Selina Concise Solutions for ICSE Class 9 Biology Chapter 11 Digestive System

Exercise 1

Question A:
Answer: 1. (iii) stomach into intestine
2. (i) HCl and pepsin
3. (iii) colon
4. (iii) Premolars, molars - Grinding.
These answers identify the movement of food through the digestive tract and the specific agents that help in chemical and mechanical breakdown. Understanding these stages helps us see how our body systematically processes nutrients for energy.
Teacher's Tip: Remember "P" for Premolars and "P" for Pulverizing/Grinding food.
Exam Tip: When writing the dental functions, always pair the tooth type with its specific action (e.g., Molars - Grinding) for full marks.

Question B.1:
Answer: (d) Dental formula of an adult human being is {2, 1, 2, 3}/{2, 1, 2, 3}.
The dental formula represents the number and types of teeth in one-half of each jaw. By multiplying these numbers, you can calculate the total 32 teeth found in an adult mouth.
Teacher's Tip: Think of the formula as a "code" for your mouth: 2 Incisors, 1 Canine, 2 Premolars, and 3 Molars.
Exam Tip: Always write the dental formula as a fraction to represent the upper and lower jaws correctly.

Question B.2:
Answer: The two reflexes which occur when a person chews and swallows food are:
1. Reflex when a person chews - Secretion of saliva
2. Reflex when a person swallows - Tongue presses upward and back against the roof (palate)
Reflexes are automatic actions that the body performs without us having to think about them. These specific actions prepare the food to be safely transported from the mouth to the stomach.
Teacher's Tip: Imagine your mouth as a "slide" where saliva is the water that helps you zip down.
Exam Tip: Mention both the action (tongue pressing) and the location (palate) to describe the swallowing reflex accurately.

Question B.3:
Answer: (ii) Both the statements are wrong. Small intestine is longer (7 metres) than large intestine (1.5 metres). Also, large intestine is so called because of its width which is larger than that of small intestine.
It is a common mistake to think the "small" intestine is actually smaller in length. The names refer to the diameter of the tubes, not how long they stretch out inside your abdomen.
Teacher's Tip: Think: Small intestine is "Small in Width," Large intestine is "Large in Width."
Exam Tip: If asked for the lengths, remember 7m for small and 1.5m for large to show precise knowledge.

Question C.1:
Answer: Digestion is the process of breaking down complex food materials into simpler substances by the action of enzymes.
Need for a digestive system:
1. Large complex molecules like carbohydrates, proteins and lipids need to be broken down to simpler molecules. These simpler molecules can then be absorbed and utilized by the body.
2. The breaking down of complex food molecules into their simpler form is possible only through the process of digestion.
3. During digestion, large complex macromolecules present in food are converted into small simpler molecules, which can be simplified in different compartments of digestive system only.
Our body cannot use a piece of bread or meat directly; it must be turned into microscopic building blocks. The digestive system acts like a chemical factory that refines raw food into usable energy.
Teacher's Tip: Think of digestion like taking apart a Lego castle to use the individual bricks for something else.
Exam Tip: Always include the word "enzymes" in your definition of digestion to gain full marks.

Question C.2:
Answer: SUBSTRATE - END PRODUCTS
Starch - Maltose
Proteins - Small peptides and amino acids
Fats - Fatty acids and glycerol
This table shows how major nutrients are transformed into their final, absorbable forms. Each substrate requires a specific enzyme to trigger the change into these simpler chemicals.
Teacher's Tip: Remember: Proteins always end up as Amino Acids, which are the building blocks of your muscles.
Exam Tip: Be sure to list both products for fats (fatty acids AND glycerol) to get complete credit.

Question C.3:
Answer: Vitamins are used in their original form by the cells. They do not require digestion. They are either water soluble or fat soluble, hence no enzyme is required to digest vitamins. They are absorbed directly from the digestive tract, transported by blood to the cells, and the cells absorb and use them whenever they need. Besides, vitamins themselves act as catalysts or enzymes in essential chemical reactions that take place in the body.
Unlike food, vitamins are so small that they can enter our blood without being broken down first. They act like "helpers" that make sure other chemical reactions in our body happen at the right speed.
Teacher's Tip: Think of vitamins as the "keys" that unlock different functions in your body's engine.
Exam Tip: State that vitamins are "absorbed directly" to explain why they don't need digestion.

