Transcript File - Biology with Radjewski

Unit 2: Chapter 5
Biological Membranes
Structure of the Cell Membrane
6 main components
1. Phospholipids
2. Proteins
3. Cholesterol
4. Carbohydrates
5. Glycoproteins
6. glycolipids
Phospholipids (review)
• Membrane is a bilayer
– One layer of polar heads – they are
hydrophilic because they love water
– One layer of nonpolar tails – they are
hydrophobic because they hate water
Membranes have proteins!
•
2 main types
1. Integral proteins
•
•
•
•
Inserted into the membrane
May be unilateral – reach only partway across
the membrane
Or they may be transmembrane – completely
span the membrane
Removal disrupts cell membrane
2. Peripheral proteins
•
•
Not embedded in membrane, but attached to the
membrane surface by either integral proteins or
filaments from the cytoskeleton
Removal has little effect on cell membrane
7 functions of membrane proteins
1.
2.
3.
4.
5.
Anchoring cell
Passive transport
Active transport
Enzyme activity
Signal transduction – transmitting info
into cell
6. Cell recognition – like ID tags
7. Junction between cells – cell adhesion
Cholesterol
• Controls the fluidity of the cell membrane
History of the Cell Membrane
•
2 models
1. Sandwich Model
2. Fluid Mosaic Model
Sandwich
Model
• Davson and Danielli in
1935
• Perceived the plasma
membrane as a sandwich
where the proteins were
the bread and the
phospholipid bilayer was
the meat
Fluid Mosaic
Model
• Singer and Nicholson in
1972
• Said the membrane is
fluid and must be
• If it solidifies, its
permeability will change
and enzymes will
denature
• Said the membrane is
mosaic – there are
proteins embedded in it
Nature of protein and lipid mobility
• Lateral movement of lipids/proteins is quick
• Lipids and proteins rarely flip across the lipid
bilayer
Basic Terms to Understand
• Selectively permeable – prevents passage
of most materials through the membrane
• Solute – what is dissolving (salt, sugar,
etc.)
• Solvent – what it is dissolving in (water
etc.)
• Solution – mixture of solvent and solute
7 ways substances can get into a
cell
1.
2.
3.
4.
5.
6.
7.
Diffusion
Bulk flow
Osmosis
Facilitated diffusion
Active transport
Vesicle mediated transport
Cell to cell junction
Diffusion
• Moves materials from a high concentration to a
low concentration
• Requires no energy – type of passive transport
• Easy passage through – oxygen, carbon
dioxide, nitrogen, and small polar molecules
• Slow passage through – large polar molecules
like glucose and charged ions
– Proteins allow movement of charged/polar molecules
• Particles move until equilibrium
Bulk Flow
• Molecules move all together in same
direction due to hydrostatic pressure
Osmosis
• ALL things undergo diffusion.
• Water also diffuses, however, the water diffusion
is not evident unless it crosses a membrane.
• Osmosis is the diffusion of water across a
membrane from high to a low concentration
• No energy is required – type of passive transport
• Since cells have membranes, osmosis is
important to cells
Membranes and Osmosis
• Tank w/ semipermeable membrane: water
may pass, solute can’t
• At first the concentration of solute is very high
on the left.
• But over time, the water moves across the
semipermeable membrane, and dilutes the
solute.
