Cell membranes + fluid mosaic Flashcards
Why is the cell membrane important
Vital structures found in all cells. Creates an enclosed space separating the internal cell environ from the external cell environ. Intracellular membranes form compartments within the cell
Act as interface for communication -cell signalling e.g. hormones
How do substances cross membranes
By diffusion, osmosis and active transport
Reasoning for Fluid Mosaic Model
Why is it described as fluid
Fluid because:
phospholipids and proteins can move around via diffusion. Phospholipids mainly move laterally but can flip-flop (transverse). The many different types of proteins interspersed throughout the bilayer move about within it although some may be fixed in position
Reasoning for Fluid Mosaic Model
Why is it described as mosaic
Mosaic because:
Scattered pattern produced by the proteins within the phospholipid bilayers looks like a mosaic from above
Reasoning for Fluid Mosaic Model
Why is it described as a model
Representation of an idea to make it easier to understand
What is a phospholipid
Phosphate head joined to a glycerol molecule
Have a saturated fatty acid tail (no double bonds) and unsaturated fatty acid head (double bonds)
Head is hydrophilic and tail is hydrophobic
Appearance of cell membrane
Under an EM it appears as a double line. Width is 7nm
Why do phospholipids form a phospholipid monolayer
Polar head and two non polar tails. Polar head is soluble in water (hydrophilic) and non polar tail is insoluble in water (hydrophobic). So if over surface of water will form a single layer with heads in water and tails sticking out
Why is membrane made of phospholipids
They act as barrier to most water-soluble substances so ensure water soluble molecules such as sugars, amino acids and proteins cannot leak out of cell
Ensure unwanted water soluble molecules cannot get in
Micelle
If phospholipids are shaken with water they form spheres with hydrophilic phosphate heads facing out towards water and hydrophobic fatty acid tails facing in towards each other
Middle of sphere carry lipid based molecules inside
Fluidity of cell surface membrane
Saturated phospholipids only = highly viscous
Mix of saturated and unsaturated phospholipids and fatty acid tails= More fluid because the unsaturated hydrocarbon tails have kinks that prevent the molecules from packing closely together enhancing membrane fluidity
Why is fluidity of cell surface membrane important
Must be a dynamic structure if the cell is to grow and respond to environmental changes e.g. phagocyte
3 factors affecting permeability of a cell membrane
Heat, ethanol and pH
A temp exceeding the optimum and going beyond the normal range of pH levels can denature the membrane’s proteins. Ethanol dissolves the lipid components of the membrane.
This all makes the membrane far more permeable acting as if it’s full of holes
Factors affecting rate of diffusion
Steepness of conc gradient: Greater diff in conc over the membrane means greater diff in number of molecules passing in two directions = faster rate
Temp: Molecules and ions have more kinetic energy at higher temps. Move faster = higher rate of diffusion
Surface area: Greater surface area means more molecules can diffuse across at any one moment = faster diffusion
Properties of molecules or ions: large molecules diffuse slower as require more energy to move. Uncharged and non polar molecules diffuse directly across phos bilayer
Channel proteins
Water filled pores. Allow charged substances to diffuse through the cell membrane.
Diffusion of these molecules doesn’t happen freely so most channel proteins are gated. Part of channel protein on inside surface of membrane can move in order to open or close the pore. Channel proteins can therefore control the exchange on ions
