Metabolism & major metabolic pathways

Question 1

What is digestion?

Answer 1

Digestion is the process of breaking food down into molecules small enough to absorb

Question 2

What is the purpose of mechanical digestion?

Answer 2

Chewing or grinding, increases the surface area of food

Question 3

What is the purpose of chemical digestion?

Answer 3

Chemical digestion splits food into small molecules that can pass through membranes; these are used to build larger molecules

Question 4

In chemical digestion, what does the process of enzymatic hydrolysis

Answer 4

It splits bonds in molecules with the addition of water

Question 5

What chemical digestion occurs in the human digestive system?

Answer 5

1) Carbohydrate Digestion
2) Protein Digestion
3) Nucleic Acid Digestion
4) Fat Digestion

Question 6

How are carbohydrates digested in the Oral cavity, Pharynx and Esophagus?

Answer 6

Polysaccharides such as starch and glycogen are broken down by salivary amylase into smaller polysaccharides or maltose

Question 7

Are carbohydrates broken down in the stomach?

Answer 7

No, Ph is too high

Question 8

How are carbohydrates broken down in the small intestine

Answer 8

1) Pancreatic amylases break down the smaller polysaccharides into disaccharides (that weren’t broken down)
2) The disaccharides (from oral cavtity) are then broken down by enzymes from inestinal epithelium into monosaccharides

Question 9

How are proteins digested in the stomach?

Answer 9

Proteins are digested by pepsin into small polypeptides

Question 10

How are proteins digested in the small intestine?

Answer 10

1) Small polypeptides are digested by pancreatic trypsin and chrymotrypsin into smaller polypeptides and then finally by pancreatic carboxypeptidase into small peptides
2) Then, the small peptides are digested into amino acids by dipeptidases, carboxypeptidase and aminopeptidase

Question 11

Are nucleic acids digested in the oral cavity, pharynx and esophagus?

Answer 11

No

Question 12

Are proteins digested in the oral cavity, pharynx and esophagus?

Answer 12

No

Question 13

Are nucleic acids digested in the stomach?

Answer 13

No

Question 14

How are nucleic acids digested in the small intestine?

Answer 14

1) DNA and RNA is digested into nucleiotides by pancreatic nucleases
2) Nucleotides are converted to nucleotidases which is then digested into nitrogenous bases, sugars and phosphates by nucleosidases and phosphatases

Question 15

Are fats digested in the oral cavity, pharynx and esophagus?

Answer 15

No

Question 16

Are fats digested in the stomach?

Answer 16

No

Question 17

How are fats digested in the small intestine?

Answer 17

Fat (triglycerides) are converted to glycerol, fatty acids and monoglycerides by pancreatic lipase

Question 18

What does over nourishment cause?

Answer 18

obesity, which results from excessive intake of food energy with the excess stored as fat

Question 19

Why is obesity bad for you?

Answer 19

Obesity contributes to type 2 diabetes, cancer of the colon and breasts, heart attacks, and strokes

Question 20

What helps regulate body weight?

Answer 20

Hormones regulate long-term and short-term appetite by affecting a “satiety center” in the brain

i. Ghrelin
ii. Insulin and PYY
iii. Leptin

Question 21

How does ghrelin help regulate body weight?

Answer 21

A hormone secreted by the stomach wall, triggers feelings of hunger before meals

Question 22

How does insulin and PYY help regulate body weight?

Answer 22

Insulin from the pancreas and PYY, a hormone secreted by the small intestine after meals, both suppress appetite

Question 23

How does leptin help regulate body weight?

Answer 23

Produced by adipose (fat) tissue, also suppresses appetite and plays a role in regulating body fat levels

Question 24

How does the small intestine have a huge surface area?

Answer 24

Due to villi and microvilli that are exposed to the intestinal lumen (The enormous microvillar surface creates a brush border that greatly increases the rate of nutrient absorption)

Question 25

What is the imprortance of digestion?

Answer 25

It allows materials needed to process the food to be localised in a single place eg, teeth in mouth
Prevents each cell having to contain a full range of digestive enzymes

Question 26

What is ingestion?

