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Blood enters the atria. It cannot pass into the ventricles because the atrioventricular valves are closed. The heart is in diastole.The atria contract. The pressure in the atria becomes greater than the p...

Leaves have a large surface area, which means more space to allow CO2 to enter. They are thin so there is less for gases to travel. Leaves contain Stomata; these are small holes found distributed througho...

Anaeobic respiration in yeast:
glucose -> carbon dioxide + ethanol

Anaerobic respiration in muscle cells:
glucose -> lactic acid

The lactic acid build up in muscles r...

For non-polar small molecules simple diffusion down a concentration gradient through the phospholipid bilayer is possible as it is soluble in the hydrophobic fatty acid tails of the membrane. An example o...

Within a population of a species there will be variation between individuals. These variations arise because of mutations causing differences in alleles and phenotypes. Some individuals will have an allel...