Answers – 8.2.9 Human Digestive System

The human body needs to digest food because food molecules are too large to pass through cell membranes and enter the bloodstream. Digestion breaks down proteins into amino acids, carbohydrates into glucose, and fats into fatty acids and glycerol. These smaller molecules can then be absorbed into the blood and used by cells for energy, growth, and repair.

Physical (mechanical) digestion: Breaking food into smaller pieces without changing its chemical structure. Example: chewing food with teeth in the mouth.

Chemical digestion: Breaking down food molecules using enzymes to change their chemical structure. Example: amylase enzyme breaking down starch into maltose in the mouth.

Ingestion involves putting food into the mouth using the lips, tongue, and teeth. The tongue helps move food around in the mouth so the teeth can break it into smaller pieces before swallowing. This is the first step of the digestive process and only takes place in the mouth.

Physical digestion in the mouth happens when teeth break food into smaller pieces through chewing. Different types of teeth have different jobs:

Incisors (front teeth): Cut food like scissors

Canines (pointed teeth): Tear food apart

Molars (back teeth): Crush and grind food into smaller pieces

The tongue helps move food around so all the teeth can work on it properly. This increases the surface area of food, making it easier for enzymes to work on it later.

Chemical digestion in the mouth uses the enzyme amylase, which is found in saliva.

Amylase breaks down starch (found in foods like bread, rice, and potatoes) into maltose (a type of sugar).

This is why bread tastes sweeter if you chew it for a long time – the amylase is converting starch into sugar.

Saliva contains:

• Water (to moisten food)
• Amylase enzyme (to break down starch)
• Mucus (to help food slide down easily)
• Antibacterial substances (to kill harmful bacteria)

There are three pairs of salivary glands that produce about 1.5 liters of saliva every day.

The stomach has thick muscular walls that contract and relax in a churning motion. This squeezes and mixes the food for 2-4 hours, breaking it into smaller pieces.

The churning action creates a thick, soupy mixture called chyme. This physical mixing helps break down food mechanically and mixes it thoroughly with digestive juices.

1. The enzyme involved: Protease enzymes
2. What type of food it breaks down: Proteins (from meat, eggs, cheese, fish)
3. The role of hydrochloric acid: Creates the acidic conditions needed for protease enzymes to work properly, kills harmful bacteria in food, and helps break down food structure

Hydrochloric acid is present in gastric secretions to help with protein digestion and food safety.

Two functions:

1. Kills harmful bacteria: The strong acid destroys dangerous microorganisms that might be in food
2. Activates protease enzymes: Creates the acidic environment (low pH) needed for protease enzymes to work effectively

The small intestine is about 6 meters long and 3 cm wide. It is divided into two main parts:

Duodenum (first 25 cm): Where most chemical digestion happens. Receives digestive juices from the pancreas and bile from the liver.

Ileum (remaining 3.5 meters): Where absorption mainly occurs. Has millions of tiny finger-like projections called villi to increase surface area for absorption.

This division allows the small intestine to complete digestion first, then focus on absorbing the nutrients efficiently.

1. Production of bile: The liver makes bile, which helps break down fat droplets into smaller pieces (emulsification). This makes it easier for lipase enzymes to digest fats. Bile is not an enzyme but acts like soap to break up fat droplets.
2. Storage of glycogen: The liver converts excess glucose from digested food into glycogen for storage. When the body needs energy, the liver converts glycogen back into glucose and releases it into the bloodstream.

Bile breaks large fat droplets into smaller droplets through a process called emulsification. This is like how soap breaks up grease on dishes.

Even though bile is not an enzyme, it is essential for fat digestion because it increases the surface area of fat droplets. This gives lipase enzymes much more area to work on, making fat digestion much faster and more efficient.

Without bile, fats would remain in large droplets that enzymes cannot break down effectively.

Bile is released when you eat fatty foods. When fat enters the duodenum, the body sends chemical signals to the gall bladder, causing it to contract and squeeze bile through a tube into the small intestine. This ensures bile is only released when needed for fat digestion.

1. The three enzymes it contains:
   • Amylase (breaks down starch)
   • Protease (breaks down proteins)
   • Lipase (breaks down fats)
2. Why the secretion is alkaline: The pancreatic juice is alkaline to neutralize the acidic chyme coming from the stomach. This creates the right pH conditions for the pancreatic enzymes to work properly.

When food leaves the stomach, it is very acidic due to hydrochloric acid. The pancreatic enzymes (amylase, protease, and lipase) work best in alkaline conditions, not acidic conditions.

The alkaline pancreatic juice neutralizes the stomach acid, creating the proper pH environment for these enzymes to function effectively. Without this neutralization, the enzymes would not work properly and digestion would be incomplete.

Absorption mainly occurs in the ileum (the second part of the small intestine). Some absorption also happens in the large intestine, but this is mainly for water and minerals.

The main function of the ileum is to absorb digested nutrients into the bloodstream.

