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Carbohydrates in Biochemistry One Shot

Carbohydrates in Biochemistry

Learn everything about carbohydrates in biochemistry, including their classification, functions, and metabolic pathways like glycolysis, gluconeogenesis, and glycogenolysis. Perfect for students and enthusiasts looking for simplified explanations and MCQs.

1. Introduction to Carbohydrates:

Carbohydrates are organic compounds composed primarily of carbon, hydrogen, and oxygen atoms. The general formula for carbohydrates is Cn(H2O)m, where n and m are integers.
They serve as a primary source of energy and include sugars, fibers, and starches.
General Formula: Cn(H2O)nC_n(H_2O)_nCn(H2O)n

A Generic Carbohydrate Structure

Basic Molecular Structure

Generic carbohydrate molecule with carbon, hydrogen, and oxygen atoms

Carbon atoms: The backbone of the molecule.
Hydrogen atoms: Attached to the carbon atoms.
Oxygen atoms: Typically found in hydroxyl groups (-OH).

Examples of Carbohydrates

1. Sugars are simple carbohydrates with a sweet taste. Common sugars include:

Glucose: The primary energy source for cells.
Fructose: Found in fruits and honey.
Sucrose: Table sugar, formed from glucose and fructose.

Glucose, Fructose, and Sucrose molecules

2. Fibers are complex carbohydrates that the body cannot digest easily. They provide bulk to the diet and aid in digestion. Examples of fibers include:

Cellulose: Found in plant cell walls.
Lignin: A structural component of plants.
Pectin: Found in fruits and vegetables.

3. Starches are complex carbohydrates that are digested and used for energy. They are found in grains, potatoes, and legumes.

2. Classification of Carbohydrates:

Carbohydrates can be classified based on the number of sugar units they contain:

Carbohydrates can be classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

flowchart classifying carbohydrates into monosaccharides, disaccharides, oligosaccharides, and polysaccharides

Monosaccharides

Single sugar unit
Examples: glucose, fructose, galactose

Disaccharides

Two sugar units joined together
Examples: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)

Oligosaccharides

3-10 sugar units
Examples: raffinose (glucose + fructose + galactose), stachyose (glucose + galactose + galactose + fructose)

Polysaccharides

Many sugar units joined together
Examples: starch (amylose, amylopectin), glycogen, cellulose, chitin

3. Monosaccharides:

Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed further.
They are classified based on the functional groups:
- Aldoses contain an aldehyde group.
- Ketoses contain a keto group.

Examples of Aldoses: Glucose, Mannose, Galactose.
Examples of Ketoses: Fructose, Xylulose, Erythrulose.

Monosaccharides are further classified based on the number of carbon atoms:
- Triose (3 C atoms): Glyceraldehyde
- Tetrose (4 C atoms): Erythrose
- Pentose (5 C atoms): Ribose
- Hexose (6 C atoms): Glucose
- Heptose (7 C atoms): Glucoheptose

Aldoses vs. Ketoses: A Comparison

Aldoses and ketoses are two types of monosaccharides, the simplest forms of carbohydrates. They differ based on the location of their carbonyl group (C=O).

Aldoses

Carbonyl group: At the end of the carbon chain (aldehyde group).
Examples: Glucose, galactose, mannose

Ketoses

Carbonyl group: Within the carbon chain (ketone group).
Examples: Fructose, dihydroxyacetone

Key differences and similarities:

Structure: Aldoses have a terminal aldehyde group, while ketoses have a ketone group within the chain.
Chemical properties: Both aldoses and ketoses can undergo reactions such as oxidation and reduction, but their specific reactions may differ due to the location of the carbonyl group.
Isomers: Many monosaccharides exist as isomers, having the same molecular formula but different structural arrangements. For example, glucose, galactose, and mannose are isomers of each other.

Common monosaccharides:

Glucose: The primary energy source for cells.
Fructose: Found in fruits and honey.
Galactose: Found in milk and dairy products.
Mannose: Found in plants and microorganisms.
Ribose and deoxyribose: Components of nucleic acids (DNA and RNA).

4. Disaccharides:

Disaccharides are composed of two monosaccharides joined together by a glycosidic bond.
They are classified into:
- Reducing sugars (e.g., Maltose, Lactose) contain free aldehyde or ketone groups.
- Non-reducing sugars (e.g., Sucrose) do not have free aldehyde or ketone groups.

