IGCSE Biology : U2 : Gas Exchange

Gas Exchange in Leaves

Occurs in stomata through diffusion; CO2 enters during photosynthesis, O2 exits; in respiration, O2 enters, CO2 exits.

Leaf Adaptations for Gas Exchange

Broad for large surface area, thin for short diffusion distance, air spaces for easy gas movement, many stomata, controlled by guard cells.

Hydrogen-carbonate Indicator

Measures CO2 levels; yellow for CO2 increase, purple for decrease, orange for no change.

Animal Cell Respiration

Obtains energy from glucose oxidation; needs constant O2 supply; CO2 is a waste product to be removed.

Human Respiration

Gas exchange in lungs, not breathing; lungs part of respiratory system; ventilation moves air in and out.

Composition of Inhaled and Exhaled Air

Inhaled - N2(78%), O2(21%), Ar(0.9%), CO2(0.04%); Exhaled - N2(78%), O2(17%), Ar(0.9%), CO2(4%).

Mechanism of Breathing

Requires air pressure difference; intercostal muscles and diaphragm change volume, causing air movement.

Alveoli in Lungs

700,000,000 air sacs for gas exchange; moist for increased diffusion rate; rich blood supply for steep gradient; one-cell thick walls for faster diffusion; permeable walls.

Alveoli Adaptations

Large surface area, moist for gas dissolution, rich blood supply, one-cell thick walls, permeable, collagen and elastic fibers for expansion and contraction.