Carbohydrates (OCR A-Level Biology A): Revision Notes

Carbohydrates

Introduction to carbohydrates

Carbohydrates are organic molecules that form a large proportion of compounds in living cells. They serve three main functions: as energy sources (e.g. glucose in respiration), as energy stores (e.g. glycogen in animals, starch in plants), and as structural components (e.g. cellulose in plant cell walls).

Key features of carbohydrates:

• All carbohydrates contain the elements carbon (C), hydrogen (H) and oxygen (O)
• The ratio of hydrogen to oxygen atoms is always $2:1$ (same as in water)
• Most carbohydrates follow the general formula $(C_x(H_2O)_y)$
• The name 'carbohydrate' literally means "hydrated carbon"

Monosaccharides

Monosaccharides are the simplest carbohydrates, consisting of single sugar molecules. They are often called simple sugars due to their small molecular size.

Properties of monosaccharides

• Small molecules that can easily pass through cell membranes
• Soluble in water, allowing transport in aqueous solutions
• Sweet-tasting
• Contain no glycosidic bonds (they are single units)
• Can exist as either ring structures or straight chains
• Serve as monomers (building blocks) for larger carbohydrates

Glucose

Glucose is the most important monosaccharide in biology. It is a hexose sugar containing six carbon atoms.

Functions of glucose:

• Main respiratory substrate - broken down during cell respiration to release energy
• Synthesised by plants during photosynthesis
• Transported in the blood of animals
• Building block for disaccharides and polysaccharides

Cell respiration is a process involving many enzyme-catalysed reactions within cells that release energy, which is used to make adenosine triphosphate (ATP).

Alpha and beta glucose (isomers)

Glucose exists in two structural forms: α-glucose and β-glucose. These are isomers - molecules with the same molecular formula but different structural arrangements.

The difference between these two forms lies in the position of the hydroxyl group ($\text{-OH}$) and hydrogen ($\text{H}$) on carbon 1 ($\text{C}_1$):

• In α-glucose: the $\text{-OH}$ group on $\text{C}_1$ is below the ring plane
• In β-glucose: the $\text{-OH}$ group on $\text{C}_1$ is above the ring plane

Other monosaccharides

Pentose sugars contain five carbon atoms and play important roles in nucleic acids:

• Ribose ($\text{C}_5\text{H}_{10}\text{O}_5$): component of RNA molecules and ATP
• Deoxyribose ($\text{C}_5\text{H}_{10}\text{O}_4$): component of DNA molecules (note the loss of one oxygen atom at carbon 2)

Triose sugars contain three carbon atoms:

• Glyceraldehyde and other trioses are important intermediate molecules in respiration and photosynthesis

Disaccharides

Disaccharides are carbohydrates formed when two monosaccharides join together. They are sometimes called complex sugars.

Formation of disaccharides

Disaccharides form through a condensation reaction:

• Two monosaccharides come together
• A molecule of water ($\text{H}_2\text{O}$) is removed
• A covalent bond called a glycosidic bond forms between the two sugars

A glycosidic bond is a covalent bond that joins two carbohydrate molecules through a condensation reaction.

When two α-glucose molecules join:

• The bond forms between $\text{C}_1$ of one glucose and $\text{C}_4$ of the other
• This creates a 1,4-glycosidic bond
• The disaccharide maltose is formed

Breaking down disaccharides

Disaccharides can be broken down into their component monosaccharides through hydrolysis:

• A water molecule is added
• The glycosidic bond breaks
• Two separate monosaccharides are released

Common disaccharides

Disaccharide	Monosaccharides	Function	Location
Maltose	glucose + glucose	Energy storage and transport in plants	Produced when starch is broken down
Sucrose	glucose + fructose	Energy storage and transport in plants	Table sugar, transported in phloem
Lactose	glucose + galactose	Energy source for young mammals	Milk sugar

Properties of disaccharides

• Small molecules but larger than monosaccharides
• Soluble in water
• Sweet-tasting
• Contain one glycosidic bond
• Easily hydrolysed to release monosaccharides
• Provide an energy source when broken down

Testing for reducing sugars (Benedict's test)

Reducing sugars (all monosaccharides and some disaccharides like maltose and lactose) can be detected using Benedict's test:

Principle: Reducing sugars donate electrons to copper(II) ions in Benedict's solution, reducing them to copper(I) oxide.

Procedure:

1. Add equal volumes of Benedict's solution (bright blue) to the sugar solution
2. Heat in a water bath at approximately $80°\text{C}$ for $5$ minutes
3. Observe colour change

Results:

• Negative result (no reducing sugar): remains blue
• Positive result (reducing sugar present): colour changes through green → yellow → orange → brick red
• The final colour indicates the approximate concentration of reducing sugar

Control: Test distilled water with Benedict's solution - should remain pale blue, showing the reagent itself doesn't change colour without reducing sugar present.