Question C.4:
Answer: It is very important to chew our food thoroughly as chewing of food helps to break down complex food materials into simpler substances. The act of chewing stimulates the salivary glands to release saliva. The saliva helps to moisten the food and form bolus, which can be swallowed easily. Saliva also contains special enzymes that help to break down carbohydrates.
Chewing is the first step of digestion and makes the work much easier for your stomach. By increasing the surface area of the food, enzymes can work faster to dissolve the nutrients.
Teacher's Tip: Chew your food until it feels like a soft paste; your stomach will thank you!
Exam Tip: Mention the term "bolus" (the soft ball of food) to describe the result of proper chewing.

Question C.5:
Answer: Rectum acts as a temporary storage site for undigested food. It has voluntary smooth muscles that remove the faeces out of the body through the anus.
The rectum holds onto waste until your body is ready to get rid of it. This control is important for maintaining regular and healthy bathroom habits.
Teacher's Tip: The rectum is like a "waiting room" for waste before it leaves the building.
Exam Tip: Identify the rectum as a "temporary storage" organ to accurately define its purpose.

Question C.6:
Answer: Roughage is a dietary fibre that largely consists of cellulose. It cannot be digested by our body as our body does not contain cellulose-digesting enzymes.
Examples of roughage:
1. Fruits
2. Green leafy vegetables
Even though we cannot digest it, roughage acts like a broom for our intestines. It helps push other food along the digestive tract to prevent constipation.
Teacher's Tip: Think of roughage as "Nature's Scrubbing Brush" for your insides.
Exam Tip: Use the term "cellulose" to explain why humans cannot digest roughage.

Question C.7:
Answer: Adaptations of ileum for the absorption of digested food:
1. Very long to provide more surface area for absorption
2. Presence of large number of villi to further increase the surface area
The ileum is the final section of the small intestine where most nutrients enter the blood. Its design ensures that as much food as possible is absorbed before it moves into the large intestine.
Teacher's Tip: Villi look like tiny fingers that grab onto nutrients as they pass by.
Exam Tip: Always link "length" and "villi" to the concept of "increased surface area" for full marks.

Question C.8:
Answer: Functions of hydrochloric acid:
1. It gets mixed with food and kills the bacteria present in food.
2. It activates pepsin to act on proteins.
This strong acid in our stomach serves as a protective barrier and a chemical starter. It keeps us safe from germs while helping the body begin the difficult task of digesting meat and other proteins.
Teacher's Tip: HCl is like the "Security Guard" and the "Battery Jump-starter" of the stomach.
Exam Tip: Be sure to mention "pepsin" as the specific enzyme that needs HCl to work.

Question D.1:
Answer: Vegetarian menu for dinner:
Foodstuffs - Weight (gm/ml)
Cereals - 320
Pulses - 70
Green leafy vegetables - 100
Root vegetables - 75
Fruits - 75
Milk - 600
Fat and oil - 30
Brown sugar and jaggery - 30
This menu provides a balanced mix of carbohydrates, proteins, and vitamins for a healthy adult. Each weight represents the correct portion size to maintain body energy and health.
Teacher's Tip: A colorful plate usually means you are getting a good variety of these food groups.
Exam Tip: When writing a diet chart, include at least one source of protein like "Pulses" and one source of calcium like "Milk."

Question D.2:
Answer: Main characteristics of an enzyme:
1. It is a protein and therefore, gets destroyed by heating.
2. It acts only on one kind of substance called the substrate. So, it is very specific.
3. It acts as a catalyst, so it can be used again and again.
4. It only affects the rate of a chemical reaction and always speeds up the reaction.
5. It always forms the same end products from the fixed substrate.
6. It acts best only at a particular pH.
7. It acts best within a narrow temperature range, usually between 35°C-40°C.
Enzymes are like specialized biological tools that only work on one specific job. Because they are not used up in the reaction, a small number of enzymes can process a lot of food.
Teacher's Tip: Think of an enzyme as a "Lock and Key"; only the right key (substrate) fits the lock (enzyme).
Exam Tip: Mention the temperature range of 35°C to 40°C$ as this is close to human body temperature.

