2.1.5 biological membranes Flashcards
what are the functions of plasma membranes
- acts as a barrier between cell and its environment. they control what enters and leaves as they are partially permeable
- cell signalling and recognition of cells
what are the functions of membranes within cells
- act as a barrier between organelle and cytoplasm making their different functions more efficient
-can form vesicles for transport
-control what enters and leaves organelles
-barriers within organelles
-the site of some chemical reactions
what model describes the structure of membranes
the fluid mosaic model
what is the fluid mosaic model
it is a model to describe the arrangement of molecules in the membrane
phospholipids form a fluid bilayer with cholesterol in between them
protein molecules are scattered within
what is the role of phospholipids in the membrane
they have a hydrophilic had and hydrophobic tails which makes them form a bilayer with the heads facing outwards.
the centre of the bilayer is hydrophobic so it does not allow water soluble substances through. it allows fat soluble substances to diffuse into the cell
what is the role of cholesterol in the membrane
it fits between the phospholipid molecules binding to the phospholipid tails causing to pack closely together. this increases rigidity and decreases fluidity
at low temperatures the cholesterol increases fluidity
what is the role of proteins in the membrane
they can control what enters and leaves the cell by forming carrier proteins or channel proteins. they can also act as receptors in cell signalling
what is the role of glycoproteins and glycoproteins in the membrane
they stabilise the membrane forming hydrogen bonds with surrounding water molecules
they are also the site where drugs, antibodies and hormones bind
they act as receptor for cell signalling and are also antigens.
what is cell signalling
how cells communicate with each other to control processes and respond to the environment
how do cells signal each other
one cell releases a messenger molecule
this travels to another cell
the messenger molecule is detected by the cell because it binds to the receptor on its cell membrane
what is the role of receptors in cell signalling
they have specific shapes that only messenger molecules with complementary shapes can bind to.
different cells have different receptors
what is a target cell
a cell that responds to a specific messenger molecule
how can we investigate membrane permeability
cut equal sized pieces of beetroot and rinse to remove pigment released during cutting
place each one in a beaker of water and place in different temperature water baths for the same period of time
run the coloured liquid through a colorimeter . the higher the permeability the more pigment is released so the absorbance of the liquid would be higher
how do temperatures below 0 degrees effect membrane permeability
phospholipids have little energy so are packed closely together however the proteins denature increasing permeability
ice crystals can form and pierce the membrane making it highly permeable when it thaws
how do temperatures between 0 and 45 degrees effect membrane permeability
the membrane is partially permeable because the phospholipids can move . as temperature increase permeability increases as he phospholipids have more energy to move
how do temperatures above 45 degrees effect membrane permeability
the phospholipid bilayer starts to breakdown and membrane becomes more permeable. water inside the cell expands putting pressure on the membrane.
channel and carrier proteins denature which increases permeability
how can different solvents effect membrane permeability
surrounding the membrane in a solvent dissolves the lipids in the cell membrane so it looses its structure making it more permeable
what is diffusion
the net movement of particles from an area of high concentration t an area of low concentration down a concentration gradient. it is a passive process
what molecules can diffuse through cell membranes
small non polar molecules
what factors effect rate of diffusion
the concentration gradient - the higher it is the faster the rate of diffusion
the thickness of exchange surface - the thinner it is the faster the rate of diffusion as there is less distance to travel
surface area - larger the surface area he faster the rate of diffusion
temperature - the higher the temperature the faster the rate of diffusion as particles have more energy
how do we investigate rate of diffusion
make up some agar jelly infused with phenolphthalein and dilute NaOH
add cubes of this agar into a beaker of dilute HCl
as the acid diffuses into the agar jelly the pink cubes turn colourless as the NAOH is neutralised
what is facilitated diffusion
the movement of particles from a high concentration to a low concentration using carrier and channel proteins
how do carrier proteins work
they move large molecules in or out of the cell
1. large molecule attaches to the carrier protein
2. the protein changes shape
3. this releases the molecule on the other side of the membrane
how to channel proteins work
they form pores in the membrane for charged particles to diffuse through.
what is active transport
uses energy to transport molecules and ions across plasma membranes against a concentration gradient
it involves carrier proteins
what is endocytosis
when a cell takes in a substance by surrounding a substance with a section of its plasma membrane. the membrane then pinches in forming a vesicle inside the cell containing the ingested substance. this requires energy
what is exocytosis
a vesicle containing the substance pinch off from the Golgi apparatus and move to the plasma membrane. the vesicles fuse with the plasma membrane and the substance is released outside. it uses atp as an energy source.
what is osmosis
the diffusion of water molecules across a partially permeable membrane from an area of high water potential to an area of low water potential
what is water potential
the likelihood of of water molecules to diffuse out or into a solution.
pure water has the highest water potential. it is 0
how are animal cells effected by water potentials of surrounding solutions
(hypotonic, isotonic, hypertonic)
in a solution with a higher water potential (hypotonic) net movement of water is into the cell so the cell bursts
in a solution with the same water potential (isotonic) movement of water is into and out of the cell so the cell stays the same
in a solution with a lower water potential (hypertonic) net movement of water is out of the cell so the cell shrinks
how are plant cells effected by water potentials of surrounding solutions (hypotonic, isotonic, hypertonic)
in a hypotonic solution net movement of water is into the cell so the vacuole swells and pushes against the wall. cell becomes turgid
in an isotonic solution movement of water is into and out of the cell so the cell stays the same
in a hypotonic solution net movement of water is out of the cell it becomes flaccid. cytoplasm and membrane pull away from the cell wall. this is called plasmolysis
how can we investigate water potential
put equal potato cylinders in different concentrations of sucrose solutions (lower water potential stronger the concentration)
find the change in mass to see if water has moved in or out. plot on a graph.