Answer 26

It’s the taking in of food into the alimentary canal eg, putting food in your mouth

Question 27

Why is digestion essential for absorption?

Answer 27

If food isn’t broken down, chemicals in food would not pass from digestive system into body tissues

Question 28

What is absorption?

Answer 28

Soluble food is passed from cells into bloodstream

Question 29

What is egestion?

Answer 29

It’s the removal of unabsorbed and undigested material from the digestive system through anus

Question 30

What is the function of villi?

Answer 30

It increases the surface area for either digestion or absorption

Question 31

Where are the villus found?

Answer 31

Lining of duodenum and Ileum

Question 32

What are the adaptions of the small intestine for absorption?

Answer 32

Very long
Numerous villi
Walls of villi are one cell thick
Rich blood supply to carry away water soluble products
Each villus has lymph supply [lacteal] to carry away fats

Question 33

The body stores energy-rich molecules that are not immediately needed, how is it stored?

Answer 33

In humans, excess energy is first stored in the liver and muscle cells in a polymer called glycogen, which is a branched chain form of glucose

Question 34

What is the purpose of the body storing energy-rich molecules?

Answer 34

When fewer calories are taken in than expended, the human body expends liver glycogen first, then muscle glycogen and fat and subsequently proteins

Question 35

What happens to the excess energy when glycogen stores become full?

Answer 35

When glycogen stores become full, excess energy is stored in fat in adipose cells in the form of triglycerides.

Question 36

What allows tryglyceride to make multiple oxidations possible?

Answer 36

Are highly reduced molecules

Question 37

What makes tryglycerides compact?

Answer 37

They are anhydrous (exclude water)

Question 38

Which tissue fuel provides fuel for the longest to shortest, in order

Answer 38

Fat, Body Protein, Muscle Glycogen, Liver Glycogen, Blood/Extracellular Glucose

Question 39

What is the fate of carbohydrates in the body (in the “non-fasting” state)

Answer 39

Carbohydrates are broken down into glucose by various enzymes.
Some glucose is used immediately, but the majority enters the blood stream triggering insulin release and the uptake of glucose into cells. Excess glucose is stored as glycogen in liver and muscle.

Question 40

What is the fate of proteins in the body (in the “non-fasting” state)

Answer 40

Proteins are broken down into individual amino acids and used in body cells to form new proteins or to join the amino acid pool.
Amino acids that are in excess of the body’s needs are converted by liver enzymes into keto acids and urea. Keto acids may be used as sources of energy, converted into glucose, or stored as fat. Urea is excreted from the body in sweat and urine.

Question 41

What is the fate of Fats in the body? (in the “non-fasting” state)

Answer 41

Fats are digested in the small intestine, and packaged into lipoproteins.
Excess fat is stored as droplets in fat cells. When fats are used as an energy source, they are broken down in cellular mitochondria through b-oxidation.

Question 42

What is the fate of Carbohydrates, fats and proteins in the body (in the fasting state)

Answer 42

Carbohydrate, fats and protein are metabolized in separate processes into a common product called acetyl-CoA.
Acetyl-CoA is a major metabolic pathways player, and is an important part of the process which creates the energy molecule called ATP (adenosine triphosphate) in the mitochondria.

Question 43

What is the function of the hormone glucagon?

Answer 43

It is released and it triggers the release of glycogen and fatty acids to fuel the body until the next meal arrives.

Question 44

What occurs approximately 2-4 hours after a meal?

Answer 44

Blood glucose concentration drops to normal baseline levels and a fasting state begins. This drop in blood sugar causes insulin levels to also decline. Another hormone, glucagon, is released and it triggers the release of glycogen and fatty acids to fuel the body until the next meal arrives.

Question 45

If you skip lunch, what will your body begin to do?

Answer 45

When you skip lunch, your glucose levels are low – you release hormone glucagon and it enters blood stream and acts on target cells in liver. It breaks glycogen to glucose, glucose is released and blood glucose levels increase

Question 46

What happens to your metabolism in the “starvation” state?

