Gas Exchange

Diffusion is required to supply all organisms with oxygen.

The efficiency of diffusion is increased if there is:

1. A large surface area over which exchange can take place.
2. A concentration gradient without which nothing will diffuse.
3. A thin surface across which gases diffuse.

Fick's Law

Fick's law is used to measure the rate of diffusion.

The larger the area and difference in concentration and the thinner the surface, the quicker the rate.

Unicellular organisms

Unicellular Organisms do not have specialised gas exchange surfaces. Instead gases diffuse in through the cell membrane.

The smaller something is, the smaller the surface area is but, more importantly, the bigger the surface area is compared to its volume.

Multicellular organisms

Multicellular Organisms are bigger than Unicellular organisms. This makes efficient diffusion of gases more difficult.

However, if they are small, or large but very thin (like the flatworms, Platyhelminths), the outer surface of the body is sufficient as an exchange surface because the surface area to volume ratio is still high.

Gas Exchange in Plants

Plants obtain the gases they need through their leaves. They require oxygen for respiration and carbon dioxide for photosynthesis.

The gases diffuse into the intercellular spaces of the leaf through pores, which are normally on the underside of the leaf - stomata. From these spaces they will diffuse into the cells that require them.

Insects have no transport system so gases need to be transported directly to the respiring tissues.

There are tiny holes called spiracles along the side of the insect.

The spiracles are openings of small tubes running into the insect's body, the larger ones being called tracheae and the smaller ones being called tracheoles.

The ends of these tubes, which are in contact with individual cells, contain a small amount of fluid in which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the surface area of air in contact with the cells. Gases diffuse in through the spiracles and down the tracheae and tracheoles.

Ventilation movements of the body during exercise may help this diffusion.

The spiracles can be closed by valves and may be surrounded by tiny hairs. These help keep humidity around the opening, ensure there is a lower concentration gradient of water vapour, and so less is lost from the insect by evaporation.

Fish use gills for gas exchange. Gills have numerous folds that give them a very large surface area.

The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills.

The gas exchange surface of a mammal is the alveolus.

There are numerous alveoli - air sacs, supplied with gases via a system of tubes (trachea, splitting into two bronchi - one for each lung - and numerous bronchioles) connected to the outside by the mouth and nose.

These alveoli provide a massive surface area through which gases can diffuse. These gases diffuse a very short distance between the alveolus and the blood because the lining of the lung and the capillary are both only one cell thick.

The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood. Ventilation movements also maintain the concentration gradients because air is regularly moving in and out of the lungs.

This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from the respiratory centre in the medulla of the brain.

Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving information about the state of the lungs.

Chemoreceptors

There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in carbon dioxide levels in the blood.