Sucrose (table sugar)

Formed from glucose and fructose.
Glycosidic bond between the C1 of glucose and the C2 of fructose.

sucrose chemical structure

Lactose (milk sugar)

Formed from glucose and galactose.
Glycosidic bond between the C1 of galactose and the C4 of glucose.

lactose chemical structure

Maltose (malt sugar)

Formed from two glucose molecules.
Glycosidic bond between the C1 of one glucose and the C4 of the other glucose.

maltose chemical structure

5. Oligosaccharides:

Oligosaccharides consist of 2 to 9 monosaccharide units.
They often occur as glycoconjugates like glycoproteins and glycolipids.

Oligosaccharides are carbohydrates containing 3-10 sugar units connected by glycosidic bonds. These bonds are formed between the anomeric carbon (C1) of one sugar and the hydroxyl group (OH) of another sugar.

diagram showing glycosidic bonds in oligosaccharides

Glycoconjugates are molecules composed of a carbohydrate (oligosaccharide or polysaccharide) linked to a non-carbohydrate component, such as a protein or lipid.

Glycoproteins

Carbohydrate: Oligosaccharide or polysaccharide
Non-carbohydrate component: Protein
Function: Cell recognition, signaling, and structural support
Examples: Antibodies, hormones, enzymes

glycoprotein structure

Glycolipids

Carbohydrate: Oligosaccharide or polysaccharide
Non-carbohydrate component: Lipid
Function: Cell recognition, signaling, and membrane stability
Examples: Blood type antigens, gangliosides

glycolipid structure

6. Polysaccharides:

Polysaccharides contain more than 10 monosaccharide units.
Examples: Starch, Glycogen, Cellulose.
- Starch consists of two components:
  - Amylose: Linear chain.
  - Amylopectin: Branched chain.
- Glycogen: Known as animal starch, it has a highly branched structure similar to amylopectin but more extensive.
- Cellulose: A structural carbohydrate and the main component of plant cell walls. It forms linear polymers with high tensile strength.

Polysaccharides can be further classified into:

Homopolysaccharides: Composed of one type of monosaccharide.
Heteropolysaccharides: Composed of different types of monosaccharides.

Chart comparing starch, glycogen, and cellulose

Feature	Starch	Glycogen	Cellulose
Structure	Branched and unbranched forms (amylose and amylopectin)	Highly branched	Linear chains
Monomer	Glucose	Glucose	Glucose
Glycosidic bond	α-1,4 and α-1,6 (amylopectin)	α-1,4 and α-1,6	β-1,4
Function	Energy storage in plants	Energy storage in animals	Structural component of plant cell walls
Source	Plants (grains, potatoes, legumes)	Animals (liver, muscles)	Plant cell walls
Molecular weight	High	Very high	Very high
Solubility	Soluble in hot water	Soluble in hot water	Insoluble

Amylose

Linear chain of glucose units connected by α-1,4 glycosidic bonds.
Forms a helical structure.

Amylopectin

Branched structure with α-1,4 glycosidic bonds in the main chain and α-1,6 glycosidic bonds at branch points.
More compact than amylose.

Cellulose

Linear chain of glucose units connected by β-1,4 glycosidic bonds.
Forms a straight, rigid structure.

7. Sources of Carbohydrates:

Starch: Found in foods like beans, lentils, peanuts, potatoes, peas, corn, and grains.
Sucrose: Extracted from sugar and honey.
Lactose: Abundant in milk.
Fructose: Found in fruits.
Maltose: Present in cereals, potatoes, processed cheese, and pasta.

8. Functions of Carbohydrates:

Energy Production

Primary fuel source: Carbohydrates are broken down into glucose, which is used by cells for energy.
ATP production: Glucose is metabolized through glycolysis, the Krebs cycle, and oxidative phosphorylation to produce ATP, the energy currency of cells.

Cell Structure

Cell walls: Cellulose is a major component of plant cell walls, providing structural support.
Cell membranes: Glycolipids and glycoproteins are embedded in cell membranes, playing roles in cell recognition, signaling, and adhesion.