Answer 46

The stored glycogen in the liver and muscles is exhausted, insulin levels drop and the body ramps up its access to stored fat.
As fatty acids flow into the blood stream, the liver takes the excess fats and more ketone bodies through ketosis.
The muscles continue to burn fatty acids, but decrease their use of ketones. The ketone bodies then build up in the blood stream to a level at which the brain begins to oxidize them for fuel. As the brain uses the ketones, it needs less glucose, so the liver decreases the rate of gluconeogenesis.

Question 47

As the brain uses the ketones, it needs less glucose, so the liver decreases the rate of gluconeogenesis, how does this help to preserve muscle tissue?

Answer 47

This helps preserve muscle tissue as the body doesn’t need to break down the amino acids to convert them to glucose.

Question 48

How can the body survive for long periods without eating?

Answer 48

Due to the ability to metabolise ketones, the human body can survive for long periods without eating.

Question 49

Where are the hormones glucagon and insulin both produced and what cells produce them?

Answer 49

Islets of the pancreas
Alpha cells -glucagon,
Beta cells(most abundant) - make insulin

Question 50

What are alpha cells?

Answer 50

Alpha cells (more commonly alpha-cells or α-cells) are endocrine cells in the pancreatic islets of the pancreas.

Question 51

Degeneration of the beta cells is the main cause of what?

Answer 51

type I (insulin-dependent) diabetes mellitus

Question 52

The synthesis and breakdown of glycogen is central to what?

Answer 52

Maintaining metabolic balance

Question 53

What regulates the breakdown of glycogen into glucose?

Answer 53

Insulin and glucagon

Question 54

What is the site for glucose homeostasis?

Answer 54

The liver

Question 55

What stimulates glycogen synthesis?

Answer 55

A carbohydrate-rich meal raises insulin levels, which triggers the synthesis of glycogen.

Question 56

What stimulates the break down of glycogen?

Answer 56

Low blood sugar causes glucagon to stimulate the breakdown of glycogen and release glucose

Question 57

What is glycogen?

Answer 57

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage, representing the main storage form of glucose in the body.

Question 58

What are the two types of alpha linkage found in glycogen?

Answer 58

a-1,4 and a-1,6-glycosidic bond

Question 59

What are alpha linkages?

Answer 59

When a compound with alpha confirmation links with another compound with alpha confirmation, the linkage is alpha linkage

Question 60

Where is the glycogen in humans stored?

Answer 60

The liver and skeletal muscle.

Question 61

How is the liver and skeletal muscle able to hold a large amount of glycogen?

Answer 61

Due to the large weight of muscle (however this glycogen is not available to the rest of the body)

Question 62

What is the function of the liver glycogen stores?

Answer 62

To maintain glucose homeostasis

Question 63

What do the liver cells use glycogen for?

Answer 63

To control glucose levels.

Question 64

What makes glucose useful that differs from fats and amino acids in terms of energy

Answer 64

Glucose is useful for shorts bursts of energy without using oxygen. This differs from the breakdown of fats and amino acids which requires oxygen

Question 65

What is the key enzyme in glycogen breakdown?

Answer 65

Glycogen phosphorylase is the key enzyme in glycogen breakdown i.e. control point.

Question 66

What is the key enzyme in glycogen synthesis?

Answer 66

The primary, regulated enzyme in glycogenesis is glycogen synthase

Question 67

What is Glycogenolysis?

Answer 67

process by which glycogen, the primary carbohydrate stored in the liver and muscle cells of animals, is broken down into glucose to provide immediate energy and to maintain blood glucose levels during fasting.

Question 68

Describe Glycogenolysis

Answer 68

In humans, excess energy is first stored in the liver and muscle cells in a polymer called glycogen, which is a branched chain form of glucose. Low blood sugar causes you to release the hormone glucagon and it enters blood stream and acts on target cells in liver. It breaks glycogen to glucose, glucose is released and blood glucose levels increase.
This drop in blood sugar causes insulin levels to also decline.
Once glucose is trapped in cells as glucose-6-phosphate, it can be converted to glucose-1-phosphate, which is then added to glycogen chains by two repetitive reactions.
Glycogen phosphorylase is the key enzyme in glycogen breakdown i.e. control point

Question 69

What is Gluconeogenesis?

Answer 69

The formation of glucose, allows the production of glucose from non-sugar molecules.

