Palisade cells: Tall cylindrical cells, packed with chloroplasts, found towards the upper surface of a leaf. The main site of photosynthesis in most plants.

Partial pressure: A measure of the amount of a particular gas in a mixture. You can’t just say ‘concentration of oxygen, or carbon dioxide’, because the rate at which it passes into our blood depends on the partial pressure. 20% of the air is oxygen, and at sea level the atmospheric pressure is 100KPa. So the partial pressure of oxygen is 20 KPa (20% of 100). If you were to climb a mountain, there would be less air pressing down on you, the atmospheric pressure would fall and so would the partial pressure of oxygen. There would be less of a partial pressure gradient between the air and your blood, so oxygen would enter more slowly. That’s why you would find it harder to breathe.

​Pathogen: Any “agent” that can causes disease, e.g. virus, bacterium, toxin, protozoan etc.

Pathogenic (pathogenicity): The ability to cause disease.

PCR: The polymerase chain reaction. DNA cloning in a test tube; a quick method of making millions of copies of a tiny DNA sample. e.g. in forensics.

Pedigree: Of known ancestry (a “family tree”). Applies to humans as well as animals

Persistent pesticide: (non-biodegradable) Pesticide which remains in the same chemical form over a long period of time.

Pest (agricultural): Organism that causes damage to crop plants.

Pesticide: Chemical toxic to living organisms that is used to control pests.

Phenotype: The observable features of an organism (genotype + environment = phenotype)

Phloem: vital vascular (transport) tissue in a plant. Transports the products of photosynthesis (e.g. sucrose) from sources to sinks in the process of translocation. . Consists of living cells, in contrast to xylem. 

Phylogenetic: In taxonomy, from origin of phyla i.e. what evolved from what? Bit like a family tree. Based on shared features, DNA and protein studies.

Pits: In plant cells; holes in adjacent cellulose cell walls, through which plasmodesmata pass

Plasma: Liquid part of the blood.

Plasmid: Tiny circle of DNA found in the cytoplasm of bacteria. Contains useful rather than essential genes e.g for antibiotic resistance. Used as a vector, to transfer genes into bacteria in genetic engineering.

Plasmodesmata: Thin strands of cytoplasm that penetrate the cells walls (at pits), connecting adjacent cells. Important in the symplast pathway, and in phloem where companion cells communicate with sieve tubes via plasmodesmata.

Point mutation: A mutation of one base (which can change the amino acid and the whole protein)

Polygenic: Controlled by many alleles (see continuous variation) e.g. height in humans

Polyploid: A cell/organism containing many sets of chromosomes e.g. wheat can have 8

​Population: All the members of the Same species living in one habitat.

Prokaryotic cell: Simple cells with no true nucleus or complex organelles. Bacteria are prokaryotic.

​Purkyne (or Purkinje) Fibres: Specialised muscle fibres (not nerves) along the ventricle walls that coordinate ventricular contraction., starting at the apex (bottom), ensuring that blood is forced up into the arteries.