Just like plants, animals are living beings and carry out several fundamental activities essential for their survival and continuation. These activities, such as obtaining nutrients (nutrition), getting energy from food (respiration), removing waste (excretion), and creating new individuals (reproduction), are collectively called life processes.

In this chapter, we will focus on two key life processes in animals: nutrition and respiration. Understanding these processes helps us appreciate how animals obtain and utilize the energy and materials needed for life.

Animals consume a wide variety of food depending on their species and habitat. Some eat plants, some eat other animals, some eat both, and some filter tiny particles from water. The food consumed by animals contains complex substances like carbohydrates, proteins, and fats. Before the body can use these complex substances, they must be broken down into simpler forms. This breakdown and utilization of food constitute nutrition.

The process of breaking down complex food components into simpler ones happens within a long tube inside the body called the alimentary canal. This tube starts at the mouth and ends at the anus. As food moves through the alimentary canal, various digestive juices are secreted, which help in this breakdown. The simpler, broken-down food is then absorbed into the body and transported to different parts to support various bodily functions.

9.1 Nutrition In Animals

Let's explore how animals, particularly humans, carry out the process of digestion and nutrient absorption. Do all animals perform digestion the same way?

9.1.1 Digestion In Human Beings

The human digestive system is a complex network that food travels through, undergoing breakdown and absorption at different stages.

Beginning With The Mouth Cavity

The journey of food begins in the mouth cavity. Here, food undergoes mechanical and chemical digestion:

• Mechanical Digestion: Teeth cut, crush, and grind food into smaller pieces (chewing).
• Chemical Digestion: Saliva, a liquid secreted in the mouth, mixes with food. Saliva contains digestive juices that start breaking down complex carbohydrates (like starch) into simpler sugars. This is why starchy food, if chewed for a while, might begin to taste sweet.

Saliva also moistens the food, making it easier to swallow. The tongue helps mix food with saliva and pushes the food towards the throat for swallowing.

Science and Society: Good oral hygiene, including brushing teeth and cleaning the tongue regularly and rinsing the mouth after meals, is essential for preventing tooth decay and maintaining a healthy mouth, which is the first part of the digestive system.

Activity 9.1: Let Us Investigate

Comparing the effect of iodine solution on plain boiled rice versus chewed boiled rice demonstrates the action of saliva. Iodine turns blue-black in the presence of starch.

Test tube	Contents	Initial colour (before iodine)	Final colour (after iodine)	Possible reason for colour change
A	Boiled rice + water	White/Off-white	Blue-black	Presence of starch, which reacts with iodine.
B	Chewed boiled rice + water	White/Off-white	No colour change or slight blue-black	Saliva broke down starch into sugar; less or no starch remaining to react with iodine.

This experiment confirms that saliva in the mouth chemically breaks down starch, an initial step in digestion. Digestion begins in the mouth.

Food Pipe (Oesophagus): A Passage From The Mouth To The Stomach

After being chewed and mixed with saliva, the softened food is swallowed and enters the food pipe or oesophagus. This is a tube that connects the mouth cavity to the stomach.

Food does not just drop down the food pipe. The muscular walls of the oesophagus contract and relax in a wave-like movement (called peristalsis) to gently push the food downwards towards the stomach. This wave-like movement continues throughout the alimentary canal to move food along.

Stomach

From the oesophagus, food enters the stomach, a J-shaped muscular organ. The stomach walls churn the food, mixing it thoroughly with secretions from the stomach lining.

Stomach secretions contain:

• Digestive juice: Contains enzymes that begin the breakdown of proteins into simpler substances.
• Acid (Hydrochloric acid): Helps break down proteins and kills harmful bacteria that might be present in the food.
• Mucus: Protects the inner lining of the stomach from the corrosive action of the acid.

In the stomach, food is partially digested and transformed into a semi-liquid mixture, preparing it for the next stage.

Fascinating Fact: Our understanding of stomach digestion was significantly advanced by chance observations made by Dr. William Beaumont on a patient named Alexis St. Martin who had a permanent opening in his stomach after a gunshot wound. Dr. Beaumont's experiments through this opening provided direct evidence of how stomach digestion works.

Small Intestine

The partially digested food from the stomach moves into the small intestine. Despite its name, the small intestine is the longest part of the alimentary canal, measuring approximately 6 meters in length. It is where most of the digestion and absorption of nutrients takes place.

The small intestine receives digestive juices from three sources:

• Its own inner lining.
• The liver, which secretes bile. Bile is stored in the gallbladder and released into the small intestine. It is slightly basic and helps neutralize the acidic food coming from the stomach. Bile is crucial for breaking down large fat globules into smaller droplets, making them easier to digest by enzymes.
• The pancreas, which secretes pancreatic juice. Pancreatic juice is also basic and helps neutralize acid. It contains enzymes that break down carbohydrates, proteins, and fats into simpler forms.

