Lipids and Biomembranes

Lipids are defined by their physical properties rather than a specific chemical structure, encompassing a diverse group of molecules.
They exhibit a wide variety of structures, including:
- Saturated and unsaturated fatty acids.
- Glycerides (mono-, di-, and triglycerides).
- Phospholipids and glycolipids.
- Steroids and sterols.
- Waxes.
Primary functions of lipids:
- Energy storage: Efficient storage due to high caloric content.
- Biomembrane composition: Essential components of cellular membranes.
- Chemical signaling: Act as hormones, signaling molecules, and participate in intracellular communication.
Four primary types of lipids:
- Triglycerides: Used for energy storage and insulation.
- Phospholipids & Glycolipids: Key components of cell membranes.
- Steroids: Act as hormones and affect membrane fluidity.

Glycerol: A three-carbon alcohol with a hydroxyl group ( $OH$ ) on each carbon.
- The structure of glycerol is: $H<em>2C-CH-H</em>2C$ with an $OH$ group attached to each carbon.
Fatty Acids:
- Saturated: Contain only single bonds between carbon atoms.
- Unsaturated: Contain one or more double bonds between carbon atoms. These can be:
- Monounsaturated: One double bond.
- Polyunsaturated: Multiple double bonds.
- Fatty acids generally have a carboxylic acid group at one end and a hydrocarbon chain.

Formed by combining 3 fatty acids with glycerol through ester bonds.
The reaction involves the formation of ester bonds between the glycerol and the fatty acids. This process is known as esterification, where water is released.

Composed of 2 fatty acids, glycerol, and a phosphate group, which is often modified with additional molecules.
Structural formula, space-filling models, and icons are used to represent phospholipids to visualize their arrangement and behavior in membranes.

There is a lot of variety in polar head groups of phospholipids, such as:
- Phosphatidylcholine (PC).
- Phosphatidylethanolamine (PE).
- Phosphatidylserine (PS).
- Phosphatidylinositol (PI).
Fatty acid tails in phospholipids can also vary in length and degree of saturation, influencing membrane fluidity.
Both the polar head groups and the fatty acid composition contribute to the diversity of phospholipids, affecting membrane properties and interactions.

Phospholipids are amphipathic, meaning they have both hydrophobic (fatty acid tails) and hydrophilic (polar head) regions, allowing them to form structures in aqueous environments.

In water, phospholipids spontaneously form micelles or bilayers due to their amphipathic nature. This is driven by the hydrophobic effect.
Micelles are spherical structures with the hydrophobic tails facing inward and the polar heads facing outward.
Bilayers are two-layered structures with the hydrophobic tails facing inward and the polar heads facing outward, forming a stable barrier.

Bilayers have exposed edges and tend to fold into liposomes, which are spherical vesicles with an aqueous cavity, useful for drug delivery and research.

Artificial bilayers can be created using purified phospholipids to study lipid properties, such as lipid mobility and membrane permeability.
This setup allows for controlled experiments to investigate the behavior of lipid membranes under specific conditions.

Water is excluded at the bilayer interface due to the hydrophobic nature of the fatty acid tails, creating a non-polar environment.
The polar head groups interact with water on the exterior surfaces of the bilayer, stabilizing the membrane structure.

Some membrane lipids are glycolipids, which contain a carbohydrate moiety attached to a lipid molecule, found on the extracellular surface of the cell membrane.

Steroids are a fourth type of lipid, characterized by a four-ring structure.
They can function as circulating hormones, such as estrogen and testosterone, or as membrane components, like cholesterol.

Animal cells have cholesterol in their biomembranes, which modulates membrane fluidity.
Plants and fungi have different types of steroids, such as phytosterols and ergosterol, while bacteria do not contain steroids but have similar compounds called hopanoids.

Biomembranes are asymmetrical, meaning the lipid composition and protein distribution differ between the two leaflets of the bilayer, influencing membrane function.

Biomembranes have associated proteins:
- Transmembrane proteins: Span the entire lipid bilayer, with hydrophobic regions interacting with the lipid core.
- Membrane-associated proteins: Interact with one leaflet of the lipid bilayer, often through hydrophobic interactions or lipid modifications.
- Lipid-linked proteins: Attached to the membrane via a lipid anchor, such as GPI anchors or prenylation.
- Peripheral proteins: Associate with the membrane through interactions with other proteins, rather than direct insertion into the lipid bilayer.

Biomembranes are selectively permeable, regulating the passage of substances.
Not everything can cross a lipid bilayer; it depends on size, charge, and polarity.
The membrane controls the passage of molecules and ions into and out of the cell, maintaining cellular homeostasis and allowing for specific transport processes.