Exchange and Transport
Single celled organisms
Absorption and release of gases by diffusion through their outer surface
Thin surface area and short diffusion pathway meaning their is no need for an exchange system
Fish
Water passes through the fishes mouth and passes through the gills
Gill filaments which are thin plates - they have a big surface area
Gill filaments are covered in lamellaes which also increases the surface area
Lamellaes are covered in blood capillaries and a thin surface area
In the lamellaes, blood flows through one way while water flows over the in the opposite direction (the counter-current system). It maintains a large concentration gradient between the water and the blood. Concentration of oxygen is always higher in the water than in the blood. Therefore, there is a constant flow of oxygen diffusing from the water into the blood.
Insects
Air filled pipes called tracheae
Air moves into the tracheae through pores in the surface called spiracles
Oxygen travels down the concentration gradient
Tracheae branch of into tracheoles which have thin, permeable walls. This means the oxygen diffuses straight into respiring cells
CO2 moves down its own concentration gradient towards the spiracles and is released into the atmosphere
Insects use rhythmic abdominal movements to more air in and out of spiracles
Plants
CO2 - photosynthesis O2 - respiration
The main gas exchange surface is the surface of the mesophyll cells (they have a large surface area)
Gas moves through the pores in the epidermis called stomata
Stomata can open to allow gas exchange but can close if the plant is losing too much water - guard cells control the opening and closing
Insects controlling exchange
If insects lose too much water, they can use their muscles to close the spiracles, they also have a waxy cuticle and tiny hairs which prevent evapouration
Plants controlling exchange
If the plants get dehydrated, the guard cells lose water and become flaccid, making them close
Xerophytes - waxy, waterproof cuticle to prevent evaporation, reduced number of stomata meaning less places for water to escape, curled leaves with stomata as protection from the wind, layer of hairs on epidermis to trap moist air around the stomata
Humans
Lungs
Air enters the trachea, splits into the two bronchi, then into the bronchioles, at the end of the bronchioles there is the alveoli which is where gas exchange occurs
The diaphragm, intercostal muscles and ribcage all work together to allow this process to happen
Inspiration
The external intercostal and diaphragm contracts, the ribcage moves upwards and outwards, diaphragm flattens and increases the volume of the thoracic cavity,
Active process - requires energy
as TC volume increases, lung volume decreases
Expiration
The external intercostal and the diaphragm relax, the ribcage moves downwards and inwards, diaphragm becomes curved again
TC volume decreases which increases air pressure
Air is forced down the pressure gradient
Expiration is a passive process unless it is forced
Alveoli gas exchange
Huge number of alveoli for big surface area, they are surrounded by a network of capillaries
Oxygen diffuses through the alveoli exthothelium and the capillary endothelium into the blood, CO2 diffuses into the alveoli to be breathed out
Alveoli are adapted to gas exchange as the exchange surface is only one cell thick so short diffusion pathway and there is a large surface area as there is millions of them in the lungs