Respiration

Form 4 Biology Chapter 7: Respiration

The Respiratory Process in Energy Production

1. Respiration is an important living process carried out by all living

organisms.

2. Respiration can be divided into two stages:

a) external respiration

b) internal respiration

3. External respiration is a mechanical process that maintains a

continuous exchange of gases between the respiratory surface of an

organism and its environment.

4. For most organisms, the exchange of gases occurs through a specialized

structure called respiratory structure.

5. Internal respiration is the biochemical process in which energy is

made available to all living cells. This process involves the oxidation

of organic molecules to release the chemical energy stored within

these molecules.

6. The energy that is released during this process is used to synthesis

energy-carrying molecules called adenosine triphosphate.

7. The main substrate for cellular respiration is glucose.

8. There are two types of cellular respiration:

a) aerobic respiration

b) anaerobic respiration

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Energy Production in Aerobic Respiration

1. Aerobic respiration requires a continuous supply of oxygen from the air

or water surrounding the organism.

2. In the cells, glucose molecules are oxidized by oxygen to release energy.

3. Aerobic respiration involves the oxidation of glucose in the presence

of oxygen to carbon dioxide, water and energy.

4. Organisms that respire aerobically are called aerobic organisms.

5. Aerobic respiration releases all the available energy stored within the

glucose molecules.

6. The entire process does not involve only a single chemical reaction, but

is also driven by a sequence of complex biochemical reactions which are

catalyzed by respiratory enzymes

7. The energy that is stored within the glucose molecules is release

gradually. This is far more useful to the organism than as sudden release

of all the energy.

8. Only a small portion of the energy is lost in maintain the body

temperature. A larger portion of the energy is used to synthesize

adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and

inorganic phosphate.

How anaerobic respiration occurs in human muscle

1. During vigorous exercise such as running a race, the muscles initially

respire aerobically.

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2. However, the muscle soon uses up all the available oxygen. In spite of

the increased breathing rate and heartbeat rate, the blood cannot supply

oxygen fast enough to meet their requirements .

3. The rate at which oxygen is used by the muscles exceeds the amount of

oxygen supplied by the blood.

4. The muscles are in a state of oxygen deficiency and an oxygen debt is

incurred.

5. The muscle obtains the extra energy from anaerobic respiration,

because oxygen is not available.

6. During anaerobic respiration, the glucose molecules break down

partially into an intermediate substance called lactic acid instead of

carbon dioxide and water.

7. Because glucose is not completely broken down, the energy released

during anaerobic respiration is much less than the energy released during

aerobic respiration.

8. In fact, for every molecule of glucose, anaerobic respiration releases

only two molecules of ATP.

9. Therefore, in terms of energy yield, anaerobic respiration is less

efficient than aerobic respiration.

10. Much of the energy is still trapped within the molecules of lactic acid.

11. The accumulation of lactic acid can reach a high level of concentration

which can cause muscle cramps and fatigue .

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12. This contributes to the exhaustion a person feels during and after a

period of intense exercise.

13. The person needs to breathe in deeply and rapidly in order to inhale

more oxygen.

14. The excess oxygen is used by the body to oxidize the accumulated

lactic acid to carbon dioxide and water .

15. Oxidation of lactic acid occurs mainly in liver . A portion of the lactic

acid is oxidized to produce energy. The remaining lactic acid is

converted into glycogen and stored in the muscle cells.

16. The oxygen debt is paid off when all the lactic acid is removed. This

happens through the increased breathing rate after vigorous exercise.

17. Therefore, an oxygen debt is the amount of oxygen needed to remove

lactic acid from the muscle cells.

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Anaerobic respiration in yeast

1. Yeast normally respires aerobically.

2. In the absence of oxygen, yeast carries out anaerobic respiration.

3. It produces ethanol, carbon dioxide and energy during anaerobic

respiration.

4. Anaerobic respiration in yeast is also known as fermentation and is

catalyzed by the enzyme zymase.

5. The ethanol produced during fermentation can be used in wine and beer

making.

6. In bread making, yeast is used as it produces carbon dioxide during

fermentation. This causes the dough to rise. Ethanol evaporates during

baking.

