Grade 12 Biology PDF - Photosynthesis Short Note

Grade 12 Biology UNIT 3 Energy Transformation Prepared By: Mr. Simon Solomon  UNIT 3 Energy Transformation At the end of this unit, learners will be able to: ✓ Discuss the process of energy transformation in cells ✓ Summarize the proce...

Grade 12 Biology UNIT 3 Energy Transformation Prepared By: Mr. Simon Solomon  UNIT 3 Energy Transformation At the end of this unit, learners will be able to: ✓ Discuss the process of energy transformation in cells ✓ Summarize the process of photosynthesis using chemical equation. ✓ Analyze an absorption spectra of chlorophyll a and chlorophyll b using graph. ✓ Discuss the mechanism as to how CO₂ is fixed in C₃,C₄ plants, and CAM Plants ✓ Justify why the rate of photorespiration is less in C4 plants as compared to C3 plants ✓ Differentiate between substrate-level phosphorylation and oxidative phosphorylation ✓ Show the mechanism electron transport system in mitochondria. ✓ calculate amount of energy yield from a given molecule of glucose.  3.1. ENERGY By the end of this section you should be able to: ✓ Define Energy ✓ Define cellular metabolism. ✓ Explain anabolic and catabolic pathways in cellular metabolism. What is energy? Definition: Energy is the capacity to do work. Energy is a property of objects which can be transferred to other object or converted into different forms, but cannot be created or destroyed. ❑Organism use energy to survive, grow, respond to stimuli, reproduce and for every type of biological process. ❑All living things require energy to carry out life processes. They can be use different forms of energy to power the biological processes. ❑The cells need energy to grow and reproduce, but even non-growing cells need energy simply to maintain themselves. ❑The study of transformation of one form of energy into another is called thermodynamics.  Cells obtain energy in many forms, but that energy can seldom be used directly to power cell processes. For this reason, cells have mechanisms that convert energy from one form to another. The ordered systems of the cell provide the information that makes these energy transformations possible. Because most components of these energy conversion systems evolved very early in the history of life, many aspects of energy metabolism tend to be similar in a wide range of organisms. In metabolism, series of chemical reactions are taking place in the cells of organisms. These reactions may aid in the transformations of energy from one form to another in cells.  What is Metabolism? ❑ Metabolism, Greek word “metabole” meaning “to change,” and consists of the chemical reactions that change or transform energy in a cell. ❑ Metabolism is the sum of chemical reactions that takes place within each cell of an organism. ❑ The chemical reactions enable cells to produce energy for vital processes and also synthesize new organic materials. ❑ The reactions of metabolism are organized into a step – by - step sequences called metabolic pathways, in which the product of one reaction becomes the starting point or substrate of another. ❑ Metabolic pathways are a series of reactions catalyzed by multiple enzymes. ❑ In a metabolic pathway, a specific molecule is altered in a series of defined steps, resulting in a certain product. ❑ A specific enzyme or a macromolecule that speeds up a chemical reaction, catalyzes each step. ❑ Enzymes enable metabolic reactions to proceed fast through to sustain life. ❑ These reactions can be divided into catabolic reactions that convert nutrients to energy and anabolic reactions that lead to the synthesis of larger biomolecules. ❖ Anabolism is the set of reactions involved in the synthesis of complex molecules, starting from the small molecules inside the cells of an organism. ✓ Anabolic reactions help in the building of macromolecules like proteins, nucleic acids, and polysaccharides. ❖ Catabolism is the set of reactions involved in the breakdown of complex molecules like proteins, glucose, and fatty acids, respectively. ✓ It is also the breakdown of monomers into carbon.  