gaseous exchange-stpm

Gaseous exchange Transport of oxygen The red blood cells contain haemoglobin, the molecule that transports oxygen. Typically, the human body requires between 250 cm 3 (at rest) and 1000 cm 3 (in strenuous exercise) of oxygen per minute. At human (37 o C) body temperature, about 0.45 cm 3 of oxygen can be dissolved in 100 cm 3 of blood plasma, but the total oxygen transporting facility of blood is about 20 cm 3 per 100 cm 3 . Thus some 98 per cent of oxygen is transported by haemoglobin. Haemoglobin and oxygen transport Haemoglobin is a compact molecule made up of four interlocking sub-units. The sub-units consist of four polypeptide chains called globin (two α-chains and two β-chains), each attached to a haem group. Haemoglobin therefore has a quaternary structure. The haem groups comprise a porphyrin ring containing an atom of iron (II) at the centre. The haem groups in haemoglobin are responsible for its red colour, and are the site of oxygen transport. An iron atom combines with a molecule of oxygen, but without oxidation of the iron (II). Thus, each haemoglobin molecule can carry four molecules of O 2 . Like all respiratory pigments, haemoglobin must bind oxygen reversibly, loading oxygen in the lungs and unloading it in other parts of the body. Loading and unloading depends on cooperation among the sub-units of the haemoglobin molecule. The binding of oxygen to one sub-unit induces the remaining sub-units to change their shape slightly so that their affinity for oxygen increases. And when one sub-unit unloads its oxygen, the other three quickly follow suit as a conformational change lowers their affinity for oxygen. The structure of haemoglobin Haemoglobin is a conjugated protein The protein part (called globin) consists of four polypeptide chains. These chains are of two types called alpha and beta. They are about the same length (about 140 amino acids) but have slightly different compositions. Each chain is combined with a non-protein prosthetic group called haem. Haem consists of an atom of iron enclosed in a ring structure (porphyrin ring) Each haem group can combine with one molecule of oxygen. This process is called oxygenation. (It is not the same as oxidation, because the iron does not lose any electrons and is not chemically oxidised). Each molecule of haemoglobin can therefore combine with a maximum of four molecules of oxygen. Oxygen tension (partial pressure) In a mixture of gases, each component gas exerts a pressure (its partial pressure or tension) in proportion to its molar percentage in the mixture. The partial pressure of a gas is the pressure contributed by one gas to the total pressure of a mixture of gases. The partial pressure of a gas is the pressure exerted by the particular gas in a mixture of gases. Partial pressures of the components of air Component gases Percentage composition Partial pressure/kPa Partial pressure/mm Hg 1

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Gaseous Exchange-STPM

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Gaseous exchange

Gaseous exchangeTransport of oxygen

The red blood cells contain haemoglobin, the molecule that transports oxygen. Typically, the human body requires between 250 cm3 (at rest) and 1000 cm3 (in strenuous exercise) of oxygen per minute. At human (37 oC) body temperature, about 0.45 cm3 of oxygen can be dissolved in 100 cm3 of blood plasma, but the total oxygen transporting facility of blood is about 20 cm3 per 100 cm3. Thus some 98 per cent of oxygen is transported by haemoglobin.

Haemoglobin and oxygen transport

Haemoglobin is a compact molecule made up of four interlocking sub-units. The sub-units consist of four polypeptide chains called globin (two -chains and two -chains), each attached to a haem group. Haemoglobin therefore has a quaternary structure. The haem groups comprise a porphyrin ring containing an atom of iron (II) at the centre. The haem groups in haemoglobin are responsible for its red colour, and are the site of oxygen transport. An iron atom combines with a molecule of oxygen, but without oxidation of the iron (II). Thus, each haemoglobin molecule can carry four molecules of O2. Like all respiratory pigments, haemoglobin must bind oxygen reversibly, loading oxygen in the lungs and unloading it in other parts of the body. Loading and unloading depends on cooperation among the sub-units of the haemoglobin molecule. The binding of oxygen to one sub-unit induces the remaining sub-units to change their shape slightly so that their affinity for oxygen increases. And when one sub-unit unloads its oxygen, the other three quickly follow suit as a conformational change lowers their affinity for oxygen.The structure of haemoglobin

Haemoglobin is a conjugated protein

The protein part (called globin) consists of four polypeptide chains. These chains are of two types called alpha and beta. They are about the same length (about 140 amino acids) but have slightly different compositions.

Each chain is combined with a non-protein prosthetic group called haem. Haem consists of an atom of iron enclosed in a ring structure (porphyrin ring)

Each haem group can combine with one molecule of oxygen. This process is called oxygenation. (It is not the same as oxidation, because the iron does not lose any electrons and is not chemically oxidised). Each molecule of haemoglobin can therefore combine with a maximum of four molecules of oxygen.Oxygen tension (partial pressure)

In a mixture of gases, each component gas exerts a pressure (its partial pressure or tension) in proportion to its molar percentage in the mixture. The partial pressure of a gas is the pressure contributed by one gas to the total pressure of a mixture of gases.

The partial pressure of a gas is the pressure exerted by the particular gas in a mixture of gases.Partial pressures of the components of air

Component gasesPercentage composition Partial pressure/kPaPartial pressure/mm Hg

Oxygen2121.2160

Carbon dioxide0.040.040.3

Nitrogen7980.0

Partial pressure of the components of alveolar air

Components gasesPercentage compositionPartial pressure/kPaPartial pressure/mm Hg

Oxygen13.814.0104

Carbon dioxide5.55.640

Nitrogen80.7

Partial pressures of the components in tissue cellsComponent gasesPercentage compositionPartial pressure/kPaPartial pressure/mm Hg

Oxygen< 5.6< 40