Question D.3:
Answer: The small intestine is the most important organ of the digestive system as it serves both, for digestion and absorption. It receives two digestive juices; the bile and the pancreatic juice in the duodenum. These two juices virtually complete the digestion of starch, proteins, carbohydrates, etc. After the breakdown of food, the small intestine absorbs simple substances such as glucose, amino acids, etc.
The small intestine is where the "real work" of getting nutrients into your body happens. Its massive surface area allows it to efficiently soak up all the building blocks the body needs to survive.
Teacher's Tip: Most of your food's "journey" into your blood happens right here in the small intestine.
Exam Tip: Mention both "digestion" and "absorption" to describe the dual role of the small intestine.

Question D.4:
Answer: Liver is an important organ in our body as it serves the following functions:
Production of bile
Control of blood sugar levels
Control of amino acid levels
Synthesis of foetal red blood cells
Produce fibrinogen and heparin
Regulate blood volume
Destroy dead red blood cells
Store vitamin and minerals
Excrete toxic and metallic poisons
Produce heat
Detoxification
The liver is the body's largest internal organ and acts as a multitasking filter and storage center. It processes everything we eat and drink to remove toxins and regulate our blood chemistry.
Teacher's Tip: Think of the liver as the "Chemical Processing Plant" of your body.
Exam Tip: If asked for three functions, "Production of bile," "Detoxification," and "Storage of vitamins" are the easiest to remember.

Question D.5:
Answer: (i) Peristalsis: Peristalsis is the rhythmic contraction and relaxation of the muscles of the alimentary canal that pushes the food along the gut.
(ii) Omnivore: Omnivores are organisms that consume both plants and animals.
(iii) Pylorus: Pylorus is the passage at the lower end of the stomach that opens into the duodenum.
(iv) Kilocalorie: A kilocalorie is a unit of energy. It is the energy required to raise the temperature of 1 kg of water by 1 Celsius.
(v) Basal metabolic rate: Basal metabolic rate refers to the minimum amount of energy in the form of calories that our body requires to complete its normal functions.
These terms describe the mechanics, classifications, and measurements used in human biology. Understanding them helps us speak scientifically about how our bodies move food and use energy.
Teacher's Tip: Think of Peristalsis as "Squeezing a tube of toothpaste" to move the contents along.
Exam Tip: Define "Kilocalorie" specifically using "1 kg of water" and "1 Celsius" for a perfect score.

Question D.6:
Answer: REGION - ENZYME - ACTION ON FOOD
Stomach - Pepsin - Acts on proteins and converts it into polypeptides
Duodenum - Amylopectin - Acts on starch and converts it into maltose
Duodenum - Trypsin - Acts on remaining proteins, proteoses and peptones to produce peptides and amino acids
Ileum - Erepsin - Acts on proteins and polypeptides and converts them into small peptides and amino acids
Ileum - Sucrase - Acts on sucrose and converts it into glucose and fructose
Ileum - Lactase - Acts on lactose and converts it into glucose and galactose
This chart tracks how different enzymes in specific parts of the intestine break down food into tiny pieces. By the time food reaches the end of the ileum, it is in its simplest form, ready for the body to use.
Teacher's Tip: Notice that many enzyme names end in "-ase" (like Sucrase and Lactase).
Exam Tip: Match the correct enzyme to the correct region (Stomach vs. Duodenum vs. Ileum) to avoid losing points.

Question D.7:
Answer: Importance of water in our body:
1. Water is the major component of blood, which carries nutrients and oxygen, to and from all the cells.
2. It is the major component of saliva and mucous, which lubricate the membranes that line our digestive system beginning with the mouth.
3. It helps in regulating the temperature of the body.
4. Water is very essential for digestion as well as absorption of food.
Water is the medium that allows every chemical reaction in our body to take place. Without enough water, our blood wouldn't flow and our digestive system would grind to a halt.
Teacher's Tip: Drink water throughout the day; your body is about 70% water, just like the Earth!
Exam Tip: List "regulation of body temperature" as a key function of water for full marks.

