SENIOR THREE

There are two types of respiration: (1) Aerobic respiration and (2) Anaerobic respiration.

1.2 Aerobic Respiration

This is the breakdown of food to release energy in the presence of oxygen. It produces energy, carbon dioxide and water, and is the most efficient process of energy release because food is completely broken down.

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (Glucose) (oxygen) (carbon dioxide) (water)

The carbon dioxide produced diffuses from the tissues into the blood and is transported to the lungs for expiration through the trachea and nostrils. In plants, the carbon dioxide produced is either lost to the atmosphere through the stomata (leaves) or lenticels (stems), or used in photosynthesis.

Experiment: To show that living organisms use oxygen in aerobic respiration

Materials:

• Conical flask

• Delivery tube

• Beaker

• Sodium hydroxide solution

• Water

• Germinating seeds

Procedure:

• Place germinating seeds in a conical flask together with a test tube containing sodium hydroxide.

• Connect a delivery tube to the flask with the other end dipped in a beaker of water.

• Leave the set-up to stand and observe the water level in the delivery tube.

Observation: After some time, water rises in the delivery tube.

Explanation: As the seeds respire they use oxygen and produce CO2, but the CO2 is absorbed by the sodium hydroxide, so it is not returned to the flask — hence a decrease in air volume, seen as rising water.

Conclusion: Oxygen is used up in aerobic respiration.

Experiment: To show that living organisms liberate CO2 during aerobic respiration

Materials:

• Soda lime (sodium hydroxide)

• Lime water

• Filter pump

• A toad (or small aerobe)

• Two delivery tubes

• Three flasks and corks

Procedure:

• A toad/rat is used as the aerobe and the apparatus is set up with three flasks in series, left to stand for 40 minutes.

• Sodium hydroxide absorbs CO2 from the incoming air.

• Limewater in flask A confirms the absence of CO2 in incoming air.

• Limewater in flask B/C tests for CO2 in exhaled air.

• A filter pump ensures air flows in one direction.

Observation: Limewater in the flask nearer the organism turns milky, while that in flask A (before the organism) remains clear.

Conclusion: Living organisms give out carbon dioxide during respiration.

1.3 Anaerobic Respiration

This is the breakdown of food to release energy in the absence of oxygen. The food is not completely broken down; part of it remains as ethanol in plants or lactic acid in animals. Less energy is released than in aerobic respiration because most of it remains locked in the intermediate substances.

Equation for anaerobic respiration in animals

C6H12O6 → 2CH3CH(OH)COOH + CO2 + 150 kJ (energy) (Glucose) (lactic acid)

Equation for anaerobic respiration in plants / yeast (fermentation)

C6H12O6 --enzyme (zymase)--> C2H5OH + CO2 + Energy (118 kJ) (Glucose) (ethanol)

Fermentation is anaerobic respiration carried out in solution, as seen in yeast. It is commercially exploited in beer brewing (to produce alcohol) and in bread baking (to raise dough).

Oxygen debt

During vigorous activity, oxygen supply to the muscles may not meet energy demand, so anaerobic respiration occurs and lactic acid accumulates. Breathing rate stays high even after exercise to supply the extra oxygen needed to oxidise this lactic acid to CO2, water and energy. The amount of oxygen needed to break down the accumulated lactic acid is called the oxygen debt.

Lactic acid rises rapidly during exercise due to increased anaerobic respiration from insufficient oxygen. At the end of exercise, lactic acid drops suddenly as it is oxidised in the liver; the extra oxygen needed is obtained by deep breathing.

Experiment: To show that CO2 is given off during anaerobic respiration (fermentation)

Materials:

• Two test tubes

• Delivery tubes

• Yeast

• Glucose

• Oil

• Lime water

Procedure:

• Boil about 20 cm³ of glucose solution to drive off dissolved oxygen; cool to room temperature.

• Add a layer of oil to prevent oxygen from dissolving back in.

• Add a small quantity of yeast suspension using a pipette.

• Pour limewater into a second test tube and connect the two with a delivery tube.

• Leave to stand in a warm place for an hour (set up a control using boiled yeast or without yeast/glucose).

Observation: Bubbles of gas are seen in the limewater, and the limewater turns milky.

Explanation: Yeast breaks down glucose in the absence of oxygen to produce ethanol, CO2 and some heat; the CO2 reacts with the limewater to form insoluble calcium carbonate, turning it milky.

