• The fish opens it mouth while muscular contraction lower the flow of the bucal cavity and the pharynx.
• This increases the volume and decreases the pressure of water in the buccal cavity.
• Water flows into the mouth from the outside because the internal water pressure is lower than the external water pressure. The flexible edges of the operculum is scaled against the outside of the fish by the higher external water pressure.
• Muscle in the operculum causes it to bulge out thereby increasing the volume and decreasing the pressure in the region of the gills, water therefore move from the mouth from cavity to the gills.
• The fish now closes it mouth while the muscle raises the flow of the buccal cavity and the pharynx.
• The remaining water is push out over the gills and through the operculum.

NB: Although there is more oxygen than air in water a fish will suffocate when brought to land because the muscular system of the mouth and the operculum which can work well in water will not function sell on land or in air. When it is out of water the gill filament stick together so that the total surface expose is very much reduce.

Aim: To find out if CO₂ is produced by yeast cell

Requirement: yeast,glucose, lime water, oil, test tube and delivery/

Procedure:

• Water is boiled in order to expel oxygen.
• The water is then cooled to restore the H₂O to a suitable temperature for the yeast.
• 5 cm³ of 5 percent glucose solution and 1 cm³ of yeast suspension is placed in a test tube
• A bit of oil is then placed on the glucose and yeast mixture to exclude atmospheric O₂ from the test tube
• The test tube is then cover with a stopper. A hole is created in the stopper and a delivery tube is filted into the test tube allowing the open end to dip into a tube containing a solution of lime water.
• Core is taken to ensure that conical flask is alright. This is  done by smearing Vaseline around the mouth of the test tube. This is test experiment(experiment A)
• A similar  experiment is setup( experiment B) with boiled yeast as control experiment.
• A setup is left for few minutes.

DIFFERENT BETWEEN INSPIRATION AND EXPIRED AIR