Biological membranes Flashcards
membrane function
-separate contents of cells from their environment
- separate organelles from each other and cytosol
compartmentalisation
formation of separate membrane bound areas in a cell
why is compartmentalisation important
- metabolism
- containing reactions
why is compartmentalisation needed for metabolism
as there are many different and often incompatible reactions happening in a cell
why is compartmentalisation needed for containing reactions
- some reactions are only needed in some parts of the cell
- cell components need to be protected
- chemical gradients need to be contained
plasma membrane
cell surface membrane which separates the cell from its external environment
structure of plasma membrane
- membranes are formed from a phospholipid bilayer
how does the phospholipid bilayer work in cell membranes
- hydrophilic phosphate heads form the inner and outer surface of a membrane
- this sandwiches the hydrophobic fatty acid tails that form a hydrophobic core inside the membrane
why is only hydrophilic heads contacting water suitable for plasma membranes
- cells are usually in an aqueous environment and the inside of cells and organelles are usually aqueous so membranes are suited
fluid mosaic model
phospholipids are free to move within the layer relative to each other giving membrane flexibility and because proteins embedded in the bilayer vary in shape size and position
cell membrane components
- contain various proteins and lipids
glycoprotien
branching carbohydrate protein
glycoprotein function
regulates site for chemical reactions
glycolipid function
acts as recognition site
cholesterol function
for stability/flexibility
types of protein in cell membranes
extrinsic protein
intrinsic protein
extrinsic protein
partly embedded
intrinsic protein
spans the whole phospholipid bilayer
which direction to hydrophobic tails point
inwards
what direction to hydrophilic heads point
outwards
intrinsic protein structure
transmembrane proteins
embedded through both layers of membrane
have amino acids with hydrophobic R groups on their external surfaces which interact with hydrophobic core of membrane keeping them in place
channel proteins
- provide a hydrophilic channel
- allows passive movement of polar molecules and ions down a concentration gradient through membranes
how are channel proteins held in position
by interactions between hydrophobic core 0f membrane and hydrophobic R groups on outside of proteins
carrier proteins
play role in passive and active transport
often involves protein shape changing
what type of protein is a glycoprotien
intrinsic
glycoprotein structure
embedded in cell surface membrane with attached carbohydrate chains
glycoprotein function
plays role in cell adhesion as receptors for chemical signals
chemical signalling
when a chemical binds to a receptor it elicits a response from the cell causing events inside the cell
e.g receptor for neurotransmitters
acetycholine at nerve cells
e.g receptor for peptide hormone
insulin
glycolipid strucure
lipids with an attached carbohydrate chains
glycolipid function
cell markers/antigens can be recognised by the cells immune system as self ( of the organisms) or non self (cells belonging to another organism)
extrinsic proteins structure
present on one side of the bilayer
normally have hydrophilic R group on their outer surface and interact with polar heads of phospholipids or with intrinsic proteins
cholesterol structure
lipid with hydrophilic and hydrophobic end
- positioned between phospholipids in a membrane bilayer :
- hydrophilic end interacts with heads
- hydrophobic end interacts with tail
this pulls them together
cholesterol function
regulates membrane fluidity
adds stability to membranes without making them rigid
prevent membranes being too solid by stopping phospholipid molecules grouping too closely
how could cell processes be disrupted in relation to membranes
- if membranes lose their structure they lose control of letting substances in and out of cells
what happens if temperature increases
- phospholipids have more kinetic energy
- move more
- membrane is more fluid and loses its structure
what happens if temperature continues to increase
cell will completely break down
what happens when a membrane loses its structure and becomes more permeable
it is easier for particles to cross it so carrier and channel proteins will denature so membrane permeability will be affected
what happens when organic solvents e.g alcohol mix with membranes
- they dissolve membranes disrupting cells
- this is why alcohol is used in antiseptic wipes
passive transport
utilises energy from natural movemnt particles instead of energy from another source
diffusion
net movement of particles from a high concentration to a low concentration
explain how diffusion is a passive process
- it continues until there is equilibrium and no concentration difference between the 2 areas
describe features of diffusion
- happens in a gas/liquid
- random movement
- over short distances diffusion is faster
why are cells ideal for diffusion
they are small
how does temperature affect diffusion
increased temperature
particles have more kinetic energy
more diffusion
how does concentration difference affect diffusion
greater concentration difference = steeper concentration gradient = more diffusion
simple diffusion
diffusion without a membrane/barrier
what does diffusion across membranes involve
passing through a phospholipid bilayer
if membrane is permeable to particles
