Why can oxygen gas O 2 and carbon dioxide gas CO 2 move through the cell membrane?
Physics of Gas DiffusionThe movement of gases in a contained space (in this case, the lungs) is random. However, overall diffusion results in movement from areas of high concentration to those of low concentration. The rate of diffusion of a gas is primarily affected by Show
Collision of the molecules of gas with the sides of the container results in pressure. This is defined by the ideal gas law, given in the following equation: (n represents the number of moles, R the gas constant (8.314), T the absolute temperature and V the volume of the container)  Fig 1 – Equation to calculate pressure of a gas in a container Diffusion of Gases Through GasesWhen gases are diffusing through other gases (such as in the alveoli), their rate of diffusion can be defined by Graham’s Law: “The rate of diffusion is inversely proportional to the square root of its molar mass at identical pressure and temperature” In other words, the smaller the mass of a gas, the more rapidly it will diffuse. Diffusion of Gases Through LiquidsWhen gases are diffusing through liquids, for example across the alveolar membrane and into capillary blood, the solubility of the gases is important. The more soluble a gas is, the faster it will diffuse. The solubility of a gas is defined by Henry’s law, which states that: “The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid”. If we assume that the conditions of temperature and pressure for all gases remain fixed (as they roughly do in the alveoli) then it is the inherent differences between different gases that determine their solubility. Carbon dioxide is inherently more soluble than oxygen, and thus diffuses much faster than oxygen into liquid. Fick’s LawFick’s law gives us a number of factors that affect the rate of diffusion of a gas through fluid:
In the lungs, whilst oxygen is smaller than carbon dioxide, the difference in solubility means that carbon dioxide diffuses roughly 20 times faster than oxygen. This difference between the rate of diffusion of the individual molecules is compensated for by the large difference in partial pressures of oxygen, creating a larger diffusion gradient than that of carbon dioxide. However, this means that in disease states which impair the ability of the lungs to adequately ventilate with oxygen, oxygen exchange is often compromised before that of carbon dioxide. Diffusion of OxygenThe partial pressure of oxygen is low in the alveoli compared to the external environment. This is due to continuous diffusion of oxygen across the alveolar membrane and the diluting effect of carbon dioxide entering the alveoli to leave the body. Despite this, the partial pressure is still higher in the alveoli than the capillaries, resulting in a net diffusion into the blood. Once it has diffused across the alveolar and capillary membranes, it combines with haemoglobin. This forms oxyhaemoglobin which transports the oxygen to respiring tissues via the bloodstream. Further information on the transport of oxygen within the blood can be found here. During exercise, blood spends up to half the normal time (one second at rest) in the pulmonary capillaries due to the increase in cardiac output moving blood around the body more quickly. However, diffusion of oxygen is complete within half a second of the blood cell arriving in the capillary. This means that exercise is not limited by gas exchange. By CNX OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons Fig 2 – Diagram showing the partial pressures of oxygen and carbon dioxide in the respiratory system Diffusion of Carbon DioxideThe partial pressure of carbon dioxide in the capillaries is much higher than that in the alveoli. This means that net diffusion occurs into the alveoli from capillaries. The carbon dioxide is then exhaled as the partial pressure in the alveoli is higher than the partial pressure in the external environment. Carbon dioxide is transported in the blood in multiple ways; including dissolved, associated with proteins and as bicarbonate ions. Further information on transport of carbon dioxide in the blood can be found here. Diffusion BarrierThe diffusion barrier in the lungs consists of the following layers:
By Cruithne9 (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons Fig 3 – Diagram showing the layers making up the diffusion barrier in the lungs Factors That Affect The Rate of DiffusionThere are many properties which can affect the rate of diffusion in the lungs. The main factors include:
Clinical Relevance – EmphysemaEmphysema is a chronic, progressive disease that results in destruction of the alveoli in the lungs. This results in a greatly reduced surface area for gas exchange in the lungs, which typically leads to hypoxia (Type 1 respiratory failure). The main symptom is of emphysema is shortness of breath, however patients may also experience wheezing, a persistent cough or chest tightness. Emphysema, alongside chronic bronchitis are the conditions that make up Chronic Obstructive Pulmonary Disease (COPD). Whilst smoking is the most common cause, other risk factors include exposure to second-hand smoke, exposure to occupational fumes or dust and living in areas with high levels of pollution. Treatment depends on the stage of the condition (i.e. the degree of symptoms and airway obstruction) but typically includes:
By Yale Rosen from USA [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons Fig 4 – Emphysematous lungs Why can o2 and CO2 cross a lipid bilayer?Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot.
How does oxygen pass through cell membrane?Oxygen diffuses through the cell membrane and is transported in blood plasma by free diffusion and by convection.
How does CO2 move in and out of the cell?CO2 is gaseous molecule and hence it moves in and out of the cell freely by simple diffusion. Diffusion is a process in which the molecules interact and move from a region of higher concentration to a region of lower concentration.
How is CO2 transported across cell membrane?Carbon dioxide (CO2) movement across cellular membranes is passive and governed by Fick's law of diffusion. Until recently, we believed that gases cross biological membranes exclusively by dissolving in and then diffusing through membrane lipid.
|
