M2, C5 Plasma Membranes Flashcards
what is compartmentalisation
the formation of separate membrane-bound areas in a cell
define plasma membrane
the cell surface membrane that separates the cell from its external envrionment
what are membranes formed from
a phospholipid bilayer
hydrophilic phosphate heads form the inner and outer surface of a membrane
fatty acid tails are sandwiched inside to form the hydrophobic core
where do cells normally exist
aqueous environments
why are phospholipid bilayers suited for aqueous environments
the outer surfaces of the hydrophilic phosphate heads can interact with water
what is the fluid-mosaic model
shows how proteins occupy various positions in the membrane
phospholipids are free to move within the layer relative to each other
this gives the membrane flexibility
what are the components in a cell membrane
glycoprotein glycolipid cholesterol hydrophilic heads of phospholipid molecules extrinsic protein pore intrinsic protein hydrophobic tails of phospholipid molecules
what are the 2 types of membrane proteins
intrinsic and extrinsic
what are intrinsic proteins
transmembrane proteins that are embedded in both layers of the membrane
have amino acids and hydrophobic R groups on the external surfaces which interact with the hydrophobic core keeping them in place
what are channel proteins
channel proteins - provide a hydrophilic channel to allow passive movement of polar molecules and ions down a concentration gradient. this is held in place by the interactions between the hydrophobic core and the R groups on the outside of proteins
they are intrinsic
what are glycoproteins
Proteins with carbohydrates attached
they play a role in cell adhesion (when cells join together) and act as receptors for chemical signals
what are glycolipids
phospholipids with carbohydrates attached
they are cell markers or antigens so the immune system recognises it as self or non-self
what is cell signalling
involves glycoproteins
when the chemical binds to the receptor on the glycoprotein, it elicits a response from the cell which can set off a cascade of events
what are extrinsic proteins
present in one side of the bilayer
have hydrophilic R groups on their outer surfaces
interact with polar heads of phospholipids or intrinsic proteins
some can move between layers
why is cholesterol in the plasma membrane
it regulates the fluidity of them
has a hydrophobic and hydrophilic end
they are positioned between the phospholipids in a membrane bilayer - the hydrophilic end interacts with the heads and the hydrophobic end interacts with the tails, pulling them together
therefore it adds stability but doesn’t make them too rigid - they stop the phospholipid molecules from grouping too closely and crystallising
what needs to happen for a chemical reaction to take place
all the organelles need to be in specific positions
what happens to phospholipids as temperature increases
they gain kinetic energy meaning they move more
the membrane becomes more fluid because it loses its structure
eventually they could break down completely
what does the loss of structure in cell membranes mean
they become more permeable meaning particles can more easily cross it
what happens to carrier and channel proteins as temp increases
denature
increases membrane permeability further
why is water essential for the formation of the phospholipid bilayer
Because the non-polar tails of the phospholipids are orientated away from water which forms a bilayer with the hydrophobic core
the charged phosphate heads interact with water which keeps it intact
what are carrier proteins
carrier proteins - passive and active transport into cells. this often involves the proteins changing shape
they are intrinsic
how do organic solvents affect membranes
they dissolve membranes which disrupts cells
give a common example of an organic solvent
alcohol
why is alcohol in anti-septic wipes
the alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection
why are pure or very strong alcohol solutions dangerous
they are toxic
they destroy cells
the molecules can enter the membranes and disrupt the membrane because they lie between phospholipids
what happens when a membrane is disrupted
it becomes more fluid and more permeable
give an example of a cell that needs intact cell membranes to function
nerve cells
if neuronal membranes are disrupted then nerve impulses are no longer transmitted as normal, hence why people behave differently when they drink alcohol
suggest an experiment to investigate how temperature affects membrane permeability
- get 5 small pieces of beetroot, all equal sizes
- thoroughly wash under water
- place in 100ml of distilled water in a water bath
- increase the water bath by 10 degrees
- samples of water taken 5 mins after the water bath has increased by 10 degrees
- measure the absorbance of each sample using a colorimeter with a blue filter
- repeat 3 times
the results show that absorbance rapidly increases after 50 degrees hence suggesting that the membrane is disrupted between 40 and 50 degrees
what microscope would you see a detailed plasma membrane
electron microscope - you would see the double line which is 7-10 nm thick
(a light microscope you would see a single line)
how can polar and non-polar molecules pass through the membrane
Polar molecules require proteins to enable them to pass through the membrane
Non-polar molecules can diffuse directly through the phospholipid bilayer
define diffusion
the net movement of particles from a region of higher concentration to a region of lower concentration
when does diffusion continue to
until there is a concentration equilibrium between the 2 areas
why does diffusion happen
particles in a gas or liquid have kinetic energy
this movement is random and unequal
what slows down the overall movement in diffusion
particles are constantly colliding with each other
this means diffusion over short distances are faster
what factors affect the rate of diffusion
temp - increases kinetic energy
concentration difference - overall movement will be larger
what is simple diffusion
diffusion when there is no barrier/ membrane
why are membranes described as partially permeable
polar molecules diffuse through a membrane at a slow rate
