Respiration in Organisms

Every living thing needs energy to carry out its life activities — to move, grow, repair itself, and keep its body working. This energy comes from the food we eat, but the energy stored in food must first be released in a form the body can use. The process by which living cells break down food to release energy is called respiration. Respiration goes on continuously, day and night, in every living cell, which is why it is one of the most basic life processes.

The actual release of energy from food takes place inside the cells, so this part of the process is called cellular respiration. In cellular respiration, glucose (a simple sugar obtained from food) is broken down to release energy. There are two types of cellular respiration, depending on whether oxygen is used. Aerobic respiration takes place in the presence of oxygen and breaks glucose down completely into carbon dioxide and water, releasing a large amount of energy. Most organisms, including humans, mainly use aerobic respiration. The word equation is: glucose + oxygen → carbon dioxide + water + energy.

Anaerobic respiration takes place in the absence of oxygen and breaks glucose down only partially, releasing a smaller amount of energy. Different organisms produce different products. In yeast, anaerobic respiration produces alcohol and carbon dioxide (this process, called fermentation, is used in baking bread and making other products). In our own muscles during heavy exercise, when oxygen runs short, anaerobic respiration produces lactic acid, whose build-up causes the muscle cramps and aches we feel after running hard.

It is important not to confuse respiration with photosynthesis or breathing. Photosynthesis (in plants) makes food and stores energy, while respiration breaks down food and releases energy — they are opposite processes. Breathing, which we study next, is only the taking in of oxygen and giving out of carbon dioxide; it is a part of respiration but not the same thing, because the actual release of energy happens inside the cells during cellular respiration.

To respire, most animals need a steady supply of oxygen and must get rid of the carbon dioxide that respiration produces. The taking in of oxygen-rich air and giving out of carbon-dioxide-rich air is called breathing. It is important to understand that breathing is not the same as respiration: breathing is only the exchange of gases (taking in and giving out air), while respiration is the release of energy inside the cells. Breathing supplies the oxygen that respiration needs and removes the carbon dioxide it makes.

Breathing has two parts. Inhalation (breathing in) is the taking in of air containing oxygen, and exhalation (breathing out) is the giving out of air containing more carbon dioxide. One complete breath consists of one inhalation and one exhalation. The number of times a person breathes in a minute is the breathing rate. At rest, an adult human breathes about 15 to 18 times per minute; this rate increases during exercise, because the active muscles need more oxygen and produce more carbon dioxide.

The mechanism of breathing in humans depends on a dome-shaped muscle below the lungs called the diaphragm and the muscles between the ribs. During inhalation, the diaphragm contracts and flattens, and the ribs move up and outward, so the chest cavity becomes larger. This increases the space for the lungs, air pressure inside falls, and air rushes in to fill the lungs. During exhalation, the diaphragm relaxes and curves up, and the ribs move down and inward, so the chest cavity becomes smaller, pushing air out of the lungs.

The composition of the air we breathe out is different from the air we breathe in. Inhaled air contains more oxygen and little carbon dioxide, while exhaled air contains less oxygen and much more carbon dioxide, along with water vapour. This is why exhaled air turns lime water milky (a test for carbon dioxide) and why a mirror held in front of the mouth becomes misty with water vapour. These changes show that gases are exchanged during breathing to support respiration in the body.

In human beings, breathing is carried out by a set of organs that together form the respiratory system. This system takes in air, carries it to the lungs, allows oxygen and carbon dioxide to be exchanged, and carries the waste air back out. Following the path of the air through these organs helps us understand how the body gets the oxygen it needs for respiration.

Air enters the body through the nostrils into the nasal cavity, where it is filtered, warmed, and moistened. Tiny hairs and sticky mucus in the nose trap dust and germs, cleaning the air before it goes deeper — which is why it is healthier to breathe through the nose than the mouth. From the nasal cavity, the air passes into the windpipe (trachea), a tube supported by rings of cartilage that keep it from collapsing. The trachea divides into two tubes called bronchi (singular: bronchus), one going to each lung.

