Cell Biology: Topic 1.3 Membrane Structure Flashcards
Outline the structure of phospholipids
Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
Because phospholipids contain both hydrophilic (water-loving) and hydrophobic (water repelling) regions, they are classed as amphipathic
Outline the arrangement of membrane
Phospholipids spontaneously arrange into a bilayer
The hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively
Outline the properties of the phospholipid membrane
- The bilayer is held together by weak hydrophobic interactions between the tails
- Phospholipid layers are stabilized by interaction of hydrophilic heads and surrounding water
- Hydrophilic / hydrophobic layers restrict the passage of many substances
- Individual phospholipids can move within the bilayer laterally, allowing for membrane fluidity and flexibility
- This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
Which two factors influences the properties of the cell membrane?
“Shorter fatty acid tails will increase fluidity as they are less viscous and more susceptible to changes in kinetic energy
Lipid chains with double bonds (unsaturated fatty acids) have kinked hydrocarbon tails that are harder to pack together, therefore increses fluidity”
What are integral proteins?
Integral proteins are permanently attached to the membrane. Integral proteins are hydrophobic on at least part of their surface and they are therefore embedded in the hydrocarbon chains in the centre of the membrane. Many integral proteins are transmembrane - they extend across the membrane, with hydrophilic parts projecting through the regions of the phosphate heads on either side.
What are peripheral proteins?
Peripheral proteins are hydrophilic on their surface, so are not embedded in the membrane. They are temporarily attached by non-covalent interactions and associate with one surface of the membrane. Some have
a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the membrane surface (serves functions like-enzymes and transporters/carriers, anchorage)
Structure of Membrane Proteins: The amino acids of a membrane protein are localised according to polarity:
Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer
Polar (hydrophilic) amino acids are located internally and face aqueous solutions
Structure of Membrane Proteins: Transmembrane proteins typically adopt one of two tertiary structures:
Single helices (recognition, receptor) Helical bundles (enzymes, transporters, receptors) Beta barrels (transporters-common in channel proteins)
List the functions of membrane proteins
Membrane proteins can serve a variety of key functions:
Junctions – Serve to connect and join two cells together (cell-adhesion)
Enzymes – Fixing to membranes (immobilised enzymes) localises metabolic pathways
Transport – Channels which allow hydrophilic particles across by facilitated diffusion and pumps for active transport which uses ATP to move particles across the membrane
Recognition – May function as markers for cellular identification
Anchorage – Attachment points for cytoskeleton and extracellular matrix
Transduction – Function as receptors (hormone binding sites) for peptide hormones or receptor for neurotransmitters at synapses.
Mnemonic: Jet Rat
Function of cholesterol in animal cell membranes (and compare it to plant cell membrane function)
Cholesterol is a component of animal cell membranes, where it functions to maintain integrity and mechanical stability. It is absent in plant cells, as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose
Explain the structure of cholesterol, hence its positioning in the cell membrane
Cholesterol is a type of lipid, but it is not a fat or oil. Instead it belongs to a group of substances called steroids. Cholesterol is an amphipathic molecule (like phospholipids): most of the cholesterol molecule is hydrophobic so it is attracted to the hydrophobic hydrocarbon tails in the centre of the membrane, but one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic. This is attracted to the phosphate heads on the periphery of the membrane. Cholesterol molecules are therefore positioned between phospholipids
in the membrane.
Why the fluidity of animal cell membranes needs to be controlled?
Phospholipid bilayers are fluid, in that the phospholipids are in constant movement relative to one another
The fluidity of animal cell membranes needs to be carefully controlled. If they were too fluid they would be less able to control what substances pass through, but if they were not fluid enough the movement of the cell and substances within it would be restricted
What is the role of cholesterol in maintaining the properties of the membrane? (5)
Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane:
- Cholesterol functions to restrict molecular motion, reducing fluidity
- It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
- It functions to separate phospholipid tails by disrupting the regular packing and so prevent crystallisation of the membrane, increasing fluidity.
- It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
- Due to its shape cholesterol can help membranes to curve into a concave shape, which helps in the formation of vesicles during endocytosis .
Outline the Danson-Danielli model of the membrane
The first model that attempted to describe the position of proteins within the bilayer was proposed by Hugh Davson and James Danielli in 1935. Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer
The model was described as a ‘lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers. When viewed under a transmission electron microscope, membranes exhibit a characteristic ‘trilaminar’ appearance: 3 layers (two dark outer layers and a lighter inner region). The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers.
They proposed this sandwich model because they thought it would explain how membranes, despite being very thin, are a very effective barrier to the movement of some substances.
There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli:
- It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
- It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
- It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)
- The temperatures at which membranes solidified did not correlate with those expected under the proposed model
