Macromolecules & Lipids

EFB325 Cell Physiology

Building blocks & macromolecules

Hierarchy of molecular structure and organization through biology

start with inorganic precursors-phosphate (PO₄^3-), ammonium (NH₄⁺), oxygen, water, carbon dioxide (CO₂)
up to 99% of cell mass is constituted by 6 elements: C, H, N, O, P, S
these are used to produce small organic molecules, which often serve as building blocks
nucleotides, amino acids, fatty acids, and sugars
building blocks (monomers) are polymerized to form macromolecules=nucleic acids, proteins, lipids, polysaccharides
macromolecules come together to form supra-molecular structures=membranes, cell wall, chromosomes
organelles=compartmentalization to allow metabolic control using specialized reactions
contain macromolecules (proteins, nucleic acids)
allow concentration of substrate molecules
allow differences in pH appropriate for the enzymes in that compartment
organelles and structures all together form the cell
combinations of cell types function together as tissues

Major classes of biological macromolecules and their biochemistry (lipids, polysaccharides, nucleic acids, proteins)

Lipids

diverse group of molecules, serve different functions=storage, membranes, signaling
aren't truly polymers like polysacharides, nucleic acids or proteins, but are a major cellular constituent

Typical lipid has a molecule of glycerol and ester bonds to 2 or 3 fatty acids

fatty acids=carbon chain with carboxyl group on the end; made from a two-carbon subunit (acetyl-CoA) so are commonly found with chain lengths in multiples of 2 carbons
fatty acids are linked to glycerol through a condensation reaction (loss of water)
the fatty acids are "activated" by a high-energy covalent linkage to CoA
lipids are highly hydrophobic
can be saturated=fully reduced (no double bonds) or unsaturated=partially oxidized (at least one double bond)
having a double bond affects the shape by introducing a kink in the chain and reduces flexibility

Triacylglycerides

lipids serve as excellent energy storage forms, also store of reduced carbon structures
storage lipids have three fatty acids linked to glycerol
will form fat droplet in water
in animals, the fatty acids are typically saturated (all the carbons are bound to max # of hydrogens), examples are palmitate (16:0) or stearate (18:0)=higher melting temperature, so solid at room temp
in plants, the fatty acids are commonly unsaturated (have some double bonds), such as linoleic acid (18:2) or oleate (18:1)=lower melting temp, liquid at room temp

Diacylglycerides

majority of lipids in membranes are phospholipids, have two fatty acids and another group linked by a phosphate group; for example phosphatidyl choline
this third group is frequently charged, resulting in an amphipathic lipid molecule (highly hydrophobic on side with fatty acids, but polar "head group")
in water, the fatty acid sides tend to coalesce together, with polar "head groups" facing the water, forming a bilayer = membrane
not all diacylglycerides are built from glycerol; sphingolipids (not glycerol-based) are also in animal membranes
membranes have the important property of acting as a barrier against the movement of macromolecules and charged molecules=semi-permeable

Steroids

includes cholesterol=membrane lipid
some steroids can act as hormones=estrogens, testosterone
synthesized by the assembly of 6 units of a 5-carbon monomer=isoprene
cholesterol has the dual effect of stabilizing the membranes at high temperature, while keeping them fluid at low temperatures
cholesterol is essentially absent in plant membranes, but plants do have steroids that can act as hormones