Carbohydrates: Functions and Types

Carbohydrate Functions

• Provides fuel for the body during exercise
• Preserves proteins in muscles
• Provides energy for the central nervous system
• Most important energy source for the brain
• Glycosylates proteins and lipids

Carbohydrate Types

• Complex carbohydrates are large polymers of simple sugars
• Simple sugars consist of disaccharides or monosaccharides.
• Carbohydrates must be broken down to monosaccharides to be absorbed in the jejunum (small intestines).

Monosaccharides

• Monosaccharides are monomers, such as glucose.
• Glucose is the body’s preferred carb-based energy source.
• Fructose, or "fruit sugar," is sweeter than glucose but has less effect on blood glucose levels.
• Fructose needs to be converted into glucose by the liver before it can be used by the body.
• The liver is the only organ that can metabolize fructose in significant amounts.
• High fructose diets can overload the liver, causing it to turn fructose into fat.

Disaccharides

• Disaccharides are dimers, such as maltose, sucrose and lactose.
• Maltose is formed from two α-glucose molecules.
• Sucrose is formed from glucose and fructose.
• Lactose is formed from glucose and galactose.

Polysaccharides

• Polysaccharides are polymers, such as starch, cellulose, and glycogen.
• Starch comes from plants.
• Cellulose is an indigestible plant polymer.
• Glycogen is a stored form of glucose found in animal muscles.

Oral Cavity (Carbohydrate Digestion)

• Chewing mixes food with salivary gland secretions.
• Salivary amylase breaks down complex carbohydrates into disaccharides and trisaccharides.

Stomach (Carbohydrate Digestion)

• Salivary amylase continues to digest carbohydrates until the stomach's pH drops below 4.5 (1–2 hours post-meal).

Duodenum (Carbohydrate Digestion)

• Chyme in the duodenum prompts secretin to release buffers, shifting the pH from acidic to alkaline.
• Cholecystokinin (CCK) triggers the release of pancreatic buffers and enzymes, including pancreatic alpha-amylase.
• Pancreatic alpha-amylase has the same function as salivary amylase, which was deactivated in the stomach.
• Chyme stimulates gastric inhibitory peptide (GIP) release, which stimulates insulin release by the pancreas.

Jejunum (Carbohydrate Digestion)

• Epithelial cells complete carbohydrate digestion.
• The brush border's plasma membrane contains maltase, sucrase, and lactase.
• Maltase breaks down maltose (glucose + glucose).
• Sucrase breaks down sucrose (glucose + fructose).
• Lactase breaks down lactose (glucose + galactose) into simple sugars for absorption.
• Monosaccharides diffuse through the cytosol and reach the interstitial fluid via facilitated diffusion across the basolateral surfaces.
• Monosaccharides diffuse into the capillaries of the intestinal villi, then transported to the liver via the hepatic portal vein.

Colon (Carbohydrate Digestion)

• Indigestible carbohydrates, such as cellulose, remain unaltered by intestinal enzymes.
• These carbohydrates serve as a nutrient source for colonic bacteria.
• Bacterial metabolic activity produces small amounts of flatus, or intestinal gas.
• Foods high in indigestible carbohydrates, like beans, increase bacterial gas production, leading to colon distension, cramps, and more frequent discharge of intestinal gases.

After Absorption (Carbohydrate Digestion)

• Glucose can be stored in the liver and the muscles as glycogen (a polymer of glucose) to be used later for energy.
• Glucose can be converted to triglycerides in the liver and can be stored as fat in adipocytes.
• Energy needs are met in 3 phases: glucose catabolism, glycogen breakdown, and fatty acids conversion to Acetyl CoA.
• One glucose can generate up to 32 ATP.
• Mitochondria can generate 129 ATP from fatty acids.
• Glucose can be used in building glycoproteins, structural materials, or nucleic acids.
• Glucose can also convert to glycerol, which is essential for synthesizing triglycerides.

Using Glucose For Energy

• Glucose is broken down into two pyruvate molecules via glycolysis in the cytoplasm of cells.
• Glycolysis is an anaerobic process.
• Glycolysis results in a net gain of 2 ATP for each glucose molecule processed.
• In the presence of oxygen, each pyruvate molecule is converted into acetyl-CoA and transported into the mitochondria.
• Processing each pyruvate molecule by mitochondria yields up to 32 ATP.
• For each molecule of pyruvate processed, 3 molecules of oxygen (O2) are consumed.
• The mitochondrial process generates 3 molecules of carbon dioxide (CO2) and 6 molecules of water per pyruvate molecule.
• Catabolizing a pair of pyruvate molecules yields 30–32 ATP for the cell.
• In the absence of oxygen, each pyruvate molecule is converted to 2 ATP and Lactic Acid in the cytosol.
• The lactic acid will leave the cell and will be sent to the liver to be recycled into glucose via the Cori cycle.

Fats Functions

• A component in cell walls
• A source of energy
• Absorbing fat-soluble vitamins, including vitamins K, E, D, and A
• Insulating your body and protecting your organs
• To make steroid hormones

Oral Cavity (Lipid Digestion)

• Chewing breaks food into smaller pieces and disrupts connective tissues.
• Saliva containing lingual lipase saturates the bolus, which splits triglycerides into monoglycerides and fatty acids.
• Dietary lipids are generally not water-soluble.

Stomach (Lipid Digestion)

• Chyme mixing in the stomach forms large lipid drops.
• Lingual lipase functions in the acidic environment of the stomach but only breaks down triglycerides on the surface of lipid drops.
• Approximately 20% of triglycerides are broken down by the time chyme exits the stomach.

