Exchange between Organisms - Gas Exchange Flashcards
How is gas exchanged in single-celled organisms?
Single-celled organisms are small and therefore have a large surface area to volume ratio. Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrane. In the same way, carbon dioxide from respiration diffuses out across their body surface. There is a thin surface membrane and a short diffusion pathway. Where a living cell is surrounded by a cell wall, this is no additional barrier to the diffusion of gases.
How does conserving water conflict with the increase in surface area?
As with all terrestrial organisms, insects have evolved mechanisms to conserve water. The increase in surface area required for gas exchange conflicts with conserving water because water will evaporate from it.
What are tracheae?
For gas exchange, insects have evolved an internal network of tubes called tracheae. The tracheae are supported by strengthened rings of chitin to prevent them from collapsing.
What are tracheae divided into?
The tracheae divide into smaller dead-end tubes called tracheoles (these aren’t lined with rings of chitin). The tracheoles extend throughout all the body tissues of the insect.
How is air brought directly to the respiring tissues?
The tracheoles (tracheal tubes) extend throughout all the body tissues of the insect. In this way, atmospheric air, with the oxygen it contains, is brought directly to the respiring tissues, as there is a short diffusion pathway from a tracheole to any body cell. So gases are transported directly between the external environment and the body cells.
What are the three ways respiratory gases move in and out of the tracheal system?
- along a diffusion gradient
- mass transport
- the ends of the tracheoles are filled with water
How do respiratory gases move in and out of the tracheal system along a diffusion gradient?
When cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls. This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells. Carbon dioxide is produced by cells during respiration. This creates a diffusion gradient in the opposite direction. This causes gaseous carbon dioxide to diffuse along the tracheoles and tracheae from the cells to the atmosphere. As diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly by this method.
How do respiratory gases move in and out of the tracheal system by mass transport?
The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out. This further speeds up the exchange of respiratory gases.
How do respiratory gases move in and out of the tracheal system by the ends of the tracheoles filling with water?
During periods of major activity, the muscle cells around the tracheoles carry out some anaerobic respiration. This produces lactate, which is soluble and lowers the water potential of the muscle cells. Water therefore moves into the cells from the tracheoles by osmosis. The water in the ends of the tracheoles decreases in volume and in doing so draws air further into them. This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid. This increases the rate at which air is moved in the tracheoles but leads to greater water evaporation.
Why is the diffusion pathway short in insects?
Every cell of an insect is only a very short distance from one of the tracheae or tracheoles and so the diffusion pathway is always short.
What are spiracles?
Gases enter and leave tracheae through tiny pores in the exoskeleton, called spiracles, on the body surface. The spiracles may be opened and closed by a valve. When the spiracles are open, water vapour can evaporate from the insect. For much of the time, insects keep their spiracles closed to prevent this water loss. Periodically they open the spiracles to allow gas exchange.
What are the limitations of the tracheal system?
The tracheal system is an efficient method of gas exchange. It 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 of a small size. As a result, the length of the diffusion pathway limits the size that insects can attain.
How are insects adapted to conserve water?
Insects have an exoskeleton:
- composed of a hard fibrous material called chitin (for protection)
- covered by a lipid-rich layer (to prevent water loss)
The spiracles contain valves that can close in order to prevent water loss, although this also limits gas exchange. The tiny hairs surrounding the spiracles also help to trap humid air reducing the concentration gradient of water vapour which reduces water loss.
What is the anatomy of the insect gas exchange system?
- Each segment of the insect (apart from the head) has a pair of openings called spiracles.
- Tracheal tubes connected to each spiracle branch into a series of tracheoles.
- Tracheoles repeatedly divide until their numerous microscopic ends penetrate into individual body cells.
How do oxygen and carbon dioxide diffuse across a concentration gradient?
- Oxygen moves down a concentration gradient from the air into body cells.
- Carbon dioxide moves down a concentration gradient from body cells into the air.
How is a concentration gradient maintained?
Rhythmic contractions of abdominal muscles compress air sacs increasing ventilation which helps to maintain a concentration gradient during vigorous activity.
What is the fluid/gas interface?
- When the insect is at rest, tracheal fluid fills the end of the tracheoles.
- It is where the fluid and gas meet (the fluid/gas interface), that exchange of gases occurs (oxygen is taken up, carbon dioxide is given off).
