Structure & Function of Sucrose
- Unique Structure. All sugars are made from three basic building blocks -- carbon, hydrogen and oxygen -- connected to form different structures.
- Energy for Your Body. Sucrose has one essential job to perform: It provides energy for your body. ...
- Impact on Blood Sugar. ...
- Health Concerns. ...
What is the unique structure of sucrose?
Unique Structure. Sucrose consists of one molecule of glucose connected to one molecule of fructose. The connection between the molecules, called an acetal oxygen bridge, makes sucrose a non-reducing sugar, which means it’s less reactive than most other sugars. This doesn't affect its digestion or function in your body,...
How are glucose and fructose linked together in sucrose?
In sucrose, the monomers glucose and fructose are linked via an ether bond between C1 on the glucosyl subunit and C2 on the fructosyl unit. The bond is called a glycosidic linkage. Glucose exists predominantly as a mixture of α and β "pyranose" anomers, but sucrose has only the α form.
What happens to sucrose when it is broken down?
This process is called a condensation reaction, and forms a glycosidic bond between the two molecules. As you can see in the image, the reaction can also go the other way. To dissolve sucrose into fructose and glucose, a molecule of water can be added back in. This is what happens to sucrose as you digest it.
What is the function of sucrose in plants?
Sucrose Uses. Sucrose is the most common form of carbohydrate used to transport carbon within a plant. Sucrose is able to be dissolved into water, while maintaining a stable structure. Sucrose can then be exported by plant cells into the phloem, the special vascular tissue designed to transport sugars.
What's the function of sucrose?
As a carbohydrate, sucrose provides your body with the energy required to perform physical and mental functions. Your body breaks down foods such as sucrose and starch into fructose and glucose during digestion. The fructose and glucose are metabolized by your body to release energy to your cells.
What is the structure of sucrose?
C12H22O11Table sugar / Formula
What is the functional group of sucrose?
Acetal Functional Group A carbon that has two ether oxygens attached is an acetal.
How does sucrose change the configuration of sucrose?
Answer and Explanation: Sucrose changes the configuration of sucrase by binding to the active site of this enzyme.
What is the structure of a sugar?
Chemically, sugar consists of carbon (C), oxygen (O), and hydrogen (H) atoms, and is classified as a carbohydrate . There are three main groups of sugars, classified according to the way the atoms are arranged together in the molecular structure. These groups are the following: Monosaccharides or simple sugars.
What are the characteristics of sucrose?
Sucrose appears as white odorless crystalline or powdery solid. Denser than water. WHITE SOLID IN VARIOUS FORMS. Hard, white, odorless crystals, lumps, or powder.
What functional group do all sugars have in common?
Sugars, or carbohydrates, have two major functional groups: an aldehyde or a ketone (both are collectively called carbonyls), and an alcohol functional group.
What functional groups are found in sugar?
All sugars are characterized by two functional groups such as carbonyl (C=O) and hydroxyl (C−OH). Here are the examples:The bonds present between two carbohydrates molecule is. ... Which of the following is sweet in taste but is not a sugar? ... Sugar is rich in.More items...
Which functional groups can be identified in sucrose table sugar the structure of which is shown below?
The structure of the sucrose molecule is shown below. The sucrose has a hydroxyl group, acetal group, and glycosidic linkage.
How does sucrose change the configuration of sucrase quizlet?
How does sucrose change the configuration of sucrase? Sucrose binds to its active site on sucrase, changing the shape of sucrase enzyme.
What type of bond is sucrose?
Hence, sucrose has strong covalent bonds holding a given sucrose molecule together but weak polar bonds between adjacent molecules.
How is sucrose formed choose the best answer?
How is sucrose formed? Choose the best answer. Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In this process, a water molecule is lost.
How do you draw sucrose structure?
0:116:20Structure of Sucrose - Biomolecules - Chemistry Class 12 - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo now let us understand that how we can draw the structure of sucrose. And what are the informationMoreSo now let us understand that how we can draw the structure of sucrose. And what are the information. Related to the structure of sucrose. This is what I'm going to talk about in this topic.
What type of bond is sucrose?
Hence, sucrose has strong covalent bonds holding a given sucrose molecule together but weak polar bonds between adjacent molecules.
What is the structure of maltose?
C12H22O11Maltose / Formula
What is the structure of lactose?
