Respiration

1. What This Chapter Covers

Food provides energy for all biological activities only after it is broken down, by the process known as respiration. Respiration is therefore the final utilization of food, and it normally takes place in an oxygen-rich environment.

The term comes from the Latin respire, meaning to breathe, and in this book it refers to the whole chain of processes from the inhalation of air to the use of oxygen in the cells. That definition is the single most important sentence in the chapter, because most errors come from treating respiration and breathing as the same thing.

The index allots the chapter 10 periods in July and runs it from page 26 to page 50. All cells of living organisms constantly use energy from food, and for that they require air, food material and certain chemicals.

2. Discovery of Gases and Respiration (Textbook 2.1)

The term respiration came into use a century after the word breathing, which goes back to the 14th century, and long before scientists knew that air is a mixture of gases. As a medical term it referred simply to the passage of air and the production of body heat.

It was not until the 18th century, when Lavoisier and Priestley did comprehensive work on the properties of gases, that anything was known about how gaseous exchange goes on in the body.

What Lavoisier established

In his early experiments Lavoisier thought that the gas liberated on heating powdered charcoal in a bell jar over water was fixed air — the name carbon dioxide went by in those days.

The next series of experiments dealt with the combustion of phosphorus in a bell jar. From these he showed that whatever in atmospheric air combined with the phosphorus was not water vapour. His own words were that the substance is "either air itself, or another elastic fluid present in a certain proportion, in the air which we breathe". This was the respirable air, the component that also helped burning.

The decisive observation was about lime water. The air we breathe out made lime water milky; the air formed after heating a metal did not; and fixed air also made lime water milky.

From this he drew the logical conclusion about respiration: either the eminently respirable air, oxygen, is changed to fixed air, carbon dioxide, in the lungs, or it is being exchanged. The amount of air reaching the lungs is completely exchanged, with almost an equal volume of fixed air given out.

The book quotes the chemist John Daper, writing a Human Physiology textbook around the mid-19th century: "The chief materials which a living being receives are matter that can be burnt, water and oxygen gas; and out of the action of these upon one another, all the physical phenomena of its life arise. What the body expels out is water, oxide of carbon, phosphorous, sulphur and others."

By the mid-19th century the role of the major compounds was known, though the events were not clearly understood, and people believed there was some relationship between the heat produced in the body and respiration. The everyday observation behind this is that the air we breathe out is warmer than the air around us.

3. The Five Steps in Respiration (Textbook 2.2)

The book is careful to say there are no strict demarcations between the steps — respiration is a complex process of several physical and biochemical processes — but for general understanding it divides it into five heads.

Reading this table in order answers the most commonly confused exam question — Mala says energy is released from glucose in the lungs, Raziya says in the muscles. Only the fifth step releases energy from glucose, and it happens in every cell, so Raziya is right.

Breathing (2.2.1)

In a setup with lime water, the water turns milky white fast when we breathe out into it, compared with a similar setup in which normal air is passed in with a syringe. Water vapour also deposits on a mirror when we breathe on it.

By respiratory system the book means the passages that transport air to the lungs and to the microscopic air sacs in them, the alveoli, where gases are exchanged between air sacs and blood vessels, and back again.

4. Pathway of Air (Textbook 2.3)

The whole passage from nostrils to alveolus is moist and warm.

The epiglottis (2.3.1)

From the pharynx there are two passages that begin with nearly the same opening and end separately, one to the lungs and one to the stomach. Air must go into one and food into the other, and food must not enter the tube to the lungs.

The epiglottis, a flap-like valve, channels this. It is partly closed when we swallow, deflecting food down to the stomach and away from the trachea, and it opens more widely when we take a breath. Nervous regulation guides its function, which is why we are advised not to talk while eating.

The book's "Do you know?" box: the interior of the lung is divided into millions of small chambers, enormously increasing the moist surface available for transfer. Spread out, all the alveoli of our lungs would cover nearly 160 m², about the area of a tennis court.

Mechanism of breathing (2.3.2)

The lungs themselves can neither draw in air nor push it out. The chest wall muscles and a flexible flattened muscle, the diaphragm, move air in and out of them.