Question 70

What are the major non-carbohydrate precursors which may be used for the formation of glucose?

Answer 70

Lactate, pyruvate, glycerol and amino acids

Question 71

What are the major non-carbohydrate precursors which may be used for the formation of glucose?

Answer 71

Lactate, pyruvate, glycerol and amino acids

Question 72

In mammals, where does glucogenesis principally occur?

Answer 72

The liver, kidney, intestine, and muscle.

Question 73

What major non-carbohydrate precursos does the
a) liver
b)kindey
c) intestines
use for the formation of glucose

Answer 73

Liver - lactate, glycerol, and glucogenic amino acids (especially alanine)
Kidney - lactate, glutamine and glycerol.
Intestine - glutamine and glycerol.

Question 74

The liver can use both glycogenolysis and gluconeogenesis to produce glucose, whereas the kidney only uses gluconeogenesis, why?

Answer 74

After a meal, the liver shifts to glycogen synthesis, whereas the kidney increases gluconeogenesis. (ask)

Question 75

Gluconeogenesis is not the reverse of glycolysis, why?

Answer 75

Glycolysis is an exergonic process whereas gluconeogeneis is endergonic. Seven of the steps in glycolysis steps are at equilibrium. However, steps 1, 3 and 10 are exergonic and cannot be reverse.

Question 76

Does gluconeogenesis and Glycolysis occur at the same time?

Answer 76

Liver can be performing gluconeogenesis (say, from a protein meal) while at the same time skeletal muscle can be performing glycolysis during a brief burst of contraction (maybe a sprint). Due to the reciprocal nature and control of these pathways, they should not be running simultaneously in the same cell.

Question 77

Where does Glucogenesis begin and how?

Answer 77

Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate by the carboxylation of pyruvate.

Question 78

What does Glucogenesis require and how is it catalysed?

Answer 78

This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase. This enzyme is stimulated by high levels of acetyl-CoA (produced in β-oxidation in the liver) and inhibited by high levels of ADP and glucose.

Question 79

To export oxaloacetate to the cytosol from the mitochondria, what has to occur?

Answer 79

To export oxaloacetate to the cytosol from the mitochondria, it has to be reduced to malate using NADH. In the cytosol, malate is again oxidized back to oxaloacetate using NAD+, where the remaining steps of gluconeogenesis take place.
Oxaloacetate is decarboxylated and then phosphorylated to form phosphoenolpyruvate using the enzyme PEPCK. A molecule of GTP is hydrolyzed to GDP during this reaction.
A molecule of GTP is hydrolyzed to GDP during this reaction.

Question 80

Oxaloacetate is decarboxylated and then phosphorylated to form phosphoenolpyruvate using what enzyme?

Answer 80

The enzyme PEPCK

Question 81

Describe Gluconeogenesis

Answer 81

Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate by the carboxylation of pyruvate. This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase. This enzyme is stimulated by high levels of acetyl-CoA (produced in β-oxidation in the liver) and inhibited by high levels of ADP and glucose.
To export oxaloacetate to the cytosol from the mitochondria, it has to be reduced to malate using NADH (not shown). In the cytosol, malate is again oxidized back to oxaloacetate using NAD+, where the remaining steps of gluconeogenesis take place.
Oxaloacetate is decarboxylated and then phosphorylated to form phosphoenolpyruvate using the enzyme PEPCK. A molecule of GTP is hydrolyzed to GDP during this reaction.
The next several steps in the reaction are at equilibrium and are the same as in glycolysis, except reversed.
Fructose 1,6-bisphosphatase converts fructose 1,6-bisphosphate (F1,6BP) to fructose 6-phosphate (F6P), using one water molecule and releasing one phosphate
In both glycolysis and gluconeogensis, this reaction is the rate-limiting step and is highly regulated by allostery. F2,6BP, AMP and Citrate all act to control this step.
The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen of the endoplasmic reticulum, where glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to produce glucose and release an inorganic phosphate i.e. again it is not a simple reversal of glycolysis.
Glucose is shuttled into the cytoplasm by glucose transporters located in the endoplasmic reticulum’s membrane.

