Biological Molecules notes
MDCAT Biology
Biological molecules are carbon-containing compounds found in living organisms. This chapter covers their classification, the importance of water, carbohydrates, proteins, lipids, RNA, conjugated molecules and the structure of DNA.
Classification of Biological Molecules
Biological molecules are chemical substances present in living organisms. They may be organic or inorganic. Water and mineral salts are mainly inorganic substances, while carbohydrates, proteins, lipids and nucleic acids are organic molecules.
Living organisms obtain many organic molecules from their environment. The survival of an animal depends on its ability to take organic molecules from the environment and use them for energy, growth, repair and reproduction.
Carbon is the basic element of most organic molecules. It forms stable covalent bonds with other carbon atoms and with hydrogen, oxygen, nitrogen, phosphorus and sulfur.
- Carbon is tetravalent, so one carbon atom can form four covalent bonds.
- The three main elements in carbohydrates are carbon, hydrogen and oxygen.
- Major organic biological molecules include carbohydrates, proteins, lipids and nucleic acids.
- Nucleotides are the building blocks of nucleic acids. ATP is an example of a nucleotide.
- Large molecules formed by joining smaller units are called macromolecules or polymers.
- Addition polymerisation joins monomers without eliminating a smaller molecule such as water.
- Condensation involves joining molecules with the removal of water.
- Puccinia is a rust fungus that commonly destroys wheat. Rose is an example of a mesophyte.
Biological Importance of Water
Water is the most abundant inorganic component of living organisms. Its molecule has two hydrogen atoms covalently bonded to one oxygen atom. Oxygen attracts the shared electrons more strongly than hydrogen, giving water a partial negative end and partial positive ends.
This unequal distribution of charge gives water its dipole nature. Water molecules form hydrogen bonds with one another. These properties explain water's solvent action, high heat capacity, cohesion and role in temperature regulation.
Water is present in different amounts in different tissues. Brain tissue contains about 85% water in its cells. Water is also a reactant in hydrolysis and is released during many condensation reactions.
- Water is a polar molecule with a dipole nature.
- Water is a very good solvent for many ionic and polar substances.
- Water acts as a thermoregulator or thermo-stabilizer because it has a high heat capacity.
- High heat capacity allows water to absorb much heat with only a small rise in temperature.
- Water provides a medium for most metabolic reactions in cells.
- Hydrolysis uses water to break chemical bonds in larger molecules.
- The cells of brain tissue contain about 85% water.
- Cohesion between water molecules and adhesion to surfaces help in the movement of water in plants.
Carbohydrates
Carbohydrates are organic molecules made mainly of carbon, hydrogen and oxygen. Hydrogen and oxygen are often present in approximately the same ratio as in water, although this ratio is not exact in every carbohydrate.
Carbohydrates are classified as monosaccharides, disaccharides and polysaccharides. Monosaccharides are single sugar units. Disaccharides contain two monosaccharides joined by a glycosidic bond. Polysaccharides contain many sugar units.
Carbohydrates provide energy and may also form structural materials. Their properties depend on the number and arrangement of their sugar units.
- Glucose is a monosaccharide and is also called dextrose.
- Fructose is a monosaccharide and is not a disaccharide.
- Galactose is a monosaccharide.
- Sucrose is formed from glucose and fructose.
- Maltose is formed from two glucose molecules and is hydrolysed by maltase to give glucose.
- Lactose is the sugar of milk and is considered a sugar of animal origin.
- Polysaccharides are generally sparingly soluble in hot water.
- Starch is a storage polysaccharide in plants, while glycogen is a storage polysaccharide in animals.
- Cellulose is a structural polysaccharide present in plant cell walls.
Proteins and Their Structure
Proteins are nitrogen-containing organic molecules made from amino acids. Amino acids are joined by peptide bonds. A peptide bond forms between the amino group of one amino acid and the carboxyl group of another, with the removal of water.
The shape of a protein determines its function. Protein structure is described at four levels: primary, secondary, tertiary and quaternary. Each level depends on different bonds or interactions between amino acids.
