CIE AS/A Level Biology -2.2 Carbohydrates and lipids- Study Notes- New Syllabus

Ace A level Biology Exam with CIE AS/A Level Biology -2.2 Carbohydrates and lipids- Study Notes- New Syllabus

Key Concepts:

describe and draw the ring forms of α-glucose and β-glucose
define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide
state the role of covalent bonds in joining smaller molecules together to form polymers
state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar
describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides
describe the breakage of a glycosidic bond in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test
describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms
describe the molecular structure of the polysaccharide cellulose and outline how the arrangement of cellulose molecules contributes to the function of plant cell walls
state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds
relate the molecular structure of triglycerides to their functions in living organisms
describe the molecular structure of phospholipids with reference to their hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails

CIE AS/A Level Biology 9700-Study Notes- All Topics

Ring Forms of α-Glucose and β-Glucose

🌱 Description of α-Glucose and β-Glucose

Both α-glucose and β-glucose are isomers of glucose with the molecular formula C₆H₁₂O₆.
They differ in the position of the -OH group attached to carbon 1 (the anomeric carbon) in the ring form.
These forms exist because glucose cyclizes (forms a ring) in aqueous solutions, creating a hemiacetal ring structure (a six-membered pyranose ring).

🔍 Key Structural Difference

Feature	α-Glucose	β-Glucose
-OH group on carbon 1 (anomeric carbon)	Points down (below the plane of the ring)	Points up (above the plane of the ring)
Common form in nature	Found in starch and glycogen	Found in cellulose

🧪 Structure Details

The ring is formed by bonding carbon 1 and carbon 5 oxygen to form a hexagonal pyranose ring.
Carbon atoms are numbered clockwise from the oxygen in the ring.
The difference between α and β is crucial because it affects how glucose polymers (like starch and cellulose) form.

✍️Diagram of α-Glucose and β-Glucose (Haworth Projections)

Note:
In α-glucose, the -OH on carbon 1 points down (same side as C6 CH₂OH group).
In β-glucose, the -OH on carbon 1 points up (opposite side to C6 CH₂OH group).

📌 Summary

Isomer	OH on C1 Position	Role in Polysaccharides
α-Glucose	Down	Building block of starch, glycogen
β-Glucose	Up	Building block of cellulose

Definitions of Key Biological Terms

🌱 Monomer

A monomer is a small, simple molecule that can join with other similar molecules to form a larger molecule called a polymer.
Example: Glucose is a monomer for carbohydrates.

🌿 Polymer

A polymer is a large molecule made up of many repeating monomers linked together.
Example: Starch is a polymer made from glucose monomers.

🧪 Macromolecule

A macromolecule is a very large molecule essential to life, often made by polymerising smaller units (monomers).
Includes carbohydrates, proteins, lipids, and nucleic acids.

🍬 Monosaccharide

The simplest form of carbohydrate; a single sugar unit (monomer).
Examples: Glucose, fructose, galactose.

🍭 Disaccharide

A carbohydrate made of two monosaccharide units joined by a glycosidic bond.
Examples: Sucrose (glucose + fructose), lactose (glucose + galactose).

🍞 Polysaccharide

A carbohydrate polymer made of many monosaccharide units linked together.
Examples: Starch, glycogen, cellulose.

📊 Summary Table

Term	Definition	Example
Monomer	Small molecule that can join to form polymers	Glucose
Polymer	Large molecule made of repeating monomers	Starch
Macromolecule	Very large molecule essential to life	Proteins, carbohydrates
Monosaccharide	Single sugar unit	Glucose
Disaccharide	Two monosaccharides joined	Sucrose
Polysaccharide	Many monosaccharides joined	Cellulose, glycogen

Role of Covalent Bonds in Forming Polymers

🌱 Key Role of Covalent Bonds

Covalent bonds are strong chemical bonds formed by the sharing of electron pairs between atoms.
They join smaller molecules (monomers) together to form larger molecules called polymers.
During polymerisation, covalent bonds form between monomers by condensation reactions, where a small molecule (often water) is removed.
These covalent bonds create stable, strong linkages that hold the polymer chain together.