Anabolism (synthesis) Anabolism is another subcategory of metabolism, which helps in the construction of molecules from smaller units. It is the chemical process in which nutrients are used in the formation of comparatively complex molecules in the living cells with moderately simpler structures. This process includes making components of cells such as proteins, carbohydrates, lipids, which require energy in the form of ATP. Anabolism is a buildup feature, whereas catabolism is a breakdown feature of metabolism. It is also known as biosynthesis. Anabolism helps in the building of macromolecules like proteins, nucleic acids, and polysaccharides. These macromolecules are produced from small molecules using enzymes and nonprotein chemical compound that is required for an enzyme’s activities.  Catabolism (degradation) Catabolism is the subcategory of metabolism, which breakdowns large or complex molecules such as proteins, polysaccharides, and fats into small molecules like amino acids, monosaccharides, and fatty acids. It is a destructive state of metabolism. This process includes glycolysis and citric acid cycle. The glycolysis is the metabolic process that converts glucose pyruvic acid and a hydrogen ion. It is a chain of ten enzyme-catalyzed reactions. The energy released in this process is used in the formation of NADH (nicotinamide adenine dinucleotide) and ATP (adenosine triphosphate). The citric acid cycle is a sequence of chemical reactions used by aerobic organisms to stored energy derived from the oxidation of carbohydrates, fats, and proteins. No. Anabolism Catabolism 1. It is the constructive phase of metabolism. It is the destructive metabolism phase degradative reactions biosynthetic reactions 2. It is the process whereby simpler substances It is the process whereby complex are joined together to form complex macromolecules are broken down to macromolecules. form simpler substances or monomers 3. The process requires energy to construct The process releases energy as a result of substances. the breakdown of molecules. 4. It is an endergonic(energy-absorbing) It is an exergonic(energy-releasing) reaction reaction 5. It occurs during photosynthesis It occurs during cellular respiration It involve the creation of bonds; it takes It involve the breaking of bonds; energy to create chemical bonds released whenever chemical bonds ; are broken, energy is released  3.4. Photosynthesis At the end of this unit, learners will be able to: ✓ Define photosynthesis. ✓ Explain the process of dark and light reaction id photosynthesis. ✓ What are the main components of sunlight? ✓ How does having multiple types of pigments benefit plants?  PHOTOSYNTHESIS A plant is an autotroph (“self-feeder”), which uses inorganic substances such as water and carbon dioxide (CO2) to produce organic compounds. The opposite of an autotroph is a heterotroph, which is an organism that obtains carbon by consuming preexisting organic molecules. Plants, multicellular algae, unicellular protests, some microbes such as cyanobacteria, and Purple sulfur bacteria harness solar energy and convert it into chemical energy are Photoautotrophs. Photosynthesis is a series of chemical reactions that use light energy to assemble CO2 into glucose (C6H1206) and other carbohydrates. The plant uses water in the process and releases oxygen gas (O2) as a byproduct. The waste product of photosynthesis, O2, is essential to much life on the earth.  IMPORTANT OF PHOTOSYNTHESIS ✓ It is the number one source of oxygen in the atmosphere ✓ It contributes to the carbon cycle among the earth, the oceans, plants and animals ✓ It contributes to the symbiotic relationship among plants, humans and animals ✓ It directly or indirectly affects most living things on earth it serves as the primary energy process for plants This process provides not only food for the plant but also the energy, raw materials, and O₂ that are used to support most heterotrophs.  External structure of the leaf The outer leaf layer is known as the epidermis. The epidermis secretes a waxy coating called the cuticle that helps the plant retain water. Leaf Lamina (Blade): is the flat and broad part of the leaf with vein and veinlets. It commonly known as Leaf Blade. ✓ Photosynthesis take place on this part of the leaf. Leaf base: is the lowermost part of the leaf. It is the part that connects the blade to the petiole. Petiole: is the stalk that joins the blade or attachment point on a stem. It known as leaf stalk. ✓ It helps transport water and nutrients.  