Exploring Gas Exchange in Different Organisms

Exploring Gas Exchange in Different Organisms
Gas exchange in mesophytes Gases enter and leave plants mainly through the stomata in the leaves and the lenticles in stems. The main gas exchange surface is the spongy mesophyll layer found inside the leaves.. The spongy mesophyll cells are loosely packed thus increasing the surface area for gas exchange. Gases are needed for both respiration and photosynthesis. The spongy mesophyll layer is specially adapted for gas exchange. The cells are coated in a layer of water in which gases can dissolve. The cell walls are thin so reducing the distance for diffusion of gases into the cell.
The many widely spaced cells present a large surface area of gas exchange, easily reached by air entering through the stomata. A concentration gradient is maintained due to the rapid diffusion of gases in air. Air enters through the stomata by diffusion; there is no ventilation process. At night, when only respiration is taking place oxygen readily dissolves and diffuses through the walls of the spongy mesophyll cells and is transported along to other cells by diffusion. This maintains a lower concentration of oxygen in the spongy mesophyll cells than is present in the air in the leaf. This difference in concentration ensures continued diffusion of oxygen into the spongy mesophyll cells Carbon dioxide produced during respiration diffuses from the spongy mesophyll leaf cells, where it is present at a higher concentration to the air spaces in the leaf where carbon dioxide is at a lower concentration. Carbon dioxide diffuses and passes out through the stomata by diffusion to the even lower concentration of carbon dioxide in the environment. During the day both respiration and photosynthesis are taking place. The plant needs oxygen for respiration and carbon dioxide for photosynthesis. In bright light, the rate of photosynthesis is greater than the rate of respiration. Therefore carbon dioxide is needed in larger quantities than is produced in respiration. A difference in concentration exists with a higher concentration of carbon dioxide in the leaf air spaces and a lower concentration in the spongy mesophyll cells. Therefore carbon dioxide dissolves and diffuses through the walls of the spongy mesophyll from the leaf air space and into the palisade cells in particular for use in photosynthesis. This keeps the concentration of carbon dioxide low in the palisade and spongy mesophyll cells. Thus a difference in concentration exists with a higher concentration of carbon dioxide in the leaf air spaces and a lower concentration in the spongy mesophyll cells. Constant usage of the carbon dioxide will maintain this difference in concentration and diffusion of carbon dioxide into the cells will continue. Oxygen will be produced in photosynthesis in larger quantities than is required for respiration. The excess oxygen will diffuse through to the spongy mesophyll cells. As a result the concentration of oxygen in these cells will be higher than the concentration in the leaf air space. Oxygen will therefore diffuse out to the lower concentration in the leaf air space, and across the narrow leaf to the stomata and out as the outside concentration of oxygen will be even lower. The removal of oxygen maintains a difference in concentration and ensures the process of diffusion continues. The spongy mesophyll is well adapted as a gas exchange surface. It is permeable to gases and it presents a large surface area for gas exchange. The diffusion of air into and out of the stomata maintains a difference in concentration or diffusion gradient, so allowing continued diffusion. [image] gas exchange IN insects In common with all arthropods insects have an outer skeleton called an exoskeleton. The mammalian skeleton, found on the inside, is called an endoskeleton. The exoskeleton made of chitin is tough light and allows movement. It is covered with a waxy waterproof cuticle in order to reduce water loss. Gas exchange surfaces are moist so an internal respiratory surface is essential to prevent excessive water loss. Insects are air breathers gaining oxygen from the air. Air enters through holes in the exoskeleton called spiracles located on both sides of the thorax and abdomen. Spiracles lead to a system of tubes extending into the insect. Larger tubes are enclosed by rings of chitin.,Chitin is impermeable, so gas exchange cannot take place in the larger tubes. These tubes branch extensively and form narrow tubes called tracheoles which run close to all the cells in the insect. The tracheoles branch into tiny tubes 0.7 um in diameter, some of which actually penetrate cells. No chitin is present in these tubes and they are freely permeable to gases. The tracheoles extend from the outside deep into the insect, so supplying oxygen directly to the tissues. Blood is not used to transport respiratory gases. Similarly carbon dioxide is removed through the tracheoles. The walls of the tracheoles are the gas exchange surface [image] How insects achieve efficient gas exchange Large surface area
Extensive system of branching tracheoles
Withdrawal of fluid in tracheoles during exercise, further
increasing surface area for gas exchange.