🔍 Example

In carbohydrates, covalent glycosidic bonds join monosaccharides.
In proteins, peptide bonds (a type of covalent bond) link amino acids.
In nucleic acids, phosphodiester bonds join nucleotides.

🧠 Summary
Covalent bonds are essential for building stable, long chains of polymers from smaller monomers.
These bonds determine the structure and properties of biological macromolecules.

Reducing and Non-Reducing Sugars

🌱 Reducing Sugars

Glucose, fructose, and maltose are reducing sugars.
They have free aldehyde or ketone groups that can donate electrons and reduce other substances like Benedict’s reagent.
This property allows them to participate in redox reactions.

🌿 Non-Reducing Sugar

Sucrose is a non-reducing sugar.
Its glycosidic bond blocks the reactive group, so it cannot reduce Benedict’s reagent directly.
Sucrose does not give a positive Benedict’s test unless it is first broken down by hydrolysis.

Sugar	Reducing or Non-Reducing	Reason
Glucose	Reducing	Has free aldehyde group
Fructose	Reducing	Has free ketone group
Maltose	Reducing	Has free aldehyde group after bond cleavage
Sucrose	Non-reducing	Glycosidic bond blocks reactive group

Formation of a Glycosidic Bond by Condensation

🌱 What is a Glycosidic Bond?

A glycosidic bond is a covalent bond that links two sugar molecules (monosaccharides).
It forms when two hydroxyl groups (-OH) on adjacent sugar molecules react.

🔬 Condensation Reaction

During glycosidic bond formation, a condensation reaction occurs:
A molecule of water (H₂O) is removed (lost).
The oxygen atom forms a bridge between the two sugar units, creating the glycosidic bond.

🍭 In Disaccharides

Example: Sucrose is formed from glucose and fructose by a glycosidic bond.
The bond forms between carbon 1 of glucose and carbon 2 of fructose, releasing water.

🌿 In Polysaccharides

Polysaccharides (e.g., starch, glycogen, cellulose) are long chains of monosaccharides linked by many glycosidic bonds formed through repeated condensation reactions.
The type of glycosidic bond (e.g., α-1,4 or β-1,4) affects the structure and properties of the polysaccharide.

Process	Description
Glycosidic Bond Formation	Covalent bond formed between two sugars
Reaction Type	Condensation (water molecule removed)
Example (Disaccharide)	Sucrose: glucose + fructose linked by bond
Example (Polysaccharide)	Starch: many glucose units linked by glycosidic bonds

🧠 Key Point:
Glycosidic bonds join sugar units into larger carbohydrates by condensation, crucial for energy storage and structural molecules in cells.

Breakage of Glycosidic Bonds by Hydrolysis

🌱 What is Hydrolysis?

Hydrolysis is the process of breaking chemical bonds by adding a water molecule (H₂O).
It is the reverse of condensation.

🔍 Hydrolysis of Glycosidic Bonds

In carbohydrates, hydrolysis breaks glycosidic bonds between sugar units in disaccharides and polysaccharides.
This splits larger sugars into smaller sugars or monosaccharides.
The addition of water breaks the bond, releasing the individual sugar units.

🍭 Example in Disaccharides and Polysaccharides

Disaccharides like sucrose can be hydrolysed into glucose and fructose.
Polysaccharides like starch and cellulose can be broken down into many glucose molecules by hydrolysis.

🧪 Relation to Non-Reducing Sugar Test

Some sugars (e.g., sucrose) are non-reducing sugars and do not react with Benedict’s reagent initially because their glycosidic bonds block the reactive group.
When hydrolysed (using dilute acid or enzymes), these sugars break into reducing sugars (e.g., glucose and fructose).
After hydrolysis and neutralisation, the Benedict’s test becomes positive, indicating reducing sugars are present.

