Introduction to Carbohydrates

• Carbohydrates are essential organic molecules
• They consist of carbon, hydrogen, and oxygen atoms
• The general formula for carbohydrates is CnH2nOn
• Carbohydrates are the main source of energy for organisms
• Glucose is the central molecule in carbohydrate metabolism

Classification of Carbohydrates

• Carbohydrates can be classified as simple or complex
• Classification depends on their chemical structure and degree of polymerization

Simple Carbohydrates

• Simple carbohydrates have one or two sugar molecules
• Monosaccharides, disaccharides, and oligosaccharides are types of simple carbohydrates.

Monosaccharides

• Monosaccharides are carbohydrate monomers
• Glucose is an example of a monosaccharide
• Other examples include mannose, galactose, and fructose

Disaccharides

• Disaccharides form when two monosaccharides combine
• Sucrose, lactose, and maltose are examples of disaccharides

Oligosaccharides

• Oligosaccharides consist of 2-9 monomers
• Trioses, pentoses, and hexoses are all considered oligosaccharides

Complex Carbohydrates (Polysaccharides)

• Complex carbohydrates have two or more sugar molecules and are starchy foods
• They consist of a large number of monomers through polymerization
• Starch, glycogen, and cellulose are examples of polysaccharides
• Polysaccharides exhibit extensive branching and are homopolymers, meaning they consist of only glucose units

Starch

• Starch has two components: amylose and amylopectin
• Amylose forms a linear chain, while amylopectin is branched

Glycogen

• Glycogen, also known as animal starch, has a structure similar to starch, but with more extensive branching

Cellulose

• Cellulose is a structural carbohydrate that is the primary component of plant cell walls
• It is a fibrous polysaccharide with high tensile strength, forming a linear polymer

Food Sources

• Simple sugars are found in fruits as fructose
• Galactose is present in dairy products
• Lactose is abundant in milk and other dairy products
• Maltose is present in cereal, beer, potatoes, processed cheese, and pasta
• Sucrose is naturally found in sugar and honey, containing vitamins and minerals

Healthy vs Unhealthy Sources

• Carbohydrates have both healthy and unhealthy sources
• Healthy sources include fresh fruits, vegetables, corn, potatoes, milk, and milk products
• Unhealthy sources include soda, white bread, artificial sugar, pastries, and highly processed foods
• Carbohydrates can be found as sugars, starch, and fibers

Simple Carbohydrate Sources

• Fresh fruits like apples, oranges, bananas, pineapple, sweet potatoes, and berries are rich sources of healthy simple carbohydrates
• Milk is also a source of simple carbohydrates
• Foods with artificial sugars and heavily processed foods are unhealthy sources

Starchy and Fibrous Carbohydrate Sources

• Grains are rich in carbohydrates, including whole grains and grain bread
• Beans, potatoes, sweet potatoes, and nuts contain high levels of carbohydrates
• Cereals are also rich in carbohydrates
• Fibrous Carbohydrates include pumpkin, carrot, tomatoes, beans, broccoli, cucumbers, and squash
• Complex Carbohydrates include beans, peas, whole grains, barley, oats, wild rice, and brown rice
• Complex carbohydrates are good because they contain starch and fiber, and help in maintaining minimal sugar level in blood

Digestion of Carbohydrates

• Digestion of carbohydrates involves breaking down complex polysaccharides like starch into glucose
• Enzymes, such as amylase and maltase, play a key role

Digestion in the Mouth

• Mechanical digestion involves chewing, which breaks down carbohydrates into smaller pieces, forming a bolus

Chemical Digestion in the Mouth

• Saliva contains salivary amylase (ptyalin) that hydrolyzes starch into maltose

Digestion in the Stomach

• Mechanical digestion involves strong peristaltic contractions to mix carbohydrates into chyme

Chemical Digestion in the Stomach

• The acidic environment of the stomach (pH 1.5) stops the action of salivary amylase

Digestion in the Small Intestine

• Mechanical digestion does not occur in the small intestine

Chemical Digestion in the Small Intestine

• The pancreas releases pancreatic juice containing pancreatic amylase
• Enzymes are secreted by intestinal cells lining the villi

Action of Pancreatic Amylase and Intestinal Enzymes

• Pancreatic amylase hydrolyzes oligo- and polysaccharides into monosaccharides
• Starch converts to Maltose + Isomaltose with pancreatic amylase
• Maltose converts to Glucose + Glucose with maltase
• Sucrose converts to Glucose + Fructose with sucrase
• Lactose converts to Glucose + Galactose with lactase

Key Enzyme Actions

• Amylases convert polysaccharides to disaccharides
• Disaccharidases convert disaccharides to monosaccharides for absorption
• Salivary amylase and pancreatic amylase are key amylases
• Maltase, sucrase-isomaltase, lactase, and trehalase are key disaccharidases

Undigested Carbohydrates

• Certain carbohydrates like cellulose are not digested due to the lack of cellulase
• Cellulose provides fibers for water absorption in the large intestine, expanding muscle walls and triggering egestion, which can relieve constipation

Absorption of End Products

• Only monosaccharides such as glucose, galactose, and fructose are absorbed in the small intestine
• Glucose accounts for 80% of the total monosaccharide absorption
• Absorption mostly occurs in the duodenum and upper jejunum of the small intestine

Mechanisms for Carbohydrate Absorption

• Passive diffusion
• Facilitated diffusion (carrier mediated)
• Active transport

Influencing Factors

• Types of carbohydrate
• Fat and carbohydrate absorption
• Protein's potential effects
• Carbohydrate absorption blockers
• Mucus membrane
• Thyroid hormones
• Adrenal cortex
• Anterior pituitary

Metabolic Roles

• Energy production
• Energy storage
• Building macromolecules
• Sparing protein
• Assisting in lipid metabolism (sparing lipids)

Fate of Carbohydrates

• Glucose can undergo: glycolysis, glycogenolysis, or gluconeogenesis

Glycolysis

• Stepwise degradation of glucose in the cytosol
• Produces pyruvate (with oxygen) or lactate (without oxygen), generating ATP
• Glycolysis is the sole source of metabolic energy in tissues and cells

Glycogenolysis

• Breakdown of glycogen into glucose
• Glycogen is the storage form of glucose in animals
• Excess glucose is converted into glycogen and stored in the cytosol of liver and muscle cells as granules

Gluconeogenesis

• Synthesis of glucose from non-carbohydrate sources
• Meets the body’s glucose needs when carbohydrate is insufficient from diet or glycogen reserves
• Essential for nervous system and erythrocytes
• Failure of gluconeogenesis is usually fatal

Diseases from Inadequate Carbohydrates

• Ideally, 45-65% of daily caloric intake should come from carbohydrates
• Deficiency can lead to: Hypoglycemia, Ketosis, Weight gain

Diseases from Excessive Carbohydrates

• Excessive carbohydrate intake can cause: Weight gain, Type 2 diabetes, Cardiovascular diseases

Key Points

• Carbohydrates are the primary energy source
• Mechanical and chemical digestion occur in the mouth, stomach, and small intestine
• Glucose accounts for 80% of monosaccharide absorption in the small intestine