122
4
CHAPTER
BIOENERGETICS Major Concepts: 4.1
Photosynthesis (8 Periods)
Number of allotted teaching periods: 14
4.1.1 Role of Light 4.1.2 Role of Photosynthetic Pigments – Absorption Spectrum and Action Spectrum 4.1.3 Role of Carbon dioxide in Photosynthesis 4.1.4 Role of Water in Photosynthesis 4.1.5 Mechanism of Photosynthesis 4.2
Cellular Respiration (5 Periods) 4.2.1 Aerobic and Anaerobic Respiration 4.2.2 Mechanism of Respiration 4.2.3 Synthesis of ATP – Chemiosmosis and Substrate-level Phosphorylation
4.3
Photorespirtion (1 Period)
Living things cannot grow, reproduce, or exhibit any of the characteristics of life without a ready supply of energy. Energy, which is the capacity to do work occurs in many forms as light energy, electrical energy, heat energy, etc. Most of the actions of an organism involve a complex series of energy transformations. For example potential energy derived from the chemical energy of food molecules is converted to kinetic energy in the muscles at work. The total energy of the universe does not change. An organism is an open system, it can exchange matter with its surroundings. All metabolic reactions involve energy transformations. So the quantitative study of energy relationships in biological system is called bioenergetics. Biological energy transformations obey the laws of thermodynamics.
96
BIOLOGY XI: Chapter 4, BIOENERGETICS
4.1 PHOTOSYNTHESIS Photosynthesis is the conversion of light energy to chemical energy. Photosynthesis comes from two Latin words photo means light and synthesis means to put together, or building up. Through photosynthesis plants and algae produce food for themselves and for all other living things. The sun is the ultimate source of almost all the energy that powers life. Plants and other photosynthetic organisms capture a tiny portion of the sun s energy, and in the process of photosynthesis convert it into chemical energy of organic molecule i.e. sugar. When photosynthesis occurs, oxygen is released and carbon dioxide is absorbed. All the life forms exist and maintained on this planet the Earth by the process of photosynthesis. Light, photosynthetic pigments, carbon dioxide and water play important role in the process of photosynthesis. The overall reaction of photosynthesis can be summarized as follows:
4.1.1 ROLE OF LIGHT Sun is the only source of energy on Earth. Sunlight is a form of energy known as electromagnetic energy also called radiation. Electromagnetic energy travels as waves. The array of electromagnetic waves coming from the sun varies in Fig: 4.1 Photons length. These wave lengths are measured from the crest of one wave to the crest of the next and are measured in nanometers. The full range of electromagnetic radiation in the universe is called electromagnetic spectrum. Visible light is only a small part of the spectrum. It is called visible light because it is the part of the spectrum that the eye can see. Visible light can be resolved into six spectral regions: violet (390-430 nm), blue (430-470 nm), green (470-560 nm), yellow (560-590 nm), orange (590-620 nm), red (620-780 nm). Although electromagnetic energy travels as waves, it also behaves like individual particles discrete packets of energy called photons. Shorter wavelength radiation has photons of a high-energy content than long wave length radiation. Photons of visible light have just the right amount of energy to promote electrons to a higher electron shell in atoms. Only about 42% of solar radiation passes through the Earth s atmosphere and
BIOLOGY XI: Chapter 4, BIOENERGETICS
97
reaches its surface. Not all the visible light falling on the leaves is absorbed. Only one percent is absorbed and the rest of the light is reflected or transmitted. During photosysthesis plants make sugar using carbon dioxide and hydrogen from water. This requires energy. This energy (ATP and NADPH) and hydrogen are supplied by the reactions, which take place in light. ATP is made when energy is used to bind another phosphate to ADP, a process called phosphorylation. energy is supplied by light, and the process is therefore called photophosphorylation. NADPH is made from NADP in a process called Fig: 4.2 The Interaction of Light with Chloroplast reduction. The hydrogen comes from water. This also required energy (photolysis), which is provided by light. When light shines on leaf, high-
Fig: 4.3 Electromagnetic Spectrum
98
BIOLOGY XI: Chapter 4, BIOENERGETICS
energy electrons are released by the chlorophyll molecules. It is the energy from these electrons that is used in making of ATP and NADPH.
4.1.2 ROLE OF PHOTOSYNTHETIC PIGMENTS Pigment is any substance that absorbs light energy. All the pigments that take part in photosynthesis are present in the chloroplasts. The pigments are carotenoids, chlorophyll and phycobilins. Carotenoids Carotenoids are lipid compounds, which are yellow, orange, red or brown pigments. They absorb light strongly in the blue-violet range. They are seen in leaves before leaf fall, present in some flowers and fruits e.g. red skin of tomato is due to carotene. There are two types of carotenoids: carotenes and xanthophylls. The most widespread and important carotene is (beta) carotene, which is familiar as the orange pigment of carrot. Xanthophylls are yellow in colour and contain oxygen along with carbon and hydrogen (C40H56O2). Lutein is a widely distributed xanthophylls which is responsible for yellow colour of foliage in autumn. The function of carotenoids are: (a) act as accessory pigment as they transfer light energy to chlorophyll a (b) protect chlorophyll from excess of light (c) protect chlorophyll from oxidation by oxygen produced in photosynthesis (d) attract insects, birds and other animals for pollination and dispersal. Chlorophyll There are several types of chlorophyll e.g. a,b,c,d,e,f. They differ in their molecular structure from one another. Chlorophylls absorb mainly violet, blue, orange and red wavelengths. Green and yellow are least absorbed and are transmitted or reflected, so plants appear green in colour, unless masked by other pigments. Chlorophyll a occurs in all photosynthetic organisms except pigment containing bacteria. Chlorophyll b occurs in all autotrophic organisms except brown, red and blue green algae. Chlorophyll c, d are found only in algae and in combination with chlorophyll a. Chlorophyll e and f are restricted to photosynthetic bacteria and are known as bacteriochlorophylls. Molecular formula of chlorophyll a and b: Chlorophyll a = C55 H72 O5 N4Mg Chlorophyll b = C55 H70 O6 N4Mg
BIOLOGY XI: Chapter 4, BIOENERGETICS
99
Science Titbits Phycobilins are mainly found in blue green algae (cyanobacteria) and red algae. These are protein linked pigments and get destroyed by heat. These are water soluble. The red phycobilins are called phycoerythrin, found in red algae. The blue one is called phycocyanin found in photosynthetic bac teria. The pigment part is called phycobilins. They are also accessory pig ments i.e. transfer the absorbed light to chlorophyll.