Question D.8:
Answer: Test for starch:
1. Take the food item to be tested. Put it into a test-tube containing water and boil to make a solution.
2. Cool the solution and add 2-3 drops of dilute iodine solution to it.
3. Blue-black colour of the solution indicates the presence of starch in the food item.
Test for proteins:
1. Take the food item to be tested in a test tube.
2. Add few drops of dilute nitric acid to it.
3. Heat the test-tube gently.
4. Rinse off the acid with water and add few drops of ammonium hydroxide to it.
5. Colour change from colourless to yellow and then from yellow to orange red indicates the presence of protein in the food item.
Chemical tests allow scientists to see what nutrients are hidden inside different types of food. These color changes are reliable indicators used in laboratories around the world.
Teacher's Tip: Remember: Iodine + Starch = Blue-Black color.
Exam Tip: Mention the specific colors (Blue-black for starch, Orange-red for protein) to prove your results.

Question E.1:
Answer: The dental formula of a normal human adult is: Human adult = {2, 1, 2, 3}/{2, 1, 2, 3} = 32 (Permanent teeth + Wisdom teeth)
As an adult, you have 32 specialized teeth designed to handle a varied diet. This includes the wisdom teeth, which often grow in much later than the rest of your adult teeth.
Teacher's Tip: You have 8 incisors total (4 top, 4 bottom) because the formula is for only one side of your face.
Exam Tip: Don't forget to mention that the formula represents "half" of the jaw when explaining the total count of 32.

Question E.2:
Answer: While swallowing saliva in the mouth, the larynx is pulled upwards to bring it close to the back of the tongue, when a flap called epiglottis closes its opening. Then, it goes towards the oesophagus.
The epiglottis acts like a safety lid for your windpipe, making sure food only goes down the "food pipe." This clever mechanism prevents us from choking while we eat.
Teacher's Tip: Think of the epiglottis as a "Traffic Controller" that directs food to the right tunnel.
Exam Tip: Use the term "epiglottis" to explain how food is kept out of the respiratory system.

Question E.3:
Answer: Organ - Enzyme - Food acted upon - Final product
Stomach - Pepsin - Proteins - Polypeptide
Mouth - Amylase - Starch - Disaccharide
Ileum - Maltase - Maltose - Glucose
This summary highlights the key points in the digestive journey where specific chemicals meet their targets. By the time food reaches the ileum, it is simplified into glucose, which is the body's primary fuel.
Teacher's Tip: Amylase starts in the mouth, which is why bread tastes sweet if you chew it for a long time.
Exam Tip: Remember that "Maltase" breaks down "Maltose"—the names are almost the same!

Question E.4:
Answer: (a) 1 - Enamel, 2 - Dentine, 3 - Pulp, 4 - Gum, 5 - Crown, 6 - Cement
(b) The tooth shown in the diagram has only one root, so it is an incisor or canine which is used for biting and piercing.
(c) The part labelled ‘3’ (pulp) is a soft connective tissue present in the pulp cavity of the tooth. It consists of blood capillaries, lymph vessels and nerve fibres. These extend from the crown of the tooth and open through the pulp cavity at the base of the root.
(d) Type of teeth in the mouth of an adult:
1. Incisors (8) - Used for biting and cutting
2. Canines (4) - Used for holding and tearing of food
3. Premolars (8) - Used for grinding and crushing of food
4. Molars (12) - Used for grinding and crushing of food
A tooth is a complex living structure with layers of protection and a soft core containing nerves. The enamel is the hardest substance in your entire body, protecting the sensitive parts inside.
Teacher's Tip: The "Pulp" is the part that feels pain because it contains all the nerves.
Exam Tip: Identify the "Incisor" by looking for a single root in a tooth diagram.

Question E.5:
Answer: Internal structure of a mammalian tooth
The internal structure shows how different layers like enamel, dentine, and pulp are organized to provide strength and feeling. This anatomy ensures the tooth can withstand the pressure of chewing while remaining a living part of the body.
Teacher's Tip: Enamel is like the "Armor," Dentine is the "Wall," and Pulp is the "Control Room."
Exam Tip: Practice labeling the "Enamel-dentine junction" for advanced diagrams in your science papers.