can substances with a +/- charge easily pass through a membrane
no, as the hydrophobic interior of membranes repels these
can polar molecules with a partial +/- easily pass through
they can, but slowly and smaller polar molecules pass through more easily than big ones
what are membranes in term of permeability
only partially permeable
what is the rate at which molecules/ions diffuse across membranes affected by
- surface area (increased sa, increased diffusion
- membrane thickness (thicker membrane, less diffusion)
facilitated diffusion
diffusion across a membrane through a protein channel
how permeable are membranes with protein channels
they are selectively permeable
why are membranes with protein channels selectively permeable
they are often specific to 1 ion or molecule
how does facilitated diffusion also involve carrier proteins
they change shape when other molecules bind so external energy isn’t required
what does diffusion of water into a solution do to the volume of that solution
increase its volume
if a solution is in a closed sytem and water has entered increasing its volume, what happens to pressure
it increases, this is hydrostatic pressure
if an animal cell is placed into a solution with a higher water potential than the cytoplasm what happens
water will move into the cell by osmosis and hydrostatic pressure increases
what happens if too much water moves into the animal cell via osmosis
cell bursts
what is cytosis
cell bursting
why does cytosis occur
cells have thin surfaces membranes and no cell walls so the membrane can’t stretch and withstand the pressure
what happens if an animal cell is placed into a solution which has lower water potential than cytoplasm
it loses water to the solution via osmosis
what happens if cell loses too much water
crenation - cell surface membrane puckers
how do animal cells prevent cytosis/crenation
they have control mechanisms to make sure cells are continuously surrounded by aqueous solutions with equal water potential
can plants control water potential of fluid around them
no
what do cells help to support them
cellulose walls
when water enters plant cells by osmosis what does the increased hydrostatic pressure do ?
push the membrane against the cell wall - turgor
how does turgor pressure increase
as it resists entry of further water
when plants are placed in a solution with lower water potential what happens
water is lost from the cells by osmosis
what happens when plant cells lose water by osmosis
a reduction of volume in the cells cytoplasm which pulls the membrane away from the cell wall - plasmolysed
what does plasmolysis mean
the cell membrane pulls away from the cell wall
osmosis
diffusion of water from an area of high water potential to an area of low water potential across a semi permeable membrane
water potential
the pressure exerted by water molecules as they collide with a membrane or container
what is water potential measured in
pascals/kilopascals
what is the water potential of pure water
0 kpa
what is the highest possible water potential
0 kpa
what happens to the water potential value as the concentration of a solution increases
it becomes more negative
when will osmosis stop
when equilibrium is reached
active transport
movement of ions/molecules into or out of a cell from a region of lower concentration to a region of higher concentration
what does active transport require
energy
carrier proteins
why does active transport need energy
particles are going against the concentration gradient
how is the energy supplied for active transport
metabolic energy supplied by ATP
why are carrier proteins needed for active transport
they act as pumps
describe the process of active transport
1- molecule/ion being transported binds to receptors in channel of carrier proteins on the outside of the cell
2- on inside of cell ATP binds to carrier protein and is hydrolysed into ADP and phosphate
3- binding of the phosphate molecule to the carrier protein causes protein to open up the cell/ change shape
4- molecule/ion is released to inside of cell
5- phosphate molecule is released from carrier protein and recombines with ADP to form ATP
6- carrier protein returns to original shape
how is active transport selective
specific substances are transported by specific carrier proteins
bulk transport
form of active transport where large molecules are too big to travel through carrier / channel proteins
types of bulk transport
endocytosis
exocytosis
endocytosis
bulk transport of material into cell
what are the 2 types of endocytosis
phagocytosis
pinocytosis
what is phagocytosis for
solids
what is pinocytosis for
liquids
except for states, are there any differences between phagocytosis and pinocytosis
no
process bulk transport
- cell surface membrane invaginates when it comes into contact with the material to be transported
- membrane enfolds the material until membrane fuses forming a vesicle
- vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell
exocytosis
opposite of endocytosis
vesicles formed by golgi apparatus fuse with the cell surface membrane and the contents of the vesicle is released outside of the cell
what is needed to move the vesicles along the cytoskeleton to change cell shape, engulf materials e.t.c
ATP (energy)
why do people’s behaviour change after drinking alcohol
- alcohol disrupts the membrane
- membrane becomes more permeable
- affects cells that need intact membranes
- when nerves are transmitted neural membranes are disrupted so nerve impulses don’t transmit normally