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Comparison between aerobic and anaerobic respiration

Aerobic Respiration Anaerobic Respiration

Present Availability of oxygen Absent

Complete oxidation of glucose

Oxidation of glucose Incomplete oxidation of glucose

Carbon dioxide, water and energy

Products of respiration Lactic acid and energy (in muscle cell) or ethanol,

carbon dioxide and energy (in yeast)

38 Number of ATP molecules released per

molecule of glucose

2

A large amount of energy is released per

mole of glucose

Amount of energy released per mole of

glucose

A small amount of energy is released per mole of

glucose

Mitochondria Where the process takes place

Cytoplasm

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The Respiratory Structures and Breathing Mechanisms in Humans

and Animals

Adaptations of the respiratory structures

Unicellular Organism

1. The entire plasma membrane of unicellular organism is the respiratory

surface.

2. This organism has a large surface area to volume ratio for the

diffusion of gases and the plasma membrane is thin and moist

3. Gaseous exchange occurs in the entire plasma membrane of Amoeba sp.

by simple diffusion.

4. The concentration of oxygen in the natural surroundings of the organism

is high causing oxygen to diffuse into the cell and the carbon dioxide

produced from respiration to diffuse into the surroundings.

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The respiratory structure and breathing mechanism of insects

1. The respiratory system in insects is a tracheal system which consists of

a series of tracheae that branch repeatedly to form very fine, thin-walled

tubes called tracheoles.

2. The respiratory system of insects is separated from the circulatory

system. Unlike mammals, the insect’s circulatory system is not

involved in gaseous transport .

3. Air enters the insect’s tracheal system through spiracles, i.e. openings in

the exoskeleton. The spiracles are located at the sides of the thorax and

abdomen of the insect, usually a pair per body segment. Air flow is

regulated by small muscles that contract and relax to control the

valves in each spiracle.

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4. After the spiracles, air enters a longitudinal tracheal trunk, then diffuses

throughout a branched network of tracheoles which are very fine thin-

walled tubules that reach into every part of the insect’s body.

5. The tracheoles are filled with liquid to facilitate gas exchange . The

thin and moist surface of tracheoles also aid in the exchange of

respiratory gases between atmospheric air and living cells.

6. The air in the trachea, full with carbon dioxide, eventually diffuses out

of the tracheal system, through the spiracles, and out into external

environment.

7. In larger insects, rhythmic contraction and relaxation of the

abdominal muscles to control body volume facilitates better

ventilation of the tracheal system. Contraction of the abdominal

muscles increases the air pressure in the tracheal system, so that the air

is forced out through the spiracles. When the same muscles relax, the

tracheal air pressure drops and air flows in through the spiracles and into

the tracheae.

8. In the tracheal system there are collapsible air sacs. In dry and warm

environments, these air sacs provide a temporary air supply allowing

the insect to close its spiracles for short periods to reduce loss of water

through evaporation. These air sacs also provide air which help regulate

buoyancy in aquatic insects.

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The structural adaptation of gills for gaseous exchange

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1. Each fish has 4 pairs of gills, each with double rows of lamellae-

covered filaments. The total surface area of the lamellae is 10-60 times

more than that of the external surface area of the whole fish.

2. The walls of the lamellae are only one-cell thick and the lamellae on the

filaments are located very close together so that most of the water

passing between them is involved in the gaseous exchange process.

This makes for very efficient gaseous exchange between the lamellae

and the water flowing past the gills.

3. The blood in the lamellae flows in the opposite direction

(countercurrent) to the flow of water. This allows the gills to absorb 80%

of the water’s oxygen content. If the blood were to flow in the same

direction as the water, only a maximum of 50% of the water’s oxygen

content can be absorbed.

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The respiratory structure and breathing mechanism of fish

1. The breathing mechanism of fish involves gill ventilation which is

achieved through a one-way flow of water, in through the mouth, past

the gill arches, and out through the opercula flaps.

a) The opercula are closed and the mouth is opened. The floor of the

buccal cavity is lowered . Pressure is lowered inside the buccal cavity.

