Welcome to Biological Molecules: Carbohydrates
Hi there! Welcome to one of the most important chapters in your AQA AS Biology course. Everything you see in the mirror and everything you eat is made of biological molecules. In this section, we are focusing on Carbohydrates.
Think of carbohydrates as the "fuel" and the "scaffolding" of life. They provide the energy you need to move and the structure that holds plants upright. Don't worry if the chemistry seems a bit scary at first—we’ll break it down into simple, bite-sized pieces!
1. Monomers and Polymers
Before we dive into sugars, we need to understand how biological molecules are built. Most of them are polymers.
- Monomers: These are small, basic molecular units. Think of them like individual LEGO bricks.
- Polymers: These are large molecules made by joining lots of monomers together in a long chain. Think of this as the LEGO castle you built.
The "Making and Breaking" Reactions
How do we join these bricks together? Biology uses two specific reactions that you must know:
1. Condensation Reaction: This joins two molecules together. When they join, a chemical bond forms and a molecule of water is released (eliminated).
Memory Aid: Think of "condensation" on a cold window—water is appearing!
2. Hydrolysis Reaction: This breaks the chemical bond between two molecules. To do this, a water molecule is used up.
Memory Aid: "Hydro" means water, and "lysis" means splitting. You are using water to split the bond.
Key Takeaway
Condensation = Joining + Water Out.
Hydrolysis = Breaking + Water In.
2. Monosaccharides: The Single Sugars
Monosaccharides are the monomers (the building blocks) of carbohydrates. They are simple sugars that are sweet and soluble.
The three common monosaccharides you need to know are: Glucose, Galactose, and Fructose.
Glucose and its Isomers
Glucose is a "hexose" sugar, meaning it has 6 carbon atoms. Its formula is \( C_6H_{12}O_6 \).
However, Glucose has two different forms called isomers. Isomers have the same "ingredients" but a slightly different "shape":
- Alpha (\(\alpha\)) Glucose: The Hydrogen (H) atom is on the top of Carbon-1.
- Beta (\(\beta\)) Glucose: The Hydrogen (H) atom is on the bottom of Carbon-1.
Simple Trick: To remember Alpha, think "Alpha is Down" (referring to the -OH group on Carbon 1). In Alpha glucose, the -OH group points down. In Beta, it points up!
3. Disaccharides: The Double Sugars
When two monosaccharides join together via a condensation reaction, they form a disaccharide. The bond that holds them together is called a glycosidic bond.
You need to know these three specific pairs:
- Glucose + Glucose = Maltose (Found in germinating seeds).
- Glucose + Fructose = Sucrose (Table sugar).
- Glucose + Galactose = Lactose (The sugar in milk).
Quick Review Box:
All disaccharides are formed by a condensation reaction and held by a glycosidic bond. To turn them back into single sugars, you would perform hydrolysis.
4. Polysaccharides: The Giants
When you join many glucose units together, you get a polysaccharide. There are three main ones you need to know for AQA:
Starch (Plants)
Starch is how plants store energy. It is made of alpha-glucose.
Why it’s good at its job:
1. It is insoluble, so it doesn't affect the water potential of the cell (it won't cause the cell to swell with water).
2. It is coiled and compact, so you can fit a lot of energy into a small space.
Glycogen (Animals)
Glycogen is the main energy storage molecule in animals (stored in your liver and muscles). It is also made of alpha-glucose.
Why it’s good at its job:
It is very heavily branched. This is important because it means enzymes can act on the ends of the branches simultaneously, breaking it down into glucose very quickly for respiration when you need a burst of energy!
Cellulose (Plant Cell Walls)
Cellulose provides structural strength to plant cell walls. It is made of beta-glucose.
Why it’s good at its job:
1. It forms straight, unbranched chains.
2. These chains run parallel to each other and are joined by hydrogen bonds to form strong fibers called microfibrils.
Analogy: A single thread is weak, but if you weave many threads into a rope, it becomes very strong. That is how cellulose works!
Key Takeaway
Starch & Glycogen = Alpha-glucose (Energy storage).
Cellulose = Beta-glucose (Strength/Structure).
5. Biochemical Tests
In the lab, you need to be able to identify these sugars using specific tests.
Testing for Reducing Sugars (Benedict's Test)
All monosaccharides and some disaccharides (like maltose) are reducing sugars.
Step-by-step:
1. Add blue Benedict's solution to your sample.
2. Heat it in a water bath that has been brought to the boil.
3. If a reducing sugar is present, the color will change from blue to green, yellow, orange, or brick-red (depending on how much sugar is there).
Testing for Non-Reducing Sugars (Sucrose)
If the Benedict's test stays blue, you might have a non-reducing sugar like sucrose.
Step-by-step:
1. Get a new sample and add dilute hydrochloric acid (this hydrolyses the sugar into its monomers).
2. Heat it in a water bath.
3. Neutralize it by adding sodium hydrogencarbonate.
4. Now perform the Benedict's test again. If it turns red now, a non-reducing sugar was originally present!
Testing for Starch (Iodine Test)
This is the easiest test!
1. Add Iodine dissolved in potassium iodide solution to your sample.
2. A color change from orange/brown to blue-black proves starch is present.
Common Mistake to Avoid:
In the Benedict's test, students often forget that heat is required. It won't work at room temperature!
Summary: The Carbohydrate Checklist
- Can you define monomer and polymer?
- Do you know the difference between condensation and hydrolysis?
- Can you draw the difference between alpha and beta glucose?
- Can you name the components of maltose, sucrose, and lactose?
- Can you explain how the structure of starch, glycogen, and cellulose relates to their function?
- Do you know the colors for the Benedict's and Iodine tests?
Don't worry if you can't answer all of these yet—re-read the sections above and try drawing the molecules yourself. You've got this!