Module - Photosynthesis

Mother Goose Special Science High School Module 1: Photosynthesis

TargetsIn the end of this module, the student must be able to: y y y y y y State the importance of photosynthesis to human life Enumerate the differences between the Light reactions and Dark Reactions Differentiate the cyclic and non-cyclic phosphorylation in the calvin cycle Identify the raw materials needed in the process of photosynthesis Classify plants according to their mode of photosynthesis State the photosynthetic formulas

Purposes of StudyPhotosynthesis is the worlds largest scale process to use solar energy and convert inorganic compounds such as carbon dioxide and water into organic compounds such as carbohydrates to release oxygen. These products of photosynthesis provides organic substances, energy and oxygen, for nearly all life activities. Therefore, without this process there would be no survival and sustainable development of human society. Today, the issues on food, energy and environment are closely linked with plant photosynthesis. As a student therefore, it is significant to understand this process to have a valuable appreciation and awareness on how the plants perform a great part for the survival of the biotic life.

1A. Introduction to PhotosynthesisWhere does the energy in food come from? This depends on the source of the food, of course. The ultimate source of all food energy is the sun. Green plants and microscopic organisms are able to trap the energy of sunlight and use that energy to build the high-energy complex molecules that make up food. Whether an animal is a meat-eater or a plant-eater, the sourcePage 1


of energy in its diet can be traced back to plants. Plants obtain the energy they need from the sun. Man obtains the energy they need from the foods produced by the plants. This food energy will now be supplied to human body which will be distributed to the cells. Without this energy, the living body may not perform its function properly.

What is Photosynthesis? Before having knowledge on the importance of Photosynthesis and its impact on our life and earth, we first need to know what Photosynthesis is. The simplest way to understand what photosynthesis would be to know that it refers to the process of producing and releasing oxygen in the air. Photosynthesis is performed by the plants to produce their own food. It is the process of transforming solar energy with the aid of water and carbon dioxide into usable chemical energy called glucose and oxygen.

Products of Photosynthesis The primary product of Photosynthesis is Glucose which is the source of carbohydrates like cellulose, starches etc. In advance photosynthesis, the final product is considered to be glyceraldehyde-3-phosphate. It is because, in the calvin cycle, 2 molecules of G3P(Glyceraldhyde-3-phosphate) produces a single molecule of glucose. The process of Photosynthesis also produces fats, proteins, water soluble sugars such as maltose and sucrose. The plants depend on this glucose for their growth and energy. All the living creatures on the earth depend on fats, proteins and carbohydrates to derive their basic source of energy and thus have a direct dependence on the process of Photosynthesis for their survival.

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Photosynthesis equation.

can

be

represented

using

a

chemical

CO2 + 2H2O ------> (CH2O) + H2O + O2Solar energy

This entails the use of carbon dioxide and water with the aid of sunlight to produce carbohydrate, water and oxygen. The overall balanced equation is...

6CO2 + 6H2O ------> C6H12O6 + 6O2Solar energy

Where: CO2 = carbon dioxide H2O = water Light energy is required C6H12O6 = glucose O2 = oxygen

Photosynthetic formula for the net consumption of water 6CO2 + 6H2O -----------Solar energy

C6H12O6

+ 6O2

Photosynthetic formula for Reduction/oxidation CO2 + H2O -------Solar energy

(CH2O) + O2

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The Leafs layers Photosynthesis primarily occurs in plant leaves. The raw materials of this process which are water and carbon dioxide, passes through small openings called stomata. Carbon dioxide and water diffuse into the chloroplasts, the organelles that carry on photosynthesis. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma. An internal membrane system within the stroma forms flattened sacs called thylakoids, which in some cases are organized into stacks to form grana. Spaces within all thylakoids are connected to form an inner compartment, the thylakoid space. Chlorophyll and other pigments involved in absorption of solar energy reside within thylakoid membranes; these pigments absorb solar energy, and energize electrons prior to reduction of CO2 to a carbohydrate.

1B. Requirements for PhotosynthesisPhotosynthesis is the process by which green plants use the energy of sunlight to produce carbohydrates. In an overall sense, it requires energy coming from the sun, chlorophyll, that traps energy from the sun to make food, carbon dioxide, thatPage 4


enters through the stomata of the leaves and water, absorbed from the roots of the plants. 1. Light The sun bathes the Earth in a steady stream of sunlight. Sunlight provides the energy to warm the Earth and to drive the process of photosynthesis. What your eyes perceive as white light from the sun is actually a mixture of different wavelengths of light. Many of these wavelengths are visible to your eyes and make up what is known as the visible light. Your eyes see the different wavelengths of the visible light as different colors. Photosynthesis in green plants uses solar energy kinds of intermediate wavelengths known as light(380-750 nm) known as light which is the tiny portion of the electromagnetic spectrum that a human can see. Most effective are violet, blue, orange, and red light: least effective is green light. Each chloroplast has several kinds of pigments, with each kind absorbing best at wavelengths different from the others.

