CBSE Class 7 Science Advanced Biology Respiration in Plants and Animals Notes Set 01

Respiration in Plants and Animals

Introduction

Every living thing needs energy to stay alive, and it gets that energy through a chemical process called respiration. Oxygen is taken in and used inside the body, and carbon dioxide is given out as a waste gas. Respiration is therefore one of the most important activities of any organism.

Respiration and breathing are not the same thing. Breathing is only the physical act of drawing air in and pushing air out, which brings oxygen into the body and removes carbon dioxide. It is just one step of respiration. In the full process of respiration, oxygen is carried to the cells of the body, where it is used to break down glucose and release energy.

The overall change can be written as:

Glucose + Oxygen → Carbon dioxide + Water + Energy

The two processes differ in the following ways.

Respiration in Plants

Plants also need energy for their life activities, and they get it by oxidising glucose. Oxidation means that a substance combines with oxygen; the substance is then said to be oxidised.

A plant makes its own food by photosynthesis, which produces glucose in the presence of sunlight. In respiration, that same glucose is broken down and its energy is set free for the plant to use.

Photosynthesis and respiration in a plant: in the day carbon dioxide goes in and oxygen comes out (photosynthesis); at night oxygen goes in and carbon dioxide comes out (respiration).

Stomata

The leaf surface has tiny pores called stomata (one pore is a stoma). Each pore is bordered by two bean-shaped (kidney-shaped) cells known as guard cells, which open and close it. Stomata let gases pass between the plant and the air during photosynthesis and respiration.

Top view of a stoma between two guard cells, and a stoma shown closed and opened, with nucleus, chloroplasts, vacuole, guard cell and cell wall marked.

Working of the Guard Cells

The wall of a guard cell on the outer side is thin, while the wall on the inner side (facing the pore) is thick. In sunlight, water enters the guard cells and makes them swollen and firm (turgid). The thin outer walls bulge outwards and pull the thick inner walls apart, so the stoma opens.

When water moves out of the guard cells, they become limp (flaccid). The inner walls straighten and come together, and the stoma closes.

Pneumatophores

Some plants have special roots that help them respire. These are pneumatophores, also called breathing roots. They rise up out of the water and let air reach the plant (aeration). Trees of swampy places, such as mangroves, have them.

Pneumatophores growing vertically upwards from the root system under the water surface, with breathing pores at the tips. (The book's photograph of a mangrove tree is not reproduced.)

Transpiration

Roots take up water from the soil and send it to all parts of the plant, including the leaves. Part of this water is used in photosynthesis, and the rest leaves through the leaves into the air. Transpiration is the loss of water as water vapour from the aerial (above-ground) parts of the plant. The water vapour escapes through the stomata.

Transpiration: water is absorbed by the roots, moves upwards through the stem, and water vapour is released from the leaf.

Significance of Transpiration

1. It cools the plant, because the water vapour leaving the leaves lowers the plant's temperature.
2. It gets rid of extra water present in the cells.
3. It helps water climb from the roots to the upper parts. Water lost from the leaves leaves their tissues with less water, so water is drawn up from the roots to the leaves to replace it.

Respiration in Animals

Animals respire in the same basic way as plants: oxygen is taken in, used to break down food (carbohydrates) and release energy, and carbon dioxide is given out. Simple, small organisms exchange gases by plain diffusion. Larger and more complex animals have a whole organ system for respiration.

Types of Respiration

There are two types: aerobic and anaerobic.

1. Aerobic respiration needs oxygen and happens only when oxygen is present. It releases a large amount of energy. Human beings respire this way.
2. Anaerobic respiration takes place without oxygen and releases much less energy. Yeast and many other microbes respire this way.

In yeast, anaerobic respiration is:

Glucose → Ethanol + 2CO₂ + Energy (2 ATP)

In most microbes the glucose is changed into ethanol, but in animals it is changed into lactic acid. Animals respire anaerobically when their muscles do not get enough oxygen.

What is ATP?

When you run and play games, your body needs a ready supply of energy, and that supply is ATP. The name stands for adenosine triphosphate. Cells make ATP inside tiny structures called mitochondria, which are known as the powerhouses of the cell because they contain the enzymes and other substances needed to release energy. The cell then uses this ATP energy to do work.

Respiration in Different Organisms

Lower organisms: Yeast, most bacteria and other lower organisms respire anaerobically. Yeast exchanges gases by plain diffusion, which is the movement of molecules (here, gases) from a region where they are more concentrated to a region where they are less concentrated.

Aquatic animals: Fish respire through special organs called gills. A fish takes water in through its mouth and the water flows over the gills. The oxygen dissolved in the water is absorbed by the blood in the gills.

Respiration in a fish: water enters through the mouth, flows over the gills, and then moves out.

