone layer have received considerable attention. The potential for forests and other photosynthetic ecosystems to mitigate increasing atmospheric CO 2 is also an active area of research.

Different species, and in some cases even different cultivars of the same species, vary widely in their ability to tolerate the multitude of environmental stresses that a photosynthesizing plant may encounter. The natural geographic distribution of plant species can be understood in terms of their ability to photosynthesize efficiently under the range of conditions that they encounter in the Earth's various ecosystems. Likewise, environmental factors govern the range over which specific agricultural crops can be successfully introduced and cultivated.

Both traditional plant breeding methods and the techniques of modern molecular biology are being used to produce plants that can maintain their vigor and sustain high rates of photosynthesis when subjected to environmental stress. These approaches have been very successful in the past and promise to further increase crop productivity and decrease the need for fertilizers, herbicides, and insecticides. These topics will be dealt with in more detail in another module.

Blankenship, R. E. 2002. Molecular Mechanisms of Photosynthesis. Blackwell Science Publishers, Oxford, UK, Malden, MA, USA.

Jansson, J. 1994. The Light-Harvesting Chlorophyll a/b-Binding Proteins. Biochimica Biophysica Acta, 1184: 1919.

Long, S. P., E. A. Ainsworth, A. Rogers, and D. R. Ort. 2004. Rising Atmospheric Carbon Dioxide: Plants FACE the Future. Annual Review of Plant Biology, 55: 591-628.

Nelson, N. and C. F. Yocum. 2006. Structure and Function of Photosystems I and II. Annual Review of Plant Biology, 57: 521-565.

Raghavendra, A. S., ed. 1998. Photosynthesis: A Comprehensive Treatise. Cambridge University Press, Cambridge, UK, New York, USA.

Raven, P. H., G. B. Johnson, J. Losos, and S. Singer. 2005. Biology, 7th. ed. McGraw-Hill, New York.

Szalai, V. A. and G. W. Brudvig. 1998. How Plants Produce Dioxygen. American Scientist, 86: 542-551.

Taiz, L. and E. Zeiger. 2006. Plant Physiology, 4th. ed. Sinauer Associates, Sunderland, MA.

Wild, A. and R. Ball. 1997. Photosynthetic Unit and Photosystems: History of Research and Current View. Backhuys Publishers, Leiden.


Section Summary

In the first part of photosynthesis, the light-dependent reaction, pigment molecules absorb energy from sunlight. The most common and abundant pigment is chlorophyll a. A photon strikes photosystem II to initiate photosynthesis. Energy travels through the electron transport chain, which pumps hydrogen ions into the thylakoid space. This forms an electrochemical gradient. The ions flow through ATP synthase from the thylakoid space into the stroma in a process called chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy carrier for the Calvin cycle reactions.