AQA biology Unit 2 Gas exchange and oxygen dissociation

More model answers you should know for the unit 2 exam

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What is the relationship between an organisms size

The larger the organism, the smaller the ratio therefore the slower the rate of diffusion across the gas exchange surface

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Features of specialized gas exchange surfaces

large S.A to volume ratio

Thin (short diffusion pathway)

Partially permiable

movement of external medium to maintain concentration gradient

movement of internal medium to maintain concentration gradient

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How have large organisms adapted to be efficient a

Flattened shape so no cell is ever far away from the surface

Specialized exchange surfaces with large S.A to increase the surface area to volume ratio(e.g lungs or gills)

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Gas exchange in insects

Air enters through open spiracles and passes through the tracheae.

There is a diffusion gradient in the tracheae causing oxygen to diffuse into cells. Ventilation replaces air in tracheae maintaining concentration gradient.

Spiracles are able to close

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How do respiratory gases move in and out of trache

Down a concentration gradient  (air moving in has a high oxygen content, at the tissues the oxygen concentration is low as it is used in respiration)

Ventilation by movement of muscles in the abdomen creates mass air movement

NOTE: Spiracles closing also conserves water and that it is the buildup of CO2 that causes the spiracles to open.

Insects have no blood system so oxygen diffuses to the tracheae straight to the tissues  

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How insects are adapted to conserve water

Small S.A to volume ratio

Waterproof coverings on body surfaces

ability to close spiracles

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How insects are adapted to be efficient at gas exc

Numerous spiracles to increase S.A

Short diffusion pathway as their is no blood 

Many traceoles to increase S.A 

Concentration gradient maintained by ventilation and the removal of oxygen by respiring tissues

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Gas exchange in fish

Water enters through the mouth and s forced across the gill filaments.

The water flow in the opposite direction to the blood in the lamellae. This is known as countercurrent flow which produces a concentration gradient to promote the diffusion of oxygen from the water along the whole length of the gill.

Water finally passes out through the operculum

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Why are fish gills efficient at gas exchange

Numerous gill filaments with numerous lamellae-increased S.A

Thin squamous epithelium cells on gill and capillaries - Short diffusion pathway 

Gills are well supplied with numerous small capillaries - increased S.A

Countercurrent flow - maintains steep concentration gradient

Constant ventilation and circulation of blood maintains concentration gradient 

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Gas exchange in plants

During respiration oxygen diffuses into the plant when the stomata are open

Carbon dioxide diffuses out of the plant

During photosynthesis the opposite happens 

In the mesophyll layer the plant has numerous air spaces to aid gaseous diffusion

GAses do not have to be dissolved in water for diffusion through the stomata 

Stomata close to conserve water. When this happens no gas exchange can occur through the stomata

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How plants are adapted to increase gas exchange

Numerous stomata (increase S.A)

Diffusion takes place in gas phase 

Long flat leaves ensure no living cell is far from external air

Air spaces in spongy mesophyll (short diffusion pathway as it doesn't have to pass through cells)

Photosynthesis and respiration maintains concentration gradient

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Structure of haemoglobin

Primary structure consists of 4 polypeptide chains built of amino acids

Secondary structure consists of these chains coiled into alpha-helices connected with hydrogen bonds

Tertiary structure provides a globular shape with futher bonding

Quaternary structure  of 4 chains linked together to form a spherical molecule.

Each polypeptide is associated with a haem group containing Iron (II) ions

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Role of haemoglobin

Readily associates with oxygen at gas exchange surfaces as it has a high affinity there

Readily dissociates from oxygen at respiring tissues as it has a lower affinity here

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Oxygen dissociation curves

Further to the left greater affinity for oxygen (takes it up readily but releases it less readily)

Further to the right lower affinity for oxygen (takes it up less readily but releases it more easily

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Effects of carbon dioxide concentration

Greater concentration of CO2 the more readily haemoglobin releases its oxygen (Bohr effect) Oxygen dissociation curve shifts right 

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Loading, Transporting and unloading of oxygen

Carbon Dioxide is constantly removed at gas exchange surfaces

pH is higher due tolow levels of CO2

Haemoglobin loads oxygen more readily

Haemoglobin has a high affinity in this state so does not release oxygen during transport

Carbon Dioxide is produced in respiring cells and is acidic so the pH is lower

Shape of haemoglobin changes into one with a lower affinity for oxygen

haemoglobin releases its oxygen to the respiring cells

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Why does haemoglobin unloads oxygen

Carbon dioxide levels increase Ph becomes more acidic

haemoglobin changes shape (ionic bonds are changed)

More oxygen is dissociated

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Explain the advantage of the curve being on the le

High percentage saturation at low partial pressures of oxygen

LOw oxygen environments include High altitudes, Underwater, burrows, in the uterus 

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Explain the advantage of the curve being further l

Haemoglobin has a lower affinity for oxygen so release oxygen more readily to respiring cells.

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Hey guys heres some revision cards for Oxygen dissociation/gas exchange



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