Gas Exchange

Gas exchange in small organisms:

For single celled orgasnisms, all their vital nutrients and oxygen diffuse directly into the cell and waste substances can diffuse straight out. This is efficiant because;

• diffusion distance from outside to innermost part of cell is very small
• SA in contact with outer environment is very large relative to volume of organism
• metabloic demands are low

Gas exchange in large organisms:

In contrast to small single cellular organism such as Aboeb, large organisms need nutrients and oxygen to travel large distances from the outside to reach the cytoplasm of the cell. The metabolic rate is also alot higher, so simple diffusion would take too long to sustain organism

Factors effecting rate of diffusion:

Surface area, concentration gradient, diffusion distance

Gas exchange:

• oxygen taken in through mouth, nasal cavity down respiratory tract
• passes through trachea and splits through left and right bronchus into bronchioles
• once inside alveoli sacs, diffusion takes place into cappilaries surrounding lungs
• alveoli specialised in several ways:
  • single cell wall of epithelial cells (short diffusion distance) with cappilaries (well vasculised)
  • very large SA-average human has 480-500 million alveoli so around 10-18 table tennis tables
  • high concentration gradient

Breathing in mammals involves other aspects as well:

Gas exchange system of insects had evolved so each cell gets a direct line of oxygen. Insecs have 3 main components to their gas exchange systems:

• Spiracles: on thorax and abdomen, site of entry and exit of gases, opened and closed by sphincters
  • sphincters keep closed as much as possible; site if water loss
• Trachea: large tubes (1mm diameter),carry oxygen along and inside body of insect, supported by spirals of chitin (hold trachea open)
  • oxygen enters by diffusion only, research shows its sufficient for oxygen demand
• Tracheoles: smaller tube of a single long cell, permable to gases, spraed through muscle tissue directly into cells
  • network of threcheoles (similar to human capillaries) form lage SA for gas exchange

Some very active insects have developed ways of coping with extra energy demands:

• Mechinal Ventilation: Air actively pumped into tracheal system, spiracles open and pumping movement of thorax/abdomen change volume and pressure inside the body, drawing air in and out
• Increase volume of air moves through system (inflate and deflate reservoirs)

Due to water having 41x less oxygen than air, fish use gills to gain oxygen. Gills are suited to water based gas exchange in several ways

• large SA- good for diffusion
• well vasculated- maintains concentration gradient
• counter current exchange system-  as diffusion happens down a concentration gradient, the blood in the gill filaments and the water flowing in different directions ensures the most effective diffusion occurs

The aquatic gas exchange system is made up of several components:

• Gill filament (overlapping)- stacks of filament that need water to keep them from sticking together 
• Lamella- main site of gas exchange 
• Buccal Chamber- contacts to ventilate water through system (for cartilagenous fish that have no operculum so have to swim constantly)
• Operculum-  a covering for gills (allows for gas exchange when stationary)

There is a constance conflict between the need to phtoosynthesise and the need for respiration. Main gas exchnage in plants happen in leaves that have the following features suitable for gas exchange

• Spongy Mesophyll:
  • large SA
  • moit
• Air spaces- allows for diffusion inside cell
• Waxy cuticle- barrier to rpevent excess diffusion of gases and water out of cell
• Stomata- underside of epidermis of leade, pores that let substnaces in and out
• Guard Cells- through turgor controll the stomata