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Carbohydrates — Structure, Digestion & Absorption

Tier 3 · #26 of 33 · moderate yield · topic 3.4 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    What is a carbohydrate , at its simplest chemical definition?
    Back
    A polyhydroxy aldehyde or ketone
  2. Front
    Glucose and galactose differ in configuration at exactly one carbon — making them ___ epimers.
    Back
    C4 epimers
  3. Front
    How are glucose and galactose absorbed across the apical membrane of the enterocyte ?
    Back
    SGLT1 ( Na⁺-coupled secondary active transport )
  4. Front
    An epimer differs from its partner sugar at how many chiral centers?
    Back
    Exactly one
  5. Front
    Which transporter carries fructose across the apical membrane of the enterocyte ?
    Back
    GLUT5 ( facilitative, passive, no Na⁺ )
  6. Front
    Glucose and mannose differ in configuration at C2 — making them ___ epimers.
    Back
    C2 epimers
  7. Front
    What glycosidic bond joins the two sugars in lactose ?
    Back
    β(1→4) ( galactose β-C1 to glucose C4)
  8. Front
    After brush-border hydrolysis, all three monosaccharides exit the enterocyte basolaterally into the portal blood via which transporter?
    Back
    GLUT2
  9. Front
    What glycosidic bond joins the two sugars in sucrose ?
    Back
    α(1→2) ( glucose α-C1 to fructose C2)
  10. Front
    Is lactose a reducing or non-reducing sugar, and why?
    Back
    Reducing — glucose 's anomeric carbon is free (not in the bond)
  11. Front
    Which two monosaccharides make up lactose ?
    Back
    Galactose + glucose
  12. Front
    Is sucrose a reducing or non-reducing sugar, and why?
    Back
    Non-reducing — both anomeric carbons are locked in the bond
  13. Front
    Which two monosaccharides make up sucrose ?
    Back
    Glucose + fructose
  14. Front
    The D/L designation of a hexose is set by the configuration of which carbon?
    Back
    C5 ( highest-numbered chiral center )
  15. Front
    Why can L-glucose not be phosphorylated by hexokinase or metabolized?
    Back
    Standard enzymatic machinery only acts on D-configured sugars
  16. Front
    Sucrase is the brush-border enzyme that cleaves sucrose into which two products?
    Back
    Glucose + fructose
  17. Front
    What is the anomeric carbon of glucose ?
    Back
    C1 — the new chiral center formed when the open chain cyclizes
  18. Front
    Lactase (β-galactosidase) cleaves lactose into which two products?
    Back
    Glucose + galactose
  19. Front
    What is mutarotation ?
    Back
    Slow interconversion of α and β anomers through the open-chain form
  20. Front
    Which glycosidic bonds make cellulose indigestible by human enzymes?
    Back
    β(1→4) bonds (human amylases cleave only α-linkages )
  21. Front
    In a Haworth projection, β-D-glucose has its C1-OH on the ___ side as the C5-CH₂OH; α-D-glucose has it on the ___ side.
    Back
    Same side (β) ; opposite side (α)
  22. Front
    Amylose is linked exclusively by which glycosidic bond?
    Back
    α(1→4) ( linear, unbranched )
  23. Front
    Glucose forms which ring type, and fructose forms which?
    Back
    Glucose = pyranose (6-membered) ; fructose = furanose (5-membered)
  24. Front
    Amylopectin has an α(1→4) backbone plus which branch linkage, and how often?
    Back
    α(1→6) branches every 24–30 residues
  25. Front
    Glucose is an aldohexose; fructose is a ___.
    Back
    Ketohexose ( carbonyl at C2 )
  26. Front
    Glycogen shares amylopectin's bonds but branches how often?
    Back
    Every 12–14 glucose residues ( denser branching )
  27. Front
    What is the functional consequence of glycogen 's denser branching versus amylopectin ?
    Back
    More non-reducing ends → faster glucose release
  28. Front
    Galactose is the C4 epimer of glucose and is processed to glucose-1-phosphate by which pathway after phosphorylation by galactokinase ?
    Back
    The Leloir pathway
  29. Front
    Which enzyme begins starch digestion in the mouth , and what bonds does it hydrolyze?
    Back
    Salivary α-amylase — internal α(1→4) bonds
  30. Front
    What inactivates salivary amylase , ending its action?
    Back
    Low pH of the stomach (stomach acid)
  31. Front
    Which enzyme is the main engine of starch digestion, and where does it act?
