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Lecture 22 – Gas Exchange IWhat is pressure?- Pressure is a force- It is the measure of the concentration of a gasComposition of Air- Oxygen – 21%- Nitrogen – 78%- Argon - 0.93%- Carbon dioxide – 0.03%Partial Pressure of a Gas- PGAS = pressure (concentration) of that gas in a mixture of gases (e.g., air)Calculate the Pressure (Concentration) of O2 in Air- O2 is 21% of air- Total pressure of air at sea level = 760 mm Hg (or 1 Atm)- Thus PO2 is calculated  (0.21) x 760 mm Hg = 160 mm HgPartial Pressures of Gases in Air at Sea Level and BelowAltitude and PressureWhat are the gas exchanges issues living organisms face? (LECTURE OVERVIEW)- Diffusion considerationso Surface/volume, diffusion distance, atmospheric pressures in different environments, etc.- Solubility in watero Other ways of carrying gases in body fluids?- Adapting to different environmentso Gas concentration (partial pressures)o Temperatureso Salinity- Organism’ metabolic demands How do O2 and CO2 get into and out of cells and organisms?- Diffusion- What do we know about it? Increases entropyDiffusion- Negative G, so it is spontaneous- Notice increase in S (entropy!)- Organisms rely on diffusion to move, obtain, and remove essential molecules.- Examples?Diffusion is a rate!- Often referred to as FLUX- Molecules moving from one point to another per unit timeo e.g., mmoles/sec- Abbreviated as J or S/TDiffusion across a membrane of area A and thickness x- J or S/t = - D A TP/xo D = Diffusion coefficiento A = area for diffusiono T = Temp in Ko P = concentration or pressure gradiento x = thickness of membranesD: Diffusion Coefficient- Determined by physical characteristics of solution and solute- Inversely proportional to solute’s molecular weight- Units are cm2/sec- D of CO2 is >> than D of O2Problem: Diffusion- Thus organisms:o are/have small cells with max surface area to volume ratioso have thin respiratory (gas exchange) surfaces with large surface areaso have moist respiratory (gas exchange) surfaces, therefore a large D Problem: water loss?o move air or fluid to maximize PProblem: Diffusion Distances- t = x2/2Do t = elapsed time since diffusion begano X is mean distance traveled by solute in time to D = diffusion coefficient of solute in free solutionProblem: Surface/Volume- As cell radius increases, the surface increases by radius2- However the volume increases by radius3- Thus the surface area available to support the increased volumeis not adequate- Sphere (SA = 4r2, V = 4r3/3)- Cube (SA = 6 L2, V = L3) - QUESTION: In Nature, what types of cells are the smallestand have the most surface area/volume? What are theircharacteristics that require them to be small?o Small cells?  brain cells, nerves, cardiac, skin cellso Large cells?  fat cells, storage cellsProblem: Diffusion- Thus organisms:o are/have small cells with max surface area to volume ratioso have thin respiratory (gas exchange) surfaces with large surface areaso have moist respiratory (gas exchange) surfaces, therefore a large Do move air or fluid to maximize P- To overcome surface/volume and diffusion distance problems, place cells in thin layers very near to circulating fluido (water currents, open fluid filled sinuses, or capillaries)Solubility of Atmospheric Gases in WaterThinking about Respiration- What gases concern aerobic organisms?o O2 o CO2- O2 is not very soluble in watero Only about 0.003 ml O2 in 100 ml water for every 1 mm HgP(O2)o What do organisms do about this?  Red Blood cells containIhemoglobin, a molecule designedto hold oxygen and carry it to  The main function of red blood cells is to take oxygenfrom the lungs to the cells of the body. Hemoglobincombines with oxygen in the lungs, where oxygen levels are high; this is thenIeasilyIreleased through the walls of the capillaries, where the oxygen level is low.  Each molecule of hemoglobin contains fourIironIatoms, and each iron atom can bind a one molecule of oxygen (which contains two oxygen atoms, called O2) for a total of four oxygen molecules (4 * O2) or eight atoms of oxygen for each molecule of hemoglobin.  The iron in hemoglobin gives blood its red color! - Organisms can carry lots of dissolved CO2o How? The same way oxygen is. It binds to/reacts with hemoglobin forming a complex called carboxy-hemoglobin, which is soluble and can be transported through the blood.Solubility of O2 & CO2 in Water- Only 0.003 ml of O2 dissolves in water for every mm Hg pressure of O2o e.g., at 160 mm Hg P of O2 only ~ 3 ml of the gas dissolves in 100 ml of your blood- In contrast, CO2 is 30 times more soluble!- What are the consequences of these facts for organisms? o Although carbon dioxide is not toxic at atmospheric concentrations, too much carbon dioxide in the air will prevent the body from giving up its carbon dioxide which turns the blood acidic. o In a closed space (closed unventilated bank vault, etc.) oxygen will be used up and carbon dioxide will build up (respiratory acidosis) and suffocation will ultimately occur without fresh air.Carbonic Anhydrase- CO2 + H2O  H2CO3 H+ + HCO3- - Present in many organisms, including plants, diatoms, and methane producing bacteria, marine cyanobacteria, and some chemolithothrophs- What does this enzyme (CA) do for an organism?o Key Point: Role of CA in carrying CO2 in body fluids & in regulation of pHCarbonic Anhydrase & pH- CO2 + H2O  H2CO3 H+ + HCO3- o Increase CO2  decrease pH o Decrease CO2  increase pHo Increase [H+]  increase CO2o Decrease [H+]  decrease CO2As temperature increases, PGAS dissolved decreases As [Solute] increases, PGAS dissolved decreasesOxygen atdepths inPacificOcean- EXAM  Question: What does this mean for organisms living in tropical seas? In the deep sea?o Varible solubilities in order to adapt to their necessitieso Changes need to be made in order to adapt to their surroundingsSo what are the issues facing organisms as they maximize the rates at which they unload CO2 and take up O2?- Diffusion and single celled organismso Smallo Membrane foldingso Move through watero Sessile cells move water over surface (cilia) or live in currents- Small Animalso Ex. Sponges, cnidarians, flatworms Small, specialized body plans- Channeled - Septate- Flattened Low metabolic rates No specialized respiratory structures – diffusion adequate for both gas exchange and distributiono Gills  external gills are in


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UMD BSCI 207 - Lecture 22 – Gas Exchange I

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