2.2- Lipids 2: Phospholipids and cholestrol

Question 1

What is the structure of Phospholipids ?

Answer 1

Phospholipids have the same structure of triglycerides, EXCEPT that one if the fatty acids is replaced by a phosphate group. A condensation reaction between an -OH group on a phosphoric acid molecule (H3PO4) and one of the three -OH groups on the glycerol forms an ester bond..

Most of the fatty acids found in Phospholipids have an even number of carbon atoms (often 16 or 18). Commonly one of these chains is saturated and one of them is unsaturated.

Question 2

How do Phospholipids behave in water?

Answer 2

When surrounded by water, the phosphate group has a negative charge, maing it polar attracted to water. However, the fatty acids tails are non- polar and so are repelled by water. It is common to refer to the head as ‘hydrophilic’ and the tail as hydrophobic, which means that the Phospholipids molecule is amphipathic.

Membrane lipids tend to be amphipathic, whereas those involved ins storage are not.

Phospholipids that are amphipathic have very distinct properties in water. They may form a layer on the surface of the water with heads in the water and tails sticking up out of the water. They may also form micelles- tiny balls with the tails tucked away inside and heads pointing outwards into the water.

Question 3

What is the Phospholipids bilayer.

Answer 3

Amphipathic Phospholipids s are excellent at forming membranes around cells and organelles. Inside and outside a cell membrane is an aquesous solution. The Phospholipids form a bilayer, with two rows of Phospholipids , tails pointing inwards and heads pointing outwards into the solution. Between 20 and 80% of membranes in plant and animals cells are made of Phospholipids. Bacterial membranes tend to contain a greater proportions of protein.

• The individual Phospholipids are free to move around in their layer but will not move into any position where their hydrophobic tails are exposed to water. This gives the membrane some stability.
• The membrane is electively permeable. It is only possible for small and non-polar molecules to move through the tails in the bilayer such as oxygen and carbon dioxide. This lets the membrane control what goes in and out of the cell, and keeps it functioning properly.

Question 4

What is cholesterol?

Answer 4

Cholesterol is a steroid alcohol- a type of lipid which is not made from glycerol or fatty acids. It consists of four carbon based rings or isoprene units. Cholesterol is a small and hydrophobic molecule, which means it can sit in the middle of the hydrophobic part of the bilayer. It regulates the fluidity of the membrane, preventing it from becoming too fluid or stiff.

• Cholesterol is mainly made in the liver in animals. Plants also have a cholesterol derivative in their membrane. It is called stigmasterol and is different from Cholesterol in only one respect: it has a double bind between carbon 22 and carbon 23.
• The steroid hormones testosterone, oestrogen and vitamin D are all made from cholesterol. Because they are small and hydrophobic, they can pass through the hydrophobic part of the cell membrane and any other membrane inside the cell.. Steroids are also abundant in plants and on ingestion and absorption some can be converted into animal hormones.