    Back
    Pancreatic α-amylase , in the duodenum/small intestine
  32. Front
    Neither salivary nor pancreatic amylase can cleave which linkage — and which brush-border enzyme handles it?
    Back
    α(1→6) branch points — cleaved by isomaltase
  33. Front
    Maltase cleaves maltose into what?
    Back
    Two glucose molecules
  34. Front
    Where in the intestine do the disaccharidases (maltase, isomaltase, sucrase, lactase) act — and are they secreted into the lumen?
    Back
    At the brush border (luminal cell surface); NOT secreted into the lumen
  35. Front
    Isomaltase (oligo-1,6-glycosidase) cleaves which bonds of limit dextrins?
    Back
    The α(1→6) branch points
  36. Front
    In lactose intolerance , why does undigested lactose cause osmotic diarrhea?
    Back
    Unabsorbed lactose draws water into the lumen by osmosis
  37. Front
    What gases and acids do colonic bacteria produce by fermenting undigested lactose ?
    Back
    CO₂, H₂ (hydrogen gas), and short-chain fatty acids
  38. Front
    Lactase activity peaks at birth, then declines by what age in most humans?
    Back
    Age 3 to 5
  39. Front
    Which populations have a high frequency of the lactase persistence allele?
    Back
    Parts of Africa and northern Europe (long dairying histories)
  40. Front
    What energy source powers SGLT1 's uphill transport of glucose , and how is it maintained?
    Back
    The Na⁺ gradient , maintained by the basolateral Na⁺/K⁺-ATPase
  41. Front
    Does insulin affect SGLT1 or GLUT2 ?
    Back
    No — insulin acts only on GLUT4
  42. Front
    Which GLUT isoform is insulin -regulated, and in which tissues?
    Back
    GLUT4 — muscle and adipose tissue
  43. Front
    What does insulin physically do to GLUT4 to increase glucose uptake?
    Back
    Triggers translocation of GLUT4 vesicles to the plasma membrane
  44. Front
    In the absence of insulin , where is GLUT4 located (the basis of type 2 diabetes insulin resistance)?
    Back
    Sequestered intracellularly (away from the membrane)
  45. Front
    GLUT1 (Km ~1 mM) is found in which tissues and serves what role?
    Back
    RBCs and blood-brain barrier — baseline glucose delivery
  46. Front
    GLUT2 (Km ~17 mM) is found in liver and pancreatic β-cells and acts as what?
    Back
    A glucose sensor (responds to postmeal surges)
  47. Front
    GLUT3 (Km ~1.5 mM, high-affinity) is the dedicated transporter of which cells?
    Back
    Neurons (brain) — guarantees neuronal supply
  48. Front
    Why does the brain take up glucose without needing insulin ?
    Back
    It uses GLUT3/GLUT1 , which are constitutively at the membrane (insulin-independent)
  49. Front
    After glucose enters a cell, it is immediately phosphorylated at C6 to form what?
    Back
    Glucose-6-phosphate (G6P)
  50. Front
    Why does phosphorylating glucose to G6P trap it inside the cell?
    Back
    The charged phosphate cannot cross the plasma membrane
  51. Front
    Hexokinase ( brain/muscle ) — Km, and is it inhibited by its product G6P ?
    Back
    Low Km (~0.1 mM) ; yes, allosterically inhibited by G6P
  52. Front
    Glucokinase (hexokinase IV) — which tissues, and its Km?
    Back
    Liver and pancreatic β-cells ; high Km (~10 mM)
  53. Front
    Is glucokinase inhibited by G6P , and what is its kinetic curve shape?
    Back
    Not inhibited by G6P ; sigmoidal kinetics
  54. Front
    Why is glucokinase ideal as a glucose sensor/buffer in the fed state?
    Back
    High Km means it activates only when blood glucose is high , clearing the postmeal load
  55. Front
    What is the normal mean blood glucose concentration to know for the exam?
    Back
    ≈5.5 mmol/L (100 mg/dL)
  56. Front
    Why does amylose form the characteristic blue-black complex with iodine?
    Back
    Its α linkages give it a helical coil that traps iodine
  57. Front
    Besides dietary restriction, what enzyme preparation lets a lactose-intolerant person tolerate dairy?
    Back
    Commercial β-galactosidase (predigests the lactose )
  58. Front
    Match each stereochemical term to its relationship: two sugars that are perfect mirror images (D vs L) are ___.
