What structural feature of animal lipids allows animals to pack them in a smaller space compared to plants?

Question

why is this important?

Answer 1

Animal lipids tend to be saturated. That means they're long, straight chains made up of single bonds. The straight chain structure allows the molecules to stack up neatly and pack into a smaller space. Imagine taking a bunch of straight wires and bundling them up -- they pack up really neatly.

Contrast this to plant fats. Plant fats are often unsaturated, and they have cis-double bonds. This causes the lipid chains to form corners and bends. Imagine taking a bunch of kinked, curled and bent-up wires and trying tp pack them. They won't stack very easily, thus they take up more space.

Why is this important? Because neatly packing fats solidify under warmer conditions than fats that don't pack so well. For example, lard is saturated fat, and it is solid at room temperature. Olive oil is unsaturated, and it is liquid at room temperature. To make olive oil solidify, you have to cool it a lot. The firmness or liquidity of fats has an effect on cell membrane structure. Many species adapt to their climates by balancing the saturated and unsaturated fats in various proportions.

As an aside, you may have heard of "trans-fats." These are unsaturated fats with trans-double bonds. They are different from cis-double bonds because they allow the lipid chain to keep a straight conformation. This means they can stack neatly like a saturated fat, even though they are unsaturated. To make matters worse, the double bonds in fats are hard to break, so having a trans-unsaturated fat in your body is bad because it's hard to metabolise, thus they stick to your arteries more.