Conclusion: Carbon dioxide is produced during anaerobic respiration.

Experiment: To show the liberation of heat during fermentation (anaerobic respiration)

Materials:

• 10% glucose solution

• 10% yeast suspension

• Two vacuum/thermos flasks

• Two thermometers

• Cooking oil

• Water bath

• Cotton wool

Procedure:

• Boil 100 cc of glucose solution to drive out dissolved oxygen, then cool.

• Pour 50 cc of the glucose solution into each flask; add oil to prevent oxygen entry.

• Add yeast suspension below the oil layer in one flask only.

• Place a thermometer in each flask and record readings hourly.

Observation: The temperature in the flask with yeast rises steadily; the temperature in the flask without yeast remains the same.

Conclusion: Anaerobic respiration of glucose by yeast produces heat.

Experiment: To show that germinating seeds release heat during respiration

Materials:

• Vacuum flask

• Germinating seeds

• Cotton wool

• Thermometer

• Sodium hypochlorite/formalin solution

Procedure:

• Soak seeds in water for 24 hours; boil (kill) one group.

• Soak both sets in formalin for 15 minutes to kill bacterial/fungal spores.

• Place unboiled germinating seeds in one flask and boiled seeds in another, each plugged with cotton wool and fitted with a thermometer.

• Fix both flasks upside down on a retort stand with the seeds near the thermometer bulb.

Observation: After three days, the temperature of the germinating (unboiled) seeds is higher than that of the boiled seeds, which remains constant.

Explanation: During germination, oxygen is absorbed for respiration, which releases energy in the form of heat.

Conclusion: Germinating seeds give out heat.

1.4 Comparing Aerobic and Anaerobic Respiration

Similarities

• Both require glucose as a raw material.

• Both produce energy.

• Both produce carbon dioxide.

• Both take place in living cells.

Differences

Aerobic Respiration

Anaerobic Respiration

A common mode of respiration in both plants and animals

A rare process, limited to a few plants and animals

Produces more carbon dioxide

Produces less carbon dioxide

Occurs throughout life

Occurs temporarily, in very active muscles

Liberates large quantities of energy

Liberates less energy

Products are water, carbon dioxide and energy

Products are carbon dioxide, energy and alcohol or lactic acid

Complete breakdown of food

Incomplete breakdown of food

Oxygen is used

Oxygen is not used

Respiratory Quotient (RQ)

The respiratory quotient is the ratio of the volume of CO2 produced to the volume of O2 used:

RQ = CO2 produced ÷ O2 used

Importance of respiration

• Respiration produces energy used to run the various activities of the body.

• It is exploited commercially in baking, brewing and making dairy products such as cheese, yoghurt and butter.

1.5 Respiration and Photosynthesis Compared

Similarities

• Both take place in living cells.

• Both involve enzymes.

• Both involve oxygen, carbon dioxide and glucose.

• Both involve energy.

Differences

Respiration

Photosynthesis

Oxygen is absorbed

Oxygen is released

Carbon dioxide is released

Carbon dioxide is absorbed

Takes place in light and darkness

Needs light to take place

Energy is released

Energy is absorbed

Does not require chlorophyll

Requires chlorophyll

Takes place in plants and animals

Takes place in plants only

1.6 Gaseous Exchange

Gaseous exchange is the exchange of respiratory gases between an organism and its environment. It takes place across specialised surfaces called respiratory surfaces, allowing an organism to get rid of CO2 produced during respiration while obtaining the oxygen needed for aerobic respiration.

Note: Breathing is an active process involving movement of air in and out of the body, whereas gaseous exchange is a passive process involving passage of gases across respiratory surfaces.

Characteristics of a good respiratory surface

• Large surface area to volume ratio to enable rapid diffusion of gases (achieved by folding/branching, e.g. alveoli, gill filaments, tracheoles).

• Moist, to allow easy diffusion of gases.

• Thin-walled, to reduce the diffusion distance.

• A good network of blood capillaries for transporting gases to and from respiring tissues.

• Well ventilated, to maintain a high concentration gradient favouring diffusion.

Note: The respiratory surfaces of insects are not supplied with blood capillaries because insect blood does not transport gases — gases are transported through the tracheole tubes instead.

Gaseous exchange in plants

Plants have no special respiratory surface. They use simple pores — stomata on leaves and lenticels on stems — for gaseous exchange, aided by abundant intercellular spaces that speed up diffusion.