Chlorophyll molecules capture energy from a range of varying wavelengths of light. The total light composition is made up of visible white light commonly known as BIVGYOR (Blue, Indigo, violet, green, yellow, orange, and red), and the visiblePage 5


spectrum which consists of ultraviolet (before violet) and infrared (after red). The spectrum of colors carry different amount of energy depending on the wavelength, the higher the energy level. In visible spectrum, colors on the right side have longer wavelengths than those on the left side. Therefore, the YOR colors contain less energy than the VIB colors. During photosynthesis, all spectra of colors are absorbed by green plants, except green which is reflected. They get most of their requirements from BIV colors although ROY may also be utilized.

2. Chloroplasts Photosynthesis occurs in the chloroplasts. These are green spherical bodies located in the middle portion of the leaves. These bodies are made up of two parts: the stroma and the grana. The stroma is a large, central compartment which contains fluids and enzymes. The grana contains the thylakoids, flattened discshaped bodies filled with chlorophyll and other pigments. The first phase of photosynthesis occurs in the grana, while the second phase occurs in the stroma.

3. Pigment Chlorophyll (green pigment) and carotenoids (yellow to orange pigment) are two pigments important in photosynthesis. Chlorophyll molecules are considered primary light gatherers because they capture light energy and use it directly to power photosynthesis. They may be chlorophyll a (blue green) or chlorophyll b ( yellow green). The carotenoids are second only to the chlorophyll in order of importance. Carotenoids are also called an accessory organ because they are not that important unlike the chlorophyll. Carotenoids absorb light energy whichPage 6


the chlorophyll cannot gather, then later these carotenoids pass the energy to chlorophyll. Plants contain pigments. Pigments are colored substances that reflect or absorb light. These pigments help plants to gather the suns energy. The principal pigment of green plants is chlorophyll. Chlorophyll absorbs light very well in the blue and in the red regions of the spectrum. However, chlorophyll does not absorb light in the green region of the spectrum. This is what gives chlorophyll its green color and explains why plants appear green to the human eye. Because light is form of energy, a compound that absorbs light also absorbs the energy from the light. When chlorophyll absorbs light, much of that energy is transferred directly to electrons in the chlorophyll molecule. In other words, chlorophyll absorbs light energy and produces its own highenergy electrons.

4. Water 5. Carbon Dioxide 1C. Phases of PhotosynthesisLight- Dependent Reactions There are two stages of photosynthesis. The first stage consists of the light-dependent reactions. Light-dependent reactions got their name because they require the direct involvement of light.Page 7


Light reaction is the first stage of photosynthesis. Light reaction takes place in the chloroplast, particularly in the thylakoid membrane. In the membrane are two photosystems, namely, Photosystem I and Photosystem II. Inside of the membrane, is the lumen and outside the membrane is the stroma. Like any other kind of membrane, the thylakoid membrane is also a lipid bilayer. Meaning, the outside, is hydrophilic and the inside is hydrophobic.

Two Electron Pathways For us to further understand the different reactions in the process photosynthesis, let us discuss first the ways on how the electron are being transported and to be able to accept energy from the sun in the form of photons.

A. Non-cyclic electron pathway The first step in the light reactions is the absorption of photons from the sun by Photosystem II. These photons energy are passed from one carotenoid to another until it reaches the chlorophyll. This energy is accepted by the primary electron acceptor. The Primary electron acceptor sends the electron to the electron transport chain passing through a proteins, Plastoquinone and plastocyanin. As the electron passes through the ETC (electron transpot chain)energy is captured and stored in the form of hydrogen gradients. When these hydrogen ions flow down their electrochemical gradient through ATP synthase, ATP production occurs. The produced ATP will be later used in the calvin cycle. After passing through the ETC, the electron goes to Photosystem I. The same thing happens. But this time, instead of photons, the electron from PSII (Photosysem II), will energize the chlorphylls. The electron acceptors in PSI pass their electrons to NADP+ molecules. Each one accepts two electrons and an H+ to become a reduced form of the molecule, that is, NADPH.

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B. Cyclic electron pathway In the non-cyclic electron pathway, also called cyclic phosphorylation, a single photosystem is involved. When an electron is energized by the absorption of light, it is ejected from the photosystem reaction center. The electron then passes through an electron transport system and finally back to the reaction center. The energy released during this electron transport is ATP. The cyclic phosphorylation occurs because in the calvin cycle, more ATP is required. The non-cyclic phosphorylation produces equal amounts of ATP and NADPH. Because of this, it is also necessary for the cyclic phosphorylation to occur because it only produces ATP.

