3.2 Gas exchange Flashcards
What are the adaptations for gas exchange across the body surface of a single-celled organism?
Large surface area relative to volume;
Short diffusion distance (across the plasma membrane and cell wall);
What are the adaptations for gas exchange in the tracheal system of an insect?
Trachaea extend from openings in the insects outer surface (‘spiracles’) all the way into the insects body, branching as they go. The finer branches are called tracheoles, and they are in direct contact with the respiring cells. The tracheoles ARE the gas exchange surface.
The many branches of these tracheoles provide a large surface area (improving the surface area to volume ratio OF THE INSECT), and their walls are thin, providing a short diffusion pathway.
Some insects exhibit abdominal pumping, which serves to push stale air out of the tracheal system and draw fresh air in (VENTILATING it) helping to maintain a concentration gradient between the air in the tracheoles and the respiring cells. Insects that can’t do abdominal pumping just have to rely on normal passive diffusion between the air in the tracheal system and the air outside.
Insects DO NOT HAVE a circulatory system, so have no bloodflow that could’ve contributed to this gradient.
In some insects spiracles can be closed to avoid water loss, but when closed ventilation stops.
The fluid in the tracheoles is harder to get your head around. Either ignore it or come ask Mr Maunder to explain it to you.
What are the adaptations for gas exchange in the leaves of dicotyledonous plants?
First don’t freak out about ‘dicotyledonous’ - it’s mentioned more as a guide for teachers than students.
The interior of the leaf has air spaces. Mesophyll cells exchange gases directly with this air, so are more like single-celled organisms than they are like insects or fish. The leaf has stomata, and all plants can close these in response to TOO MUCH water loss (but be careful - plants deliberately allow OVER 90% of the water they absorb from the soil to evaporate by transpiration out of their leaves). Leaves themselves have large surface areas, but this is more about harvesting sunlight than gas exchange.
What are the adaptations for gas exchange in the xerophytic plants?
(6) (plus one extra for Y13s)
Xerophytes are plants that are adapted to dry conditions. That might mean hot, arid places, or it might mean places where the water is present but frozen for much of the year.
- Stomata in pits/grooves to trap water vapour, and so decrease the water potential gradient between the air spaces and the air outside.
- Hairs in those pits, or on the leaf surface, to trap water vapour and so decrease the water potential gradient between the air spaces and the air outside.
- Thick cuticle to increases diffusion distance, and so reduce transpiration.
- Rolled/folded/curled leaves to trap water vapour and so decrease the water potential gradient between the air spaces and the air outside.
- Leaves have evolved into spines/needles to reduce the surface area to volume ratio of the leaf and so of the plant itself.
- Alternative metabolic pathways for photosynthesis that avoid the need to open stomata during the day. These pathways involve opening the stomata at night and reacting carbon dioxide with something other than RuBP. The Calvin Cycle needs products from the light dependent reaction and so can’t happen at night, but if the plant reacts carbon dioxide with something ELSE at night, and then, during the day while its stomata are closed, it can REVERSE that reaction, and release carbon dioxide directly into the mesophyll cells without the need to risk losing water through its stomata. Several of these alternative pathways for fixing carbon have evolved.
The student then compared the rate of transpiration from the two species of plant. She did this by measuring the rate of water uptake by each plant species.
- What is transpiration?
- Suggest two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.
- Evaporation of water from the aerial parts of the plant
- Water used for turgidity, water is used in photolysis during the light dependent stage of photosynthesis, water is used for hydrolysis, and water is PRODUCED during condensation reactions, and during aerobic respiration.
What are the adaptations for gas exchange in the gills of fish?
There are many gill filaments, each covered in many lamellae, which provides a large surface area.
The lamellae are thin so there is a short diffusion pathway.
Water flowing between the lamellae is travelling in the opposite direction to the blood flow in the lamellae - described as counter current. This means blood is always passing water with a higher oxygen concentration, so the concentration gradient is maintained along the full length of the gill lamella. If water and blood flowed in same direction equilibrium would be reached.
Name the components of the gross structure of the human gas exchange system, in the order they are encounter by inhaled air.
Trachea
Bronchus
Bronchiole
Alveolus
What are the adaptations for gas exchange in the lungs?
The walls of the alveolar epithelium itself themselves are composed of squamous epithelial cells - these cells are very thin, providing a short diffusion pathway.
There are many alveoli so the total surface area is large, increasing the rate of diffusion.
There are elastic fibres in the walls of the alveoli (made using a protein called elastin) - although these make the alveoli harder to inflate, the recoil they provide helps to expel air.
The intercostal muscles and diaphragm allow ventilation to occur - replacing stale air with fresh air and so maintaining the concentration gradient between air and blood.
There is a network of many capillaries running through the lungs, so that each alveolus is very close to deoxygenated blood in pulmonary circulation. This helps to maintain the concentration gradient between air and blood.
Describe the mechanism of breathing in the lungs.
Breathing in:
Diaphragm contracts, which flattens (lowers) it.
External intercostal muscles contract, lifting ribcage up and out.
This causes the thoracic volume to increase, which lowers the pressure in the thoracic cavity (space in side your ribcage).
When the thoracic pressure is lower than the atmospheric pressure, air will move in, down this pressure gradient
Breathing out:
Diaphragm relaxes, which raises (curves) it, and external intercostal muscles relax.
Internal intercostal muscles (might) contract (if you’re blowing out).
Elastic fibres in lung tissue recoil.
This causes the thoracic volume to decrease, which raises the pressure in the thoracic cavity above atmospheric pressure, resulting in air moving out down the pressure gradient.
