Carbohydrate Metabolsim

You will be able to… Explain how Carbs are digested & absorbed Draw the steps involved in Glycolysis Compare and contrast aerobic respiration to two different types of fermentation Discuss the 3 possible fates of Pyruvate

Mass composition data for the human body Carbohydrates don’t make the man or woman

Carbs do run the man or woman: Carbohydrate recommendation is 45 to 65 percent of total calories. Sugars and starches supply energy to the body as glucose. Only energy source for red blood cells. Preferred energy source for brain & CNS

Energy sources Structural molecules

Salivary glands secrete saliva which contains mucus, water, ions, and amylase enzyme. Food is only briefly in the mouth, so carbohydrate digestion just begins. Starch polymers are broken down into smaller chains and into some disaccharides: maltose, sucrose, lactose. starch - found in plants.  It's a polymer of glucose linked in a main chain through a 1->4 links with a 1->6 branches.  Amylose is starch with no branches, while amylopectin has branches.  Starch granules consist of about 20% amylose and 80% amylopectin. glycogen - the main CHO storage in animals.   Muscle and liver glycogen consists of glucose residues in a 1->4 links with lots of a 1->6 branches (many more branches than in starch).

Small intestine > 3 m long, 2 to 3 cm wide. Coils, folding, & villi give surface area of 500-600 m long tube. Upper part (duodenum) most active in digestion: villi cells produce enzymes which complete digestion of carbohydrates. The small intestine is the major site for digestion and absorption of nutrients. The small intestine is up to 6 meters long and is 2-3 centimeters wide. The upper part, the duodenum, is the most active in digestion. Secretions from the liver and pancreas are used for digestion in the duodenum. Epithelial cells of the duodenum secrete a watery mucus. The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate. The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum. Most absorption occurs in the duodenum and jejeunum (second third of the small intestine). The inner surface of the intestine has circular folds that more than triple the surface area for absorption. Villi covered with epithelial cells increase the surface area by another factor of 10. The epithelial cells are lined with microvilli that further increase the surface area; a 6 meter long tube has a surface area of 300 square meters. Each villus has a surface that is adjacent to the inside of the small intestinal opening covered in microvilli that form on top of an epithelial cell known as a brush border. Each villus has a capillary network supplied by a small arteriole. Absorbed substances pass through the brush border into the capillary, usually by passive transport. Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli. Starch is broken down into two-glucose units (maltose) elsewhere. Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. Lactose intolerance results from the genetic lack of the enzyme lactase produced by the intestinal cells.

Plasma membrane of microvilli use these enzymes to complete digestion: Disaccharidases: disaccharides converted into monosaccharide subunits. Maltase: hydrolyzes maltose into glucose. Sucrase: hydrolyzes sucrose into glucose & fructose. Lactase: hydrolyzes lactose into glucose & galactose.

Common Disaccharides: Maltose: (14) of two D-Glucose molecules Lactose  (14) Milk sugar: galactose and glucose connected Sucrose: -Glucose and -Fructose ,  (12) glycosidic linkage

Small intestine’s second job: Absorption Uses increased S. A Small intestine’s second job: Absorption Uses increased S.A. with folds projecting into lumen (plicae circulares), villi and microvilli. Small intestine Absorption is promoted by increasing the active surface area. This occurs through increase in the length of the tube during development, folds projecting into the lumen (plicae circulares), villi and microvilli. The brush border of absorptive cells is coated with a glycoprotein, the glycocalyx. Lactase, sucrase, peptidase, lipase and alkaline phosphatase enzymes are embedded in the glycocalyx. The central core of each villus contains connective tissue, blood vessels and lymphatics responsible for transport of absorbed materials Sugars into bloodstream: Fructose diffuses into villi, glucose & galactose absorbed by active transport.

Summary of carbohydrate digestion in the human body.

What to do with those sugars? Turn them into ATP = Cellular respiration Aerobic respiration Requires molecular oxygen Includes series of redox reactions -Anaerobic respiration *Fermentation *Does not require oxygen Cellular Respiration Cellular respiration - Process by which cells convert the energy of fuel molecules (eg. glucose) into usable bond energy in ATP. (Break the $100 dollar bill into small change) Anaerobic respiration - Cellular respiration in the absence of oxygen. Aerobic respiration - Cellular respiration which uses oxygen to completely oxidize fuel molecules. All are exergonic (occur spontaneously) Use a lot of coupled reactions

Reaction Types in Cellular Respiration Dehydrogenation - Hydrogens transferred to a coenzyme (an energy carrier molecule). 2. Decarboxylations - Carboxyl groups (COO-) removed from substrates as carbon dioxide (CO2). 3. Preparation reactions - Molecules rearranged in preparation for dehydrogenation or decarboxylation. 4. Phosphorylation – phosphates added to provide energy or transform molecules.

