Adaptations for gas exchange
Unicellular Organisms
Multicellular animals
Vertebrate groups
Amphibians
Reptiles
Fish
Bony fish
The human breathing system
Ventilation of the lungs
Inhalation
Exhaation
Gas exchange in the alveolus
The alveoli have walls made of squamous epithelium, only one cell thick, so the diffusion pathway for gases is short.
A capillary network surrounds alveoli and maintains diffusion gradients, as CO2 is rapidly brought to the alveoli and oxygen rapidly carried away.
Gases dissolve in the surfactant moisture lining the alveoli.
The capillary walls are also 1 cell thick, contributing to the short diffusion pathway for gases.
They provide a large surface area relative to the volume of the body.
Insects
Plants
Stomata
Mechanism of opening and closing
Gasses diffuse through the stomata down a concentration gradient. One inside the leaf, the gasses in the sub-stomatal air chambers diffuse through the intercellular space between the spongy mesophyll cells and into cells.
Small pores on the above-ground parts of plants and occur mostly on the lower surfaces of leaves. Each pore is bounded by 2 guard cells (which are unusual as they are the only epidermal cells with chloroplasts and have thick inner walls) The width of the stomata can change and so stomta control the exchange of gases between the atmosphere and internal tissue of the leaf.
Respire to generate energy constantly.
Plants respire only at night an need oxygen from the atmosphere. Some oxygen enters the stem and roots by diffusion, but most gas exchange takes place at the roots.
Day
Night
If water leaves the guard ells, they become flaccid and the pore closes
The chloroplasts in guard cells photosynthesis, producing ATP.
If water enters guard cells, they become turgid and swell, and the pore opens.
This ATP provides energy for active transport of potassium ions into the guard cells from the surrounding epidermal cells.
Stored starch is converted to malate.
Potassium and malate ions lower water potential in guard cells, making it more negative; water enters by osmosis.
Guard cell wall thicker in places, they expand (when they absorb water) but less where wall is thick. As the guard cells stretch. a pore appears between these areas with less stretching. This is the stomata.
Reverse process and pores close.
Plants lose water through transpiration
Water evaporates from their body surface and they risk dehydration. Efficient gas exchange requires a thin, permeable surface and a large surface area, which conflicts with the need to conserve water.
Most insects reduce water loss with a waterproofing layer covering the body.
Insects have a small surface area to volume ratio so could use their body surface to exchange enough gasses by diffusion.
Gas exchange occurs through spiracles alongside the body. They lead into a system of branched chitin-linear-tubes called tracheae, which branch into smaller tubes, tracheoles.
The external intercostal muscle contracts
The ribs are pulled upwards and outwards
At same time, the diaphragm muscle contract, so it flattens
Both actions increase the thorax volume
This reduces the pressure in the lungs
Atmospheric air pressure is now greater than the pressure in the lungs, so air forced into the lungs.
At same time diaphragm muscles relax, so it domes upwards
Both actions decrease thorax volume
The ribs move downwards and inwards.
This increases the pressure in the lungs.
Air pressure in the lungs now greater than atmospheric pressure so air is forced out of the lungs.
The external intercostal muscles relax
Gas exchange takes place across the gills as they have...
A one-way current of water, kept flowing by a specialised ventilation mechanism.
Large surface area maintained by water passing through, stops gills collapsing on each other.
Many folds, providing large surface area
Cartilaginous fish
Have gills in 5 spaces on each side called gill pouches, which open to the outside at gill slits.
Sharks
Ventilation system not as efficient as bony fish
Parallel flow. Oxygen diffuses from where more concentrated (in water) to less concentrated (blood) until concentration equal. So bloods oxygen concentration limited to 50%.
Gas exchange in parallel flow doesn't occur across whole gill lamella, only part, until oxygen concentration in blood and water is equal.
No special mechanism to force water over gills, must keep swimming for ventilation to happen.
Have an internal skeleton made of bone and gills covered with a flap called the operculum.
Ventilation
Counter current flow
Taking in water
Force out water
The operculum closes
Floor of mouth is lowered
Mouth opens
The volume inside mouth cavity increases
The pressure inside mouth cavity decreases
The floor of mouth is raised
Volume inside mouth cavity decreases
Operculum opens
The pressure inside mouth cavity increaes
Mouth closes
Water flows out over gills because pressure in mouth cavity is higher than in the opercular cavity and outside.
Water flows in, as the external pressure is higher than the pressure inside the mouth.
Water moves from mouth cavity to opercular cavity and into the gill pouches, here it flows between gill lamellae. The blood in gill capillaries flows in the opposite direction to the water flowing over gill surface.
Problems for terrestrial oranisms
Gas exchange surfaces must be thin and permeable with large surface area and so are moist so lots of water lost.
Water evaporates from body surfaces, which could result in dehydration
Birds
Frogs, toads and newts
Moist skin an permeable with well developed capillary network just below surface. Gas exchange takes place through skin and in lungs.
Crocodiles, lizards and snakes
Their lungs have complex internal structure, increasing the surface area for gas exchange.
Lungs process large volumes of oxygen because flight requires lots of energy.
No diaphragm, but their ribs and flight muscles ventilate their lungs more efficiently.
Cell membrane is thin so diffusion rapid
Single cell is thin so diffusion distances inside cell are short.
Large surface area to volume ratio
Can absorb enough oxygen to meet their needs for respiration
Can remove CO2 fast to prevent building up high concentration and making cytoplasm too acidic for enzymes to function.
Have a lower surface area to volume ratio so diffusion across their surfaces is not efficientt enough for their gas exchange.
Have high metabolic rate. Need to deliver more oxygen to respiring cells and remove more CO2.
With an increase in size and specialisation of cells, tissues and organs become more interdependent.