2.2 Membrane Structure and Function page 81-86 ques. 1,2,5,6,9,12, page 86
CELL MEMBRANES:
Permeable à allows movement across the membrane
Impermeable à no movement across the membrane
Selectively permeable à allows certain substances to enter
or leave but not others
small molecules pass easily through the cell, medium-
sized ones pass less easily, large ones cannot pass through at all. Cells, therefore, Have a selectively permeable membrane
selective permeability diagram
THE FLUID MOSAIC MEMBRANE MODEL
Remember: cell membranes are composed of a
phospholipid bilayer that has fluid consistency
and is dotted with protein molecules
ß the polar head of the membrane is attracted to water
(phosphorus end)
ß the hydrocarbon chains in the fatty-acid tail (non-polar)
are not attracted to water but are compatible with other
lipids
There are many proteins built into the membrane:
Ø Proteins that carry sugars are glycoproteins and are used to identify different cells
à this is why transplanted organs are often rejected
FOUR ROLES OF MEMBRANE PROTEINS
1) transport protein channels allow substances through the membrane
2) Enzymatic activity used in respiration and photosynthesis
3) Triggering signals hormones bind on and cause changes on the inner surface of the membrane.
4) Attachment and Recognition points where cytoskeleton can attach or used in cell identity
Ø Peripheral proteins are suspended in the bilayer(on the outside or inside layers only)
Ø Integral proteins are deeply embedded (go all the way through)
2.4 Transport across Membranes pgs 89 -96 qu. 1,2,3,5,7,8 pg 97
Definition à solute: the substance that is dissolved in solution
Definition à solvent: the substance that does the dissolving of
another substance (e.g. H2O)
PASSIVE TRANSPORT
Ø No energy required to move molecules along a concentration gradient (goes from high concentration [] to low)
Definition à concentration gradient: a difference in concentration between two areas
3 types of Passive Transport
1. DIFFUSION à the movement of molecules from an area of
high concentration to an area of low concentration
Ø it is fueled by the constant random motion of particles
Ø e.g. Air Freshener, perfume, the smell of coffee
Ø This can also happen across a selectively permeable membrane (it only allows certain substances to pass through them)
Diffusion in Cells
High
Concentration Low Low
O2 O2 High Concentration
CO2 CO2
O2 is in high concentration outside the cell and low concentration inside the cell è oxygen diffuses inside the cell
Dynamic Equilibrium
à State of balance where particles move at equal rates in all directions
2. OSMOSIS à the movement of water through a semi-
permeable membrane from high concentration to low concentration
Maintaining the correct proportion of water inside and
outside the cell is essential for cell survival
Osmosis occurs in three different solutions:
· Hypertonic à the solute is greater outside the cell than
Inside
à water exits the cell
· Isotonic à the solute is equal inside and out
à the rate of water entering the cell equals the
rate it exits the cell
· Hypotonic à the solute is less outside the cell than inside
à water enters the cell
Note that animal cells do best in isotonic solutions while plant cells prefer hypotonic ones
WHY?
Osmosis and Animal Cells
· Animal cells Do Not have a cell wall
Isotonic Hypotonic Hypertonic
Normal Lysed Shriveled
E.g. Red Blood Cells (RBCs)
· Blood serum (liquid part of the blood) is isotonic when compared to red blood cell cytoplasm
· This helps RBCs to maintain their shape
Hemolysis à if the blood serum is diluted (hypotonic
solution)
à the RBCs will swell and may burst. This will
be fatal
Crenation à if the blood serum becomes concentrated (hypertonic solution)
à RBCs lose water and stick to one another
à this clogs veins and arteries and may be fatal
Osmosis and Plant Cells
· Plant cells have a cell wall made of cellulose
Turgor Pressure à in a hypotonic solution, water pressure
pushes the cytoplasm up against the
cell wall
à the rigid cell wall keeps the cell from
bursting
Plasmolysis à in a hypertonic solution, the cytoplasm
pulls away from the cell wall and the cell
(and plant) wilts
3. FACILITATED DIFFUSION à Diffusion across a
membrane with help from trans-membrane proteins
Trans-membrane proteins àthese span the thickness of the
phospholipid bilayer
à movement is still from an area of high concentration
to an area of low concentration
à needed for:
· large polar molecules such These particles cannot
as glucose diffuse across
· large ions such as sodium membranes on their
and chloride ions own
Carrier protein à trans-membrane protein that assists the
diffusion of certain substances through a membrane
Outside the Cell
Inside the Cell
Read Pgs. 50-53
Read Pgs. 56-58
# 1, 2 Pg 58
ACTIVE TRANSPORT
Ø Cells use their own energy to transport materials from an area of low concentration to high concentration
Ø Just like facilitated diffusion, this requires a trans-membrane carrier protein
Ø Adenosine triphosphate (ATP) provides the energy needed for this process
Ø As this molecule is broken apart, energy is created
Ø 30% to 40% of all energy used while sleeping is for active transport
Why is this important to us?
· Nutrients must be absorbed quickly in the small intestine or they will be lost as waste.
· If our bodies relied on simple or facilitated diffusion, about half of the nutrients would be lost
· Therefore cells pump materials against a concentration gradient to maximize nutrient absorption
FOCUS ON NERVE AND MUSCLE CELLS
THE SODIUM POTASSIUM PUMP
Ø Note that for every three Na+ (sodium ions) pumped out of the cell, two K+ (potassium ions) are pumped in
Ø Energy is created when the ATP molecule transfers a phosphate group to the transport protein
All methods of transport studied to this point involved the movement of dissolved materials
Bulk Transport à used to move materials that are not dissolved in solution
2 Types of Bulk Transport
1. ENDOCYTOSIS à the process of the cell membrane
folding in on itself to form a vesicle that brings a substance into a cell
à the cell also often extends its cytoplasm to do this
Pinocytosis: liquid droplets are engulfed
e.g. Cells bring cholesterol into the cytoplasm
Phagocytosis: solid particles are engulfed
E.g. White blood cells (called macrophages) use this process to engulf harmful bacteria
2. EXOCYTOSIS à movement of products outside the cell
membrane
E.g. used to export materials such as hormones or enzymes
“THE PATH OF THE PROTEIN”
o Made by ribosomes
o Travel through the rough ER to the Golgi Body
o Travels though the Golgi Body
o Packaged into a vesicle
o Vesicle leaves the Golgi body and attaches to the cell membrane
Protein exits the cell and enters the extracellular fluid (fluid outside the cell) TEA INFUSION DIFFUSION
è Brewing of tea involves the diffusion of solutes into water
Materials: 3 tea bags, 3 beakers, cold, hot and room temp. H2O
1. fill each beaker with a different temperature of water
2. gently place a tea bag on the surface of each beaker at the same time. Do not disturb.
a) Observe the results in each beaker and describe what happens
b) Does the filter paper of the tea bag act as a selectively permeable membrane? Explain.
c) How did the temperature of the water affect the diffusion of solutes is each of the three beakers?
d) Describe two other examples of diffusion that occurs while preparing food or cleaning clothes.
o