Factors affecting diffusion across cell membranes

Cell biology • Transport in cells

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Describe diffusion of carbon dioxide in the lungs.

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Carbon dioxide diffuses from blood into alveoli because tissue cells produce CO2, the blood arriving at the lungs has higher CO2 concentration, and CO2 moves down its concentration gradient into alveolar air .

Key concepts

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Definition and substances moved by diffusion

Diffusion describes net particle movement from high to low concentration without external energy input. Gases and small soluble molecules cross membranes by diffusion, including oxygen and carbon dioxide between alveoli and blood, and waste urea from cells into the blood fileciteturn0file10turn0file16. Diffusion operates across cell membranes and exchange surfaces that are thin and moist. Structures such as alveoli, villi and gills provide thin barriers and large areas to allow efficient diffusion of respiratory gases and dissolved substances fileciteturn0file11turn0file6.

Concentration gradient: cause and effect

A steeper concentration gradient causes a larger net movement of particles per unit time because more particles move from the high-concentration region into the low-concentration region. Increasing the difference in concentration therefore increases the rate of diffusion . Blood flow and ventilation maintain steep gradients in organisms. Rapid removal of diffused molecules (for example, oxygen carried away by blood) sustains the gradient and keeps diffusion efficient across membranes such as alveolar and capillary walls fileciteturn0file11turn0file16.

Temperature: cause and effect

Higher temperature increases the kinetic energy of particles so they move faster and collide more frequently. Faster movement and more collisions increase the probability of particles crossing membranes, and so the rate of diffusion rises with temperature . Lower temperature reduces kinetic energy and slows diffusion. Biological systems maintain conditions (for example, warm-blooded organisms maintaining body temperature) to keep diffusion rates sufficient for respiration and waste removal .

Surface area of the membrane: cause and effect

A larger surface area gives more space for particles to cross the membrane simultaneously, increasing the overall rate of diffusion. Highly folded or extensively branched surfaces such as alveoli, villi or gill lamellae increase surface area and therefore increase diffusion rates fileciteturn0file6turn0file11. Reduced surface area decreases the number of diffusion sites and lowers diffusion rate. Diseases or damage that reduce surface area in exchange organs (for example, emphysema in lungs) impair oxygen uptake because less membrane area is available for gas exchange .

How substances move across cell membranes via diffusion

Small nonpolar molecules and some small polar molecules cross the phospholipid bilayer or pass through channel proteins by moving down their concentration gradient. Movement occurs from the side with higher concentration to the side with lower concentration until equilibrium or continuous removal by transport systems exists fileciteturn0file10turn0file11. Exchange surfaces combine thin diffusion distances, large surface area and mechanisms that maintain concentration gradients. For example, oxygen diffuses from alveoli into blood because alveolar air has higher oxygen concentration than capillary blood; blood flow then removes oxygen, restoring the gradient and allowing continued diffusion .

Key notes

Important points to keep in mind

Diffusion = net movement from high to low concentration, passive process .

Steeper concentration gradient → faster diffusion; removal of diffused molecules maintains gradient fileciteturn0file1turn0file11.

Higher temperature → increased particle kinetic energy → faster diffusion .

Larger membrane surface area → more simultaneous diffusion sites → higher rate (alveoli, villi, gills) fileciteturn0file6turn0file11.

Thin diffusion distance (one-cell-thick) increases rate; exchange surfaces are thin and moist .

Small molecules and gases (O2, CO2, urea) diffuse; larger or charged particles often need carrier proteins or active transport fileciteturn0file16turn0file10.