The digestive system

This statement claims that hydrochloric acid ($HCl$) in the stomach carries out the last step of protein digestion, producing individual amino acids. To judge it, we must remember what $HCl$ actually does in the stomach and which parts of digestion are done by enzymes versus acids in MYP Biology digestion content.

Inside the stomach, $HCl$ mainly helps in two ways:

1. Unfolding proteins (denaturation): the very low $pH$ makes protein molecules lose their shape, so enzymes can reach them more easily.
2. Switching on an enzyme: it changes pepsinogen (inactive) into pepsin (active). Even though it is important, $HCl$ is not an enzyme and it does not directly cut peptide bonds between amino acids.

Protein digestion in the stomach is mainly carried out by pepsin. Pepsin chops large proteins into shorter chains known as polypeptides. However, pepsin does not complete digestion into separate amino acids.

The step that produces individual amino acids happens in the small intestine, not the stomach. Pancreatic enzymes such as trypsin and enzymes on the small-intestine lining (peptidases) finish breaking polypeptides down. Since $HCl$ only helps prepare proteins and activate enzymes, it is not responsible for the final conversion to amino acids.

Selecting True would wrongly suggest that $HCl$ performs the last chemical splitting needed to make amino acids. In reality, $HCl$ supports digestion by denaturing proteins and enabling pepsin activity, but it does not complete protein hydrolysis.

Because hydrochloric acid does not carry out the final step of protein digestion into amino acids, the statement is false.

Protein digestion starts in the stomach, but it does not end there. Hydrochloric acid ($HCl$) makes the stomach very acidic, which unfolds proteins and activates pepsin. Pepsin then cuts proteins into smaller pieces called polypeptides, not into single amino acids.

The next stages occur in the small intestine. Enzymes released from the pancreas (for example, trypsin) and peptidases on the intestinal “brush border” continue breaking polypeptides down until they become individual amino acids, which can then be absorbed.

The statement says $HCl$ in the stomach causes the "final breakdown" into amino acids. But in the digestion sequence, the stomach mainly creates polypeptides, and $HCl$ works as a helper condition rather than the enzyme that finishes the process.

Because the last step happens in the small intestine using peptidases (not stomach acid), the statement cannot be correct.

Since the stomach only supports the early part of protein digestion, the correct response is False.

This question is asking about saliva’s main job specifically when it wets food. In Processes: Digestion, saliva is known to help in more than one way: it has chemical effects and also helps physically handle food.

Here we are concentrating on the physical (mechanical) effect—how adding moisture changes food so it can move to the next part of the digestive system.

Choice A claims saliva provides the right \text{pH} for proteins to unfold, and choice C suggests saliva makes the mouth strongly acidic.

But saliva is usually close to neutral (about $6.2$ to $7.6$). Protein denaturation and killing many pathogens using very low pH are mainly roles of the stomach, where hydrochloric acid ($HCl$) is released—not the mouth area in normal conditions.

Choice D says saliva activates pepsin. Pepsin is a protease that works in the stomach and only becomes active in a very acidic environment (around $pH\approx 2$).

In the mouth, the key enzyme in saliva is salivary amylase, which starts digesting starch, not proteins, and it does not involve pepsin.

Saliva includes water and mucus that make chewed food slippery and easier to shape. This allows the tongue to gather the food into a soft, rounded ball called a bolus.

A bolus matters because it can be swallowed more easily and travel down the esophagus smoothly, reducing the chance of scraping, sticking, or discomfort.

We need the main reason saliva moistening food is helpful. We can answer by using Systematic Elimination, comparing each option with what is true about the mouth and early digestion.

Consider the mouth’s normal conditions. Saliva is generally near neutral, around $6.5$ to $7.5$.

• Option A doesn’t fit because protein unfolding (denaturation) typically happens in very acidic conditions, which are found in the stomach, not in the mouth.
• Option C doesn’t fit because saliva is not “highly acidic.” The strongly acidic environment used to kill many pathogens is created by hydrochloric acid in the stomach.

Option D claims pepsin is activated in the mouth. Pepsin is made for the stomach and needs the stomach’s low pH to function. Saliva’s main enzyme is amylase, which begins starch digestion rather than protein digestion.

Once the other choices are eliminated, Option B remains.

When saliva wets food, it works like a lubricant. That lubrication helps the tongue and teeth shape chewed food into a soft, rounded bolus. This bolus is important because it can be swallowed safely and move down the esophagus smoothly without getting stuck or causing damage.

The question is asking about the special conditions needed for the enzyme pepsin to work.

Pepsin is a protease, meaning it digests proteins by cutting them into shorter peptide chains. In Processes: Digestion, we learn that enzymes only work well when factors like pH are suitable for their shape and activity.

From basic digestive system knowledge, pepsin is made in and acts in the stomach. It is unusual compared with many enzymes because it needs a very acidic pH. That low pH helps change pepsin from its inactive form (pepsinogen) into an active enzyme and allows it to function efficiently.

Check why the other options cannot provide the low-pH environment pepsin requires:

• Maltase is itself an enzyme that digests maltose into glucose; it doesn’t set the stomach’s conditions.
• Saliva is in the mouth and contains amylase; it is close to neutral or slightly basic (about pH 6.5–7.5), which would stop pepsin by denaturing it.
• Bile is made by the liver and stored in the gallbladder. It is alkaline and helps neutralize stomach acid in the small intestine, so it would prevent pepsin from working.

