Class 7 Science Respiration in Plants and Animals Chapter Notes

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Class 7 Science: Respiration in Plants and Animals Chapter Notes

A living organism requires energy for performing various activities to survive. This energy is released by the breakdown of digested food stuffs during the process of respiration. Thus, respiration is the fundamental process of energy release in our body. Respiration occurs in all living cells. The energy-rich food stuffs like glucose and fructose are oxidised in our body (in the presence of oxygen) and changed to water and carbon dioxide. This reaction with oxygen is known as oxidation. Thus, respiration is the process of taking in oxygen, using it for the release of energy by oxidation of food and elimination of the waste products, carbon dioxide and water.

The respiration process can be summarised as:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Glucose + Oxygen → Carbon dioxide + Water + Energy

Respiration is carried out in two steps:

1. External respiration or breathing
2. Internal respiration or cellular respiration.

BREATHING

It is the process of breathing in oxygen and breathing out carbon dioxide. The exchange of oxygen and carbon dioxide is brought about by the breathing mechanism differently in different animals. In higher animals, from frog to man, this exchange of gases takes place in the respiratory organ, the lungs. Here oxygen is absorbed and distributed by the blood to each and every cell of the organism.

CELLULAR RESPIRATION

Oxygen present in the blood combines with the food present in the form of glucose in the cells, to release energy. The energy released during respiration takes place only with the help of several chemical reactions, generally catalysed by enzymes. This energy is utilised for various metabolic activities. This part of respiration is called internal or cellular respiration and occurs in the mitochondria or the power house of the cell. Cellular respiration takes place in all living organisms, from bacteria to man. This shows functional unity in spite of structural diversity in living organisms.

Cellular respiration

It primarily takes place in the bag-like structures called mitochondria of all living cells. It is a two-stage process: glycolysis and Krebs cycle.

Glycolysis

It occurs in the cell cytoplasm and does not require oxygen. In this process, one molecule of glucose is split into two molecules of pyruvic acid and a small amount of energy is released.

Krebs cycle

Also known as the TCA or Tricarboxylic acid cycle. It occurs inside the mitochondria and requires oxygen. One molecule of glucose produces eight ATP molecules during glycolysis and 30 ATP molecules during the TCA cycle in mitochondria. That is why mitochondria are called the power house of a cell.

Table 1: Differences between cellular respiration and breathing

Breathing or external respiration	Cellular or internal respiration
It is a physical process of diffusion. Oxygen diffuses in, and carbon dioxide diffuses out.	It is a biochemical process in which glucose is oxidised and carbon dioxide and water are released.
It takes place outside the cells, in the lungs.	It takes place within the cells.
There is no release of energy.	There is a gradual and step-wise release of energy.
Enzymes are not involved.	Several respiratory enzymes are involved in the process.

TYPES OF RESPIRATION

Depending on whether oxygen is used in the process or not, respiration is of two types:

1. Aerobic respiration
2. Anaerobic respiration

Aerobic Respiration: This type of respiration occurs in the presence of oxygen, where food (glucose) is broken down into carbon dioxide and water and energy is released, which is stored as ATP molecules.

Glucose + Oxygen → Carbon dioxide + Water + Energy (38 ATP)

Anaerobic Respiration: This type of respiration takes place in the absence of oxygen. It is a normal feature in certain microscopic organisms like bacteria and yeast. Such organisms that can survive in the absence of oxygen are called anaerobes. In the absence of oxygen, glucose breaks down partially into carbon dioxide and ethyl alcohol. The energy released during anaerobic respiration is much less than in aerobic respiration.

Glucose → Ethyl alcohol + Carbon dioxide + Energy (2 ATP)

The process whereby micro-organisms like yeast and bacteria respire anaerobically to produce alcohol and carbon dioxide from food is called fermentation.

Table 2: Differences between aerobic and anaerobic respiration

Aerobic respiration	Anaerobic respiration
It occurs in the presence of oxygen.	It occurs in the absence of oxygen.
Complete oxidation of glucose produces carbon dioxide, water and energy.	Incomplete oxidation of glucose produces ethyl alcohol, carbon dioxide and energy.
It is a two-stage process and occurs in the cytoplasm and mitochondria of the living cell.	It occurs only in the cytoplasm of the living cell.
A large amount of energy (38 ATP molecules per glucose molecule) is released.	Very little energy is released during anaerobic respiration.
Water is formed as an end product.	Water is not formed as an end product.
It takes place in all living organisms.	It takes place in some lower organisms like yeast and bacteria.