The higher external water pressure forces oxygen-rich water into the

buccal cavity.

b) The mouth is then closed and the floor of the buccal cavity is raised.

This forces the water to flow backwards , between the gill arches and

past the filaments and lamellae.

c) The flow of water is aided by the opening of the opercula which

helps to draw the water backwards . The mouth skin flaps (valves)

closes due to the high pressure in the buccal cavity, preventing outward

flow of water through the mouth.

d) As water passes over the gill filaments and lamellae, gaseous exchange

occurs.

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The respiratory structure and breathing mechanism of amphibians

The structural adaptation of the skin for gaseous exchange

1. The respiratory surface of a frog is adapted for efficient gaseous

exchange in the lungs and the skin.

2. The frog often breathes through its skin either on land or in water.

3. The skin of the frog is thin and very permeable to respiratory

gaseous. The skin is kept moist by the secretion of mucus.

4. Beneath the skin, there are many blood vessels which receive and

transport oxygen to the body cells.

5. The lungs of the frog are a pair of thin-walled sacs that are connected to

the mouth.

6. The membranes of the lungs are thin, moist and surrounded by a

network of blood capillaries.

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The Mechanism of Breathing of a Frog

The nostril open

The base of the oral cavity is lowered

The glottis closes

Air enters the oral cavity

The nostrils close

The glottis opens

The base of the oral cavity is raised

The air pressure increases

Air enters the lungs

The lungs expand

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The respiratory structure and the breathing mechanisms of humans

1. The respiratory system of humans consists of the air passages, a pair of

lungs and the respiratory muscles that facilitate the movement of air into

and out of the lungs.

2. Within the lungs, the air passages end at thin walled alveoli surrounded

by capillaries. Here, gaseous exchange of oxygen and carbon dioxide

occurs between the alveolar space and the blood by passive diffusion.

3. The respiratory system functions to oxygenate the blood and to remove

carbon dioxide from deoxygenated blood. In removing carbon dioxide,

the respiratory system helps to maintain the pH balance of body fluids.

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4. Air is drawn into the body through the nose. It then enters the nasal

passages. The nasal passages filter, warm and moisten the air. Foreign

particles such as dust are filtered by the nostril hairs and trapped by

mucus secretions.

5. The air then passes the pharynx (the region behind the oral cavity) and

enters the trachea. The opening of the trachea is protected by the

epiglottis that prevents the entry of food.

6. The air first flows through the larynx, and then the trachea which then

branches into two bronchi that enter the lungs.

7. The trachea and bronchi are reinforced with semi-circular cartilage

rings to prevent their collapse during inhalation , when the internal air

pressure drops.

8. The inside walls of the trachea and bronchi are lined with ciliated

epithelial cells and mucus-secreting goblet cells. The mucus traps

small particles present in the air and the cilia sweep the mucus and

trapped particle to the throat to be swallowed or coughed out.

9. The bronchi branch into smaller tubes known as bronchioles which end

in grape-like sacs known as alveoli. Gaseous exchange occurs at the

alveoli.

10. The air that enters the lungs is free of small particles and microbes

to protect the respiratory surfaces from disease and contamination. The

air is also warmed and moistened by the surrounding tissue.

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Structural adaptation of the alveoli for gaseous exchange

(a) A large surface area

There are 300-500 million alveoli in an adult human’s pair of lungs.

Together, these provide a total surface area of 70 – 90 m2, 80 times of

the total area of an average adult human’s skin.

(b) A rich supply of blood

The alveoli are supplied with dense networks of blood capillaries. This

allows efficient diffusion of oxygen into the blood and carbon dioxide

out of the blood.

(c)Thin walls

The wall of each alveolus is a single layer of epithelial cells. Each

alveolus is surrounded by a network of one-cell thick capillaries. Only

0.2 µm separate the air in the alveolus from the blood in the capillaries.

Diffusion of respiratory gases between the two surfaces is very rapid

(d) Moist surface

The inside surfaces of the alveoli are lined with a layer of moisture

secreted by the epithelial cells. Respiratory gases dissolve in this

moisture and diffuse efficiently through the walls of the alveoli and

capillaries.