Basic Knowledge of Chemistry Related to Chlorophyll Benzene is an aromatic com pound, C6H6 having alternate single and double bonds. One or more carbon atoms in ben zene can be replaced by a heteroatom (i.e. other than C and H) e.g. Pyridine, where a carbon atom has been re placed by nitrogen. Pyrrole is a five sided unsaturated ni trogen containing compound. Porphin consists of four pyr role (tetrapyrrole) like rings linked by four CH group (methene bridge CH =) in an alternating double and single Pyrrole Rings In Porphin bonds. Porphyrins are the de rivatives of Porphin. The por phyrins that are found in nature are compounds in which side chains are substituted for the eight hydrogen atoms, numbered in the pyrrole rings in porphin. The porphin structure contains 16 membered ring formed of 12 carbon and four nitrogen atoms contributed by four pyrrole rings. Mg or Fe may be added to porphin. Mg porphyrin e.g. chlorophyll. Fe porphyrin e.g. haeme, cytochrome.
100
BIOLOGY XI: Chapter 4, BIOENERGETICS
Chemical Structure of Chlorophyll It consists of four pyrrole rings having Mg in the centre. The chlorophyll is a magnesium porphyrin. There are two side chains in the chlorophyll molecule. Acid chain is a methyl (CH3) ester:
Hydrocarbon chain is a long chain of alcohol phytol i.e. C20H39 (is an ester linkage with propionic acid. Phytol consists of four isoprene units). Phytol is insoluble and serves to anchor the molecule in the membrane of the granum.
Science Titbits Prokaryotes have no chromoplasts. The photosynthetic prokaryotes have unstacked photosynthetic membrane which works like thyla koid.
Fig 4.4 Structure of Chlorophyll
Role of Photosynthetic Pigments in the Absorption and Conversion of Light Energy When a molecule of chlorophyll or other photosynthetic pigment absorbs light it is said to become excited. The energy from the light is used to boost electrons to a higher energy level. The energy of the light is now trapped in the chlorophyll and has been transferred to chemical energy. This excited state is unstable and the moelcule will tend to return to its unexcited state. During photosynthesis energy is released. In the living plant the energy that is released can be passed to another chlorophyll molecule. Alternatively, the excited electron itself may pass from the chlorophyll
BIOLOGY XI: Chapter 4, BIOENERGETICS
101
molecule to another molecule called an electron acceptor. As electrons have a negative charge, this will leave a positively charged ‘hole’ in the chlorophyll molecule.
Loss of electrons is known as oxidation and gaining electrons is reduction. Chlorophyll is therefore oxidized and the electron acceptor is reduced. Chlorophyll replaces its electrons by removing low energy electrons from another molecule described as an electron donor. The wavelengths of light appear in different colours when passed through a prism .You must have seen a rainbow. It shows the colours of visible light. Spectrophotometer (fig. 1.4) is an instrument, which is used to measure relative abilities of different pigments to absorb different wavelength of light. Absorption Spectrum Different pigments absorb different wavelengths of light and these wavelengths are not absorbed at the same rate. A curve obtained by plotting the amount of absorption of different wavelengths of light by a particular
Fig: 4.5 Absorption Spectrum
102
BIOLOGY XI: Chapter 4, BIOENERGETICS
pigment is called absorption spectrum of that pigment. The main photoreceptors are chlorophyll a and b and both absorb blue violet (430nm) and orange-red (670 nm), region of the visible spectrum. Chlorophyll a shows two minor peaks at about 680 and 700 nm. The peak is more for chlorophyll b near 450 475nm. Q. How does the absorption spectrum of chlorophyll a differ from that of chlorophyll b ? Action Spectrum It is a graph showing the measure of effectiveness of light of various wavelengths in photosynthesis. How to obtain an action spectrum? First the plant is illuminated with light of different wavelength. Because photosynthesis gives off oxygen, we use the production rate of oxygen as a means to measure the rate of photosynthesis at each wavelength of light or consumption relation of carbon dioxide can also be used.
Fig: 4.6 Action Spectrum
The first action spectrum was obtained by German biologist T.W. Engelmann in 1883. He passed light through a prism. This light illuminated a filament of Spirogyra. Aerobic bacteria moved toward the blue and red portion of the spectrum, as the cells of this region produced most of the oxygen. Arrangement of Photosynthetic Pigments in the form of Photosystems Photosynthetic pigments are organized into clusters, called photosystems. Each photosystem has a pigment complex composed of chlorophyll a and b molecules and accessory pigments such as carotenoid pigment. The closely packed pigment molecules in the photosystem serve as an antenna for gathering solar energy. Solar energy is passed from one
BIOLOGY XI: Chapter 4, BIOENERGETICS
103
Fig: 4.7 Light Harvesting Photosystem
pigment to the other until it is concentrated into one of two particular chlorophyll a molecules the reaction centre chlorophylls. Electrons in the reaction centre chlorophyll molecules, become so excited that they escape and move to a nearby primary electron acceptor molecule. Reaction centre has one or more molecules of chlorophyll, primary electron acceptor, and associated electron carriers of electron transport system. Electron transport system plays a role in generation of ATP by chemiosmosis. Light energy absorbed by the pigment molecules of antenna complex is transferred ultimately to the reaction centre. There, the light energy is converted into chemical energy. The light dependent reactions that occur in the thylakoid membranes require the participation of two light gathering units called photosystem I (PSI) and photosystem II (PSII). The photosystems are named for the order in which they were discovered and not for the order in which they occur in the thylakoid membrane. Both the systems contain an antenna complex or light harvesting complex. The light harvesting complex contains 200 to 300 pigment molecules and collect light energy as shown in fig. 4.7. Different pigments collect light of different wavelengths, making the photosystem more efficient. All the energy is transferred from molecule to molecule and finally to reaction centre of a specialized form of chlorophyll a known as P700 in PSI and P680 in PSII. P stands for pigment, their absorption peaks are at wavelengths of 700nm and 680nm respectively. Both the wavelengths are of red light.