Four stages of aerobic respiration Reaction Types in Cellular Respiration 1. Dehydrogenation - Hydrogens transferred to a coenzyme. 2. Decarboxylations - Carboxyl groups removed from substrates as carbon dioxide. 3. Preparation reactions - Molecules are rearranged in preparation for dehydrogenation or decarboxylation. Note location of each stage & amount of ATP formed Product of one stage becomes reactant of next stage

(organic molecule) Phosphorylation Glycolysis: Glyco = sugar Lysis = to split One 6 C glucose split into two 3 C pyruvates (eventually) Substrate (organic molecule) Phosphorylation of glucose occurs to make the reactions exergonic Also stops glucose from leaving the cell. This process actually involves 4 separate subpathways: glycolysis, transition reaction, the Kreb's Cycle, and the electron transport system. 1. Glycolysis This first step is actually anaerobic and occurs in the cytosol of the cell. It involves the breakdown of glucose into pyruvate (pyruvic acid) 2ATP 2ADP 2NAD 2NADH Glucose ———————————————> PGAL —————————————————> 2 Pyruvate

An overview of glycolysis. 6 C stages & 3 C stages Energy absorbing Energy producing

Fructose 1,6 bisphosphate Glycolysis: energy investment Phase: Phosphorylation ATP kick start: Glucose turned into Fructose 1,6 bisphosphate this turns eventually into 2 glyceraldehyde-3-phosphate molecules Step 1 Energy input phase. The cell uses 2 molecules of ATP as a source of energy to do some chemical rearrangements resulting in a 6 carbon sugar called fructose 1,6 biphosphate. The ATP here serves as activation energy. Generally in cellular respiration when molecules need to be rearranged ATP needs to be used. In this case the chemical rearrangements result in a molecule which can be easily split into two three carbon molecules. Inorganic phosphate Its important to realize that ATP production by electron transport phosphorylation uses inorganic phosphate from the cytoplasm as the source of phosphate. In contrast substrate level phosphorylation uses phosphate transferred from other organic compounds to the ADP to make ATP. Step 2 Breaking the fructose 1,6 biphosphate into to two three carbon molecules called PGAL The PGAL is a molecule from which energy can easily be harvested. Glycolysis actually involves a series of separate reactions. The end products of glycolysis include 2 molecules of pyruvate, 2 molecules of NADH and a net gain of 2 ATP molecules.

Structural relationships among glycerol and acetone and the four C3 intermediates in the process of glycolysis. This converts into That (isomers)

The two glyceraldehyde-3-phosphates eventually turn into two pyruvates Glycolysis: E capture phase The two glyceraldehyde-3-phosphates eventually turn into two pyruvates (with what type of rxn?) The end products of glycolysis include: 2 molecules of pyruvate 2 molecules of NADH 2 ATP molecules net gain (4 made / 2 used) Step 3 Energy harvesting stage 1. 2 ADP are used to make two molecules of ATP. Step 4 Energy harvesting stage 2. Two more ADP and 2 NAD+ molecules are used to make two molecules of NADH and two more molecules of ATP. This step also yields two pyruvate molecules. The pyruvate still have most of the original energy that was found in the original glucose molecule and the point of the of aerobic cellular respiration will be to harvest as much of that energy as possible! What are the net results of glycolysis? 2 ATPs, 2 NADH, and a glucose turned into 2 pyruvate If O2 is present for the completion of cell respiration, the pyruvic acid now undergoes a transition reaction, preparing it for the Kreb's Cycle: 2NAD 2NADH 2 Pyruvic Acid———> 2 Acetyl coenzyme A + CO2

Glycolysis animation simplistic version Glycolysis animation Let’s go through the steps in detail: Glycolysis animation simplistic version Glycolysis animation more detailed steps Glycolysis animation details http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html http://programs.northlandcollege.edu/biology/Biology1111/animations/glycolysis.html http://www.johnkyrk.com/glycolysis.html Enzymes involved in glycolysis