Question 82

What does the enzyme Fructose 1,6-biphosphate convert?

Answer 82

Fructose 1,6-bisphosphatase converts fructose 1,6-bisphosphate (F1,6BP) to fructose 6-phosphate (F6P), using one water molecule and releasing one phosphate

Question 83

In both glycolysis and gluconeogensis, the equilibrium reaction is highly regulated, how?

Answer 83

Highly regulated by allostery. F2,6BP, AMP and Citrate all act to control this step.

Question 84

In both glycolysis and gluconeogensis, the equilibrium reaction is highly regulated, how?

Answer 84

Highly regulated by allostery. F2,6BP, AMP and Citrate all act to control this step.

Question 85

Describe the final reaction of gluconeogenesis

Answer 85

The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen of the endoplasmic reticulum, where glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to produce glucose and release an inorganic phosphate i.e. again it is not a simple reversal of glycolysis.
Glucose is shuttled into the cytoplasm by glucose transporters located in the endoplasmic reticulum’s membrane.

Question 86

Describe glucose-6-phosphate in gluconeogensis

Answer 86

Glucose-6-phosphate is formed from fructose 6-phosphate by phosphoglucoisomerase (the reverse of step 2 in glycolysis).
Glucose-6-phosphate can be used in other metabolic pathways or dephosphorylated to free glucose by glucose-6-phosphatase.
Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form (glucose-6-phosphate) is locked in the cell, a mechanism by which intracellular glucose levels are controlled by cells.

Question 87

How are fats broken down in the gut?

Answer 87

In the lumen of the gut triglycerides aggregate into fat globules as a consequence of the aqueous environment i.e. they are hydrophobic
The liver produces bile salts (cholesterol like molecules that are amphipathic meaning they are both hydrophilic and hydrophobic), that are stored in the gall bladder
The bile salts break down the fat globules so that the triglycerides can be accessed
The pancreases secretes lipases to breakdown fats into monoacylglycerols and fatty acids that can now be absorbed by epithelial cells of the gut.

Question 88

In the lumen of the gut triglycerides aggregate into fat globules, why?

Answer 88

As a consequence of the aqueous environment (they are hydrophobic)

Question 89

What actually breaks down the fat globules and triglycerides in the gut? [Digestion and absorption of fats]

Answer 89

The bile salts break down the fat globules so that the triglycerides can be accessed
The pancreases secretes lipases to breakdown fats into monoacylglycerols and fatty acids that can now be absorbed by epithelial cells of the gut.

Question 90

Explain the uptake of fats by epithelial cells [Digestion and absorption of fats]

Answer 90

Epithelial cells absorb fatty acids and monoglycerides and then recombine them back into triglycerides
These fats are coated with phospholipids, cholesterol, and proteins to form water-soluble chylomicrons
Chylomicrons are transported into a lacteal, a lymphatic vessel in each villus
Lymphatic vessels deliver chylomicron-containing lymph to large veins that return blood to the heart

Question 91

In the uptake of fats by epithelial cells, where are chylomicrons are transported into

Answer 91

A lacteal, a lymphatic vessel in each villus

Question 92

What are chylomicrons and what do they arise from?

Answer 92

They are fats coated with phospholipids, cholesterol, and proteins and is water-soluble. Chylomicrons arise solely from the intestine, and contain 90% triglyceride primarily of dietary origin and other phospholipids, cholesterol, and fat soluble vitamins and are coated with a layer of apolipoprotein (apo A and B types).

Question 93

Where do the lymphatic vessels deliver chylomicron-containing lymph?

Answer 93

Lymphatic vessels deliver chylomicron-containing lymph to large veins that return blood to the heart

Question 94

What is a lacteal?

Answer 94

A lymphatic vessel in each villus

Question 95

How are the tryglicerides transported to adipose and muscle cells

Answer 95

Predominately triglycerides are stored in fat cells (adipose cells) as fat globules in the cytoplasm.
Muscle cells also store triglycerides but for ATP production
In the gut, the triglycerides are placed into carrier particles called Chylomicrons
The chylomicrons bind to the membranes of fat and muscle cells and then are again broken down to the fatty acids and monoglycerides
They are then reformed back into fat globules to function as high energy stores

Question 96

Where are tryglycerides stored?