C₁₂H₂₂O₁₁Lactose / Formula
How does sucrose transport carbon?
Sucrose is the most common form of carbohydrate used to transport carbon within a plant. Sucrose is able to be dissolved into water, while maintaining a stable structure. Sucrose can then be exported by plant cells into the phloem, the special vascular tissue designed to transport sugars. From the cells in which it was produces, the sucrose travels through the intercellular spaces within the leaf. It arrives at the vascular bundle, where specialized cells pump it into the phloem. The xylem, or vascular tube which carries water, adds small amounts of water to the phloem to keep the sugar mixture from solidifying. The sucrose mixture then makes its way down the phloem, arriving at cells in the stem and roots which have no chloroplasts and rely on the leaves for energy.
What is the difference between sucrose and ribose?
Humans use sucrose as a sweetener, while plants use it to transport carbon they have acquired and imbued with energy. Ribose, a five-carbon sugar, is used in the construction of DNA, which does store information. The uses of carbohydrates are truly incredible. 3.
What is the name of the substance that is extracted from plants?
Sucrose was one of the first substances to be extracted from plants on a mass-scale, creating the white table sugar we know today. These sugars are extracted and purified from large crops, including sugar cane and sugar beets. To extract the sugar, the plants are usually boiled or heated, releasing the sugar.
Why do plants use sucrose?
Plants use sucrose as a storage molecule. For quick energy, cells may store the sugar for later use. If far too much is accumulated, plants may begin to combine the complex sugars like sucrose into even large and denser molecules, like starches. These molecules, and oily lipids, are the main storage chemicals used by plants. In turn, animals eat these sugars and starches, break them back down into glucose, and use the energy within the bonds of glucose to power our cells.
How is glucose broken down?
The six-carbon glucose and fructose can be broken down into 3-carbon molecules, which are imported into the mitochondria, where they go through the citric acid cycle (AKA the Krebs Cycle ). This process reduces coenzymes, which are then used in oxidative phosphorylation to create ATP.
Why is sucrose important?
Sucrose has been an important sugar for humans because it is easy extracted from plants such as sugar cane and sugar beets. These plants tend to store an excess of sugar, and from this we produce the majority of the sugar that we use.
What tube is responsible for sucrose?
The xylem, or vascular tube which carries water, adds small amounts of water to the phloem to keep the sugar mixture from solidifying. The sucrose mixture then makes its way down the phloem, arriving at cells in the stem and roots which have no chloroplasts and rely on the leaves for energy.
What is Sucrose?
Other, more common names for sucrose are table sugar, beet sugar, refined sugar, cane sugar, or just plain sugar.
What are carbohydrates made of?
Carbohydrates are biological molecules made up largely of carbon, hydrogen, and oxygen. Monosaccharide sugars are rings of carbon, sometimes with an oxygen atom in the ring, with additional carbon, hydrogen, and oxygen branches. The molecular formula of sucrose is C12H22O11. The fructose and glucose rings are linked to each other by an oxygen atom.
What is table sugar?
Lesson Summary. Sucrose, or table sugar, is a common type of sugar made of glucose and fructose. It is a disaccharide, meaning that it is made up of two monosaccharide sugar units and a simple carbohydrate. Sucrose is a crystal at room temperature that can be caramelized by heating.
What happens when you heat sucrose?
In chemistry, we call this a combustion reaction. If you heat sucrose gently to the right temperature, you will get caramel. Through a process called caramelization the sucrose breaks down and reforms different sugars giving you the distinct flavor and color of caramel.
Why do we add sugar to food?
Food manufacturers add sugar to make food s last longer and to make them taste better.
What is the plant that makes the most sugar?
Sugarcane and sugar beets are the two plants that are used to make most sucrose, or refined table sugar. Hot water is used to extract sucrose. The extract can then be concentrated into syrup and then crystallized to make table sugar.
How to know if a product has sucrose?
To familiarize yourself with everyday products that contain sucrose, look through kitchen and medicine cabinets at your house for items that contain sucrose. Carefully examine the ingredient list and find five different products that contain sucrose.
What is Sucrose (C12H22O11)?
Sucrose is a molecule composed of two monosaccharides, namely glucose and fructose. This non-reducing disaccharide has a chemical formula of C 12 H 22 O 11.
What happens when sucrose is heated?
