Gas exchange in organisms

• Unicellular organisms ?

  small volume

  large SA:V

  concentration gradient maintained:

  O2 kept low inside by respiration

  CO2 kept high inside by respiration

  short diffusion pathway:

  thin plasma membrane

  lack nuclei

  irregular shape increases SA
• Insects?

  Large SA:V-trachea branching into many tracheoles

  Water diffuses out of tracheole tips when cells have more lactate (due to anaerobic respiration)= increased tracheole wall exposed for exchange

  large concentration gradient:

  -O2 and CO2 in respiration

  -continual flow of air through trachae

  short diffusion pathway:

  -thin, moist tracheole walls and short

  -lack of chitin=permeable to gases and water

  spiracles open up to rising CO2 levels but close sometimes to reduce water loss
• Anaerobic respiration in insects?

  -lactate is produced which lowers the water potential of muscle cells

  -water moves into the muscle cells from tracheole cells by osmosis

  -fluid level in the tracheole tip decreases

  -more air drawn into tracheole

  -gas exchange rate increases via diffusion and water loss in tracheoles increases
• Ventilation in insects?

  -flattening and expanding of thorax and abdomen=pressure differences

  -high CO2 air squeezed out and new air enters increasing the concentration gradient

  -diffusion=tracheoles need to be short

  -size of insects limited
• Large fish have gills with filaments and many lamellae, arranged in 8 gill arches and 560 filaments.
• The blood capillaries in gills increase surface area for gas exchange.
• The countercurrent system in gills ensures that the whole lamellae surface is used for gas exchange as equilibrium is not reached, maximising surface area for gas exchange.
• Large fish have a large concentration gradient in gills.
• The constant flow of aerated water over lamellae in gills is a key aspect of gas exchange.
• Large fish increase the volume of the buccal cavity by opening their mouth, creating a low pressure inside the cavity.
• Large fish increase the pressure in the buccal cavity by closing their mouth and raising the floor of the cavity.
• Water is squeezed out over gills in large fish.
• Capillaries in gills carry oxygen-rich blood away.
• The countercurrent system in gills maintains a difference in concentration between water and blood.
• The lamellae, epithelium and capillary walls in gills are one cell thick, allowing for a small diffusion pathway for gas exchange.
• Blood is close to the surface of lamellae in gills.
• Large SA:V plants have mesophyll cells in contact with many air spaces in between them.
• A leaf has a large internal surface in relation to its volume.
• Large concentration gradient in plants due to photosynthesis which keeps CO2 levels low inside.
• Large numbers of stomata in plants allow for good volumes of gas to be diffused.
• Plants have a continuous air supply through air spaces via stomata.
• Cell walls in plants are thin and permeable, allowing for water and gas to move through them.
• Mesophyll cells in plants are close to stomata, facilitating the exchange of water and gases.
• Plants lose water vapour via stomata in transpiration.
• Xerophytes have adaptations such as a thick cuticle and hairs around stomata to limit water loss.
• Rolling up leaves, as in marram grass, creates a humid atmosphere inside, decreasing the water potential gradient.
• Human ventilation ?

  Inspiration:

  -diaphragm contracts/flattens

  -external intercostal muscles contact=ribcage up/out

  -thoratic volume increases

  -lung pressure decreases below atmosphere

  -air enters down concentration gradient

  Expiration

  -diaphragm relaxes/dome

  -EIM relax=ribcage down/in

  -thoratic volume decreases

  -lung pressure rises above atmosphere

  -down pressure gradient
• Forced expiration?

  -ICM contract=ribcage down/in

  -abdominal muscles contract, increasing pressure in abdominal cavity

  -diaphragm upwards
• Human gas exchange system?

  Trachea

  -goblet cells, mucus to trap particles

  -ciliated epithelial cells

  Alveoli

  -elastic fibres which stretch and recoil

  -strong collagen fibres, no bursting
• Tidal volume?

  volume of air breathed in and out in a single breath

  residual volume-volume of air remaining in the lungs after maximum expiration due to bronchi and trachea trapping air

  pulmonary ventilation rate:

  volume of air moved into lungs in 1 min

  PV=TVxBR
• Ficks law?

  rate of diffusion = SA x conc. grad.

  Diffusion length
• Alveoli adaptions?

  Thin walls and large surface area

  One cell thick endothelium walls

  Capillaries and ventilation

  RBC pressed against capillary walls

  RBC move slowly