Land (terrestrial) animals: They have respiratory organs and passages for air, and the organs are kept inside the body because they are delicate. The surface of these organs is very thin and fine, which helps gases pass across it.

Cutaneous respiration: Here gases are exchanged through the skin and not through a special organ. For some animals this is the only way of respiring, and for others it is used along with another method. Amphibians such as the frog respire through both skin and lungs, whereas an earthworm respires through its skin alone.

Respiration in a frog: oxygen and carbon dioxide are exchanged at the lungs, and also through the skin (cutaneous respiration).

Human Respiration

The human respiratory system is made of a pair of nostrils, the pharynx, trachea, bronchi, bronchioles and alveoli. The main organs of respiration are the two lungs. (Fact: the total surface area of the lungs is about as large as a tennis court.)

1. Nostrils: Air carrying oxygen enters the body through the pair of nostrils when we inhale, and carbon dioxide leaves through them when we exhale. Fine hairs and a layer of mucus in the nose filter the air we breathe in.
2. Pharynx: Air passes from the nasal passage into the pharynx, which is a passage shared by food and air. It opens into the food pipe (digestive system) through the gullet and into the respiratory system through the glottis.
3. Trachea: From the pharynx, air goes into the trachea, a tube also called the windpipe. The trachea divides into two bronchi, the right and the left.
4. Bronchus and bronchiole: Each bronchus (plural: bronchi) goes into the lung on its own side and splits into smaller branches called bronchioles. Each bronchiole ends in tiny balloon-like sacs called alveoli.
5. Alveoli: These small balloon-like sacs at the ends of the bronchioles are where gases are exchanged between the respiratory system and the circulatory system. They are linked to the circulatory system by very thin blood vessels called capillaries, and gases pass between the alveoli and the blood.
6. Lungs: The lungs are the organs of respiration. We have two, a left lung and a right lung, and the left lung is smaller than the right. The branching network of bronchi, bronchioles and alveoli makes up the lungs. Each lung is wrapped in a covering of two thin layers called pleural membranes, with a liquid, the pleural fluid, filling the space between them. Together, the membranes and fluid protect the lungs from injury and shock.
7. Ribs: These are long curved bones around the chest that protect internal organs such as the heart and lungs.
8. Diaphragm: A muscular sheet that separates the chest cavity from the abdominal cavity. Along with the ribs, it plays a major role in breathing.

The lungs: trachea, bronchi, bronchioles and alveoli, with an enlarged view of a terminal bronchiole and its alveoli.

The human respiratory system.

The path taken by air and the gases exchanged at each stage can be shown as a flow chart.

Steps of respiration: oxygen travels from the nostrils through the pharynx, trachea, bronchus, bronchioles and alveoli to the capillaries, blood and the different parts of the body; carbon dioxide travels back the same way.

Mechanism of Respiration

Breathing has two stages, inspiration (inhalation) and expiration (exhalation).

Inspiration is the movement of air with oxygen into the lungs. It happens in these steps:

1. On inhaling, the ribs move up and the diaphragm flattens.
2. The chest cavity becomes larger.
3. The pressure inside the lungs falls, so air rushes into the lungs.
4. When the oxygen-rich air reaches the alveoli, oxygen passes into the blood and carbon dioxide passes out of it. The blood then carries the oxygen to the different parts of the body.

Expiration is the release of carbon dioxide from the lungs. Its steps are:

1. The ribs move down and inwards to their original position.
2. The diaphragm relaxes and goes back up to its dome shape.
3. The chest cavity and the lungs become smaller.
4. The pressure on the air inside the lungs rises, so the air is pushed out.

Blood supply of the alveoli: bronchioles end in clusters of alveoli, each wrapped in a network of capillaries carrying blood.

Breathing in: ribs up, diaphragm down, air in. Breathing out: ribs down, diaphragm up, air out.

Transport of Gases

Gases are carried efficiently with the help of respiratory pigments. In humans the respiratory pigment is haemoglobin, a red, iron-containing pigment found in the red blood cells (RBCs). It carries oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs.

Haemoglobin inside a red blood cell, which carries oxygen throughout the body.

Points to Remember

1. Respiration is the process that breaks down food to release energy.
2. Energy is released as ATP when oxygen combines with glucose.
3. Respiration is of two kinds: aerobic (with oxygen) and anaerobic (without oxygen).
4. Fish respire with gills.
5. In humans, respiration includes breathing, transport of gases and respiration inside the cells.
6. Breathing means taking in oxygen and giving out carbon dioxide.
7. Haemoglobin is found in the red blood cells and carries oxygen from the lungs to the rest of the body.
8. Plants respire too, to get energy.
9. Gas exchange in plants happens through the stomata of the leaves.
10. Transpiration is the loss of extra water from plants as water vapour.
11. Transpiration cools the plant.