Margin: is the boundary or edge of the leaf blade. Midrib: is the central main vein that connects to the leaf stalk Veins: are the vascular tissue bundles that support the leaf and transport nutrient. Apex: is the tip of the leaf blade  Internal Structure of Leaf The internal structure of the leaf contains three main parts: ✓ Stomata ✓ mesophyll cells and ✓ vascular system (xylem vessels and phloem) Stomata: are the tiny openings or pores in plant tissue (epidermis) of leaves that allow for gas exchange. The epidermis in plant leaves also contains special cells called guard cells that regulate gas exchange between the plant and the environment. The mesophyll is made up of specialized parenchyma cells found between the lower and upper epidermis of the leaf. They are specialized for photosynthesis and therefore contain chloroplasts. They are of two types of mesophylls, palisade mesophyll and spongy mesophyll. ❖ Spongy mesophyll is called sponge because in three dimensions it is spongy in appearance, because it has many large air spaces between the cells. ❖ Palisade mesophyll cells are near the upper surface of the leaf where they receive more sunlight. ✓ They contain more chloroplasts than spongy mesophyll cells ✓ In plants, the highest density of chloroplasts is found in the mesophyll cells of leaves.  The vascular system is a complicated network of conducting tissues that interconnecting tissues that interconnects all organs and transports water, minerals, nutrients, organic compounds through the plant body. ❖ Xylem: is the tissue of vascular plants that transports water and nutrients from the soil to the stems and leaves. ❖ Phloem: is the vascular tissue that transports carbon from the leaves to basal parts of the plant.  The place/site of photosynthesis All green parts of a plant, including green stems and un-ripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants. CO2 enters the leaf, and O2 exits, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning “mouth”). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other non-photosynthetic parts of the plant. A typical mesophyll cell has about 30–40 chloroplasts, each measuring about 2–4 μm to 4–7 μm.  Structure of chloroplast In plants, the highest density of chloroplasts is found in the mesophyll cells of leaves. A chloroplast has two membranes surrounding a dense fluid called the stroma. ❖ The stroma is a fluid-filled matrix where the light-independent stage of photosynthesis takes place. ❖ Within the stroma, another set of membranes form disk-shaped compartments known as thylakoids. which segregates the stroma from the thylakoid space inside these sacs and another set of membranes form disk-shaped compartments within the stroma, ❖ The interior of a thylakoid is called the thylakoid lumen. In most plant species, the thylakoids are interconnected to form stacks called this thylakoid sacs are called grana (singular, granum). Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.  3.4.2. Photosynthetic pigments Photosynthetic cells contain special pigments that absorb light energy. Different pigments respond to different wavelengths of visible light. Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their particular colors. Photosynthesis pigment interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more solar energy.  There are three basic classes of pigments. 1. Chlorophylls: are greenish pigments which contain a porphyrin ring. This ring has the potential to gain or lose electrons easily and whereby providing energized electrons to other molecules. There are several kinds of chlorophyll, which the most important one is chlorophyll "a". It is a green pigment found in all plants, algae, and cyanobacteria. The second kind of chlorophyll, chlorophyll "b" occurs only in "green algae" and in plants. The third form of chlorophyll called chlorophyll "c", is found only in the photosynthetic members of the Chromista and dinoflagellates. 2. Carotenoids: are usually red, orange, or yellow pigments, and they include the familiar compound carotene, which gives carrots their color. Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin, the brown pigment whose colors keep in other brown algae as well as the diatoms. 3. Phycobilins: are water-soluble pigments, and are, therefore, found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta.  