Gas Exchange with the environment

Why do organisms need to exchange gases with their environments? Carbon Dioxide and Oxygen levels must be regulated Cellular respiration depends on the rate of oxygen entering the cells So why is this crucial? Because the amount of energy released will be reduced if not enough oxygen is available

Carbon dioxide is a waste product for the cells Carbon dioxide is a waste product for the cells. What happens if this is allowed to increase in cells and body tissues? When dissolved in water Carbon dioxide forms carbonic acid and even though it is weak, it changes the pH and that’s not a good thing to do!!

So how do animals exchange gases with their environment The answer is DIFFUSION Remember that CO2 and O2 are small molecules and so move easily across all membranes by diffusion. Does this process involve energy and do they always move this way? No energy involved and yes they always use diffusion. Then how do they move? Down the concentration gradient. High to Low.

What properties do you think the gas exchange surface should have What properties do you think the gas exchange surface should have? Think about the lungs Large Surface Area / Volume ratio ( I told you this concept would be back!!) The barrier (membrane) to cross should be wafer thin. An decent amount of gas to be transferred, this maintains the concentration gradient and keeps the exchange rate going Some way of removing the substance after the transfer has occurred. Eg: In the lungs, the O2 is whisked away by the blood and the CO2 is breathed out.

There is a formula for this – for the Maths inclined

Do you think the gas exchange surface should be moist, dry, or in-between? Definitely moist. In fact one of the biggest problems faced by terrestrial animals is the evaporation of water from this surface

The getting of oxygen Where you live and your size usually determines what type of gas exchange mechanism you will use Smaller animals like amoeba and underwater molluscs can exchange gas directly between their cells and the environment, Most animals however are way to big for this to be efficient and have devised ingenious ways to get around this problem

Water Vs Air In which do you think Oxygen is more abundant? Its air. Air is always about 21% oxygen. Although at high altitude because it is less dense, there are less oxygen molecules Water generally has about 7ml (10mg) of O2 per litre Air has 210ml (285mg) per litre

The amount of O2 in water will also decrease as: - The pressure increases (like in deep water) - The Temperature increases - The Salinity increases So basically it is a lot harder work to get oxygen out of water than air.

Gills Vs Lungs Which do you think extracts the greater percentage of oxygen, Gills or Lungs? Actually it is Gills. They can get a massive 90% out of water. Lungs can only get 25%. So which is the most efficient? The lungs. 25% of 210ml is more than 90% of 7ml However, both are designed to give the animal enough 02 for maximum energy efficiency

Gills Gills are basically wafer thin membranes surrounded by heaps of blood vessels. What would the surface are be like? As the fish swims around with its mouth open, water rushes past the gills and the oxygen diffuses out of the water Blood flows around the gills in opposite directions a process called Counter current flow. This basically keeps the concentration gradient really high so it allows the maximum amount of O2 to get absorbed from the water

Counter current flow in the gills

Insects Breathe through holes in their skin called spiracles These connect direct to the trachea and branch into tracheoles

http://evolution.berkeley.edu/evolibrary/article/_0_0/constraint_14

Transporting gases Oxygen does not actually dissolve that well in the plasma and so the amount that can be carried is very small. So how do animals get the large amount of O2 from the lungs to the tissues? By using oxygen carrier molecules These are usually proteins with a metal in the middle of the molecule Mammals have haemoglobin as the oxygen carrier and it has which metal in the middle? Iron

Mammals have haemoglobin for two reasons, what are they? It has the ability to bind 4 molecules of oxygen at a time and can easily release it to the tissues It increases the oxygen carrying capacity of the blood so the animal can have less blood to pump around and still meet its energy needs

Getting rid of CO2 It all ends up in the bloodstream 7% dissolved in plasma 23% carried by Haemoglobin 70% converted into Hydrogen carbonate and carried in the red blood cells When the blood gets to the alveoli the CO2 diffuses across the walls into the alveloar spaces, where it will be breathed out by the lungs

Consider the following diseases Emphysema, Iron deficiency anaemia, Asthma and pneumonia All of these give very similar symptoms. Tiredness, inability to exercise, shortness of breath. Explain why. Tip: It has something to do with how they affect the respiratory surface