Proteins perform many functions, including catalysis, transport, defence, movement and structural support. Enzymes are mostly globular proteins that act as biological catalysts.
- The primary structure is the specific sequence of amino acids in a polypeptide chain.
- The secondary structure results from hydrogen bonding in the polypeptide backbone.
- Alpha helix and beta pleated sheet are common forms of secondary structure.
- The secondary structure helps maintain the helix shape of many enzymes.
- The tertiary structure is the three-dimensional folding of one polypeptide chain.
- The quaternary structure occurs when two or more polypeptide chains combine.
- Antibodies are defensive proteins that help protect the body from foreign substances.
- Fibrin is an insoluble fibrous protein involved in blood clotting and cannot be crystallised easily.
- Fibrous proteins are generally long and structural, while globular proteins are compact and usually functional.
Lipids
Lipids are a diverse group of organic compounds that are insoluble in water but soluble in non-polar organic solvents. They include fats, oils, waxes, phospholipids and steroids. Lipids contain mainly carbon, hydrogen and oxygen, but phospholipids also contain phosphorus.
A neutral lipid is formed by a chemical reaction between an alcohol and a fatty acid. Water is released during this reaction, so it is a condensation or esterification reaction. The products contain ester bonds.
Lipids store energy, reduce heat loss, protect organs and form important parts of cell membranes. Oils are usually liquid at room temperature, while fats are usually solid.
- A neutral lipid is commonly formed from glycerol and three fatty acid molecules.
- The reaction of an alcohol with an acid, with the release of water, forms an ester.
- A fatty acid has a carboxyl group and a hydrocarbon chain.
- The solubility of carboxylic acids decreases as their molecular mass increases.
- Saturated fatty acids contain no carbon-carbon double bond in their hydrocarbon chains.
- Unsaturated fatty acids contain one or more carbon-carbon double bonds.
- Phospholipids contain a phosphate group and have hydrophilic and hydrophobic regions.
- Phosphatidylcholine is a common phospholipid.
- Lipids yield more energy per gram than carbohydrates, but they are not usually the first immediate energy source.
RNA and Nucleotides
Ribonucleic acid, or RNA, is a nucleic acid made of ribonucleotides. Each ribonucleotide contains a ribose sugar, a phosphate group and a nitrogenous base. RNA usually consists of a single polynucleotide strand.
The nitrogenous bases in RNA are adenine, guanine, cytosine and uracil. RNA is formed by phosphodiester bonds between adjacent nucleotides. It participates in protein synthesis and in some organisms can also act as genetic material.
Different types of RNA perform different roles in protein synthesis. Messenger RNA carries information from DNA, transfer RNA brings amino acids, and ribosomal RNA forms part of ribosomes.
- The sugar found in RNA is ribose, which is a pentose sugar.
- RNA contains the bases adenine, guanine, cytosine and uracil.
- RNA generally has a single-stranded structure.
- mRNA carries the genetic message from DNA to ribosomes.
- tRNA carries specific amino acids to the ribosome.
- rRNA is a structural and functional part of ribosomes.
- tRNA comprises about 10 to 20% of cellular RNA.
- ATP is a nucleotide composed of adenine, ribose and three phosphate groups.
- A nucleotide differs from a nucleoside because a nucleotide contains phosphate, while a nucleoside does not.
Conjugated Molecules
Conjugated molecules are formed when a non-protein part combines with a protein part. The protein portion is called the apoprotein, while the non-protein portion is called the prosthetic group. The complete molecule is called a conjugated protein.
The prosthetic group gives the molecule a special property or function. Conjugated molecules are important in transport, respiration, cell membranes and other biological processes.
The non-protein component may be a carbohydrate, lipid, mineral ion, pigment or nucleic acid. Examples include glycoproteins, lipoproteins and chromoproteins.
- A conjugated protein consists of an apoprotein and a prosthetic group.
- A glycoprotein contains a carbohydrate group attached to a protein.
- A lipoprotein contains a lipid component and a protein component.
- A chromoprotein contains a coloured prosthetic group.
