A2. Gas Exchange Flashcards
Gas exchange surfaces
Gas exchange occurs over a gas exchange surface - a boundary between the outside environment and the internal environment of an organism. Organisms need oxygen and carbon dioxide to diffuse across gas exchange surfaces as quickly as possible. Most gas exchange surfaces have two things in common that increase the rate of diffusion: (2 + 1 things)
- They have a large surface area.
- They’re thin (often just one layer of epithelial cells)—this provides a short diffusion pathway across the gas exchange surface.
The organism also maintains a steep concentration gradient of gases across the exchange surface, which increases the rate of diffusion.
Gas exchange in single-celled organisms
What do single-celled organisms do?
How are they adapted for gas exchange and what does this mean?
Single-celled organisms absorb and release gases by diffusion through their cell-surface membranes. They have a relatively large surface area, a thin surface and a short diffusion pathway (oxygen can take part in biochemical reactions as soon as it diffuses into the cell) so there’s no need for a specialised gas exchange system.
Gas exchange in fish
Why are special adaptations needed?
There’s a lower concentration of oxygen in water than in air. So fish have special adaptations to get enough of it. In a fish, the gas exchange surface
is the gills.
Gas exchange in fish - Structure of gills
What happens in the gills?
How are adapted to function? (3 things)
Figure 2: A section of a fish’s gill.
Water, containing oxygen, enters the fish through its mouth and passes out through the gills.
- Each gill is made of lots of thin plates called gill filaments, which give a large surface area for exchange of gases (and so increase the rate of diffusion).
- The gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more.
- The lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and the blood.
Gas exchange in fish - The counter-current system in 3 steps
Figure 3: The counter-current system across a lamella.
- In the gills of a fish, blood flows through the lamellae in one direction and water flows over them in the opposite direction. This is called a counter-current system.
- The counter-current system means that the water with a relatively high oxygen concentration always flows next to blood with a lower concentration of oxygen.
- This in turn means that a steep concentration gradient is maintained between the water and the blood-so as much oxygen as possible diffuses from the water into the blood.
Gas exchange in dicotyledonous plants
Why do plants need gases and what are they used for?
What is the main gas exchange surface and are how are they adapted and what do they do?
What else can help with exchange of gases?
Figure 4: Structure of a dicotyledonous plant leaf.
- Plants need CO2 for photosynthesis, which produces O2 as a waste gas.
- They need O2 for respiration, which produces CO2 as a waste gas.
- The main gas exchange surface is the surface of the mesophyll cells in the leaf.
- They’re well adapted for their function - they have a large surface area.
- The mesophyll cells are inside the leaf.
- Gases move in and out through special pores in the epidermis (mostly the lower epidermis) called stomata (singular = stoma). The stomata can open to allow exchange of gases, and close if the plant is losing too much water. Guard cells control the opening and closing of stomata.
Exam Tip
In the exam it’s not enough to write that the counter-current system creates a steep concentration gradient - you need to say that the concentration gradient … to get the marks.
Exam Tip
In the exam it’s not enough to write that the counter-current system creates a steep concentration gradient -you need to say that the concentration, gradient is maintained over the whole length of the gill to get the marks.
Tip: The normal circulation of the fish replaces the _____________blood that leaves the gill with more ________________blood. The normal ________________of the fish ensures that _____ _____with a relatively high _________ concentration is taken in. Both of these help to maintain the steep concentration ___________.
Tip: The normal circulation of the fish replaces the oxygenated blood that leaves the gill with more deoxygenated blood. The normal ventilation of the fish ensures that more water with a relatively high oxygen concentration is taken in. Both of these help to maintain the steep concentration gradient.
Gas exchange in insects
What do insects use for gas exchange?
Where does air move in?
Carry on the process (4 steps)
What do insect use to move air in and out of…?
- Terrestrial insects have microscopic air-filled pipes called tracheae which they use for gas exchange.
1) Air moves into the tracheae through pores on the surface called spiracles.
2) Oxygen travels down the concentration gradient towards the cells.
3) The tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells. This means that oxygen diffuses directly into the respiring cells - the insect’s circulatory system doesn’t transport O2.
4) Carbon dioxide from the cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere.
- Insects use rhythmic abdominal movements to move air in and out of the spiracles.
Figure 7: Gas exchange across the tracheal system of an insect.
Control of water loss
Exchanging gases tends to make you lose water - there’s a sort of trade-off
between the two. Luckily for plants and insects though, they’ve evolved
adaptations to minimise water loss without reducing gas exchange too much. (4 adaptations)
- If insects are losing too much water, they close their spiracles using
muscles. - They also have a waterproof, waxy cuticle all over their body to reduce evaporation
- tiny hairs around their spiracles to reduce evaporation.
- If the plant starts to get dehydrated, the guard cells lose water and become flaccid, which closes the pore.
Examples of xerophytic adaptations include:
- Stomata sunk in pits to trap water vapour, reducing the concentration gradient of water between the leaf and the air. This reduces evaporation of water from the leaf.
- A layer of ‘hairs’ on the epidermis to trap water vapour round the stomata.
- A reduced number of stomata, so there are fewer places for water to escape.
- Thicker waxy, waterproof cuticles on leaves and stems to reduce evaporation.
- Curled leaves with the stomata inside, protecting them from wind (windy conditions increase the rate of diffusion and evaporation).
Figure 9: Adaptations of a xerophytic plant.
