Class 7 Science Respiration in Plants and Animals Chapter Notes

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Respiration in Plants & Animals
A living organisms 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 the waste products - carbondioxide and water.

Respiration process can be summarized as
C6H12O6 + 6O2 ®6CO2 + 6H2O + Energy
Glucose + Oxygen ® Carbondioxide + 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 of carbondixoide. The exchange of oxygen and carbondioxide is brought about by 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 organisms.

CELLULAR RESPIRATION
Oxygen present in the blood combines with the food present in the form of glucose in the cells, to release energy. This energy releases during respiration takes place only with the help of several chemical reactions, generally catalysed by enzymes. This energy is utilized 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
Primarily takes place in the bag like structure 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.

Krebscycle
Also known as 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 TCA cycle in mitochondria. That is why mitochondria are called power house of a cells.

Table 1: Differences between cellular respiration and breathing

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 carbondixoide and water and energy is released which is stored as ATP molecules.
Glucose + Oxygen ®Carbondioxide + Water + Energy (38 ATP)

Anaerobic Respiration: This type of respiration takes place in the absence of oxygen. It is 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 partially into carbondioxide and ethyl alcohol. The energy released during anaerobic respiration is much less than in aerobic respiration.
Glucose ® Ethylalcohol + Carbondioxide + 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.

Human beings have a more extensive and complex respiratory system. This is because our metabolic rate isnhigh and so is the energy demand. Since our body cannot store oxygen so we need to breathe day and night to move air into and out of our system. The respiratory system of human being includes
1. Respiratory Tract    2. Respiratory Organ 

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 windpipe while swallowing or drinking thus preventing the food from entering the lungs.
Trachea: The pharynx leads to trachea, a tube lying in front of the food pipe. It passes through the neck and extends into the chest cavity. Its walls is supported by C-shaped cartilaginous ring.
Bronchi & Bronchioles: At the lower end trachea splits into two bronchi that leads 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 air, we breathe in goes into the blood and CO2 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-ordinate action of diaphragm and ribcage help in the mechanism of drawing oxygen (inhalation) and expelling the CO2 (exhalation).

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, intercostal muscles contract and push the ribs upwards and outwards, making the cone - shaped diaphragm flat. This causes an increase in the air pressure inside the alveoli. The atmospheric air, which is at higher pressure, rushes in through the respiratory tract to equalize the pressure. Finally, the exchange of air takes place in the alveoli surrounded by fine blood capillary. Oxygen is taken up by haemoglobin of 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 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 the haemoglobin (the red coloured pigment of Red blood cells) to form oxyhaemoglobin (Fig, 4).

The oxygenated blood goes from 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, the oxygen they need to breakdown food (glucose) and release energy.
Carbondioxide and water are produced as waste products during cellular respiration, which need to be eliminated from the body. This carbondioxide is now absorbed by the blood in the blood capillaries which combine to form veins. This carbondioxide is exchanged with oxygen in the alveoli. The carbondioxide 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 breaks down to be obsorbed through the small tiny blood vessels called capillaries just under the skins 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 of carbondioxide back out the same capillaries through the skin and away from the worm.

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