Absorption spectra of photosynthetic pigments An absorption spectrum is a graph that shows absorption from a spectrophotometer. shows absorption at wavelengths from 400-700 nm by three pigments; Chlorophyll a, Chlorophyll b, and the carotenoids. Chlorophyll a absorbs violet-blue and reddish orange-red wavelengths. Chlorophyll b absorbs mostly blue and yellow light. Both Chlorophyll a and Chlorophyll b also absorb light of other wavelengths with less intensity. However, none of them absorbs green, so that the leaf looks green because light is reflected to our eyes instead of being absorbed by the leaf. Carotenoids are ubiquitous and essential pigments in photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio) chlorophylls, and thereby expanding the wavelength range of light that is able to drive photosynthesis. Only absorbed light (largely blue and red) is useful in photosynthesis.  Light-dependent and light-independent reactions Figure 3.28. Light-dependent reaction (left) and light-independent reaction (right)  Light-dependent and light-independent reactions Inside a chloroplast, photosynthesis occurs in two stages: 1. the light-dependent reactions and 2. the light-independent (or Calvin Cycle) reactions. 1. Light dependent reaction (cyclic and non-cyclic photophosphorylation) The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H+ ) and giving off O2 as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored.  (The electron acceptor NADP + is first cousin to NAD+ , which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP + molecule.) The light reactions use solar energy to reduce NADP + to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP. NADPH, a source of electrons, acts as “reducing power” that can be passed along to an electron acceptor, reducing it, while ATP is the versatile energy currency of cells. ❑ Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.  Photosystem I and photosystem II The chlorophyll molecules accessory pigments and associated electron acceptors are organized into unit called Photosystem (PS). Photosystem biochemical mechanism in plant by which chlorophyll absorbs light energy for photosynthesis and a site where light reaction begun. There are two such mechanisms. Thus are: I. Photosystem I II. Photosystem II Sensitive to different light wavelength PsI (700λ) and PsII (680 λ) and linked to different ETC. In eukaryotic two photosystem exist the first called photosystem II. Which named for the order of its discovery rather than the order function. ❖ Photosystem I and photosystem II the following major reactions take place inside PsI and PsII respectively 1. Electrons (e–) in chlorophyll molecules in photosystem II are excited by the energy in photons of light – they become more energetic. Because of the extra energy, they escape from the chlorophyll and pass to an electron acceptor (the primary electron acceptor). 2. The conditions created in the chloroplast cause the following reaction to occur: 2H2O → O2 + 4H+ + 4e– This light-dependent splitting of water is called photolysis. The electrons replace those lost from the chlorophyll molecule. 3. The primary electron acceptor passes the electrons to the next molecule in an electron transport chain (plastoquinone or ‘Pq’). The electrons then pass along a series of cytochromes (similar to those in the mitochondrial electron transport chain) and finally to plastocyanin (Pc) – the last carrier in the chain. The electrons lose energy as they are passed from one carrier to the next. 4. One of the molecules in the cytochromes complex is a proton (hydrogen ion) pump. As electrons are transferred to and then transferred from this molecule, the energy they lose powers the pump which moves protons from the stroma of the chloroplast to the space inside the thylakoid. This leads to an accumulation of protons inside the thylakoid, which drives the chemiosmotic synthesis of ATP. 5. Electrons in chlorophyll molecules in photosystem I are excited (as this photosystem absorbs photons of light) and escape from the molecule. They are replaced by the electrons that have passed down the electron transport chain from photosystem II. 6. The electrons then pass along a second electron transport chain involving ferredoxin (Fd) and NADP reductase. At the end of this electron transport chain, they can react with protons (hydrogen ions) and NADP in the stroma of the chloroplast to form reduced NADP. ❑ A summary of the light-dependent reactions Light energy is used to excite electrons