Adaptations Of Gas Exchange

• When a cell increases in size, the diffusion pathway becomes longer. This means that the diffusion from the outer cell surface to the inner cell is longer. 
• When the cell becomes larger, diffusion won't be able to meet the cells needs such as :
  • Supplying nutrients such as Glucose and Oxygen. 
  • Removing waste such as Urea and Carbon Dioxide

• Gas Exchange is the process by which Oxygen reaches cells and Carbon Dioxide is removed from them
• Unicellular organisms have a large surface area to volume ratio.
• Gas exchange occurs across the whole surface.
• The permeable membrane allows gases to diffuse.
• Special organs are not required as diffusion is sufficient to meet the oxygen requirements of the organism.

• Flatworms have adapted a flattened shape to overcome the problem of an increase in size. 
• This increases their surface area to volume ratio therefore no cell in the body is far from the surface meaning there is no need for specialised gas exchange organs. 
• They exchange gases directly with the environment via diffusion. 
• Diffusion across the permeable membrane meets the oxygen requirements of the organism. 

• Earthworms have developed a tubular shape and is restricted to damp enviroments.
• They secrete mucus to keep the body surface cells moist. 
• This allows gases to diffuse and dissolve
• The elongated shape provides a large surface area to volume ratio compared with organisms of similar volume. 
• They exchange gases directly with the environment via diffusion across the moist surface
• Blood vessels are close to the body surface so gases can diffuse in and out of the blood across the body surface. 

• As size increases surface area to volume ratio decreases.
• Diffusion across the body surface is insufficient to provide enough oxygen to survive. 
• Large organisms are more metabolically active meaning so they require more oxygen.
• Diffusion pathways are too large so rate of diffusion is very slow. This means that they need a specialised gas exchange surface and a method of circulation. 
• Many animals have a toughened body surface so they have internal gas exchange surfaces.
• Blood circulates in the vessels, this maintains a concentration gradient for the diffusion of oxygen into the cell and carbon dioxide out of the cells. 
• Blood contains the respiratory pigment haemoglobin to carry oxygen to body cells. 

• Many animals and plants have specialised gas exchange surfaces so tat gas exchnage is rapid and efficient. 
• Gill lamella in fish, Alveoli in Lungs and Tracheoles are examples. 
• To macimise the rate of diffusion, these surfaces nust contain 
  • Large SA:V ratio. 
  • Thin so diffusion pathways are short.
  • Partially permeable. 
  • Moist to allow gases to disolve and diffuse. 

• Insects fly so they need alot of energy so they need a good supply of oxygen.
• Air diffuses into the insect through paired holes called spiracles running along each side of the body.
• The spiracles lead to a system of branched, chitin-lined air tubes called Trachae. 
• Spiracles can open and close to allow oxygen and carbon dioxide to enter and leave. 
• When resting, diffusion meets oxygen demands, whereas during flight, abdomen movement ventilates the trachae.

• The Trachae branch repeatedly until they end as very fine, thin walled, Tracheoles. 
• Oxygen diffuses from the ends of tracheoles into the cells and Carbon Dioxide out of the cells and into the tracheoles. 
• Ends of tracheoles are fluid filled and close to the muscle fibres. The fluid is where gas exchange takes place. 
• This fluid provides a moist surface for oxygen to dissolve and when muscles contract fluid is drawn to the muscle cells. 

• Oxygen is supplied directly to the tissues. 
• No respiratory pigment is required. 
• Oxygen diffuses faster in the air than blood.
• Spiracles close in order to reduce water loss. 
• Chitin is a structural feature which allows the trachae to remain open. 
• Gas exchange organs are inside the body of a terrestrial organism as it reduces oxygen and water loss. It is protected by ribs and exoskeletons in insects. 

• Fish can be caategorised into 2 groups determined by the material that makes up the skeleton and their gill ventilation mechanisms. 
• There are many problems caused by living in water:
  • Water contains less oxygen than air.
  • Rate of diffusion is slower in water than in air
  • Water is a dense medium so it doesnt flow as freely

• They have a skeleton made entirely of cartilage. 
• Nearly all live in Sea water
• Just behind the head are 5 Gill Clefts which open at Gill Slits. 
• Water is taken in by the mouth and forced through the Gill Slits when the floor of the mouth is raised. 
• Gas exchange involnves parralell flow. This means that blood circulates the same direction as water travels over the gills. 

• These types of fish have an internal skeleton made up of bones. 
• The gills are covered by a flap called the operculum. 
• Gas exchaneg involves counter current flow. This means that blood in the gill capillaries circulates in the opposite direction as water flowing over the gills. 