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The Light-Independent Reactions/ calvin cycle The second set of reactions in photosynthesis is called the light-independent reactions or calvin cycle (named after Melvin Calivin). This set of reactions does not directly involve light. The light-independent reactions convert the energy from the ATP and NADP+ into a form that can be stored indefinitelysugars. The calvin cycle is a series of reactions that results in the conversion of carbon dioxide to into organic molecules. It occurs in the stroma, which is the area that surrounds the thylakoid membrane Step 1: Carbon Fixation During this reaction, carbon dioxide from the atmosphere is attached to RuBP, a five carbon molecule. Which result to a six carbon molecule which then eventually splits into two molecules of 3-Phosphoglycerate.

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Step 2: Carbon Dioxide Reduction The first 3-carbon molecule in this cycle called 3PG(3phosphoglycerate) undergoes reduction to G3P in two steps. As 3PG becomes G3P, ATP becomes ADP + P and NADPH becomes NADP+. This signifies the reduction of carbon dioxide to a carbohydrate. Energy and electrons needed for this reduction reaction which are supplied by ATP and NADPH.

Step 3: RuBP Regeneration Take note that in the calvin cycle reactions, it needs three turns of the cycle to allow 1 G3P to exit. For every turn of the calvin cycle, five molecules of G3P are used to reform three molecules of RuBP and the cycle continues. This reactions also uses some of the ATP produced by the light reactions. The carbohydrate produced by the calvin cycle is now the ultimate nutrient source for most living thigs on earth.

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1E. ATP SYNTHESISATP plays an important role in every living cell. ATP is also a central compound in both respiration and photosynthesis. As you have read, ATP is produced from ADP in both mitochondria and chloroplasts. In both places, ATP is made by an electronPage 12


transport chain that is associated with a membrane. Is this just a coincidence or is there some special connection between these membranes and ATP production?

1F. Membranes and ATPIn the early 1960s, British scientist Peter Mitchell thought that he could explain the connection between the membranes and ATP. Mitchell suggested that the changes that took place in the membrane during electron transport were essential in the production of ATP. In fact, Mitchell argued, the purpose of electron transport is to produce different electrical charges on each side of the membrane and then to use those differences to power the production of ATP.

1G. ATP and PhotosynthesisRecall that during light-dependent reactions of photosynthesis, some of the high-energy electrons are passed to an electron- carrying chain. At the end of the chain, some of the energy is used to split water. When the water molecules is broken apart, the oxygen is released and the electrons are returned to chlorophyll. But where do the hydrogen ions go? The hydrogen ions are released inside the photosynthetic membrane when the electrons are removed. This produces a high concentration of hydrogen ions (H+) inside the membrane, giving it a positive charge. Outside the membrane, there is low concentration of hydrogen ions, which gives it a negative charge.

1H. ATP and RespirationDuring the process of respiration, something very similar takes place in the mitochondrion. The electron carriers, NADPH and FADH2, pass their electrons to the electron transport chain. As these electrons are passed down the chain, they provide the energy to pump hydrogen ions out of the membrane. Because there are more hydrogen ions outside the membrane, there is a morePage 13


positive charge. Inside the membrane, where hydrogen ions, there is a negative charge.

there

are

fewer

1I. ChemiosmosisThe difference in electrical charges across the photosynthetic and mitochondrial membranes is a source of energy. This energy could be used to attach a phosphate to ADP to make ATP. The photosynthetic and mitochondrial membranes are impermeable to hydrogen ions. However, there is an enzyme that seems to have a channel right through its center. The channel allows the hydrogen ions (H+) to pass through it, drawn by the strong negative charges (from OH- ions) on the other side of the membrane. This enzyme is called an ATP-synthesizing enzyme , meaning an ATP-maker. As the hydrogen ions pass through this channel, the energy from the movement is used to attach a phosphate to ADP, making ATP. This process of ATP formation in chloroplasts and mitochondria is called chemiosmosis.

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1J. Importance of PhotosynthesisThe importance of Photosynthesis can be understood with respect to our breathing process. The breathing process keeps us alive and Photosynthesis provides us oxygen to breathe in. The process of Photosynthesis and Respiration are inter-related and serve one another. While Photosynthesis requires carbondioxide and releases oxygen to produce glucose, Respiration needs oxygen while inhaling and releases carbon-dioxide while exhaling. Photosynthesis happens during the day time when the sun shines because the plants require sunlight to produce energy. On the other hand Respiration happens all the time as long as a living creature is alive. However, unlike other living creatures, the plants breathe once in a day. During night, when there is no sunlight, the stomata (pores through which sunlight and carbon-dioxide enter the leaves) are closed and the leaves breathe releasing carbondioxide in the air.

