Molecular Biology

Water

Water is a small molecule. It consists of two hydrogen (H) atoms, bonded to an oxygen atom (O).
Their difference in electron attraction gives the molecule its polarity. 

Hydrogen poles are slightly positive whereas oxygen poles are slightly negative. Hydrogen bounds can link two water molecules between a H from the first and O from the second. These hydrogen bounds is what makes water cohesive
The bipolarity of water brings many substances to dissolve. Water is thus a good solvent.
Two molecules can link together, releasing a molecule of water, or one molecule can be split into two thanks to water support. Those reactions are named condensation and hydrolysis.

Lipids

Lipids are organic molecules, predominantly made of carbon (C) and hydrogen (H), that are mostly hydrophobic.

Fatty acids, the main bricks of lipids, consist of long hydrophobic hydrocarbon chains. They can be saturated (all C atoms are connected by a single covalent bound) or unsaturated (two C atoms are connected by a double bound).

We can distinguish:

1. Triglycerides = 3 fatty acids condensed with a glycerol molecule
2. Phospholipids = 2 fatty acids and a phosphate group linked to a glycerol molecule
3. Steroids = lipids made of 4 rings of hydrocarbon chains, cholesterol and molecules derived from it (estrogens, testosterone…)

Carbohydrates

Carbohydrates, or sugars, are organic molecules built on CH2O units.

We can identify:

1. Monosaccharides = small molecules made of six C tops which can be ringed (glucose, fructose, ribose)
2. Disaccharides = molecules made of a bound between two monosaccharides (sucrose = glucose + fructose)
3. Polysaccharides = macromolecules made of many monosaccharides that are linked together (cellulose, starch, glycogen), with a role of structure or food storage in cells

Proteins

Proteins are organic macromolecules made of amino acids, containing C, O, H but also nitrogen atom (N) 

In each molecule of amino acids, there is a central C atom (carbon). This atom, unique to each amino acid, forms bonds with other atoms (an H atom, an amino group (NH₂), a carboxyl group (COOH) and a radical (R)), which differ from one amino acid to another. There are 20 main amino acids in living organisms with properties that depend on the atoms and the R group‘s polarity.
Amino acids are linked together by a peptide bound, involving carboxyl group from one amino acid to another.
Small chains of amino acids are called peptides, whereas long chains are named proteins. 
The sequence of amino acids giving protein specificity is coded by genes. 

DNA and RNA

Besides lipids, carbohydrates and proteins, there is a fourth type of organic molecules: nucleic acids. They are units of polymers, called nucleotides.

A nucleotide is made of a pentose sugar, a phosphate group and a base (4 possibilities). DNA and RNA nucleotides differ by their sugar, the first contain a deoxyribose while the second one has a ribose. This difference gives polymers, DNA and RNA, distinct properties.
DNA structure has been hypothesized by Watson and Crick and different experiments confirmed this antiparallel two strands coiled in a double helix.
In this molecule, each nucleotide is link to the next one by a pentose / phosphate group bound.
Specific hydrogen bounds between the bases (Adenine(A)-thymine(T) and Cytosine(C)-Guanine(G)) of two strands is what makes DNA stable.
In RNA, Thymine is replaced by Uracil. RNA is made of one strand and the molecule is less stable. 
Nucleic acids carry information thanks to their base sequences.

DNA replication

DNA replication is a mechanism giving two identical DNA molecules from an original one. It takes place in the S phase of the cell cycle.

Different models made in order to explain DNA replication but the Meselson and Stahl experiment, using different N isotopes, confirmed that replication was semi-conservative:
each one of the two parental strands, when split, gives a matrix for a new strand thanks to complementary base pairing (A-T and C-G). Both daughter molecules are made of an old and a new strand.
DNA replication requires many enzymes to uncoil DNA, synthetize new strands and rewind new molecules. The DNA polymerase, an enzyme which is necessary for DNA synthesis, can cause mistakes and thus induce mutations in DNA sequencing.

From DNA to proteins

Genes, DNA base sequences, code for proteins is a process that includes DNA transcription into RNA, and RNA translation into polypeptide.  

Transcription requires the uncoiling of DNA before the RNA polymerase is able to make a copy of a DNA sequence thanks to complementary base pairing. 
After the RNA synthesis, messenger RNA (mRNA) is matured and sent to the cytoplasm, where translation can begin on ribosomes.
Translation requires different RNA types and proteins. According to the genetic code, the sequencing of RNA gives the sequencing of amino acids. Base triplets found on mRNA is code for a specific amino acid brought by transfer RNA (tRNA) to the ribosome and linked by a peptide bond to the peptide in construction. 
After translation, polypeptides are matured. That‘s when they get their shape and function.

Enzymes

Cells reactions are sped up by biological catalysts called enzymes.

Enzymes, mostly proteins, can race cellular reactions thanks to their particular 3D shape. 
A specific substrate can bind to the active site of a specific enzyme (substrate specificity). This binding reduces the energy needed to convert substrate into specific products (specificity of action). After the reaction, products are released from the enzyme.
Enzyme setting depends on genes and their expression. Enzymes can be affected by mutation, their amino acids sequence being crucial for protein shape. Enzymes can also be affected by the environment, as temperature (or pH) changes proteins shape.

Respiration

Cells require energy to function. The main form of energy they use is ATP (adenosine triphosphate). Because it cannot be stored, ATP has to be produced perpetually. 

To produce ATP, cells break down biological molecules (glucose, fatty acids…). This process is known as cellular respiration.

• Aerobic respiration = when oxygen is available, organic breakdown can be total. ATP production is important and only carbon dioxide and water are produced
• Anaerobic respiration = when oxygen is lacking, organic breakdown isn‘t total. ATP production is limited and other products accumulate (lactate or ethanol)

When exercised, muscle cells use anaerobic respiration when the oxygen supply isn‘t sufficient and then switch to aerobic respiration as soon as the oxygen transport matches their needs.

Photosynthesis

Cell photosynthesis is a phenomenal key for life. 
It produces new organic molecules using light energy.

Photosynthesis uses carbon dioxide and water to produce glucose and oxygen. The reaction isn‘t spontaneous and requires energy.
To collect light energy, light is absorbed by photosynthetic pigments. Those pigments (mainly chlorophyll A and B) absorb blue and red wavelength mostly. In this way, red and blue lights are much more efficient for photosynthesis. 
This process is linked to water photolysis. Electrons taken from it can be transferred to carbon dioxide. This reduction causes organic compounds to form. 
As a result, oxygen is a byproduct of photosynthesis. Photosynthetic organisms have played a crucial part in the evolution of the Earth’s atmosphere and oxygen increase.