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The Breathin Mechanism

Inhalation Exhalation

Diaphragm contracts and descends Diaphragm relaxes and ascends

External internal muscles contract External intercostal muscles relax

Internal intercostal muscles relax Internal intercostal muscles contract

Ribcage expands Ribcage contracts

Thoracic volume and lung volume increase

Thoracic volume and lung volume decrease

Air pressure in lungs decreases Air pressure in lungs increases

Higher external air pressure forces air to flow into the lungs

High air pressure in lungs forces air to flow out to the exterior

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SBP Trial 2009

a) Based on Diagram 3, explain one adaptation of alveolus for efficient

gases exchange. (2 marks)

b) i) Name P (1 mark)

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ii) Explain the role of P to prevent dirt and bacteria from entering the

alveolus. (2 marks)

c) i) On Diagram 3, draw labeled arrow ( ) to show the direction of:

Blood flow

Oxygen diffusion

Carbon dioxide diffusion (3 marks)

ii) Explain why the diffusion of oxygen occur at the alveolus. (2 marks)

d) A hard mass of food passing down the oesophagus might indirectly

interrupt the air supply to lung by pressing on P. Explain how P

overcome this problem. (2 marks)

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Gaseous Exchange across the Respiratory Surfaces and Transport of

Gases in Humans

Partial pressure of gases and gaseous exchange across respiratory

surfaces

1. The partial pressure of a specific gas is a measure of the

concentration of that particular gas in a mixture of gases. It is

shown as the pressure that the particular gas exerts in gas mixture.

2. Atmospheric pressure at sea level is approximately 760 mm Hg.

Oxygen comprises 21% of atmospheric gas, its partial pressure is

around 159.6 mm Hg. Carbon dioxide which comprises 0.03% of the

atmosphere has a partial pressure of 0.23 mm Hg.

3. In the lungs, respiratory gases diffuse between the alveoli and the

blood; the net direction is dependent on the difference of the partial

pressures of the gas in the two areas.

4. Figure 7.15 shows the partial pressures of oxygen and carbon dioxide

in an alveolus and in the surrounding blood capillaries.

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5. The inspired air in the alveolus has a higher oxygen partial

pressure than the dexoygenated blood in the capillaries. Thus,

oxygen diffuses into the blood. Conversely, carbon dioxide diffuses

into the alveolus as inspired air in the alveolus has a lower carbon

dioxide partial pressure than the deoxigenated blood in the

capillaries.

6. Figure 7.16 shows the partial pressure of oxgen and carbon dioxide

respiring cells and the surrounding blood capillaries.

7. Similarly, diffusion of oxygen from blood in the capillaries into

respiring cells in body tissues occurs, as the carbon dioxide partial

pressure in the blood is higher than in the cells. Diffusion of carbon

dioxide from the respiring cells into the blood also occurs as the

carbon dioxide partial pressure in the cells is higher than in the

blood .

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8. The Transport of Respiratory Gases in Humans

Transport of Oxygen

1. In the lungs, oxygen diffuses from the alveoli into the blood, resulting in

oxygenated blood.

2. In oxygenated blood, molecular oxygen is transported in two ways:

a) 98.5% of the oxygen inhaled is bound to haemoglobin in red blood

cells, forming oxyhaemoglobin.

Hb + 4O2 Hb8 (oxyhaemoglobin)

b) 1.5% of the oxygen is dissolved in the plasma

3. The oxygenated blood is then transported to the body tissues. At the

body tissues, the partial pressure of oxygen is higher in the blood than in

the respiring cells.

a) The dissolved oxygen diffuses out from the blood plasma

b) The oxyhaemoglobin in the red blood cells dissociates, forming

haemoglobin and molecular oxygen.

HbO8 Hb + 4O2

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Transport of Carbon Dioxide

1. In the body tissues, cellular respiration uses oxygen and produces carbon

dioxide. Thus, the partial pressure of carbon dioxide. Thus, the partial

pressure of carbon dioxide is higher in the cells than in the blood; this

causes carbon dioxide to diffuse into the blood.

