Lipids, Membranes, and the First Cells Review Set (BIOL 203 Module 1/2)

lipids

A class of macromolecules grouped together by their *hydrophobicity* (they don't dissolve well in water!) and/or by their ability to dissolve well in nonpolar substances

Lipids are nonpolar (hydrophobic) because they contain *many more C-H bonds than polar covalent bonds / functional groups*

Not considered true polymers (lipid structures are not made of a single repeating unit / monomer), structures can vary widely

Used in energy storage, cell communication / signaling, membranes

Fatty acid

A lipid consisting of a long hydrocarbon chain (consists of C and H atoms) bonded to a carboxyl group

Can be saturated or unsaturated

Saturated fatty acids

Follow the line of reasoning...

*Fatty acid molecules that have the maximum number of hydrogen atoms attached* ∴ only have single C-C bonds ∴ no kinks exist in the hydrocarbon chain ∴ are stackable

van der Waals forces can easily interact across these stackable, saturated fatty acid chains ∴ substances with many saturated fatty acids tend to be *solid* at room temp

Unsaturated fatty acids

Follow the line of reasoning...

*Fatty acid molecules that do not have the maximum number of hydrogen atoms attached* ∴ have some double C=C bonds ∴ kinks exist in the hydrocarbon chain ∴ are not easily stackable

van der Waals forces cannot interact across these unstackable, unsaturated fatty acid chains ∴ substances with many unsaturated fatty acids tend to be *liquid* at room temp

hydrogenation

Bombardment of hydrogen atoms onto a fatty acid to turn C=C bonds into C-C bonds for preservation (liquids spoil faster than solids)

May also result in trans C=C bonds that can lead to atherosclerosis (formation of plaque in blood vessels)

glycerol

A 3-carbon alcohol (has three hydroxyl groups) that forms the backbone of triglycerides, waxes, and phospholipids

Ester bonds

The bonds commonly found within lipids, formed between the hydroxyl group of one molecule (e.g., glycerol) and the carboxyl group of another molecule (e.g., fatty acid)

Triglycerides

Lipid made of 1 glycerol molecule and 3 fatty acid molecules

Primarily as energy storage molecules: C-H and C-C bonds have more potential energy (nonpolar bonds) in comparison to other molecules; store twice as much energy per gram than carbohydrates

Also has insulative and protective functions

Aka fats

steroids

Lipid made of 4 fused carbon rings

Serve as components of the plasma membrane and as precursors to other molecules, like hormones used in cell signaling

Ex. cholesterol

phospholipids

Lipid made of 1 glycerol molecule, 2 fatty acid molecules, and 1 polar phosphate / choline "head"

Considered *amphipathic* molecules: both hydrophilic and hydrophobic

Serve as a key component in forming the lipid bilayer of the plasma membrane

Selective permeability

Refers to the property of membranes (such as those formed by a lipid bilayer) that allow some substances to pass more readily through them than other substances

This property arises from phospholipids' tendency to form lipid bilayers in water. Phospholipids' polar heads face outward and interact with water, while their hydrophobic fatty acid tails are shielded on the inside. *Substances most able to cross the lipid bilayer must be able to 1) physically move past the polar heads and 2) interact with the inner fatty acid tails.*

Small, nonpolar molecules' permeability

Have very high permeability (readily cross lipid bilayer)! Can "squeeze" past the polar heads and interact with the inner hydrophobic fatty acid tails.

Ex. neutral gases like O2, CO2, and N2

Small, polar molecules' permeability

Have decent permeability (can cross lipid bilayer somewhat). Though they have weak interactions with the inner hydrophobic fatty acid tails, these molecules can still "squeeze" past phospholipids' polar heads.

Ex. H2O, glycerol

Large, polar molecules' permeability

Have minimal permeability (barely cross lipid bilayer). These molecules possess difficulty in "squeezing" past the polar heads and have weak interactions with the inner hydrophobic fatty acid tails. Will often require assistance in moving across lipid bilayer (e.g., transport protein).

Ex. glucose, sucrose

Charged ions' permeability

Have little to no permeability (practically cannot cross lipid bilayer). These molecules cannot interact with inner hydrophobic fatty acid tails. Will always require assistance in moving across lipid bilayer (e.g., transport protein).

Ex. Cl-, K+, Na+