B3.1 Gas exchange

Outline need for gas exchange in living organisms

gas exchange supplies O2 for aerobic respiration and removes CO2 as waste product

in plants: CO2 is needed for photosynthesis - O2 is released

in animals: O2 is absorbed for respiration - CO2 is excreted to prevent toxicity

Describe properties or gas-exchange surfaces

large surface area: maximises diffusion

thin walls: short diffusion distance

moist: allows gases to dissolve for diffusion

permeable: enables gas molecules to pass easily

explain how concentration gradients are maintained at exchange surfaces in animals

metabolism maintains internal gradients: O2 is used, CO2 is produced

ventilation refreshes air/water → keeping external O2 is high, CO2 is low

circulation brings deoxygenated blood to exchange surface & carries oxygenated blood away

fish - countercurrent flow of water and blood ensures that blood always meets water with higher O2 concentration - maintain constant gradient

describe adaptations of mammalian lungs for gas exchange

• millions of alveoli increase surface area

• alveoli & capillary walls are one cell thick - short diffusion path

• rich capillary network maintains concentration gradient

• moist alveolar lining allows gas solubility

• surfactant prevents alveolar from collapsing by reducing surface tension

explain process of ventilation

inhalation:

• diaphragm contracts

• external intercostal muscles lift ribs

• increase thoracic volume

• lowers pressure

• air enters lungs

exhalation:

• diaphragm relaxes

• internal intercostal muscles & abdominal muscles contract

• decreases thoracic volume

• increases pressure

• air exits

determine measurements of lung volume

tidal volume: air in/out during normal breath

vital capacity: maximum air exhaled after full inhalation

inspiratory reserve volume: extra air inhaled after tidal inhalation

expiratory reserve volume: extra air exhaled after tidal exhalation

ventilation rate = breaths per minute

outline adaptations of leave for gas exchange

stomata allow CO2 in and O2 out - open/close via guard cells

air spaces in spongy mesophyll facilitate internal diffusion

moist mesophyll cells enable gas solubility

way cuticle prevents water loss but limits gas exchange

veins (xylem) maintain hydration for continuous evaporation

outline process of transpiration and the factors that affect rate of transpiration

water evaporates from mesophyll - diffuses through stomata

creates tension, pulling water up xylem

temperature: increase in temp increases evaporation rate

humidity: high humidity decreases rate of diffusion - if air outside is saturated, transpiration stops

wind: still air allows saturated pockets to form near stomata - reduces transpiration

moving air removes these pockets- increases the rate

very strong winds - may cause stomata to close (reduce transpiration)

stomatal density

Stomatal density = (Average number of stomata per field of view) ÷ (Area of field of view in mm² or µm²)

adaptations of foetal and adult haemoglobin for transport of oxygen

• Foetal haemoglobin (HbF) has a higher affinity for O₂ than adult haemoglobin (HbA).

• Allows O₂ transfer from maternal to foetal blood across placenta.

• Foetal Hb becomes saturated at lower partial O₂ pressure

• After birth, HbF is replaced with HbA over several months.

explain Bohr shift and benefits for respiring tissues

• Bohr effect: CO₂ lowers blood pH → Hb releases O₂ more easily.

• In active tissues (high CO₂), O₂ is released faster.

• In lungs (low CO₂), Hb has high affinity → O₂ uptake.

• Curve shifts right with increased CO₂ or low pH