HUMAN RESPIRATORY SYSTEM

Human beings have a more extensive and complex respiratory system. This is because our metabolic rate is high and so is the energy demand. Since our body cannot store oxygen, we need to breathe day and night to move air into and out of our system.

The respiratory system of a human being includes:

Respiratory Tract	Respiratory Organ
Nostrils and nasal passage Pharynx Trachea Bronchi and bronchioles Alveoli	Lungs

Respiratory Tract

Nostrils and Nasal Passage: Nostrils are a pair of slits that open into left and right nasal chambers. The nasal chamber possesses a border of hairs, which trap dust particles and germs in the air and filter them. It is richly supplied with blood vessels that warm the incoming air. The sticky mucus lining of the nasal chamber moistens the air and filters dust particles.Pharynx: Pharynx is a common passage at the back of the mouth for air and food. Air enters the front tube called the windpipe. The entrance of the food pipe and the windpipe is guarded by a stiff flap-like structure, the epiglottis. It helps in controlling and directing food to the food pipe and air along the windpipe respectively. It opens during breathing, but closes the passage of the windpipe while swallowing or drinking, thus preventing the food from entering the lungs.Trachea: The pharynx leads to the trachea, a tube lying in front of the food pipe. It passes through the neck and extends into the chest cavity. Its wall is supported by C-shaped cartilaginous rings.Bronchi & Bronchioles: At the lower end the trachea splits into two bronchi that lead to the lungs on their respective side. Each bronchus further divides and subdivides into smaller tubes known as bronchioles. Each bronchiole ends in tiny air sacs called alveoli. It is here that exchange of gases takes place. Each lung of an adult human contains about 300-400 million alveoli. Alveoli have very thin walls and are richly supplied with blood capillaries. The oxygen present in the air we breathe in goes into the blood and the CO

 

₂ present in the blood passes out of the blood into the alveoli. Thus exchange of gases takes place.

Respiratory Organ

Lungs: These are a pair of respiratory organs that are spongy, bag-like structures, lying in the chest cavity on either side of the heart. The left lung is two-lobed and slightly smaller than the right lung, which is three-lobed.

The two lungs are protected by the flexible ribcage and sternum on the front and the vertebral column at the back. Just beneath the lungs is a dome-shaped muscular sheet, the diaphragm. The co-ordinated action of the diaphragm and ribcage helps in the mechanism of drawing in oxygen (inhalation) and expelling the CO₂ (exhalation).

 

Mechanism of Breathing
Inhalation or breathing in	Exhalation or breathing out

The process of breathing is aided by the rhythmic expansion and contraction of the thoracic cavity. It is the intercostal muscles between the ribs and the diaphragm that make this possible. Breathing involves two steps:

1. Inspiration or Inhalation
2. Expiration or Exhalation

Inspiration: Breathing in, or the entry of air from the atmosphere to the lungs, is called inspiration. During inspiration, the intercostal muscles contract and push the ribs upwards and outwards, making the cone-shaped diaphragm flat. This causes a decrease in the air pressure inside the alveoli. The atmospheric air, which is at higher pressure, rushes in through the respiratory tract to equalise the pressure. Finally, the exchange of air takes place in the alveoli surrounded by fine blood capillaries. Oxygen is taken up by haemoglobin of the RBCs of blood and carbon dioxide is given up by blood to the alveoli.

 

Expiration: The process of breathing out or expelling carbon dioxide and water from the lungs through the respiratory tract is called expiration. During expiration, the intercostal muscles relax, the ribs move inward and the diaphragm becomes cone-shaped again. The volume of the chest cavity decreases and the air pressure inside the alveoli increases. Thus air is pushed out under increased pressure through the respiratory tract.

Cellular Respiration

In inhalation, there is more oxygen in the atmospheric air in the alveoli than in the blood capillaries. So the oxygen moves from the air to the blood. Here it combines with haemoglobin (the red coloured pigment of red blood cells) to form oxyhaemoglobin.

 

The oxygenated blood goes from the lungs to the blood vessels and returns to the heart. The heart pumps this blood throughout the body through the arteries. The arteries further subdivide many times to form capillaries, which supply the cells of the body with the oxygen they need to break down food (glucose) and release energy.

