the gorter and grendel model of the 1920s showed that the phospholipids in the membrane of cells were arranged into a bilayer
gorter and grendel
evidence:
the number of phospholipids extracted from redbloodcell membranes was double the area of the plasma membrane if it was arranged as a monolayer
gorter and grendel
problems
their model did not explain the location of proteins or how molecules that were insoluble in lipids moved into and out of the cell
davson and danielli's model of the membrane from the 1930s suggested that the proteins were arranged in layers above and below the phospholipid bilayer
davson and danielli
evidence:
membranes were effective at controlling the movement of substances in and out of cells
electron micrographs showed the membrane had two darklines with a lighter band between. in electron micrographs, proteins appear darker than phospholipids.
davson and danielli
problems
freezeetched electron micrographs of the centre of the membrane showed globular structures scattered throughout
improvements in technology used to analyse the proteins in the membranes showed that proteins were globular, varied in size and had parts that were hydrophobic
singer and nicolson proposed the fluid mosaic model in the 1970s; the model stated that membranes were fluid and that the globular proteins were both peripheral and integral
singer and nicolson
evidence:
analysis of freezeetched electron micrographs showed proteins extending into the centre of membranes
biochemical analysis of the plasmamembrane components showed that membraneproteins are free to move within the bilayer
cell surface membrane creates an enclosed space separating the internal cell environment from the external environment and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and endoplasmic reticulum
membranes do not only separate different areas but also control the exchange of substances from one side of a membrane to the other, as well as acting as an interface for communication
membranes are partially permeable, substances can cross membranes by diffusion and active transport, they contain receptor proteins e.g for binding to hormones and antigens
phospholipids consist of a molecule of glycerol, a phosphate group which forms the phosphate head and two fattyacid tails making up the lipid tail
phospholipids have a polar hydrophilic head and a non polar hydrophobic tail
proteins in cell membranes are involved with cell transport and communication
cholesterol in the cell membrane increases fluidity of the membrane at low temperatures preventing it from becoming too rigid
it stops the phospholipid tails packing too closely together
interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid.
cholesterol molecules bind to the hydrophobic tails of phospholipids stabilising them and causing phospholipids to pack more closely together
cholesterol increases mechanical strength and stability of membranes; without it membranes would break down and cells would burst
glycolipids and glycoproteins
on thesurface of the cell
they aid cell-to-cell communication
glycoproteins: proteins with carbohydrate attached
glycolipids: lipids with carbohydrate attached
they both bind with substances at the cell's surface e.g hormones
some glycolipids and glycoproteins act as cell markers or antigens for cell-to-cell recognition
fluid mosaic phospholipid bilayer
mosaic: scattered pattern produced by proteins and phospholipids
fluid: molecules can move around within the membrane by diffusion
membrane is partially permeable
smallnon polar molecules can pass through the gaps between the phospholipids
largepolar molecules must pass through specialised membrane proteins called channel and carrier proteins
3 functions of the phospholipid bilayer
provides fluidity to membrane
creates enclosed space separating internal cell environment from external environment