Maintaining a diffusion gradient
Use of oxygen by cells in respiration keeps the concentration low
in cells
Production of carbon dioxide by cells increases the concentration
assisting its diffusion out of the insect
Some insects have a ventilation system which increases the
concentration gradient
Thickness of membrane (diffusion distance) A short diffusion is achieved by:
Thin tracheole walls
A lack of chitin, increasing permeability
Withdrawal of fluid enabling air to reach closer to ore cells so
reducing diffusion distance.
Specialised gaseous exchange surfaces In fishes gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. A gill has two rows of gill filaments with lamellae (gill plates)Gill arches are present in bony fish which are supports to which gill filaments are attached.Two rows of gill filaments are attached to each gill arch forming a V shape. The following are the ways in which gas exchange take place. The large surface area for gas exchange is achieved by:
Each gill having many filaments
Each filament having many lamellae
A countercurrent mechanism enabling all the lamellae to be used
for gas exchange as equilibrium is not reached
Many blood capillaries maximising the surface for gas exchange
All these features increase the surface area for gas exchange The thickness of the exchange surface is minimised by:
The lamellae having walls only one cell thick (squamous
epithelium)
The capillary wall being only one cell thick (squamous epithelium)
Blood being close to the surface of the plate
Gill plates being very thin so water is close to the blood.
All these features reduce the diffusion distance, so increasing the rate of diffusion A difference in concentration is maintained by:
The constant flow of aerated water over the lamella; this is
caused by the ventilation mechanism
The constant movement of blood removing oxygen from and bringing
carbon dioxide to the gills
The countercurrent mechanism maintaining a difference in
concentration between the water and blood by preventing
equilibrium being reached.
These features ensure that a difference in concentration is maintained over the whole lamella increasing the rate of diffusion (Fick?s Law). [image] Gas exchange in the alveoli The membrane around each alveolus is composed of a thin layer, one cell thick. These thin flat cells are called squamous or pavement epithelia. They form an efficient exchange surface and allow a rapid rate of diffusion due to the reduced distance for gases to travel. Gases can diffuse across the moist, permeable membrane of the alveoli. Capillaries closely surround each lobe or ?grape? of the alveolus, and the capillary wall is also only one cell thick. Therefore gases only have to diffuse across two thin cells between the alveolus and the blood. The short diffusion distance speeds up the rate of diffusion. Oxygen Oxygen dissolves in the moisture lining of the alveoli and then diffuses through the squamous epithelium of the alveolus and capillary into the blood. The blood carries oxygen away from the alveoli, so keeping the partial pressure of oxygen in the blood low. Gases diffuse from the higher partial pressure of the gas to an area with a lower partial pressure. The rate of diffusion depends on the difference in partial pressure between the air and the blood. In the blood, oxygen continues to diffuse through the plasma and into the red blood cells, where it combines with haemoglobin to form oxyhaemoglobin. Carbon dioxide The higher partial pressure of carbon dioxide in the blood capillaries causes diffusion of carbon dioxide in solution across to the alveolus where the partial pressure is lower. Carbon dioxide diffuses across the squamous epithelium of the bollod capillary and alveolus, into the alveoli. From here diffusion of carbon dioxide continues into the tidal air where the concentration of carbon dioxide is lower, and air with the extra carbon dioxide is breathed out. Rate of diffusion The rate of diffusion depends on the surface area, the difference in partial pressure and thickness of the exchange surface. The surface area is increased by:
The large number of alveoli
The difference in partial pressure is maintained by:
The attraction of haemoglobin for oxygen
The removal of oxygen by the blood, so keeping the partial
pressure low near the alveoli
Ventilation providing a constant supply of fresh air.
These factors all maintain a difference in the partial pressure of gases between the alveoli and the blood. The thickness of the exchange surface is minimised by the following: ? The walls of the alveoli are made of thin squamous epithelia ? The walls of the capillary are also made of squamous epithelia and are very thin ? The capillary is very narrow and close to the alveoli, so red blood cells are forced close to the alveolar wall.

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