Question

Sketch the straight-chain structures of the following monosaccharides. a. glucose c. ribulose b. ribose d. galactose

Short answer

The straight-chain structures of the monosaccharides are as follows: 1. D-Glucose: An aldohexose with six carbons, an aldehyde group at C1, and hydroxyl groups at C2, C3, and C4 on the right, and C5 on the left. 2. D-Ribose: An aldopentose with five carbons, an aldehyde group at C1, and hydroxyl groups at C2, C3, and C5 on the right, and C4 on the left. 3. D-Ribulose: A ketopentose with five carbons, a ketone group at C2, and hydroxyl groups at C1, C3, and C5 on the right, and C4 on the left. 4. D-Galactose: An aldohexose with six carbons, an aldehyde group at C1, and hydroxyl groups at C2, C4, and C5 on the right, and C3 on the left.

Step-by-step solution

1. Glucose straight-chain structure

Glucose is an aldohexose, which means it has an aldehyde group at one end (C1) and six carbon atoms. To draw the structure: 1. Write down the carbon chain with six carbons. 2. Add the aldehyde functional group at the first carbon (C1). 3. Add hydroxyl groups on C2, C3, C4, and C5. 4. Place hydrogen atoms to complete the structure, remembering that each carbon atom should form four bonds. The glucose straight-chain configuration is D-glucose, so the hydroxyl groups on C2, C3, and C4 should be on the right, whereas the hydroxyl on C5 should be on the left.

2. Ribose straight-chain structure

Ribose is an aldopentose, which means it has an aldehyde group at one end (C1) and five carbon atoms. To draw the structure: 1. Write down the carbon chain with five carbons. 2. Add the aldehyde functional group at the first carbon (C1). 3. Add hydroxyl groups on C2, C3, C4, and C5. 4. Place hydrogen atoms to complete the structure, remembering that each carbon atom should form four bonds. The ribose straight-chain configuration is D-ribose, so the hydroxyl groups on C2, C3, and C5 should be on the right, whereas the hydroxyl on C4 should be on the left.

3. Ribulose straight-chain structure

Ribulose is a ketopentose, which means it has a ketone functional group on the second carbon atom (C2) and five carbon atoms. To draw the structure: 1. Write down the carbon chain with five carbons. 2. Add the ketone functional group at the second carbon (C2). 3. Add hydroxyl groups on C1, C3, C4, and C5. 4. Place hydrogen atoms to complete the structure, remembering that each carbon atom should form four bonds. The ribulose straight-chain configuration is D-ribulose, so the hydroxyl groups on C1, C3, and C5 should be on the right, whereas the hydroxyl on C4 should be on the left.

4. Galactose straight-chain structure

Galactose is an aldohexose, which means it has an aldehyde group at one end (C1) and six carbon atoms. To draw the structure: 1. Write down the carbon chain with six carbons. 2. Add the aldehyde functional group at the first carbon (C1). 3. Add hydroxyl groups on C2, C3, C4, and C5. 4. Place hydrogen atoms to complete the structure, remembering that each carbon atom should form four bonds. The galactose straight-chain configuration is D-galactose, so the hydroxyl groups on C2, C4, and C5 should be on the right, whereas the hydroxyl on C3 should be on the left.

Key concepts

Glucose Structure

Glucose is one of the most essential monosaccharides, commonly known as blood sugar. Chemically, glucose is an aldohexose, which means it's a six-carbon sugar with an aldehyde group at its first carbon (C1). It's often found in nature as D-glucose, where the hydroxyl groups are positioned on the right for the second (C2), third (C3), and fourth (C4) carbons in its linear form. The hydroxyl on the fifth carbon (C5) is positioned on the left. Understanding the glucose structure is crucial because it plays a vital role in energy production in living organisms.

When depicting its straight-chain form, each carbon atom is connected linearly, with hydrogen atoms and hydroxyl groups attached to create a structure following the general formula C6H12O6. It's important to note that in aqueous solutions, glucose prefers to adopt a ring structure which is more stable and thus commonly found in biological systems.

Ribose Structure

Ribose is another pivotal monosaccharide, especially in the context of genetic material. It is an aldopentose, meaning it has five carbons and includes an aldehyde functional group at its first carbon (C1). In its linear form, D-ribose features hydroxyl groups on the right side for the second (C2), third (C3), and fifth (C5) carbons, while the hydroxyl on the fourth carbon (C4) is on the left.

Ribose is the backbone of RNA (ribonucleic acid), which is crucial for coding, decoding, and expression of genes. In RNA structures, ribose is found as part of a ribonucleotide and forms a ring structure rather than the straight-chain form, which makes it more chemically reactive and suited for its biological roles.

Ribulose Structure

Similar to ribose, ribulose is a five-carbon monosaccharide, but with a ketone group on the second carbon (C2), which classifies it as a ketopentose. The straight-chain configuration of D-ribulose places hydroxyl groups on C1, C3, and C5 on the right and the hydroxyl on C4 on the left.

Ribulose plays an integral role in the photosynthetic process in plants where it is involved in carbon fixation. The ribulose molecule commonly exists in a ring-form rather than its straight-chain structure, forming ribulose bisphosphate in the Calvin cycle, enabling the transformation of carbon dioxide to organic compounds.

Galactose Structure

Galactose is often mentioned together with glucose as they are both aldohexoses, consisting of six carbon atoms. Similar to glucose, it has an aldehyde group at C1. The straight-chain form of D-galactose differs from glucose by the orientation of the hydroxyl group on the fourth carbon (C4), which is on the right side, while the hydroxyl group on the third carbon (C3) is on the left side, in contrast to glucose.

Despite its similarity to glucose, galactose plays a different role and is a constituent of lactose, found in milk. In human metabolism, the liver converts galactose to glucose, which can then be used for energy generation. This demonstrates the importance of understanding galactose's structure, as it impacts nutritional biochemistry and metabolic disorders, such as galactosemia.