- As activity increases, the fluid is removed from the tracheoles.
- This means that the exchange of gases occurs nearer the cells.
- In the extreme case of fatigued flight muscle, the exchange interface lies within the muscle cells.
What is the difference between insects at rest and at flight?
When the insect is at rest, tracheal fluid seeps into the tracheoles from the surrounding cells and fills the end of it.
When insects are flying, the water diffuses into the muscle:
- The muscles draw up the tracheal fluid which:
- provides them with oxygen-containing fluid for respiration
- lowers the pressure in the tracheoles which draws more air in through the spiracles from the outside
- increases the surface area available for oxygen to diffuse through tracheal walls directly - Muscle cells respire anaerobically and produce lactic acid.
- Lowers the water potential in muscle cells.
- Water moves by osmosis from the tracheoles into the muscle cells.
- Reduces the distance for diffusion so diffusion of oxygen much faster.
How are the respiratory demands of most insects met?
- Diffusion alone does not meet the respiratory demands in most insects.
- In slightly larger insects gases are moved, to a large extent, by pumping actions of the body segments.
- The respiratory system of an insect is very efficient for small organisms.
- As body size increases, the efficiency decreases.
- When body diameter exceeds about 3cm, the respiratory needs cannot be met.
- Hence it is the respiratory system of insects which restricts their body size.
What are air sacs?
Some larger species (e.g. locusts) have special collapsible tracheae called air sacs:
- Inflated and deflated by ventilation movements of the abdomen → draws air into/out of the tracheal system.
- Movements increase with increased levels of activity.
- Increases concentration gradients.
What is a specialised breathing mechanism in some insects?
- Some insects have evolved a more specialized breathing mechanism.
- Firstly their abdomen is expanded by different muscles and this closes some of the spiracles at the back of their body whilst opening those at the front.
- Oxygen enters at the front spiracles at the front.
- When they contract their abdomen, they open up the spiracles at their rear and close those at the front.
- Spiracles at the back let carbon dioxide out.
- It’s an alternating opening of back spiracles or front spiracles that drives oxygen in and then carbon dioxide out and this is a specialized breathing.
What are insect exoskeletons covered in to prevent them losing water?
The outer cuticular layer (epicuticle) is a protein-polyphenol complex made up of lipoproteins, fatty acids, and waxy molecules, and is the insect’s primary defence against water loss.
What is the countercurrent system?
A mechanism by which the efficiency of exchange between two substances is increased by having them flowing in opposite directions.
What is the exoskeleton?
The external skeleton that supports and protects an animal’s body. It also helps with water retention, but it means that waxy exoskeletons don’t allow for effective gas exchange. It is found in insects and some other invertebrates.
What are filaments?
A slender thread-like structure found in high numbers extending from fish gill arches.
What are lamellae?
Thin membranous structures found in high numbers lining fish gill filaments. Each lamellae consists of a network of capillaries covered by a single layer of epithelial cells.
Why do insects need a tracheal system?
Because they don’t have lungs and their waxy exoskeletons don’t allow for effective gas exchange. To overcome this issue, insects evolved an exchange surface - called a tracheal system - that delivers oxygen directly to every tissue in the body.
Where does gas exchange in the tracheal system occur?
It occurs mostly through the tracheal fluid at the ends of the tracheoles.
How is the tracheal system ventilated?
Larger insects are able to ventilate their tracheal system and have evolved several different mechanisms for doing so.
- Air sacs in the tracheal system can be squeezed by flight muscles to push air in and out.
- Flight muscles can alter the volume of the insect thorax (or chest cavity) to ventilate the tracheal systems.
What specialised internal gas exchange surface have fish evolved? Why?
Fish have a waterproof, and therefore a gas-tight, outer covering. This impermeable membrane means that gases can’t diffuse through their skin. Being relatively large, they also have a small surface area to volume ratio. Their body surface is therefore not adequate to supply and remove their respiratory gases and so they have evolved a specialised internal gas exchange surface: the gills.
What is the structure of the gills?
The gills are located in a cavity within the body of the fish, on the sides behind the head. Four pairs of gill arches are on each side (gill arches are a series of bony “loops” present in fish that support the gills). On each arch are two series of stacked gill filaments in a pile splayed out as a V. At right angles to the filaments are gill lamellae, which are disc-like projections on the upper and lower surfaces of each gill filament. They increase the surface area of the gills. Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.
