1.Entry of useful substances. Eg oxygen, glucose,nutrients and vitamins, hormones and growth substances, water
2.Disposal of excretory substances. Eg urea, carbon dioxide.
3. Secretion of substances. Eg vitamins, extracellular enzymes.
4. Maintenance of conditions essential for cellular function. Eg to maintain pH and ionic concentration
Transport across cells occur by four main processes. They are:
(i) Diffusion
(ii) Osmosis
(iii) Active Transport
(iv) Bulk Transport