All Free Biology MCQs with Answers

• A. Glucose, fructose, galactose, maltose, and lactose
• B. Glucose, sucrose, and cellulose
• C. Lactose, starch, glycogen, and trehalose
• D. Sucrose

Explanation: Reducing sugars are those sugars that can donate electrons to another chemical, effectively reducing it, while they themselves become oxidized. This property is due to the presence of a free aldehydic or ketonic group. All monosaccharides are reducing sugars, as are some disaccharides, such as maltose and lactose, which have a free anomeric carbon. Sucrose does not exhibit this behavior because both anomeric carbons are involved in the glycosidic bond, rendering it non-reducing. Similarly, polysaccharides like starch and cellulose lack free anomeric carbons needed for reduction.

• A. Erythrose
• B. Ribulose
• C. Heptose
• D. Ribose

Explanation: The correct answer is Ribose. Deoxyribose is similar to ribose but lacks an oxygen atom at the second carbon. When this deficiency is corrected by adding an oxygen atom to the second carbon, deoxyribose becomes ribose. The other options are incorrect: Erythrose is a tetrose with only 4 carbon atoms, Ribulose is a ketopentose rather than an aldopentose, and Heptose has 7 carbon atoms, which doesn't match the structural framework of deoxyribose or ribose.

• A. Fructose
• B. Sucrose
• C. Glucose
• D. Monellin

Explanation: Fructose is the sweetest naturally occurring carbohydrate and is significantly sweeter than both glucose and sucrose. This is why it is often used to sweeten foods. Sucrose, although sweet, does not match the sweetness level of fructose. Glucose is even less sweet compared to fructose. Monellin, despite its high sweetness, is a protein and not a sugar, thereby disqualifying it from being the correct answer.

• A. C5H10O5
• B. C6H12O6
• C. C5H10O4
• D. C6H12O5

Explanation: Deoxyribose is represented by the formula C5H10O4, indicating it is a 5-carbon sugar with one less oxygen atom than ribose, which has the formula C5H10O5. This loss of an oxygen atom is what gives it the name 'deoxy'. Option A is ribose, option B is glucose, and option D is not a standard sugar formula.

• A. Glucose
• B. Fructose
• C. Glucose & Fructose
• D. Glucose & Galactose

Explanation: Maltose is a disaccharide composed of two glucose molecules. When hydrolyzed by the enzyme maltase, it breaks down into two glucose monosaccharides. Option A is correct because maltose is made up of glucose units. Option B is incorrect because fructose is not part of maltose; it is a component of sucrose. Option C is incorrect as maltose does not contain fructose. Option D is incorrect because glucose and galactose form lactose, not maltose.

• A. Fats
• B. Typhoid
• C. Malaria
• D. Carbohydrates

Explanation: The iodine test is a chemical test for the presence of starch, a type of carbohydrate. When iodine is added to a sample containing starch, it turns a blue-black color, indicating a positive result. This is because iodine interacts with the helical structure of amylose, a component of starch. The other options (fats, typhoid, and malaria) are unrelated to the iodine test: fats are a different class of biomolecules, and typhoid and malaria are diseases not detected by this test.

• A. 5
• B. 6
• C. 3
• D. 7

Explanation: The correct answer is 6. A pyranose ring is a six-membered structure in organic chemistry, consisting of five carbon atoms and one oxygen atom. This configuration forms a stable ring, which is a common feature in carbohydrates such as glucose. Options A, C, and D are incorrect because they do not accurately represent the number of atoms in a pyranose ring.

• A. Maltose, Sucrose, Lactose
• B. Glycogen, Lactose, Sucrose
• C. Maltose, Lactose, Glucose
• D. All of the above

Explanation: The correct answer is Option A: Maltose, Sucrose, Lactose. Each of these is a disaccharide, composed of two linked monosaccharides. Maltose consists of two glucose molecules, sucrose is made of glucose and fructose, and lactose is composed of glucose and galactose. Option B is incorrect because glycogen is a polysaccharide, not a disaccharide. Option C is incorrect because glucose is a monosaccharide, not a disaccharide. Option D is incorrect because not all options are correct; only Option A includes only disaccharides.

• A. C3
• B. C5
• C. C1
• D. C2

Explanation: The correct answer is C1 because α and β-glucose differ in the orientation of the hydroxyl (-OH) group at the anomeric carbon, which is C1. In α-glucose, the -OH group on C1 is positioned on the opposite side of the ring compared to the CH2OH group, while in β-glucose, it is on the same side. The other carbon positions (C2, C3, and C5) do not affect the anomeric nature of glucose, thus these options are incorrect.

• A. Fructose
• B. DHAP
• C. Lactose
• D. PGA

Explanation: The correct answer is Lactose. Lactose is a disaccharide sugar consisting of glucose and galactose. It is predominantly found in milk and dairy products, thus making it a sugar of animal origin.Fructose is a monosaccharide found in plants, particularly fruits, and not of animal origin. DHAP is not a sugar but a metabolic intermediate in glycolysis and gluconeogenesis, and it is not specific to animals. PGA refers to phosphoglyceric acid, which is involved in photosynthesis in plants, not in animals.