104
BIOLOGY XI: Chapter 4, BIOENERGETICS
4.1.3 ROLE OF CARBON DIOXIDE IN PHOTOSYNTHESIS Organisms that have an inorganic source of carbon namely carbon dioxide are called autotrophic organisms (autos, self). Air contains about 0.03 to 0.04 percent of carbon dioxide. Land plants use this atmospheric carbon dioxide for photosynthesis. Dissolved carbon dioxide, bicarbonates, and soluble carbonates are present in water, which are used by aquatic photosynthetic organisms as carbon source. Carbon dioxide from the environment is accepted by the 5C sugar ribulose bisphosphate (RuBP). The 6C product is unstable. It breaks into two 3C structure. At the end of the reaction it becomes PGAL (phosphoglycereldehyde). PGAL is used to reform RuBP so that absorption of carbon dioxide continues. The PGAL is used to make glucose and other organic compound. The role of carbon dioxide is that carbon provided by carbon dioxide becomes part of glucose.
4.1.4 ROLE OF WATER IN PHOTOSYNTHESIS In 1930, Van Niel hypothesized that plants split water as a source of hydrogen, releasing oxygen as a byproduct. This observation was based on investigations on photosynthesis in bacteria that make carbohydrates, from carbon dioxide, but do not release oxygen. Neil's hypothesis was confirmed in 1940, when for the first time O18 in biological research was used. In first experiment water was made of O18. The water tagged O18 was added to an alga suspension. The oxygen, evolved during photosynthesis, was found to be radioactive. It was separated and identified. In another experiment carbon dioxide with tagged O18 was added. The oxygen evolved contained none of the isotopes. Thus the source of evolved oxygen was proved to be water.
Importance of Water in Photosynthesis 1. Water is one of the raw materials for photosynthesis. Water replaces the electron lost by the P680 during photolysis. Water broken by light gives
BIOLOGY XI: Chapter 4, BIOENERGETICS
105
H+ and the ultimate end of H+ is phosphorylation of ADP to ATP and NADP to NADPH. 2. Carbon dioxide is absorbed due to film of water over mesophyll. 3. Oxygen is produced by photolysis of water.
4.1.5 MECHANISM OF PHOTOSYNTHESIS The study of biochemistry of photosynthesis shows that it is a process that provides a link between the two worlds, the living and nonliving. The nonliving world provides water, carbon dioxide and energy from the sunlight. Photosynthesis is a redox process. It takes place according to the chemical reaction shown in fig. 4.8.
Fig. 4.8 Redox Process of Photosynthesis
Fig: 4.9 An Overview of Photosynthesis
106
BIOLOGY XI: Chapter 4, BIOENERGETICS
This is a complex chemical process, completed by a series of simple steps or reactions. The process of photosynthesis has been divided into two sets of reactions. The first set of photosynthetic reactions is called light dependent reactions (light reactions) because they can take place only in the presence of light. The light-dependent reactions occur in the thylakoid membrane where the pigment chlorophyll a and chlorophyll b are located. These pigments absorb violet, blue and red light better than the light of other colours. The light dependent reactions are the energy capturing reactions. The light dependent reactions capture solar energy. The second set of reactions is called the light independent reactions (dark reactions) because these can take place whether light is present or not provided NADPH and ATP of the light reactions are available. Light Dependent Reactions (Light Reactions) It occurs in the grana of a chloroplast. ATP production during photosynthesis is sometimes called photophosphorylation because light is involved. Photosystem II: When light strikes the chlorophyll molecules, its energy causes an electron in the reaction centre chlorophyll P680 to be boosted. The electron is said to be excited because it possesses greater energy than the normal one. This excited electron is captured by the primary electron acceptor of PSII. Photolysis: The activated P680 is an oxidizing agent so strong that it is capable of oxidizing an oxygen atom that is part of a water molecule. In a reaction catalyzed by a unique enzyme water is split by a process called photolysis (breaking by light) into its components; two electrons, two protons + (H ) and oxygen. Each electron is donated to a P680 molecule and protons are released into the thylakoid interior space. Because oxygen does not exist in atomic form the oxygen produced by splitting one water molecule is ½ O2. Two water molecules must be split to yield one molecule of oxygen, which is released into the atmosphere. The name photolysis is somewhat misleading because it implies that water is broken by light. Actually, light breaks water indirectly by activating P680 molecules.
