Download Membranes

ASSESSMENT STATEMENTS
2.4.1
Draw and label a diagram to show the structure of a membrane
2.4.2
Explain how the hydrophobic and hydrophilic properties of phospholipids help
to maintain the structure of cell membranes
2.4.3
List the functions of membrane proteins
2.4.4
Define DIFFUSION and OSMOSIS
2.4.5
Explain passive transport across membranes by simple diffusion and facilitated
diffusion
2.4.6
Explain the role of protein pumps and ATP in active transport across
membranes
2.4.7
Explain how vesicles are used to transport materials within a cell between the
rough ER, Golgi apparatus, and plasma membrane
2.4.8
Describe how the fluidity of the membrane allows it to change shape, break,
and re-form during endocytosis and exocytosis
2.4.2 EXPLAIN HOW THE HYDROPHOBIC AND
HYDROPHILIC PROPERTIES OF PHOSPHOLIPIDS
HELP TO MAINTAIN THE STRUCTURE OF CELL
MEMBRANES.
Phospholipid molecules make up the cell membrane
•
Hydrophilic phosphate head (attracted to water)
•
Two hydrophobic hydrocarbon tails (not attracted to water, but attracted to each
other)
Cell membranes are made up of a double layer (bilayer) of these phospholipid
molecules.
This makes the membrane very stable but also allows flexibility. The phospholipid in
the membrane are in a fluid state which allows the cell to change it’s shape
easily.
Cholesterol can be found in the hydrophobic region (fatty acid tails) in
animal cells.
They determine membrane fluidity, which changes with temperature,
allowing effective membrane function at a wider range of temperatures
Plant cells, don’t have cholesterol, so they rely on saturated or unsaturated
fatty acids to maintain proper membrane fluidity
The membrane also contains proteins:
Integral proteins go completely from one side of the
membrane to the other (a & b)
Peripheral proteins are attached to the membrane but
only on one side (c, d, & e)
2.4.3 LIST THE FUNCTIONS OF MEMBRANE
PROTEINS.
Membrane proteins can act as:
1. hormone binding sites
2. electron carriers
3. pumps for active transport
4. channels for passive transport
5. enzymes
In addition they can be used for cell to cell communication as well
as cell adhesion.
HORMONE BONDING SITES
These proteins may detect hormones arriving at the cells to signal
changes in function. They may also be involved in other cell and
substance recognition as in the immune system
ELECTRON CARRIERS
Help catalyze chemical reaction an important
role in photosynthesis and cell respiration.
PROTEINS AS ENZYMES
Break down chemicals for use within the cell
PUMPS FOR ACTIVE TRANSPORT
 Requires energy, in the form of ATP, or adenosine triphosphate
 Molecules are ‘pumped’ across the membrane UP the
concentration gradient
 Pumps fit specific molecules
 The pump changes shape when ATP activates it, this moves the
molecule across the membrane
CHANNELS FOR PASSIVE TRANSPORT
These proteins span the
membrane from one side to
another. They allow the
movement of large
molecules across the
plasma membrane.
2.4.4 DEFINE
DIFFUSION
Diffusion is the passive
movement of small particles
across a cell membrane. The
particle moves from a region
of high concentration to a
region of low concentration.
This process continues until
the concentration of the
substance is the same on
both sides of the membane
2.4.4 DEFINE OSMOSIS
Osmosis is the diffusion of water molecules across a semipermeable
membrane (allows only some molecules through) from a region of
lower solute concentration to a region of higher solute
concentration.
The movement of water into and out of cells depends on osmosis.
2.4.5 EXPLAIN PASSIVE TRANSPORT ACROSS
MEMBRANES BY SIMPLE DIFFUSION AND
FACILITATED DIFFUSION.
Simple Diffusion
Facilitated Diffusion
Both methods = Passive transport (do not require ATP)
Molecules moving from high concentration to low concentration
Diffusion of molecules through the
phospholipid bilayer
Uses channel proteins embedded in
the membrane
How hydrophobic (lipid soluble)
molecules will pass through
How hydrophilic molecules and
charged particles will pass through.
MEMBRANE TRANSPORT ANIMATIONS
http://www.northland.cc.mn.us/biology/BIOLOGY1111/animations/passive1.swf
2.4.6 EXPLAIN THE ROLE OF PROTEIN PUMPS
AND ATP IN ACTIVE TRANSPORT ACROSS
MEMBRANES.
Active transport involves the movement of substances through the membrane using
energy from ATP.
The advantage of active transport is that substances can be moved against the
concentration gradient, meaning from a region of low concentration to a region of
high concentration.
This is possible because the cell membrane has protein pumps embedded it which
are used in active transport to move substances across by using ATP. Each protein
pump only transports certain substances so the cell can control what comes in
and what goes out.
EXAMPLE: SODIUM POTASSIUM PUMP
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodi
um_potassium_pump_works.html
2.4.7 EXOCYTOSIS
Vesicles are used to transport materials within a cell between the rough
endoplasmic reticulum, Golgi apparatus and plasma membrane.
After proteins have been synthesized by ribosomes they are transported to
the rough endoplasmic reticulum where they can be modified.
Vesicles carrying the protein then bud off the rough endoplasmic reticulum
and are transported to the Golgi apparatus to be further modified.
After this the vesicles carrying the protein bud off the Golgi apparatus and
carry the protein to the plasma membrane.
2.4.7 EXOCYTOSIS CONTINUED…
At the plasma membrane the vesicles fuse with the membrane expelling their content
(the modified proteins) outside the cell.
The membrane then goes back to its original state
2.4.7 ENDOCYTOSIS
Endocytosis is a similar process which involves the pulling of the plasma membrane
inwards so that the pinching off of a vesicle from the plasma membrane occurs
and then this vesicle can carry its content anywhere in 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.
The phospholipids in the cell membrane are not solid but are in a fluid state
allowing the membrane to change its shape and also vesicles to fuse
with it.
This means substances can enter the cell via ____________________ and
exit the cell via ______________. The membrane then returns to its
original state.
EXERCISES
Why is the term ‘equilibrium’ used with passive but not active transport?
Why are exocytosis and endocytosis know as examples of active transport?