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The roles of membranes within cells and at the surface of cells
To include the roles of membranes as,
• Partially permeable barriers between the cell and its environment, between organelles and the cytoplasm and
within organelles
• Sites of chemical reactions
• Sites of cell communication (cell signalling)
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Membranes at the surface of cells separate cell components from the outside environment, controls what substances enter
and leave the cell and used in cell recognition and signalling
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All membranes are partially permeable
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The cell surface membrane, also known as
the plasma membrane, surrounds the cytoplasm
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For example, electron carriers and the enzyme ATP synthase have to be in the correct positions
within the cristae for the production of ATP in respiration
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Cell signalling is communication between cells
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Cytokines are an example of cell signals
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Cells work together to trigger responses inside the cell
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Glycoproteins and glycolipids act as receptors
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The attachment of signal molecules causes changes inside the cell
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Glycoproteins and glycolipids may be involved in cells signalling that they are ‘self’ (of the organism), to allow recognition by
the immune system
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To include phospholipids, cholesterol, glycolipids, proteins and glycoproteins and the role of membrane-bound
receptors as sites where hormones and drugs can bind
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It was proposed by American scientists Singer and
Nicholson to show how all the components found in cell
membranes might be arranged to form a biological
membrane
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Made up of a bilayer of phospholipid molecules forming the
basic structure with proteins floating in the phospholipid
bilayer, some completely freely and some bound to other
components or to structures within the cell
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The cell surface membrane/plasma membrane refers to membranes that enclose cells in order to distinguish them from the
membranes that from organelles
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The hydrophilic
phosphate heads of the phospholipids form both the inner and outer surface with hydrophobic fatty acid tails on the inside of
the membrane forming the hydrophobic core
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The movement of the phospholipids causes the movement of the other components
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Cholesterol stabilises the membrane, maintains its fluidity and
reduces its permeability to polar molecules/ions
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They are called cell markers or antigens and can be
recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
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Intrinsic proteins, or integral proteins, are transmembrane proteins embedded through both layers of a membrane
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Extrinsic proteins or peripheral proteins are present in one side of the bilayer
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They can be present in
either layer or move between layers
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They provide a hydrophilic channel that allows the passive movement of polar
molecules and ions (large and too hydrophilic molecules) down a concentration gradient through membranes
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Carrier proteins actively move some substance across the membrane
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This involves the shape of
the protein changing
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They are embedded in the cell-surface membrane with attached carbohydrate (sugar)
chains of varying lengths and shapes
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Receptors are surface ‘sensors’ that are capable of receiving signals
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Signal molecules can fit into receptor molecules on cell surface membrane because their shapes are complementary to
each other
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A target cell is any cell with a
receptor for the hormone molecules
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Some receptor sites allow hormones to bind with the cell so that a cell ‘response’ can be carried out
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Cell membrane receptors are also
important in allowing drugs to bind, and so affect cell metabolism
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Binding of the hormone and receptor causes the target cell to respond in a certain way
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Insulin is released in response to increased blood sugar levels
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Some medicinal drugs have been developed that are complementary to the shape of a type of receptor molecule
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Beat-blockers are used to prevent heart muscle from increasing the heart rate
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Factors affecting membrane structure and permeability
To include the effects of temperature and solvents
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This is because phospholipids in a
cell membrane are constantly moving
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This makes a membrane more fluid and it begins to lose its structure
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Carrier and channel proteins will be denatured at higher temperatures
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Membranes are not denatured at high temperatures – their structure is disrupted or destroyed
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The non-polar tails of the phospholipids are
orientated away from the water, forming a bilayer with a hydrophobic core
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Many organic solvents are less polar that water (alcohols and benzene)
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Non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between
the phospholipids disrupts the membrane
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Some cells need intact cell
membranes for specific functions
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Practical investigations into factors affecting membrane structure and permeability
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When beetroot cell membranes are disrupted, the
red pigment is released and the surrounding solution is coloured
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The
temperature of the water bath is increased in 10-20°C intervals
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The absorbance/transmission of each sample is
measured using a colorimeter with a blue filter
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The percentage absorbance increased steadily and then quickly from about 50°C (or percentage transmission decreased…)
because the cell membranes have been disrupted so the membrane permeability increases
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Diffusion only happens between different concentrations of the same substance
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They have their own kinetic energy
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Equilibrium doesn’t mean the particles stop moving, just that the movements are equal in both directions
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Over a short distance, diffusion
is fast but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place
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The greater the difference in concentration (concentration gradient) between two regions, the faster the rate of diffusion
because the overall movement from the higher concentration to lower concentration will be larger
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They diffuse down a concentration gradient
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g
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Some water
molecules can pass through as they are small enough, even though they are polar
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The movement is down a
concentration gradient and doesn’t require external energy
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The thicker the area for exchange surface, the lower rate of diffusion
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Membranes with protein channels are selectively permeable as most as specific to one molecule or ion
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The
hydrophobic interior of the membrane repels ions, so they cannot move easily through the bilayer
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It changes shape when a specific molecule binds
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Active transport uses more energy to move substances because more energy is needed to move substances up a
concentration gradient
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They are similar to the protein carriers in facilitated
diffusion
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They carry large or charge molecules and ions
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Molecule/ions to be transported binds to receptors in the channel of the carrier protein on the outside of the cell

