MEMBRANE STRUCTURE AND FUNCTION
CH 7
I) The structure of membranes
A. The phospholipid bilayer
1. The bilayer
Composed of two layers of phospholipids
The hydrophobic tails face in and the polar heads face out
prevents soluble material from passing across the membrane and allows for
diffusion of small nonpolar molecules across the membrane
B. The phospholipid bilayer is fluid
1. purpose: TO ALLOW EFFICIENT TRANSPORT ACROSS THE MEMBRANE
2. Factors that affect fluidity
a. 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
b. Temperature
The higher the temp the more fluid the membrane
c.
Amount of cholesterol
The more cholesterol the more fluid the membrane cuz it prevents
hydrophobic interactions
3. The membrane is a fluid mosaic
a. components of the membrane
1. phospholipid
forms the bilayer
2. proteins
partially or completely span the membrane
kinds of membrane proteins:
o transport proteins act as channels to let things in and out
o enzymes on the interior surface of membrane carry out
reactions
o receptors on cell surface receive signals from other cells
o identity markers let other cells know who they are
o adhesion proteins to stick cells together
3. Cytoskeleton fibers on the inner surface of the membrane
http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1101
II. Mechanisms of transport across the membrane
A. Passive transport
1. general features
high to low concentration (down the concentration gradient)
no energy required
2. Types of passive transport
a. 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 plant
cells go the other way
Rate of diffusion is dependent on the differences in solute
concentration on either side of the membrane
b. facilitated diffusion
movement of slightly larger molecules through transport proteins
down the gradient
no energy
transport is saturable:
rate of transport
rate of transport
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solute concentration (mM)
as solute concentration increases, rate of transport increases, till all transport
proteins are saturated and rate levels off
c. 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
If a cell is placed in a hypotonic solution it will _________________
If a cell is placed in a hypertonic solution it will _________________
http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html
B. Active transport
1. Features
Movement of ions across the membrane
Ions move UP the concentration gradient (low to high)
requires the expenditure of energy
requires a transport protein which is specific to the ion being transported
The energy changes the shape of the transport protein so ions can be transported
http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#
Compare passive and active transport:
Energy used to pump ions up the concentration gradient is stored in the gradient
As the ions diffuse down their gradient, the stored energy is used to drive another cellular
process
1. Sources of energy used in active transport
a. ATP
b. electrons (in photosynthesis and respiration)
2. Cellular processes that use the energy stored in an ion gradient (and therefore are
dependent on active transport)
a. opening and closing of stomata on plant leaves to let oxygen, carbon dioxide, and
water in and out of plant
b. formation of ATP during photosynthesis and aerobic respiration
c. generation of nerve impulses
d. muscle contraction
C. Bulk movement
Transport of large molecules across the membrane using vesicles
Requires energy
1. Endocytosis
movement of material into the cell
stuff ingested is engulfed in a vesicle
membrane gets smaller during endocytosis
Types of endocytosis
o Phagocytosis
 Movement of solid material into cell
Single celled organisms eat like this
White blood cells destroy invaders like this
o Pinocytosis
 Movement of large molecules dissolved in water into cell
o Receptor mediated endocytosis
 Large molecule enters cell by binding to a specific cell surface
protein
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#
2. Exocytosis
Movement of large molecules out of cell thru vesicles especially proteins
The vesicles from endocytosis fuse with lysosomes for digestion
The vesicles with proteins to be secreted by exocytosis came from RER and Golgi
Membrane lost during endocytosis is replaced by exocytosis
III. The relationship between cellular transport mechanisms and vertebrate organ systems
A. The digestive system
after digestion is complete in the small intestine nutrients like glucose, amino acids
and acetyl CoA pass across the cells of the small intestine into the blood via
facilitated diffusion
they are carried through the circulatory system to all of the cells and enter the cells
by facilitated diffusion where they enter into aerobic respiration
B. Respiratory System
oxygen that is inhaled enters the lungs to the alveoli where it diffuses across the
cell membranes of the alveoli and enter into the blood
carbon dioxide from the blood diffuses across the cell membrane into the alveoli to
be exhaled
oxygen is carried to all of the cells by being bound to hemoglobin in the RBC
at the cells of the body oxygen diffuses into the cells
CO2 diffuses out of the cells to go back to the lungs
C.
Excretory System
Ions, glucose are passively transported into filtrate in the Glomerulus by ion
channels and facilitated diffusion
Water osmoses in too
Ions are actively transported back out into blood and water follows by osmosis
This prevents terrestrials from peeing too much water