S1) Energy Production I — Carbohydrates

Question 1

Define the terms endergonic and exergonic

Answer 1

• Exergonic: a reaction where the energy released is greater than the energy input
• Endergonic: a reaction where the energy input is greater than the energy released

Question 2

What is the ATP-ADP cycle?

Answer 2

The ATP-ADP cycle is the cycle that couples endergonic reactions with exergonic reactions

Question 3

What does the ATP-ADP cycle do?

Answer 3

• It allows for the controlled release of energy by oxidation
• Some of the energy is conserved by the formation of ATP from ADP and Pi and the remainder is lost as heat

Question 4

What is cell metabolism?

Answer 4

Cell metabolism is the highly integrated network of chemical reactions in distinct metabolic pathways that occur within cells

Question 5

Define the terms catabolism and anabolism

Answer 5

• Catabolism involves the breakdown of larger molecules to smaller ones
• Anabolism involves the synthesis of larger molecules from smaller ones

Question 6

Compare and contrast catabolic and anabolic pathways

Answer 6

• Catabolic pathways are oxidative, release large amounts of free energy, and produce some intermediary metabolites
• Anabolic pathways are reductive, and use energy and intermediary metabolites from catabolism to synthesise cell components

Question 7

Identify the 3 major carrier molecules

Answer 7

• NAD
• NADP
• FAD

Question 8

Represent the 3 major carrier molecules in their oxidised and reduced forms

Answer 8

• Oxidised forms: NAD⁺, NADP⁺, FAD
• Reduced forms: NADH + H⁺, NADPH + H⁺, FAD₂H

Question 9

Why must carrier molecules cycle between oxidative and reductive processes to maintain cell function?

Answer 9

To allow for both oxidation reactions (require NAD⁺) and reduction reactions (require NADH + H⁺) to occur

Question 10

What is Free Energy?

Answer 10

Free Energy is the energy released in an exergonic reaction that is available to do work (ΔG)

Question 11

What are the four steps in carbohydrate metabolism?

Answer 11

• Stage 1: Breakdown to monomers
• Stage 2: Breakdown to metabolic intermediates
• Stage 3: TCA (Kreb’s) cycle
• Stage 4: Oxidative phosphorylation

Question 12

In stage 1 of metabolism, carbohydrates are digested at the salivary glands, pancreas and small intestines

Identify the respective enzymes involved

Answer 12

• Saliva: amylase
• Pancreas: amylase
• Small intestine: lactase, sucrase, pancreatic amylase, isomaltase

Question 13

Why can’t cellulose be digested by humans?

Answer 13

No enzymes to break down the β 1-4 linkages present in dietary fibres i.e. no cellulase

Question 14

Identify 3 clinical features of primary lactase deficiency

Answer 14

• Absence of lactase persistence allele
• Only occurs in adults
• Highest prevalence in Northwest Europe

Question 15

Identify 3 clinical features of secondary lactase deficiency

Answer 15

• Caused by injury to small intestine
• Occurs in both infants and adults
• Generally reversible

Question 16

Identify four clinical conditions which could cause injury to the small intestine, leading to secondary lactase deficiency

Answer 16

• Gastroenteritis
• Coeliac disease
• Crohn’s disease
• Ulcerative colitis

Question 17

Describe 3 features of congenital lactase deficiency

Answer 17

- Extremely rare

• Autosomal recessive defect in lactase gene
• Cannot digest breast milk

Question 18

What foods should someone with a lactase deficiency abstain from?

Answer 18

• Milk
• Cream
• Yoghurt
• Cheese

Question 19

Identify 4 symptoms associated with lactase deficiency?

Answer 19

• Bloating/cramps
• Flatulence
• Diarrhoea
• Vomiting
• Rumbling stomach

Question 20

How are monosaccharides absorbed?

Answer 20

• Active transport by SGLT1 into intestinal epithelial cells
• Active transport by GLUT2 into blood supply
• Facilitated diffusion by transport proteins GLUT1-GLUT5 into cells

Question 21

Identify 4 tissues which have an absolute requirement for glucose

Answer 21

• Neutrophils
• Lens of eye
• Red blood cells
• Innermost cells of kidney medulla

Question 22

Describe the glucose dependency of the brain

Answer 22

The CNS prefers glucose as a fuel but can use ketone bodies for some energy requirements in times of starvation

Question 23

Where does glycolysis occur?

Answer 23

• Occurs in all tissues
• Occurs in cytosol

Question 24

What kind of process is glycolysis?

