Download MEMBRANE STRUCTURE AND FUNCTION CH 7

MEMBRANE STRUCTURE AND
FUNCTION
CH 7
I. The membrane is a fluid mosaic
A. The phospholipid bilayer
• Composed of two layers of phospholipids
• hydrophobic tails face in, polar heads face
out
• prevents soluble material from passing across
the membrane and allows for diffusion of
small nonpolar molecules across the
membrane
B. proteins
• partially or completely span the membrane
• 6 main functions of membrane proteins
– Transport
– Enzymes
– Cell recognition
– Cell communication
– Cell anchorage
– Attach to cytoskeleton and extracellular matrix
C. Membranes are fluid
• To allow efficient transport across the
membrane
1. Factors that affect fluidity
• The amount of unsaturated fatty acids
– Unsaturated fatty acids have carbon-to-carbon
double bonds
– Double bonds form kinks
– Kinks reduce hydrophobic interactions between
tails
– Less hydrophobic interactions the more fluid
• Temperature
– The higher the temp the more fluid the
membrane
• Amount of cholesterol
– Animal membranes
– At high temps it inhibits mobility
– At low temps it maintains fluidity by preventing
tight packing of phospholipids
2. Variations in lipid composition of
cell membranes
• Membranes of different species can have
different lipid compositions due to different
environments
• This is an adaptation
• Many species that live where temps vary can
alter the lipid composition of their
membranes
•
II. Membranes are Selectively
Permeable
• They regulate what enters and leaves
• Hydrophobic molecules can easily slip thru
the bilayer (gases, steroids)
• Polar molecules and large molecules can’t
cross as easily (sugar, ions, proteins)
• Hydrophobic molecules slip thru bilayer
• Smaller hydrophilic molecules must pass thru
transport proteins
III. Mechanisms of transport across
the membrane
A. Passive transport
• general features
– high to low concentration (down the
concentration gradient)
– no energy required
– Types:
• Diffusion
• Osmosis
• Facilitated diffusion
1. Diffusion
• Movement of small
nonpolar molecules
across the
phospholipid bilayer
• Down the gradient
• No energy
• Ex: O2 into cells CO2
out of cells. Only
photosynthetic cells
go both ways
2. facilitated diffusion
• movement of slightly larger molecules
through transport proteins
• down the gradient
• no energy
• as solute concentration increases, rate of
transport increases, till all transport proteins
are saturated and rate levels off
3. osmosis
• movement of WATER across the phospholipid
bilayer thru aquaporins
• no energy
• down the gradient (hypotonic to hypertonic;
high water potential to low water potential)
• a hypotonic solution has more water and less
solute than a hypertonic solution
• Water potential = solute potential + pressure potential
(Ψ=Ψs +Ψp)
– Water potential: tendency for water to move across a
membrane from higher water potential to lower water
potential
– In a living cell it is zero or negative
– solute potential results from the presence of solutes and is
always negative (Ψs = -iCRT)
– the greater the solute concentration (C) the smaller the solute
potential
– pressure potential is zero unless a force (like turgor pressure
from cell wall) is applied. In plant cells as water flows in Ψp
becomes positive
– therefore Ψ=Ψs
– the higher the concentration of solute, the lower the water
potential and the greater the tendency of water to move
•
• If a cell is placed in a hypotonic solution it
will _________________
• If a cell is placed in a hypertonic solution it
will _________________
http://www.northland.c
c.mn.us/biology/biology
1111/animations/active1
.swf
B. Active transport
• Movement of ions UP the concentration
gradient (low to high) thru transport protein
specific to ion
• requires the expenditure of energy
• The energy changes is the shape of the
transport protein so ions can be transported
http://www.northland.cc.
mn.us/biology/biology11
11/animations/active1.s
wf
• Many active transport pumps are
electrogenic pumps
– They help maintain membrane potential by
generating voltage across the membrane
– Voltage is due to differences in positive and
negative charges across the membrane
– Electrogenic pumps store energy for cellular work
• Proton pump in ATP synthesis
• Na+/K+ pump in nerve impulse transission
C. Bulk movement
• Transport of large molecules across the
membrane in vesicles using energy
1. Endocytosis
– movement of material into the cell
– stuff ingested is engulfed in a vesicle
– The vesicles from endocytosis fuse with
lysosomes for digestion
2. Exocytosis
• Movement of large molecules out of cell thru
vesicles especially proteins
• The vesicles with proteins to be secreted by
exocytosis came from RER and Golgi