The chest wall is made of ribs, muscles and skin, and the ribs are attached at an angle to the spine, extending downward from it. When we inhale, the chest wall moves up and expands, increasing the volume of the chest cavity.

Think of the chest cavity as a room and the diaphragm as its floor. When relaxed it is dome-shaped with the convex side pushing up into the chest cavity. When it contracts during inhalation it flattens and the dome moves downward, so the volume of the cavity increases.

With greater volume, the internal pressure decreases and air from outside rushes into the lungs. This is inspiration. The reverse then occurs: the chest wall is lowered and moves inward, the diaphragm relaxes back into its dome, pressure on the lungs increases, and their elastic tissue contracts and squeezes the air out. This is expiration.

Breathing is slow and shallow at rest and deeper and faster during hard exercise, because the pattern is coordinated moment by moment with the body's need for oxygen and removal of carbon dioxide. Scientists have found that all breathing movements stop at once when the nerves leading from the brain to the respiratory muscles are cut.

Another "Do you know?": lungs are spongy and not of the same size — the left lung is slightly smaller to make space for the heart. They are protected by two membranes called pleura, with fluid between them that prevents injury and helps the elastic lung slide as it expands.

The top row is the chapter's own flow chart of the five steps. The bottom half is the fork that opens inside step five, and knowing which product belongs to which branch is worth more marks than any other single fact in the chapter.

5. Gaseous Exchange in the Lungs (Textbook 2.3.3)

Exchange takes place within the lungs by diffusion, from the alveoli to the blood capillaries and back. The alveoli are numerous and only one cell thick, and they are surrounded by capillaries that are also only one cell thick.

Deoxygenated blood, dark red in colour, flows from the heart through these capillaries and collects oxygen from the alveoli. At the same time carbon dioxide passes out of the capillaries into the alveoli, and we get rid of it when we breathe out. The bright red, oxygen-rich blood returns to the heart and is pumped to all parts of the body.

The book gives these as approximate values. Nitrogen does not change, which tells you it takes no part in the exchange, while oxygen falls by five points and carbon dioxide rises by more than a hundredfold.

Total lung capacity is nearly 5800 ml. At rest we inhale and exhale only about 500 ml, and 1200 ml remains in the lungs even after complete exhalation.

6. Transport of Gases (Textbook 2.3.4)

When oxygen in air is within normal limits, around 21 per cent, almost all of it is carried in the blood bound to haemoglobin in the red blood cells.

Haemoglobin is a coloured pigment like chlorophyll, the major difference being that it carries an iron atom where chlorophyll carries magnesium. As oxygen diffuses into the blood it rapidly combines with haemoglobin to form oxyhaemoglobin, and the reverse happens at the tissues.

Hb + 4O₂ gives Hb(O₂)₄ in the lungs

Hb(O₂)₄ gives Hb + 4O₂ in the tissues

Carbon dioxide is usually transported as bicarbonate, while some combines with haemoglobin and the rest dissolves in the blood plasma.

At sea level nearly every haemoglobin molecule exposed to air combines with oxygen. At a height of 13 km, about 8 miles, the oxygen concentration is about one fifth of that at sea level, and only about half as many oxygen molecules combine with haemoglobin. Human life is impossible at that altitude without a supplementary supply, which is why modern aircraft have pressurized cabins.

At the tissues (2.3.5)

In the capillaries over the tissues, haemoglobin meets a very different environment. The tissues are continuously using oxygen, so its concentration there is low — perhaps only one third of that in the lungs.

Because the concentration is so low, oxyhaemoglobin releases the oxygen molecule, which enters the cells. Inside the cells, reactions produce carbon dioxide and water and release energy, and the carbon dioxide enters the blood capillaries.

7. Cellular Respiration (Textbook 2.4)

Cellular respiration is the pathway, or chain of chemical reactions, by which cells release energy by breaking the chemical bonds of glucose molecules. All living cells carry it out.

It may be aerobic, in the presence of oxygen, or anaerobic, in its absence. In animals, anaerobic respiration leads to the formation of lactic acid from glucose, and only a few ATP molecules are produced.

In prokaryotic cells such as bacteria it occurs within the cytoplasm. In eukaryotic cells both the cytoplasm and the mitochondria are sites of the reactions, and the energy produced is stored in the mitochondria as ATP. This is why mitochondria are called the power houses of the cell.

