Unit 1: Carbohydrate metabolism
Biochemical Metabolism notes · PTU syllabus (BMLS202-18)
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Unit summary
Carbohydrates are the body's main fuel. This unit covers their importance and classification, digestion and absorption, glycolysis, the citric acid cycle, gluconeogenesis, glycogenolysis and glycogenesis, and disorders of carbohydrate metabolism.
After this unit you can
- Classify carbohydrates and describe their digestion and absorption
- Explain glycolysis and the citric acid cycle
- Explain gluconeogenesis and glycogen metabolism
- Describe disorders of carbohydrate metabolism
PTU syllabus topics
- Introduction
- importance and classification
- digestion and absorption
- glycolysis
- citric acid cycle
- gluconeogenesis
- glycogenolysis
- glycogenesis
- disorders of carbohydrate metabolism
- 1Glycolysis
Glucose to 2 pyruvate, in cytoplasm
- 2Link reaction
Pyruvate to acetyl-CoA
- 3Citric acid (TCA) cycle
In mitochondria: NADH, FADH2
- 4Electron transport chain
Most ATP made here
Topic 1
Importance and classification of carbohydrates
- Carbohydrates: polyhydroxy aldehydes or ketones; general formula (CH₂O)n. They provide energy (4 kcal/g), store energy (glycogen), form structures (cellulose) and are parts of nucleic acids (ribose).
Monosaccharides
Aldoses (glucose, galactose, ribose) and ketoses (fructose)
Disaccharides
Sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)
Oligosaccharides
3–10 units
Polysaccharides
Homopolysaccharides (starch, glycogen, cellulose) and heteropolysaccharides (hyaluronic acid, heparin)
Topic 2
Digestion and absorption
- 1
Mouth
Salivary amylase starts starch digestion
- 2
Stomach
Acid stops amylase
- 3
Small intestine
Pancreatic amylase → maltose and dextrins
- 4
Brush border
Maltase, sucrase, lactase → monosaccharides
- 5
Absorption
Glucose and galactose by Na⁺-glucose co-transport (SGLT1); fructose by facilitated diffusion (GLUT5)
- 6
Portal blood
To the liver
- Lactose intolerance: lactase deficiency → undigested lactose fermented in the colon → bloating and diarrhoea.
Topic 3
Glycolysis
- 1Glucose → glucose-6-phosphate (hexokinase; uses ATP)
- 2Fructose-6-phosphate → fructose-1,6-bisphosphate (phosphofructokinase-1; rate-limiting; uses ATP)
- 3Split into two triose phosphates
- 4Oxidation produces NADH; substrate-level phosphorylation makes ATP
- 5Phosphoenolpyruvate → pyruvate (pyruvate kinase; ATP)
- Net yield: 2 ATP and 2 NADH per glucose (aerobic: pyruvate enters mitochondria). Anaerobic: pyruvate → lactate (lactate dehydrogenase) to regenerate NAD⁺ — in red cells and exercising muscle.
Topic 4
Citric acid cycle
- Link reaction: pyruvate → acetyl-CoA by pyruvate dehydrogenase (needs vitamins B1, B2, B3, B5 and lipoic acid).
- Per acetyl-CoA: 3 NADH, 1 FADH₂, 1 GTP and 2 CO₂. Complete oxidation of glucose yields about 30–32 ATP through oxidative phosphorylation. The cycle is amphibolic — it also supplies intermediates for synthesis.
Topic 5
Gluconeogenesis
- Gluconeogenesis: making glucose from non-carbohydrate sources — lactate, glycerol and glucogenic amino acids (alanine) — mainly in the liver (and kidney) during fasting.
- Pyruvate carboxylase and PEP carboxykinase
- Bypass pyruvate kinase
- Fructose-1,6-bisphosphatase
- Bypasses phosphofructokinase
- Glucose-6-phosphatase
- Bypasses hexokinase; releases free glucose
- Cori cycle: lactate from muscle → liver → glucose → muscle.
Topic 6
Glycogenesis and glycogenolysis
Meaning
Synthesis of glycogen from glucose
Breakdown of glycogen to glucose
Key enzyme
Glycogen synthase (with branching enzyme)
Glycogen phosphorylase (with debranching enzyme)
Hormone
Insulin (fed state)
Glucagon and adrenaline (fasting, stress)
Site
Liver and muscle
Liver releases glucose to blood; muscle uses it locally
Topic 7
Disorders of carbohydrate metabolism
Diabetes mellitus
Insulin deficiency (type 1) or resistance (type 2)
Fasting glucose ≥ 126 mg/dL, 2-hour OGTT ≥ 200 mg/dL, HbA1c ≥ 6.5%
Galactosaemia
Galactose-1-phosphate uridyl transferase deficiency
Reducing sugar in urine, cataracts, liver damage in infants
Glycogen storage diseases
E.g., von Gierke's disease (glucose-6-phosphatase deficiency)
Hypoglycaemia, enlarged liver, lactic acidosis
Lactose intolerance
Lactase deficiency
Diarrhoea after milk; hydrogen breath test
Hereditary fructose intolerance
Aldolase B deficiency
Hypoglycaemia after fructose
- Normal fasting glucose: 70–100 mg/dL; impaired fasting glucose: 100–125 mg/dL (ADA criteria).
Key terms
- Glycolysis
- Breakdown of glucose to pyruvate
- Citric acid cycle
- Mitochondrial cycle oxidising acetyl-CoA
- Gluconeogenesis
- Synthesis of glucose from non-carbohydrates
- Glycogenolysis
- Breakdown of glycogen
- HbA1c
- Glycated haemoglobin reflecting average glucose over 2–3 months
Quick revision
- Classification; digestion by amylases and disaccharidases; SGLT1, GLUT5.
- Glycolysis: hexokinase, PFK-1, pyruvate kinase; net 2 ATP; lactate.
- PDH; TCA cycle products; 30–32 ATP.
- Gluconeogenesis bypass enzymes; Cori cycle; glycogen synthase and phosphorylase.
- Diabetes criteria; galactosaemia; glycogen storage diseases; lactose intolerance.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Name the rate-limiting enzyme of glycolysis.
- Q2.What is the net ATP yield of glycolysis?
- Q3.Why is the TCA cycle called amphibolic?
- Q4.Name two substrates of gluconeogenesis.
- Q5.Which hormone stimulates glycogenolysis?
- Q6.State the diagnostic criteria for diabetes.
Long-answer questions
- Q1.Describe glycolysis with its regulation and energetics.
- Q2.Describe the citric acid cycle.
- Q3.Explain gluconeogenesis and glycogen metabolism.
- Q4.Describe disorders of carbohydrate metabolism.
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