topic 2 - cell membrane structure Flashcards
cgp (topic 12B) 38 - 39
what do cell surface membranes do
surround cells - acting as a barrier between the cell and its environment, controlling which substances enter and leave the cell - they’re partially permeable
what are the three ways substances can move across the cell surface membrane
(1) diffusion
(2) osmosis
(3) active transport
what does the membranes around organelles do
divides the cell into different compartments - they act as a barrier between the organelle and the cytoplasm
what is the basic structure of all cell membranes
composed of lipids, proteins and carbohydrates
what is the fluid mosaic model
a model formulated in 1972 which describes describes the structure of the plasma membrane as a mosaic of components - including phospholipids, cholesterol, proteins, and carbohydrates
in the fluid mosaic model what do the phospholipid molecules form
a continuous double layer (bilayer) - the bilayer is fluid as the phospholipids are constantly moving
why was the fluid mosaic model given this name
because proteins are scattered through the bilayer like tiles in a mosaic
what two proteins allow large molecules and ions to pass through the membrane
carrier proteins and channel proteins
what do the receptor proteins on the cell surface membrane do
they allow the cell to detect chemicals released from other cells - the chemicals signal to the cell to respond in some way
what are the six features of the cell membrane
(1) protein
(2) cholesterol
(3) protein channel
(4) phospholipid
(5) glycolipid
(6) glycoprotein
what are glycoproteins
proteins that have a polysaccharide (carbohydrate) chain attached
what are glycolipids
lipids with polysaccharide chains called
what do phospholipids do
act as a barrier to separate the inside of a cell from its outside environment
what is the difference between the head and tail of a phospholipid
the head is hydrophilic - it attracts water
the tail is hydrophobic - it repels water
in what way do the phospholipids arrange themselves into a bilayer
the heads face out towards the water on either side of the membrane
how is the phospholipid bilayer able to act as a barrier to dissolved substances
since the centre of the bilayer is hydrophobic so the membrane doesn’t allow water soluble substances through it
what is cholesterol a type of
lipid
what is cholesterol not present in
bacterial cell membranes
how does cholesterol make the membrane more rigid
the cholesterol molecules fit between the phospholipids - they bind to the hydrophobic tails of the phospholipids causing them to pack more closely together which restricts the movement of the phospholipids - making the membrane less fluid and more rigid
what are two examples of different conditions that could affect the permeability of cell membranes
(1) temperature
(2) solvent concentration
why do we use beetroot to investigate how conditions affect permeability
beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell
what is the first step to investigating how temperature affects beetroot membrane permeability
use a scalpel to carefully cut five equal sized pieces of beetroot - rinse the pieces to remove any pigment released during cutting
what is the second step to investigating how temperature affects beetroot membrane permeability
add the five pieces to five different test tubes, each containing 5cm3 of water - use a measuring cylinder or pipette to measure the water
what is the third step to investigating how temperature affects beetroot membrane permeability
place each test tube in a water bath at a different temperature for the same length of time (measured using a stopwatch)
what is the fourth step to investigating how temperature affects beetroot membrane permeability
remove the pieces of beetroot from the tubes, leaving just the coloured liquid
what is the fifth step to investigating how temperature affects beetroot membrane permeability
use a colourimeter - a machines that passes light through the liquid and measures how much of that light is absorbed - the higher the absorbance, the more pigment released, so the higher the permeability of the membrane
what is the sixth step to investigating how temperature affects beetroot membrane permeability
connect the colourimeter to a computer and use software to collect the data and draw a graph of the results
what happens to the membrane permeability when temperature is below 0C
the phospholipids don’t have much energy, so they can’t move very much - they’re packed closely together and the membrane is rigid, but channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane - ice crystals may form and pierce the membrane making it highly permeable when it thaws
what happens to the membrane permeability when temperature is between 0 and 45C
the phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable - as the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane
what happens to the membrane permeability when temperature is above 45C
the phospholipid bilayer starts to melt and the membrane becomes more permeable - water inside the cell expands, putting pressure on the membrane - channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell - this increases the permeability of the membrane