Answer 96

Triglycerides are stored in fat cells (adipose cells) as fat globules in the cytoplasm.

Question 97

Describe the digestion of fats

Answer 97

• In the lumen of the gut triglycerides aggregate into fat globules as a consequence of the aqueous environment i.e. they are hydrophobic
• The liver produces bile salts (cholesterol like molecules that are amphipathic meaning they are both hydrophilic and hydrophobic), that are stored in the gall bladder
• The bile salts break down the fat globules so that the triglycerides can be accessed
• The pancreases secretes lipases to breakdown fats into monoacylglycerols and fatty acids that can now be absorbed by epithelial cells of the gut.

Question 98

What happens to triglycerides when the body needs energy?

Answer 98

When the body needs energy, adipose cells mobilise the triglycerides into free fatty acids and glycerol, which are released into the blood.

Question 99

What is serum albumin?

Answer 99

Serum albumin is found in human blood. It is the most abundant protein in human blood plasma.

Question 100

What are Chylomicrons?

Answer 100

Chylomicrons are lipoprotein particles that consist of triglycerides, phospholipids, cholesterol, and proteins.

Question 101

Describe the absorption of fats

Answer 101

• Epithelial cells absorb fatty acids and monoglycerides and then recombine them back into triglycerides. These fats are coated with phospholipids, cholesterol, and proteins to form water-soluble chylomicrons
• Chylomicrons are transported into a lacteal, a lymphatic vessel in each villus
• Lymphatic vessels deliver chylomicron-containing lymph to large veins that return blood to the heart
  Predominately triglycerides are stored in fat cells (adipose cells) as fat globules in the cytoplasm.
  In the gut, the triglycerides are placed into carrier particles called Chylomicrons
• The chylomicrons bind to the membranes of fat and muscle cells and then are again broken down to the fatty acids and monoglycerides
  -They are then reformed back into fat globules to function as high energy stores

Question 102

Do muscle cells store triglycerides?

Answer 102

Yes, but for ATP production

Question 103

What cells absorb fatty acids and monoglycerides and then recombine them back into triglycerides?

Answer 103

Epithelial cells

Question 104

Where are triglycerides stored predominantly?

Answer 104

Triglycerides are stored in fat cells (adipose cells) as fat globules in the cytoplasm.

Question 105

What breaks down the fat globules so that the triglycerides can be accessed

Answer 105

Bile salts

Question 106

In the breakdown of FAs they must be activated by the addition of ______ and replaced by ______, why?

Answer 106

Coenzyme A (CoA), which is then exchanged for a Carnithine molecule so that the FA can pass across the mitochondrial membrane

Question 107

Describe fatty acid breakdown

Answer 107

i. A hydrogen is extracted and a double bond is created
ii. A hydroxyl group is added to the double bond giving an alcohol
iii. The alcohol group is oxidised to carbonyl group, a ketone
iv. CoA cleaves the activated FA, removing 2 carbons and generating Acetyl-CoA

Question 108

Are FA breakdown and synthesis the reverse of eachtother and why?

Answer 108

Yes, the inverse steps oxidation vs reduction

Question 109

Where does fatty acid synthesis and breakdown occur?

Answer 109

Fatty acid synthesis occurs in the cytoplasm in contrast to β-oxidation which occurs in the mitochondria

Question 110

What is fatty acid breakdown?

Answer 110

Fatty acid degradation is the process in which fatty acids are broken down into their metabolites, in the end generating acetyl-CoA, the entry molecule for the citric acid cycle, the main energy supply of animals.

Question 111

Is FA breakdown oxidation or reduction?

Answer 111

FA breakdown is an oxidative process that consists of 4 steps. (known as β-oxidation)

Question 112

Is FA breakdown cyclic or not?

Answer 112

It is a cyclic process that is repeated.

Question 113

To facilitate FA breakdown what is added?

Answer 113

A CoA molecule is added (i.e. activation) making more reactive.

Question 114

Is fatty acid synthesis oxidation or reduction and what sort of conditions does it require?

