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Cells and Systems 6-8 / Week 09 / Tuesday
2/6
Week 09 · Food Becomes You

Tuesday

The digestive system, step by step
// Molecules broken apart and put back together
⏱ about 20 min

Tuesday: The Digestive System, Step by Step

Raven has drawn a long winding tube on the whiteboard at the Windowsill. It starts at a mouth and ends far below.

"A bite travels this whole way," she says. "What do you notice about the shape?"

Rocket traces it with his finger. "It is one long tube. With a bag in the middle."

"The bag is the stomach," Raven says. "And below it, the tube is longer than this room."

Nova dims her light so the drawing stands out. "Each part has a job," she says. "One part is extremely sour. One part is folded like a towel."

"Sour?" Rocket says. "Like a lemon?" Nova hums. "Even more. Read on and find the number."

One long path

Digestion begins in the mouth. The teeth chew food into smaller particles. Enzymes in saliva also begin to chemically break down food.

Swallowed food enters the esophagus, a long tube that connects the mouth to the stomach. Its muscles squeeze in waves, called peristalsis, pushing the food along.

The stomach contents are extremely acidic, with a pH between 1.5 and 2.5. This acid breaks down food tissues and switches on digestive enzymes.

Further breakdown happens in the small intestine. Bile from the liver, and enzymes from the pancreas and the small intestine itself, finish the job.

The human small intestine is over 6 meters long. Its lining is highly folded, with finger-like projections called villi that soak up the small molecules.

What is left travels to the large intestine. There water is absorbed, and the drier waste is stored until it leaves the body.

Mouth: teeth chew, saliva enzymes start chemical digestion
Esophagus: muscle waves push food down
Stomach: strong acid and enzymes break food apart
Small intestine: digestion finishes, small molecules are absorbed
Large intestine: water is absorbed, waste is stored
A BITE'S JOURNEY, IN ORDER
  • ?Small intestine
  • ?Mouth
  • ?Stomach
  • ?Esophagus
  • ?Large intestine
WHY THIS EXERCISEThe organs of digestion form one system, and each organ does its job in order.
WHICH ORGAN?
  • Read the question.
  • Tap your answer.
Where does chemical digestion begin?
Which organ is extremely acidic, with a pH between 1.5 and 2.5?
Where are most small molecules absorbed into the body?

Why the folds matter

The small intestine is where the digestion of protein, fats and carbohydrates is completed.

Its surface is folded into villi, and each villus is covered with even smaller microvilli.

All those folds increase the surface area of the small intestine. More surface means more places to absorb nutrients.

Think of a towel. Folded, it fits in your hand. Spread out, it covers a table. The small intestine is spread out on the inside.

The human small intestine is over ____ meters long.
The finger-like projections lining the small intestine are called ____.
StatementTrue or false?
Peristalsis is the wave of muscle squeezes that pushes food along.?
The large intestine is where most digestion happens.?
Bile from the liver helps break down fats in the small intestine.?
Chewing is the only kind of digestion in the mouth.?
WHY THIS EXERCISEEach organ in the system has a different job, and the jobs happen in order.
Try it
Fold a kitchen towel as small as you can. Then spread it flat on the table.
Compare how much table it covers each way. That is what folding does for the small intestine.
Draw the digestive system as a labeled tube: mouth, esophagus, stomach, small intestine, large intestine. Mark where acid works and where villi absorb.

Strong work. Tomorrow is Windowsill Lab: you build a paper-chain model of breaking and relinking, and catch your breath on a mirror.

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