Lecture 1: Membrane structure and function 1 Flashcards
Learning Objectives
- Have an appreciation of the fundamental importance of the lipid membrane
- Understand the broad classifications of lipids
- Understand why lipid bilayers and cell membranes form
Give some of the functions of the cell membrane?
- Partition cells and cellular organelles
- Mediates molecular traffic across the boundary
- Organize complex enzyme sequences (e.g. ETC)
- Key in energy generation (electrochemical gradient, ETC)
- Cell-cell communication
What is an integral membrane protein?
A protein which passes all the way through the membrane.
What is the cell membrane composed of?
Lipids and proteins
What are some of the roles of lipids in the cell?
- Energy storage (triacylglycerols, aka triglycerides)
- Signalling molecules
- Cofactors
- Pigments
- MOSTLY: component of membranes
What is the general structure of a phospholipid?
Phospholipids are amphipathic: they have a hydrophilic head and two hydrophobic hydrocarbon tails.
The hydrophilic head is typically composed of choline, phosphate and glycerol. It is the choline that is substituted for other (normally positively) charged molecules in other phospholipids. These charges form stabilizing interactions with the aqueous environment. Polarity is therefore an important feature.
The two hydrophobic tails vary in length, but are normally 14-24 carbons long and can be saturated or unsaturated. In a typical phospholipid, one chain is saturated and lies flat, taking up less space. The other is unsaturated and so has a kink (especially if is it a cis C=C double bond) and is bulkier. This long hydrocarbon portion is oily or greasy.
What are the names of the three commonest phospholipids?
- Phosphatidylethanolamine (PE)
- Phosphatidylserine (PS)
- Phosphatidylcholine (PC)
What is the structure of sphingomyelin?
Sphingomyelin is built from sphingosine, which has one fatty acid chain attached to a glycerol-like molecule (the middle OH on glycerol has been replaced with an NH3+).
The OH attached to the carbon bearing the fatty acid is left available to form hydrogen bonds with the aqueous environment (stabilizing interactions). The other OH, on the other end of the molecule, undergoes a condensation reaction to form a bond to the phosphate group of the choline-phosphate addition. The NH3+ on the middle carbon is used to from an amide link to a second fatty acid chain.
Give an example of a sterol.
Cholesterol
What is the structure of cholesterol?
Sterols are much more compact than phospholipids. Sterols are also amphipathic, with a single hydroxyl -OH group as the polar (hydrophilic) head, a rigid steroid ring structure (4 rings) and then a short hydrocarbon tail (hydrophobic).
Which sterols are present in which organisms?
Prokaryotes
Sterols are not present in prokaryotes, but bacteria do have sterol-like molecules called hopanoids, which, like sterols, stabilize the membrane.
Eukaryotes
- Mammals: cholesterol
- Yeast: Ergosterol
- Maize leaf: Sitosterol
- Protozoa (e.g. paramecium): Stigmasterol
What are the functions of cholesterol in the membrane?
Cholesterol slots into the membrane between two phospholipids creating a cholesterol-stiffened region near the polar head. This makes the membrane more durable, but the middle of the membrane (lower part of each phospholipid) is still fluid.
Cholesterol makes the membrane less permeable to very small molecules due to the tighter packing.
Cholesterol helps to prevent crystallization (phase-transition) when the temperature drops.
The amount of cholesterol in the membrane can be changed by the cell.
Which membranes in the cell have the greatest percentage composition of sphingolipids?
Lysosomal membranes have the highest percentage, with the plasma membrane having the next highest percentage of sphingolipids.
Give two other names for phospholipids.
Glycerophospholipids, phosphoglycerides
What causes phospholipids to form membranes?
Shape (vaguely cylindrical) and amphipathic nature (bury hydrophobic tails away from water)