Detoxification

Liver function: The liver uses carbohydrates to synthesize molecules involved in detoxification processes, such as bile acids and glutathione.
Neutralizing toxins: Carbohydrates can help neutralize harmful substances in the body.

Other Functions

Storage: Starch in plants and glycogen in animals store excess carbohydrates for energy.
Structural support: Chitin is a carbohydrate found in the exoskeletons of insects and crustaceans.
Cell recognition: Carbohydrates on the surface of cells act as markers for cell recognition and communication.
Fat metabolism: Carbohydrates participate in fat metabolism.

Key Points:

Carbohydrates play essential roles in various biological processes.
They provide energy, contribute to cell structure, and aid in detoxification.
The specific function of a carbohydrate depends on its structure and location in the body.

9. Stereochemistry of Carbohydrates:

Carbohydrates can exist in D and L forms:
- D-type: Found abundantly in nature and rotate plane-polarized light clockwise.
- L-type: Also abundant in nature but rotate plane-polarized light anticlockwise.

10. HMP Shunt (Pentose Phosphate Pathway):

This is an anabolic pathway occurring in the cytosol of most organisms and in plastids of plants.
Divided into two phases:
1. Oxidative Phase:
  - Converts glucose-6-phosphate into ribulose-5-phosphate.
  - Produces NADPH used for anabolic reactions.
2. Non-oxidative Phase:
  - Converts ribulose-5-phosphate into fructose-6-phosphate and glyceraldehyde-3-phosphate.
  - These intermediates are used in glycolysis.

The Hexose Monophosphate Shunt (HMP Shunt)

The HMP shunt, also known as the pentose phosphate pathway, is an alternative pathway for glucose metabolism that produces NADPH and pentoses. It consists of two phases: the oxidative phase and the non-oxidative phase.

Oxidative Phase

Oxidative phase of the HMP shunt

Glucose-6-phosphate dehydrogenase: Catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconate, producing NADPH.
6-phosphogluconate dehydrogenase: Catalyzes the conversion of 6-phosphogluconate to ribulose-5-phosphate, producing another NADPH.

Non-Oxidative Phase

Nonoxidative phase of the HMP shunt

Ribulose-5-phosphate isomerase: Converts ribulose-5-phosphate to xylulose-5-phosphate.
Ribose-5-phosphate isomerase: Converts ribulose-5-phosphate to ribose-5-phosphate.
Transketolase: Catalyzes the transfer of a two-carbon unit from xylulose-5-phosphate to ribose-5-phosphate, producing sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate.
Transaldolase: Catalyzes the transfer of a three-carbon unit from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate, producing erythrose-4-phosphate and fructose-6-phosphate.
Transketolase: Catalyzes the transfer of a two-carbon unit from xylulose-5-phosphate to erythrose-4-phosphate, producing fructose-6-phosphate and glyceraldehyde-3-phosphate.

Key points:

The HMP shunt produces NADPH, a reducing agent essential for various cellular processes, including fatty acid synthesis and drug detoxification.
The non-oxidative phase can also produce pentoses, such as ribose-5-phosphate, which is needed for nucleotide synthesis.
The HMP shunt is especially active in tissues with high rates of fatty acid synthesis, such as the liver and adipose tissue.

11. Glycogenolysis:

The breakdown of glycogen into glucose.
Key enzymes: Glycogen phosphorylase, Phosphoglucomutase, Glucose-6-phosphatase.

Glycogen Breakdown: Glycogenolysis

Glycogenolysis is the process of breaking down glycogen into glucose-1-phosphate, which can then be converted to glucose-6-phosphate and enter glycolysis for energy production.

step by step diagram of glycogen breakdown

Steps involved:

Glycogen phosphorylase: This enzyme catalyzes the sequential removal of glucose residues from the non-reducing end of glycogen, producing glucose-1-phosphate.
Phosphoglucomutase: Converts glucose-1-phosphate to glucose-6-phosphate.
Glucose-6-phosphatase (in liver and kidney): This enzyme can further convert glucose-6-phosphate to glucose, which can be released into the bloodstream.

Key points:

Glycogen breakdown is primarily regulated by hormones, such as glucagon and epinephrine, which stimulate glycogen phosphorylase activity.
The liver is the main organ responsible for glycogen breakdown and glucose release into the bloodstream.
Glycogenolysis is essential for maintaining blood glucose levels during fasting or exercise.