Water Moves Because It is Polar
• Because water is polar, it binds to the solute by
hydrogen bonds
• The concentration of water is higher on the right
• Water will then flow across the membrane, down its
concentration gradient, to the left
Three osmotic environments
• Hypertonic
• Hypotonic
• Isotonic
– Same solute concentration inside and outside
the cell
– Water flows in and out of the cell equally in
both directions
– Most cells in our body are isotonic
Hypertonic
• High concentration of solute outside the
cell
• Therefore there is more water inside the
cell
• Water will move out of the cell
• If this process continues, the cell collapses
and dies – this is called plasmolysis
Hypotonic
• Low concentration of solute outside the
cell
• Therefore there is more water outside the
cell
• Water will move inside the cell
• This causes the cell to expand, causing
turgor pressure in plant cells Animal cells
could burst – this is called cytolysis
Plant Cells and Osmotic Pressure
• In plants, hypotonic solutions produce osmotic
pressure that produces turgor pressure
– Turgor means “tight or stiff owing to being very full”
– Keeps plant upright; in hypertonic conditions plants
wilt
Vacuole fills
Vacuole
shrinks
Hypotonic solution
Hypertonic
Hypertonic
Dialysis
• Dialysis is the diffusion of solutes across a membrane
• The selectively permeable membrane allows small
sugar molecules to move across the membrane, but
large proteins cannot
Facilitated Diffusion
•
•
Transport proteins move materials
through membrane
3 kinds of transport proteins
1. Uniport – carries a single molecule across
the membrane
2. Symport – moves 2 different molecules at
the same time in the same direction
3. Antiport – exhanges 2 molecules in opposite
directions
Vesicle Mediated Transport
•
•
When vesicles or vacuoles fuse with the
membrane to move substances in or out
of the cell
2 main types
1. Exocytosis – when vesicle expels contents
outside the cell
2. Endocytosis – when vesicles bring
substances into the cell
•
3 types
3 types of Endocytosis
1. Phagocytosis – solid being taken into the
cell
2. Pinocytosis – liquid being taken into the
cell
3. Receptor Mediated Endocytosis –
substance binds to a specific receptor on
the cell before it is brought in
Active Transport
• Energy is needed
• Moves materials against the concentration
gradient
• Main example: sodium potassium pump
Sodium Potassium Pump
• Occurs in animal cells
• Required ATP (active transport)
• Exchanges 3 Na+ ions on inside for 2 K+
ions on outside
• This exchange is uneven so an electric
potential is generated and so the
membrane is now considered to be
polarized
• Let’s see this in action
How do cells stick together and let
materials?
1.
2.
3.
4.
Plasmodesmata
Gap junctions
Desmosomes
Tight junctions
Plasmodesmata
• They are
channels
that allow
movement
of certain
molecules
and ions
between
plant cells
Gap Junctions
• Cytoplasmic channels between
neighboring animal cells
• Let adjacent cells communicate
• Small dissolved molecules and electrical
signals may pass from one cell to the
other
• Very similar to plasmodesmata
Desmosomes
• Function as rivets
and join animal
cells together
• They are
reinforced by
intermediate
filaments made of
keratin
• Still permits
materials to move
around them in the
intercellular space
Tight Junctions
• Continuous belt
around animal cells
that fuse
membranes of
neighboring cells
• It is leak proof
• Contains no
intercellular space
Water Potential
• Chemical potential of water
• Measure of the energy available for
reaction or movement
• Measures the ability of water to move and
water always moves from areas of higher
potential to areas of low potential
• Has the symbol Ψ (psi)
Water Potential Continued…
• Is measured in the unit bars
• The formula for calculated water potential
is osmotic potential (solute) + pressure
potential
So, the equation is
Ψ = Ψs + Ψp
Equation Components
Ψp = pressure on the system
= 0, if the system or container is open
** all of our problems will be open so
Ψp will always equal 0.
Ψs = change in water potential due to solute
molecules
The more solute, the lower the water
potential
Ψs = -iCRT
Where
i = ionization constant
= # of ions in the solute
= 1 when there are no ions present
C = Molar Concentration
usually given in problem
equal to Molarity (M), or moles/volume
R = pressure constant
= 0.0831 Liters X Bars/moles x Kelvin
this number never changes
T = temperature
needs to be in Kelvin
Conversion of Celsius to Kelvin is
K = 273 + Celsius
Suppose we have a beaker of distilled water at
room temperature. (0M) What is the water
potential?
Ψ= Ψs + Ψp
Ψ = -iCRT + 0
Ψ = -(1)( 0) (0.0831)(23 +
273) + 0
Ψ=0+0
Ψ=0
Suppose we have an open beaker A
that contains Sucrose (2.5M) at 25 C.
What is the water potential?
Ψ=Ψs+Ψp
Ψ = -iCRT + 0
Ψ = -(1)(2.5)(0.0831)(25 + 273)
Ψ = -61.91 bars