The breakdown does not happen as a single reaction but as a series of small steps, because the change in the chemical nature of the molecule from one stage to the next is slight and only a small amount of energy is released at each.

ATP

Energy from the breakdown of glucose is stored in ATP, adenosine triphosphate, called the energy currency of the cell. It is capable of supplying energy wherever it is needed within the cell, and each ATP molecule gives 7200 calories. The energy is stored in phosphate bonds, and breaking a bond releases it.

The two stages

Glucose is the most commonly used sugar for deriving energy in plants, animals and microorganisms, and in all of them it is oxidized in two stages.

In the first stage, glucose is converted into two molecules of pyruvic acid, a three-carbon compound. This step is glycolysis.

In the second stage, if oxygen is available, pyruvic acid is oxidized to CO₂, water and a large amount of energy. If oxygen is inadequate or not utilised, pyruvic acid is converted into ethanol — this is fermentation — and in some bacteria lactic acid is formed instead, with very little energy released, nearly one tenth of what adequate oxygen would give.

8. Oxygen Debt and Muscle Fatigue (Textbook 2.4.1)

When you sprint a hundred yards you do a considerable amount of muscular work, yet you do not stand on the track panting beforehand to stock up on oxygen. The fastest sprinters do not breathe at all while running a hundred yards. Afterwards, though, you pant for some minutes.

The simple explanation that the race runs on ATP already present fails, because a muscle carries only enough ATP to last about half a second of vigorous exercise. There must be another way to produce energy first and use up oxygen later.

The investigation described in the book had an athlete pedal a stationary bicycle or run on a treadmill for nine minutes of vigorous exercise while regular blood samples were taken. One compound, lactic acid, varied greatly in concentration, and the book asks you to read its graph for the starting value, the peak, and how long recovery would take if the rate of fall continued.

The interpretation is that glucose in the working muscles was changed to lactic acid. The energy stored in lactic acid is less than in glucose, and the difference is used to rebuild ATP from ADP and phosphate.

The athlete therefore builds up an oxygen debt during the sprint, which must be repaid later; that is what the panting afterwards is for. In a longer race athletes breathe throughout, so some lactic acid is removed while they run and they can go on longer before becoming exhausted.

The presence of lactic acid in the blood is the main cause of muscle fatigue, and the tiredness disappears if the body gets enough rest. The book notes that the same effect scales with effort: walking, brisk walking, slow jogging and running over the same distance give increasing levels of pain.

9. Anaerobic Respiration and Fermentation (Textbook 2.5, 2.6)

Lab Activity — yeast without oxygen

The question is whether a rise in temperature and the production of carbon dioxide can be detected when living organisms are kept away from oxygen. Yeast grows rapidly if supplied with glucose in solution; wild yeasts grow naturally on the skins of grapes and apples.

Remove dissolved oxygen from the glucose solution by heating it for a minute and then cooling it without shaking. Add some yeast, and cut off the air supply by pouring a one centimetre layer of liquid paraffin on top.

To check that the oxygen has gone, add a few drops of diazine green (Janus Green B) to the yeast suspension before pouring the paraffin. The blue dye turns pink when oxygen is in short supply.

Attach another bottle containing bicarbonate indicator solution or lime water. The carbon dioxide released during anaerobic respiration passes through the tube and turns the lime water milky, and a thermometer shows the temperature change at the same time. On a smaller scale in test tubes, warm them to about 37 °C to speed up the test.

Fermentation (2.6)

If yeast and sugar solution are left to stand without oxygen for some days, they develop a characteristic smell caused by a new compound, ethanol, manufactured by the yeast from the sugar. The book points to the same smell in idly and dosa dough left out of the refrigerator.

The ethanol can be separated from the yeast-glucose mixture by fractional distillation, because ethanol boils at a lower temperature, 70 °C, than the sugar solution. Like aerobic respiration, this is a process of producing energy — here with no supply of oxygen.

10. Respiration versus Combustion (Textbook 2.7)

Lavoisier, in a compilation of 1783, wrote that "respiration is a combustion process. It is a very slow process and here oxygen is not only combines with carbon but also with hydrogen." Robinson also stated that respiration is a type of combustion and that combustion is the source of heat in animals.