when: A. The light-dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH. B. ATP and NADPH provide the energy needed to build high-energy sugars from low-energy carbon dioxide. ❑ There are two types phosphorylation in light dependent reaction. These are cyclical and Non-cyclical phosphorylation. ❑ Both process produces ATP because of:- ✓ There is an accumulation of protons in the interior of a thylakoid membrane. ✓ This creates a concentration gradient between the thylakoid and the stroma of the chloroplast ✓ Protons move through ATP synthase causing the rotor to spin for phosphorylation. Non-cyclical phosphorylation Said non-cyclical as electron travel in non-cyclical manner. Involves one way of electron flow from water to NADP. First electron donor is water and final electron acceptor is NADP It end with net product of ATP,NADP and Oxygen Involves both PsII and PsI  Cyclical phosphorylation Said cyclical as electron travel cyclical manner( Back-back) Is simple and rare process of phosphorylation. Involves only PsI Initial donor and final electron acceptor is chlorophyll molecule. The net product is only ATP molecules ❑ Plants rarely synthesis ATP by cyclical phosphorylation when:- ▪ No oxygen and reduced NADP availability. ▪ Sugar cannot synthesize as lack of carbodioxide. E.g. Bacteria Comparison between cyclic and Non-cyclical phosphorylation  Light-independent reactions (Calvin cycle) In the light-independent reactions or Calvin cycle the energized electrons from the light dependent reactions provide energy to form carbohydrates from carbon dioxide molecules. The light independent reactions are sometimes called the Calvin cycle because of the cyclical nature of the process. Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function. The light independent molecules depend on the energy carrier molecules, ATP and NADPH, to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return to the light-dependent reactions to obtain more energized electrons.  In addition, several enzymes of the light-independent reactions are activated by light. The second part of photosynthesis or the Calvin cycle is light-independent and takes place in the stroma of the chloroplast. The Calvin cycle captures CO2 and uses the ATP and NADPH to ultimately produce sugar by which chloroplasts coordinate the two stages of photosynthesis. Photosynthesis releases oxygen and sugars the basis of plant biomass which directly or indirectly feeds most living things on earth. The Calvin cycle reactions can be divided into three main stages: A. carbon fixation B. reduction, and C. regeneration of the starting molecule.  A. CARBON FIXATION In the stroma of plant chloroplasts in addition to CO2, two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphophate carboxylase/oxygenase (Rubisco), and three molecules of ribulose biphosphate(RuBP) CO2 molecules with five carbon acceptor molecules, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). Rubisco catalyzes a reaction between CO2 and RuBP. For each CO2, molecules that reacts with one RuBP two molecules of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same as the atoms move to form bonds during the reaction (3 atoms from 3 CO2 + 15 atoms from 3 RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation ,because CO2, is fixed from an inorganic form into an organic molecules  B. REDUCTION ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3 phosphate(G3P)- a carbon compound that is also found in glycolysis. Six molecules of both ATP and NADPH are used in the process. The exergonic process of ATP hydrolysis is in effect driving the endergonic redox reactions, creating ADP and NADP+. Both of these spent molecules (ADP and NADP+) return to the near by light dependent reactions to be recycled back into ATP and NADPH.  C. REGENERATION Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions, liked to call the "carbohydrate scramble." In order for one G3P to exit the cycle (and go towards glucose synthesis), three CO2 molecules must enter the cycle, providing three new atoms of fixed carbon. When three molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP acceptor.  Summary of Calvin cycle reactants and products ❑ Three turns of the Calvin cycle are needed to make one G3P molecule that can exit the cycle and go towards making glucose. When we summarize the quantities of key molecules that enter and exit the Calvin cycle as one net G3P is made. In three turns of the Calvin cycle: A. Carbon. 