1. (stick pic of fish head here) (stick pic of counter-current here)
(stick pic of gills here)
Counter-Current Mechanism:
Water moves across gill plates in an opposite direction to blood flowing through them.
So water always has a higher oxygen conc. Than in the blood past which it is flowing. So oxygen will diffuse from a high conc in water, to a low conc in blood.
In water there is a lower concentration of oxygen and it diffuses slower.
How Fish Breathe: Fish suck water in through mouth, into pharynx. The water then flows between gills, out through the operculum.
Adaptations for diffusion:
large surface area created by gill filaments covered in lamella
lamella have several blood capillaries (rich blood supply)
short diffusion pathway as gill plates are very thin, so blood passing through them is close to water.
high conc. Gradient between blood in lamella and water passing through.

2. (stick pic of insect outline here) (stick pic of tracheoles here)
(draw pic of amoeba here)
Conditions that favour gaseous exchange from body also cause water loss.
So insects have an outer layer impermeable to water (a cuticle), however this also means no gas exchange can occur either.
Therefore insects have tiny pores (spiracles) to allow gaseous exchange.
Spiracles are opened/closed by valves when needed.
These lead into trachea, strengthened by chitin for support.
Trachae branch off into tracheoles which provide a large surface area and create a short diffusion pathway as close to cells.
Single-celled organisms
Adaptations:
Large surface area
Thin membrane so short diffusion pathway.

3. (stick pic of stomata here)
(draw pic of leaf shape here)
(stick pic of leaf structure here)
(stick pic of stomata here)
Photosynthesis requires CO2 and produces O2. these gases need to be exchanged.
Adaptations of a leaf:
flat shape increases surface area
thin to create a short diffusion pathway
palisade cells close to top to capture sunlight for p.syn
stomata in lower epidermis to allow gases in and out
When open, stomata connect air spaces inside the mesophll layer with the air outside.
Photosynthesis:
CO2 diffuses into leaf, O2 diffuses out
Respiration:
O2 diffuses into leaf,
CO2 diffuses out.
Like with insects, plants cant keep their pores open constantly as they would lose too much water.
Adaptation:
Guard cells control the opening and closing of stomata, and will close if the plant is losing too much water.