- Haemoglobin is a conjugated protein containing a haem group with iron.
- Phospholipids are conjugated lipid molecules containing phosphate.
- Conjugated molecules often have functions that cannot be performed by the protein portion alone.
Structure of DNA
Deoxyribonucleic acid, or DNA, is the hereditary material in most organisms. It stores genetic information and passes it from one generation to the next. DNA also provides instructions for the formation of proteins.
DNA is made of two polynucleotide strands twisted around each other to form a double helix. The two strands run in opposite directions and are held together by hydrogen bonds between complementary nitrogenous bases.
Each DNA nucleotide contains deoxyribose sugar, a phosphate group and one nitrogenous base. Adjacent nucleotides in a strand are joined by phosphodiester bonds. The sugar and phosphate form the backbone of each strand.
- DNA contains deoxyribose, which is a pentose sugar.
- The four nitrogenous bases of DNA are adenine, guanine, cytosine and thymine.
- Adenine pairs with thymine by two hydrogen bonds.
- Guanine pairs with cytosine by three hydrogen bonds.
- The two DNA strands are complementary because the sequence of one determines the sequence of the other.
- The DNA double helix has antiparallel strands, meaning the strands run in opposite directions.
- The base sequence stores genetic information.
- DNA is the hereditary material and can make an identical copy of itself before cell division.
- DNA and RNA both contain adenine, guanine and cytosine, but DNA has thymine while RNA has uracil.
Key terms
- Biological molecule
- A chemical substance produced by or found in a living organism.
- Macromolecule
- A very large molecule formed from many smaller units.
- Monosaccharide
- A carbohydrate consisting of a single sugar unit.
- Disaccharide
- A carbohydrate formed by joining two monosaccharides.
- Polysaccharide
- A carbohydrate made of many monosaccharide units.
- Peptide bond
- The bond joining amino acids in a protein chain.
- Amino acid
- An organic compound containing amino and carboxyl groups that acts as a unit of proteins.
- Denaturation
- The loss of a protein's natural shape and function due to heat, pH or chemicals.
- Lipid
- A water-insoluble organic compound such as a fat, oil, wax or phospholipid.
- Fatty acid
- An organic acid with a carboxyl group attached to a hydrocarbon chain.
- Phospholipid
- A lipid containing phosphate, with a hydrophilic head and hydrophobic tails.
- Nucleotide
- A molecule made of a pentose sugar, phosphate group and nitrogenous base.
- Nucleoside
- A molecule made of a pentose sugar and nitrogenous base without phosphate.
- RNA
- A usually single-stranded nucleic acid involved mainly in protein synthesis.
- DNA
- A double-stranded nucleic acid that stores and transmits hereditary information.
- Glycosidic bond
- The bond joining sugar units in carbohydrates.
- Ester bond
- The bond formed between an alcohol and an acid during condensation.
- Prosthetic group
- The non-protein component permanently attached to a conjugated protein.
Test yourself on Biological Molecules
Free Biological Molecules MCQs with an explanation on every answer. No account needed.
More for Biological Molecules in the MDCAT pack
- A one-page revision sheet for this chapter
- 5 Biological Molecules mnemonics
- Chapter-wise Ratta Cards and a Quiz Builder for your own tests
Biology shortcuts
Recognising acellular organisms
If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.
- Check whether it has cytoplasm, ribosomes and independent metabolism.
- Viruses contain nucleic acid inside a protein coat but lack cellular structure.
- Example: An infectious particle without cytoplasm or ribosomes is a virus.
- Answer: Non-cellular virus.
This shortcut does not apply to bacteria, which are cellular prokaryotes.
Separating prokaryotic and eukaryotic cells
Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.
- Look for the feature present in both groups.
- Ribosomes occur in prokaryotes and eukaryotes for protein synthesis.
- Example: Which structure is common to both? Ribosomes.
- Answer: Ribosomes.
Do not use mitochondria, chloroplasts or a membrane-bound nucleus as common features.
15 more Biology shortcuts are in the MDCAT pack. Already have it? See all shortcuts