Process	Description	Example
Hydrolysis	Breaking glycosidic bonds by adding water	Sucrose → Glucose + Fructose
Effect on Tests	Converts non-reducing sugars to reducing sugars	Positive Benedict’s after hydrolysis

🧠 Key Point: Hydrolysis is essential for digesting complex carbohydrates and for detecting non-reducing sugars via chemical tests.

Molecular Structure and Function of Polysaccharides: Starch and Glycogen

🌱 Starch (Energy Storage in Plants)

Starch is a storage polysaccharide in plants made of two components:

Amylose
Structure:
- Long, unbranched chains of α-glucose units.
- Glucose molecules linked by α-1,4 glycosidic bonds.
- Forms a helical (coiled) structure.
Function:
- Compact helical shape makes amylose efficient for energy storage.
- Insoluble in water, so it doesn’t affect cell water balance.
Amylopectin
Structure:
- Branched chains of α-glucose.
- Has α-1,4 glycosidic bonds along the chain and α-1,6 glycosidic bonds at branch points every ~25 glucose units.
Function:
- Branching provides many ends for enzyme action, speeding up glucose release during respiration.

🌿 Glycogen (Energy Storage in Animals and Fungi)

Structure:
- Similar to amylopectin but more highly branched (branches every 8–12 glucose units).
- Made of α-glucose linked by α-1,4 bonds (chains) and α-1,6 bonds (branches).
Function:
- High branching allows rapid release of glucose to meet sudden energy demands.
- Compact and insoluble, making it ideal for storage without affecting cell water potential.

Feature	Starch (Amylose & Amylopectin)	Glycogen
Monomer	α-Glucose	α-Glucose
Chain Type	Amylose: unbranched; Amylopectin: moderately branched	Highly branched
Bond Types	α-1,4 (chains), α-1,6 (branches in amylopectin)	α-1,4 (chains), α-1,6 (branches)
Branching Frequency	Less frequent (~25 units)	More frequent (~8–12 units)
Function	Energy storage in plants	Energy storage in animals and fungi
Structural Role	Compact, helical structure in amylose; easier enzyme access in amylopectin	Rapid glucose release due to many branch ends

🧠 Summary
Starch is the main plant energy store, combining compact amylose and branched amylopectin for storage and quick energy release.
Glycogen is the animal energy store, highly branched for faster glucose availability.
Their molecular structures directly influence how effectively organisms store and mobilize energy.

Molecular Structure and Function of Cellulose in Plant Cell Walls

🌱 Molecular Structure of Cellulose

Cellulose is a polysaccharide made of many β-glucose units.
Glucose monomers are linked by β-1,4 glycosidic bonds.
Each adjacent glucose unit is rotated 180°, creating a straight, unbranched chain.
Many cellulose chains align parallel to each other and form strong microfibrils through hydrogen bonds between hydroxyl (-OH) groups on adjacent chains.

🔍 Arrangement and Function

The microfibrils bundle tightly to form a rigid network.
This structure provides high tensile strength, preventing the plant cell from bursting under turgor pressure.
The cellulose network is insoluble and stable, giving structural support and rigidity to plant cell walls.
Allows plants to maintain shape and stand upright.

Feature	Description
Monomer	β-glucose
Bond Type	β-1,4 glycosidic bonds
Chain Structure	Straight, unbranched chains
Interactions	Hydrogen bonds between chains
Higher Structure	Microfibrils and fibres
Function	Provides strength and rigidity to plant cell walls

🧠 Key Point
The straight chains and hydrogen bonding between cellulose molecules form strong microfibrils, crucial for the mechanical strength and protection of plant cells.

Triglycerides: Structure and Properties

🌱 Key Properties

Triglycerides are non-polar, hydrophobic molecules.
They do not mix with water because they lack charged (polar) groups.

