Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Which of the following statements about starch is false? (a) Starch is a polymer of a-D-glucose. (b) It consists of \(20 \%\) amylose and \(80 \%\) amylopectin. (c) Glycosidic linkages between glucose units are always formed only between \(\mathrm{C}_{1}\) and \(\mathrm{C}_{4}\). (d) Amylose is water soluble and amylopectin is insoluble.

Short answer

a) Starch is a polymer of a-D-glucose. b) It consists of \(20 \%\) amylose and \(80 \%\) amylopectin. c) Glycosidic linkages between glucose units are always formed only between \(\mathrm{C}_{1}\) and \(\mathrm{C}_{4}\). d) Amylose is water soluble and amylopectin is insoluble. Answer: (c) Glycosidic linkages between glucose units are always formed only between \(\mathrm{C}_{1}\) and \(\mathrm{C}_{4}\).

Step-by-step solution

Verify Statement (a)

(a) Starch is a polymer of a-D-glucose. Starch is indeed a carbohydrate polymer consisting of alpha-D-glucose monomers. This means that statement (a) is true.

Verify Statement (b)

(b) It consists of \(20 \%\) amylose and \(80 \%\) amylopectin. Starch is composed of two different glucose polymers: amylose and amylopectin. The proportion may vary in different sources of starch, but on average, the composition is approximately \(20\%\) amylose and \(80\%\) amylopectin. This means that statement (b) is true.

Verify Statement (c)

(c) Glycosidic linkages between glucose units are always formed only between \(\mathrm{C}_{1}\) and \(\mathrm{C}_{4}\). Starch is composed of alpha-1,4-glycosidic bonds, linking the glucose units together. In amylopectin, there are also alpha-1,6-glycosidic bonds for branch points, creating a more branched structure. This means that glycosidic linkages are not only formed between C1 and C4 atoms. Statement (c) is false.

Verify Statement (d)

(d) Amylose is water soluble and amylopectin is insoluble. Amylose forms a helical structure and has some water solubility, while amylopectin has a highly branched structure, which reduces its water solubility. This means that statement (d) is true. Based on the evaluation of each statement, the answer to the question is (c): Glycosidic linkages between glucose units are always formed only between \(\mathrm{C}_{1}\) and \(\mathrm{C}_{4}\), as this statement is false.

Key concepts

Alpha-D-Glucose Polymer

Starch is a fascinating molecule that serves as an energy reserve in plants. It is a polymer, meaning it consists of many repeating units linked together. These units are alpha-D-glucose, a simple sugar that is the basic building block of starch. When these glucose molecules bond together, they form long chains through a process called polymerization.

Understanding the structure of alpha-D-glucose is crucial because it explains how starch behaves in biological systems and how it can be processed in food production. Each glucose unit in the polymer is identical, with six carbon atoms arranged in a ring. This common ring shape is shared by many sugars, but it is the particular orientation of the hydroxyl (OH) groups in alpha-D-glucose that dictates how it connects with other glucose molecules to form starch.

Starch, when viewed under a microscope, shows a granular structure. Each granule is a tightly packed arrangement of glucose chains, showing just how efficient nature can be at storing energy.

Amylose and Amylopectin Composition

Starch is not just one structure but a combination of two distinct polymers of glucose: amylose and amylopectin. This composition is vital for its functionality and properties.

Amylose accounts for about 20% to 30% of starch and has a relatively linear structure due to the alpha-1,4-glycosidic linkages. It tends to form a helix, which is important for trapping ions or molecules, and this contributes to its ability to gelatinize in hot water, a key quality in food preparation.

Amylopectin, on the other hand, makes up the remainder – roughly 70% to 80% – of starch and is highly branched. These branches arise from additional alpha-1,6-glycosidic bonds that occur every 24 to 30 glucose units. The branched structure of amylopectin increases its resistance to gelatinization, making it less soluble in hot water compared to amylose.

Varying Characteristics

These two components collectively impact the texture, digestibility, and reaction to heat and enzymes of starchy foods. Starches with more amylose will be firmer, less sticky, and can form stronger gels.

Glycosidic Linkages

The structure and properties of starch are determined by glycosidic linkages. These are the bonds that hold the glucose units together in starch.

The most common linkage in starches is the alpha-1,4-glycosidic bond, which creates the linear chains of amylose and the long strands in amylopectin. These linkages are formed when the OH group on the first carbon (C1) of one glucose molecule reacts with the fourth carbon (C4) on an adjacent glucose molecule, releasing a molecule of water in the process. This type of reaction is called a condensation reaction.

In addition to the alpha-1,4 bonds, amylopectin contains alpha-1,6 bonds that create branching points. These occur at intervals along the chain, leading to the bush-like structure that is characteristic of amylopectin. The existence of these two types of bonds within starch molecules results in their varied solubility and reactivity, which affect how they are processed in the body and used in the food industry.

Solubility of Starch Components

The solubility of starch is a critical property that affects how it is used in food and industry. Amylose and amylopectin, the two components of starch, exhibit different solubility behaviors due to their structure.

Amylose has a linear conformation that allows it to interact more easily with water, making it moderately soluble in hot water. Upon heating, amylose molecules can unwind and engage with water molecules, leading to a solution that can form a gel as it cools.

Amylopectin, with its highly branched structure, has more difficulty interacting with water molecules and is less soluble. When heated, the branches in amylopectin restrict the movement of water and make it harder for the structure to dissolve.

Practical Implications

These solubility characteristics are crucial, for example, in the production of sauces and gravies where a controlled thickening process is needed. Starches are selected based on their amylose to amylopectin ratio to fine-tune the texture and stability of the product.