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Biology 12
3.5 The Permeability of the Plasma Membrane
The plasma membrane is selectively permeable, allowing passage of only certain molecules.
Passage of Molecules Across the Membrane
• Some substances freely cross the membrane. They move “down” their concentration
gradient (from high concentration to low concentration).
• Some substances are unable to freely cross and are transported by proteins or vesicles.
They may go “up,” or against, their concentration gradient.
Diffusion
Diffusion is the movement of molecules down their concentration gradient. It does not require
energy. The rate of diffusion is affected by factors such as temperature, pressure, and molecule
size.
• A solution contains a solute in a solvent. Diffusion occurs until there is an equal
distribution of solute and solvent.
Biology 12
Diffusion of Oxygen
Only a few types of molecules can diffuse across the plasma
membrane.
• Gases can diffuse across the bilayer
• Oxygen enters cells and carbon dioxide leaves
• In lungs, oxygen moves from the alveoli to blood
in the capillaries
Osmosis
Osmosis is the diffusion of water molecules across a selectively permeable membrane due to a
difference in concentration.
• There is a net movement of water and changes in solute concentration on both sides of
the membrane
Biology 12
Isotonic, Hypotonic, and Hypertonic Solutions
Isotonic solutions have the same concentration of solute and
solvent as the solution inside the cell, and water will not
enter or leave the cell.
Hypotonic solutions have a lower concentration of solute
than solution inside the cell, and water will enter the cell.
Hypertonic solutions have a higher concentration of solute
than solution inside the cell, and water will leave the cell.
Prefixes:
iso: the same as
hypo: less than
hyper: more than
tonicity: refers to osmotic
pressure
Transport by Carrier Proteins
The plasma membrane stops the passage of most molecules into and out of the cell. However,
biologically important molecules do pass. They do so because of carrier proteins that exist in
the plasma membrane.
• Carrier proteins are specific and each binds to specific molecules
• Carrier proteins are required for both facilitated transport and active transport of
substances across the plasma membrane
Facilitated Transport
• Assists in transport of molecules across the membrane by binding to those molecules
• Occurs down a concentration gradient and does not require ATP
Active Transport
• Assists transport of substances across the membrane by binding to them
• Occurs against a concentration gradient and requires energy, usually in the form of ATP
Proteins involved in active transport are often called pumps because they use energy to pump
substances against their concentration gradient.
• One important carrier protein pump is the sodium-potassium pump. It moves sodium
ions to the outside of the cell and potassium ions to the inside of the cell.
Biology 12
Bulk Transport
Macromolecules are transported into and out of the cell by vesicle formation, called
membrane-assisted transport in energy-dependent processes.
• Exocytosis is a way substances can exit a cell
• Endocytosis is way substances can enter a cell
Exocytosis
During exocytosis, a vesicle fuses with the
membrane and the substance it is carrying is
secreted outside of the cell.
• Neurotransmitters, hormones, and
digestive enzymes are examples of
substances secreted in this way
Endocytosis
During endocytosis, cells take in substances by vesicle formation.
• The plasma membrane folds in on itself and then pinches off to form an intracellular
vesicle
Endocytosis occurs in one of three ways.
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis
Phagocytosis
During phagocytosis, the material being taken into
the cell is large, such as a food particle or another
cell.
• Common in unicellular organisms and
occurs in certain types of human white
blood cells
Pinocytosis
During pinocytosis, vesicles form around liquid or very small particles.
• Common in blood cells, intestinal cells, and plant root cells
Biology 12
Receptor-Mediated Endocytosis
Receptor-mediated endocytosis is a type of pinocytosis. It involves receptor proteins that only
bind to certain molecules.
• The receptors are in coated pits. Once vesicles form, they become uncoated and fuse
with lysosomes. Empty vesicles fuse with the plasma membrane and receptors return to
their previous locations.