GASEOUS EXCHANGE AND ITS CONTROL• Gaseous exchange and its control in mammals.

– Structure of an alveolus– Structure & characteristics of haemoglobin– Transport of O2 & CO2 by haemoglobin– O2 dissociation curves of haemoglobin & myoglobin.– O2 dissociation curves of haemoglobin as in fetus and adult– Bohr effect

• Role of chemoreceptors in controling breathing

• Gaseous exchange and control in plants– Structure and functions of guard cells– Regulation of the stomatal opening and closing

• Starch-sugar hyphothesis• Potassium ion hyphothesis

OBJECTIVES

• describe how gaseous exchange occurs in lungs

• describe the structure of haemoglobin and its affinity for oxygen by using the oxygen dissociation curves

• explain roles of haemoglobin in oxygen and carbon dioxide transportation in blood

• explain the control of ventilation

• describe lung volumes and capacities

• describe the mechanism of stomatal opening and closing

9.1.1 : GASEOUS EXCHANGE

Microscopic structure of the alveolus as the basic unit of lung

• consists of tiny air sacs where gaseous exchange occurs

• each lung packed with 1.5-2.5 million alveoli

• diameter is 0.2 millimeter each

• enmeshed in capillaries

• wall of each alveolus is only one cell thick

• thin layer of watery fluid lining each alveolus

Adaptation of the lung for gaseous exchange

• consists of millions alveoli to maximize respiratory surface area

• thin-walled alveoli resemble tiny bubbles, therefore provided enormous surface areas of diffusion

• alveoli’s surface remains moist, thus gasses can easily dissolve in thin fluid and diffuse through the alveolar and capillary membranes

• both alveolar wall and the adjacent capillary walls are only one cell thick, the air is extremely close to the blood in the capillaries

Structure of heamoglobin

• the oxygen-carrying protein in red blood cells

• consists of two pairs of very similar peptides, held together by hydrogen bonds

• each peptide holds an iron-containing organic molecule called a haem that can bind one molecule of oxygen

• therefore, one haemoglobin (Hb) molecule binds up to four oxygen molecules

Hb + 4O2 Hb(O2)4 Oxyhaemoglobin

HHbNH2 + CO2 HHbNH2CO2

Carbamino-haemoglobin

Hb + CO COHb carboxyhaemoglobin

Characteristics of haemoglobin as respiratory pigment

• an effective respiration pigment due to its high affinity to O2 although the partial pressure of O2 is below 20mmHg

• release O2 easily when the partial pressure of O2 is low

O2 dissociation curve of Hb• an S- shaped curve

obtained when the percentage O2 saturation of blood is plotted against the partial pressure of O2

• an S-shaped curve is obtained due to the way that Hb binds to O2

• the first O2 molecules that combines with the Hb alters the shape of the molecule in such a way that it is easier for the next O2 molecule to join

• conversely, when one molecule of O2 dissociates from the oxyhaemoglobin, the Hb shape is adjusted to make release of successive molecules of O2 increasingly easy

• at a partial pressure of zero, no O2 is attached to the Hb molecule

• this is due to the ability of Hb to release O2 easily when the partial pressure of O2 is low

• at an O2 partial pressure of approximately 30 mm Hg, 50 % of the Hb is present as HbO8

• this is caused by the high affinity Hb to O2 at partial pressure of 73 mm Hg, the Hb is completely saturated with O2 this point is called loading tension

• at higher partial pressures of O2, further uptake of O2 can occur, but 100% saturation of Hb is rarely achieved

O2 dissociation curve of Hb and myoglobin in comparison

• myoglobin molecule is widely distributed in animals and is particularly common in skeletal muscle tissues of mammals

• very similar to the Hb subunits with respect to both amino acid sequence and three dimensional structure

• this pigment may also bind to O2, but since there is only one haem group there can be no cooperative binding

• displays great affinity for O2

• its O2 dissociation curve is displaced well to the left of Hb it only begins to release O2 when the partial pressure of O2 is below 20 mm Hg

• in this way, it acts as a store of O2 in resting muscle, only releasing it when supplies of HbO8 have been exhausted

• the myoglobin-oxygen dissociation curve is hyperbolic rather than sigmoidal

Oxygen dissociation curves of haemoglobin in fetus and

mother • the fetus has a very high

O2 demand

• the fetal Hb is of a type which has a higher affinity for O2 than the mother’s Hb

• O2 is therefore readily unloaded from the mother’s blood to the fetal blood

• a graph for fetal Hb shows a shift to the left from the curve for adult Hb

O2 dissociation curves of Hb in fetus and mother

PO2 (mmHg)

O2 s

atur

atio

n of

Hb

(%)

Fetus

Mother

Effect of partial pressure of carbon dioxide towards O2 dissociation

(Bohr’s effect)

• once blood has travelled to the body tissues, O2 is released

• this is due to the drop in the partial pressure of the O2 and a rise in the partial pressure of carbon dioxide in respiring cells

• a rise in the partial pressure of the carbon dioxide lowers the affinity of Hb for O2 which is therefore released

