Every living cell must exchange substances with its surroundings to stay alive. This is why cells need transport mechanisms.Substances a cell must take in include:
• oxygen and glucose for aerobic respiration, which releases energy as ATP;
• amino acids, fatty acids and nucleotides to make proteins, membranes and nucleic acids;
• ions such as sodium, potassium and calcium, and water.Substances a cell must get rid of or send out include:
• waste products such as carbon dioxide and urea, which must be removed before they build up to harmful levels;
• useful products made by the cell, such as enzymes, hormones and antibodies, which are released by secretion.Transport also lets a cell keep its internal conditions, such as ion concentrations and water content, different from the outside. Everything entering or leaving must cross the cell surface membrane, which controls what passes.Cells are small partly because transport relies heavily on diffusion, which is only fast over short distances. Size also matters because volume grows faster than surface area.Worked example: if a cell's diameter doubles from 20 µm to 40 µm, its surface area (proportional to diameter2) is multiplied by 4, but its volume (proportional to diameter3) is multiplied by 8. Each unit of volume now has half as much membrane to supply it, so the cell could not exchange substances fast enough.
• oxygen and glucose for aerobic respiration, which releases energy as ATP;
• amino acids, fatty acids and nucleotides to make proteins, membranes and nucleic acids;
• ions such as sodium, potassium and calcium, and water.Substances a cell must get rid of or send out include:
• waste products such as carbon dioxide and urea, which must be removed before they build up to harmful levels;
• useful products made by the cell, such as enzymes, hormones and antibodies, which are released by secretion.Transport also lets a cell keep its internal conditions, such as ion concentrations and water content, different from the outside. Everything entering or leaving must cross the cell surface membrane, which controls what passes.Cells are small partly because transport relies heavily on diffusion, which is only fast over short distances. Size also matters because volume grows faster than surface area.Worked example: if a cell's diameter doubles from 20 µm to 40 µm, its surface area (proportional to diameter2) is multiplied by 4, but its volume (proportional to diameter3) is multiplied by 8. Each unit of volume now has half as much membrane to supply it, so the cell could not exchange substances fast enough.