• The head of a bony fish with the operculum pulled back will reveal gills. There are usually 4 on each side. 
• The flap covering the goll is called the operculum and these enclose the gill arches. 
• Water is taken in through the mouth, passes over the gills and is expelled out the operculum. 
• Movements of the buccal cavity floor and operculum allow a one way current of water to flow through the gills for a gas exchange of Oxygen and Carbon Dioxide.

• Along each gill arch there are many filaments and on these there are gill lamellae. 
• The gill filaments have a large surface area for gas exchange
• Blood circulates through the gill lamellae creating a concentration gradient. Oxygen diffuses through the gill lamellae into the capillaries and carbon dioxide diffuses out into the water. 

• Blood always meets oxygen at a higher concentration. 
• The gradient for diffusion of oxygen into the blood from the water is maintaines over the whole length of the gill lamellae. 
• Counter current flow is more efficient as it results in a higher oxygen saturation level. 

• Water is taken into teh mouth and blood is flowing through the gill capillaries in the same direction as the water. 
• Gas exchange is very effiecent at first as there is a very steep concentration gradient. 
• However halfway along the gill lamellae, equilibrium is reached and diffusion of Oxygen and Carbon Dioxide is no longer possible.

• When water enters the fish, the mouth opens and the operculum closes. The buccal cavity lowers and volume inside the buccal cavity increases. However pressure decreases.
• When water leaves the fish the mouth is closed and the operculum opens. The buccal cavity rises and volume inside the buccal cavity decreases. Therefore pressure increases. 

• Vertebrates include five classes: 
  • Amphibians, Reptiles, Birds, Fish and Mammals
• Life is thought to have evolved in water with animals adapting in order to colonise the land.

Amphibians

• Tadpoles live in water so they have gills for gas exchange. 
• In adults, when resting diffusion occurs across the moist skin. When active it happens at the lungs. 

• During Inspiration muscles contract meaning ribs move upwards and out.
• The outer pleural membrane is pulled outwards, reducing the pressure in the pleural cavity.
• The inner pleural membrane is pulled outwards and the lung surface is drawn out causing alveoli to fully expand.
• Pressure in the alveoli is lower than atmospheric pressure so the air moves into the alveoli.

• During expiration muscles relax meaning the ribs move downwards and inwards. 
• The outer pleural membrane returns inwards, increasing the pressure in the pleural cavity. 
• The inner pleural membrane returns inwards. 
• The lungs retract so the alveoli deflate. 
• The alveolar pressure is higher than the atmospheric pressure so the air moves out. 

• During inspiration the diaphragm flattens and the rib cage expands pulling on the outer pleural membrane which lowers pressure in the pleural cavity. 
• The inner pleural membrane pulls on the lungs which increases the volume of the lungs 
• This decreases the pressure in the alveoli. 
• Alveolar pressure is lower than atmospheric pressure so air moves in. 

Alveoli are suited for gas exchange because:

• Alveoli have a large surface area for gas diffusion

• Moist so that gases can disolve and diffuse easily
• Permeable so oxygen and carbon dioxide can diffuse
• One call thick to provide a short diffusion pathway. 
• Extensive capillary network provides blood circulation with a maintained concentration gradient for CO₂ and O₂ diffusion. 

• The leaf blade is flat so that the diffusion pathway is short and SA:V is also increased. 
• The spongy mesophyll is permeated with air spaces to allow diffusion and circulation of gases which maintains concentration gradients. 
• Mesophyll walls are moist to allow dissolving and diffusion of gases. 
• Stomata pores allow exchange of oxygen and carbon dioxide. 

• Large SA to absorb as much light as possible. 
• Leaves can orientate themselves towards sunlight. 
• Leavs are thin to allow light to reach lower levels. 
• Cuticle and epidermis are transparent so light can pass to the mesophyll below. 
• Palisade cells are long and densely packed and have many chloroplasts. 
• Chloroplasts rotate and move in the mesophyll to maximise light absorption. 
• Intercellular air spaces allow CO2 to diffuse into the cells and O2 to diffuse out of the cells. 

• The tomata is found at the lower epidermis. 
• Their role is to allow water and gases to pass through. 
• The wicth of the pores can change to control gas exchange between atmosphere and internal tissues of the leaf. 
• There are always 2 guard cells which may contain chloroplasts and have unevenly thickened cell walls. 

• The inner cell wall is thick and outer is thin. When guard cells are rigid the pore opens and when guard cells are flaccid the pore closes. 
• K+ ions are transported from epidermal cells to guard cells. 
• Insoluble starch from the guard cells are converted to soluble maltase by an enzyme in the cytoplasm. 
• Water potential in the guard cells are lowered so water enters via osmosis. 
• The guard cells become turgid and curve apart because outer cell walls are thinner than inner cell walls.