Answer 114

FA synthesis is a reduction process and occurs under conditions where energy supply is large

Question 115

Is β-oxidation catabolic or anabolic?

Answer 115

Catabolic

Question 116

What is fatty acid synthesis?

Answer 116

Fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases

Question 117

What is added for fatty acid synthesis?

Answer 117

An activated Malonyl-CoA unit and Acetyl-CoA is added.

Question 118

How is fatty acid synthesis regulated?

Answer 118

Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, and is subject to both phosphorylation and allosteric regulation.

Question 119

What is Malonyl-CoA and what is it’s function?

Answer 119

Malonyl-CoA is a key inhibitor of FA oxidation and contols the switch between fatty acid synthesis and oxidation.
Malonyl CoA inhibits the enzyme carnitine palmitoyltransferase 1 (CPT1) thereby stopping FAs from entering the mitochondria

Question 120

What is the first step of fatty acid synthesis?

Answer 120

Release of fatty acids from Adipose cells

• Adipose cells mobilise the triglycerides into free fatty acids and glycerol in response to glucagon or adrenaline
• The FAs are released into the blood
• Serum albumin picks up the FAs and brings them to target cells

Question 121

Describe stage two of fatty acid synthesis

Answer 121

Uptake and activation of fatty acids

• In the cytoplasms of the target cells the FA must be activated (enzyme) and taken up into the mitochondrial of the cell to undergo b-oxidation. This process breaks them down to Acety-CoA molecules that can be fed into the citric acid cycle, generating ATP
• Firstly, an enzyme on the outer membrane of the mitochondria called Fatty Acyl CoA synthetase activates the fatty acid by forming a thioester bond between the sulphur atom on CoA and the carboxyl carbon of the FA.
• The enzyme catalyses this reaction in two steps
  i. An AMP molecule is transferred onto the fatty acid with the release of pyrophosphate.The hydrolysis of pyrophosphate to inorganic phosphate provides the driving force for the reaction
  ii. A CoA molecule is attached to the FA displacing the AMP molecule

Question 122

An enzyme on the membrane of the mitochodria activates the fatty acid, what is this enzyme and how does it do so?

Answer 122

An enzyme on the outer membrane of the mitochondria called Fatty Acyl CoA synthetase activates the fatty acid by forming a thioester bond between the sulphur atom on CoA and the carboxyl carbon of the FA.

i. An AMP molecule is transferred onto the fatty acid with the release of pyrophosphate.The hydrolysis of pyrophosphate to inorganic phosphate provides the driving force for the reaction
ii. A CoA molecule is attached to the FA displacing the AMP molecule

Question 123

How are fatty acids activated?

Answer 123

Fatty acids are activated by reaction with CoA to form fatty acyl CoA. (The reaction normally occurs in the endoplasmic reticulum or the outer mitochondrial membrane.) This is an ATP-requiring reaction, yielding AMP and pyrophosphate (PPi).

Question 124

Describe stage 3 of fatty acid synthesis

Answer 124

Translocation of fatty acids into the mitochondrial matrix

• For the oxidation of FAs to occur they must be transported from the cytoplasm to the inner mitochondrial space.
• Following activation of a long-chain fatty acid to a FA-CoA, the CoA is exchanged for carnithine by the enzyme carnithine palmityltransferase 1 (CPT-1).
• The FA-carnithine molecule is then transported to the inside of the mitochondrion through the action of the Carnithine AcylCarnithine Translocase (CACT) where a reversal exchange takes place through the action of CPT-2.
• Once inside the mitochondrion the FA-CoA can serve as a substrate for the β-oxidation machinery, which we discussed in an earlier slide

Question 125

For the oxidation of FAs to occur they must be transported from the cytoplasm to where and how?

Answer 125

The inner mitochondrial space through the action of the Carnithine AcylCarnithine Translocase (CACT) where a reversal exchange takes place through the action of CPT-2.

Question 126

What is stage 4 of fatty acid synthesis?

