Cell membranes Flashcards
Structure of cell membrane
50% lipids, the rest is proteins
membrane is very thin compared to cell diameter
Hydrophobic lipid bilayer - restricts movement of polar compounds
different composition of membrane lipids in outside layer vs inside layer
different lipid proportions between plasma membrane and organelle membranes
dynamic compositions - lipids from inner layer may move to the outer layer and vice versa to maintain fluidity
association of sphingo lipids and cholesterol produce micro domain enriched with proteins
- Involved in signalling
Purpose of cell membranes
Separates organelles from external environment or cytosol
This allow enzymes to be compartmentalised and biomolecules conc gradients to be established
therefore they play a role in organising reaction sequences, energy conservation and cell-cell conservation
Describe the fluid mosaic model
Lipids may move laterally
proteins can be integral(like channel proteins) or peripheral(like carrier proteins)
Structure of fatty acids
Building blocks of lipids
contain a hydrophobic hydrocarbon tail
they have a carboxylate group on the end which ionises at physiological pH becoming hydrophilic
Structure of phospholipids
Phosphate group joined by phosphodiester linkage in hydrophilic region
Structure of cholesterol
Sterols characterised by 4 rigid fused hydrocarbon rings - extra rigidity
small hydrophobic head
only one hydroxyl group so low solubility
cholesterol sticks between hydrophobic fatty acids tails and helps phospholipids pack more closely - decreases membrane fluidity
Properties of cell membrane [3]
Flexible
self-sealing
selectively permeable
Identify which molecules will be able to pass straight through the phospholipid bilayer and which will be unable to
Phospholipid bialyer has a hydrophobic centre
so large polar molecules and ions will be repelled
What is the glycolax and what is its functions ?
Layer of associated carbohydrates that attach via glycolipids or glycoproteins to the plasma membrane
Act as cell recognition markers
also protects cell from digestion
restricts uptake of hydrophobic compounds
Name the 3 classes of membrane lipids
Phosphoglycerides (A type of phospholipid)
Sphingolopids (some are phospholipids, some are glycolipids)
cholesterol (a type of sterol;steroid alcohols)
Structure of phosphoglycerides
Polar head alcohol group attached to a phosphate group which attaches to the gyclerol molecule at position 3
sometimes the alcohols may have additional phosphate groups attached to it
two fatty acids join to the glycerol backbone via 2 ester bonds
phosphate head group is hydrophilic
the diagram depicts phosphotidic acid
Structure of membrane sphingolipids
Consist of 1 Fatty acid joined to sphingosine
sphingosine is a fatty amine - 18-carbon amino alcohol with an unsaturated hydrocarbon chain
Some are phospholipids(they have a phosphate group attached to a hydroxyl group of sphingosine) , some are glycolipids (they have a carb chain attached to the hydroxyl group if sphingosine )
2 hydrophobic tails:
- one from the fatty acid residue located at ‘R’
- The other comes from the hydrocarbon tail of sphingosine
myelin sheath made from sphingomyelin
Types of membrane proteins
Peripheral proteins attached by electrostatics attractions with polar head groups/integral proteins so can be removed by ionic solvent
intergreal proteins held by hydrophobic interactions between lipids and hydrophobic domains of proteins
proteins on ones side different from those on other side
lipid anchored proteins are covalent bound to inner or outer surface via a specific lipid (GPI anchor)
Functions of membrane proteins
Transporters(carrier proteins) and channels
provide anchors to cell cytoskeleton and ECM
receptors to extracellular signals that are impermeable to the membrane such as hormones
may be enzymes
Differences in ionic composition of extracellular vs intracellular fluid
Interstitial (Extracellular) fluid contains a high [Na+] and low [K+]/[Ca]
intracellular fluid low [Na+], high [K+], high [Mg2+] and low [Ca]
