Topic 2
2.4
Membrane Structure and Function
2.4.1Draw and label a diagram to show the
structure of membranes
The plasma membrane is selectively permeable
this is fundamental to life.
2.4.2 Explain how the hydrophobic and hydrophilic
properties of phospholipids help to maintain the
structure of cell membranes
Phospholipids are amphiphatic molecules: both
hydrophobic and hydrophilic regions
The glycerol end of the phospholipid is polar,
and therefore hydrophilic. The fatty acid tails are
non-polar and therefore hydrophobic. Individual
phospholipids to not bond to adjacent phospholipids,
they are held in place by the polar force of water.
They phospholipids can therefore move around each
other, the fluid mosaic model of membranes. This
fluidity is important in allowing material to move
across the membrane.
Proteins imbed themselves both within and on the
surface of the membrane according to their
polarity.
Fluid Mosaic Model- unsaturated hydrocarbon
tails have more " kinks" which keep adjacent
phospholipids separated and result in a less
viscous, more fluid membrane.
Cholesterol within the membrane can slow the
fluid membrane movement at higher temps by
clogging things up. (See figure 8.4 page 141)It can
keep a membrane more fluid at low temps by
separating the phospholipids. Life needs a fluid
membrane. Plants and animals can alter their
membranes seasonally to optimize fluidity during
differences in temperature. ( winter wheat)
Change the percentage of saturated
hydrocarbons, or add cholesterol and you can
alter the fluidity of the membrane.
2.4.3 List the functions of membrane proteins
Listed by the IB syllabus: the functions are
hormone binding sites, immobilized proteins, cell
adhesion, cell to cell communication, channels for
passive transport, and pumps for active transport
Membrane proteins main functions ( my list):
Transport ( both active and passive), enzyme
activity, intercellular joining, chemical signal, cell
recognition, maintaining cell shape by attaching to
cytoskeleton, and electron carriers ( mitochondria
and chloroplasts)
Proteins within the membrane can be anchored in
place by cytoskeleton. Otherwise they can drift
across the membrane.
Proteins provide functionality of the membrane
Check out page 72 and 73 in chapter 5 which
shows the characteristics of the 20 amino acids of
life. Integral proteins- have hydrophobic amino
acids that remain within the hydrocarbon tails of
the phospholipids. Hydrophilic amino acids would
extend to either side of the membrane, or both
sides ( transmembrane protein)
Peripheral proteins- have hydrophilic amino
acids and are more loosely bound to either the
inside or outside of the membrane. The
cytoskeleton can anchor them to the inside.
Carbohydrates are also associated with the
membrane proteins. Usually on the outside of the
membrane. This combination of carbohydrates
( glucose )and proteins is called a Glycoprotein.
Carbohydrates on membrane proteins are used
for cell recognition. Oligosaccarides ( few sugars)
Human blood types show variation in these
glycoproteins. (Type A,B,AB, and O)
2.4.4 Define diffusion and osmosis
Diffusion is the passive movement of particles
from a region of high concentration to a region of
low concentration.
Osmosis is the passive movement of water
molecules, across a partially permeable
membrane, from a region of lower solute
concentration to a region of higher solute
concentration.
Terms related to osmotic processes:
contractile vacuole, turgid, flaccid, plasmolysis
Hypertonic, Hypotonic, Isotonic
2.4.5 Explain passive transport across membranes
by simple diffusion and facilitated diffusion.
Crossing a membrane layer:
Hydrophobic molecules… hydrocarbons, CO2,
and O2 diffuse across easily
Ions have a hard time crossing the center of a
membrane- including water and glucose. These go
through transport proteins. ( facilitated diffusion)
Passive transport
Diffusion down a concentration gradient… no
ATP is expended. Each substance moves
independent of the other substances.
Facilitated diffusion: helps polar molecules cross
the membrane. Transport membranes span the
membrane. Have limits to amount of material
passed… can be saturated. Can also be inhibited
by analogs to the target molecule.
Can be simple open channels, or gated
channels that open upon a stimulus.
2.4.6 Explain the role of protein pumps and ATP in
active transport across membranes
Active transport:
Uses ATP to move molecules against the diffusion
gradient. Classic example is the Na/K pump.
(See figure 8.15 page 149) 3Na+ are pumped out
and 2 K+ move in… creating a separation of
charge. The inside of the cell is more negative.
This electrochemical gradient helps change the
diffusion gradient for passive transport of other
ions.
Note: there are other protein pumps, such as the
H+ pumps used in photosynthesis and respiration.
2.4.7 Explain how vesicles are used to transport
materials within a cell between the rough
endoplasmic reticulum, Golgi apparatus and plasma
membrane
The rough endoplasmic reticulum contains the
ribosomes that produce proteins that are meant to
be secreted from a cell. Polypeptides move in
vesicles to the Golgi apparatus where they complete
their construction ( by folding, adding other
molecules like Fe and Mg, or adding carbohydrates)
The vesicles contained the finished protein product
then join with the plasma membrane and the
protein is released from the cell.
2.4.8 Describe how the fluidity of the membrane
allows it to change shape, break and re-form during
endocytosis and exocytosis.
Exocytosis
Large molecules can be brought out of a cell by
combining the vacuole that contains the molecules
with the plasma membrane. Cell walls are made
this way in plants, nerve signals are released into
the synapse…etc
Endocytosis
Molecules are brought in by engulfing them with
the membrane, it can either simply sink in to the
plasma membrane and be pinched off as a vesicle or
be actively engulfed like an amoeba.
pinocytosis… engulfs liquids
phagocytosis… engulfs particles
receptor mediated endocytosis… the ligand, or
target molecule binds to the receptor protein. Once
attached to the protein, it sinks into a vesicle. Cells
can concentrate molecules that are rare outside the
cell.