Inside the lungs, each bronchus branches again and again into finer and finer tubes called bronchioles, like the branches of a tree. At the end of the smallest bronchioles are millions of tiny, balloon-like air sacs called alveoli (singular: alveolus). The alveoli are the place where gas exchange actually happens: their thin walls are surrounded by fine blood vessels, so oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out. The huge number of alveoli gives the lungs an enormous surface area for this exchange.

The lungs are two spongy organs in the chest, protected by the rib cage. Below them lies the diaphragm, the muscle that helps in breathing. Once oxygen has passed into the blood at the alveoli, it is carried to all the body's cells for respiration, while the carbon dioxide collected from the cells is brought back to the alveoli and breathed out. Thus the respiratory system — from the nose through the trachea and bronchi to the alveoli in the lungs — works smoothly to supply oxygen and remove carbon dioxide for the whole body.

Humans breathe with lungs, but the animal world shows a wonderful variety of breathing organs, each suited to the animal's body and the place it lives. All animals need to take in oxygen and remove carbon dioxide, but they do so in different ways. Looking at a few examples shows how breathing organs are adapted to different environments — land, water, soil, and air.

Fish live in water and breathe using special organs called gills. Water, which has dissolved oxygen in it, enters through the fish's mouth and passes over the gills. The gills are richly supplied with blood and have a large surface area, so they can absorb the dissolved oxygen from the water and release carbon dioxide into it. This is why fish can "breathe" underwater without lungs, and why a fish taken out of water soon dies — its gills cannot take oxygen from the air.

Insects such as grasshoppers and cockroaches breathe through a network of fine air tubes called tracheae (tracheal tubes). Air enters the insect's body through tiny openings on the sides of its body called spiracles, then travels through the tracheae directly to all the tissues, supplying them with oxygen and carrying away carbon dioxide. Insects do not have lungs; this tracheal system does the job instead.

Other animals show further variety. An earthworm has no special breathing organs; it breathes through its moist skin, through which oxygen passes into its body — which is why earthworms must keep their skin moist and come to the surface when the soil is flooded. Frogs can breathe both through their moist skin (especially in water) and through their lungs (on land). Land animals like mammals, birds, and reptiles all use lungs. These examples — gills, tracheae, skin, and lungs — show that although the breathing organ differs, the purpose is always the same: to take in oxygen for respiration and give out carbon dioxide.

Plants, like all living things, also respire — they break down food to release energy, just as animals do. This often surprises people, because plants are better known for photosynthesis. But respiration and photosynthesis are two different processes: in photosynthesis a plant makes food and stores energy, while in respiration it breaks down food to release energy for its life activities. A plant needs respiration to power its growth, the uptake of minerals, and all its living processes.

An important point is that plants respire all the time, day and night, because their cells always need energy. Photosynthesis, however, happens only during the day, when there is sunlight. During the day, a plant carries out both processes at once; because photosynthesis is usually faster, the plant overall takes in more carbon dioxide and gives out more oxygen, so its respiration is "hidden". During the night, when there is no sunlight, only respiration takes place, so the plant takes in oxygen and gives out carbon dioxide, like animals do.

Plants do not have lungs or special breathing organs; instead, different parts exchange gases directly with the air. The leaves exchange gases through the tiny pores called stomata, the same pores used in photosynthesis. The stems, especially of woody plants, have small openings called lenticels for gas exchange. The roots take in oxygen from the air present in the spaces between soil particles; this is why waterlogged soil, which has no air spaces, can harm or kill plant roots, and why farmers loosen the soil to let air reach the roots.

Each plant cell carries out its own respiration, taking in oxygen and giving out carbon dioxide. Because the gases simply diffuse in and out through stomata, lenticels, and root surfaces, plants do not need a transport system for gases as animals do. Understanding that plants respire — continuously, and especially noticeably at night — completes the picture of respiration as a process common to all living things, from the smallest microbe to the largest tree.