ECDA

September 2009

DIGESTION OF CARBOHYDRATES Digestion is the breakdown of food into

smaller particles or individual nutrients. It is accomplished through six basic

processes, with the help of several body fluids—particularly digestive juices that are made up of compounds such as saliva, mucus, enzymes, hydrochloric acid, bicarbonate, and bile.

DIGESTION OF CARBOHYDRATES

The processes of digestion involve: (1) the movement of food and liquids(2) the lubrication of food with bodily secretions(3) the mechanical breakdown of carbohydrates

molecules(4) the reabsorption of nutrients—especially water(5) the excretion of waste products.

Comprehension of the tasks or processes needed to break down food are essential to an understanding of how and when food really begins to function within the body.

DIGESTION OF CARBOHYDRATES

Digestion begins in the mouth with the action of salivary amylase.

Chewed food moves by peristalsis, from the mouth to the pharynx, and then past the epiglottis that covers the larynx.

The food material then progresses past the esophagus and into the stomach.

DIGESTION OF CARBOHYDRATES

Food mixtures leaving the stomach are called chyme, and this empties into the small intestine after about 2 to 4 hours in the stomach.

In the stomach, carbohydrates in foods turn to starch, but it is not until the chyme reaches the small intestine and becomes more neutralized that starch turns to simple sugars that are then absorbed into the portal vein, which transports them to the liver.

DIGESTION OF CARBOHYDRATES Most digestion occurs in the upper portion of

the small intestine, called the duodenum. Below the duodenum is the jejunum, and

then there is the last segment, called the ileum. About 5 percent of undigested food products are

broken down in the ileum. This is why some people can have a small part of their intestine removed by surgery and still seem to digest most foods with little problem.

DIGESTION OF CARBOHYDRATES The pancreas makes pancreatic juice consisting of

enzymes (amylases, lipases, and proteases) and bicarbonate, which helps neutralize acidic secretions produced during digestion.

The pancreas delivers the pancreatic juice to the small intestine, in response to a signal of food in the intestine and the release of the hormone secretin.

The pancreas also has another function, the secretion of the hormones insulin and glucagon, which helps maintain a steady state of blood sugar in the body insulin decreases blood glucose concentrationglucagon increases glucose in the blood

DIGESTION OF CARBOHYDRATES Digestion of food in

the small intestine is usually complete after three to ten hours.

Once digestion is essentially finished, waste products leave the ileum with the help of fiber, and these solids then enter the large intestine (the colon).

GLYCOLYSIS

GLYCOLYSIS Glycolysis, through anaerobic respiration,

is the main energy source in many prokaryotes, eukaryotes without mitochondria (e.g., mature RBCs) and eukaryotic cells under low-oxygen conditions (e.g., heavily-exercising muscle cells or fermenting yeasts).

Glycolysis takes place within the cytosol of the cell.

GLYCOLYSIS EMP is the initial process of most glucose

metabolism, and it serves 3 principal functions:1. Generation of high-energy molecules (ATP and

NADH) as cellular energy sources as part of aerobic respiration and anaerobic respiration

2. Production of pyruvate molecules for the Kreb’s cycle as part of aerobic respiration.

3. Production of a variety of six- and three-carbon intermediate compounds, which may be removed at various steps in the process for other cellular purposes.

GLYCOLYSIS Glycolysis is the sequence of reactions

that converts one glucose molecule into two pyruvate molecules with the concomitant production of a relatively small amount of ATPs.

The most common and well-known type of glycolysis is the Embden-Meyerhof pathway

GLYCOLYSIS

Preparatory phase(INVESTMENT PHASE)

The first five steps are regarded as the preparatory (or investment) phase since they consume energy to convert the glucose into two three-carbon sugar phosphates (Glyceraldehyde-3-phosphate).

Preparatory Phase (EMP)

FIRST STEPThe first step in glycolysis is phosphorylation of

glucose by a family of enzymes called hexokinase to form glucose-6-phosphate (G6P).

This reaction consumes one ATP moleculePhosphorylation of glucose blocks the glucose

from leaking out - the cell lacks transporters for G6P.

Preparatory Phase (EMP)

glucose G6P

hexokinase

ATP

ADP

Preparatory Phase (EMP)

SECOND STEPG6P is then rearranged into fructose-6-

phosphate (F6P) by glucose phosphate isomerase. The change in structure is an isomerization reaction.

