Transport Across Cell
Membranes
The lipid bilayer is only permeable to small and nonpolar substances.
● Oxygen and carbon dioxide are nonpolar gases that freely diffuse from their areas of higher
concentration to areas of lower concentration.
● Steroid hormones have a cholesterol-based structure and are able to pass through cell membranes,
although more recent discoveries have demonstrated that even steroid hormones often make use of
a variety of protein channels and transporters.
● Ethanol alcohol has a slow and limited ability to cross cell membranes, but it does help to explain
some of its toxic effects.
● Even water , although small, is too polar to easily diffuse across the cell membrane.
Passive transport is movement down a concentration gradient.
● Simple diffusion is when the substance can pass through the lipid bilayer .
○ Gas exchange of oxygen and carbon dioxide is the best example for healthcare
students of substances that don’t need a transport channel.
Facilitated diffusion is passive transport that uses a channel.
● Aquaporins are extremely complex channels, formed from the coordinated interaction of 6
different proteins
○ They are inserted into the membrane under the influence of vasopressin - it
allows the body to retain more water in the body and this raises blood pressure.
■ This hormone is also known as antidiuretic hormone (ADH, made in the pituitary gland) to
emphasize that by retaining water , we make less urine.
■ Aquaporins facilitate osmosis, the diffusion of water . The channels allow water to move either
direction. Sometimes water leaves cells, and sometimes it enters cells, always following the rules of
diffusion. Understanding the movement of water under different medical circumstances will help you
learn about fluid shifts such as what occurs in pulmonary edema or circulatory shock.
Membranes
The lipid bilayer is only permeable to small and nonpolar substances.
● Oxygen and carbon dioxide are nonpolar gases that freely diffuse from their areas of higher
concentration to areas of lower concentration.
● Steroid hormones have a cholesterol-based structure and are able to pass through cell membranes,
although more recent discoveries have demonstrated that even steroid hormones often make use of
a variety of protein channels and transporters.
● Ethanol alcohol has a slow and limited ability to cross cell membranes, but it does help to explain
some of its toxic effects.
● Even water , although small, is too polar to easily diffuse across the cell membrane.
Passive transport is movement down a concentration gradient.
● Simple diffusion is when the substance can pass through the lipid bilayer .
○ Gas exchange of oxygen and carbon dioxide is the best example for healthcare
students of substances that don’t need a transport channel.
Facilitated diffusion is passive transport that uses a channel.
● Aquaporins are extremely complex channels, formed from the coordinated interaction of 6
different proteins
○ They are inserted into the membrane under the influence of vasopressin - it
allows the body to retain more water in the body and this raises blood pressure.
■ This hormone is also known as antidiuretic hormone (ADH, made in the pituitary gland) to
emphasize that by retaining water , we make less urine.
■ Aquaporins facilitate osmosis, the diffusion of water . The channels allow water to move either
direction. Sometimes water leaves cells, and sometimes it enters cells, always following the rules of
diffusion. Understanding the movement of water under different medical circumstances will help you
learn about fluid shifts such as what occurs in pulmonary edema or circulatory shock.
