Carbohydrate Metabolism CONT.

What macronutrient is digested in the oral cavity by salivary amylase, and what is the action?

Carbohydrates (CHO).

Action: Breaks down polysaccharides into smaller polysaccharides, oligosaccharides, and disaccharides

What macronutrient is digested in the oral cavity by lingual lipase?

Lipids

What macronutrient is acted upon by HCl and pepsin in the stomach, and what is the result?

Proteins

Action: HCl denatures them, and pepsin makes them into smaller peptides

What enzyme continues to digest lipids in the stomach, and what are the products?

Lingual lipase and gastric lipase.

Products: Fatty acids and monoglycerides

What enzyme in the small intestine digests carbohydrates, and what does it break them down into?

Pancreatic amylase

Action: Breaks down polysaccharides and oligosaccharides into disaccharides

What are the three brush border enzymes in the small intestine and what disaccharides do they break down?

• Sucrase (breaks down sucrose)

• Lactase (breaks down lactose)

• Maltase (breaks down maltose)

What are the product of sucrose digestion by sucrase?

Glucose and fructose

What are the products of lactose digestion by lactase

Galactose and glucose

What are the product of maltose digestion by malatase

Glucose

What enzyme further digest oligosaccharides in the small intestine?

Dextrinase and glucoamylase

What substance in the small intestine helps in fat digestion by emulsifying fats?

Bile salts

What happens to absorbed sugars during the fed state?

ransported to the liver by the hepatic portal system

What happens to most glucose after it reaches the liver?

Passes through the liver and becomes available to cells throughout the body

What happens to excess glucose in the liver during the fed state?

Absorbed by the liver and converted into glycogen or fat

What happens to fats during the fed state?

Enter the lymph as chylomicrons

What are chylomicrons?

Lipoproteins that transport dietary fats

What does lipoprotein lipase do?

Removes fats from chylomicrons for tissue uptake

Which tissues use fats as primary energy sources during the fed state?

• Hepatocytes

• Adipocytes

• Muscle cells

What happens to most fat synthesized by the liver?

Released into circulation

What happens to most amino acids during the fed state?

Pass through the liver and go to other cells for protein synthesis

What can amino acids be used for in liver cells during the fed state?

• Protein synthesis

• Fuel for ATP synthesis

• Fatty acid synthesis

What is the main function of amino acids in other body cells during the fed state?

Protein synthesis

What regulates the absorptive (fed) state?

Insulin and several intestinal hormones

What is the main function of insulin during the absorptive state?

Regulates glucose uptake by most cells

Which cells do not require insulin for glucose uptake?

• Neurons

• Kidney cells

• Erythrocytes

What effect does insulin have on plasma glucose levels?

Increases cellular glucose uptake, causing plasma glucose to fall

What processes does insulin stimulate?

• Glucose oxidation

• Glycogenesis

• Lipogenesis

What process does insulin inhibit?

Gluconeogenesis

What effect does insulin have on amino acids?

Stimulates active transport of amino acids into cells

What effect does insulin have on proteins?

Promotes protein synthesis

What happens to most dietary carbohydrates after absorption?

Burned as fuel within hours

What is oxidative carbohydrate metabolism?

Glucose catabolism

What is the function of glucose catabolism?

Transfers energy from glucose to ATP

What is glucose catabolism?

Breakdown of glucose to release energy

What are the three major pathways of glucose catabolism?

• Glycolysis

• Anaerobic fermentation

• Aerobic respiration

What happens during glycolysis?

Glucose is split into two molecules of pyruvate

What happens during anaerobic fermentation?

Pyruvate is reduced to lactate without oxygen

What happens during aerobic respiration?

Pyruvate is oxidized to carbon dioxide and water using oxygen

What do enzymes remove from intermediate compounds during glucose catabolism?

Electrons as hydrogen atoms

What do coenzymes do during glucose catabolism?

Accept and transfer hydrogen atoms during reactions

What is glycolysis?

Metabolic pathway that splits glucose into two pyruvate molecules

What is the first step of glycolysis?

Phosphorylation

What enzyme phosphorylates glucose during glycolysis?

Hexokinase

What does hexokinase do?

Transfers a phosphate group from ATP to glucose

What is produced when glucose is phosphorylated?

Glucose 6-phosphate (G6P)

Why is glucose converted to glucose 6-phosphate?

