S17) Energy Production — Carbohydrates Flashcards
Define the terms endergonic and exergonic
- 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
What is the ATP-ADP cycle?
The ATP-ADP cycle is the cycle that couples endergonic reactions with exergonic reactions
What does the ATP-ADP cycle do?
- 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
What is cell metabolism?
Cell metabolism is the highly integrated network of chemical reactions in distinct metabolic pathways that occur within cells.
Define the terms catabolism and anabolism
- Catabolism involves the breakdown of larger molecules to smaller ones
- Anabolism involves the synthesis of larger molecules from smaller ones
Compare and contrast catabolic and anabolic pathways
- 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
Identify the 3 major carrier molecules
- NAD
- NADP
- FAD
Represent the 3 major carrier molecules in their oxidised and reduced forms
- Oxidised forms: NAD+, NADP+, FAD
- Reduced forms: NADH + H+, NADPH + H+, FAD2H
Why must carrier molecules cycle between oxidative and reductive processes to maintain cell function?
To allow for both oxidation reactions (require NAD+) and reduction reactions (require NADH + H+ to occur)
What is Free Energy?
Free Energy is the energy released in an exergonic reaction that is available to do work (ΔG)
What are the four steps in carbohydrate metabolism?
- Stage 1: Breakdown to monomers
- Stage 2: Breakdown to metabolic intermediates
- Stage 3: TCA (Kreb’s) cycle
- Stage 4: Oxidative phosphorylation
In stage 1 of metabolism, carbohydrates are digested at the salivary glands, pancreas and small intestines
Identify the respective enzymes involved
- Saliva: amylase
- Pancreas: amylase
- Small intestine: lactase, sucrase, pancreatic amylase, isomaltase
Why can’t cellulose be digested by humans?
No enzymes to break down the β 1-4 linkages present in dietary fibres i.e. no cellulase
Identify 3 clinical features of primary lactase deficiency
- Absence of lactase persistence allele
- Only occurs in adults
- Highest prevalence in Northwest Europe
Identify 3 clinical features of secondary lactase deficiency
- Caused by injury to small intestine
- Occurs in both infants and adults
- Generally reversible
Identify four clinical conditions which could cause injury to the small intestine, leading to secondary lactase deficiency
- Gastroenteritis
- Coeliac disease
- Crohn’s disease
- Ulcerative colitis
Describe 3 features of congenital lactase deficiency
- Extremely rare
- Autosomal recessive defect in lactase gene
- Cannot digest breast milk
What foods should someone with a lactase deficiency abstain from?
- Milk
- Cream
- Yoghurt
- Cheese
Identify 4 symptoms associated with lactase deficiency?
- Bloating/cramps
- Flatulence
- Diarrhoea
- Vomiting
- Rumbling stomach
How are monosaccharides absorbed?
- Active transport by SGLT1 into intestinal epithelial cells
- Facilitated active transport by GLUT2 into blood supply
- Facilitated diffusion by transport proteins GLUT1-GLUT5 into cells
Identify 4 tissues which have an absolute requirement for glucose
- Neutrophils
- Lens of eye
- Red blood cells
- Innermost cells of kidney medulla
Describe the glucose dependency of the brain
The CNS prefers glucose as a fuel but can use ketone bodies for some energy requirements in times of starvation
Where does glycolysis occur?
- Occurs in all tissues
- Occurs in cytosol
What kind of process is glycolysis?
- Exergonic process
- Oxidative process
Identify the 4 functions of glycolysis
- Oxidation of glucose
- NADH production
- Synthesis of ATP (2 ATP per glucose molecule)
- Produces C6 and C3 intermediates (no loss of CO2)
Identify the enzymes at point 1, 3 and 10
1 - Hexokinase
3 - Phosphofructokinase-1
10 - Pyruvate kinase
Describe Phase 1 of Glycolysis (reactions 1-3)
- Phosphorylation of glucose to G-6-P
- Polarised glucose is more reactive & cannot return down plasma membrane
- Input: 2 moles ATP per mole glucose
Describe Phase 2 of Glycolysis (reactions 4-10)
- Reaction 4: C6 –> 2C3 units
- Reaction 6: Reducing power captured (NADH)
- Reaction 7&10: ATP synthesis (substrate level phosphorylation)
Identify the irreversible reactions in glycolysis
Large -ΔG:
- Reactions 1
- Reaction 3
- Reaction 10
Explain the allosteric regulation of glycolysis by phosphofructokinase
Allosteric regulation (muscle):
- Inhibited by high ATP
- Stimulated by high AMP
Explain the hormone regulation of glycolysis by phosphofructokinase
Hormonal regulation (liver):
- Stimulated by insulin
- Inhibited by glucagon
Describe the regulation of glycolysis through hexokinase and pyruvate kinase
Explain the dependency of glycolysis on NAD+
- 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
In light of the dependency of glycolysis on NAD+, explain why lactic acid (lactate) production is required
- RBC have no stage 4 to regenerate NAD+
- Stage 4 requires oxygen, (limited in muscles and gut)
- NAD+ is regenerated using Lactate Dehydrogenase (LDH)
Explain how the blood concentration of lactate is controlled
Which three factors account for elevations of plasma lactate concentration?
- Rate of production
- Rate of utilisation
- Rate of disposal
Identify 3 clinical features of hyperlactaemia
- Lactate concentration 2-5 mM
- Below renal threshold
- No change in blood pH (buffering capacity)
Identify 3 clinical features of lactic acidosis
- Lactate concentration above 5 mM
- Above renal threshold
- Blood pH lowered
Outline galactose metabolism
- 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
What is galactosaemia?
Galactosaemia is a clinical condition wherein one cannot use galactose obtained from the diet because of a lack of the kinase or transferase enzyme
Describe the galactosaemia that occurs when the kinase enzyme is absent
Kinase absence:
- Rare
- Build-up of galactose in tissues
Describe the galactosaemia that occurs when the transferase enzyme is absent
Transferase absence:
- Common and more serious
- Build-up of galactose and galactose-1-P
What happens when galactose accumulates in the tissues?
Accumulation of galactose in tissues leads to its reduction to galactitol by the activity of the enzyme aldose reductase
Outline fructose metabolism
- 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)
Describe the clinical features of essential fructosuria
- Fructokinase missing
- Fructose in urine, no clinical signs
Describe the clinical features of fructose intolerance
- Aldolase missing
- Fructose-1-P accumulates in liver (leads to liver damage)
- Treatment: remove fructose from diet
Describe the input and output of the pentose phosphate pathway
- Input: Glucose-6-Phosphate
- Output:
I. CO2
II. Ribose - synthesis of nucleotides, DNA and RNA
III. No ATP
What is the rate limiting enzyme for the pentose phosphate pathway?
Glucose-6-phosphate-dehydrogenase (G6PDH)
The pentose phosphate pathway is an important source of NADPH for which processes?
- Reducing power for biosynthesis
- Maintenance of GSH levels
- Detoxification reactions
Where does the pentose phosphate pathway occur?
In the cytosol