When heated to temperatures above 186 degrees Celsius, sucrose undergoes a decomposition reaction to give rise to caramel. In a manner that is similar to other carbohydrates, sucrose undergoes combustion in the presence of oxygen to yield water and carbon dioxide as the products. It can also be noted that sucrose can be reacted with potassium nitrate (a powerful oxidizing agent with the chemical formula KNO3) to yield a special type of fuel known as rocket candy. The chemical equation for the reaction between sucrose and potassium nitrate is provided below.
What is sucrose used for?
This compound is used in many pharmaceutical products. It serves as a chemical intermediate for many emulsifying agents and detergents. It also serves as a food thickening agent and as a food stabilizer.
Why is sucrose used in baking?
The use of sucrose in baking results in the brown colour of the baked products. This compound also serves as an antioxidant (a compound that inhibits oxidation). Sucrose is widely used as a food preservative.
Why is sucrose subjected to dehydration?
Sucrose can be subjected to dehydration in the presence of sulfuric acid in order to obtain a black solid that is rich in carbon. The idealized chemical equation for this process is provided below.
What is the most common type of carbohydrate used for the carriage of carbon in a plant?
Sucrose is the most common type of carbohydrate used for the carriage of carbon in a plant. Sucrose can be dissolved in water, thus retaining a stable structure. Sucrose will then be transported into the phloem by plant cells, the special vascular tissue intended for sugar transport. Thus, the structure of a sucrose molecule, ...
What is the chemical formula for rocket candy?
It can also be noted that sucrose can be reacted with potassium nitrate (a powerful oxidizing agent with the chemical formula KNO3) to yield a special type of fuel known as rocket candy. The chemical equation for the reaction between sucrose and potassium nitrate is provided below.
What was the first substance to be synthesized?
Sucrose was first synthesized enzymatically in the laboratory from potassium D-glucosyl-1-phosphate and D-fructose. The first chemical synthesis was accomplished by reaction of 3,4,6-tri-O-acetyl-1,2-anhydro-alpha-D-glucopyranose with 1,3,4,6-tetra-O-acetyl-D-fructofuranose.
What is a sweetener?
Sweetener in foods and soft drinks, manufacture of syrups, source of invert sugar, confectionary, preserves and jams, demulcent, pharmaceutical products, caramel, chemical intermediate for detergents, emulsifying agents, and other sucrose derivatives. Lewis, R.J., Sr (Ed.).
What are the special hazards of combustion products?
Special Hazards of Combustion Products: Irritating fumes may form in fires. Behavior in Fire: Melts and chars (USCG, 1999)
How is a substance absorbed into the body?
The substance can be absorbed into the body by inhalation and by ingestion.
What is the starting material in fermentative prodn?
Starting material in fermentative prodn of ethanol, butanol, glycerol, citric and levulinic acids. Used in pharmaceuticals as a flavor, as a preservative, as antioxidant (in form of invert sugar), as demulcent, as substitute for glycerol, as granulation agent and excipient for tablets, as coating for tablets.
How to dispose of a chemical spill?
SMALL SPILLS AND LEAKAGE: If you spill this chemical, you should dampen the solid spill material with water, then transfer the dampened material to a suitable container. Use absorbent paper dampened with water to pick up any remaining material. Seal your contaminated clothing and the absorbent paper in a vapor-tight plastic bag for eventual disposal. Wash all contaminated surfaces with a soap and water solution. Do not reenter the contaminated area until the Safety Officer (or other responsible person) has verified that the area has been properly cleaned. STORAGE PRECAUTIONS: You should store this material under ambient temperatures. (NTP, 1992)
What temperature does decomp occur?
Decomp @ 160-186 °C; does not reduce Fehling's soln, form osazone, or show mutarotation; hydrolyzed to glucose and fructose by dil acids and by invertase, a yeast enzyme; upon hydrolysis optical rotation falls and is negative when hydrolysis is complete.
What is sucrose made of?
Chemical compound. Sucrose is made up of one molecule of glucose and one molecule of fructose joined together. It is a disaccharide, a molecule composed of two monosaccharides: glucose and fructose. Sucrose is produced naturally in plants, from which table sugar is refined. It has the molecular formula C 12 H 22 O 11 .
How is sucrose formed?
