Download Plasma Membranes

Topics included in this booklet are;
 Plasma membranes
 Diffusion
 Osmosis
 Active Transport
Plasma Membranes
All cells are surrounded by a membrane (called the plasma membrane), a
barrier which is responsible for controlling the entry and exit of
substances to and from the cell. The plasma membrane has two parts;
Bimolecular leaflet of phospholipids which make up the bulk of the
membrane.
Proteins which are found at intervals along the membrane.
These two parts are found in the ……………………………………………. Model of
membrane structure.
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There are two types of proteins in the plasma membrane;
EXTRINSIC PROTEINS – these proteins are found on the outer and
inner surfaces of the membrane but do not penetrate the whole
membrane.
INTRINSIC PROTEINS – these proteins penetrate the whole plasma
membrane.
The functions of the membrane proteins are;
1. To provide channels by which water soluble molecules may pass across
the membrane.
The bulk of the membrane is made up of phospholipids. Lipid soluble
molecules can pass across the membrane freely as they can dissolve in the
membrane. Molecules that are not lipid soluble, particularly charged
molecules must pass across the membrane via pores crated by intrinsic
protein molecules in order to avoid the hydrophobic tails of the
phospholipid molecules. These molecules use carrier proteins to enter the
cell. Each molecule has its own specific carrier determined by the shape
of the protein molecules. Because the movement of the molecule is aided
by the protein this is called FACILITATED DIFFUSION.
NB Small non-lipid soluble molecules can pass between the phospholipid
molecules.
2. To enable cells to recognise molecules on the outside of the cell.
Some molecules are very large and cannot enter cells directly e.g.
hormones, which are made of proteins. The metabolism of the cell may
need to be altered due to the presence of one of these molecules on the
cell surface. Proteins are able to recognise certain molecules outside the
cell and signal their presence to the inside of the cell. This process is
aided by the presence of the glycocalyx.
3. To act as pumps
Some of the proteins pump molecules into the cell against the
concentration gradient. This is ACTIVE TRANSPORT.
The plasma membrane is called a PARTIALLY PERMEABLE MEMBRANE
because it is permeable to some molecules and impermeable to others.
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There are three ways in which molecules may pass across the membrane;
DIFFUSION
OSMOSIS
ACTIVE TRANSPORT
Why is the plasma membrane
structure described as the fluid
mosaic model?
What property of proteins allows
them to recognise molecules?
What is the glycocalyx?
Which of the processes of entry
into the cell are
Active?
Passive?
Examples of molecules which may
enter the cell by
Diffusion
Osmosis
Active transport
What are the functions of the
plasma membrane?
Diffusion
All particles in liquids and gases are in constant random motion. This
motion results in a net movement of particles from a region of high
concentration to a region of lower concentration. Diffusion is the passive
movement of substances along a concentration gradient.
Diffusion continues until EQULIBRIUM is reached. At this point the
particles are spread evenly in the available space. Random movement of
particles still occurs but there is no net movement in any particular
direction.
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Particles can also diffuse across a membrane as long as the membrane has
pores that are larger than the particles.
The rate of diffusion can be affected by a number of factors;
FACTOR
EFFECT ON DIFFUSION
Temperature
Physical movement
Concentration Gradient
Distance
Pressure
Size of Molecules
These factors affect diffusion across membranes too. There are also
additional factors that can affect the rate of diffusion across a
membrane;
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FACTOR
EFFECT ON DIFFUSION ACROSS
A MEMBRANE
Thickness of membrane
Diameter and number of pores
(permeability)
Surface area of membrane
FICKS LAW is a formula by which we can work out the rate of diffusion;
Rate of diffusion =
surface area x concentration difference
Thickness of membrane
Facilitated Diffusion
Large lipid insoluble molecules such as glucose and amino acid cannot
enter the cell by diffusion across the plasma membrane. They must be
aided across the membrane by intrinsic proteins called carrier proteins.
These proteins provide channels through which these molecules can pass
across the membrane in a process called FACILITATED DIFFUSION.
Specific carrier protein molecules transport each substance across the
membrane. The proteins recognise the molecules they interact with by
their shape.
Facilitated diffusion only occurs down a concentration gradient and is a
passive process.
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Gated channel proteins – a special case of facilitated diffusion
Mineral ions also move across membranes by facilitated diffusion but
they travel along an electrochemical gradient. This is a gradient which
occurs between two regions with different charges. If there is a high
concentration of Na + ions in one area that area becomes positively
charged. A high concentration of Cl- ions will make an area negatively
charged. If these regions are either side of a membrane then the Na+
will move from the positive to the negative region and Cl- form the
negative to the positive region, down electrochemical gradients. Proteins
in the membrane called CHANNEL PROTEINS allow ions to pass through
the membrane. Each channel protein has a particular shape and electrical
charge and will only allow one type of ion through. Channel proteins can
be open or closed and so are called GATED CHANNELS.
Osmosis
Not all substances can pass through the cell surface membrane. Water
molecules do but larger solute molecules do not. This means that the
membrane is partially permeable.
The movement of water molecules through a partially permeable
membrane from a region of high concentration of water molecules to a
region of lower concentration of water molecules is called osmosis.
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Key terms in describing osmosis are
 Water potential
 Solute potential
 Pressure potential
Water Potential
Water molecules, like all other molecules are mobile (they move around
constantly). In pure water or in solutions containing very few solute
molecules the water can move freely the water molecules have
……………………………………………………….
In a solution with many solute molecules, the movement of water
molecules is restricted because of the interactions (bonding) between the
solute
and
water
molecules
the
water
molecules
have
…………………………………………………
Therefore any solution in which the water molecules have high average
kinetic energy will have a greater tendency to lose water than those with
low free kinetic energy because the water molecules are able to move
around more.
When describing free kinetic energy biologists use the term
…………………………..(Symbol )
Define osmosis using the term water potential.
Measuring water potential
The water potential of pure water (symbol w) (at atmospheric pressure)
is 0 KPa (……………….………………)
Any addition of solute molecules decreases the water potential of the
solution therefore makes the water potential negative.
Match each of the solutions with the correct measurements of water
potential
Description
Water potential
Pure water
 = 50KPa
0.5M glucose solution
 = 0KPa
1M glucose solution
 = 200KPa
2M glucose solution
 = 100KPa
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Look at the following diagram
Label the water and solute molecules
Label one of the solutions pure water and the other glucose solution
Which solution has the highest water potential?
In which direction will osmosis occur?
Look at the following diagram
X
Y
-405 KPa
Z
-395KPa
-380KPa
In which direction(s) will osmosis occur between these three cells?
Between which pair of cells will the net rate of water movement be
greatest? Explain your answer.
Solute potential
The effect that solute molecules have on water potential is referred to a
solute potential. Any addition of solute molecules decreases the water
potential of a solution so solute potential (symbol s) is always a negative
value.
The water potential of a solution can be calculated as follows;
Water potential of solution = water potential of pure water + solute potential

