Describe how the digestive system breaks food into absorbable molecules and how the immune system defends the body against pathogens, including the roles of white blood cells and antibodies (MA STE HS-LS1-2, HS-LS1-3, structure and function).

What this topic is asking

The Massachusetts STE framework (HS-LS1-2) asks you to explain how body systems carry out the functions of life. Two systems feature here: the digestive system, which breaks food into molecules the body can absorb and use, and the immune system, which defends the body against disease. On the High School Biology MCAS, digestion is tested by asking why large molecules must be broken down and what does the breaking, while immunity is tested by asking how white blood cells and antibodies work and why immunity makes a second infection milder. The crosscutting concept is structure and function.

The digestive system

Food is made of large molecules, the carbohydrates, proteins, and lipids you met in the chemistry of life. These molecules are far too big to cross the gut wall into the blood. The job of digestion is to break them into small molecules that can be absorbed:

• Carbohydrates (starch) are broken down into simple sugars (glucose).
• Proteins are broken down into amino acids.
• Lipids are broken down into fatty acids and glycerol.

This breakdown is sped up by digestive enzymes, which catalyze the reactions (the same enzyme principles from enzymes and biochemical reactions apply). Once broken down, the small molecules are absorbed into the blood and carried to every cell, where they are used for energy (in respiration) and as building materials. The MCAS often asks why large molecules must be digested first: because only small molecules can be absorbed.

The immune system

The immune system defends the body in stages. The skin and other barriers keep most pathogens out. If a pathogen gets in, white blood cells respond in two main ways:

• Some white blood cells engulf and digest pathogens directly.
• Others produce antibodies, proteins that bind to a specific pathogen (matching its shape, like a lock and key) and mark it for destruction. Because each antibody fits one pathogen, this defense is highly specific, another example of structure and function.

This is how the body usually clears an infection. The specificity of antibodies links straight back to protein shape: an antibody's binding site is precisely shaped for one target.

Immunity and memory

Here is the idea the MCAS asks about most. After an infection is cleared, the immune system keeps memory cells that remember the pathogen. If the same pathogen invades again, the memory cells let the body make the right antibodies faster and in greater number, so the pathogen is destroyed before it causes illness. This is immunity, and it is also how vaccines work: a vaccine exposes the body to a harmless form of a pathogen so memory cells form without the person getting sick.

Try this

Q1. Explain why large food molecules must be broken down before they can be absorbed. [2]

• Cue. Large molecules are too big to pass across the gut wall into the blood, so they must be broken into small molecules (such as glucose and amino acids) that can be absorbed.

Q2. Explain how antibodies help defend the body. [2]

• Cue. Antibodies are proteins that bind to a specific pathogen, matching its shape, and mark it for destruction by white blood cells.