The digestive juices from the small intestine wall complete the breakdown of carbohydrates into simple sugars (like glucose), proteins into amino acids, and fats into fatty acids and glycerol.

Once food is fully digested into these simple forms, the process of absorption occurs. The inner surface of the small intestine is specially adapted for efficient absorption. It is lined with thousands of tiny, finger-like projections called villi.

The villi greatly increase the surface area available for absorption. Within the villi are numerous blood vessels. The simple, digested nutrients pass through the thin walls of the villi into these blood vessels, which then transport them via the blood to all parts of the body. These absorbed nutrients are used for energy, growth, repair, and maintaining bodily functions.

Science and Society: Conditions like Celiac disease highlight the importance of the small intestine's ability to absorb nutrients. In this condition, the body's reaction to gluten damages the villi, impairing nutrient absorption. Avoiding gluten is the primary way to manage this disease.

Large Intestine

After the small intestine absorbs most of the nutrients, the remaining undigested food passes into the large intestine. The large intestine is shorter than the small intestine (about 1.5 meters long) but is wider, which is why it's called 'large'.

The primary function of the large intestine is to absorb excess water and some salts from the undigested material. This process concentrates the waste, forming a semi-solid substance called stool or feces.

The stool is stored temporarily in the final part of the large intestine, called the rectum. Finally, the waste material is eliminated from the body through the anus, a process called egestion or defecation.

Eating foods rich in dietary fiber (from fruits, vegetables, whole grains) is beneficial for the large intestine, as fiber helps in the formation and passage of stool, promoting healthy bowel function.

Fascinating Fact: The large intestine is home to many types of bacteria. These bacteria are beneficial; they help break down remaining undigested food (especially fiber) and produce some vitamins (like Vitamin K and certain B vitamins). Consuming fiber-rich and fermented foods can support a healthy population of these beneficial gut bacteria.

Science and Society: Ancient health systems like Ayurveda (Charaka Samhita) recognized the importance of digestion for overall well-being and recommended easily digestible foods and spices to enhance digestive fire. Modern science also emphasizes proper meal timings, mindful eating, and avoiding overeating for digestive health.

9.1.2 Do All Animals Digest Food The Same Way As Humans Do?

While the basic principle of breaking down complex food into simpler forms for absorption is common, the digestive systems and processes vary significantly across different animal species, adapted to their specific diets.

• Ruminants (e.g., cows, buffaloes): These grass-eating animals have a unique digestive process called rumination. They partially chew grass, swallow it into a special stomach chamber called the rumen where microorganisms begin to break down tough plant fiber. The partially digested food (cud) is then regurgitated back into the mouth for thorough chewing. This allows them to extract nutrients from fibrous plant matter that other animals cannot.
• Birds: Birds lack teeth and have a specialized organ called a gizzard in their digestive tract. Food is mechanically ground in the gizzard, often with the help of small stones (grit) swallowed by the bird, replacing the function of chewing.

These examples show that animals have evolved diverse digestive systems and processes to efficiently obtain nutrients from their varied food sources.

Once food is digested and nutrients are absorbed, the body needs a way to convert these nutrients, particularly simple sugars (like glucose), into usable energy for all life activities. This process is called respiration.

9.2 Respiration In Animals

Respiration is another fundamental life process carried out by all living organisms to obtain energy. Does respiration happen the same way in all animals?

9.2.1 Respiration In Humans

In humans, respiration involves taking in oxygen and releasing carbon dioxide. This is facilitated by the respiratory system.

How Do We Breathe?

Breathing is the physical process of inhaling (taking in) air and exhaling (giving out) air. It's a continuous process vital for life.

The pathway of air in the human respiratory system begins with the nostrils (nasal openings). Air enters through the nostrils into the nasal passages. Tiny hairs and mucus in the nasal passages filter dust and dirt, cleaning the inhaled air. From the nasal passages, air travels down the windpipe (trachea), which branches into two tubes entering the lungs.

Within the lungs (protected by the rib cage), these tubes further divide into smaller branches, ending in tiny air sacs called alveoli.

Science and Society: The respiratory system's delicate nature means it can be affected by airborne particles and pathogens, like viruses (e.g., SARS-CoV-2 causing COVID-19), leading to serious lung issues.

The mechanism of breathing involves the movement of the rib cage and a dome-shaped muscular sheet located below the lungs called the diaphragm.

Activity 9.2: Let Us Make A Model

A simple model using a plastic bottle, a Y-shaped tube with balloons attached, and a rubber sheet covering the open base can demonstrate the breathing mechanism. The balloons represent the lungs, and the rubber sheet represents the diaphragm.

Pulling the rubber sheet downwards increases the volume inside the bottle, causing the balloons to inflate (like inhalation). Releasing the rubber sheet moves it upwards, decreasing the volume and causing the balloons to deflate (like exhalation).

In the human body:

• Inhalation: The rib cage moves up and outwards, and the diaphragm moves downwards. This increases the space in the chest cavity, allowing air to rush into the lungs.
• Exhalation: The rib cage moves down and inwards, and the diaphragm moves upwards. This decreases the space in the chest cavity, pushing air out of the lungs.

