Digestion and Absorption in the Mammalian Gut · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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How nutrients cross the gut lining

Science · Organisms & Life Processes · ages 16-17
Name ______________________   Date ____________

  1. Which substances does the large intestine absorb from the remaining gut contents?

    • Intact starch and proteins
    • Undigested food particles
    • Water and salts

  2. Where do most chemical digestion and nutrient absorption occur?

    • In the small intestine
    • In the large intestine
    • In the mouth and stomach

  3. Villi and microvilli increase the surface area available for nutrient absorption.

    Circle one:   True   False

  4. Glucose concentration is higher inside a gut lining cell than in the lumen. What can drive more glucose into that cell?

    • Glucose moving alone by diffusion up its concentration gradient
    • Sodium moving down its gradient through a glucose co-transporter
    • Blood glucose entering the lumen through the sodium-potassium pump

  5. For a substance diffusing across a lining, surface area doubles and diffusion distance triples. Concentration difference and all other factors stay fixed. What is the new rate?

    • Two thirds of the original rate
    • Six times the original diffusion rate
    • Three halves of the original rate

  6. Which mechanism directly maintains the low sodium concentration inside a gut lining cell?

    • The sodium-glucose co-transporter uses ATP to pump sodium out
    • The glucose exit carrier exports sodium while glucose leaves the cell
    • The sodium-potassium pump uses ATP to move sodium out

  7. A sodium-glucose co-transporter is changed so glucose movement is no longer coupled to sodium. With no other energy coupling, can it drive glucose uptake against its gradient?

    • No; energy from sodium's downhill movement is no longer coupled to glucose
    • Yes; glucose binding alone supplies the energy needed for uphill movement
    • Yes; a concentration gradient always pushes glucose towards higher concentration

  8. In coeliac disease, villi are damaged, but small nutrients remain available in the gut. Which chain explains possible weight loss through reduced absorption?

    • Less starch is digested in the stomach, so the gut receives fewer small nutrients
    • Less nutrient crosses the lining, reducing the supply available to body cells
    • Less ATP is made from glucose inside muscle cells, although gut uptake stays unchanged
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How nutrients cross the gut lining W1-mt_xZ2-3ZClDa-s1

  1. Water and salts · The large intestine absorbs water and salts from contents remaining after passage through the small intestine.
  2. In the small intestine · The small intestine is the main site of chemical digestion and nutrient absorption.
  3. True · Villi are tissue projections, and microvilli are membrane projections. Both increase the absorbing surface area.
  4. Sodium moving down its gradient through a glucose co-transporter · Sodium's downhill movement through the co-transporter supplies energy for glucose to enter against its own gradient.
  5. Two thirds of the original rate · The rate multiplier is the area multiplier divided by the distance multiplier: 2 ÷ 3.
  6. The sodium-potassium pump uses ATP to move sodium out · The blood-facing sodium-potassium pump uses ATP to export sodium, helping maintain the gradient used by co-transport.
  7. No; energy from sodium's downhill movement is no longer coupled to glucose · Uphill glucose movement requires energy. Removing sodium coupling removes this co-transporter's energy source.
  8. Less nutrient crosses the lining, reducing the supply available to body cells · Villus damage can reduce the absorbing surface. Less nutrient uptake can reduce the body's nutrient and energy supply, contributing to weight loss.
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