Monosaccharides and Carbohydrates

• Carbohydrates provide fuel for the body during exercise
• Carbohydrates preserve the breakdown of proteins in muscles
• Carbohydrates provide energy for the central nervous system
• Carbohydrates are the most important energy source in the brain
• Carbohydrates glycosylate proteins and lipids
• Carbohydrates are ingested as either complex polymers of simple sugars or simple sugars consisting of disaccharides or monosaccharides
• Carbohydrates must be broken down to monosaccharides to be absorbed in the small intestines (jejunum)

Monosaccharides

• Monosaccharides are the monomer of carbohydrates
• Glucose is a simple sugar (monosaccharide) and 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 the body can use it
• Every cell in the body can use glucose
• The liver is the only organ that can metabolize fructose in significant amounts
• A diet high in calories and fructose can overload the liver, causing it to turn fructose into fat

Disaccharides

• Disaccharides are dimers

• 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

• Starch is a polysaccharide that comes from plants

• Cellulose is an indigestible plant plymer

• Glycogen is the stored form of glucose found in animal muscles

Oral Cavity

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

Stomach

• Salivary amylase continues to digest carbohydrates until the stomach's pH drops below 4.5
• Salivary amylase remains active for 1–2 hours post-meal

Duodenum

• Chyme in the duodenum prompts secretin to release buffers
• Buffers shift the duodenal pH from acidic to alkaline, essential for intestinal enzyme function
• 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 from the stomach, rich in carbohydrates, stimulates gastric inhibitory peptide (GIP) release
• GIP stimulates insulin release by the pancreas

Jejunum

• Epithelial cells in the jejunum 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
• Enzymes transport monosaccharides across the plasma membrane
• 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 before being transported to the liver via the hepatic portal vein

Colon

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

After Absorption

• Glucose can be stored in the liver and the muscles as glycogen (a polymer of glucose)
• Glucose can be converted to triglycerides in the liver
• Glucose can be stored as fat in adipocytes

Energy Needs

• Energy needs are met in the body in 3 phases:
  • Early- glucose catabolism; 1 glucose can generate up to 32 ATP
  • Glycogen breakdown followed by glucose catabolism
  • Fatty acids are converted to Acetyl CoA and enter the Krebs (Citric acid cycle) and mitochondria generate 129 ATP from fatty acids through this process
• Inside cells, glucose can also 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

• When energy is needed, glucose is broken down into two pyruvate molecules via glycolysis in the cytoplasm of cells
• Glycolysis is an anaerobic process which mean it does not need oxygen
• 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
• The processing of 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 Function

• Fats are a component in cell walls
• Fats are a source of energy
• Fats are responsible for absorbing fat-soluble vitamins, including vitamins K, E, D, and A
• Fats insulate your body and protect your organs
• Fats are needed to make steroid hormones

Lipid Digestion

• Dietary lipids are generally not water-soluble

Oral cavity

• 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

Stomach

• 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

• 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
• Chylomicrons diffuse into intestinal lacteals of the lymphatic system, which lack basement membranes and have gaps between endothelial cells

Lipid Absorption

• 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
• 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
• This allows the chylomicrons to 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 to make steroid hormones and compounds like Vitamin D3, used for energy, or it can form plaques that 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

Proteins Essential Functions

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

Protein Structure

• 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

• 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

• 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, which expose 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

• 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

• 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

• 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