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Diffusion
The exchange of substances between cells and the environment occurs in ways that require
metabolic energy and in ways that do not.

Active Transport Requires energy

Passive Transport Does not require energy



To understand diffusion and other forms of passive transport it is necessary to understand
that:

All particles…

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Simple Lipid Diffusion

A few substances can diffuse directly through the lipid bilayer of the membrane.

The only substances that can do this are hydrophobic (lipid-soluble) molecules such as
water, oxygen and carbon dioxide.

Lipid diffusion is a passive process, no energy is involved and substances can only move
down…

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Facilitated Diffusion
Facilitated diffusion is a passive process.

It relies only on the kinetic energy of the diffusing molecules.

There is no external input of energy.

It occurs down a concentration gradient.

It occurs at specific points on the plasma membrane where there are special protein
molecules.



Channel Proteins
These…

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