BIOLOGY XI: Chapter 4, BIOENERGETICS
107
1st Electron Transport Chain (ETC): The absorbed light energy causes the chlorophyll molecule P680 to give up a pair of electrons. Each of the photo excited electrons passes from primary electron acceptor of PSII to PSI via an electron transport chain. This chain consists of (i) an electron acceptor molecule plastoquinone (PQ) (ii) two cytochromes: cytochrome b (cyt b) and cytochrome f (cyt f) (iii) a copper containing protein called plastocyanin (PC). Production of ATP: As electrons pass through the chain their energy goes on decreasing and is used by the thylakoid membrane to produce ATP from phosphate and ADP. This ATP generated by light reactions will provide chemical energy for the synthesis of sugar during Calvin cycle. Photosystem I: When P700 molecule absorbs a photon of light, electrons are boosted to a higher energy level. P700 molecule passes the electron to a primary acceptor, creating a “hole”. The hole of P700 is filled by the pair of electrons received from the P680 (photosystem II) via electron transport chain of PSII. 2nd Electron Transport Chain: The primary electron acceptor of photosystem I passes the photoexcited electrons to a second electron transport chain. The electrons are accepted by ferredoxin (Fd). It is an iron containing protein. An enzyme called NADP reductase (flavoprotein enzyme) transfers
Fig: 4.10 Z Scheme
108
BIOLOGY XI: Chapter 4, BIOENERGETICS
the electrons from Fd to NADP. This is a redox reaction. NADP combines with electrons and hydrogen ions to form NADPH (reduced). The NADPH will provide reducing power for the synthesis of sugar in the Calvin cycle. Z Scheme: The path of electron transport through the two systems during non-cyclic photophosphorylation is known as Z-Scheme due to its shape. It takes place in the granum of the chloroplast. Light Independent Reactions (Dark Reactions) The light independent reactions are the second stage of photosynthesis. They take their name from the fact that light is not directly required for these reactions to proceed. These reactions occur when CO2 has entered the leaf and ATP and NADPH have been produced during the light dependent reactions. In this stage of photosynthesis, NADPH and ATP are used to reduce carbon dioxide. CO2 becomes CH2O within a carbohydrate molecule. Electrons and energy needed for this reduction synthesis are supplied by NADPH and ATP. The reduction of carbon dioxide occurs in the stroma of a chloroplast by a series of reactions known as the Calvin cycle. Calvin Cycle Carbon fixation has been explored by Melvin Calvin and co-workers at the University of California. Melvin Calvin, won the Nobel Prize in 1961 for this work. The Calvin cycle can be divided into three phases, carbon fixation, reduction, regeneration of carbon dioxide acceptor RuBP. Carbon Fixation: One of the key substance in this process is a five carbon phosphorylating sugar called ribulose bisphosphate (RuBP). It is capable of combining with carbon dioxide with the help of RuBP carboxylase (an enzyme) also known as rubisco. Six carbon intermediate molecules are formed during the incorporation process. It is unstable and exists for such a short time that, it has not been possible to isolate it. It breaks down to form two molecules of 3-phosphoglycerate i.e., phosphoglyceric acid (PGA), a phosphorous containing compound with three carbon atoms. The initial reaction sequence, in which carbon dioxide combines with organic molecule i.e. RuBP is called carbon fixation. Because the initial carbon fixation reaction is three carbons compound that Calvin cycle is also known as C3 pathway. Reduction: Each molecule of 3-phosphoglycerate (PGA) is converted to 1, 3-bisphosphoglycerate (PGAP). These are then transferred to glyceraldehyde 3-phosphate (PGAL) and in the process a phosphate group is given off. The reducing agent is NADPH. Water of light reaction is also formed in the reaction.
BIOLOGY XI: Chapter 4, BIOENERGETICS
109
The NADP and ADP are available again for the light reaction to accept hydrogen electrons or high energy bonds. Since combining each molecule of CO2 with RuBP gives rise to two molecules of PGA. This sequence of reactions requires two ATP and two NADPH units for each molecule of CO2 that is incorporated into carbohydrate.
Fig 4.11 Calvin Cycle
Fig 4.12 Fate of the Atoms in Photosynthesis
For the CO2 incorporation process to continue, the majority of the PGAL molecules produced are used to regenerate the supply of RuBP molecules. The return to original point means that the process is cyclic. RuBP Regeneration: Starting with three molecules of RuBP, six molecules of PGAL are formed. Out of these five are ultimately used to reform a molecule of RuBP i.e. five three-carbon molecules (5 PGAL, C3) are transformed to three five carbon molecules (RuBP, C5). This process involved a complex cycle, containing 3,4,5,6 and 7 sugar phosphates. It is here that remaining ATP is used converting 5C sugar phosphate to RuBP. Nine ATP and six NADPH coming from the light reactions must be used in dark reactions to produce a net gain of one PGAL, which can be used to form glucose.
110
BIOLOGY XI: Chapter 4, BIOENERGETICS Skills: Analysing, Interpreting and Communication Draw the molecular structure of chlorophyll, showing the porphyrin head and phytol tail. Develop the graphical interpretation of the wavelengths of light along with the percentage absorption by chlorophyll a and b. Develop a flow chart for explaining the events of light independent re足 actions.
4.2
CELLULAR RESPIRATION
When an atom or molecule loses an electron, it is oxidized and the process by which it occurs is called oxidation. When an atom or molecule gains an electron, it is reduced and the process is called reduction. Energy stored in chemical bonds can be transferred to new bonds, with electrons shifting from one energy level to another. Oxidation-reduction reaction plays a key role in energy flow through biological system, because electrons that pass from one atom to another carry their potential energy position, i.e. they maintain their distance from the nucleus. In biological systems electrons do not travel alone from one atom to another but rather in the company of a proton. A proton and an electron together make-up a hydrogen atom. Thus oxidation-reduction is a chemical reaction usually involves the removal of hydrogen atom from one molecule and the gain of hydrogen atom by another molecule. Respiration is a series of complex oxidation-reduction reactions by which living cells obtain energy through the breakdown of organic matter. There are two kinds of respirations: aerobic respiration and anaerobic respiration.
4.2.1 AEROBIC AND ANAEROBIC RESPIRATION Aerobic respiration takes place in the presence of abundant gaseous oxygen. The overall reaction may be indicated as follows:
BIOLOGY XI: Chapter 4, BIOENERGETICS
111
Fig. 4.13 Anaerobic Respiration
It involves many steps. The energy stored in a glucose molecule is released. The glucose is oxidized. Hydrogen atoms are lost by the glucose and gained by oxygen to produce water. Anaerobic respiration takes place in many microorganisms (bacteria, yeast), muscle cells of vertebrates and in the cells of higher plants. Anaerobic respiration is incomplete oxidation-reduction reaction. It is also known as fermentation. It consists of glycolysis followed by the reduction of pyruvate by NADH to either lactic acid or alcohol and carbon dioxide. NADH is oxidized to NAD. The pathway operates anaerobically because after NADH transfers its electron to the pyruvate, it is “free” to return and pick up more electrons during the earlier reaction of glycolysis. Ethyl alcohol and CO2 is produced by anaerobic respiration in the yeast cells. In bacteria and animal tissue only lactic acid is produced. 1/5th of lactic acid is oxidized to CO2 and water with the release of energy. While 4/5th is converted to pyruvic acid and water in the presence of oxygen. Pyruvic acid is used to resynthesize glucose which is stored as glycogen (the animal starch).