• this is called the Bohr effect or the Bohr shift

• an increase in the partial presure of CO2 shifts the O2 dissociation curve to the right

CO2 TRANSPORTATION

IN BLOOD

CO2 TRANSPORTATION IN BLOOD

• the red blood cell and Hb both play a significant part in this process as well as in the transport of O2

• CO2 is carried by the blood in three different ways

– in aqueous solution (5%)

– combined with protein (10-20%)

– as hydrogencarbonate (85%)

CO2 is carried by the blood in three different ways

i) in aqueous solution (5%)

• most of the CO2 carried in this way is transported in physical solution

• a very small amount is carried as carbonic acid (H2CO3)

ii) combined with protein (10-20%)

• CO2 combines with the amine (NH2) group of Hb to form a neutral carbamino-haemoglobin compound

HHbNH2 + CO2 HHbNH2CO2

Carbamino-haemoglobin

• the amount of CO2 that is able to combine with Hb depends on the amount of O2 already being carried by the Hb

• the less the amount of O2 being carried by the Hb molecule, the more CO2 that can be carried by the Hb

• CO2 produced by the tissues diffuses passively into the bloodstream and passes into the erythrocytes where it combines with water to form carbonic acid

• this process is catalysed by the enzyme carbonic anhydrase found in the erythrocytes and take less than one second to occur

• carbonic acid then proceeds to dissociate into hydrogen and hyrogenbicarbonate ions;

iii) as hydrogenbicarbonate ion, HCO3- (85%)

• when the erythrocytes leave the lungs their oxyhaemoglobin is weakly acidic and associated with potassium ions (KHbO2)

• in areas of high CO2 concentrations (as at the tissue), O2 is easily given up by oxyhaemoglobin

• when this happen, the Hb becomes strongly asidic

• it dissociates from the carbonic acid forming haemoglobinic acid (HHb)

• the potassium ions associate with hydrogencarbonate ions to form potassium hydrogencarbonate

• by accepting hydrogen ions, Hb acts as a buffer molecule

• this enables large quantities of carbonic acid to be carried to the lungs without any major alteration in blood pH

KHbO2 KHb + O2

H+ + HCO3- + KHb HHb + KHCO3

• the cell surface membrane of an erythrocytes is relatively impermeable to the passage of sodium and potassium ions

• however a cation pump operates and expels large numbers of sodium ions into the plasma

• the majority of hydrogenbicarbonate ions formed within the erythrocytes diffuse out into the plasma along a concentration gradient and combine with sodium to form sodium hydrogencarbonate

• the loss of hydrogenbicarbonate ions from the erythrocytes is balanced by chloride ions diffusing into erythrocytes from the plasma

• electrochemical neutrality is maintained

• this is called chloride shift

• potassium hydrogencarbonate which formed in the erythrocytes is also capable of dissociating

• some of the chloride ions which enter the erythrocytes combine with potassium ions to form potassium chloride, while the hydrogencarbonate ions diffuse out

• when hydrogencarbonate leaves the erythrocytes, the excess H+ ions which remain decrease the pH within the erythrocyte

• this caused the dissociation of potassium oxyhaemoglobin (KHbO2) into O2 and potassium Hb

• when the erythrocytes reach the lungs, the reverse process occurs

LUNG VOLUME AND CAPACITIES• lung capacity of an average man is approximately 5 dm3

• during quiet breathing he will breathe in and out about 450 cm3 of air

• this is called tidal volume

• if after a normal tidal inspiration he continues to inhale, he can take in a further 1500 cm3 of air

• this is called inspiratory reserve volume

• if after a tidal expiration the man continues to exhale, he can force out a further 1500 cm3 of air

• this is termed as expiratory reserve volume

• the amount of air exchanged after a forced inspiration followed immediately by a forced expiration is termed the vital capacity even after forced expiration, 1500 cm3 of air remain in the lungs

• this cannot be expelled and is called residual air

• during inspiration about 300 cm3 of the tidal volume reaches the lungs, while the remaining 150cm3 remains in the respiratory tubes, where gaseous exchange does not occur

• when expiration follows, this air is expelled from the body as unchanged room air and is called dead space air

• the air that reaches the lungs mixes with the 1500 cm3 of air already present in the alveoli

• its volume is small compared to that of the alveolar air and complete renewal of air in lungs is therefore a necessarily slow process

• the air that comes into close contact with the blood is alveolar air

• it contains less O2 than inspired air, but more CO2

LUNG VOLUME AND CAPACITIES• Tidal volume

– The volume of air that is moved in and out with each breath during normal rhythmic breathing

• Inspiratory reserve volume– The additional volume of air that can be forced out after a normal tidal inspiration

• Expiratory reserve volume– The additional volume of air exchanged after a forced inspiration followed immediately

by a forced expiration

• Residual air– The volume of air which remains in the lungs and cannot be expelled even after forced

expiration• Dead space

– The tidal volume which remains in the respiratory tubes during inspiration and gaseous exchange does not occur