• Keeps intracellular glucose concentration low

• Promotes continued glucose diffusion into the cell

• Prevents glucose from leaving the cell

Why can glucose 6-phosphate not leave the cell?

Phosphorylated compounds cannot pass through the membrane

What can happen to glucose 6-phosphate?

• Converted to fat

• Converted to amino acids

• Polymerized into glycogen

• Oxidized for energy extraction

Where does glycolysis occur?

Cytosol

What molecule is the starting point for glycolysis, and where does it come from?

Glucose, which can enter the cell directly or be released from glycogen

How many ATP molecules are inititally invested in glycolysis to trap glucose in the cell

2 ATP

What is the end product of the break down of the 6-carbon sugar during glycolysis?

Two 3-carbon pyruvate molecules

What are the net products yielded from glycolysis?

2 ATP, 2 Pyruvate, 2 NADH

What is glycogen and where is it stored?

Glycogen is a polysaccharide of glucose, stored in the muscles and liver

What is the first reaction in glycolysis?

Glucose is phosphorylated to glucose 6-phosphate

What enzyme catalyzes the conversion of glucose to glucose 6-phosphate

Hexokinase

What is the net ATP gain from glycolysis?

2 ATP per glucose

Why is the net gain of glycolysis only 2 ATP?

4 ATP produced but 2 ATP used to start glycolysis

Where is most of glucose’s energy after glycolysis?

In the pyruvate molecules

What determines the fate of pyruvate?

Oxygen availability

Why can glycolysis not continue without anaerobic fermentation?

NAD+ must be regenerated

What does NADH do during anaerobic fermentation?

Donates electrons to pyruvate

What is produced when pyruvate is reduced during anaerobic fermentation?

Lactate

What happens to lactate after it is formed?

Travels to the liver through the blood

What does the liver do with lactate when oxygen becomes available?

Oxidizes it back to pyruvate

Why do we breathe harder after exercise?

To provide oxygen needed to process lactate

What can the liver do with lactate besides converting it to pyruvate?

• Convert it to G6P

• Store it as glycogen

• Release glucose into the blood

Why is anaerobic fermentation considered wasteful?

Most energy from glucose remains in lactate

Which muscle tolerates anaerobic fermentation best?

Skeletal muscle

Which tissue does not use anaerobic fermentation?

Brain

What happens to pyruvate in the presence of oxygen?

Enters mitochondria for aerobic respiration

What is required for aerobic respiration?

Oxygen

What are the two main steps of aerobic respiration?

• Matrix reactions

• Membrane reactions

Where do matrix reactions occur?

Mitochondrial matrix

Where do membrane reactions occur?

Mitochondrial cristae

What starts the citric acid cycle?

Acetyl-CoA combines with oxaloacetic acid to form citric acid

What happens to the carbon atoms from glucose during aerobic respiration?

Released as CO2 and exhaled

Where is most glucose energy stored after matrix reactions?

NADH and FADH2

What are the purposes of membrane reactions?

• Transfer energy to ATP

• Regenerate NAD+ and FAD

What is the mitochondrial electron transport chain?

Series of compounds that transfer electrons

What is the final electron acceptor in the electron transport chain?

Oxygen

What is formed when oxygen accepts electrons and protons?

Water

What is the body’s primary source of metabolic water?

Water formed during aerobic respiration

What creates the proton gradient in mitochondria?

Proton pumps in the electron transport chain

What is the chemiosmotic mechanism?

Protons flowing through ATP synthase drive ATP production

How much ATP can one NADH produce?

About 2.5 ATP

How much ATP can one FADH2 produce?

About 1.5 ATP

How many ATP are produced by complete aerobic oxidation of one glucose molecule?

32 ATP

What is glycogen?

Energy storage molecule

What is glycogenesis?

Synthesis of glycogen

What hormone stimulates glycogenesis?

Insulin

What is glycogenolysis?

Breakdown of glycogen

What hormones stimulate glycogenolysis?

• Glucagon

• Epinephrine

What is gluconeogenesis?

Synthesis of glucose from noncarbohydrates

What substances can be used for gluconeogenesis?

• Glycerol

• Amino acids

Where does gluconeogenesis mainly occur?

Liver

What is lipogenesis?

Synthesis of fat

What molecules can be used to make fat?

• Sugars

• Amino acids

What is lipolysis?

Breakdown of fat for fuel