Photosynthesis and Environment The level of carbon-dioxide in the environment largely depends on the process of Photosynthesis. The process of Photosynthesis again depends on the number of plants and trees we have. Excessive increase or decrease in the level of carbon-dioxide can bring forth disastrous results on the planet earth. Industrial revolutions and technical progress have led to too many factories, production houses, buildings, roads etc thereby increasing the use of fuel and release of industrial waste and carbon-dioxide which can be very harmful for the environment.Page 15


Just the way, an increase in the carbon-dioxide level may harm the environment; similarly decrease in the level may cause the planet to freeze as CO2 helps in keeping our planet warm and live-able. Photosynthesis helps in maintaining the balance of the carbon-dioxide level in nature by taking in CO2 in the day time (and simultaneously supplying oxygen for other living beings) and breathing it out in the night.

Photosynthesis and Life

Photosynthesis is directly related to the life and survival of all the other living creatures on earth. It not just supplies oxygen without which breathing and being alive would be difficult, but it also supplies food and energy to all.

Among all the living organisms on planet earth only plants are capable of producing their own food and deriving energy from it. No other living creature can produce their food and thus, depend on plants or other creatures which feed on plants to survive. Therefore, by producing energy the plants supply all the necessary nutrients and energy directly and/or indirectly to the other living creatures. The production of this energy is possible through Photosynthesis.

Conclusion Going by all the above the facts, it is evident how Photosynthesis plays an important role in regulating the life cycle on earth. Now you may realize the importance of Photosynthesis every time you breathe in and out and take a look at the plants at your backyard or neighborhood or front garden. The sunlight, water and the plants work together to supply thePage 16


raw source of energy to us and help us breathe in oxygen to live on. Life without Photosynthesis, would, thus be impossible.

Test YourselfChoose the best answer for each question 1. The reactants of photosynthesis include all of the following except ________. a)Solar energy B) Carbohydrates C) Water D) Carbon Dioxide

2. Which of the following is NOT a product of the lightdependent reactions? a) ATP b) Oxygen c) Glucose d) NADPH

3. The light-dependent reactions take place in the ______. a) Stroma b) Thylakoids c) Cytosol d) Cristae

4. The First step in the calvin cycle is ________. A) Carbon Fixation b) PGAL reduction c) RuBP regeneration d) ATP synthesis

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5. The Calvin Cycle takes place in the ______ of chloroplasts. a) Thylakoid membrane b) Stroma c) Inner membrane d) Outer membrane

6. The Calvin Cycle uses _______, which are products of the light reactions of photosynthesis. a)Carbon Dioxide and Water b)RuBP and Oxygen c) ADP and NADH d)ATP and NADPH

7. The calvin cycle begins by the attachment of carbon dioxide to which of the following? a) RuBP b) glucose c) 3-phosphoglycerate d) none of the above

8. Glyceraldehyde-3- phosphate and 3-phosphoglycerate are three carbon molecules produced during the calvin cycle. a) True b) False

9. The Calvin Cycle functions to construct carbohydrates from carbon dioxide. a) True b) FalsePage 18


10. Which of the following is a requirement for photosynthesis? a) Sunlight b) Water c) Pigment d) All of the above

Answer the following: 1. What is the overall reaction for photosynthesis? 2. Where does the energy for photosynthesis come from? 3. What plant pigments are involved in photosynthesis? 4. Explain why chlorophyll appears green to us in terms of what happens to different wavelengths of light that strike a chlorophyll molecule. 5. How does the amount of energy in light change as the wavelength increases? 6. Which colors of light are most effective for photosynthesis? Explain why. 7. In what organelle of a plant cell does photosynthesis take place? 8. What are the two stages of photosynthesis? 9. In which part of the chloroplast does each stage occur?Page 19


10.

What happens to water molecules in the light reactions?

11. What photosynthesis waste product is formed in the light reactions? 12. What two products of the light reactions are used up in the Calvin cycle? 13. What happens to carbon dioxide molecules in the Calvin cycle reactions?14. How can cells store the sugar that is produced in photosynthesis?

BibliographyBooks: Campbell Biology, Neil A. Campbell, Jane B. Reece Cell Biology- Short Course- Stephen R. Bolsover, Elizabeth A. Shephard, Hugh A. White, Jeremy S. Hyams Biolgy- Sylvia S. Mader

Websites: http://biology.clc.uc.edu/courses/bio104/photosyn.htm http://www.johnkyrk.com/photosynthesis.html http://www.youtube.com/watch?v=slm6D2VEXYs http://www.youtube.com/watch?v=-rsYk4eCKnA&feature=relmfu http://www.youtube.com/watch?v=yfR36PMWegg&feature=relmfu http://www.youtube.com/watch?v=7ynX_F-SwNY&feature=relmfuPage 20


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Module - Photosynthesis