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=
w
+
s
Remembering that w = 0 what is the s of cells x,y & z above?
Cell X =
Cell Y =
Cell Z =
Pressure potential
A
w = 0
B
w = 0
Both of these cells contain pure water.
will osmosis occur between these two cells?
will there be any movement of water between these two cells?
What happens if one of the cells is squeezed?
A
=0
B
=0
What happens to the pressure inside cell b?
What happens to the free kinetic energy of the water molecules inside
cell b?
What happens to the water potential inside cell b?
Will osmosis now occur? In which direction?
The contribution of external pressure on water potential is called
pressure potential (symbol p). At atmospheric pressure p = 0. p is
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always a positive value because any increase in pressure increases water
potential.
As p also contributes to water potential the equation for the water
potential of a solution is now as follows;
Water potential = water potential + solute potential + pressure potential
of pure water
of solution
Write this equation using the appropriate symbols.
Since the water potential of pure water is always 0 the equation can be
simplified as
Note that this equation only applies to the calculation of  in plant cells
as this the only instance in which pressure potential can develop due to
the presence of the cell wall.
Calcuation of  - worked example
A
s = -600 KPa
p = 400 KPa
Cell A
Cell B

=
s = p

=
=
-600 + 400
=
=
-200 KPa
=
Osmosis will occur from cell A to cell B.
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B
s = -800KPa
p = 400 KPa
s + p
-800 + 400
-400 KPa
Calculate  and the direction of osmosis for the following cells.
A
A
A
s = -600 KPa
p = 300 KPa
B
s = -700KPa
p = 200 KPa
s = -400 KPa
p = 100 KPa
B
s = -600KPa
p = 600 KPa
s = -700 KPa
p = 500 KPa
B
s = -700KPa
p = 500 KPa
Water potential and animal cells
Isotonic solutions have the same concentration of water molecules as
animal cells, so the rate at which water molecules diffuse into and out of
the cell is the same. There is no NET movement of water molecules
Hypertonic solutions have a lower concentration of water molecules
compared to the inside of a cell and so there is a net movement of water
molecules out of the cell. The cell shrinks this is called crenation.
Hypotonic solutions have a higher concentration of water molecules
compared to the inside of the cell so there is net movement of water
molecules into the cell. This causes the cell to swell. A cell which is full
of water but which has not burst is said to be turgid. Lysis is the term
given to the bursting of the cell.
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Plant cells and osmosis
The vacuole of a plant cell usually has a high concentration of ions and so
has a very negative water potential. This low water potential causes
water to enter the cell by osmosis. The cytoplasm and vacuole both swell
and the cell develops a high pressure potential. The pressure exerted by
the vacuole on the cell wall is called TURGOR PRESSURE. This pressure
keeps the cell TURGID (rigid) and is the main method of support in young
plants. If the cells lose water the leaves and stem wilt.
If a fully turgid plant cell is placed into a hypertonic solution (one that
has a lower concentration of water molecules compared to the inside of
the cell) then the following changes occur;
The cell starts to lose water by osmosis
The pressure potential of the cell drops to zero
The cytoplasm shrinks and the plasma membrane starts to peel away from
the cell wall this is the point of INCIPIENT PLASMOLYSIS.
Further loss of water results in the plasma membrane pulling away from
the cell wall completely, this is FULL PLASMOLYSIS.
Active Transport
Active transport is the movement of molecules across the plasma
membrane against a concentration gradient. This is an active process and
uses intrinsic protein molecules as molecular pumps. It requires energy in
the form of ATP produced by respiration therefore cells which carry out
lots of active transport have many mitochondria.
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Active transport is dependent on the energy produced by respiration.
Any factor that increases the rate of respiration will increase the rate of
active transport. Poisons such as cyanide which act as respiratory
inhibitors will stop active transport.
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