    Back
    Enantiomers
  59. Front
    Two sugars differing in configuration at exactly one chiral center are called ___ (not anomers or enantiomers).
    Back
    Epimers
  60. Front
    Stereoisomers that differ only at the C1 anomeric carbon created on ring closure are called ___.
    Back
    Anomers (α vs β)
  61. Front
    The open-chain form versus the ring form of the same sugar are related as ___.
    Back
    Tautomers
  62. Front
    A Leloir-pathway / galactokinase defect that blocks conversion of galactose to glucose-1-phosphate causes which clinical disease?
    Back
    Galactosemia
  63. Front
    Lactose intolerance has three causes — name all three.
    Back
    (1) Physiologic decline with age , (2) genetic/congenital absent lactase , (3) secondary loss from mucosal injury
  64. Front
    Which cause of lactose intolerance results from mucosal injury after gastroenteritis, celiac disease, or Crohn's disease?
    Back
    Secondary (acquired) lactase loss
  65. Front
    By unit count: a monosaccharide has ___ unit(s) and cannot be hydrolyzed further; a disaccharide has ___.
    Back
    1 (non-hydrolyzable) ; 2
  66. Front
    By unit count, an oligosaccharide contains how many sugar units, and a polysaccharide how many?
    Back
    Oligosaccharide = 3–10 units ; polysaccharide = more than 10
  67. Front
    By chain length, a 3-carbon sugar is a ___, a 5-carbon sugar (e.g. ribose) is a ___, and a 6-carbon sugar is a ___.
    Back
    Triose; pentose; hexose
  68. Front
    Which 5-carbon sugar is the classic example of a pentose ?
    Back
    Ribose
  69. Front
    At what developmental stage does lactase activity FIRST appear in the fetus?
    Back
    Only during the late stages of gestation
  70. Front
    Why is yogurt often better tolerated than milk by a lactose-intolerant person?
    Back
    Yogurt contains bacterial β-galactosidase that stays partly active in the gut and predigests the lactose
  71. Front
    What defines an aldose versus a ketose ?
    Back
    Aldose = carbonyl (aldehyde) at C1 ; ketose = carbonyl (ketone) at C2
  72. Front
    Why is glucose classified as an aldose but fructose as a ketose ?
    Back
    Glucose 's carbonyl is an aldehyde at C1 ; fructose 's is a ketone at C2
  73. Front
    What fraction of starch is amylose versus amylopectin ?
    Back
    Amylose 15–20%; amylopectin 80–85%
  74. Front
    Unlike muscle hexokinase , the brain 's hexokinase is NOT inhibited by which molecule, and why does that matter?
    Back
    Not inhibited by ATP — ensures continuous glucose phosphorylation for neural energy
  75. Front
    Besides glucose , which other sugars can hexokinase phosphorylate?
    Back
    Galactose, fructose, and mannose
  76. Front
    Hexokinase can phosphorylate galactose , but which enzyme is the dedicated kinase for galactose?
    Back
    Galactokinase
  77. Front
    What is a 'limit dextrin'?
    Back
    A short branched fragment from pancreatic amylase digestion that still contains the intact α(1→6) branch points
  78. Front
    What is the ΔG°' of the hexokinase reaction ( glucose → G6P ), and what does this imply about reversibility?
    Back
    ΔG°' = −16.7 kJ/mol — essentially irreversible under cellular conditions
  79. Front
    Beyond trapping glucose in the cell, what else does phosphorylation to G6P accomplish?
    Back
    Commits the sugar — it is the entry ticket into glycolysis, glycogen synthesis, or the pentose phosphate pathway
  80. Front
    Which enzymes interconvert epimers, allowing galactose -derived intermediates to rejoin the glucose pathway?
    Back
    Epimerases
  81. Front
    Glycogen , the animal storage polysaccharide, is concentrated in which two tissues?
    Back
    Liver and muscle
  82. Front
    Pancreatic amylase drives starch down to which three products?
    Back
    Maltose, maltotriose, and (branched) limit dextrins
  83. Front
    Besides not carrying fructose , does GLUT5 transport glucose or galactose ?
    Back
    No — GLUT5 is fructose-specific (carries neither glucose nor galactose)
  84. Front
    SGLT1 moves glucose UPHILL against its gradient using the Na⁺ gradient, not direct ATP. This class of transport is called ___.
    Back
    Secondary active transport

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