Lipids and Membranes

What are lipids

Non-polar organic compounds, not soluble in water

What are the 3 main biological functions of lipids

• membrane forming/ structural

• energy storage

• Precursors to signalling molecules

What are fatty acids composed of

Carboxylic acid with long carbon chains

can be saturated or unsaturated

What fatty acids have higher melting points saturated or unsaturated and why

Saturated fatty acids have higher melting points than unsaturated fatty acids

Saturated fatty acids have linear chains, this means they pack more closely together, this means there are more hydrophobic interactions

So more energy is needed to break the hydrophobic interactions

Unsaturated fatty acids have non-linear chains and chains are bent, this means they can’t pack closely together, there are less hydrophobic interactions, so less energy is needed to break the interactions

What are polyunsaturated fatty acids used for

Signal molecule precursors e.g

Arachidonic acid is the precursor for prostaglandins

What are the main functions of triacylglycerols

How are triacylglycerols formed

The main function is energy storage

Formed by condensation reactions of 3 fatty acid molecules with one glycerol

This forms ester bonds and 3 mol of water is released

What are phospholipids and what is their function

An amphipathic molecule

Hydrophilic head group ( contains heteroatoms)

Hydrophobic lipid tail group ( fatty acids)

They are structural lipids used to form membranes

What is the function of cholesterol (2)

It increases fluidity of the membrane as it decreases packing making the membrane more rigid

It is a precursor for biosynthesis

E.g

It is a precursor for testosterone, cholic acid, Vitamin D3

What is compartmentalisation for membranes

Lipids membranes create compartments where different reactions occur and is an essential part of cellular machinery and so competing metabolic processes can be carried out simultaneously

What is the role of compartmentalisation and membranes

• structure and containment

• Control internal conditions of compartment

• Control passage of molecules

• Receive and transmit messages

What do phospholipids do in aqueous environments

They self-assemble and form non-covalent interactions with each other

What are the 3 main structures lipids form when they self assemble and why are these favourable

Micelles

Phospholipid bilayer

Liposomes

They maximise favourable interactions and expel water

What are the main characteristics of the lipid bilayer?

What does it let in, what doesn’t it let in

• it is fluid ( cholesterol)

• impermeable to water and polar compounds

• permeable to small uncharged compounds, lipid-soluble compounds and water (via aquaporins)

What is the eukaryotic cell membrane composed of

• phospholipids

• Carbohydrates used for signalling ( glycoproteins/ glycolipids)

• Proteins : used for transport and signalling ( peripheral or integral)

• Cholesterol

What are the two different membrane proteins

Integral proteins

Peripheral proteins

Are membranes asymmetrical or symmetrical and why

Asymmetrical

as different lipids, proteins and carbohydrates are unevenly distributed between the inner and outer layer of the membrane

It is important to have the correct protein orientation for function

What are the two main methods of membrane transport

• transmembrane proteins selectively transporting specific molecules and ions

• Endocytosis or exocytosis ( bulk transport and required membrane fluidity)

What is passive diffusion

The solute/ion is conveyed down it’s concentration gradient

Doesn’t require metabolic energy

What are the three different types of passive diffusion

• simple passive diffusion across a membrane

• Simple passive diffusion via a channel ( water and aquaporins)

• Facilitated passive diffusion via carrier proteins

What is active transport

Solute/ion travels against its concentration gradient bua a carrier protein

Requires metabolic energy (ATP)

What happens to the carrier protein in active transport

ATP binds to the carrier protein and is hydrolysed causing a conformational change in the carrier protein

What are the three different types of carrier proteins

• uniport : transport of one type of specific solute

• symport : transport of two solutes in the same direction

• Antiport: transport of two solutes in different directions

How is glucose transported into the blood from the ileum

How is a high Ca2+ concentration maintained in the ER

What happens to the membrane during Endocytosis

The membrane creates a vesicle by sealing itself around the substance

The membrane then self heals

Examples of Endocytosis

Phagocytosis