Ch22: Carbohydrate Metabolism

SPR26 OSU BIOPHRM 3312

Stage I of food catabolism occurs in the digestive tract; the digestion of _______ into their respective monomers.

• lipids (triacylglycerol)

• Carbohydrates

• proteins

lipid - triacylglycerol products

fatty acids and glycerol

carbohydrates products

glucose and other simple sugars (fructorse and galactose)

protein products

standard amino acids (20)

The digestive system consists of the________

digestive tract and several accessory organs.

The digestive tract, starting from the mouth, includes the following organs in sequence:

Mouth → stomach → small intestine (duodenum, jejunum, ileum) → large intestine → rectum → anus

Accessory digestive organs include the ______ aid digestion.

salivary glands, liver, gallbladder, and pancreas

Digestion involves both _______processes, such as chewing in the mouth and churning in the stomach, and _______processes facilitated by enzymes that break down food.

• mechanical

• chemical

The small intestine is the primary site of absorption for water-soluble nutrients, such as simple sugars, amino acids, and short-chain fatty acids, which enter the bloodstream and are delivered to the liver through the ____.

hepatic portal vein

large fat molecules do not enter the bloodstream, rather they enter the _______ system.

lymphatic

alpha-amylase is produced by the salivary gland and the pancreas. Maltase is produced by the small intestine.

No digestion of ______ in the stomach

carbohydrates

The first stage in carbohydrate catabolism is digestion, the breakdown of _______

polysaccharides and disaccharides into monosaccharides (mostly glucose, also fructose and galactose)

Digestion entails the mechanical breakdown (physical grinding, softening, and mixing) of food, as well as the _________.

enzyme-catalyzed hydrolysis of glycosidic bonds

Digestion begins in the mouth with ________, continues in the stomach (mechanical only), and concludes in the small intestine (_______ where most digestion occurs).

• saliva alpha-amylase

• duodenum

______released from the pancreas continues the breakdown of polysaccharides into smaller oligosaccharides, disaccharides, and maltose.

Pancreatic amylase

Maltase, sucrase, and lactase, present in enterocytes breakdown maltose, sucrose, and lactose, respectively, into their monosaccharide components _________

(glucose, fructose, and galactose).

only the form ____ can be absorbed by microvilli, as ____ are too large.

• monosaccharides

• disaccharides

The small intestine (jejunum) maximizes nutrient absorption through its large surface area enhanced by three structural features: in this order, from outer to inner:

folds → villi → microvilli

Folds:

deep, permanent folds that slow down the movement of chyme, providing more time for digestion and absorption.

Villi:

inner surface of the small intestine is covered with tiny, finger-like projections - each is lined with enterocytes (specialized epithelial cells) that help absorb nutrients

Microvilli:

On the apical surface of enterocytes, these are even smaller hair-like structures — forming the brush border, which increases surface area further and contains enzymes that complete the final stages of digestion.

Monosaccharides (glucose, galactose and fructose) are absorbed into enterocytes by __________

specific transporters.

where do sugars enter after the absorption by small intestine

portal circulation and are delivered to the liver

what are sugars used for in the liver

• energy production

• stored glycogen

• fatty acid conversion (when excess)

• released into bloodstream (maintain glucose homeostasis)

the peripheral tissues take up glucose, why

• ATP production

• glycogen storage

• biosynthetic pathways

what is the role of the microbiota

in colon, microorganisms ferment cellulose, undigested carbohydrates and unabsorbed sugars into short-chain fatty acids and gases such as CO2, CH4, and H2 and H2S

digestion of what causes abdominal cramps and pain, bloating, gas, diarrhea

fermentation of cellulose

Each human adult harbors ______ bacteria in the gut (called gut microbiota), ~ 3 times the number of human cells in our body.