2
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Binding of the phosphate molecule to the carrier protein causes the protein to change shape – opening up to the inside of
the cell
4
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Phosphate molecule released from carrier protein and recombines with ADP to form ATP
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Carrier protein returns to original shape
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Bulk transport is a form of active transport where large molecules or whole bacterial cells are moved into or out of a cell by
endocytosis or exocytosis
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There are two types: phagocytosis for solids and pinocytosis for
liquids – the process is the same
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The membrane enfolds the material until eventually the membrane fuses, forming a vesicle
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For
example, vesicles containing bacteria are move towards lysosomes, where the bacteria are digested by enzymes
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Vesicles, usually formed by the Golgi apparatus, move towards and
fuse with the cell surface membrane
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For example, insulin
is processed and package into vesicles in the Golgi apparatus
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Energy in the form of ATP is required for the movement of vesicles along the cytoskeleton, changing the shape of cells to
engulf materials, and the fusion of cell membranes as vesicles form or as they meet the cell surface membrane
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Dialysis tubing is partially permeable, with pores similar size to that of the membrane
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The tubing is a barrier to large molecules
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The other end is tied and the
model cell is placed in a beaker of distilled water
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This
can then be used to calculate the rate of diffusion
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Starch can’t pass through and can be tested using
iodine solution
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Changes in mass/volume can be measured over
time to calculate the rate of osmosis
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Other variables such as concentration must be kept constant
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Osmosis is diffusion of water through a partially permeable membrane down a water potential gradient
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A solute is a solid that dissolves in a liquid
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A solution is a liquid containing dissolved solids
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Water has a water potential of 0kPa
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When solutions of different concentrations are separated by a partially permeable membrane, the water molecules can
move between the solutions but the solutes usually cannot
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This will
continue until the water potential is equal – equilibrium is reached
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If the solution is in a closed system e
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a cell, this results in an
increase in pressure
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It has the same units
as water potential, kPa
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An isotonic solution is a solution of equal concentration to a cell
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A hypertonic solution is a solution of higher concentration than a cell
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A hypotonic solution is a solution of lower concentration than a cell
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If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell
by osmosis, increasing the hydrostatic pressure inside the cell
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The cell surface membrane cannot withstand the increased pressure and will break causing the cell to burst –
cytolysis
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If an animal cell is placed in a solution that has a lower water potential than the cytoplasm, it will lose water to the solution
by osmosis down the water potential gradient
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Crenation is when the cell shrinks by osmosis because water leaves the cell
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In blood, the aqueous solution is
blood plasma
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However plants are unable to
control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water
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When placed in a hypotonic solution, water
enters by osmosis, the increased hydrostatic pressure pushes the membrane against the rigid cell walls – turgor
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As the turgor pressure increases, it
resists the entry of further water and the cell is said to be turgid
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Plasmolysis is the shrinking of protoplasm away from the cell wall of a plant due to water loss
from osmosis, thereby resulting in gaps between the cell wall and cell membrane
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Water potential
of solution
Net movement
of water
Condition of
protoplast

Higher (hypotonic)

Equal (isotonic)

Lower (hypertonic)

Enters cells

Water constantly enters and
leaves at equal rates
No change

Leaves cell

Swells and becomes turgid
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Practical investigations into the effects of solutions of different water potential on plant and animal cells
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The mass before
and after placing the pieces in the sugar/salt solutions can be measured (mass difference/% mass change)
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For animal cells, eggs can be used
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