2. Carbon dioxide is transported in the blood in three ways.

a) 7% to 10% of the carbon dioxide is transported in a dissolved state in

blood plasma.

b) Some of the carbon dioxide is reversibly bound to haemoglobin in red

blood cells forming carbaminohaemoglobin. About 20% to 30 % of the

carbon dioxide is transported in this way.

c) 60% to 70% of the carbon dioxide is transported in the form of

bicarbonate ions in plasma. In the red blood cells, carbon dioxide

combines with water to form carbonic acid (H2CO3). This reaction is

catalyzed by the enzyme carbonic anhydrase, found in red blood cells.

Carbonic acid then dissociates to form bicarbonate ions (HCO3-) and

hydrogen ions (H+).

CO2 + H2O H2CO3 H+ + HCO3-

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SBP Midyear Trial 2008

a) i) Based on Diagram 3, name structure X. (1 mark)

ii) State the adaptation of X in order to perform the following

function:

Facilitating the diffusion of gases (1 mark)

Transportation of oxygen to all the body cells (1 mark)

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b) Explain why the concentration of carbon dioxide in the body cells

is always higher than in the blood. (2 marks)

c) i) Compare the concentration of the respiratory gases in the blood

vessels labelled P and Q. (1 mark)

ii) Describe how the following respiratory gases are transported in

the blood.

Oxygen (2 marks)

Carbon dioxide (2 marks)

d) A boy used to smoke cigarettes during his leisure time. Explain

how his bad habit affects his lungs. (2 marks)

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Composition of Inhaled and Exhaled Air in Humans

Component Inhaled Air Exhaled Air

Explanation

Nitrogen 78% 78%

Oxygen 21% 16% Oxygen is absorbed from inhaled air and used for cellular respiration.

Carbon dioxide

0.03% 4% Carbon dioxide is produced during cellular respiration and excreted from the body during exhalation.

Water vapour Variable Saturated Exhaled air is saturated with water vapour because water evaporates from the mucus and cells lining the air passages. Besides that, one of the waste products of cellular respiration is water which will be eliminated out through the lungs.

Temperature External environmental temperature, usually lower than body temperature

Body temperature

Exhaled air comes from the body which has been warmed by body temperature.

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The Regulatory Mechanism of Respiration

Control of the breathing rate

1. Changes in the breathing rate can be caused by afferent information

received by the respiratory centre. This information includes information

about the partial pressure of oxygen, information about the partial

pressure of carbon dioxide, blood pH.

2. This information comes from specialized cells called chemoreceptors

that respond to chemical stimuli.

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3. There are two types of chemoreceptors.

a) Central chemoreceptors

These are cells in the medulla oblongata that are sensitive to changes in

the partial pressure of carbon dioxid e and pH in the cerebrospinal

fluid.

If the level of carbon dioxide in the blood increases, for example,

during vigorous exercise, then the carbon dioxide and carbonic acid

concentrations increase in the cerebrospinal fluid.

The respiratory centre is stimulated. The centre instructs the

diaphragm and external intercostal muscles to increase their

contraction rate. This increases the breathing rate.

More carbon dioxide is removed from the blood. The blood’s carbon

dioxide level and pH return to normal. Negative feedback then causes

the respiration rate to decrease to normal.

b) Peripheral chemoreceptors

These are located in the aortic bodies on the aorta, and the carotid

bodies on the carotid arteries.

They are connected by nerves to the respiratory centre and are sensitive

to changes in the partial pressures of oxygen and carbon dioxide, and

also to the pH of the blood leaving the heart.

During vigorous exercise, in the blood, the oxygen partial pressure

decreases and the carbon dioxide partial pressure increases.

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If the arterial blood reaching the carotid body has a too low oxygen

partial pressure or a too high carbon dioxide partial pressure, the

peripheral chemoreceptors are stimulated.

Information is sent to the respiratory centre to increase the breathing

rate.

The increased breathing rate allows the intake of more oxygen and the

removal of more carbon dioxide from the blood.

When the partial pressures have returned to normal, the respiratory rate

returns to normal.

4. The respiratory centre also receives information from proprioreceptors

in the muscles, tendons and joints.

