Gas exchange

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What is gas exchange?
The exchange of substances between and organism and their environment
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Why does gas exchange require a mass transport system?
The majority of cells are too far away from exchange surfaces for diffusion alone to supply or remove their tissue fluid with the exchange substances
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Name 2 factors affecting the rate of exchange
Metabolic rate and size of organism
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What substances are often exchanged?
Respiratory gases, nutrients, heat and excretory products
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Define active transport
Movement of a substance from a region of low conc to a region of high conc. The process requires ATP (the expenditure of metabolic energy)
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Define diffusion
Movement of a substance from a region of high conc to a region of lower conc. Passive process
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What is facilitated diffusion?
Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances across plasma membranes
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Define osmosis
Movement of water from an area of higher water potential to a region of lower water potential through a selectively permeable membrane
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Name some features of specialised exchange surfaces
large SA:V ratio which increases rate of exchange as there is more surface for molecules to collide with, thin so short diffusion pathway, selectively permeable, movement of environmental medium to maintain diffusion gradient, transport system
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Why is diffusion not suitable to be the only method of substance transport?
It would take too long for substances to reach the middle of the organism if diffusion was the only method of trasport
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Why is a flattened shape of an organism helpful in gas exchange?
No cell is far from the surface
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What internal network do insects have for gas exchange?
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How are tracheae adapted for their function?
They are supported by strengthened rings to prevent them from collapsing
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How are gases transported to the tissues of the insect?
The tracheae divide into smaller tubes called tracheoles which extend throughout the body tissues of the ijsect
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What is good about this system?
The atmospheric air is brought directly to the respiring tissues so there is a short diffusion pathway from a tracheole to any body cell
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How do gases enter and exit the tracheae?
Tiny pores called spiracles on the body surface
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Why do insects keep spiracles closed most of the time?
When spiracles are open water vapour can escape so spiracles are kept closed most of the time to prevent water loss
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How do gases move in and out of the tracheae along a diffusion gradient?
When cells are respiring O2 is used up so its conc towards the ends of the tracheoles falls. This creates a diffusion gradient that causes O2 to diffuse from the atmosphere along the tracheae and tracheoles to the cells. CO2 is produced by the cont
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cells during respiration which creates a diffusion gradient along the opposite direction. This causes CO2 to diffuse along the tracheoles and tracheae from the cells to the atmosphere
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How do gases move in and out of tracheae by mass transport?
The contraction of muscles in insects can squeeze the tracheae enabling mass movements of air in and out. This further speeds up the exchange of respiratory gases
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How does water in the ends of tracheoles aid gas exchange?
During major activity cells respire anaerobically which produces lactate. Lactate is soluble and lowers the water potential of the muscle cells so water therefore moves into the cells from the tracheoles by osmosis. The water in the ends of the cont
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tracheoles decreases in volume and in doing so draws more air into them. This means that the final diffusion pathway is a gas rather than a liquid so the diffusion is more rapid. This increases the rate at which air is moved into the tracheoles cont
but leads to greater water evaporation
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Give a limitation of the tracheal system
Relies mostly on diffusion to exchange gases between the environment and the cells. For diffusion to be effective, the diffusion pathway needs to be short which is why insects are small. The length of the diffusion pathway limits the size of insects
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Why is the body surface of a fish not adequate to supply and remove respiratory gases?
Fish have a waterproof (and therefore gas-tight covering) and are also relatively large so have a small SA:V ratio
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What is the specialised exchange surface of the fish?
The gills
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Where are the gills located in the body of a fish?
Behind the head
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What are gills made up of?
Gill filaments which are stacked up in a pile
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What increases the surface area of gills?
Gill lamellae which are at right angles to the gill filaments
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How does water reach the gills?
Water is taken in through the mouth and is forced over the gills and out through an opening of the body
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What is countercurrent flow?
The flow of water over the gill lamellae and the flow of blood in opposite directions
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Why is countercurrent flow important?
Important for ensuring maximum possible gas exchange is achieved. If water and blood flowed in the same direction far less gas exchange would take place
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Explain the countercurrent arrangement
Blood that is already well loaded with oxygen meets water which has its max conc of O2. Therefore, diffusion of O2 from water into the blood takes place. Blood with little O2 in it meets water which has had most (but not all) of its O2 removed cont
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Again, diffusion of O2 from water to blood
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What adaptations do plant leaves have for gas exchange?
Many small pores (stomata) so no cell is far from a stomata which means there is a short diffusion pathway. Numerous interconnecting air spaces throughout the mesophyll so that gases can readily come into contact with the mesophyll cells. Large cont
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SA of mesophyll cells for rapid diffusion
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Where are stomata located?
Mainly on the underside of the leaf
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What is the function of the guard cells?
Can open and close the stomata pres which is important as they can control the rate of gaseous exchange as well as reducing water loss be evaporation
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What can cause guard cells to open or close?
Changing the water potential