• Vital capacities– The volume of air exchanged after a forced inspiration followed immediately by a forced

expiration

9.1.2 CONTROL

Control of ventilation

• Nervous system usually controls respirations automatically to meet body’s demands without our conscious concern

• Respiratory centre is the area from which nerve impulses are sent

• It is located in the brain stem

• It consists of groups of neurons in the brain stem

• It can be divided into three areas: i. The medullary rhythmicity area in medulla oblongata

ii. The pneumotaxic area in the pons

iii. The apneustic area in the pons

Medullary rhythmicity area

• It controls the basic rhythm of respiration

• both inspiratory and expiratory areas are located here

• the basic rhythm of respiration is determined by nerve impulses generated in the inspiratory area

• nerve impulses from the active inspiratory area travel via nerves to the muscles of inspiration (diaphragm and external intercostal muscles)

• these muscles contract and inspiration occurs

• the expiratory neurons remain inactive during normal quiet respiration

• when the inspiratory neurons become inactive again, the muscles relax, expiration occurs and the cycle repeat itself over and over

2) pneumotaxic area

• located in the upper pons

• transmits inhibitory impulses to the inspiratory area

• it help turn off the inspiratory area before the lungs become too full of air

3) the apneustic area

• located in the lower pons

• stimulates the inspiratory area to prolong inspiration, thus inhibiting expiration

• The main stimulus that controls the breathing rate is the concentration of CO2 in blood

• when CO2 levels increase, chemoreceptors in the carotid and aortic bodies of the blood system are stimulated to discharged nerve impulses which pass to the inspiratory centre

• the inspiratory centre sends out impulses via the phrenic and thoracic nerves to the diaphragm and intercostal musles

• these muscles increase the rate at which they contract

• this automatically increases the rate at which inspiration takes place

• inspiratory activity inflates the alveoli, and stretch receptors located here and in the bronchial they are stimulated to discharge impulses to the expiratory centre which automatically cuts off inspiratory activity

• the respiratory muscles therefore relax and expiration takes place

• after this had occurred, the alveoli are no longer stretched and the stretch receptors no longer stimulated

• the expiratory centre becomes inactive and inspiration can begin again

• the whole cycle is repeated rhythmically throughout the life of the organism

That’s all for

today

GASEOUS EXCHANGE AND CONTROL IN

PLANT

Stomata Structure and function of guard cells in the stomatal opening and closing mechanisms

– stoma is a micro pore that located between a specialised epidermal cells called guard cells

– the guard cells occur in pairs side by side

– guard cells have a distinctive shape

• the only epidermal cells that contain chloroplasts

• Guard cell has a thinner outer wall and a thicker, less elastic, inner wall

• the size of stoma is adjusted by the turgidity of the guard cells

Regulation of stomatal opening and closing

• due to the guard cells turgidity

three mechanisms

i. Starch-sugar hypothesis (photosynthesis in the guards cells and accumulation of sugar)

ii. CO2 concentration in the leaf (related to starch-sugar hyphothesis)

iii. Potassium ion concentration of the guard cells

i. Starch-sugar hypothesis (photosynthesis in the guards cells and accumulation of sugar)

• an increase in sugar concentration (from phothosynthesis) in guard cells during the day lead to – the decrease in their solute potential and

– entry of water by osmosis

– cells become turgid

• The outer wall of guard cell is thinner and more elastic than the inner wall.

• The increase in turgor pressure causes the guard cells to curve outward

• Stoma open

• The decrease in sugar concentration (light dependent reaction not occur) in guard cells during the night lead to – the increase in their solute potential and

– outflow of water by osmosis

– cells become flaccid

– causes the closing of stoma

ii. CO2 concentration in the leaf (related to starch-sugar hyphothesis)

• at night, carbonic acid is accumulated in the interstitial cells

• carbonic acid is dissociate into hydrogen and hydrogencarbonate ions

H2CO3 HCO3- + H+

• this will increase the concentration of H+

• enzyme catalyse the transformation of glucose-1-phosphate into starch

• this decrease the turgidity of guard cells, thus the pore is close

• day time, CO2 is used during photosynthesis, the concentration of H+ is decreased

• phosphorilase catalyse the transformation of starch into glucose-1-phosphate in the guard cells

• the turgidity of the guard cells increase, the entry of water by osmosis

• the pore is open

iii. Potassium ion concentration of the guard cells

• during the day, photosynthesis occurs in the guard cells , ATPs are synthesizes

• the ATPs activate the ions pump, increasing the concentration of potassium ions in the guard cells

• this leads to the decreasing in the solute potential of the adjacent cells and the entry of water by osmosis

• the pore is open

• during the night, photosynthesis (light dependent reaction) not occurs,

• ATPs are not synthesizes• The ions pump become inactive,

– The potassium ion diffuse out of the guard cell

– The concentration of potassium ions in the guard cells decrease.

• this leads to the increasing in the solute potential of the adjacent cells and the outflow of water by osmosis

• the pore is close

THE END