Download Membranes

Membranes
Cell Membranes
-
all cells are delineated by a membrane
-
play a crucial role in almost all cellular activity
-
separate the cell from its surroundings
-
regulates the movement of substances in and out of the cell
o (i.e. a selective barrier – controls concentration variation of molecules in cell)
Internal membranes
-
enclose:
o nucleus
o mitochondria
o chloroplasts etc.
o (i.e. compartmentalise special metabolic activities)
Analysis of extracts of cell membranes
(a)
made of lipid
(b)
associated with proteins which contribute to their function
i. selective permeability
ii. trans-membrane transport
iii. structural integrity
The cell membrane is a structure common to both animal and plant cells.
There used to be a theory of the structure as shown on the following page. However, this
was later not accepted and a new theory was created which is now accepted to be correct.
Davson Danielli (1930) model – unit membrane
Original model
phospholipid bilayer
3.5nm
The problem seen in this model was that it did not explain how the
substances got through the membrane.
(a)
(b)
(c)
hydrophilic bond
2nm
Total: 7.5 nm
7.5 nm thick
not seen under light microscope
forms anchorage for component protein
Phospholipids
(a)
(b)
(c)
Composed of fatty acid chains attracted to glycerol
have polar heads (hydrophilic)
non polar tails (hydrophobic – do not mix with water)
If a thin layer of polar lipids is spread on the surface of water, the molecules orientate into a
monolayer.
water
polar heads
Singer and Nicholson – Fluid Mosaic Model – 1972 – model now accepted
cholesterol
glycoprotein
protein
phospholipidic tail
carrier
protein
head
pore protein
• “proteins flowing in a sea of lipids”
• fluid mosaic model – i.e. membrane is not static
• both proteins and lipids have considerable freedom of movement: mainly lateral
Even with an electron microscope it is not possible to see he molecular structure of a cell
membrane. Thus it is necessary to construct a model to explain its various properties.
The matrix of the membrane is composed of a bilayer of phospholipids and cholesterol
molecules.
Proteins are embedded in the bilayer.
(a)
some penetrate only part of the way through, while others penetrate all the
way through
(b)
Lipids
functions:
i. structure
ii. transport
iii. act as enzymes
-
composition similar to olive oil
variation affects fluidity and permeability
i.
unsaturated lipids have curled tails to prevent close
packing and to make membrane structure more open and
fluid
ii.
cholesterol is important in regulating fluidity
The structure is asymmetric as there are different proteins in either half of the bilayer.
Carbohydrates close t exterior forming “glycocalyx” – receptor molecule / cell recognition
Proteins are involved in the transport of molecules across membrane specific receptors for
hormones or act as enzymes.
Evidence for the structure is proposed from:
(a)
(b)
(c)
freeze fracture studies
isotopic labelling of proteins
electron microscope studies
Movement through membranes
(a)
(b)
(c)
(d)
(e)
(a)
diffusion
facilitated diffusion
active transport
osmosis
exocytosis / endocytosis
Diffusion
-
“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:
rate
α
1
distance
2
-
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 – positive (+) ions move in more readily than
negative (-)ions. Ions of a higher charge are attracted more into the cell
Fick’s law of diffusion
rate of diffusion
α
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
(b)
Facilitated Diffusion
-
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
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
(c)
down a gradient
(c)
Active Transport
-
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 uptake of products of digestion
o uptake of mineral ions from the soil
o nerve impulses
o 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
antiport