3CO2 combine with 3 RuBP acceptors, making 6 molecules of glyceraldehyde-3- phosphate (G3P). B. 1- G3P molecule exits the cycle and goes towards making glucose. C. 5- G3P molecules are recycled, regenerating 3- RuBP acceptor molecules. D. ATP. 9 ATP are converted to 9 ADP (6 during the fixation step, 3 during the regeneration step). E. NADPH. 6 NADPH are converted to 6 NADP+ (during the reduction step). ❑A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle, or 6 CO2, 18 ATP, and 12 NADPH, to produce one molecule of glucose.  C3, C4, and CAM plants use different carbon fixation pathways At the end of this unit, learners will be able to: ✓ Define types of Photosynthesis ✓ Explain the process of C3, C4 and CAM photosynthesis. ✓ List an example of C3, C4 and CAM plants ✓ Explain photorespiration of plant ✓ Differentiate each types of photosynthesis  C3 PHOTOSYNTHESIS The method of photosynthesis that takes place in plants living in temperate environments, such as those found in Europe is called C3 photosynthesis – because the first compound formed in the light-independent reactions of the Calvin cycle is GP, which contains three carbon atoms. C3 Plants are plants capable of fixing CO₂ into a 3-Carbon sugar called Phosphoglycerate (PGA). The Calvin cycle is also known as the C3 pathway because a three-carbon molecule 3- phosphoglyceric acid (3-PGA), is the first stable compound in the pathway. RuBP + CO2 Rubisco PGA About 95% of plant species are C3, including cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees, and some lawn grasses, Tomatoes, Potatoes, Spinach, Cassava, Rice, Barley.etc.  Most of the plants use the Calvin cycle, C3 plants use only this pathway to fix carbon from CO2. The photosynthesis process can take place only when the stomata on leaves are open C3 plants exhibit the C3 pathway. Leaves of C3 plants have the following major adaptation for photosynthesis: ✓ Have broad leaves to capture more light ✓ Palisade cell has chloroplast at upper surface of leaves to absorb more light. ✓ Their stomata are mainly at lower surface. ✓ To minimize water loss (has waxy cuticle) ✓ They open their stomata mainly at day time, to allow entry of CO2 and close if water loss is great on hot day. ✓ Spongy mesophyll has air space. ✓ hat allows easy diffusion of CO2 and O2 between palisade and stomata. ✓ Bundle sheath cell lacks chloroplast and CO2 fixed ones  PHOTORESPIRATION (C2 PHOTOSYNTHESIS) It involves oxidation of carbon. Is catalyzed by Rubisco/oxygenase (oxygenation) instead of carboxylase. In hot tropical conditions ,C3 plants do not yields enough carbon dioxide for photosynthesis ,this is because their stomata closed to prevent water loss that prevent entry of carbon dioxide. As a result, the concentration of carbon dioxide in the leaves falls. Rubisco behave in unusual way that RuBP binds with oxygen instead of carbon dioxide. Then RuBP is oxidized to one molecule of GP (not two) and one molecule of phosphoglycolate. Ribulose biphosphate + oxygen Rubisco 1GP + 1phosphoglycolate The product of this reaction does not continue in the Calvin cycle. The plant therefore loses CO2 that it fixed in previous turns of the cycle, wasting both ATP and NADPH.  Where 1GP formed in photorespiration re-enter the Calvin cycle and phosphoglycolate converted into GP for use in the Calvin cycle. These reactions (involves chloroplast, peroxisomes and a mitochondrion) catalyzed by complex network of enzymes. In addition, carbon dioxide is produced in the process. Photorespiration reduces efficiency of photosynthesis by 25% than C3 plants.Because: ❑Carbon is oxidized, which is the reverse of photosynthesis (reduction of carbon to carbohydrate). ❑Ribulose biphosphate must be resynthesized and the phosphoglycolate removed. ❑It also cost ATP and NADPH in the resynthesize of RuBP. The reaction of Photorespiration  C4 PHOTOSYNTHESIS This pathway gets round the problem of photorespiration that reduces efficiency of photosynthesis. Takes place in plants that grow in tropical and sub tropical environment like Ethiopia. Examples of C4 plants include; maize, crabgrass, sorghum and sugarcane. Named C4, because the first compound formed in the light independent reactions of photosynthesis contain carbon four (oxaloacetate), not GP that of C3 plants. In the C4 pathway, CO2 combines with a three-carbon “ferry” molecule to form a four-carbon compound (hence the name C4). This molecule then moves into adjacent bundle-sheath cells that surround the leaf veins. The CO2 is liberated inside these cells, where the Calvin cycle fixes the carbon a second time. Meanwhile, at the cost of two ATP molecules, the three-carbon “ferry” returns to the mesophyll to pick up another CO2.  