Activity 2 tests this directly. Heat glucose in a test tube over a flame with a delivery tube into lime water. The glucose melts, and on further heating it burns, producing carbon dioxide, water and heat.

So combustion of glucose gives the same products as the respiratory equation. But the book gives three reasons why the processes must differ:

1. Glucose must be burnt at high temperature in the laboratory to liberate energy. If that happened in our cells, all the cells would be burnt.
2. Once glucose starts burning we cannot easily stop the process, but living cells exercise control over this kind of burning.
3. Water normally stops combustion, yet cells contain a lot of water and respiration still goes on.

Heat production (2.7.1)

Living animals and plants usually produce energy in the form of heat. A sweater keeps us warm because it prevents the loss of heat energy produced by the body — which tells you that heat is constantly lost from the body surface and must be continuously generated to keep the body temperature constant.

The rate is not always the same. In vigorous activity a greater amount of heat is generated, which is why we feel hot after running. During cellular respiration some energy is stored as ATP, some is used in day-to-day activities, and the excess is released as heat.

If oxygen is insufficient during vigorous exercise, muscles start anaerobic respiration, lactic acid forms and the muscles hurt. We return to normal after rest, and deep breathing helps restore energy.

11. Respiratory Organs Across Animals (Textbook 2.8)

Exchange of gases is common to all living organisms but is not carried out the same way in all of them.

Single-celled Amoeba and multicellular Hydra, Planaria, roundworms and earthworms obtain oxygen and expel carbon dioxide directly through the body surface by diffusion.

In other multicellular animals special organs have evolved. The factors the book names are body size, availability of water, and the type of circulatory system, and the habitat matters most.

Insects such as cockroach and grasshopper have a tracheal system: a series of tubes called trachea divided into fine branches, the tracheoles, which carry air directly to the cells in the tissues.

Fishes have gills or branchiae, supplied with thin-walled capillaries — branchial respiration. The fish opens its mouth and lowers the floor of the oral cavity, drawing in water; then it closes the mouth and raises the floor, pushing water into the pharynx and through the internal branchial apertures to the gill pouches, where the gill lamellae absorb dissolved oxygen.

Respiration through the skin is cutaneous respiration. The frog, an amphibian, respires cutaneously, through lungs (pulmonary) and through the bucco-pharyngeal cavity. Reptiles, birds and mammals respire through lungs.

12. Respiration in Plants (Textbook 2.9)

Gaseous exchange in plants happens at the stomata in leaves, and also at the surface of aerial roots and the lenticels on stems. Mangrove plants have specialized breathing roots. Plants absorb oxygen to produce energy and release carbon dioxide, exactly as animals do.

How the gases move (2.9.1)

The stomatal openings lead to a series of spaces between the cells that form a continuous network through the plant — very large in the leaves, much smaller elsewhere.

These air spaces are lined with water. Oxygen dissolves in that film and passes through the porous cell walls into the cytoplasm, where glucose is broken into carbon dioxide and water with the liberation of energy, stored in the mitochondria as ATP. Carbon dioxide passes back out the same way.

The whole system works by diffusion. As cells use up oxygen, a gradient develops between the cells and the air in the spaces, and between those spaces and the air outside the stomata and lenticels, so air passes in. As carbon dioxide is released, a gradient forms in the reverse direction and it passes out.

Respiration through roots (2.9.2)

Plants aerate their roots by taking in oxygen through the surface of the root hairs, whose walls are very thin, drawing it from the air spaces between soil particles.

Plants in very wet places such as ponds or marshy soils cannot get oxygen that way. They are adapted by having much larger air spaces connecting the stems with the aerial roots, and the most usual adaptation is a hollow stem. The book suggests cutting the stems of pond plants and counting how many are hollow compared with the same number of species from normal soil.

The problem is harder for trees, and few survive with roots permanently in water. The exception is the mangrove tree of the tropics, which has aerial roots above the soil surface and takes in oxygen through them.

Activity 3 — carbon dioxide from germinating seeds

Soak a handful of moong or Bengal gram seeds a day ahead, keep them in a tied cloth pouch in a corner of the classroom for two days, then put the sprouts into a glass or plastic bottle of about 200 ml. Stand a small beaker three-quarters full of lime water inside the bottle and close it tightly.