The energy for the reaction is gained by the cleavage of uridine diphosphate (UDP). Sucrose is formed by plants, algae and cyanobacteria but not by other organisms. Sucrose is the end product of photosynthesis and is found naturally in many food plants along with the monosaccharide fructose. In many fruits, such as pineapple and apricot, sucrose is the main sugar. In others, such as grapes and pears, fructose is the main sugar.
What is the linkage between sucrose and glucose?
In sucrose, the monomers glucose and fructose are linked via an ether bond between C1 on the glucosyl subunit and C2 on the fructosyl unit. The bond is called a glycosidic linkage. Glucose exists predominantly as a mixture of α and β "pyranose" anomers, but sucrose has only the α form. Fructose exists as a mixture of five tautomers but sucrose has only the β- D -fructofuranose form. Unlike most disaccharides, the glycosidic bond in sucrose is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the non-reducing end of the other. This linkage inhibits further bonding to other saccharide units, and prevents sucrose from spontaneously reacting with cellular and circulatory macromolecules in the manner that glucose and other reducing sugars do. Since sucrose contains no anomeric hydroxyl groups, it is classified as a non- reducing sugar .
How to measure the purity of sucrose?
The purity of sucrose is measured by polarimetry, through the rotation of plane-polarized light by a sugar solution. The specific rotation at 20 °C using yellow "sodium-D" light (589 nm) is +66.47°. Commercial samples of sugar are assayed using this parameter. Sucrose does not deteriorate at ambient conditions.
Where is sucrose extracted?
For human consumption, sucrose is extracted and refined from either sugarcane or sugar beet. Sugar mills – typically located in tropical regions near where sugarcane is grown – crush the cane and produce raw sugar which is shipped to other factories for refining into pure sucrose. Sugar beet factories are located in temperate climates where the beet is grown, and process the beets directly into refined sugar. The sugar refining process involves washing the raw sugar crystals before dissolving them into a sugar syrup which is filtered and then passed over carbon to remove any residual colour. The sugar syrup is then concentrated by boiling under a vacuum and crystallized as the final purification process to produce crystals of pure sucrose that are clear, odorless, and sweet.
When was sucrose invented?
The word sucrose was coined in 1857, by the English chemist William Miller from the French sucre ("sugar") and the generic chemical suffix for sugars -ose. The abbreviated term Suc is often used for sucrose in scientific literature.
How is sugar extracted from cane sugar?
Since the 6th century BC, cane sugar producers have crushed the harvested vegetable material from sugarcane in order to collect and filter the juice. They then treat the liquid (often with lime (calcium oxide)) to remove impurities and then neutralize it. Boiling the juice then allows the sediment to settle to the bottom for dredging out, while the scum rises to the surface for skimming off. In cooling, the liquid crystallizes, usually in the process of stirring, to produce sugar crystals. Centrifuges usually remove the uncrystallized syrup. The producers can then either sell the sugar product for use as is, or process it further to produce lighter grades. The later processing may take place in another factory in another country.
What is Sucrose?
Sucrose is a simple carbohydrate with limited reactivity that is used as a transport and energy storage molecule in most plants and as an energy source for animals.
How does sucrose help plants?
Plants make sucrose by condensing the glucose and fructose molecules produced by photosynthesis through light energy . Sucrose helps to transport carbon throughout the plants and also acts as a storage for energy. It can easily be broken down back down into fructose and glucose to be used for ATP production.
Why is sucrose important in plants?
In plants, sucrose is very important for the compression of energy for transportation and storage. Photosynthesis produces glucose and fructose using light energy and then enzymes facilitate a condensation reaction to form the glycosidic bond.
What is the most common disaccharide?
Sucrose is the most common disaccharide, which means it is made up of two monosaccharides. Its monomers are glucose and fructose. The monosaccharides are bonded together through a glycosidic linkage.
How does glucose break down into fructose?
This molecule can be broken back down into glucose and fructose by adding a water molecule, which happens during digestion.
Why is sucrose used to transport carbon within the plant?
Sucrose is also used to transport carbon within the plant because it can travel through the phloem.
How many ring in a molecule of maltose?
This molecule can be easily distinguished from the other major disaccharides (glucose and maltose) because it is made up of one 5-membered ring and one 6-membered ring.
What is the name of the enzyme that polymerizes the glucosyl moiety of sucrose?
