Gas exchange, digestion and absorption GapFill

While unicellular organisms can often exchange all the substances they need to with their environment by  osmosisactive transportsimple diffusiontranslocation, larger organisms have a smaller  salt toleranceconcentration gradientwater potentialsurface-area-to-volume ratio and need specialised exchange surfaces to maintain their higher metabolic rate. Insects have a system of internal passages called  tracheaevenulesalveolicapillaries to exchange respiratory gases, while fish use  tracheolesswim bladdersgillslungs, a series of stacked filaments, to increase the surface area for gas exchange. Dicotyledonous plants exchange gases through pores called  chloroplastsstomatamesophyllalveoli. These are found on the underside of leaves and can be closed in order to limit  heathydrogenwatercarbon dioxide loss at night.

Humans have specialised organs for gas exchange called  lungskidneystracheabronchi. These contain small air sacs called  lamellaebronchiolesalveolivacuoles, which can stretch and recoil during inhalation and exhalation due to the presence of collagen and  inflexibleelasticglycogenglobular fibres.

The human digestive system also involves specialised exchange surfaces. Food molecules that have been partially digested by the stomach are absorbed and digested further in the lining of the   pancreasileumkidneyslarge intestine. For example, proteins are digested by  peptidaseslipaseslysozymeslactases, producing amino acids, and  carbohydratespolypeptidesfatsphospholipids are digested to glucose molecules, both of which are absorbed via  simple diffusiontranslocationco-transportfacilitated diffusion. Lipids are emulsified by  phosphatesribosomeslysosomesbile salts after being broken down, forming  micellesalveolipeptidesemulsates.