Plants do not need special respiratory surfaces or a blood transport system because:

• They utilise the CO2 produced by their cells in photosynthesis, preventing its accumulation.

• They produce oxygen as a by-product of photosynthesis, which is then used in respiration.

• They have numerous stomata and lenticels that favour fast gaseous exchange.

• They have large intercellular spaces that favour fast circulation of gases without blood.

• They have a low demand for oxygen because of their low metabolic rate (they are immobile).

Gaseous exchange in simple organisms

Small organisms such as amoeba, paramecium, hydra and jellyfish have a large surface area to volume ratio, so gaseous exchange takes place over the whole body surface — diffusion alone is sufficient because of their small body volume.

Larger organisms (insects, vertebrates) have a small surface area to volume ratio, so gaseous exchange occurs at a specialised respiratory surface which is part of a respiratory organ.

Surface area to volume ratio and gaseous exchange

Surface area to volume ratio (SA:V) is obtained by dividing the total surface area of an object by its volume. Consider two boxes:

Box A (2 cm × 1 cm × 2 cm): Surface area = 2(2×1) + 2(1×2) + 2(2×2) = 4+4+8 = 16 cm²; Volume = 2×1×2 = 4 cm³; SA:V = 16 ÷ 4 = 4

Box B (3 cm × 2 cm × 4 cm): Surface area = 2(3×2) + 2(3×4) + 2(2×4) = 12+24+16 = 52 cm²; Volume = 4×2×3 = 24 cm³; SA:V = 52 ÷ 24 = 2.3

Since box A (smaller) has a larger SA:V ratio than box B (larger), we conclude that smaller organisms have a larger surface area to volume ratio than larger organisms. Key implications:

• Smaller organisms have a larger SA:V ratio, facilitating a faster rate of diffusion to supply all body tissues with respiratory gases.

• Smaller organisms have a short diffusion distance — many are single-celled or have only one layer of cells.

• Larger organisms have a smaller SA:V ratio and a greater diffusion distance (many cell layers), so diffusion alone cannot meet their respiratory demands.

• Mammals have therefore developed a blood circulatory system with highly branched vessels to transport gases to all cells.

• Insects have developed a tracheal system with finely divided tracheoles that carry gases directly to and from all cells.

Respiratory organs and surfaces of common organisms

Animal

Respiratory organ

Respiratory surface

Amphibians

Lungs

Alveolus

Amphibians

Skin

Skin surface

Amphibians

Buccal cavity

Buccal cavity epithelium

Birds

Lungs

Alveolus

Fish

Gills

Gill filaments

Insects

Tracheal system

Tracheoles

Mammals

Lungs

Alveolus

Tadpoles

Gills

Gill filaments

Note: The movement of gases (and water) to and from a respiratory surface is called ventilation (breathing).

1.7 Gaseous Exchange in Insects

The respiratory organs of insects consist of a network of tubes — tracheal tubes — making up the tracheal system, which reaches all body tissues like capillaries do in mammals.

Ventilation mechanism

Inhalation: The abdominal wall expands, reducing internal pressure and increasing volume. This forces air containing oxygen into the insect through the spiracles, to the trachea and then the tracheoles. Between the tracheoles and muscles, gaseous exchange occurs: oxygen enters the tissues while CO2 diffuses into the fluid in the tracheoles.

Exhalation: The abdominal wall contracts, decreasing internal volume and increasing pressure, forcing air with a high CO2 concentration out through the spiracles.

1.8 Gaseous Exchange in Fish

Fish use water as the medium for gaseous exchange; their respiratory surface is the internal gill. Fish absorb dissolved oxygen from water using gills; in bony fish, a pair of gills on each side is covered by a gill plate (operculum).

Parts of the gill

Gill bar: Provides attachment and support to the gill filaments.

Gill raker: Hard projections from the gill bar that trap suspended food and protect the gill filaments by filtering out sand particles.

Gill filaments: The sites of gaseous exchange — finger-like projections that increase surface area, with a network of capillaries whose blood flows in the opposite direction to the water (counter-current flow) to maintain a high concentration gradient. They have a thin membrane, are well ventilated and numerous.

Mechanism of ventilation in bony fish

Inward movement of water: The fish closes the operculum and opens the mouth, then lowers the mouth-cavity floor, increasing volume and lowering pressure below that of the surrounding water. Water flows in through the mouth. The mouth then closes and the buccal cavity floor rises, decreasing volume and increasing pressure (the gullet stays closed), forcing water into the gill chamber. As water passes over the gill filaments, oxygen diffuses into the blood while CO2 diffuses into the water.