Fructose can also enter the glycolytic pathway by phosphorylation at this point.

This reaction is freely reversible under normal cell conditions.

Preparatory Phase (EMP)

G6P F6P

ISOMERASE

Preparatory Phase (EMP) THIRD STEP

This step spends another ATP moleculeThe reaction is now irreversible, and the

energy supplied destabilizes the molecule.Because the reaction catalyzed by

phosphofructokinase-1 (PFK-1) is energetically very favorable, it is essentially irreversible, and a different pathway must be used to do the reverse conversion during gluconeogenesis.

Preparatory Phase (EMP)

F6P F-1,6-BP

PFK-1

ATP

ADP

Preparatory Phase (EMP) FOURTH STEP

Destabilizing fructose-1,6-biphosphate allows the hexose ring to be split into two triose sugars, dihydroxyacetone phosphate (DHAP), a ketone, and glyceraldehyde-3-phosphate (G3P), an aldehyde.

This split reaction is mediated by an enzyme lyase, fructose biphosphate aldolase

G3P DHAP

FBP Aldolase

Preparatory Phase (EMP) The enzyme Triosephosphate isomerase

rapidly interconverts DHAP with G3P that proceeds further into glycolysis. This is advantageous, as it directs DHAP down the same pathway as G3P, simplifying regulation.

Preparatory Phase (EMP)

REVIEW!Starting molecule: GLUCOSEProduct: DHAP and G3P (2 triose molecules)Energy spent: 2 ATP molecules

GLYCOLYSIS

Pay-off phase(REAPING PHASE)

This 2nd half of glycolysis pathway is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH.

Since glucose leads to 2 triose sugars (G3P and DHAP) in the preparatory phase, each reaction in the pay-off phase occurs twice (2x) per 1 glucose molecule.

Pay-off Phase (EMP)

FIRST STEPThe triose sugars (2 G3P molecules) are

dehydrogenated and inorganic phosphate is added to them, forming 1,3-biphosphoglycerate (1,3-BPG)

The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give two NADH + H+.

The enzyme used here is glyceraldehyde phosphate dehydrogenase

Pay-off Phase (EMP)

G3P 1,3-BPG

G3PDH

NAD+ + Pi

NADH + H+

Pay-off Phase (EMP) SECOND STEP

This step is the enzymatic transfer of a one phosphate group from 1,3-BPG to ADP by phosphoglycerate kinase (PGK), forming ATP and 3-phosphoglycerate (3PG).

During this step, 2 new ATPs are formed (equal to the 2 ATPs consumed in the preparatory phase)

This step requires ADP; thus, when the cell has plenty of ATP (and little ADP), this reaction does not occur.

Pay-off Phase (EMP)

1,3-BPG 3PG

PGK

ADP ATP

Pay-off Phase (EMP)

3PG 2PG

THIRD STEPIn this reaction, the enzyme Phosphoglycerate mutase (PGM)

mediates in the formation of 2-phosphoglycerate (2PG)

PGM

Pay-off Phase (EMP)

2PG PEP

FOUTH STEPIn this reaction, the enzyme Enolase, a type of lyase, then forms

phosphoenolpyruvate (PEP) from 2PG.

ENOLASE

Pay-off Phase (EMP)

PEP Pyruvate

LAST (5TH) STEP A substrate-level phosphorylation forms a pyruvate and a

molecule of ATP by means of the enzyme pyruvate kinase (PK).

PK

ADP ATP

Pay-off Phase (EMP)

REVIEW!Starting molecule: Two Glyceraldehyde-3-P

(G3P) moleculesProducts: 2 Pyruvate molecules

4 ATP molecules

2 NADH moleculesEnergy spent: 2 ATP molecules (Prep phase)Net ATP gained: 2 ATPs per 1 glucose molecule

GLYCOLYSIS The products of EMP all have important

cellular uses:ATP provides an energy source for many cellular

functions.NADH + H+ provides reducing power for other

metabolic pathways or further ATP synthesis.Pyruvate is used in the citric acid cycle in aerobic

respiration to produce more ATP, or is converted to other small carbon molecules in anaerobic respiration.