Respiration I. Introduction Oxygen and Carbon Dioxide.

Respiration I.

Introduction

Oxygen and Carbon Dioxide

Cells use O2 and Produce CO2.

• In order to produce energy most cells must obtain O2 from the external environment.

• As a result of O2 utilization and energy production CO2 is produced.

• The cells must get rid of this CO2.

• Q: How do they do it?• A: By diffusion and convection.

Diffusion.

• Unicellular organism can exchange O2 and CO2 by simple diffusion.

Remember Diffusion?

• Movement of molecules from one location to another solely as a result of their random motion.

• Fick diffusion equation• (dQs/dt) = DsA (dCs/dx)• dQs/dt is the rate of diffusion or flux (J).• Ds is the diffusion coefficient.• A is the cross sectional area.• dCs/dx is the concentration gradient.

Flux is flow of materials through a pathway

• Factors that affect flux– Concentration gradient.– Temperature.– Mass of molecule.– Surface area.

• Small multicellular organisms face a challenge in supplying the cells at their center with O2.

Multicellular Organisms Use Diffusion Plus Convection.

• Convection is movement in bulk.

• O2 and CO2 are moved in masse.

• The force is provided by pumps.

In air breathing animals(mammals)

• Two convection and two diffusion mechanisms at play.

• Alveolar ventilation.• Alveolar gas exchange.• Circulation.• Tissue gas exchange.

In water breathing animals(fish)

• Gill perfusion replaces alveolar ventilation. The gills are perfused by the water in which the animals lives. Usually a pump forces water through the gills.

O2 and CO2 in Air.

• Normal dry atmospheric air.• O2 = 20.95% (21%).

• CO2 = 0.03% (0.0 %).

• N2 = 78.08% (79 %).

• H2O = 0.0%.

• Inert gases; pollutants = the rest.

Pressures

• Gas molecules have mass and acceleration.• Therefore, they exert a force that is made

evident when they hit a surface.• This results in pressure. • Gases in the atmosphere exert the so called

barometric pressure.• At sea level PB = 760 torr (mm Hg).

PB=PO2+PCO2+PN2

• In dry air barometric pressure is equal to the sum of the partial pressures of O2, CO2 and N2.

• PB = 760 torr.

• PO2= 0.21 (760) = 159 torr.

• PCO2= 0.00 (760) = 0 torr

• PN2= 0.79 (760)= 600 torr.

Pressures Change With Altitude.

Pressures Change With Altitude.

• Molar fraction of O2 is 0.21 at sea level or at any elevation.

• PO2 at sea level is 160 torr.

• PO2 in Denver is 132 torr.

• Why? • Because PB in Denver is 630 torr.

Why does PB change?

• Because the concentration of the gases changes. Gravity pulls molecules lower

• Concentration of a gas in air is related to its pressure.

O2 and CO2 in solution.

• Gases in solution do not exert atmospheric pressure.

• When we say the pressure of O2 in a solution is 100 torr, what we are saying is:

• The concentration of O2 in the liquid is such that it is in equilibrium with a gaseous phase in which the pressure IS 100 torr.

O2 and CO2 in solution.

O2 and CO2 in solution.

• The concentration of a gas in solution is • [G] = PGas x SGas

• [G] is the molar concentration.• PGas is the pressure of the gas.

• SGas is the solubility coefficient.

• This is Henry’s Law