Testing for non-reducing sugars

Non-reducing sugars (like sucrose) do not react with Benedict's solution initially because their glycosidic bonds prevent them from donating electrons. However, they can be detected by first hydrolysing them:

Procedure:

1. Heat the sugar solution with dilute hydrochloric acid ($\text{HCl}$) for $5$ minutes
   • This hydrolyses the glycosidic bond, breaking the disaccharide into monosaccharides
2. Add sodium hydrogen carbonate to neutralise the acid
   • Wait for fizzing to stop
3. Now perform the Benedict's test as normal

Results: If non-reducing sugar was present, the solution will now turn brick red with Benedict's solution, as the hydrolysis has released reducing sugars (monosaccharides).

Polysaccharides

Polysaccharides are large polymer molecules formed when many monosaccharide units join together through condensation reactions. Each monosaccharide is attached by a glycosidic bond.

Properties of polysaccharides

• Very large molecules containing hundreds to thousands of monosaccharide units
• Insoluble in water - this is advantageous for storage as they don't affect water potential or osmotic balance of cells
• Not sweet
• Contain many glycosidic bonds
• Form long chains which may be branched or unbranched, coiled or straight
• Often stabilised by many hydrogen bonds between chains
• Functions include energy storage (starch, glycogen) and structural support (cellulose)

Starch

Starch is the main carbohydrate storage molecule in plants. It is a polymer of α-glucose molecules.

Structure: Starch actually consists of two different molecules:

1. Amylose (10-30% of starch):
   • Unbranched chain of α-glucose molecules
   • Joined by 1,4-glycosidic bonds
   • Coils into a helix (spring-like spiral shape)
   • Forms due to the angle of glycosidic bonds and shape of α-glucose
2. Amylopectin (70-90% of starch):
   • Branched chain of α-glucose molecules
   • Main chains joined by 1,4-glycosidic bonds
   • Branch points formed by 1,6-glycosidic bonds (between $\text{C}_1$ and $\text{C}_6$)
   • Creates a branched, tree-like structure

Storage locations:

• Amyloplasts (plastids specialised for starch storage) in plant organs like potato tubers
• Chloroplasts in leaves

Testing for starch (iodine test)

Starch can be detected using iodine solution (iodine in potassium iodide):

Procedure:

1. Add two drops of iodine solution (straw yellow) to sample
2. Observe colour change

Results:

• Positive result (starch present): blue-black colour appears
• Negative result (no starch): remains straw yellow/brown

Glycogen

Glycogen is the main carbohydrate storage molecule in animals. Like starch, it is a polymer of α-glucose molecules.

Structure:

• Similar to amylopectin but with more frequent branching
• Shorter chains between branch points
• Even more compact than starch
• Contains both 1,4-glycosidic bonds (main chains) and 1,6-glycosidic bonds (branches)

Storage locations:

• Stored as granules in animal cells
• Main storage sites are liver cells and muscle cells

Cellulose

Cellulose is the main structural carbohydrate in plants, forming a major component of plant cell walls. It is a polymer of β-glucose molecules.

Structure:

• Long, unbranched chains of β-glucose
• Due to the position of the $\text{-OH}$ group on $\text{C}_1$ in β-glucose, adjacent glucose molecules can only join if alternate molecules are inverted (rotated $180°$)
• This creates straight chains rather than coils
• Many $\text{-OH}$ groups project outwards from the chains

Hydrogen bonding and fiber formation:

1. Many hydrogen bonds form between $\text{-OH}$ groups on adjacent cellulose chains
2. Multiple chains held together form a microfibril
3. Multiple microfibrils held together by more hydrogen bonds form a cellulose fiber
4. These fibers are extremely strong and rigid

Why can't most organisms digest cellulose?

• Most organisms lack the enzyme cellulase needed to break the β-1,4-glycosidic bonds in cellulose
• Only some bacteria, fungi, and protists produce cellulase
• Herbivores rely on gut bacteria containing cellulase to digest plant material

Other polysaccharides

• Chitin: Found in exoskeletons of insects and cell walls of fungi
• Peptidoglycans: Found in bacterial cell walls

Relating structure to function

The structure of each carbohydrate determines its function in organisms:

Monosaccharides as energy sources

• Small size → easily diffuse through membranes
• Soluble → readily transported in blood and sap
• Simple structure → easily broken down to release energy quickly
• Ring structure → chemically stable but still reactive

Disaccharides for storage and transport

• Larger than monosaccharides but still relatively small
• Soluble → can be transported in solution
• More stable than monosaccharides → suitable for short-term storage
• Easily hydrolysed → can quickly release monosaccharides when needed

Polysaccharides for long-term storage

• Large, compact molecules → store many glucose units in small space
• Insoluble → don't affect water potential or cause osmotic problems
• Chemically stable → suitable for long-term storage
• Branched structures (in glycogen and amylopectin) → provide many sites for rapid glucose release
• Helical structures (in amylose) → further compaction

Cellulose for structural support

• Long, straight chains → can be packed tightly in parallel
• Extensive hydrogen bonding → provides tensile strength
• Insoluble and unreactive → provides stable, permanent structure
• Fiber arrangement → strength in multiple directions

Remember!