Ch. 7: Membrane Structure and Function. Introduction. The plasma membrane is selectively permeable . The macromolecules that make up the PM are lipids, proteins, and carbohydrates. Phospholipids make up most of the PM. Phospholipids are amphipathic molecules.
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Structure and Function
depends on size and attachment to cytoskeleton.
Hydrophobic region of protein
acids are more fluid.
(~107 times per second)
(~ once per month)
Movement of phospholipids
tails with kinks
Cholesterol within the animal cell membrane
B. The six major functions of membrane
Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)
C. The cytoplasmic and exterior sides of the
PM differ. This difference is determined
as the ER builds the PM. Vesicles fuse
with the PM enlarging the PM.
1. Cells recognize other cells by keying on
surface molecules, often carbohydrates,
on the plasma membrane.
Example: Four human blood types
(A, B, AB, O)
2. Ions and polar molecules pass through
Ex. Water & glucose
transport of ions and polar molecules.
Molecules of dye
Membrane (cross section)
Diffusion of one solute
Diffusion is random, until equilibrium
Diffusion of two solutes
1. A solution with the higher concentration
of solutes is hypertonic.
2. A solution with the lower concentration
of solutes is hypotonic.
3. Solutions with equal solute
concentrations are isotonic.
Water molecules will move from a hypotonic
solution to a more hypertonic solution. It
will move across a selectively permeable
membrane until the solutions are isotonic.
Plant cells have a cell wall that contribute to its water balance.
If a cell is isotonic to it surroundings, there is no movement of water into the cell and the cell is flaccid and the plant may wilt.
In a hypertonic solution, a cell wall has no
advantages. Plasmolysis will take place.
Example: Paramecium are hypertonic to
Paramecium have a specialized organelle, the contractile vacuole, that functions as a pump to force water out of the cell.
3. There are two models of facilitated
Fig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.
This pump maintains higher
concentrations of K+ inside the cell and
lower concentrations of Na+ inside the
The sodium-potassium pump uses the
energy of one ATP to pump three Na+
ions out and two K+ ions in.
called membrane potential.
will diffuse into the cell down the
voltage across the membrane are
called electrogenic pumps.
Proton pumps are found in mitochondria
and chloroplasts. These pumps store
energy that is later used for cellular work.
A substance that has been pumped across
a membrane can do work; as it diffuses
back, it can power another transport
This type of endocytosis is very
specific. Special receptors bind to
ligands that are in the extracellular
Receptor proteins are usually clustered
in regions of the membrane called
coated pits, which are lined on their
cytoplasmic side with protein.
An example of receptor-mediated
endocytosis is cholesterol. Cholesterol
is taken in for the synthesis of
membranes and as a precursor for
Cholesterol travels in blood in particles
called low-density lipoproteins (LDLs).
These particles bind to LDL receptors
and then enter the cells by endocytosis.
Passive Transport Active Transport
Polar – Yes
Nonpolar - No