~10¹⁴

short chain fatty acids

• acetate

• propionate

• butyrate

glycolysis function

ATP production & converts glucose to pyruvate for CAC

gluconeogenesis function

Glucose synthesis from non-carb precursors — pyruvate, lactate, glycerol (from lipids), and glucogenic amino acids (from proteins)

Glycogenesis (Glycogen Synthesis) function

Storage glucose as glycogen

Glycogenolysis (Glycogen Degradation)

Breakdown of glycogen to glucose-6-phosphate or glucose

Pentose Phosphate Pathway (PPP)

Converts glucose to NADPH & ribose-5-phosphate (ribose)

glycolysis location

cytoplasm of all cells

gluconeogenesis location

liver (main) & kidneys

glycogenesis (glycogen synthesis) location

liver and muscle cytoplasm

glycogenolysis (glycogen degradation) location

liver and muscle cytoplasm

pentose phosphate pathway location

cytoplasm

glycolysis process

Glucose → 2 pyruvate + 2 ATP + 2 NADH (per glucose).

Gluconeogenesis process

Lactate, glycerol, and amino acids → glucose

Glycogen Synthesis (Glycogenesis) process

Glucose → glucose-6-phosphate → UDP-glucose → glycogen

Glycogen Degradation (Glycogenolysis) process

Glycogen → glucose-1-phosphate → glucose-6-phosphate → glucose (liver only)

Pentose Phosphate Pathway (PPP) process

Glucose-6-phosphate → NADPH + ribose-5- phosphate + glycolytic intermediates

Glycolysis purpose

Provides ATP, intermediates for other pathways, and pyruvate for the citric acid cycle

Gluconeogenesis purpose

Maintains blood glucose during prolong fasting or starvation.

Glycogen Synthesis (Glycogenesis) purpose

Glucose storage for energy during fasting (liver) or exercise (muscle).

Glycogen Degradation (Glycogenolysis) purpose

Maintain blood glucose (liver) or provide energy for muscle contraction

Pentose Phosphate Pathway (PPP) purpose

NADPH for biosynthesis (fatty acids, cholesterol), ribose-5-phosphate for nucleotide synthesis, and redox balance

Glucose serves as a ________ source for all living cells, with most cellular energy derived from its bonds

primary energy

While various tissues can also utilize fats or proteins for energy, the _______ rely predominantly on glucose.

brain and red blood cells

________, a linear metabolic pathway occurring in the cytoplasm of all human cells involves a series of 10 enzyme- catalyzed reactions.

Glycolysis

Glycolysis breaks down one glucose molecule into _________

2 pyruvates, 2 ATP & 2 NADH

glucose metabolism aerobic conditions:

Pyruvate is transported into mitochondria and converted to acetyl-CoA by pyruvate dehydrogenase to enter the citric acid cycle for ATP production.

glucose metabolism anaerobic conditions:

Pyruvate is reduced to lactate-by-lactate dehydrogenase to regenerate NAD⁺ allowing glycolysis to continue.

Oxidation of Pyruvate under aerobic conditions, where does this occur?

mitochondrial matrix

Oxidation of Pyruvate under aerobic conditions, what is the enzyme complex used to decarboxylate pyruvate (release CO2)?

pyruvate dehydrogenase

Oxidation of Pyruvate under aerobic conditions, what are the products?

acetyl-CoA and CO2

Oxidation of Pyruvate under aerobic conditions, what is this reaction coupled with?

reduction of NAD+ to NADH

Oxidation of Pyruvate under aerobic conditions, what happens to the acetyl-CoA?

enters the CAC, generates reduced enzymes, which product ATP via oxidative phosphorylation

Reduction of Pyruvate under anaerobic conditions, what is the overall rxn?

lactate dehydrogenase catalyzes the reduction of pyruvate to lactate, which is coupled to the oxidation of NADH to NAD⁺
— crucial for regenerating NAD⁺, which is required for the continuation of glycolysis (Step 6)

3 NAD+ is converted to 3 NADH in the _________

citric acid cycle

NAD+ is regenerated from NADH via the __________

electron transport chain

what are the rxns that convert NAD+ to NADH?

• glycolysis (step 6)

• pyruvate → acetyl-CoA (by pyruvate dehydrogenase)

• citric acid cycle (steps 3, 4, 8)

what rxns regenerations NAD+ from NADH?

• ETC

• lactate fermentation

Lactate Fermentation: When oxygen availability is insufficient for the electron transport chain, an alternative pathway to regenerate _________

NAD+ from NADH involves the reduction of pyruvate to lactate catalyzed by alcohol dehydrogenase

anaerobic glycolysis energy output

2 ATP

aerobic glycolysis energy output

32 ATP