112
BIOLOGY XI: Chapter 4, BIOENERGETICS
4.2.2 MECHANISM OF RESPIRATION It takes place in four stages: Glycolysis, Oxidation of Pyruvic Acid, Krebs Cycle, and Electron Transport Chain. Glycolysis Glycolysis (glyco , glucose, lysis , to breakdown) is the breakdown of glucose to pyruvic acid. It takes place in cytoplasm. It does not need oxygen. Glycolysis can be divided into two phases, preparatory phase and oxidative phase. Preparatory Phase: In this phase breakdown of glucose occurs and energy is expended. First glucose is converted to glucose 6 phosphate during which a phosphate group from ATP is transferred to glucose. Glucose 6 phosphate is converted to its isomer fructose 6 phosphate with the help of an enzyme. Another ATP molecule transfers a second phosphate group forming fructose 1, 6 bisphosphate. Then enzymatic splitting of fructose 1, 6 bisphosphate into two fragments takes place. Each of these molecules contains three carbon atoms. One is called 3 phosphoglyceraldehyde (PGAL) or glyceraldehyde 3 phosphate (G3P) while the other is dihydroxy acetone phosphate. These two molecules are isomers and in fact, readily interconverted by yet another enzyme of glycolysis. Oxidative Phase: Two electrons or two hydrogen atoms are removed from the molecule of 3 phosphoglyceraldehyde (glyceraldehyde 3-phosphate) (PGAL), which is oxidized and electron/H+ is transferred to a molecule of NAD which is reduced. Inorganic phosphate is present in the cell. From which a second phosphate is donated to the molecule forming 1,3 bisphosphaglycerate (PGAP). PGAP is converted to 3 phosphoglycerate (3PGA). During which a phosphate bond is transferred from PGAP to ADP forming ATP. 3 PGA is converted to 2-phosphoglycerate (2PGA). From 2 PGA a molecule of water is removed and phosphoenol pyruvate (PEP) is formed. PEP then gives up its “high energy” phosphate which converts ADP to ATP. The product is pyruvate (or pyruvic acid C3H4O3). It is equivalent to half glucose molecule that has been oxidized to the extent of losing two electrons as hydrogen atoms. Oxidation of Pyruvic Acid The oxidation of pyruvic acid takes place in two stages: Oxidation of pyruvic acid to form acetyl Coenzyme A and oxidation of the acetyl CoA.
BIOLOGY XI: Chapter 4, BIOENERGETICS
PGAL
PGAP
PGA
PEP
Fig 4.14 Glycolysis
113
114
BIOLOGY XI: Chapter 4, BIOENERGETICS
Formation of Acetyl Coenzyme A A carbon atom is split off, and is removed as carbon dioxide. Pair of electrons and their associated hydrogen is released which reduce NAD to NADH. A 2C fragment acetyl is produced. The 2C fragment is added to a cofactor, a carrier molecule called Coenzyme A (CoA), forming a compound called Acetyl CoA.
Pyruvic acid
Acetyle Coenzyme A
Oxidation of Acetyl Coenzyme A It begins with the binding of Acetyl CoA to 4C compound. The resulting 6C then passes through a series of electron yielding oxidation reactions. Two CO2 molecules are split off regenerating the 4C compound which is free to bind another acetyl group. The process is a continuous cyclic flow of carbon. In each turn of the cycle a new acetyl group comes into replace the two CO2 molecules, that are lost and more electrons are extracted. This cycle is called Citric acid cycle or Krebs cycle. It was discovered by British scientist Sir Hans Kreb. A complex oxidation-reduction involves NAD or NADP. NAD and NADP act as intermediate in cellular reactions involving electron transfer. Many of the electrons removed from reduced carbon compounds in various enzyme-catalyzed reactions are transferred to NAD to produce NADH. When a molecule of NAD or NADP gains electrons and becomes re足 duced, a hydrogen ion combines with it as well. Thus the reduced form is symbolized as NADH or NADPH. In actuality, another hydrogen ion be足 comes closely associated with each reduced molecule. Technically it is + more accurate to represent the reduced form as NADH + H or NADPH + + H . For convenience the simpler form NADH or NADH2 or NADPH or NADPH2 is generally used.
BIOLOGY XI: Chapter 4, BIOENERGETICS
115
Krebs Cycle The cycle, which oxidizes acetyl CoA, consists of nine reactions. It begins and ends with an oxaloacetate. At every turn of the cycle, an acetyl CoA enters and is oxidized to CO2 and H2O. The extracted electrons are temporarily housed within NADH molecules. In one reaction a different coenzyme called reduced Flavin Adenine Dinucleotide (FADH2) is used to carry the electrons. Steps of Krebs Cycle 1. The first step is the combination of acetyl CoA with oxaloacetate (4C) forming citrate (6C) or Citric Acid. 2. Citrate is converted to Isocitrate (6C).
1 2
8
3 7
4
6 5
Figure: 4.15 Krebs Cycle
116
BIOLOGY XI: Chapter 4, BIOENERGETICS
3. Isocitrate is oxidized by NAD to (alpha) Ketoglutarate (5C). NAD is converted to NADH and CO2 is released. 4. Oxidation of Ketoglutarate by NAD forms Succinyl CoA (4C), NADH is produced and CO2 is released. The coenzyme NADH carries electrons to the electron transport system. 5. Succinyl CoA is oxidized to Succinate (4C). GTP (Guanine triphosphate) is formed. GTP reacts with ADP to form ATP. 6. Succinate is oxidized to Fumarate (4C). FAD (Flavin Adenine Dinucleotide) is converted to FADH2.This coenzyeme also carries electrons to the electron transport system. 7. Fumarate combines with water to form Malate (4C). 8. Malate is oxidized by NAD to oxaloacetate which is the starting material of Krebs cycle and NADH is formed. This coenzyme NADH also carries electrons to the electron transport system. Electron Transport Chain Electrons from reduced coenzymes are passed to oxygen through series of electron carriers. It is called electron transport chain or system. Whenever hydrogen is removed from a substrate there are seven intermediate hydrogen acceptors to catch the atom. They are NAD, FAD, Ubiquinone also called coenzyme Q and four cytochromes i.e. b, c, a and a3. The electrons from NADH and FADH2 are passed to coenzyme Q. At this step an electron is split off the hydrogen atom. The proton which is released is free while the electron is passed successively from coenzyme Q to cyt. b, c, a and a 3. The steps in the hydrogen-electrons transfer can be summarized as follows: 1. The substance in the chain event is alternately oxidized and reduced. Oxidation is accomplished by the loss of hydrogen in the case of NAD, FAD and the coenzyme oxidation is accomplished by the loss of electrons in case of cytochrome b, c, a and a3. 2. Since two H atoms are released at a time and cytochrome b through a3 can accept only one electron at a time, so there is two cytochrome system to capture the electrons. 3. An electron and a proton are brought together after the final transfer from
BIOLOGY XI:
117
cyt a3. It produces hydrogen. 4. Molecular O2 is the hydrogen acceptor, and water is the final product. Four of the electrons are used to reduce a molecule of O2 gas and form water.