Dextransucrase (sucrose: 1,6-α-D-glucan 6-α -D-glucosyltransferase, EC 2.4.1.5) polymerizes the glucosyl moiety of sucrose to form dextran, an α-1→6 linked glucan with α-1→2, α-1→3, or α-1→4 branch linkages depending on the producing enzyme. When other carbohydrates in addition to sucrose (such as maltose, isomaltose, etc.) are present in dextransucrase digests, some of the glucosyl groups of sucrose are transferred to the carbohydrates —acceptor reaction— and are diverted from forming dextran [1,2].
What is the role of sucrose in plants?
Sucrose (α-d -glucopyranosyl β- d -fructofuranoside) is one of the most abundant products in nature. Not only is it the main compound derived from photosynthesis and the predominant molecule of carbon translocation in most plants, but it also plays a central role in the plant's biological functions and responses to environmental stress ( Vargas and Salerno, 2010 ). In plants, glucose and fructose are involved in signaling pathways in which sucrose concentration functions as a key sensor of the nutritional status of plants. Therefore, invertase plays a key role in the control of cell differentiation and development. Although animals, including man, show a marked preference for diets containing sucrose, their genomes do not code for invertase. Instead, they use a different and unrelated enzyme to hydrolyze sucrose, sucrose-glucosidase (EC 3.2.1.48). The genomes of human intestinal microorganisms such as Bacteriodes thetaiotamicron ( Xu, 2003) and Bifidobacterium longum ( Schell et al., 2002) have invertase genes, demonstrating that these organisms benefit from the consumption of sucrose by humans. The use of sucrose as a source of carbon and energy depends on the rupture of the α-1-β-2-glucosidic bond by the action of invertases that irreversibly hydrolyze the disaccharide in glucose and fructose. There are two types of enzymes that hydrolyze sucrose: the typical invertases named β-fructosidases (β- d -fructofuranosido fructohydrolase, EC 3.2.1.26) and the α-1,4-glucosidases glucosidases (α- d -glucoside glucohydrolase, EC 3.2.1.20) and oligo-α-1,6-glucosidases (EC 3.2.1.48) with a wide range of substrate specificity. Invertase can be classified into two classes according to its activity, initially differentiated by the optimum pH in vitro: (i) acidic invertase (Ac-InVS, EC 3.2.1.26, β-fructofuranosidases) with an optimum pH of between 4.5 and 5, and (ii) alkaline/neutral invertase (A/N-InVS) with an optimum pH in the range of 6.5–8.0 ( Tymowska-Lalanne and Kreis, 1998 ). Invertases are defined as enzymes that hydrolyze sucrose in glucose and fructose, as shown above. Some invertases are reported to be highly specific to sucrose, such as the alkaline invertebrate carrot ( Lee and Sturm, 1996 ), but the strict specificity for a substrate is an exception among these enzymes. In contrast, most β-fructosidases have a relatively broad specificity of substrates and can hydrolyze not only sucrose but also β-fructosid bonds in one or more of the following saccharides: sucrose 6-phosphate, raffinose, inulin, and levan ( Lee and Sturm, 1996 ). Depending on their preferred substrates, β-fructosidases are often named in the literature as sucrose, invertase, fructanase, inulinase, or levanase. Invertases are found in the GH32 family of glycosyl hydrolases according to sequence-based classification (afmb.cnrs-mrs.fr/CAZY). This family, which includes > 370 members of plant, fungal and bacterial origin, contains not only invertase but also other fructofuranosidases such as inulinase (EC 3.2.1.7), levanase (EC 3.2.1.65), and exo -inulinase (EC 3.2.1.80), and transfructosidases such as sucrose: sucrose 1-fructosyltransferase (EC 2.4.1.99) and fructan: fructan 1-fructosyltransferase (EC 2.4.1.100) ( Alberto et al., 2004 ). Glycosyl hydrolases or glucosidases are a broad group of enzymes that displays a wide variety of protein folding and substrate specificities. They share a common feature, two critical sites of acid residues, which constitute the catalytic mechanism responsible for the breakdown of glycosidic bonds. In yeast invertase, these two invariant residues have been identified as an aspartate located near the N-terminal that acts as a nucleophile and a glutamate that acts as the general acid/base ( Reddy and Maley, 1996 ). With no known exception to date, the molecular mechanism appears conserved among members of the same sequence-based family. The analysis of the sequence coupled to structural comparisons has revealed significant similarities between representatives of different families, accompanied by a conservation of the catalytic machinery and the stereochemical reaction result, reflecting an old divergence of a common ancestor to acquire new substrate specificities.
