BIOL103 WK2

**Why are lipids important in biology?

Lipids store large amounts of energy, form self‑sealing membranes, act as hormones and signalling molecules, and contribute to membrane fluidity and stability.**

**Why are lipids hydrophobic?

They consist mostly of long hydrocarbon chains (C–H) with very little oxygen, making them non‑polar and insoluble in water.**

**Why do lipids release more energy than carbohydrates?

Their hydrocarbon chains contain many C–H bonds that release large amounts of energy when oxidised.**

**What does “like dissolves like” mean?

Polar molecules dissolve in polar solvents (e.g., water). Non‑polar molecules dissolve in non‑polar solvents. Lipids are non‑polar → hydrophobic.**

**Example of a hydrophilic molecule

Ethanol (polar).**

**Example of a hydrophobic molecule

Methane (non‑polar).**

**What is a fatty acid?

A molecule with a carboxyl group (COOH) attached to a long hydrocarbon tail. Tail length and saturation determine properties.**

**Saturated fatty acid

No C=C double bonds

**Unsaturated fatty acid

One or more C=C double bonds

**Monounsaturated vs polyunsaturated

Mono = one C=C

**cis fatty acids

Hydrogens on same side of double bond → kink → lower melting point → common in nature.**

**trans fatty acids

Hydrogens on opposite sides → straighter chain → packs tightly → higher melting point → formed during partial hydrogenation → unhealthy.**

**How does chain length affect melting point?

Longer chains have stronger dispersion forces → higher melting point → less soluble in water.**

**What does 18:1(Δ9) mean?

18 carbons, 1 double bond, double bond starts at carbon 9 from the carboxyl end.**

**What is a triacylglycerol?

A glycerol molecule bonded to three fatty acids via ester linkages. Main energy storage lipid.**

**Are triacylglycerols polar?

No — esterification removes polarity → fully hydrophobic.**

**What does amphipathic mean?

A molecule with both hydrophilic (polar) and hydrophobic (non‑polar) regions. Essential for membrane formation.**

**Three major membrane lipid types

Phospholipids (~65%), glycolipids (~10%), cholesterol (~25%).**

**Structure of a phospholipid

Glycerol backbone + 2 fatty acids + phosphate + head group. Hydrophilic head + hydrophobic tails.**

**Why phospholipids form bilayers

Hydrophilic heads face water

**What is a glycolipid?

A lipid with one or more sugar residues attached via a glycosidic bond. Sugar head = polar. Backbone = glycerol or sphingosine.**

**Where are glycolipids found?

Plants (glycoglycerolipids), bacteria & humans (glycosphingolipids). Important for cell recognition (e.g., ABO blood groups).**

**What is a sphingolipid?

Lipid with a sphingosine backbone + fatty acid (amide bond) + head group (phosphate or sugar).**

**Difference between phosphosphingolipid and glycosphingolipid

Phosphosphingolipid has phosphate

**Structure of cholesterol

Four fused carbon rings (sterol), hydroxyl group (polar head), hydrocarbon tail (non‑polar).**

**Role of cholesterol

Modulates membrane fluidity and stability

**Why are proteins essential?

They perform nearly every dynamic function in living organisms: enzymes, transport, signalling, structure, movement, immunity, storage.**

**Percentage of cell dry mass that is protein

More than 50%.**

**What is an amino acid?

A molecule with an amino group (NH₂), carboxyl group (COOH), hydrogen, and variable R‑group attached to a central α‑carbon.**

**What determines amino acid properties?

The R‑group (side chain).**

**Categories of amino acids

Non‑polar (hydrophobic), polar (hydrophilic), charged (acidic or basic), aromatic.**

**Conservative substitution

Replacing one amino acid with another of similar properties (e.g., hydrophobic → hydrophobic) with minimal functional impact.**

**At pH 7, what are the charges?

Amino group = NH₃⁺

**Acidic residues

Donate protons

**Basic residues

Accept protons

**Cysteine

Forms disulfide bonds (S–S) when oxidised → stabilises tertiary and quaternary structure.**

**Aromatic amino acids

Absorb UV light (Trp & Tyr at 280 nm

**How are peptide bonds formed?

Condensation reaction between amino group of one amino acid and carboxyl group of another → releases water.**

**What does primary structure represent in peptide bonds?

The linear sequence of amino acids linked by peptide bonds.**

**four levels of protein structure - Primary structure

Linear amino acid sequence.**

**four levels of protein structure- Secondary structure

α‑helices and β‑sheets formed by hydrogen bonding between backbone atoms (C=O and N–H).**

**α‑helix

Coiled structure

**β‑sheet

Extended strands connected by H‑bonds

** four levels of protein structure - Tertiary structure

Overall 3D shape formed by interactions between R‑groups (hydrophobic interactions, ionic bonds, H‑bonds, disulfide bridges).**

** four levels of protein structure - Quaternary structure

Assembly of multiple polypeptide subunits into a functional protein (e.g., haemoglobin).**

**Normal haemoglobin structure

Tetramer of 2 α and 2 β subunits

**Sickle‑cell mutation

Single amino acid change: Glutamic acid → Valine in β‑chain.**

**Why does this cause disease?

Valine is hydrophobic

**Why must proteins fold correctly?

Function depends entirely on correct 3D structure. Misfolding leads to diseases (Alzheimer’s, Parkinson’s, mad cow disease).**

**Role of chaperones

Prevent aggregation of unfolded proteins

**What is denaturation?

Loss of protein structure (secondary, tertiary, quaternary) → loss of function.**

**Causes of denaturation

Heat, pH changes, pressure, detergents, salt concentration.**

**Is denaturation reversible?

Sometimes (e.g., haemoglobin). Primary structure remains intact unless chemically broken.**