Answer 126

Actual synthesis itself

• To build FAs Acetyl-CoA is needed, which comes from the matrix of the mitochondria.
• Fatty acid synthesis takes place in the cytosol of the cell. Therefore need to move Acetyl-CoA into the cytoplasm.
• Citrate synthase converts oxaloacetate to citrate and transports it across the membrane
• When ATP is high in the mitochondria it inhibits the CAA enzyme isocitrate dehydrogenase thereby increasing the citrate levels
• Citrate is not used in FA synthesis it is recycled back to oxaloacetate
• The enzyme malate dehydrogenase converts oxaloacetate to malate via malate dehydrogenase using a molecule of NADH than can subsequently by converted to pyruvate and moved back into the mitochondria
• Malate enzyme generates the NADPH that will be used in fatty acid synthesis

Question 127

To build Fatty acids, what is needed?

Answer 127

FAs Acetyl-CoA is needed, which comes from the matrix of the mitochondria.

Question 128

What promotes fatty acid synthesis?

Answer 128

When ATP is high in the mitochondria it inhibits the CAA enzyme isocitrate dehydrogenase thereby increasing the citrate levels. Citrate is not used in FA synthesis it is recycled back to oxaloacetate

Question 129

In humans, the first opportunity for ingested food to undergo enzymatic hydrolysis is in the _____.

a) mouth
b) stomach
c) liver
d) small intestine
e) large intestine

Answer 129

a) mouth

Question 130

What is Enzymatic hydrolysis?

Answer 130

Enzymatic hydrolysis is a process in which enzymes facilitate the cleavage of bonds in molecules with the addition of the elements of water. It plays an important role in the digestion of food.

Question 131

Increased appetite is the typical result of increased levels in the blood of _____.

a) leptin
b) PYY
c) ghrelin
d) insulin
e) secretin

Answer 131

C) Ghrelin is released in greater quantities by an empty, shrunken stomach than by a full stomach.
Leptin (a), PYY (b), and insulin (d) are associated with satiety, not hunger. Secretin (e) is not linked to appetite regulation.

Question 132

The bile salts function in fat digestion by _____.

a) hydrolyzing fat molecules to glycerol and fatty acids
b) separating individual fat molecules from each other
c) dissolving fats in water
d) dispersing big droplets of fats to small droplets
e) triggering the secretion of pancreatic lipase

Answer 132

D) When fat globules enter the small intestine, they are mixed with water and bile salts. Bile salts coat smaller fat globules, emulsifying them. Options b and c do not occur in water.

Question 133

A fatty acid absorbed into an intestinal cell _____.

a) is hydrolyzed to smaller fragments
b) enters a lacteal vessel as a carbohydrate
c) becomes part of a chylomicron
d) is bound to bile salts
e) exits the cell to directly enter the blood

Answer 133

C) Fats are insoluble in water, and fat droplets are too large to pass through the membranes of capillaries, so chylomicrons enter the lacteal vessels of the lymph system.

Question 134

Glucagon, a pancreatic hormone, functions to _____.

a) stimulate hunger
b) inhibit activity in the small intestine
c) increase fat storage in fat cells
d) reduce glucose levels in the blood
e) stimulate the liver to release glucose

Answer 134

e)

Question 135

Increased glucose levels in blood, after a meal, trigger _____.

a) glucagon release from the pancreas
b) insulin release from the pancreas
c) secretin release from the duodenum
d) cholecystokinin release from the pancreas
e) activation of amylase in the blood

Answer 135

b) The main response to high blood glucose is release of pancreatic insulin, triggering its target cells to take up the excess glucose. Glucagon secretion will be reduced during this time. Secretin and cholecystokinin are digestive hormones, not related to blood glucose levels. Amylase is a digestive enzyme mainly in saliva, not in the blood.

Question 136

In which of the following structures does absorption of water, vitamin K, and salt occur?

a) large intestine
b) small intestine
c) liver
d) stomach
e) pancreas

Answer 136

A) The final absorption of those three key elements occurs in the large intestine. This is why treatment of diarrhea, which is watered-down stool from rapid expulsion of food from the body, includes replenishing water, sodium, and potassium immediately.

Question 137

Which statement about glycolysis is true?

a) It splits water.
b) It produces FADH2.
c) It occurs in the cytosol.
d) It makes the most ATP compared to the other steps in the breakdown of glucose.
e) It splits lipids.

Answer 137

C)