Lipids

Lipids

• Hydrophobic or non-polar molecules (=they don’t mix well with water)

• Held together by London Dispersion Force

• For Insulation (endotherms) and energy (because fat stores energy, increases volume, insulates)

  • Important for cell membranes, hormones, insulation and protection

  • Can be saturated and non-saturated

Why do lipids need London Dispersion Force?

Most lipids are non-polar and hydrophobic— they lack strong polar interactions, therefore rely on LDF

How LDF affects lipid behavior?

1. How tight the packing is

• Saturated v.s. unsaturated

• How close molecules are

2. Surface area contact

• long v.s. short fatty acid chains

Affect of tighter packing on LDF (lipids)

LDF are weak forces caused by temporary shifts in electrons = they only work well when molecules are very close together

• Tightly packed = chains line up straight and close together

• Reduces the distance between molecules

• Shorter distance = stronger dispersion forces

Affect of surface area on LDF (lipids)

Long, straight fatty acid chains have large surfaces that can lie next to eachother

• more surface contact —> more temporary dipoles interacting —> higher LDF

  • this is why saturated fats (straight chains) are solid at room temp.

Affect of kinks on LDF (lipids)

Unsaturated fats have double bonds that create “kinks”

• The kinks stops the chains from lining up neatly

• Less surface contact = weaker LDF

  • e.g. why oils (unsaturated fats) stay liquid

Saturated v.s. Unsaturated Fatty Acids

1. Saturated

• SINGLE CARBON BONDS = straight

• Single bonds can rotate

2. Unsaturated

• Has a double carbon bond among the single carbon bonds = kinks/bends

• The double bond = stuck in position

Phospholipids

Has a polar (+ and - charged), hydrophilic head AND a hydrophobic tail = Can interact with polar & non-polar molecules and ions

• Head:

  • Can form ionic and ion-dipole interactions

    • Strong ions can remove water molecules to get closer to the phospholipid head (strong attraction), causing the heads to tilt/stretch towards the ions = changes how the membrane behaves

• Tail:

  • Can form LDF with other non-polar molecules = temporary dipoles = stabilize the membrane + allow non-polar substances to dissolve into it

Why does the membrane get formed?

The hydrophilic tails touch water…

• …Forces the water to form a stiff, cage-like structure around the tails

  • BUT water needs to be free-roaming (high entropy)

• So water pushes the tails to other tails = hydrophilic tails form together to prevent this

What is it about phospholipids that lead to a cell membrane?

Water naturally pushes the tails towards other tails

• The heads already come with the tails — it’s just because the tails are exposed

  • = Creates a bubble

What is it about cellulose v.s. starch— that we can digest starch as humans but not cellulose?

We have enzymes that are specifically shaped to snap COILED bonds (alpha bonds).

• Since cellulose has glucose facing every other direction = creating a straight + tough chain——- we can’t digest it for energy