Respiration I. Introduction Oxygen and Carbon Dioxide

Cells use O 2 and Produce CO 2. In order to produce energy most cells must obtain O 2 from the external environment. As a result of O 2 utilization and energy production CO 2 is produced. The cells must get rid of this CO 2. Q: How do they do it? A: By diffusion and convection.

Diffusion. Unicellular organism can exchange O 2 and CO 2 by simple diffusion.

Remember Diffusion? Movement of molecules from one location to another solely as a result of their random motion. Fick diffusion equation (dQ s /dt) = D s A (dC s /dx) dQ s /dt is the rate of diffusion or flux (J). D s is the diffusion coefficient. A is the cross sectional area. dC s /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 O 2.

Multicellular Organisms Use Diffusion Plus Convection. Convection is movement in bulk. O 2 and CO 2 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.

O 2 and CO 2 in Air. Normal dry atmospheric air. O 2 = 20.95% (21%). CO 2 = 0.03% (0.0 %). N 2 = 78.08% (79 %). H 2 O = 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 P B = 760 torr (mm Hg).

O2 P B =P O2 +P CO2 +P N2 In dry air barometric pressure is equal to the sum of the partial pressures of O 2, CO 2 and N 2. P B = 760 torr. P O2 = 0.21 (760) = 159 torr. P CO2 = 0.00 (760) = 0 torr P N2 = 0.79 (760)= 600 torr.

Pressures Change With Altitude.

Molar fraction of O 2 is 0.21 at sea level or at any elevation. P O2 at sea level is 160 torr. P O2 in Denver is 132 torr. Why? Because P B in Denver is 630 torr.

Why does P B change? Because the concentration of the gases changes. Gravity pulls molecules lower Concentration of a gas in air is related to its pressure.

O 2 and CO 2 in solution. Gases in solution do not exert atmospheric pressure. When we say the pressure of O 2 in a solution is 100 torr, what we are saying is: The concentration of O 2 in the liquid is such that it is in equilibrium with a gaseous phase in which the pressure IS 100 torr.

O 2 and CO 2 in solution.

The concentration of a gas in solution is [G] = P Gas x S Gas [G] is the molar concentration. P Gas is the pressure of the gas. S Gas is the solubility coefficient. This is Henry’s Law