small polar molecules pass through more easily
what is the rate at which molecules or ions diffuse across membranes is affected by
surface area
thickness
what is facilitated diffusion
diffusion across a membrane through protein channels
what are membranes with protein channels known as
selectively permeable
what is the rate of facilitated diffusion dependent on
temperature concentration gradient membrane surface area thickness number of channel proteins present
what is active transport
the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration
what is needed for active transport
metabolic energy is supplied by ATP
what do carrier proteins act as in active transport
pumps
give the process of active transport into the cell
- the molecule binds to receptors in the channel of the carrier protein on the ourside of the cell
- ATP binds to the carrier protein inside the cell and is hydrolysed into ADP and phosphate
- The binding of the phosphate molecule to the carrier protein causes the protein the change shape which opens up to the inside of the cell
- molecule is released to the inside of the cell
- phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- Carrier protein returns to its original shape
why is active transport into the cell selective
because specific substances are transported by specific carrier proteins
what is bulk transport
form of active transport
large molecules and whole cells are too large to move through channel or carrier proteins so are moved into and out of cell by bulk transport
what are two types of bulk transport
endocytosis - bulk transport into cells
exocytosis - bulk transport out of cells
what are the two types of endocytosis
phagocytosis - for solids
pinocytosis - for liquids
give the process for endocytosis
- cell surface membrane invaginates when it comes into contact with material
- the membrane enfolds the material
- the membrane fuses which forms a vesicle
- vesicle pinches off and moves into cytoplasm to transfer material
what are the basic steps for exocytosis
its the opposite of endocytosis
vesicles formed by the Golgi apparatus move towards and fuse with the cell surface membrane
the contents of the vesicle are then released outside of the cell
why is ATP needed for bulk transport
the move vesicles along the cytoskeleton
change the shape of the cell to engulf materials
fusion of cell membranes as vesicles form or as they meet the cell surface membrane
what is a solute
a substance dissolved in a solvent forming a solution
what is water potential
the pressure exerted by water molecules as they collide with a membrane or container
it’s measured in pascals
symbol is the greek letter psi
define pure water
has a water potential of 0kPa which is the highest possible value for water potential
what happens when solutions with different concentrations/ different water potentials are separated by a partially permeable membrane
the water molecules can move between the solutions but solutes usually can’t
there is a net movement of water from the solution with high water potential to the solution with lower water potential
is a solution more or less concentrated if the water potential is high
less
what is hydrostatic pressure
the diffusion of water into a solution leads to an increase in volume of this solution
if the solution is in a closed system, pressure increases
what are the units of hydrostatic pressure
kPa
what happens if an animal cell is placed in a solution with a higher water potential than that of the cytoplasms
water will move into the cell by osmosis, increasing hydrostatic pressure inside the cell
what is cytolysis
when the cell-surface membrane of an animal cell can’t stretch enough to withstand the hydrostatic pressure so breaks and the cell will burst
what happens 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
causes a reduction in the cell volume and the cell-surface membrane ‘puckers’ (crenation)
how is cytolysis or crenation prevented
animals have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic)
what is the aqueous solution in blood
blood plasma
in a red blood cell, what happens if it is in a high water potential solution
water enters cell
it swells and bursts
haemoglobin is released
in a red blood cell what happens if it is a low water potential solution
water leaves cell
haemoglobin is more concentrated so the cell appears darker
cell shrinks and shrivels
in a red blood cell what happens if it is an equal water potential solution
water constantly enters and leaves at equal rates
red blood cell is normal
where are solutes stored in a plant cell
dissolved in the vacuole
what differs plants and animals in terms of osmosis
plants are unable to control water potential of fluid around them eg. roots are usually surrounded by almost pure water
what happens when water enters a plant cell by osmosis
hydrostatic pressure increases
membrane is pushed against the rigid cell wall
the pressure against the cell wall is called turgor
as turgor increases it resists entry of further water and the cell is said to be turgid
what is turgor pressure
pressure against the cell wall
what is a cell called when it stops the entry of water into it
turgid
what happens when plant cells are placed in a solution with a lower water potential
water is lost from the cells by osmosis
there is a reduction in the volume of the cytoplasm
what does it mean when a cell is said to be plasmolysed
when water is lost from the cell through osmosis
if this doesn’t stop then volume of cytoplasm keeps reducing which pulls the cell-surface membrane away from the cell wall
how can you investigate osmosis in plant cells
place pieces of potato or onion into sugar or salt solutions with different concentrations/different water potentials
water will move into or out of cells depending on water potential of the plant tissue
as the plant tissue gains or loses water it will increase or decrease in mass and size
how can you investigate osmosis in animal cells
eggs are used
boil the egg, remove the shell and there is a single membrane bound structure
place the eggs in different concentrations of sugar syrup
overtime osmosis takes place and there will have been a net movement of water into or out of the eggs
how thick is the plasma membrane
7-10nm
