Carbohydrates: Simple vs Complex

Carbohydrates

• Carbohydrates are composed of carbon, hydrogen, and oxygen, represented by the formula (CH2O)n.
• They provide energy.
• They should make up about 70-80% of an animal's diet and must be provided daily.

Carbohydrate Types

• Simple carbohydrates include monosaccharides (single sugars) and disaccharides (double sugars).
• Complex carbohydrates are polysaccharides (many sugars).

Monosaccharides

• Simple carbohydrates include hexoses (six-carbon sugars) and pentoses (five-carbon sugars).
• Glucose, fructose, galactose, and mannose are examples with the formula C6H12O6.

Glucose

• Glucose, also known as "dextrose" or "blood sugar," is a primary fuel source for the body.
• Found in all disaccharides and polysaccharides.
• Only fructose and sucrose are sweeter than glucose.
• In nature it is found in combination with other compounds.

Fructose

• Fructose, known as "fruit sugar," is found in fruit, honey, and syrup.
• It converts to glucose in the body.

Galactose

• Galactose forms part of lactose and is found in milk.
• It converts to glucose in the body.
• Galactose is found in combination with lipids in nervous tissue.

Disaccharides

• Simple carbohydrates include sucrose, maltose and lactose

Sucrose

• Sucrose is known as table sugar
• Made up of glucose plus fructose
• Sucrose is refined from sugar beets and cane

Lactose

• Lactose is found in milk
• Made up of glucose and galactose
• Lactose intolerance is when someone is missing the digestive enzyme to split lactose into monosaccharides which are then absorbed

Maltose

• Maltose is also known as malt sugar
• It is made up of glucose and glucose
• Maltose is found in germinating seeds
• Used in fermentation to produce malted beverages

Polysaccharides

• Complex carbohydrates include glycogen, starches, and fibers.

Glycogen

• Glycogen consists of long chains of glucose found in animals.
• It is stored in the liver and muscles.
• It helps maintain blood glucose levels and serves as a quick energy source, lasting about 12 hours, especially during exercise.

Starch

• Starch is composed of long chains of glucose found in plants.
• It is found in cereal grains, legumes, and root vegetables.

Fiber

• Fiber consists of mostly indigestible carbohydrates like lignin.
• It is a component of plant cell walls.
• Fiber is classified based on its solubility in water.
• It is abundant in wholegrains, legumes, and fruits.

Fiber Types

• Insoluble fiber is non-viscous with examples including cellulose and lignin.
• Soluble fiber is viscous and fermentable with examples including pectins.

Carbohydrate Digestion in Monogastrics

• NFE refers to soluble carbohydrates like starches and sugars.
• Digestion involves enzymatic breakdown into hexoses such as glucose.
• This happens in monogastric animals, with primary breakdown in the small intestine.

Digestion in the Mouth

• Digestion begins here
• Salivary glands secrete α-amylase (ptyalin) to hydrolyze starches.
• During mastication, salivary α-amylase acts briefly on dietary starch, randomly breaking α-(1 → 4) bonds, hydrolyzing starch into dextrins.

Digestion in the Stomach

• Carbohydrate digestion halts temporarily in the stomach.
• This is because the high acidity inactivates salivary α-amylase.

Digestion in the Intestine

• Further digestion of carbohydrates occurs in the small intestine, aided by pancreatic enzymes.
• Involves digestion due to pancreatic α-amylase.
• Also involves intestinal enzymes such as sucrase, maltase, lactase, and isomaltase.

Digestion due to Pancreatic α-amylase

• Function is to degrade dextrins further into maltose, isomaltose, and α-limit dextrin.
• α-limit dextrins are short oligosaccharides consisting of 3 to 5 glucose units.

Digestion due to Intestinal Enzymes

• Enzymes for the final phase of carbohydrate digestion are in the brush-border membrane.
• Maltase breaks down maltose into glucose.
• Isomaltase breaks down isomaltose into glucose.
• Sucrase breaks down sucrose into glucose and fructose.
• Lactase breaks down lactose into glucose and galactose.
• Dextrinase breaks down α-limit dextrin into glucose and maltose.

End Products of Carbohydrate Digestion

• Glucose, fructose, and galactose are readily absorbed through intestinal mucosal cells into the bloodstream.

Absorption of Carbohydrates

• Carbohydrates are absorbed as monosaccharides from the intestinal lumen.
• Absorption relies on active transport and facilitative transport.
• Active transport moves against a concentration gradient from low to high.
• Facilitative transport moves with the concentration gradient from high to low.

Active Transport

• The transport of glucose and galactose across the brush border membrane of mucosal cells relies on active transport.
• Key features include energy requirement, specific transport proteins, and the presence of sodium ions.
• A sodium-dependent glucose transporter (SGLT-1) binds both glucose and Na+ at separate sites, transporting them across the plasma membrane of the intestinal cell.
• Sodium is transported down its concentration gradient (higher to lower), glucose then moves against its concentration gradient at the same time.
• This active transport requires free energy from the hydrolysis of ATP, linked to a sodium pump that expels Na+ from the cell in exchange for K+.

Facilitative Transport

• Fructose and mannose are transported across the brush border by a Na+-independent facilitative diffusion process, requiring a specific glucose transporter, GLUT-5.
• Glucose and galactose can also use it if the concentration gradient is favorable.

Carbohydrate Transport

• GLUT-2, a sodium-independent transporter, facilitates sugar transport out of mucosal cells into the portal circulation for delivery to the liver.

Carbohydrate Digestion in Ruminants

• The majority of a ruminant’s diet includes cellulose, hemicellulose, and lignin.
• Enzymes secreted by rumen microbes break down complex carbohydrates.
• VFAs and gases are produced

Carbohydrate Digestion in Ruminants

• Digestion occurs through microbial fermentation in the rumen.
• Plant material is fermented into simple sugars.
• Microbes utilize these simple sugars and produce waste products.
• Waste includes gases, heat, and VFAs (volatile fatty acids).
• Ratios depend on the kind of diet consumed.

Composition of VFAs in the Rumen

• Acetic acid: 60-70%
• Propionic acid: 15-20%
• Butyric acid: 10-15%

Compostition of Gases in the Rumen

• Carbon dioxide: 76%
• Methane: 22%
• Hydrogen: 2%
• Small amounts of oxygen and nitrogen from ingested air.
• VFAs get absorbed straight from the rumen, reticulum, and abomasum.

Microbial Populations

• Cellulolytic bacteria ferment plant fiber, particularly cellulose, producing cellulase.
• Acetate with some propionate is their main product
• They thrive at pH 6-7 and are most common in animals fed roughage diets.
• Pectinolytic bacteria ferment pectins.
• Hemicellulolytic bacteria ferment hemicellulose.
• Amyolytic bacteria digest starches and sugars, preferring pH 5-6.
• They produce mainly propionate but sometimes lactate.
• This bacteria predominates in animals fed grain diets.
• Rapid changes to grain diets can cause lactic acidosis and rapidly decrease pH.
• An example is Streptococcus bovis.
• Ureolytic bacteria will ferment urea
• Proteolytic bacteria breaks down proteins

The Small Intestine

• Secretes digestive enzymes, receives digestive secretions from the pancreas and liver, and further digests carbohydrates.
• It also absorbs water, minerals, amino acids, glucose, and fatty acids.

The Large Intestine

• Also known as the Caecum
• The bacterial population ferments unabsorbed products of digestion.
• It absorbs water and VFA, and forms faeces.