During physical activity, the muscles, tendons and joints are

stretched; this stimulates the proprioreceptors which then send

impulses to the respiratory centre to bring about an immediate increase

in the breathing rate so that the body can rapidly increase its oxygen

intake.

5. Breathing can also be influenced by other parts of the brain. A person

can consciously breathe more deeply and more rapidly.

6. Receptors in the skin that are sensitive to stimuli when suddenly

stimulated can also cause immediate increases in the breathing rate. This

allows greater oxygen intake that may be necessary for the ‘fight-or-

flight’ response.

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The human respiratory response in different situations

At rest

1. When at rest, the body’s energy needs are low. The levels of blood pH,

carbon dioxide partial pressure and oxygen partial pressure are within

the normal range. All the receptors are not abnormally stimulated.

2. Therefore, the respiratory centre only affects its normal stimulation of

the respiratory muscles. The respiration rate and heartbeat rate are

normal.

During vigorous activity

1. Vigorous physical activity requires greater energy expenditure and the

rate of cellular respiration increases to generate this energy. This higher

respiration rate requires more oxygen and glucose, and will produce

more carbon dioxide.

2. An increased breathing rate allows more oxygen to be absorbed into the

bloodstream and more carbon dioxide to be expelled from the body.

3. An increased heartbeat rate allows more blood carrying oxygen and

glucose to be pumped to the body’s tissues, and more blood carrying

carbon dioxide to be pumped to the lungs for excretion.

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Fear

1. At the face of a threat, a person readies himself for a fight or for flight.

This is aptly named ‘fight-or-flight’ response. It is mediated by hormone

adrenaline that is secreted by the adrenal glands.

2. The effects of adrenaline on the circulo-respiratory system include

An increase in the heartbeat rate, 5 times as much output as its

normal volume to pump more oxygen and glucose to tissues such as

the muscles.

Arteries constricting to maximize pressure around the body’s

systems and veins dilating to speed up the return of blood to the heart.

The breathing rate speeds up so that more oxygen can be absorbed

into the bloodstream and more carbon dioxide can be excreted.

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At high altitudes

1. As altitude increases, atmospheric pressure decreases as the air becomes

thinner. There is less oxygen in the atmosphere.

2. At high altitudes, with each normal breath, a person takes in relatively

less air, and thus, less oxygen into the lungs, compared to breathing at

sea level.

3. The result is lower blood oxygen levels (hypoxia) which can causes

altitude sickness. The symptoms include headaches, breathlessness,

fatigue, nausea, and swelling of the face, hands and feet.

4. For mountain climbers, acclimatization to high altitudes starts with an

increase in both the breathing rate and the heartbeat rate.

5. After a few weeks at high altitudes, the body further adapts by

increasing red blood cell production in the bone marrow to increase the

blood’s oxygen-carrying capacity. Production of myoglobin, the

oxygen-carrying protein in cardiac muscles, also increases. The body

also develops more capillaries in response to altitude as this allows faster

diffusion of oxygen to the cells.

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The Contents of Cigarette Smoke and The Dangers of Smoking

1. The chemicals in cigarette smoke include:

Benzene: a colourless cyclic hydrocarbon obtained from coal and

petroleum, used as a petrol additive and as a solvent in chemical

manufacture. It is a known carcinogen and is associated with

leukaemia.

Formaldehyde: a colourless, highly poisonous liquid, used to

preserve dead bodies. It is known to cause cancer, respiratory, skin

and gastrointestinal problems.

Ammonia: used in cleaning fluids. In cigarettes, it turns nicotine

from tobacco into a gaseous form which is easily absorbed in the

lungs.

Tar: a particulate drawn into the lungs with cigarette smoke. Once

inhaled, the smoke condenses and the tar is deposited in the airways,

paralyzing the cilia of the epithelial lining.

Nicotine: one of the most addictive substances known to man. It is

the main cause of addiction to smoking. It is also a powerful and fast-

acting poison and can be used as an insecticide.

Carbon Monoxide: an odourless, tasteless and poisonous gas.