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Why is water loss an issue for insects?
Water easily evaporates from their body surface as most insects are terrestrial so they can become dehydrated. Efficient gas exchange requires a thin, permeable surface with a large area and these features conflict with the need to conserve water.
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What adaptations to insects have to reduce water loss?
Small SA:V ratio to minimise the area over which water is lost. Waterproof coverings over the body surfaces. Insects have a rigid outer skeleton of chitin that is covered with a waterproof cuticle. Spiracles can close to prevent water loss
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Why can't plants have a small SA:V ratio?
Photosynthesis requires a large leaf surface area for the capture of light and for the exchange of gases
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What have xerophytes (plants with a restricted water supply) done to reduce water loss?
They have evolved to limit water loss via transpiration (evaporation of water from a plant)
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Explain how a thick/waxy cuticle can limit water loss
A waxy cuticle forms a waterproof barrier but the thicker the cuticle, the less water that can escape
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How does rolling up leaves reduce water loss?
Rolling leaves protect the lower epidermis (location of the stomata) as it helps trap a region of still air within the rolled leaf. The region becomes saturated with water vapour and so has a high water potential. There is no water potential cont
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gradient between the inside and outside of the leaf so therefore no water loss
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Explain how hairy leaves can reduce water loss
A thick layer of hair on the leaf traps still, moist air next to the leaf surface. The water potential gradient between the inside and outside of the leaf is reduced and therefore less water is lost by evaporation
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How does having pitted/grooved stomata limit water loss?
Traps still, moist air next to the leaf and reduce the water potential gradient
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Explain how a reduced SA:V ratio can help reduce water loss
Leaves that are small and circular in cross-section, rather than leaves that are broad and flat, the rate of water loss can be reduced. The reduction in SA is balanced against the need for a sufficient area for photosynthesis
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Why is the volume of oxygen that has to be absorbed and the volume of carbon dioxide that has to be removed large in mammals?
They are relatively large organisms with a large volume of living cells (so lots of O2 needs to be absorbed for respiration and CO2 removed as a product). Also, mammals maintain a high body temp which is linked to high respiratory/metabolic rate
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What protects the lungs?
The ribcage
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What are the lungs?
A pair of lobed structures made up of a series of tubules (bronchioles) which end in alveoli
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What is the trachea?
A flexible airway that is supported by c-shaped rings of cartilage. The cartilage prevents the trachea from collapsing when the air pressure inside falls when breathing in. The trachea walls are made up of muscle lined with goblet cells
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What are the bronchi?
2 divisions of the trachea, each leading to one lung. Produce mucus to trap dirt particles and have cilia that move mucus towards the throat. Supported by cartilage
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What are the bronchioles?
A series of branching subdivisions of the bronchi. Muscle walls allow them to constrict so that they can control the flow of air into and out of the alveoli
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Define inhalation
When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs
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Define exhalation
When the air pressure in the lungs is greater than that of the atmosphere and air is forced out of the lungs
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Breathing in is an active/passive process
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Describe the process of inhalation
The external intercostal muscles contract while the internal ones relax. The ribs are pulled upwards and outwards increasing the volume of the thorax. The diaphragm muscles contract causing it to flatten which also increases the vol. Increased cont
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vol results in a reduction of the pressure in the lungs. Atmospheric pressure is now greater than pulmonary pressure so air is forced into the lungs
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Describe the process of exhalation
The internal intercostal muscles contract while the external ones relax. Ribs move downwards and inwards deceasing the vol of the thorax. Diaphragm muscles relax so it is pushed up and vol further decreases. Decreased vol of thorax increases cont
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pressure in lungs. Pulmonary pressure now greater than that of the atmosphere so air is forced out of the lungs
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What occurs during normal breathing?
The recoil of the elastic tissue in the lungs is the main cause of air being forced out. Only under more strenuous conditions do the various muscles play a part
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What is the pulmonary ventilation rate?
The total volume of air that has moved in to the lungs in 1 minute
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What is the calculation for pulmonary ventilation rate?
Pulmonary ventilation rate (dm3min-1)=tidal volume (dm3)x breathing rate (min-1)
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What is the tidal volume?
The volume of air taken in at each breath when the body is at rest (usually 0.5 dm3)
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What is the breathing rate?
The number of breaths taken in 1 minute (normally about 15-20 in a healthy adult)
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What are alveoli covered in?
A network of capillaries
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Why is the diffusion of gases between the alveoli and blood very rapid?
RBCs are slowed as they pass through pulmonary capillaries allowing more time for diffusion. The distance between the alveolar air and the RBCs is reduced as the RBCs are flattened against the capillary walls. The walls of the alveoli and cont
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capillaries are very thin so there is a short diffusion pathway. Alveoli and capillaries have a very large total SA. Constant ventilation of the lungs and circulation of blood around the alveoli ensure a steep conc gradient of the gases to be cont
exchanged. Blood flow through the pulmonary capillaries maintains a conc gradient
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Other cards in this set

Card 2


Why does gas exchange require a mass transport system?


The majority of cells are too far away from exchange surfaces for diffusion alone to supply or remove their tissue fluid with the exchange substances

Card 3


Name 2 factors affecting the rate of exchange


Preview of the front of card 3

Card 4


What substances are often exchanged?


Preview of the front of card 4

Card 5


Define active transport


Preview of the front of card 5
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well good :D

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