Role of CCK and Bile (Lipid Digestion)

• When chyme enters the duodenum, CCK is released, triggering secretion of pancreatic enzymes like pancreatic lipase.
• CCK also stimulates gallbladder contraction, releasing bile into the duodenum.
• Bile salts emulsify large lipid drops into smaller droplets for better pancreatic lipase access.
• Pancreatic lipase breaks triglycerides into fatty acids, monoglycerides, and glycerol.
• Released molecules interact with bile salts, forming lipid-bile salt complexes known as micelles.

Micelle and Lipid Absorption

• Micelles contact intestinal epithelium, and lipids diffuse across the plasma membrane into the cytosol.
• Intestinal cells synthesize new triglycerides from monoglycerides, fatty acids, and glycerol.
• These triglycerides combine with absorbed cholesterol, phospholipids, and other lipid-soluble materials and are coated with proteins, forming chylomicrons.

Chylomicron Transport

• Intestinal cells secrete chylomicrons into interstitial fluid via exocytosis.
• The protein coating keeps chylomicrons suspended in interstitial fluid.
• Size of chylomicrons prevents them from diffusing into capillaries.
• Instead, chylomicrons diffuse into intestinal lacteals of the lymphatic system, which lack basement membranes and have gaps between endothelial cells.

Lipid Absorption Process

• Lipids diffuse across the plasma membrane and enter the cytosol when a micelle contacts the intestinal epithelium.
• Intestinal cells synthesize new triglycerides from monoglycerides, fatty acids, and glycerol.
• Triglycerides, cholesterol, phospholipids, and lipid-soluble materials are coated with proteins, forming chylomicrons.
• Chylomicrons are lipoproteins that contain insoluble lipids.
• The phospholipid and protein coating makes chylomicrons water soluble.
• Intestinal cells secrete chylomicrons into interstitial fluid via exocytosis.
• The protein coating keeps chylomicrons suspended in interstitial fluid.
• Chylomicrons are too large to diffuse into capillaries.
• Chylomicrons diffuse into the intestinal lacteals of the lymphatic system after being released.
• Intestinal lacteals lack basement membranes and have large gaps between endothelial cells so that the chylomicrons can move into the lymphatic ducts.

Chylomicron Movement and Breakdown

• Chylomicrons move from lacteals to the thoracic duct.
• They then drain into systemic blood through the left thoracic duct on the left subclavian vein.
• Chylomicrons are either broken down into triglycerides by lipoprotein lipase in capillary walls or transported into the liver.
• Triglycerides diffuse into tissues and are stored in adipose cells or used by muscle for energy.

Cholesterol and Lipoproteins

• Chylomicrons in the liver form LDL, which carries cholesterol to tissues.
• Released cholesterol can be used in three ways: to make steroid hormones and compounds like Vitamin D3, for energy, or can form plaques and clog blood vessels.
• Excess cholesterol is carried back to the liver to be recycled or excreted in bile by binding to high density lipoproteins (HDL).
• The ratio of LDL to HDL is an important indicator of health.
• Having more HDL and less LDL is optimal for cholesterol management.

Protein Functions

• Tissue structure
• Hormone system
• Metabolic system
• Transport system
• Enzymes that regulate metabolism
• Balancing the acid/base environment

Protein Information

• Proteins (a polymer) are macromolecules composed of amino acid subunits (the monomers ).
• Polymers (2 or more covalently linked amino acids) cannot be absorbed by the body.
• Proteins must be broken down to the single amino acid level to get into the body.
• Protein digestion is complex and time-consuming due to the complex structures of proteins.

Initial Breakdown (Protein Digestion)

• The initial step involves breaking down solid food, beginning in the mouth through chewing and mixing with saliva.
• Mechanical processing continues in the stomach through churning and mixing.

Stomach Environment (Protein Digestion)

• The acidic environment in the stomach kills pathogens and breaks down connective tissues and plant cell walls.
• Stomach acids denature proteins, disrupting tertiary and secondary structures, exposing peptide bonds for enzymatic action.
• Chief cells secrete pepsinogen, an inactive proenzyme.
• HCl in the stomach converts pepsinogen to pepsin.
• Pepsin breaks down complex proteins into smaller peptide and polypeptide chains.
• Protein digestion is not completed in the stomach because pepsin works on specific peptide bonds and time is limited.

Duodenum Activity (Protein Digestion)

• Acidic chyme in the duodenum stimulates the production and release of pancreatic enzymes via CCK (Cholecystokinin).
• Pancreatic enzymes are secreted as inactive proenzymes and activated in the duodenum.
• Enteropeptidase, an enzyme from the duodenal epithelium, converts trypsinogen to trypsin.
• Trypsin converts other proenzymes to chymotrypsin, carboxypeptidase, and elastase.
• These enzymes break down proteins into dipeptides, tripeptides, and amino acids.
• Each enzyme targets specific peptide bonds.

Small Intestine Absorption (Protein Digestion)

• Epithelial surfaces in the small intestine contain peptidases, including dipeptidases.
• Dipeptidases break dipeptides into individual amino acids.
• Amino acids are absorbed through facilitated diffusion and cotransport mechanisms.
• Amino acids enter intestinal capillaries for transport to the liver through the hepatic portal vein.

Liver and Body Usage (Protein Digestion)

• In the liver, amino acids are sometimes used to synthesize needed proteins.
• Many amino acids leave the liver where they are distributed throughout the body and used for protein synthesis.
• When glucose levels are deficient, amino acids can be used in gluconeogenesis in the liver.