Carbon dioxide and water are produced as waste products during cellular respiration, which need to be eliminated from the body. This carbon dioxide is absorbed by the blood in the blood capillaries, which combine to form veins. This carbon dioxide is exchanged with oxygen in the alveoli. The carbon dioxide is then expelled from the body through exhalation or expiration.

Respiration in Other Animals

We have lungs for respiration. But all animals do not have lungs. Frogs, snakes, crocodiles, birds and mammals possess lungs. They have nostrils (not the nose) to inhale air. However, primitive animals like earthworm, cockroach, mosquito, snail and fish are without lungs. Their respiratory organs are much different.

Respiration in Earthworm

Worms breathe through their skin, as they don’t have any lungs or nose. The mouth is used for eating organic and rotting material along with soil. Breathing through their skin allows them to stay underground for long periods. They also don’t have any eyes or ears but rather sense their way along with chemical and light sensitive cells.

The skin’s moisture plays a key role in how oxygen travels into the worm. The worm exposes itself to oxygen by either lying out in the open or burrowing into the soil. Oxygen meets the dampness of the skin and dissolves to be absorbed through the small tiny blood vessels called capillaries just under the skin’s surface. The oxygen then travels through these blood streams and is pushed throughout the body with their five larger blood vessels that resemble hearts. Once through the body, the capillaries push the waste carbon dioxide back out the same capillaries through the skin and away from the worm.

Worms often spend most time underground or underneath things as their skin has to stay moist. It cannot be allowed to dry out or the pores will not enable oxygen to be dissolved and then absorbed. There is usually enough oxygen in the soil to keep worms alive. During rainy times, it is suspected that the ground is so moist that there is not enough oxygen to breathe in, so they often come to the surface in an effort to avoid suffocating. Worms also come to the surface more at night to avoid the heat of the day. They are unable to breathe underwater as the water blocks off the pores. Staying moist but not wet is the key to their survival.

Respiration in Cockroach

Cockroaches, like all insects, breathe through a system of tubes called tracheae. The tracheae of insects are attached to the spiracles, excluding the head. Thus cockroaches, like all insects, are not dependent on the mouth and windpipe to breathe. The valves open when the CO₂ level in the insect rises to a high level; then the CO₂ diffuses out of the tracheae to the outside and fresh O₂ diffuses in. Unlike in vertebrates that depend on blood for transporting O₂ and CO₂, the tracheal system brings the air directly to cells, the tracheal tubes branching continually like a tree until their finest divisions, tracheoles, are associated with each cell, allowing gaseous oxygen to dissolve in the cytoplasm lying across the fine cuticle lining of the tracheole. CO₂ diffuses out of the cell into the tracheole.

While cockroaches do not have lungs and thus do not actively breathe in the vertebrate lung manner, in some very large species the body musculature may contract rhythmically to forcibly move air out and in through the spiracles; this may be considered a form of breathing.

Respiration in Fishes (Aquatic Respiration)

Fishes and other animals living in water are called aquatic animals. They obtain oxygen dissolved in water. Their respiratory organs are gills. Gills are projections of skin. They have a network of fine blood capillaries. The oxygen dissolved in water diffuses into the blood. Carbon dioxide present in the capillary blood diffuses into water. The oxygenated blood from capillaries is taken to the heart and pumped to different parts of the body, and carbon dioxide-rich blood from body tissue is taken to the gills for the exchange of respiratory gases.

RESPIRATION IN PLANTS

Like animals, plants also need energy to grow and perform life functions. Plants get this energy through the process of respiration. Plants use atmospheric oxygen to break down glucose into carbon dioxide and water, accompanied by a release of energy. Generally, plants do not have any organs for breathing. The exchange of gases in plants is performed by a simple process of diffusion.

In higher plants, oxygen and carbon dioxide move in and out of the roots and stem through the outer layers of cells. But the majority of gas exchange takes place through small openings in the leaves called stomata (singular: stoma). Stomata with nearby cells are known as the stomatal apparatus.

On each side of a stoma is an oval-shaped guard cell and several accessory or subsidiary cells. The expansion and shrinking of the guard cells due to the flow of water from surrounding cells result in the opening and closing of stomata. Under normal conditions, the stomata remain open during the daytime and close at night. The stomata help in the exchange of carbon dioxide and oxygen. All parts of the plant produce carbon dioxide as a result of respiration. However, the rate of exchange of gases in plants is much slower than in animals.

Respiration process can be summarized as
C6H12O6 + 6O2 ®6CO2 + 6H2O + Energy
Glucose + Oxygen ® Carbondioxide + Water + Energy