Download Movement through membranes

SICM Tuition
Biology AS
Movement through membranes
(a)
(b)
(c)
(d)
(e)
diffusion
facilitated diffusion
active transport
osmosis
exocytosis / endocytosis
We have already talked about most of these at
GCSE. (and in the previous weeks!) so
hopefully this shouldn’t take that long. But
then again…that’s assuming you remember any
of it!
Diffusion
(a)
-
“the net movement of uncharged molecules (e.g. CO2, O2, urea) from a region
where they are in high concentration to a region where they are in low
concentration”
-
i.e. DOWN THE CONCENTRATION GRADIENT
-
it occurs wherever a concentration gradient exists and would continue until the
diffusing substance is evenly distributed
-
e.g. O2 through cell membrane of plant and animal cells or released from
photosynthesising chloroplasts.
Rate of diffusion depends on…
-
concentration gradient
-
the distance – shorter the distance, the faster the rate:
-
area – the larger the surface area, the greater the rate of diffusion
-
nature of the structure – the greater the number/size of pores, the greater the
rate of diffusion
-
the size and nature of the diffusing molecule – the smaller the molecule, the
greater the rate of diffusion. e.g. fat soluble molecule – diffuses fast
-
size of charge on ion – posit ive (+) ions move in more readily than
negative (-)ions. Ions of a higher charge are attracted more into the cell
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rate
∝
1
distance2
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Biology AS
Fick’s law of diffusion
(I would talk about how this law is “Ficksed”, but that would be as bad as the CGP books
and we don’t want that now do we?!)
rate of diffusion
∝
surface area × concentration difference
thickness of membrane
Routes of diffusion
(a)
(b)
-
-
CO2, O2, H2O(even though it is polar), urea, ethanol (i.e. small uncharged
molecules)
-
the smaller / the more fat soluble the molecule, the faster the rate (e.g. O2,
CO2, urea, ethanol – very rapid)
-
molecules squeeze between polar phospholipids heads – they dissolve on the
lipid on one side and emerge from the other
O2
-
Thus, K+, Na+, Cl-, HCO3- and glucose cannot cross in this way. They have to
be aided by proteins.
Glucose, amino acids and ions travel through in different ways:
through water filled pores in the channel proteins
-
pores are selective in determining which substance will move across (by size of
pore)
-
pores may be gated – open / closed by nerves.
Conclusion
Diffusion:
down a concentration gradient
no energy needed
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Biology AS
Facilitated Diffusion (diffusion that is facilitated…simple enough?)
(b)
-
for substances (e.g. glucose) the rate of diffusion is speeded up by the presence of
protein carriers in the membrane
rate of reaction
glucose
concentration
limiting rate
number of carrier
molecules limits rate
concentration of glucose
-
down a concentration gradient (i.e. from high conc. → low conc.)
-
there are 2 types of protein membrane for facilitated diffusion:
i.
specific carrier proteins
forming a “gate” allowing solute to go
through (e.g. to transport glucose)
carrier protein for glucose – permease
glucose binds to permease on one side of the membrane and is
released from the other
carrier protein specific to particular molecules
ATP
ADP
the binding of the solute molecule to the carrier protein alters the
conformation of the carrier so that its position in the membrane
changes and the solute molecule is discharged to the other side of the
membrane
ii.
ion channels
protein pores that open / close to control the passage
of selected ions (e.g. Na+, K+
hydrophilic channel allows solutes through
Summary – facilitated diffusion:
(a)
Carrier proteins
(b)
No energy
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(c)
down a gradient
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Biology AS
Active Transport
(c)
-
the movement of substances across a membrane using energy (usually A.T.P.)
-
occurs against a concentration gradient – involves a specific carrier protein
-
typically, A.T.P. is hydrolysed and the binding of the phosphate group to the
carrier molecule changes the protein’s confirmation in such a way that the solute
is delivered / transferred across the membrane.
-
the specific carrier protein also acts as ATPase – releasing the energy for A.T.P.
Key facts
factors that affect the rate of respiration (and therefore A.T.P. production) will
affect the rate of active transport
-
in cells where active transport is particularly important, there are large numbers of
mitochondria (to produce A.T.P.)