5. Energy is released at three places, NAD-CoQ, Cyt. b-c, aa3. The released energy is captured by ADP to form ATP, by chemiosmosis. 6. Electron transport chain is the main producer of ATP. For every pair of electrons that enters by way of NADH, three ATP results. For every pair of electrons that enters by way of FADH2, 2 ATP results.
b
c
Oxidative phosphorylation is the synthesis of ATP in presence of oxygen as a result of energy released by the electron transport system.
Science Titbits Ubiquinone is not a pro tein, but a small molecule solu ble in lipids and insoluble in water Cytochromes literally means cell colour . The re duced cytochromes are pink in colour. They are protein plus pigment molecules containing iron. They can gain or lose an electron.
Fig. 4.16 Electron Transport System
118
BIOLOGY XI: Chapter 4, BIOENERGETICS
4.2.3 SYNTHESIS OF ATP, CHEMIOSMOSIS AND SUBSTRATE LEVEL PHOSPHORYLATION Virtually every cell in every organism relies on energy from ATP molecules. Cells generate ATP by phosphorylation. A cell has two ways to do this: chemiosmosis and substrate level phosphorylation. Chemiosmosis The flow of electron in electron transport is usually tightly coupled to the production of ATP and does not occur unless the phosphorylation of ADP can also proceed. For a long time just how ATP synthesis is related to electron transport remained a mystery. In 1961 Peter Mitchell, a British biochemist proposed the chemiosmotic model. Because the respiratory electron transport chain is located in the plasma membrane of anaerobic bacterial cell, the bacterial plasma membrane can be considered comparable to the inner mitochondrial membrane. Mitchell was able to show that if bacterial cell were placed in an acidic environment (i.e. an environment with high hydrogen ion, or proton, concentration), the cell would synthesize ATP even if no electron transport were taking place. On the basis of these and other experiments Mitchell proposed that electron transport and ATP synthesis are coupled by means of a proton gradient across the inner mitochondrial membrane in eukaryotes (or across the plasma membrane in bacteria). Chemiosmosis is the production of ATP due to hydrogen ion gradient across a membrane; according to chemiosmotic model (fig. 4.17) the electron transport chain in the inner mitochondrial model + includes the proton pums. Protons are actually hydrogen ions (H ). The electron transport system consists of three protein complexes and two mobile carriers. The complexes are NADH dehydrogenase complex, cytochrome b-c complex and cytochrome oxidase complex. The two mobile carriers transport electrons between the complexes. As redox occurs the protein complexes use energy released from the electrons to actively transport + H ions from matrix into the intermembrane space of the mitochondrion. This established a strong electrochemical gradient; there are about ten times as many hydrogen ions in the intermembrane space than there are in the matrix. + The result is H gradient in the intermembrane space. This is a form of potential energy. In accordance with the general principle of diffusion the highly concentrated protons are expected to diffuse out. However they are prevented from doing so because the inner mitochondrial membrane is impermeable to H+ except through certain channels formed by an enzyme called ATP synthase. The cristae contain ATP synthase.
BIOLOGY XI: Chapter 4, BIOENERGETICS
119
Fig. 4.17 The electron transport system is located in the cristae. As electrons move from one complex to the other, hydrogen ions (H+) are pumped from the matrix into the intermembrane space. As hydrogen ions flow down their concentration gradient from the intermembrane space into the matrix, ATP is synthesized by the enzyme ATP synthase, which is a part of the ATP synthase complex. ATP leaves the matrix by way of a channel protein.
The potential energy of the H+ gradient is the source of energy needed to synthesize ATP. ATP synthase also contains the enzyme that catalyses the phosphorylation of ADP to form ATP. As the H+ ion move through the ATP synthase, driven inward by diffusion, their passage induces the formation of ATP. Thus by chemiosmosis, a cell couples the exergonic reactions by
120
BIOLOGY XI: Chapter 4, BIOENERGETICS
electron transport to the endergonic synthesis of ATP. Once formed ATP molecules diffuses out of the mitrochondrial matrix by the way of channel protein. Because the chemical formation of ATP is driven by a diffusion force similar to osmosis, so this process is called chemiosmosis. Peter Mitchell, received a Nobel Prize in 1978 for his chemiosmosis theory of ATP production in mitochondria and chloroplast. Substrate Level Phosphorylation The addition of inorganic phosphate to any organic molecule is called phosphorylation. When phosphate is enzymatically transferred from an organic substrate molecule it is called substrate level phosphoralytion. (Gk. phos, light and phoreus, carrier). The substrate is one of several substances produced as cellular respiration converts glucose to carbon dioxide. The reaction occurs because the bond holding the phosphate molecule in the substrate molecule is less stable than the new bond holding it in ATP. The reaction products are a new organic molecule and a molecule of ATP. Substrate level phosphorylation accounts for only a small percentage of the ATP that a cell generate. The coupled reactions link exergonic with endergonic reactions (fig.4.18b). Cells control energy flow by coupling reactions, so that energy released by exergonic reactions is used to drive endergonic reactions. Most energy in living cells involve pairs of coupled reactions linked by ATP. In the first coupled reaction, energy released by an exergonic reaction drives ATP synthesis, in the second, ATP hydrolysis drives an energy requiring reaction. Coupled reactions catalyzed by enzymes provide the energy and the specificity necessary to construct the different types of molecules needed by the cell (fig.4.18b). Importance of PGAL Glyceraldehydes 3-phosphate is an important step of glycolysis. Its oxidation produces 1,3-bisphosphoglycerate (PGAP) and 2NADH molecules, which leads to the formation of pyruvate. In the Krebs cycle of photosynthesis, one PGAL molecule is converted to glucose phosphate within the chloroplast. Glucose phosphate is then converted to starch. Fixed carbons leave the chloroplast in the form of dihydroxyacetone phosphate. It is formed from PGAL. In cytoplasm the chloroplast, dihydroxyacetone phosphate can be used to make the six-carbon sugars, glucose and fructose, which are then joined to form sucrose. It is transported to other parts of the plants.