What Do We Breathe Out?

Activity 9.3: Let Us Explore

By blowing exhaled air through lime water in a test tube and comparing it to passing normal air through lime water (using a syringe), you can see a difference. Exhaled air turns lime water milky, while normal inhaled air does not significantly change it.

Since lime water turns milky in the presence of carbon dioxide, this experiment indicates that exhaled air contains a higher concentration of carbon dioxide than inhaled air. Carbon dioxide is a waste product of respiration.

Science and Society: Various breathing practices, like Pranayama, Tummo breathing, and deep breathing techniques, are used across cultures for improving respiratory function, relaxation, and mental well-being. These practices highlight the conscious control and benefits associated with optimizing breathing.

How Does The Exchange Of Gases Happen?

The primary site for the exchange of gases (oxygen and carbon dioxide) in the lungs is the alveoli. These tiny air sacs have very thin walls and are surrounded by a network of fine blood vessels (capillaries).

Oxygen from the air that fills the alveoli passes through their thin walls and the capillary walls into the bloodstream. At the same time, carbon dioxide (a waste product transported by the blood from different parts of the body) passes from the blood into the alveoli to be exhaled.

The energy needed for bodily functions is released through respiration, a chemical process that occurs within the cells. In this process, oxygen is used to break down simple sugars (glucose) obtained from digested food.

$ \text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water} + \text{Energy (released)} $

So, breathing is the physical act of bringing air into the lungs and expelling it. Respiration is the chemical process of using oxygen to release energy from food within the cells, producing carbon dioxide and water as byproducts. Both breathing and respiration are essential for survival.

Note that while inhaled air contains about 21% oxygen, exhaled air still contains around 16-17% oxygen. This means we don't use up all the oxygen we inhale.

Nutrients and oxygen are transported throughout the body by the circulatory system, which includes the heart, blood, and blood vessels. The heart pumps blood, delivering oxygen and nutrients to cells and carrying away carbon dioxide and other waste products.

Science and Society: Smoking severely damages the respiratory system, increasing the risk of lung diseases and cancer. Secondhand smoke (passive smoking) is also harmful to non-smokers, emphasizing the importance of avoiding smoking for personal and public health.

9.2.2 Do Other Animals Breathe The Same Way As Humans Do?

Different animals have evolved diverse structures and mechanisms for breathing, adapted to their specific environments and lifestyles.

• Most land animals, including mammals (like elephants, lions, cows), birds, reptiles (like lizards, snakes), breathe using lungs. However, the structure of lungs varies among these groups.
• Most aquatic animals, like fish, breathe using gills. Gills are specialized organs rich in blood vessels that extract dissolved oxygen from water and release carbon dioxide into the water.
• Amphibians, like frogs, often have different breathing methods at different life stages. Tadpoles (young frogs) breathe with gills. Adult frogs can breathe using lungs on land and can also exchange gases directly through their moist skin when in water.
• Animals like earthworms breathe through their moist skin.

These variations in breathing mechanisms demonstrate how animals are adapted to suit their unique habitats and ensure the exchange of gases necessary for respiration and energy production.

Besides the digestive and respiratory systems, animals have other vital systems (like circulatory, nervous, excretory systems) that work together to perform all life processes.

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In a Nutshell:

• Life processes (nutrition, respiration, excretion, reproduction, etc.) are essential for the survival of living organisms.
• Nutrition involves breaking down complex food into simpler forms (digestion) and absorbing nutrients.
• In humans, digestion occurs in the alimentary canal (mouth, oesophagus, stomach, small intestine, large intestine, anus) with the help of digestive juices from the stomach, liver, and pancreas.
• Digestion begins in the mouth (starch breakdown by saliva) and continues in the stomach (protein breakdown) and small intestine (main breakdown of carbs, proteins, fats).
• Most nutrient absorption occurs in the small intestine, specifically through villi.
• The large intestine absorbs water and salts from undigested food, forming stool for elimination (egestion).
• Digestive systems vary across animals (e.g., ruminants with rumen, birds with gizzard) adapting to their diets.
• Respiration is the process of breaking down glucose with oxygen to release energy, producing carbon dioxide and water.
• Breathing is the physical act of inhaling and exhaling air, facilitated by the respiratory system (nostrils, nasal passages, windpipe, lungs, alveoli, diaphragm, rib cage).
• Gas exchange (oxygen in, carbon dioxide out) happens in the lungs, specifically in the alveoli.
• Exhaled air contains more carbon dioxide than inhaled air.
• Breathing brings oxygen for respiration; respiration uses oxygen to produce energy. Breathing is physical, respiration is chemical.
• The circulatory system (heart, blood, blood vessels) transports nutrients and oxygen.
• Different animals have evolved various breathing mechanisms (lungs, gills, skin) suited to their habitats.

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