# Biology 2601A/B Lecture Notes - Gas Exchange, Partial Pressure, Fick'S Laws Of Diffusion

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16 Oct 2011
School
Department
Organismal Phys Lecture 15
Gas Exchange 1: Principles & Animals Breathing in Water
In general, it is important to note that metabolism and photosynthesis are dependent on O₂ and
CO₂ exchange with the environment
Gas Exchange in Respiration
Involves getting O₂ out of the external medium and into the cells
o This is often done via the circulatory system
Also involves getting CO₂ out of the cells and into the external medium
Gas Exchange in Photosynthesis
Getting CO₂ out of the external medium and into the cells
o Usually via diffusion or stomata
Getting O₂ out of the cells and into the external medium
o Or can also be used in respiration
Remember: water vapour is also a gas (important when considering water loss in plants)
Although this lecture discusses O₂, the principles also apply to CO₂ and water vapour
Where is the Oxygen?
Most of the O₂ is in the air rather than in water
o Cold sea water: Approximately 0.8% O₂ in solution
o Air: Approximately 21% O₂ as a gas
The amount of O₂ that is able to dissolve in water is dependent upon temp
o Warmer water temperature = less O₂ dissolved
Solutes also affect O₂ dissolving
Partial Pressure
Each gas in a mix (either as a gas or in solution) exerts pressure
Partial pressure is the amount of pressure that the gas of interest exerts
Partial Pressure of Gas X
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Partial Pressure
Air has
o 20.95% O₂
o 78.09% N₂
o 0.93% Ar
o 0.038% CO₂
Ex. PO₂ = 0.2095 x 101.3 kPa = 22.22 kPa
Note that when altitude changes (ex. when on Mt. Everest) the proportional composition of the
air is still the same; the thing that differ is the air pressure
At higher altitudes = lower pressure
o Remember that partial pressure will influence the rate of diffusion across the cell
Gases Dissolve in Liquids
Not the same as having air bubbles
P(liquid) is proportional to P(air)
The amount of gas in a solution will depend on
o 1. Temperature
o 2. Salinity
o 3. Gas
Gases that have reacted chemically do not contribute to partial pressure in solution
Under high partial pressures there will be more gases in liquid
Henry’s Law
States that the amount of gas that will dissolve in a liquid is determined by the partial pressure
of the gas and the solubility of the gas in the liquid
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