Gastric glands in the stomach release hydrochloric acid (HCl). This drops the stomach pH to roughly $1.5$–$2.0$. That very acidic setting is exactly what pepsin needs to break down proteins effectively.

To answer, first recall what pepsin is and where it acts. Pepsin is a protease (protein-digesting enzyme) that works in the stomach. The question is really asking which option produces the right pH conditions for pepsin to be active.

We can solve it by crossing off options that do not match these requirements.

Consider maltase. Maltase is another digestive enzyme, not something that creates an acidic environment. It works on carbohydrates (maltose) in the small intestine. Since it doesn’t set stomach pH or activate pepsin, it can be removed from consideration.

Saliva is made in the mouth by salivary glands. It begins starch digestion with amylase and is around neutral pH ($pH\approx 7$). Because pepsin operates in the stomach and needs strong acidity, saliva cannot be the correct answer.

Bile is produced by the liver and stored in the gallbladder. It helps emulsify fats in the small intestine. It is also slightly basic, which helps reduce acidity coming from the stomach. Since pepsin depends on a very acidic pH, bile would interfere with pepsin rather than help it.

After removing the other choices, the remaining substance is hydrochloric acid. In the stomach, hydrochloric acid (HCl) is secreted to create a very low pH (about $1.5$–$2.0$). This acidity helps uncoil proteins and activates pepsin so protein digestion can begin, matching the required environment.

The prompt is asking for the name given to food after it has been chewed and mixed with saliva so it becomes easy to swallow. This idea belongs to the Digestion part of the Processes topic, describing how food is prepared for movement through the digestive system.

Check why the other choices don’t match:

• Chyme refers to a soupy, partially digested mixture created in the stomach, not the mouth.
• Micelles are microscopic fat-related structures that form in the small intestine to help fats get absorbed.
• Peptides are small molecules (amino acid chains) formed when proteins are chemically digested, not a lump of food you swallow.

A bolus is the compact mass of food formed when chewing breaks food up and saliva mixes in. Saliva adds moisture and lubrication (including mucus), helping the food move down the esophagus smoothly through peristalsis.

Because the question is describing the saliva-moistened ball of food formed in the mouth that travels easily through the digestive tract, the correct choice is C.

We need the term for food that has been dampened and shaped so it can pass smoothly through the digestive tract. Thinking about where each term fits in digestion helps us remove options that happen in other places or describe something else.

Option A doesn’t fit. Chyme is the thick, semi-fluid blend of partly digested food and stomach secretions. It is produced in the stomach after more digestion has occurred, so it is not the first swallowed mass from the mouth.

Option B is also incorrect. A micelle is a very small structure made when bile salts surround fatty acids. It is important for fat absorption in the small intestine, not for the physical movement of a mouthful of food down the esophagus.

Option D doesn’t match either. A peptide is a short chain of amino acids, created as a result of protein digestion. That describes a chemical breakdown product, not the clump of food being swallowed.

That leaves Option C. A bolus is formed in the mouth during chewing. Saliva moistens it and the tongue shapes it, making it easy to swallow and push along the esophagus by peristalsis.

We’re looking for the enzyme in the small intestine that takes peptides (small pieces of proteins) and finishes the job by producing single amino acids. That means the enzyme must be able to break the bonds that hold amino acids together in protein fragments.

Amylases act on starch and other complex carbohydrates, cutting them into smaller sugars. Since peptides are protein fragments, amylase is not the right enzyme here.

Nucleases break DNA and RNA into smaller units called nucleotides. They are involved in nucleic acid digestion, not in splitting peptides into amino acids.

Lipases are designed to digest lipids, producing fatty acids and glycerol. Their active sites fit fat molecules rather than peptide chains, so they won’t complete peptide breakdown.

Peptides are short amino-acid chains linked by peptide bonds. Enzymes that hydrolyze peptide bonds to release individual amino acids are peptidases (a kind of protease), which is exactly what the question describes.

Once the carbohydrate, nucleic-acid, and fat-digesting enzymes are eliminated, the only enzyme type left that correctly completes protein digestion is peptidases.

The task is to find the enzyme group in the small intestine that performs the last part of protein digestion: converting peptides into individual amino acids that can be absorbed.

Amylases specialize in breaking down starch and other carbohydrates into smaller sugars (such as maltose). They do not act on peptide chains.

Nucleases target DNA and RNA, cutting them into nucleotides. Because peptides come from proteins, nucleases are not involved in this step.

Lipases break lipids into glycerol and fatty acids. Since they work on fats rather than proteins, they are not responsible for peptide digestion.

After excluding the other enzyme types, peptidases remain. Their role is to split peptide bonds, completing digestion by producing single amino acids the body can absorb.

The key word is peptides—the molecule being broken down. Enzyme names often reflect the substrate they act on, which helps us predict the correct type.

A typical naming rule is: use the substrate name (or its root) and add the ending -ase.

$\text{Substrate Root}+\text{-ase}=\text{Enzyme Name}$

Check the roots:

• Amylases: amyl- refers to starch, so these digest carbohydrates.
• Nucleases: nucle- points to nucleic acids (DNA/RNA).
• Lipases: lip- refers to fats (lipids).

The root that matches peptides is peptid-, and adding -ase gives the enzyme name:

$\text{peptid-}+\text{-ase}=\text{peptidase}$

So peptidases are the enzymes that complete protein fragment digestion by releasing individual amino acids.