Outward movement of water: The operculum muscles relax and water flows out, while the buccal floor stays raised and the mouth stays closed; the buccal floor then lowers to repeat the cycle.

1.9 Gaseous Exchange in Amphibians

Tadpoles

Tadpoles first use external gills and later internal gills as the surface for gaseous exchange. Water taken in through the mouth passes over the gills and out through the gill slit; oxygen diffuses from the water into the blood while CO2 diffuses from the blood into the water.

Adult amphibians

Adults exchange gases through the skin, the lining of the mouth cavity (buccal cavity), and the lungs. They depend mostly on the skin and buccal cavity while in water; lungs are used mainly on land.

1. The skin: Thin-walled, moist, with a good network of blood capillaries. Used when oxygen demand is low. On land, atmospheric oxygen dissolves in the moisture layer and diffuses into the blood while CO2 diffuses out; in water, dissolved oxygen diffuses in across the skin while CO2 diffuses out.

2. The buccal cavity: Thin, moist lining with a good capillary network. During inhalation, the mouth floor lowers (closed mouth), increasing volume and reducing pressure, drawing air in through the nostrils; oxygen diffuses in and CO2 diffuses out. During exhalation, the floor muscles relax, raising the floor, reducing volume and increasing pressure, pushing air out through the nostrils.

3. The lungs: Sac-like, with a large surface area, a good capillary network, and thin walls. During inspiration, air is drawn into the buccal cavity through the nostrils, then — as the buccal floor rises and abdominal muscles contract — forced through the glottis and trachea into the lungs, where oxygen diffuses in and CO2 diffuses out. During expiration, the abdominal muscles relax, the lungs contract, and waste air is pushed back through the buccal cavity and out through the nostrils.

1.10 Gaseous Exchange in Birds

Because of their high metabolic rate, birds need an efficient gaseous exchange mechanism. Their respiratory system consists of lungs and air sacs. During inhalation, air passes through the trachea and bronchus to the posterior air sac and the lungs; on exhalation it moves to the anterior air sac and out through the trachea, giving birds a continuous, one-way flow of fresh air through the lungs.

1.11 Gaseous Exchange in Mammals (e.g. Man)

The respiratory organs in man are the lungs, and the respiratory surfaces are sac-like structures called alveoli.

The respiratory tract (air passage)

Air enters through the nostrils into the nasal cavity, where it is warmed to body temperature, then passes to the back of the mouth, the pharynx, the larynx and the trachea, and finally through the bronchus, bronchioles, to the alveolus.

The nasal cavity membrane is lined with cilia and goblet cells (which produce mucus). Dust and germs are trapped in the mucus and swept by ciliary action to the back of the mouth, where they are swallowed — so by the time air reaches the lungs it is dust- and germ-free, warm and moist.

The trachea

A tube running from the pharynx to the lungs, kept open by rings of cartilage that prevent it from collapsing. Cilia and goblet cells continue into the trachea to sweep dust and germs back to the mouth. At its lower end the trachea divides into bronchi, which divide repeatedly into bronchioles, which divide into alveolar ducts ending in alveoli.

There are about 300 million alveoli in a human lung, greatly increasing the surface area for gaseous exchange. The lungs lie in the thoracic cavity, enclosed by the thorax wall and the diaphragm.

The alveoli

An alveolus is a moist, thin-walled, sac-like structure covered on the outside by a network of blood capillaries. Oxygen diffuses from the alveolus into the blood in the capillaries and is carried around the body; CO2 diffuses from the blood into the alveolus and is expired through the alveolar duct, bronchiole, bronchus, trachea and nostrils.

The mammalian lung

The lungs are two elastic, spongy structures in the thoracic cavity, protected by the rib cage. Intercostal muscles (between the ribs) move the rib cage; below the lungs lies a muscular sheet, the diaphragm.

1.12 Breathing Mechanism in Mammals (Lung Ventilation)

Inspiration: The external intercostal muscles contract while the internal ones relax, moving the rib cage outwards and upwards; the diaphragm contracts and flattens. This increases thoracic cavity volume and reduces its pressure below atmospheric pressure, so air enters through the nostril, trachea, bronchi and bronchioles until it reaches the alveoli.

