Biological Molecules

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Notes

Chemical Elements in Living Organisms

Most biological molecules are **organic**, meaning they contain **carbon**.
The three main groups of organic molecules are **carbohydrates**, **proteins**, and **lipids**.
Carbohydrates contain **carbon**, **hydrogen**, and **oxygen** only.
Proteins contain **carbon**, **hydrogen**, **oxygen**, and **nitrogen** (some also contain sulfur).
Fats (lipids) contain **carbon**, **hydrogen**, and **oxygen** only.

Carbohydrates

Carbohydrates are made of **simple sugars** (monosaccharides) like **glucose**.
Two glucose molecules join to form **maltose** (a **disaccharide**).
Many glucose molecules join to form **starch**, **glycogen**, or **cellulose** (**polysaccharides**).
**Starch** is the storage carbohydrate in plants; **glycogen** is the storage carbohydrate in animals.
**Cellulose** is a structural polysaccharide used in plant cell walls.

Fats (Lipids)

Most fats are **triglycerides**: one **glycerol** molecule bonded to three **fatty acid** chains.
Fatty acids vary in size and structure.
Fats are **solid** at room temperature; oils are **liquid** at room temperature.

Proteins

Proteins are long chains of **amino acids**.
There are about **20** different amino acids found in living organisms.
All amino acids have the same basic structure but differ in their **R group**.
The order of amino acids determines the protein's structure and function.
Even a small change in the amino acid sequence can produce a **different protein**.

Food Tests

**Benedict's test** for reducing sugars: add Benedict's solution, heat at 80°C for 5 min; positive: blue → **orange/brick red**.
**Iodine test** for starch: add iodine solution; positive: orange-brown → **blue-black**.
**Biuret test** for protein: add Biuret solution; positive: blue → **violet/purple**.
**Ethanol emulsion test** for lipids: shake sample with ethanol, then add cold water; positive: **cloudy emulsion** forms.
**DCPIP test** for vitamin C: add DCPIP solution to sample; positive: blue colour **disappears** (becomes colourless).

DNA Structure (Extended)

DNA is a **double helix** of two strands wound around each other.
The monomer of DNA is a **nucleotide**, consisting of a **phosphate**, **deoxyribose sugar**, and a **base**.
There are four bases: **Adenine (A)**, **Thymine (T)**, **Cytosine (C)**, and **Guanine (G)**.
Base pairing: **A pairs with T**, **C pairs with G** (held by hydrogen bonds).
The sugar–phosphate backbone is on the **outside**; the bases form the **rungs** of the ladder.
The sequence of bases codes for the formation of **proteins**.

Structure of a triglyceride: one glycerol molecule bonded to three fatty acid chains.

General structure of an amino acid: central carbon bonded to an amino group, carboxyl group, hydrogen, and an R group.

Benedict's test for reducing sugars: colour change from blue to orange/brick red.

DNA double helix: sugar–phosphate backbones with complementary base pairs (A–T, C–G) joined by hydrogen bonds.

Practice questions

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1.Which of the following is a monosaccharide?
Easy
- AGlucose
- BMaltose
- CStarch
- DGlycogen
2.What are the chemical elements present in all carbohydrates?
Easy
- ACarbon, hydrogen and oxygen
- BCarbon, hydrogen, oxygen and nitrogen
- CCarbon, hydrogen, oxygen, nitrogen and sulfur
- DCarbon, hydrogen and nitrogen
3.Which food test gives a colour change from blue to violet/purple for a positive result?
Easy
- ABiuret test
- BBenedict's test
- CIodine test
- DEmulsion test
4.What is the basic unit of a fat molecule?
Easy
- AOne glycerol and three fatty acids
- BOne glycerol and one fatty acid
- CThree glycerols and one fatty acid
- DAmino acids
5.Which bases pair together in DNA?
Easy
- AA with T and C with G
- BA with C and T with G
- CA with G and C with T
- DA with U and C with G
6.Which of the following is a polysaccharide?
Easy
- ACellulose
- BGlucose
- CMaltose
- DSucrose
7.A food sample gives a blue-black colour with iodine solution. Which substance is present?
Medium
- AStarch
- BGlucose
- CProtein
- DLipid
8.Which of the following elements is found in proteins but not in carbohydrates or lipids?
Medium
- ANitrogen
- BCarbon
- CHydrogen
- DOxygen

Slides

Biological Molecules – Overview

Living organisms contain three main types of organic molecules: carbohydrates, proteins and lipids.
All three contain carbon, hydrogen and oxygen; proteins also contain nitrogen.
These large molecules are made from smaller building blocks.

Which elements are found in all carbohydrates, proteins and lipids?

Carbon, hydrogen, oxygen
Carbon, hydrogen, nitrogen
Carbon, oxygen, nitrogen
Hydrogen, oxygen, nitrogen

Carbohydrates

Carbohydrates are long chains of simple sugars (monosaccharides).
Glucose is a monosaccharide; two glucose molecules join to form maltose (a disaccharide).
Many glucose molecules join to form starch, glycogen or cellulose (polysaccharides).

Glucose is a ____ . Maltose is a ____ . Starch is a ____ .

Fats (Lipids)

Most fats are triglycerides: one glycerol molecule bonded to three fatty acid chains.
Fats are solid at room temperature; oils are liquid.
Lipids contain carbon, hydrogen and oxygen only.

What are the building blocks of a triglyceride?

One glycerol and three fatty acids
Three glycerols and one fatty acid
Amino acids
Glucose molecules

Proteins

Proteins are long chains of amino acids.
There are about 20 different amino acids, each with a variable R group.
Amino acids can be arranged in any order, creating a huge variety of proteins.

There are about ____ different amino acids. They are joined together to form ____ .

Food Tests – Glucose and Starch

Benedict's test for reducing sugars (e.g. glucose): add Benedict's solution, heat at 80°C for 5 minutes. Positive: blue → orange/brick red.
Iodine test for starch: add iodine solution. Positive: orange-brown → blue-black.

What colour change indicates a positive Benedict's test for glucose?

Blue to orange/brick red
Orange-brown to blue-black
Blue to violet
Colourless to blue

Food Tests – Protein and Lipids

Biuret test for protein: add Biuret solution. Positive: blue → violet/purple.
Ethanol emulsion test for lipids: mix food with ethanol, then add cold water. Positive: cloudy white emulsion forms.

Which statements about food tests are true?

Biuret test turns blue to violet for protein.
Ethanol test gives a blue-black colour for lipids.
Iodine test turns orange-brown to blue-black for starch.
Benedict's test requires heating to 80°C.

Food Test for Vitamin C

DCPIP test for vitamin C: add DCPIP solution to food sample.
Positive result: blue colour of DCPIP disappears (becomes colourless).

What is the positive result for vitamin C using DCPIP?

Blue colour disappears
Blue to violet
Cloudy emulsion forms
Orange-brown to blue-black

DNA Structure (Extended)

DNA is a double helix: two strands wound around each other.
Each strand is made of nucleotides: phosphate, deoxyribose sugar, and a base.
Four bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
Base pairing: A pairs with T, C pairs with G (held by hydrogen bonds).

In DNA, A pairs with ____ and C pairs with ____ .

DNA Backbone and Function

The sugar and phosphate form the backbone of each DNA strand.
The base sequence codes for protein synthesis.
The two strands are held together by hydrogen bonds between bases.

Which part of a nucleotide forms the backbone of DNA?

Phosphate and sugar
Base and sugar
Phosphate and base
Hydrogen bonds

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