Make a similar set with unsprouted seeds and leave both undisturbed for a day or two, watching the colour of the lime water. The set in which it changes faster tells you which seeds are respiring.

Activity 4 — heat from germinating seeds

Put the sprouts in a thermos flask, make a cork of thermocol or rubber with a hole bored for a thermometer, and set it so the bulb dips into the sprouts. Close the flask tightly and record the temperature every two hours for at least 24 hours, then graph the readings.

The questions to answer are whether the temperature rises, whether it rises steadily or abruptly at some time of day, and where the heat comes from.

13. Photosynthesis versus Respiration (Textbook 2.10)

Photosynthesis is a process of synthesis, an anabolic process, occurring in the chloroplasts:

6CO₂ + 12H₂O --(light, chlorophyll)--> C₆H₁₂O₆ + 6H₂O + 6O₂

Respiration is the breakdown of complex food molecules, a catabolic process, producing chemical or potential energy:

C₆H₁₂O₆ + 6O₂ --> 6CO₂ + 6H₂O + Energy

The two appear to be opposing reactions, but they have very different biochemical pathways and both are essential for a plant's metabolism.

Photosynthesis takes place in the chloroplast to produce sugars, starch and other carbohydrates for the plant's needs; cellular respiration occurs in the mitochondria, where those carbohydrates are oxidised to produce chemical energy for work at the cellular level.

During the day the rate of photosynthesis is usually higher than that of respiration; at night it is the reverse in most plants. Temperature, humidity and light intensity all affect the ratio. This is why the statement "plants photosynthesize by day and respire by night" is wrong: plants respire all the time.

Pranayama — the book's annexure

The annexure notes that only human beings can control the process of breathing. At each breath we move only 500 ml of a lung capacity of about 5800 ml, so most breathing happens in the upper lobes and the lungs are not used to their fullest; about 4600 ml is usable.

Patanjali proposed Astanga yoga, 195 yogic principles in eight divisions: Yama, social discipline; Niyama, individual discipline; Asana, body posture; Pranayama, expansion of vital energy; Prathyahara, withdrawal of senses; Dharana, concentration; Dhyana, meditation; and Samadhi, self-realisation.

In Pranayama — prana meaning gas, ayama meaning journey — air is allowed to enter three lobes of the lungs, increasing the oxygen diffusing into blood. Deep breaths reduce the rate from 20-22 breaths per minute to about 15, making more oxygen available to the brain and tissues.

Key words from the chapter

Aerobic respiration, anaerobic respiration, alveoli, pharynx, trachea, bronchi, bronchioles, epiglottis, anabolic process, catabolic process, aerial roots, lenticels, fermentation, energy currency.

14. Summary

Respiration is the whole chain from the inhalation of air to the use of oxygen in the cells, and the book divides it into five steps: breathing, gaseous exchange in the lungs, gas transport by blood, gaseous exchange at the tissue level, and cellular respiration.

Lavoisier found that the air we breathe out precipitates lime water, and concluded that respirable air is changed to fixed air in the lungs.

Air passes from nostrils to nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli and the blood, and returns by the same route. The epiglottis keeps food out of that route; the diaphragm and chest wall drive the air along it.

During inhalation the diaphragm contracts and flattens, the chest cavity's volume increases, its internal pressure falls and air rushes in. During exhalation the chest wall lowers, the diaphragm resumes its dome, pressure rises and elastic lung tissue squeezes the air out.

Aerobic respiration occurs with an adequate supply of air, producing a lot of energy along with carbon dioxide and water. Anaerobic respiration and fermentation occur when oxygen is absent or inadequate, producing much less.

Cells break down the three-carbon compound pyruvate either aerobically or anaerobically depending on the oxygen available, and in multicellular organisms cells usually cannot carry on anaerobically for long.

Respiration is not combustion: laboratory burning needs high temperature, cannot easily be stopped once started, and is stopped by water — none of which is true of the controlled, watery, stepwise oxidation inside a cell.

Photosynthesis is anabolic and happens in the chloroplast; respiration is catabolic and happens mainly in the mitochondria. They look like opposite reactions but run on entirely different biochemical pathways, and both are essential to the plant.