🔍 Molecular Structure of Triglycerides

A triglyceride molecule consists of:
- One glycerol molecule (a 3-carbon alcohol with three –OH groups).
- Three fatty acid molecules, each attached to glycerol.

🍃 Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end.
Types of fatty acids:
- Saturated fatty acids: No double bonds between carbon atoms; chains are straight.
- Unsaturated fatty acids: One or more double bonds; chains have kinks (bends).

🔗 Formation of Ester Bonds

Each fatty acid joins to glycerol by a condensation reaction.
An ester bond forms between the carboxyl group of the fatty acid and a hydroxyl group (-OH) of glycerol, releasing water.
This results in three ester bonds per triglyceride molecule.

Component	Description
Glycerol	3-carbon molecule with –OH groups
Fatty acids	Long hydrocarbon chains (saturated or unsaturated)
Bond type	Ester bonds (formed by condensation)
Property	Non-polar and hydrophobic

🧠 Key Point
The non-polar hydrocarbon tails of fatty acids make triglycerides hydrophobic, ideal for energy storage without affecting water balance in cells.

Relationship Between Molecular Structure of Triglycerides and Their Functions

🌱 Key Structural Features of Triglycerides

Composed of one glycerol molecule and three fatty acids linked by ester bonds.
Fatty acids have long non-polar hydrocarbon tails, making triglycerides hydrophobic (water-insoluble).
Fatty acids can be saturated (straight chains) or unsaturated (kinked chains).

🔍 How Structure Relates to Function

Structural Feature	Functional Benefit
Hydrophobic nature	Triglycerides form energy-dense fat stores that do not dissolve in water, preventing osmotic problems inside cells.
Long hydrocarbon chains	Provide a high amount of chemical energy when broken down (more than carbohydrates).
Ester bonds	Allow stable storage of fatty acids and release via hydrolysis when energy is needed.
Saturated vs unsaturated tails	Saturated fats pack tightly for solid energy storage; unsaturated fats provide fluidity and prevent packing, influencing membrane properties and energy use.
Compact structure	Efficient storage of energy in small volume, important for animals with limited space.
Insulation and protection	Stored fat in organisms provides thermal insulation and cushioning for organs.

🧠 Summary
The non-polar, hydrophobic nature of triglycerides makes them excellent for long-term energy storage without interfering with water balance.
Their energy-rich hydrocarbon chains supply more than twice the energy per gram compared to carbohydrates.
Their compact, stable structure supports energy storage, insulation, and protection in living organisms.

Molecular Structure of Phospholipids

🌱 Key Components

One glycerol molecule (3-carbon alcohol).
Two fatty acid tails (long hydrocarbon chains).
One phosphate group attached to the glycerol.

🔍 Hydrophilic Head

The phosphate group is polar and hydrophilic (“water-loving”).
This head interacts readily with water because it carries a negative charge, making it soluble in water.

🔍 Hydrophobic Tails

The two fatty acid tails are non-polar and hydrophobic (“water-fearing”).
These long hydrocarbon chains repel water and tend to avoid contact with aqueous environments.

🔄 Overall Structure

Phospholipids are amphipathic molecules (having both hydrophilic and hydrophobic parts).
This dual nature causes them to arrange into bilayers in water, with heads facing outward toward water and tails inward, away from water.

Part	Nature	Property	Function in Structure
Phosphate Head	Polar	Hydrophilic	Interacts with aqueous environments
Fatty Acid Tails	Non-polar	Hydrophobic	Avoids water; forms bilayer core

🧠 Key Point
The hydrophilic heads and hydrophobic tails enable phospholipids to form the cell membrane’s bilayer, providing a selective barrier in cells.

CIE AS/A Level Biology -2.2 Carbohydrates and lipids- Study Notes

CIE AS/A Level Biology -2.2 Carbohydrates and lipids- Study Notes- New Syllabus