BIOLOGY XI: Chapter 4, BIOENERGETICS
(a)
121
(b)
(c) Fig:4.18 (a)Substrate level Phosphorylation. (b) Because ATP is responsible for coupling many endergonic and exergonic reactions it is an important link between anabolism and catabolism in living cells. (c) ATP Production
122
BIOLOGY XI: Chapter 4, BIOENERGETICS
Amino acid R
Fig: 4.19 The Matabolic Pool Concept: When they are used as energy sources carbohydrates, fats and proteins enter degradative pathways at specific points. Degradation produces metabolites that can be used for synthesis of other compounds.
Skills: Analyzing, Interpreting and Communication Draw the flow charts showing the events of glycolysis and Krebs cycle. Illustrate the net energy output during glycolysis, oxidation of pyruvate and Krebs cycle.
BIOLOGY XI: Chapter 4, BIOENERGETICS
123
Respiration of Fats and Proteins Fats: We already know that glucose is broken down during aerobic cellular respiration. However, other molecules can also undergo catabolism. When a fat is used as an energy source, it breaks down to glycerol and three fatty acids. As figure 4.19 indicates, glycerol is converted to PGAL, a metabolite in glycolysis. The fatty acids are converted to acetyl-CoA, which enters the Krebs cycle. An 18-carbon fatty acid results in nine acetyl-CoA molecules. In the human body, oxidation of these acetyl-CoA molecules can produce a total of 109 ATP molecules. For this reason, fats are an efficient form of stored energyﾂ葉here are, after all, three long fatty acid chains per fat molecule. When they are used as energy sources, carbohydrates, fats and proteins enter degradative pathways at specific points. Degradation produces metabolites that can also be used for synthesis of other compounds. Proteins: The carbon skeleton of amino acids can also be broken down. The hydrolysis of proteins results in amino acids whose R-group size determines whether the carbon chain is oxidized in glycolysis or the Krebs cycle. The carbon chain is produced in the liver when an amino acid undergoes deamination, i.e. the removal of the amino group. The amino group becomes ammonia (NH3), which enters the urea cycle and becomes part of urea, the primary excretory product of humans. Just where the carbon skeleton begins degradation is dependent on the length of the R group, since this determines the number of carbon left after deamination.
4.3 PHOTORESPIRATION Decker (1959) observed that rate of respiration in the leaves of the green plants was much more in light than that in the dark. Thus respiration that occurs in green cells in the presence of light resulting in excess of carbon dioxide is termed as photorespiration. It needs oxygen and produce CO2 and H2O like aerobic respiration. However ATP is not produced during photorespiration. Rubisco Photorespiration is related to the functioning of the enzyme ribulose bisphosphate carboxylase. It is often called rubisco because it can have an oxygenase activity in addition to carboxylase activity. When more oxygen is present it acts as oxygenase and photorespiration starts. If carbon dioxide is more it acts as carboxylase and adds CO2 to ribulose bisphosphate to start the Calvin cycle.
124
BIOLOGY XI: Chapter 4, BIOENERGETICS
RuBP Reacts with Oxygen When rubisco acts as oxygenase it adds oxygen to ribulose 1,5 bisphosphate (RuBP). When RuBP reacts with oxygen, the reaction does not produce two molecules of phosphoglycerate but one molecule of phosphoglycerate and one molecule of phosphoglycolate. Phosphoglycolate loses its phosphate group to become glycolate. These reactions take place within the chlorplast. In peroxisomes glycolate is converted to glycine. Glycine is the simplest amino acid. Soon after its formation glycine diffuses into mitochondria. In mitochondria two glycine molecules are converted into serine and a molecule of carbon dioxide is formed. The pathway in which RuBP is converted into serine is called photorespiration. This pathway is named photorespiration because in the presence of sunlight (photo) oxygen is taken up and CO2 is produced. The process of photorespiration uses ATP and NADPH produced in the light reaction just like Calvin cycle. Photorespiration is reverse to Calvin cycle. Disadvantage of Photorespiration 1. It reduces the photosynthetic process. In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate by 25 percent. 2. Photorespiration is not essential for plant. Why Photorespiration Exists? The answer is that the active site of rubisco is evolved to bind both CO2 and O2 together. Originally it was not a problem as there was little oxygen in the atmosphere and the CO2 binding activity was the only one used. When the quantity of oxygen became more, the photorespiration started. Effect of Temperature on the Oxidative Activity of Rubisco A characteristic of RuBP carboxylase is that with increase in temperature and oxygen concentration, its affinity for carbon dioxide
BIOLOGY XI: Chapter 4, BIOENERGETICS
Glycolate
Glycolate
125
Glyoxalate
Fig: 4.20 Schematic representation of pathway involved in photorespiration in chloroplast, peroxisomes and mitochondria
Science, Technology and Society Connections Analyze the impact of photorespiration on the agriculture yield in the tropic climates. Photorespiration decreases net photosynthesis because a portion of CO2 fixed in photosynthesis escapes from the leave after it is fixed. Under cer足 tain conditions, up to 5% of the photosynthetic potential is lost in photores足 piratory metabolism. Thus photorespiration reduces dry mater production and agricultural yield in tropical climate. decreases and for oxygen increases. Thus with the increase in temperature more photosynthetically fixed carbon is lost by photorespiration. An Outline of C4 Photosynthesis Some plants use the enzyme pep-carboxylase instead of RuBP to fix CO2 to PEP (phosphoenolpyruvate- a C3 molecule), and the result is oxylate, a C4 molecule. It takes place in cytoplasm of mesophyll cells.