About 1% of plants use the C4 pathway. All are flowering plants growing in hot, sunny environments, including crabgrass and crop plants such as sugarcane and corn. C4 plants are less abundant, however, in cooler, moister habitats. In those environments, the ATP cost of ferrying each CO2 from a mesophyll cell to a bundle-sheath cell apparently exceeds the benefits of reduced photorespiration. The light-dependent reactions are the same as in the C3 plants, but differ in how glucose is synthesized in the light-independent reactions.  The structure of the leaf of C4 is essentially similar to that of a C3 plant, but differ in that: ✓ Cells of the bundle sheath contain chloroplasts that C3 plants lacks. ✓ No thylakoid means that the light-dependent reactions cannot occur here and oxygen is not produced in these chloroplasts. ✓ This helps to prevent photorespiration and allows Calvin cycle to take place. ✓ The light-dependent reactions in C4 pathway take place in the mesophyll cells, which have chloroplasts with thylakoid. The following reactions take place: 1. CO2 reacts with a C3 compound called PEP (Phosphoenol pyruvate) to form the C4 compound oxaloacetate in mesophyll cell. Catalyzed by the enzyme PEP carboxylase (Pepco). PEP + CO2 PEP carboxylase Oxaloacetate 2. Oxaloacetate is converted into another C4 compound (malate in bundle sheath cell. Oxaloacetate Malate 3. In the bundle sheath cell, malate is converted to pyruvate with the release of a molecule of CO2 that starts Calvin cycle by binding with RuBP. Malate CO2 Pyruvate 4. The pyruvate is back converted to PEP that enters Calvin cycle to synthesis sugar; this reaction requires ATP. ❑C4 cycle uses two more molecules of ATP to deliver a molecule of CO2 to Rubisco than does the C3 cycle. ❑This is not a problem in tropic, as the high light intensity that generates much ATP from the light dependent reactions. ❑C4 photosynthesis is most efficient under the following conditions: ✓ Low carbon dioxide concentration ✓ High light intensity ✓ High temperature ❑Plants grow in Ethiopia are C4 plants and well adapted to hot and bright days. So, produce high yield when compared to C3 plants. ❖ Why C4 plants experience low carbon dioxide concentration? ▪ This is not due to composition air in tropics is different from other regions. But, C4 plants (grass, maize...etc) grow very close together and compete for CO2 in the air reduces  CRASSULACEAN ACID METABOLISM (CAM photosynthesis) Another energy- and water-saving strategy is crassulacean acid metabolism (CAM). This carbon fixation pathway experienced in s plants adapted to desert conditions. Example: Succulent, xerophytes cacti, Agaphe...etc. In this area their stomata closed during day time when temperature is high to reduce rate of evapotranspiration. They have temporal adaptation. By opening stomata at night for carbon fixation the mesophyll cells. Plants that use the CAM pathway add a new twist: They open their stomata only at night, fix CO2, then fix it again in the Calvin cycle during the day. Unlike in C4 plants, both fixation reactions occur in the same cell.  A CAM plant’s stomata open at night, when the temperature drops and humidity rises. CO2 diffuses in. Mesophyll cells incorporate the CO2 into a four-carbon compound, which they store in large vacuoles. The stomata close during the heat of the day, but the stored molecule moves from the vacuole to a chloroplast and releases its CO2. The chloroplast then fixes the CO2 in the Calvin cycle. The CAM pathway reduces photorespiration by generating high CO2 concentrations inside chloroplasts. About 3% to 4% of plant species, including pineapple and cacti, use the CAM pathway. All CAM plants are adapted to dry habitats. In cool environments, however, CAM plants cannot compete with C3 plants. Their stomata are open only at night, so CAM plants have much less carbon available to their cells for growth and reproduction.