Explain counter-current flow in gills, its advantages, and give examples in humans and adaptations in aquatic plants for gas exchange.

a)

i) The type of flow shown is counter-current flow.

ii) The advantage of counter-current flow is that it maintains a steep concentration gradient for oxygen diffusion across the entire length of the gill plate. This maximizes the efficiency of oxygen transfer from the water to the blood, allowing for a much higher percentage of oxygen to be extracted compared to a co-current system.

b) Two organs in humans that display a similar type of flow are:

• Kidneys (specifically in the loop of Henle for water reabsorption)
• Placenta (for exchange of gases, nutrients, and waste products between mother and fetus)

c) Two ways in which floating leaves of aquatic plants are adapted to gaseous exchange are:

• They have stomata located primarily on their upper surface, allowing direct access to atmospheric air for gas exchange, as the lower surface is in contact with water.
• They possess large air spaces (aerenchyma) within their leaves and petioles, which provide buoyancy and facilitate the diffusion of gases throughout the plant.