In temperate regions C3 crops such as wheat, potato, tobacco, sugar beat and soya bean grow more efficiently than C4 crops. The examples of C4 monocot plants are maize, sugar cane, sorghum and dicot plants are
126
BIOLOGY XI: Chapter 4, BIOENERGETICS
oxaloacetate
Fig. 4.21 In C4 plants with Kranz anatomy, CO2 is initialiy fixed in mesophyll cells by the enzyme PEP-carboxylase, 4-carbon compound, malic acid, transfers CO2 to bundle sheath cells where it is further transferred to the Calvin cycle. Bundle sheath cells transfer the sugar they make to phloem tube transported through the body.
Amaranthus, Atriplex, paddy etc. grow better in tropical climate. As the first product of CO2 fixation is a 4-carbon compound oxaloacetate, so that plants are called C4 plants.Oxaloacetate is then transported to the chloroplasts of mesophyll cells. It is then converted to another 4-C compound, the malic acid (malate), with the help of NADH, produced in the photochemical phase. This reaction is catalysed by malic dehydrogenase. The malic acid is then transported to the chloroplasts of bundle sheath cells. Here malic acid (C4) is converted to pyruvic acid (C3) pyruvate with the release of CO2. Thus concentration of CO2 increases in the bundle sheath cells. These cells contain enzymes of Calvin cycle. Because of high concentration of CO2, RuBP carboxylase participates in Calvin cycle and not in photorespiration. Sugar formed in Calvin cycle is transported into the phloem. Pyruvic acid generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenol pyruvic acid (PEP) by consuming one ATP.
BIOLOGY XI: Chapter 4, BIOENERGETICS
127
Skills: Analyzing, Interpreting and Communication Justify why photorespiration is interference in the successful perfor足 mance of the Calvin cycle.
SECTION I : MULTIPLE CHOICE QUESTIONS Select the correct answer 1. Removal of the source of carbon dioxide from photosynthesizing chloroplasts results in rapid changes in the concentration of certain chemicals. Which one of the following represents the correct combination of concentration changes?
2. What are the products of the light reactions in photosynthesis? A) ATP and NADP B)
ATP, NADPH2 and oxygen
C)
ATP, PGA (phosphoglyceric acid) and NADH2
D)
ATP, PGA and oxygen
Which of the following identifies the four graphs? 3. During the light stage of photosynthesis, the photoactivated pigment removes an electron from the hydroxylation derived from the water molecule. The fate of the free hydroxyl radical is that it A)
is broken down into oxygen and a free radical of hydrogen
B)
is used to raise the activation level of chlorophyll by donating a positive charge
128
BIOLOGY XI: Chapter 4, BIOENERGETICS
C) is used to produce adenosine triphosphate from adenosine diphosphate D)
reduces carbon dioxide to sugar
4. Carbon dioxide labeled with 14C has been used to identify the intermediate compounds in the Calvin cycle, the light-independent stage in photosynthesis. Which compound would be the first to contain the 14C? A)
glucose
B)
PGA
C)
RuBP
D)
starch
5. The rate of photosynthesis of a freshwater plant is measured using five spectral colours. Which sequence of colours would give an increasing photosynthetic response?
6. During dark reactions the three carbon atoms of 3-PGA are derived from A)
RuBP only
B)
CO2 only
C)
RuBP + CO2
D)
RuBP + CO2 + PEP
7. Chlorophyll is soluble in A)
water
B)
C)
water and organic solvent D)
organic solvent not in any solvent
8. Photorespiration takes place only in A)
root
B)
mitochondria
C)
green parts of the plant
D)
all cells of the plant
9. In C4 plants, fixation of CO2 occurs in A)
palisade tissue
B)
cortex of stem
C)
spongy mesophyll and bundle of sheath
D)
phloem tissue
10. ATP synthesis during light reactions is A)
oxidative
B)
photolysis
BIOLOGY XI: Chapter 4, BIOENERGETICS C)
129
substrate phosphorylation
D)
photophosphoryation
11. In C3 plants first stable product of photosynthesis during dark reaction is A)
PGA
B)
PGAL
C)
RuBP
D)
oxaloacetate
12. The diagram shows the Krebs cycle. At which numbered stages does decarboxylation take place?
A)
1 and 2
B)
1, 2 and 3
C)
1, 3 and 4
D)
1, 2, 3 and 4
SECTION II : SHORT QUESTIONS 1. Why does someoneÂ’s blue shirt looks blue? 2. In what part of photosynthesis sugar is produced? 3. Compare CO2 fixation in C3 and C4 plants. 4. What will happen if plants are exposed to green lights? 5. Define:
(a) Photophosphorylation, (b) substrate phosphorylation, (c) chemiosmosis.
6. Name two carbon compound formed during photorespiration. 7. Distinguish between: (a) action spectrum and absorption spectrum, (b) photosystem I and II (c) photophospshorylation and oxidative phosphorylation (d) chlorophyll a and chlorophyll b. 8. Name the four main steps of oxidation of cellular respiration.
130
BIOLOGY XI: Chapter 4, BIOENERGETICS
9. The cellular cycle of ATP-ADP + Pi + energy = ATP is occurring continuously in all of our cells. Why is the input of food energy needed to run this cycle? 10. The roots of plants are not expose to sunlight they are under the ground. How do they manufacture ATP?
SECTION III : EXTENSIVE QUESTIONS 1. Describe the role of light in photosynthesis. 2. Explain the role of photosynthetic pigments. 3. Explain the role of carbon dioxide and water in photosynthesis. 4. Describe glycolysis. 5. Describe Krebs cycle. 6. Explain electron transport chain. 7. Give an account of photorespiration. 8. Write a note on C4 photosynthesis. 9. Describe the mechanism of photosynthesis. 10. ATP is known as the energy currency of the cell . Explain what this means?
ANSWER MCQS 1. C 2. B 3. A 11. A 12. B
4. B
5. B
6. C
7. C
8. C
9. C
10. C
SUPPLEMENTARY READING MATERIAL 3. Madar, S.S. Biology, 6ht edition, WCB, McGraw-Hill, USA, 1998. 4. Taylor, D.J., Green, N.P.O. and Stout, G.W. Biological science 3rd Ed. Cambridge university press, reprint, 2004.
USEFUL WEBSITES 1. www.trueorigin.org/atp.asp 2. www.pnas.org/cgi/content/abstract/77/4/1783