What is the dextran NRRL B-512F?
The dextransucrase from Leuconostoc mesenteroides NRRL B-512F synthesizes a dextran containing 95% α-1→6 and 5% α-1→3 osidic bonds. This protein is an extracellular enzyme that is induced by sucrose. In that way, dextran polymer is co-produced with the enzyme and a dextran-enzyme complex is thus recovered at the end of the culture.
Why is it necessary to subclone the amylosucrase gene?
As the present study demonstrates the potential of this recombinant amylosucrase, it appears necessary to subclone the gene in order to overproduce the enzyme and to use it as a glucosylation tool. Subcloning and sequencing of the gene are already initiated. The identification of the amino acid sequence will allow the amylosucrase to be compared with other glucosyltransferases such as glycogen and starch synthase, glucosyltransferases from L. mesenteroides or Streptococcus sp. and cyclodextringlucanotransferases. This work will undoubtedly be of particular interest for the general study on structure/function relationships of glucosyltransferases.
What solvents are used to immobilize dextransucrase?
In this work, the stability and activity of dextransucrase immobilized in alginate was assayed in the presence of different organic solvents —DMSO, DMA, DMF, acetone and t-amyl alcohol —. The effect of solvent concentration on enzyme stability was also studied.
What is sugar in the world?
Sucrose is the sugar of global commerce. It is synthesized in most plants as a transport and temporary storage product, typically converted to starch for long-term storage. In sugar beet and sugarcane, sucrose accumulates, providing nearly all edible sugar. Productions of cane and beet sugar differ. Sugarcane is a tropical, perennial, C4 grass, whereas sugar beet is a temperate, biennial, C3 broad leafed plant. Over 100 countries produce sugar beets, sugarcane, or both. Approximately 160 million tons of sugar is produced annually, for direct consumption, and for conversion to ethanol and chemical feedstocks. Both crops rely on breeding and agronomic research for sustainable production.
What is the activation of SPS?
Light activation of SPS is the result of a dephosphorylation of the enzyme that, in vivo, is accompanied either by increases in Vmax or by a higher affinity for the substrates, UDPglucose and fructose-6-P, and the activator, glucose-6-P, and decreased inhibition by P i, depending on the species ( Huber et al., 1989). Much of what is known about the regulation of SPS and its kinase and phosphatase comes from studies of the enzyme from spinach ( Huber et al., 1994; Huber and Huber, 1996) and is summarized in Fig. 7. Until recently, the only SPS from a C 4 plant that has been studied in any depth is that from maize. Sicher and Kremer (1985) first showed that maize leaf SPS was activated by light, and Kalt-Torres et al. (1987b) showed pronounced diurnal changes in SPS activity in maize, which has similarly been shown to be regulated by protein phosphorylation ( Huber and Huber, 1991 ). In spinach, activation results from the dephosphorylation of a serine residue (Ser 158 ). In the deduced sequence of maize SPS ( Worrell et al., 1991; Huber et al., 1994), it appears that the regulatory seryl residue found in spinach has been conserved, indicating that, in maize, Ser 162 is a likely candidate for regulatory phosphorylation. The maize enzyme has been classified in Group I by Huber et al. (1989) because it shows light-dependent changes in Vmax. This is in contrast to the enzyme from spinach, which shows changes in metabolite modulation, but no change in Vmax following dephosphorylation (Group II) and that from soybean, which lacks both covalent modification and shows only weak control by metabolites (Group III). It remains to be established whether SPS from different C 4 plants shows such regulatory differences, although a survey of a number of Panicum species by Ohsugi and Huber (1987) showed that only one C 4 species, P. virgatum, did not show dark inactivation of SPS. The importance of these differences in vivo is emphasized by the fact that overexpression of maize SPS in tomato leads to considerable changes in leaf carbohydrate partitioning because the activity of the maize enzyme is not down-regulated in tomato ( Worrell et al., 1991; Galtier et al., 1993), whereas spinach SPS expressed in tobacco is down-regulated ( Stitt and Sonnewald, 1995 ).