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Camden County College
Biology 010
Audersirk, Life on Earth
Chapter 5: Cell Membrane Structure and Function
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The Plasma Membrane
a. The cell is the smallest unit of life. Each cell is surrounded by a thin plasma
membrane, which isolates the cell’s contents from the external environment.
The Structure of the Plasma Membrane
The overall organization of membranes can be described as proteins floating in a
double layer of lipids. The lipids are responsible for the isolation function of the
membrane.
a. Membranes are fluid mosaics. With the phospholipid bilayer serving as the semifluid layer.. The phospholipid molecule has a hydrophilic head – water loving
(the phosphate portion) and a hydrophobic tail – water fearing (the fatty acids
chains). There are also cholesterol molecules (made by your liver) embedded in
the phospholipid bilayer. Cholesterol makes the membrane stronger, more flexible
and more impermeable to water. The proteins embedded within the bilayer are
like mosaic patterns of tiles seen in some art.
The Plasma Membrane as Gatekeeper
a. The phospholipid bilayer blocks the passage of most molecules. Most biological
molecules including salts, amino acids, and sugars are polar (a least slightly
charged) and water soluble (dissolve in water). These substances cannot easily
pass through the nonpolar hydrophobic tails. However, very small molecules such
as water and nonpolar (uncharged), lipid-soluble molecules can freely.
b. The embedded proteins selectively transport, respond to and recognize molecules.
Membrane proteins fall into 3 categories – transport proteins, receptor proteins,
and recognition proteins.
i. Transport proteins – allow the movement of hydrophilic (water-soluble)
molecules through the plasma membrane.
ii. Receptor proteins – deliver chemical messages (like hormones) to the cell.
iii. Recognition proteins – (normally with sugars molecules attached) serve as
identification tags and cell surface attachment sites.
Diffusion
The concentration of a substance is the number of molecules of that substance in a
given volume. A gradient is a physical difference between two regions. A gradient
can cause molecules to move from one region to another.
a. Diffusion is the movement of molecules (which are in a liquid or a gas) from
regions of higher concentration to regions of lower concentration. Perfume
molecules for example, move in response to a concentration gradient. (There are
more perfume molecules within the perfume bottle than outside of it. The perfume
diffuses out of the bottle and can be smelled in the room.) Also a drop of dye
dropped in water illustrates diffusion as well.
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Osmosis
Osmosis is the movement of water across membranes from areas of higher water
concentration to areas of lower water concentration..Osmosis moves water across
selectively permeable membranes.
Diffusion/Osmosis and Active Transport Across a Plasma Membrane
There are two main types of transport across membranes: passive transport (diffusion
and osmosis) and energy requiring transport (active transport, endocytosis and
exocytosis)..
a. Passive transport is movement of substances across a membrane down a
gradient which does not require energy.
i. Simple diffusion – The diffusion of water, dissolved gases, or lipidsoluble molecules through a phospholipid bilayer.
ii. Facilitated diffusion – Diffusion of molecules through a channel or a
carrier protein.
iii. Osmosis – Diffusion of water across a selectively permeable membrane.
b. Active transport is movement across a membrane of substances that travel
against a concentration gradient. This requires the use of energy.
i. Active transport – Movement of individual small molecules or ions
through membrane-spanning proteins, using cellular energy. (ATP)
ii. Endocytosis –Movement into a cell of large particles, which are engulfed
as the plasma membrane forms vesicles that enter the cytoplasm.
iii. Exocytosis – The movement of molecules enclosed in a vesicle out of the
cell. The vesicle is formed in the cell, it moves to the surface of the cell,
fuses with the plasma membrane, opens to the outside, and its contents are
released.