passive transport
Passive transport mechanisms include diffusion, which is the flow of molecules across the membrane from a region where they are in high concentration to where they are in low concentration. This is accomplished primarily only by small, hydrophobic molecules because the oily core of the phospholipid bilayer poses a barrier to others. An exception is water, in which case the diffusion process is typically referred to as osmosis. In "facilitated diffusion" specialized carrier molecules, such as ion channels, catalyze the passive flow of their substrates across the membrane. Facilitated diffusion of water, for example in the kidneys, occurs via aquaporins.

peptide linkage
A peptide linkage is a chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, releasing a molecule of water (H₂O). This is a dehydration synthesis (or condensation) reaction and usually occurs between amino acids.  Polypeptides and proteins are chains of amino acids held together by peptide bonds.

phenotype
The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. Phenotype is determined in large part by genotype, or by the identity of the alleles that an individual carries at one or more positions on the chromosomes. Many phenotypes are determined by multiple genes and influenced by environmental factors. Thus, the identity of one or a few known alleles does not always enable prediction of the phenotype.

Nevertheless, because phenotypes are much easier to observe than genotypes (it doesn't take chemistry or sequencing to determine a person's eye color), classical genetics uses phenotypes to deduce the functions of genes. These inferences can then be checked by breeding experiments. In this way, early geneticists were able to trace inheritance patterns without any knowledge whatsoever of molecular biology.

phospholipid
A phospholipid is a lipid whose head contains a positively charged (polar) group that is linked to its two non-polar tails by a negatively charged phosphate group. Both tails consist of a fatty acid, each 14-24 carbon groups long. Phospholipids are the building blocks of biological membranes.

ATP, the "high-energy" exchange medium in the cell, is synthesized in the mitochondrion by addition of a third phosphate group to ADP in a process referred to as oxidative phosphorylation. ATP is then used at various points in the series of reactions that constitute glycolysis, to transfer energy to other small molecules. ATP is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts of plant cells.

ploidy
Ploidy indicates the number of repetitions of the basic number of chromosomes.

A haploid cell bears one set of chromosomes, a diploid cell bears two sets, and a polyploid cell bears many sets.

The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), though sex cells (sperm and oocytes) are haploid.

Aneuploidy is when a cell contains an abnormal or non-integer ploidy number. This may lead to problems in cell development. Most forms of aneuploidy in humans are lethal, but trisomy (three copies) of the sex chromosome (the cause of Klinefelter's syndrome and others) and of chromosome 21 (the cause of Down syndrome) are relatively common.

Many forms of cancer have incorrect ploidy numbers, due to the accumulation of mutations which increase chromosome missegregation.

potential energy
Energy that matter possesses because of its location or structure, energy stored that has the potential to do work. Examples: chemical bonds, such as the phosphate bonds of ATP, concentration gradients, height, water backed up behind a dam.

proton
The proton is a subatomic particle with a positive fundamental electric charge of 1.6 × 10^-19 coulomb. The nucleus of the most common isotope of the hydrogen atom, H, is a single proton. The nuclei of other atoms are composed of neutrons and protons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and what chemical element it is.

A proton has a mass of 1.6726231 × 10^-27 kg, which is about 1800 times heavier than an electron.

Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons.

Because the electromagnetic force is many orders of magnitude stronger than the gravitational force, the charge on the proton must equal to the charge on the electron, otherwise the net repulsion of having an excess of positive or negative charge (depending on which charge was numerically greater - atoms would not be electrically neutral) would cause a noticeable expansion effect on the universe, and indeed any gravitationally aggregated matter (planets, stars, etc.). It is taken that the positron (antielectron) has the same magnitude charge as the electron but opposite in sign; the same applies for the antiproton and proton.

In chemistry and biochemistry, the term proton may refer to the hydrogen ion in aqueous solution (in other words, the hydronium ion). In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).