Cambridge IGCSE Biology - Chapter 4: Biological Molecules

Vocabulary practice flashcards covering water, carbohydrates, fats, proteins, DNA structure, and relevant biochemical identification tests from Cambridge IGCSE Biology Chapter 4.

Water

An inorganic nutrient that does not contain carbon. In animals, it serves as a medium for chemical reactions, a temperature regulator, and a transport medium for blood, digested food, and hormones. In plants, it is essential for photosynthesis, keeping plant cells turgid, and transporting mineral ions and food substances.

Carbohydrates

Organic molecules made up of the elements carbon, hydrogen, and oxygen, where hydrogen and oxygen atoms are present in a $2 : 1$ ratio.

Single sugars

Simple carbohydrates that cannot be further digested into smaller molecules and can pass directly through cell membranes to be absorbed into cells, such as glucose and fructose.

Double sugars

Carbohydrate molecules made up of two single sugars bonded together chemically, such as maltose and sucrose.

Maltose

A double sugar produced from the combination of two glucose molecules according to the reaction: $\text{Glucose} + \text{Glucose} \rightarrow \text{Maltose} + \text{Water}$.

Sucrose

A double sugar composed of one glucose molecule and one fructose molecule joined together; it is the sugar transported by the phloem in plants and is abundant in sugarcane stems, sweet fruits, and carrots.

Benedict's test

A chemical test used to identify reducing sugars such as glucose, fructose, galactose, maltose, and lactose. The solution remains blue if no reducing sugar is present, and turns into a green, yellow/orange, or brick-red precipitate depending on the quantity of reducing sugar present.

Polysaccharides (Complex carbohydrates)

Carbohydrate polymers consisting of many monosaccharide molecules joined together, such as starch, cellulose, and glycogen, all of which are constructed from glucose units.

Iodine test

A chemical diagnostic test for starch in which iodine solution turns from yellowish-brown to blue-black upon contact with starch.

Fats

Organic compounds composed of carbon, hydrogen, and oxygen containing much less oxygen in proportion to hydrogen compared to carbohydrates. Each fat molecule is built from one glycerol molecule joined to three fatty acid molecules.

Lipids

A class of biological compounds that are soluble in organic solvents such as ethanol and chloroform; examples include fats, waxes, and steroids.

Ethanol emulsion test

A test for the presence of fats and oils where the sample is mixed with ethanol and then diluted with water; a cloudy white emulsion confirms the presence of fats.

Proteins

Organic molecules made up of carbon, hydrogen, oxygen, and nitrogen (and sometimes sulfur), which are folded into unique three-dimensional shapes that enable specific functions such as enzyme catalysis and immune defense.

Amino acids

The simpler organic sub-units that join together in chains to build polypeptides and proteins.

Polypeptide

A long chain of amino acids linked together, comparable to a long thread that subsequently folds into a specific three-dimensional shape to make a protein.

Biuret test

A chemical test used to identify proteins, where biuret reagent remains blue in the absence of protein and turns violet (deep purple) in its presence.

Pepsin

An enzyme made of protein that contains a specific binding site to attach to a protein molecule and break it down during digestion.

Antibody

A protective protein molecule that features specific binding sites designed to attach to viruses or bacteria in order to destroy them.

DNA (Deoxyribonucleic Acid)

A nucleic acid molecule that carries genetic information and instructions for the development and functions of an organism, organized as two strands coiled into a double helix.

Base pairing

The specific matching rule of nitrogenous bases forming the rungs of a DNA double helix, where base $\text{A}$ always pairs with $\text{T}$, and base $\text{C}$ always pairs with $\text{G}$.