Movement into and out of cells
Substances may pass through the cell membrane either passively by diffusion or
actively by some form of active transport .
Diffusion :- The movement of particles from an area of high concentration to an
area of low concentration till equilibrium has been achieved .
Examples :-
Gas Exchange in Living Organisms
● Oxygen is essential for respiration in most organisms and enters cells by diffusion
down a concentration gradient. ( A concentration gradient is a difference in the
concentration of a substance between two areas, leading to the movement of that
substance from an area of higher concentration to an area of lower concentration.
This movement, often driven by diffusion, is crucial in many biological and chemical
processes . )
● Small animals with a large surface area to volume ratio absorb oxygen through their
body surface.
● Larger animals require specialized gas exchange organs (like lungs or gills) and a
circulatory system to transport oxygen efficiently.
● Carbon dioxide (CO₂), a waste product of aerobic respiration, is toxic in high amounts
and is also removed by diffusion.
Gas Exchange in Plants
● Plants absorb carbon dioxide (CO₂) for photosynthesis through stomata in their
leaves.
● CO₂ diffuses into air spaces in the mesophyll and reaches palisade cells.
● Oxygen (O₂), produced during photosynthesis, and water vapour, from transpiration,
exit through stomata.
● The rate of water vapour diffusion depends on temperature, humidity, and wind
speed.
● During respiration, O₂ is used, and CO₂ is released, with both gases also diffusing
through stomata.
Nitrogen and Decompression Sickness ("The Bends")
● Nitrogen makes up 78% of air and enters the bloodstream by diffusion, but it is not
used by the body.
● It is an inert gas and usually harmless under normal conditions.
● Divers are at risk because:
○ Deeper water = higher pressure, which increases nitrogen pressure in the air
tank.
○ This causes more nitrogen to dissolve into body tissues over time.
● When surfacing too quickly, pressure drops and nitrogen comes out of solution,
forming bubbles in the blood.
● These bubbles can:
○ Block blood flow
○ Cause joint pain
○ Lead to decompression sickness (also called "the bends")
● Decompression sickness can be fatal if untreated.
● To avoid it, divers must ascend slowly in stages.
● Recompression (rapidly increasing pressure again) is needed to stop the symptoms.
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