Chapter 1.5 Movement of substances into and out of cells - Summary Edexcel International GCSE Biology Student Book Second Edition - Biology

1.5 Movement of substances into and out of cells

Concentration gradient
● Difference in concentration of net overall movement of molecules (normally
molecules move from area of higher concentration to area of lower concentration,
moving down concentration gradient)


● Cell membrane acts as boundary for cell contents, barrier for substances moving
into and out of cells (often semi/partially-permeable)




Diffusion (擴散)
● Net movement of molecules/ions from an area of higher concentration to an area of
lower concentration (of that molecule/ion)
● Plants - oxygen and carbon dioxide into and out of leaf through stomata, water
vapour out of leaf through stomata during transpiration (蒸騰作用)
● Humans - oxygen from air to alveoli in lungs, carbon dioxide from blood to alveoli,
dissolved substances (e.g.: sugars, amino acids) from blood to cells




Osmosis (滲透作用)
● Special case of diffusion, ***only involves water molecules

● From an area of higher water potential (e.g.: more watery/dilute solution) to an
area of lower water potential (e.g.: less watery/more solute-concentrated
solution)
● Plants: water into root hair cells from soil

● Animals: if too much water enters red blood cell, the cell swells (脹) and may burst
due to lack of cell wall; else the cell shrinks
● ***Osmoregulation (滲透調節): correct osmotic balance in human body (adjust of
level of water concentration in blood)




Active transport (主動運輸)
● ***Against concentration gradient (from an area of lower concentration to an area of
higher concentration)


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