12. Gluconeogenesis:

Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors, such as amino acids, lactate, and glycerol. It occurs primarily in the liver and kidneys.
Key steps involve enzymes such as Glucose-6-phosphatase, Fructose-1,6-bisphosphatase, and Phosphoenolpyruvate carboxykinase.

Gluconeogenesis Flowchart

Key steps and enzymes involved:

Pyruvate carboxylase: Converts pyruvate to oxaloacetate, requiring ATP and biotin.
Phosphoenolpyruvate carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP), requiring GTP.
Enolase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, triosephosphate isomerase: These enzymes catalyze the reversal of glycolysis steps to convert PEP to fructose-1,6-bisphosphate.
Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.
Glucose-6-phosphatase (in liver and kidney): Converts glucose-6-phosphate to glucose, which can be released into the bloodstream.

Important points:

Gluconeogenesis is a crucial process for maintaining blood glucose levels during fasting or exercise.
It requires energy input in the form of ATP and GTP.
The enzymes involved in gluconeogenesis are often regulated by hormones, such as glucagon and cortisol.
Gluconeogenesis is inhibited by insulin.

13. Glycogenesis:

The synthesis of glycogen from glucose.
Key enzymes: Hexokinase, Phosphoglucomutase, Glycogen synthase, and Branching enzyme.

Glycogen Synthesis

Glycogen synthesis is the process of building glycogen, a storage form of glucose, primarily in the liver and muscle cells. It is stimulated by insulin.

schematic showing glycogen synthesis

Key steps and enzymes involved:

Glucose phosphorylation: Glucose is converted to glucose-6-phosphate by hexokinase in most tissues or glucokinase in the liver.
Glucose-6-phosphate isomerase: Converts glucose-6-phosphate to glucose-1-phosphate.
Uridine diphosphate glucose pyrophosphorylase (UDPGPP): Converts glucose-1-phosphate to UDP-glucose, using UTP.
Glycogen synthase: Adds glucose residues from UDP-glucose to the non-reducing end of a glycogen primer (a small pre-existing glycogen molecule).
Glycogen branching enzyme: Transfers a block of several glucose residues from the non-reducing end of a glycogen branch to a new branch point, creating a branched structure.

Important points:

Glycogen synthesis is essential for storing excess glucose in the liver and muscle cells.
It is regulated by hormones, such as insulin and glucagon.
Insulin stimulates glycogen synthesis, while glucagon inhibits it.
Glycogen branching is important for increasing the solubility and storage capacity of glycogen.

14. Epimers and Isomers:

Epimers are monosaccharides that differ in configuration around one specific carbon atom.
- Example: Glucose and Galactose.

Epimers: Glucose and Galactose

Epimers are stereoisomers that differ in the configuration at only one chiral carbon atom. Glucose and galactose are epimers that differ at the C4 carbon.

glucose and galactose structures

Key differences:

C4 configuration: In glucose, the hydroxyl group at C4 is below the plane of the molecule (α configuration). In galactose, it is above the plane (β configuration).
Biological activity: While both glucose and galactose are monosaccharides, they have different biological functions. Glucose is the primary energy source for most cells, while galactose is primarily found in milk and dairy products.

Epimerization:

Glucose and galactose can be interconverted through a process called epimerization. Enzymes such as epimerases catalyze this reaction, allowing for the conversion of one epimer into the other.

Other epimer pairs:

Mannose and fructose
Ribose and xylose

Note: The configuration at the C4 carbon plays a crucial role in the biological activity and recognition of these monosaccharides.

15. Reducing and Non-Reducing Sugars:

Reducing sugars (e.g., Glucose, Lactose) can donate electrons due to the presence of a free aldehyde or ketone group.
Non-reducing sugars (e.g., Sucrose) do not have free aldehyde or ketone groups.

Reducing and Non-Reducing Sugars

Reducing sugars are monosaccharides or disaccharides that can donate electrons and reduce other compounds. They contain a free aldehyde or ketone group. Non-reducing sugars lack a free aldehyde or ketone group and cannot reduce other compounds.

Examples of Reducing Sugars

Glucose
Fructose
Maltose
Lactose

Examples of Non-Reducing Sugars

Sucrose
Trehalose
Cellobiose