-
active transport is important in many aspects of physiology. E.g.:o
o
o
o
uptake of products of digestion
uptake of mineral ions from the soil
nerve impulses
reabsorption in kidney tubules
Summary
AGAINST concentration gradient
uses energy (A.T.P.)
specific carrier protein
More on active transport…
-
there are 3 methods of carrier proteins:
(a)
Uniport
-
where a single substance is transported across a membrane
(i.e. in one direction) (e.g. calcium pumps in the muscles)
(b)
Symport
-
where 2 substances are transported in the same direction
(e.g. glucose/sodium pumps)
(c)
Antiport
-
where 2 substances are transported in opposite directions
at the same time (e.g. sodium/potassium pumps - nerve impulse)
uniport
symport
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antiport
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Biology AS
Osmosis
(d)
“the movement of water from a region of high water potential to a region of low water
potential through a differentially permeable memrane.”
lets molecules through pores at different rates
e.g. sucrose (big) = 0 m.p.h
water (small) = fast!
Water potential (φ)
This is a measure of the free energy of water molecules. Molecules of water are in a
constant state of random motion. This is greatest in pure water – water potential = 0.
When a solute is added (i.e. a solution is formed) the solute molecules slow down the water
molecules so that they have less energy – the solution therefore has a lower water potential
than pure water.
The amount by which the solute particles lower the water potential is called the solute
potential. All solutions have a lower value than 0.
Osmosis therefore can be redefined:
“The movement of water from a region area of less negative water potential to a region of
more negative water potential through a differentially permeable membrane.”
(i)
animal cells
a. in pure water
i.
water moves in by osmosis from a higher water potential to a lower
water potential.
ii.
the cell expands and bursts.
b. in concentrated sugar solution
i.
water moves out of the cell by osmosis and the cell shrivels.
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(ii)
Biology AS
plant cells
a. in pure water
i.
water moves into the vacuole through the tonoplast, which increases
in size, forcing the cytoplasm against the cell wall.
ii.
the cell is turgid and no more water enters due to the presence of the
cell wall
iii.
as there is now no net movement of water into the cell, the water
potential inside the cell must be the same as outside the cell
iv.
as pure water surrounds the cell, the water potential within the cell is
equal to 0.
v.
inside the cell, there is till a solute potential trying to draw the water
into the cell – but the effect is counteracted due to a force called the
pressure potential due to the cell wall.
vi.
Thus: water potential = solute potential + pressure potential
b. in concentrated sugar solution
i.
water leaves the cell due to osmosis as the water potential is higher
inside the cell
ii.
the cell contents shrink away from the cell wall (i.e. the cytoplasmic
membrane loses contact with the cell wall)
iii.
the cell is called plasmolysed
Incipient plasmolysis
-
the cell wall will not be pressed upon by the cytoplasm (i.e. cell membrane has
just begun to shrink from the cell wall)
-
thus the pressure potential = 0
-
therefore: water potential = solute potential
Some words to know…
Hypertonic = high solute concentration
Hypotonic = low solute concentration
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Biology AS
Endocytosis and Exocytosis
Endocytosis
-
moving large quantities of material into a cell
stuff which is too big for a membrane
object
membrane buckles
inward
membrane surrounds
object
object in vesicle
-
(e.g. phagocytosis)
-
this is called pinocytosis when liquid (which would normally be water in
living organisms).
-
when the object is in the cell, the lysosomes (which contain enzymes) go to
the object to digest it
-
the buckling of the membrane is caused by protein receptors.
Exocytosis
object to be
removed
fuses with membrane
released
-
(e.g. releasing enzymes: e.g. pancreatic cells or wall in the digestive system)
-
endocytosis would cause the membrane to be smaller, but this des not happen:
therefore, exocytosis and endocytosis must equal out.
SYLLABUS CHECKLIST
Unit 1.3 – Transport across membranes
- understand how molecules and ions move into and out of cells;
-
understand the principles involved in passive transport by diffusion and facilitated
diffusion;
-
understand the principles of osmosis in terms of the diffusion of water molecules
from a higher to a lower water potential through a partially permeable membrane;
understand the factors which affect water potential;
-
understand the principles involved in active transport; endocytosis and exocytosis.
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