Expiration: The internal intercostal muscles contract while the external ones relax, moving the rib cage downwards and inwards; the diaphragm becomes dome-shaped. This reduces thoracic cavity volume and raises pressure above atmospheric pressure, forcing the lungs to release CO2 through the bronchi, trachea and nostrils.

Gaseous exchange in the alveolus

This occurs across the walls of alveoli and blood capillaries by diffusion. Inspired air, rich in oxygen and low in CO2, dissolves in the moisture of the alveolar epithelium and diffuses across it and the capillary walls into red blood cells, where it combines with haemoglobin to form oxyhaemoglobin. At the same time, CO2 carried as bicarbonate ion in the blood diffuses through the capillary walls into the alveoli and leaves in expired air.

Volume of gas carried by 100 cc of blood

Gas

Entering lungs

Leaving lungs

Nitrogen

Oxygen

Carbon dioxide

Blood flowing towards the lungs carries more CO2 and less oxygen; as it leaves the lungs, oxygen has been added and some CO2 removed — evidence of gaseous exchange in the lungs.

Approximate air composition during breathing

Component

Inhaled

Exhaled

Nitrogen

Oxygen

Carbon dioxide

0.03%

Water vapour

Less saturated (variable)

Saturated

Temperature

Atmospheric temperature

Body temperature

Nitrogen composition stays the same because it plays no part in body chemical reactions. Inhaled air has more oxygen than exhaled air because oxygen is used up in respiration, which produces CO2 — so exhaled air has more CO2. Note that gaseous exchange in the alveoli does not remove all the CO2 and O2 from the air.

Experiment: To demonstrate breathing in mammals (model lung apparatus)

Materials:

• Glass tubing

• Cork

• Rubber tubing

• Y-tube

• Bell jar

• Two balloons

• Rubber sheet

• Thread

Procedure:

• Fix a cork with glass tubing into the mouth of a bell jar.

• Connect a Y-tube to the glass tubing using rubber tubing.

• Tie a balloon on each end of the Y-tube to act as lungs.

• Tie a rubber sheet over the open end of the bell jar (using a rubber band) to act as the diaphragm, and attach a thread to its centre.

• Pull the thread to represent inhalation; release it to represent exhalation.

Observation: Pulling the thread stretches the rubber sheet, increasing the volume in the bell jar and reducing pressure — air enters and inflates the balloons. Releasing the thread returns the rubber sheet to its flat shape, reducing volume and increasing pressure, forcing air out and deflating the balloons.

Explanation: The bell jar represents the thoracic cavity (walls = rib cage), the glass tubing represents the trachea, and the ends of the Y-tube represent the bronchi.

Conclusion: Pulling the thread represents inspiration; releasing it represents expiration.

Important terms related to breathing

Lung capacity: The total volume of the lungs when fully inflated — about 5 litres in an adult man.

Tidal volume: The (small) volume of air breathed in and out at rest.

Vital lung capacity: The larger volume of air taken in when the body is very active.

Residual volume: The air that remains in the lungs even at maximum expiration, preventing the lungs from collapsing.

Experiment: To show that expired air contains carbon dioxide

Materials:

• Two test tubes

• Two corks

• T-tube

• Two right-angled capillary tubes

• Lime water

Procedure:

• Place the T-tube in the mouth and breathe in and out normally.

• Inhalation air is drawn from the atmosphere through a capillary tube and limewater in tube A.

• Exhaled air passes through limewater and a capillary tube at the B end.

Observation: Limewater in tube B turns milky, while that in tube A remains clear.

Explanation: Only carbon dioxide changes colourless limewater to milky; since tube B received expired air, this proves expired air contains CO2.

Conclusion: Expired air contains carbon dioxide.

Experiment: To measure the volume of expired air (depth of breathing)

Materials:

• Trough

• Calibrated bell jar

• Cork

• Glass tube

• Rubber tube

• Water

Procedure:

• Completely fill a bell jar (calibrated in litres) with water and place it in a trough.

• Connect a rubber tube from inside the bell jar to a glass tube.

• The demonstrator breathes out once into the bell jar via the glass tube, at rest and again immediately after exercise.

Observation: Water is displaced from the bell jar each time; the volume displaced after exercise is greater than at rest. The volume of water displaced equals the volume of air expired.

Conclusion: The volume of expired air is greater immediately after exercise than at rest — exercise increases the depth of breathing.

End of chapter one. 2 more chapters and about 9,381 words in the full book.