Compared to oxygen, haemoglobin has a higher affinity for carbon

dioxide. In the blood, it reduces the supply of oxygen to the body’s

tissues.

Arsenic: a rat poison

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Hydrogen cyanide: a respiratory enzyme inhibitor used as a gas

chamber poison.

2. The effect of smoking include:

a) Cancer

The risk of dying from lung cancer is more than 20 times higher

among men who smoke cigarettes, and about 10 times higher

among women who smoke cigarettes, compared o non-smokers.

Because toxins from cigarette smoke are transported all round he

body, smoking also causes cancers of the bladder, oral cavity,

oesophagus, cervix, kidney, lung, pancreas and stomach.

b) Cardiovascular Disease

Smoking can result in coronary heart disease because it causes

narrowing of the arteries, thus reducing blood circulation.

It increases the risk of coronary heart disease and stroke by 2 -4

times. Smokers are 10 times more likely to develop peripheral

vascular disease, compared to nonsmokers.

c) Respiratory Disease

Cigarette smoking causes a ten-fold increase in the risk of dying

from obstructive lung disease (chronic bronchitis and emphysema)

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d) Birth Defects

Cigarette smoking among expectant mothers causes an increased

risk of infertility, stillbirth, low birth weight and sudden infant

death syndrome (SIDS).

Respiration in Plants

The intake of oxygen by plants

1. In green plant tissues that are capable of photosynthesis, the oxygen for

respiration comes from photosynthesis and the carbon dioxide from

respiration is used for photosynthesis.

2. However, in non-green parts of the plants, and in all parts of the plant

when light intensity is low, there is either insufficient or no oxygen from

photosynthesis.

3. Oxygen for respiration has to be obtained from the external

environment, and the carbon dioxide from respiration has to be expelled

to the external environment.

4. Gaseous exchange occurs between a plant and the external environment

through diffusion. Leaf surfaces have openings called stomata that

allow gaseous exchange to occur.

5. All other plant surfaces such as the roots, stems, branches and twigs

have openings called lenticels which also allow gaseous exchange.

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Lenticel on the plant twig

6. In the leaves, when light intensity is low, photosynthesis becomes very

slow or stops completely.

7. However cellular respiration continues. As oxygen concentration in the

leaves’ intercellular spaces becomes lower than in the external

environment, external oxygen diffuse through the stomatal pores into

the intercellular spaces, and into the cells for respiration.

8. Cellular respiration produces carbon dioxide, which diffuses out of

the cells into the intercellular, which diffuses out of the cells into the

intercellular spaces. The carbon dioxide concentration in the

intercellular spaces becomes higher than in the external environment;

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carbon dioxide diffuses out through the stomatal pores, and out of the

leaves.

9. In the plant’s roots, stems, branches and twigs, the living tissue does not

photosynthesise but does respire. External oxygen constantly diffuses

down to the concentration gradient, through the lenticels, into the

plant for aerobic respiration. Similarly, carbon dioxide constantly

diffuses down its concentration gradient out through the lenticels,

and out of the plant.

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Respiration and photosynthesis in plants

1. A plant will absorb or release gases (oxygen or carbon dioxide)

depending on light intensity.

2. Figure 7.24 shows a graph depicting respiration and photosynthesis rate

in a plant at different light intensities.

a) In bright light,

The rate of photosynthesis is faster than the rate of respiration

Thus, plant produces more oxygen than it uses, and uses more carbon

dioxide than it produces.

Under very high light intensity, the photosynthesis rate does not

increase further due to limiting factors such as carbon dioxide

concentration.

b) As light intensity decreases,

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The rate of photosynthesis drops, but the rate of respiration

remains constant.

The rate of photosynthesis decreases until a point when it is equal

to the rate of respiration. This is the compensation point.

c) As light intensity decreases further (until it reaches zero)

The rate of photosynthesis drops to zero is tandem, but respiration

continues at its constant rate.

When the rate of photosynthesis becomes lower than the rate of

respiration, plants produce more carbon dioxide than is produced.

3. If the photosynthesis rate falls below the respiration rate for an extended

period of time, the plants would eventually die when their energy stores

are depleted.

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Respiration

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