the

Cell membrane permeability - Animated membrane physiology

with their hydrophobic cores and charged

surfaces cell membranes are very

efficient at blocking passage of many

molecules some molecules such as water

dissolved oxygen and simple alcohols are

able to pass through cell membranes

freely by passive diffusion

however charged molecules and large

molecules cannot pass directly through

cell membranes and hydrophobic molecules

tend to become stuck inside the

hydrophobic interior of cell membranes

in order to allow these complex

molecules to enter and exit the cell

organisms rely on two forms of membrane

transport passive transport and active

transport passive transport occurs along

an electrochemical gradient it does not

require the expenditure of energy in the

form of ATP for transport active

transport requires the expenditure of

ATP in order to transport molecules

across the membrane active transport and

some forms of passive transport require

the use of transport proteins such as

the integral protein seen here