Bio 161 W3 -Lipids & Membranes

What is the key chemical feature of lipids?

Mostly nonpolar hydrocarbon chains (C–H bonds)

How do lipids interact with water?

They are hydrophobic and avoid water (cluster together) (hydrophobic effect)

What type of interactions hold lipids together?

Hydrophobic interactions + van der Waals forces

—> No strong covalent bonding between lipid molecules in membranes

Lipids come in many shapes/sizes but backbone is….

composed of carbon & hydrogen

Why are lipids NOT considered true polymers?

• not built from long chains of repeating, similar monomer subunits

• Instead composed of a few distinct parts, like glycerol & fatty acids

Which of the four biological macromolecules are considered "true polymers"?

Polypeptides (proteins), nucleic acids, and polysaccharides

(Polymer) Polypeptides

(Monomer) Amino Acids

(Polymer) Nucleic Acids

(Monomer) Nucleotides

(Polymer) Carbohydrates/Polysaccharides

(Monomer) Monosaccharides

Which elements are always found in polypeptides and nucleic acids, but are often missing in many common lipids like triglycerides?

Nitrogen (found in both) and Phosphorus (found in nucleic acids)

Important lipids found in the cell

• Triglycerides

• Fats

• Phospholipids

Lipid Type: STERIODS

• Lipids with 4 fused carbon rings

• small modifications along backbone

Steroid hormones

cell signaling

—> many hormones are steroids, BUT not all steroids are hormones & not all hormones are steroids

Type of Steroid: Cholesterol

• unique to eukaryotes

• still has that same 4 ring carbon structure

Cholesterol can be carried in blood…

• inside of larger particles

• WE NEED CHOLESTEROL!! —> vital for life b/c of fluidity & structure it allows membranes

Why cholesterol vital for life?

It allows membranes fluidity & membrane structure

The Fatty acids….

Some of the most important lipids that give rise to life!

Fatty acids are

unbranched hydrocarbon chains joined to a carboxyl group

What is a saturated fatty acid?

No double bonds, straight chains, tightly packed

—> carbons FULLY bound to hydrogens

What is an unsaturated fatty acid?

Has double bonds (usually cis), causes kinks

—> one or more double carbon bonds, not fully saturated w/hydrogens

What do double carbon bonds cause in unsaturated fatty acids?

kinks!

Saturation level of Fatty Acids…

determines fluidity

Effect of unsaturation on membranes?

Increases membrane fluidity

Saturated Lipids…(butter, beeswax)

• no 3D kinks, packed very tightly

• compact, less fluid

• form solids @room temp

Unsaturated lipids (oil!)

• liquid @ room temp bc of kinks! (in backbone)

• less compact, more fluidity

Type of Lipids: Fats

• richest energy source in our bodies!

• Insulation: highest amount of fat near major organs b/c vital organs require a lot of energy

• Waterproofing: honeycombs made up of triglycerides —> structure waterproofs

Type of Fat: Triglyceride

• Triglycerides form through condensation reaction

• Free fatty acids attach to glycerol

What are triglycerides used for?

Energy storage

• stored in body as adipose tissue

What are triglycerides made up of?

• Glycerol

• 3 free fatty acids

What is glycerol?

3 carbon structure

Triglycerides are transported by

• molecules covered in protein & phospholipids

• triglycerides on inside! (orange)

Type of Lipid: Phospholipids

• are amphipathic

• charged/polar head (hydrophilic)

• nonpolar tail (hydrophobic)

What are phospholipids made of?

Glycerol + 2 fatty acids + phosphate group

Why are phospholipids important?

They form cell membranes

• more polarity, more ability to interact w/aqueous environment

What does amphipathic mean?

Has both hydrophilic and hydrophobic regions

Why are phospholipids amphipathic?

Polar head + nonpolar tails

Lipid Micelles

• Most simple arrangement of phospholipids

• Hydrophilic heads interact w/water

• hydrophobic tails interact w/one another

Formation of membranes in aqueous solution are

spontaneous

Lipid Bilayers

• what cells are made of!

• 2 layers form inverted to each other spontaneously

• can compartmentalize

Why do lipid bilayers form?

Hydrophobic effect (tails avoid water)

Phospholipid bilayer membrane allow…

for life as we know it!

• can compartmentalize cell’s inside/outside

In the membrane proteins are

conduits in the membrane

Cholesterol & saturation of hydrocarbons of fatty acids (membrane wise)

influence attachment of proteins + fluidity

What is the Fluid Mosaic Model?

• proteins & cholesterol embedded in membrane

• good model, still stands today BUT doesn’t specify micro domains present in membranes

Lipid Raft Model

• membrane micro-domain

• clustering of cholesterol & specific transmembrane proteins

Cells use lipid rafts for

Regulatory function

—> being able to separate signaling molecules/cascades so they aren’t turned on all the time

Cholesterol exists in two main states in the body

• transported (packaged) within lipoproteins in the blood

• structurally embedded within cell phospholipid membranes.

Proteins can be amphipathic too…

• can be polar or nonpolar so they can embed themselves in membranes w/non polar amino acids & polar amino acids on outside

The lipid raft model is considered a "better" or more accurate refinement of the Fluid Mosaic Model because

• it acknowledges that the cell membrane is not a homogeneous, random sea of lipids and proteins

• shows that membranes contain specialized, ordered, and functional microdomains that act as signaling platforms, providing a more complex and biologically realistic view.

• essentially a more advanced version

permeability

selective due to lipid & membrane structure

• based on fatty acid saturation/unsaturation

Membranes: Kins/Unsatured

• more spaces between phospholipids —> HIGHER permeability & fluidity

Membranes: Straight/Saturated

• more tightly packed lipids —> LOWER permeability & fluidity

Introduction of cholesterol in membrane

decreases fluidity

What is membrane permeability?

What substances can cross the membrane

What is membrane fluidity?

• flexible & dynamic nature of the cellular lipid bilayer, allowing lipids and proteins to move laterally within the membrane

• Movement of lipids within the bilayer

Are fluidity and permeability the same?

no

What is cholesterol’s role in membranes?

Regulates fluidity and stabilizes membrane

How does cholesterol affect permeability?

Decreases permeability to small molecules

Concentration gradient

always trends toward equilibrium

• concentration gradients of molecules on sides of membranes

What has the highest permeability?

small, nonpolar molecules

What has the lowest permeability?

small ions

Order of permeability

small non polar molecules —> small uncharged polar molecules —> large uncharged polar molecules —> small ions