What are the two ways substances are interchanged in organisms?
Passively (no energy required) by diffusion and osmosis
Actively (energy required) by active transport.
What does the SA:volume ratio need to be like in an organism for efficient exchange?
SA must be large compared to volume.
How have larger organisms evolved to overcome the problem of a smaller SA:volume ratio? (2)
-flattened shape so that no cell is ever far from the surface.
-Specialised exchange surface with large areas to increase SA:volume ratio.
What are the 5 features of a specialised exchange surface?
-Large SA:vol ratio to increase rate of exchange.
-v. thin so that diffusion distance is short so rapid exchange.
-Partially permeable to allow selected materials to cross.
-movement of environmental medium (e.g. air) to maintain a diffusion gradient.
-movement of internal medium (e.g. blood) to maintain a diffusion gradient.
What is Fick's Law?
Diffusion∝ surface area X difference in concentration/ length of diffusion path
How is gas exchanged in SINGLE CELLED ORGANISMS?
Small so large SA:vol ratio. O2 absorbed by diffusion across their body surface, which is covered only by a cell surface membrane. CO2 released in the same way. If surrounded by cell wall- completely permeable so no barrier.
What is the problem for all terrestrial organisms?
water easily evaporates from the surface of their bodies and they can become dehydrated. HOWEVER Efficient gas exchange requires thin, permeable membrane with large area- conflict with need to conserve water. Has to balance opposing forces.
How do terrestrial organisms reduce water loss? (3)Specifically insects.
-Waterproof coverings over body surface
- Insects: rigid outer skeleton covered in a waterproof cuticle.
-Small SA:volume ratio- minimise area over which water is lost.
Describe the gas exchange system in insects.
Gas enters and leaves via tiny pores called spiracles on the body surface- can be opened and closed by a valve.
Internal network of tubes called tracheae- supported by strengthened rings to prevent them from collapsing.
Tracheae divide into smaller tubes called tracheoles which extend throughout all the body tissues of the insect. So air is brought directly to respiring tissues.
How do spiracles work?
When open water can evaporate from the insect- usually closed to prevent water loss. Opened periodically to allow gas exchange.
What are the two ways that respiratory gases move in and out of the tracheal system?
-ALONG DIFFUSION GRADIENT-
O2 conc. towards end of tracheoles in low because used up by respiring cells.
Creates diffusion gradient that causes O2 to diffuse from atmosphere along trachea and tracheoles to the cells. CO2 produced by cells during respiration.
Creates diffusion gradient in opposite direction.
Causes gaseous CO2 to diffuse along tracheoles to tracheae from cells to atmosphere.
Diffusion in air much quicker than in water so gases exchange more quickly.
-VENTILATION- The movement in muscles in insects create mass movements of air in and out of the tracheae, further speeding up exchange.
What are the limitations of the insect gas exchange system?
-Relies mostly on diffusion to exchange gases between environment and cells. For diffusion to be effective diffusion pathway needs to be short SO limits the size insects can be.
Describe the structure of gills.
Within the body of fish.
Made up of gill filaments- stacked up in a pile. At right angles to filaments are gill lamellae- increase SA of the gills.
Water taken in through mouth , forced over gills and leaves through opening on each side of body.
What is countercurrent flow?
Flow of water over gill lamellae and flow of blood within gill lamellae are in opposite directions.
How does countercurrent flow ensure maximum gas exchange?
-Blood already well loaded with O2 meets water, which has it's maximum conc. O2. Therefore diffusion of O2 from water to blood takes place.
-Blood with little or no oxygen meets water which has had most, but not all, of its oxygen removed. Diffusion takes place from water to blood. Therefore fairly constant rate of diffusion across entire length of gill lamellae. Around 80% absorbed.
What would happen if blood and water flowed in the same direction across gill lamellae? (Parallel flow)
Diffusion gradient would only be maintained across part of the length of the gill lamellae so only 50% of O2 would be absorbed into blood.