Answer 24

• Exergonic process
• Oxidative process

Question 25

Identify the 4 functions of glycolysis

Answer 25

- Oxidation of glucose - NADH production - Synthesis of ATP (2 ATP per glucose molecule) - Produces C₆ and C₃ intermediates (no loss of CO₂)

Question 26

Identify the enzymes at point 1, 3 and 10

Answer 26

1 - Hexokinase 3 - Phosphofructokinase-1 10 - Pyruvate kinase

Question 27

Describe Phase 1 of Glycolysis (reactions 1-3)

Answer 27

- **Phosphorylation** of glucose to G-6-P - Polarised glucose is more reactive & cannot return down plasma membrane - **Input:** 2 moles ATP per mole glucose

Question 28

Describe Phase 2 of Glycolysis (reactions 4-10)

Answer 28

- **Reaction 4:** C₆ --\> 2C₃ units - **Reaction 6:** Reducing power captured (NADH) - **Reaction 7&10:** ATP synthesis (substrate level phosphorylation)

Question 29

Identify the irreversible reactions in glycolysis

Answer 29

Large -ΔG: - Reactions 1 - Reaction 3 - Reaction 10

Question 30

Explain the allosteric regulation of glycolysis by phosphofructokinase

Answer 30

**Allosteric regulation (muscle):** - Inhibited by high ATP - Stimulated by high AMP

Question 31

Explain the hormone regulation of glycolysis by phosphofructokinase

Answer 31

**Hormonal regulation (liver):** - Stimulated by insulin - Inhibited by glucagon

Question 32

Describe the regulation of glycolysis through hexokinase and pyruvate kinase

Answer 32

Question 33

Explain the dependency of glycolysis on NAD⁺

Answer 33

- Glycolysis pathway **requires NAD⁺ **(NADH is produced) - NAD⁺ and NADH is **constant** in cells, so glycolysis would end when all NAD⁺ is converted to NADH - Hence, NAD⁺ is **regenerated in stage 4** of metabolism

Question 34

In light of the dependency of glycolysis on NAD⁺, explain why lactic acid (lactate) production is required

Answer 34

- **RBC** have no stage 4 to regenerate NAD⁺ - Stage 4 requires **oxygen** (limited in **muscles** and **gut**) - NAD⁺ is regenerated using **Lactate Dehydrogenase** (LDH)

Question 35

Explain how the blood concentration of lactate is controlled

Answer 35

Question 36

Which three factors account for elevations of plasma lactate concentration?

Answer 36

- Rate of production - Rate of utilisation - Rate of disposal

Question 37

Identify 3 clinical features of hyperlactaemia

Answer 37

- Lactate concentration 2-5 mM - Below renal threshold - No change in blood pH (buffering capacity)

Question 38

Identify 3 clinical features of lactic acidosis

Answer 38

- Lactate concentration above 5 mM - Above renal threshold - Blood pH lowered

Question 39

Outline galactose metabolism

Answer 39

- Dietary lactose is hydrolysed by **lactase** to release **glucose** + **galactose** which absorbed into the blood stream - Galactose is metabolised largely in the liver: Galactose + ATP → Glucose 6-phosphate + ADP

Question 40

What is galactosaemia?

Answer 40

**Galactosaemia** is a clinical condition wherein one cannot use galactose obtained from the diet because of a lack of the *kinase* or *transferase* enzyme

Question 41

Describe the galactosaemia that occurs when the kinase enzyme is absent

Answer 41

**Kinase absence:** - Rare - Build-up of galactose in tissues

Question 42

Describe the galactosaemia that occurs when the transferase enzyme is absent

Answer 42

**Transferase absence:** - Common and more serious - Build-up of galactose and galactose-1-P

Question 43

What happens when galactose accumulates in the tissues?

Answer 43

Accumulation of galactose in tissues leads to its reduction to galactitol by the activity of the enzyme **aldose reductase**

Question 44

Outline fructose metabolism

Answer 44

- Dietary sucrose is hydrolysed by **sucrase** to release **glucose + fructose** which are absorbed into the blood stream - Fructose is metabolised largely in the liver and is converted to **glyceraldehyde 3-phosphate** (an intermediate of glycolysis)

Question 45

Describe the clinical features of essential fructosuria

Answer 45

- Fructokinase missing - Fructose in urine, no clinical signs

Question 46

Describe the clinical features of fructose intolerance

Answer 46

- Aldolase missing - Fructose-1-P accumulates in liver (leads to liver damage) - Treatment: remove fructose from diet

Question 47

Describe the input and output of the pentose phosphate pathway

Answer 47

- **Input**: Glucose-6-Phosphate - **Output**: I. CO₂ II. Ribose – synthesis of nucleotides, DNA and RNA III. No ATP

Question 48

What is the rate limiting enzyme for the pentose phosphate pathway?

Answer 48

Glucose-6-phosphate dehydrogenase (G6PDH)

Question 49

The pentose phosphate pathway is an important source of NADPH for which processes?

Answer 49

- Reducing power for biosynthesis - Maintenance of GSH levels - Detoxification reactions

Question 50

Where does the pentose phosphate pathway occur?

Answer 50

In the cytosol