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1. Diffusion
· The net movement of molecules from a
higher concentration to a lower
concentration down a concentration
gradient until they are equally distributed
· The rate of diffusion is increased by a
­ greater concentration gradient
­ large surface area (microvilli)
­ short diffusion distance…read more

Slide 3

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Diffusion of a small molecule across a
concentration…read more

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Fick's Law
surface concentration
Rate of diffusion
area of
difference X
a substance =
across a membrane
thickness of membrane
1. Increased surface area = increased rate of diffusion
2. Increased concentration difference =
increased rate of diffusion
3. Increased membrane thickness =
reduced rate of diffusion…read more

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2. Facilitated Diffusion
· Does not require energy
· Diffusion is speeded up by specific
carrier proteins and ion channels in cell
membranes which transport ions and polar
· Occurs along a concentration gradient
from a high to a low concentration…read more

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Carrier proteins
take particular
substances eg
glucose from one
side of the
membrane to
another. No ATP
required.…read more

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