Structural adaptations for absorption:

• Villi: Millions of finger-like projections that increase surface area
• Rich blood supply: Many blood vessels to carry absorbed nutrients away
• Thin walls: Allow nutrients to pass through easily
• Long length: About 3.5 meters provides lots of time for absorption

The large intestine mainly absorbs water from the remaining food waste. When material enters the colon, it is still quite watery. The walls of the large intestine absorb most of this water back into the body.

The large intestine also absorbs some vitamins (especially vitamin K) that are made by helpful bacteria living there, and any remaining minerals like sodium.

This water absorption is important to prevent dehydration and to form solid waste (feces).

Egestion occurs at the end of the digestive system, involving the rectum and anus. This process removes materials that the body cannot digest or absorb, such as fiber, dead bacteria, and other indigestible substances.

Rectum: Acts as a temporary storage area for solid waste (feces). It has sensors that detect when it becomes full and sends signals to the brain.

Anus: The opening at the end of the digestive system. It has two sets of muscles (sphincters) – one automatic and one that can be controlled. This allows conscious control over when waste is expelled from the body.

1. Starch from the bread: Starts being broken down by amylase in the mouth into maltose. Continues in the small intestine where pancreatic amylase breaks down remaining starch. Maltase then converts maltose into glucose, which is absorbed into the blood.
2. Protein from the cheese: No digestion in mouth. Broken down by protease enzymes in the stomach with help of hydrochloric acid. Further broken down by pancreatic protease in small intestine into amino acids, which are absorbed.
3. Fat from the butter: No digestion in mouth or stomach. In small intestine, bile breaks fat into smaller droplets, then lipase enzymes break it down into fatty acids and glycerol, which are absorbed.

1. What enzyme is used in each location: In the mouth: salivary amylase. In the small intestine: pancreatic amylase and maltase.
2. What products are formed in each location: In the mouth: starch → maltose. In the small intestine: starch → maltose → glucose.
3. Why starch digestion continues in the small intestine: Food doesn’t stay in the mouth very long, so only partial digestion occurs there. The small intestine completes the process because there is more time and the right conditions for enzymes to work fully.

Protein digestion does not begin in the mouth because saliva does not contain protease enzymes – it only contains amylase for starch digestion.

Protein digestion begins in the stomach because:

• The stomach produces protease enzymes
• The stomach produces hydrochloric acid, which creates the acidic conditions that protease enzymes need to work
• The acidic environment also helps unfold protein molecules, making them easier for enzymes to break down

1. How this might affect their digestion: They might have difficulty digesting large amounts of fatty foods because bile is not stored and released when needed.
2. Why they can still digest fats normally: The liver continues to produce bile, which flows directly into the small intestine instead of being stored first.
3. What dietary advice might be helpful: Eat smaller portions of fatty foods more frequently rather than large fatty meals. This prevents overwhelming the digestive system with more fat than can be handled at once.

1. The condition of the food when it leaves the stomach: The food is a thick, acidic, soupy mixture called chyme. It has been partially digested by protease enzymes and stomach acid.
2. How the pancreas helps neutralize the acidity: The pancreas releases alkaline pancreatic juice containing bicarbonate ions that neutralize the hydrochloric acid from the stomach.
3. Why this neutralization is necessary: The enzymes in the small intestine (amylase, protease, lipase) work best in alkaline conditions. If the environment remained acidic, these enzymes would not function properly and digestion would be incomplete.

Starch → Maltose → Glucose

Enzymes responsible:

• Starch → Maltose: Amylase (from saliva and pancreas)
• Maltose → Glucose: Maltase (from small intestine)

Mechanical digestion: Physical breakdown of food into smaller pieces without changing the chemical structure.

Examples: Chewing with teeth in the mouth; churning action of stomach muscles

Chemical digestion: Breaking down food molecules using enzymes to change their chemical structure into smaller molecules.

Examples: Amylase breaking down starch in the mouth; protease breaking down proteins in the stomach

This statement is incorrect because saliva has many important functions:

1. Contains amylase enzyme: Starts the chemical digestion of starch into maltose
2. Moistens food: Makes food easier to chew and swallow
3. Kills bacteria: Contains antibacterial substances that help protect against harmful microorganisms
4. Helps with taste: Dissolves food chemicals so taste buds can detect flavors

Carbohydrates (starch):

• Mouth: Salivary amylase breaks starch into maltose
• Small intestine: Pancreatic amylase continues breakdown, maltase converts maltose to glucose
• Absorption: Glucose absorbed in ileum

Proteins:

• Stomach: Protease enzymes break proteins into smaller chains
• Small intestine: Pancreatic protease completes breakdown into amino acids
• Absorption: Amino acids absorbed in ileum

Fats:

• Small intestine: Bile emulsifies fats, lipase breaks them into fatty acids and glycerol
• Organs involved: Liver (makes bile), gall bladder (stores bile), pancreas (makes lipase)
• Absorption: Fatty acids and glycerol absorbed in ileum